JP7548355B1 - Foamed sheet, method for producing foamed sheet, and molded product - Google Patents

Abstract

[Problem] To provide a foamed sheet having excellent moldability, heat resistance, heat insulation, and biodegradability. [Solution] A foamed sheet made of a composition containing a polylactic acid resin, in which the polylactic acid resin contains 98 mol % or more of either the D-form of lactic acid or the L-form of lactic acid, which are constituent monomer units of the polylactic acid resin, in the polylactic acid resin, the content of the polylactic acid resin relative to the total amount of organic matter in the foamed sheet is 98 mass % or more, the bulk density of the foamed sheet is 0.063 g/cm3 or more and 0.250 g/cm3 or less, and the ratio [Ra/RSm] of the arithmetic mean roughness Ra calculated in accordance with JIS B 0601:2013 to the average length RSm of the roughness curve elements calculated in accordance with JIS B 0601:2013 is 0.050 or less for the shape in the TD direction of at least one surface of the foamed sheet as viewed from a cross section in the thickness direction and TD direction of the foamed sheet. [Selected Figure] Figure 1

Description

The present invention relates to a foam sheet, a method for producing a foam sheet, and a molded body.

Plastics are processed into various product shapes such as bags and containers and are widely distributed. However, plastic products are difficult to decompose in nature, making disposal of them after use a problem. In recent years, with growing environmental awareness, there has been active development of materials to replace non-biodegradable plastics, which are difficult to decompose in nature, with biodegradable biomass plastics, which are easily decomposed in nature.

Among biodegradable biomass plastics, polylactic acid resin has attracted attention as an alternative to non-degradable plastics because its properties are similar to polystyrene, a plastic that has been used traditionally. One form of use for polystyrene is expanded polystyrene, which is made by foaming polystyrene to give it properties such as light weight, cushioning, and insulation, and is widely used. Expanded polylactic acid, which uses polylactic acid resin, a biodegradable plastic, has also been proposed as an environmentally friendly alternative to expanded polystyrene (see, for example, Patent Documents 1 to 4).

It is generally pointed out that polylactic acid resin has low heat resistance when used for food containers due to its low glass transition temperature (approximately 60°C). However, it has been confirmed that heat resistance can be improved by increasing the crystallinity of polylactic acid resin during the molding process (see Patent Documents 5 and 6).

The objective of the present invention is to provide a foamed sheet that has excellent moldability, heat resistance, insulation, and biodegradability.

The foam sheet of the present invention as a means for solving the above-mentioned problems is a foam sheet made of a composition containing a polylactic acid resin, wherein the polylactic acid resin contains 98 mol % or more of either a D-form of lactic acid or an L-form of lactic acid, which are constituent monomer units of the polylactic acid resin, in the polylactic acid resin, and the content of the polylactic acid resin relative to the total amount of organic matter in the foam sheet is 98 mass % or more, the bulk density of the foam sheet is 0.063 g/cm ³ or more and 0.250 g/cm ³ or less, and the ratio [Ra/RSm] of the arithmetic mean roughness Ra calculated in accordance with JIS B 0601:2013 to the average length RSm of roughness curve elements calculated in accordance with JIS B 0601:2013 is 0.050 or less, with respect to a shape of at least one surface of the foam sheet in a direction perpendicular to the extrusion direction of the foam sheet, as viewed from a cross section in the thickness direction of the foam sheet and in a direction perpendicular to the extrusion direction of the foam sheet.

The present invention provides a foam sheet that is excellent in moldability, heat resistance, heat insulation, and biodegradability.

FIG. 1 is a schematic diagram illustrating the outermost surface shape in the TD direction of the cross section of the foamed sheet of the present invention. FIG. 2 is a schematic explanatory diagram showing an example of a continuous kneading apparatus used in the method for producing a foamed sheet of the present invention. FIG. 3 is a schematic diagram showing an example of a tandem type continuous foamed sheet production apparatus used in the foamed sheet production method of the present invention.

The foam sheet, the method for producing the foam sheet, and the molded product of the present invention are described in detail below. Note that the present invention is not limited to the embodiments shown below, and may be modified within the scope of what a person skilled in the art can imagine, such as other embodiments, additions, modifications, or deletions, and any embodiment is within the scope of the present invention as long as it provides the functions and effects of the present invention.

(Foam sheet)
The foamed sheet of the present invention is a foamed sheet made of a composition containing a polylactic acid resin (hereinafter, may be referred to as a "polylactic acid resin composition"), wherein the polylactic acid resin contains 98 mol % or more of either a D-form of lactic acid or an L-form of lactic acid, which are constituent monomer units of the polylactic acid resin, in the polylactic acid resin, the content of the polylactic acid resin with respect to the total amount of organic matter in the foamed sheet is 98 mass % or more, the bulk density of the foamed sheet is 0.063 g/cm ³ or more and 0.250 g/cm ³ or less, and the shape of at least one surface of the foamed sheet in a direction perpendicular to the extrusion direction of the foamed sheet, as viewed from a cross section in the thickness direction of the foamed sheet and in a direction perpendicular to the extrusion direction of the foamed sheet, is calculated in accordance with JIS B 0601:2013 and the arithmetic average roughness Ra is calculated in accordance with JIS B The ratio [Ra/RSm] to the average length RSm of the roughness curve elements calculated in accordance with JIS H0601:2013 is 0.050 or less.

In the present invention, the "extrusion direction of the foam sheet" may be referred to as the "MD (machine direction) direction," and the "direction perpendicular to the extrusion direction of the foam sheet" may be referred to as the "TD (transverse direction) direction." The TD direction of the foam sheet is synonymous with the width direction of the foam sheet.

The present inventors have found that when attempting to increase the expansion ratio of a foamed sheet to improve functions such as heat insulation or weight reduction, i.e., when producing a foamed sheet with low bulk density, corrugated wrinkles may occur on the surface of the foamed sheet during production, and that when corrugated wrinkles exist on the surface of a foamed sheet, molding defects such as tears at the corrugated portions occur when the foamed sheet is molded into a container shape.

In response to these problems, the inventors conducted extensive research and discovered that the above-described configuration makes it possible to provide a foamed sheet that is excellent in heat resistance, heat insulation, and biodegradability, and also has excellent moldability that prevents molding defects such as breakage when molded into container shapes such as food containers.

Since the foamed sheet of the present invention is made of a polylactic acid resin composition, the foamed sheet of the present invention may be called a "polylactic acid foamed sheet", an "expanded polylactic acid composition sheet", etc. As will be described in detail later, the foamed sheet of the present invention has good heat resistance and can be used, for example, as a heat-resistant food container.
The foamed sheet of the present invention refers to a sheet-like product obtained by foaming a polylactic acid resin composition.

[Physical properties of foam sheet]
Since the foamed sheet is obtained by foaming the polylactic acid resin composition, the physical properties of the foamed sheet and the physical properties of the polylactic acid resin composition are synonymous with each other, except for the physical properties that characterize the thermal history and shape of the foamed sheet. The details will be described later, but the physical properties that characterize the thermal history of the foamed sheet include cold crystallization enthalpy, and the physical properties that characterize the shape include bulk density, foam diameter, basis weight, Ra, and RSm.

- Bulk density -
The bulk density of the foamed sheet is 0.063 g/cm ³ or more and 0.250 g/cm ³ or less, preferably 0.063 g/cm ³ or more and 0.125 g/cm ³ or less, and more preferably 0.063 g/cm ³ or more and 0.098 g/cm ³ or less. If the bulk density of the foamed sheet is less than 0.063 g/cm ³ , the moldability is deteriorated, and if it exceeds 0.250 g/cm ³ , the heat insulation is insufficient. On the other hand, by setting the bulk density of the foamed sheet to 0.063 g/cm ³ or more and 0.250 g/cm ³ or less, the foamed sheet contains many bubbles having a high heat insulation effect, which leads to high heat insulation.

The bulk density of the foamed sheet can be adjusted by changing the expansion ratio by the foaming temperature, the amount of foaming agent, the type of die, etc., when producing the foamed sheet. Specifically, the expansion ratio can be increased by lowering the foaming temperature when producing the foamed sheet, increasing the amount of the foaming agent, using a circular die, etc., and therefore the bulk density of the foamed sheet can be reduced.

The bulk density of the foamed sheet in the present invention is a value measured as follows.
The foamed sheet is left to stand for 24 hours or more in an environment adjusted to a temperature of 23° C. and a relative humidity of 50%, and a test piece of 50 mm×50 mm is cut out. The bulk density of the cut test piece is measured by a liquid weighing method using an automatic specific gravity meter (e.g., DSG-1 manufactured by Toyo Seiki Seisakusho Co., Ltd.).
In the liquid weighing method, the mass (g) of a test piece of the foamed sheet in air is precisely weighed, and then the mass (g) of the test piece of the foamed sheet in water is precisely weighed, and the mass can be calculated according to the following formula (1).
Bulk density [g/cm ³ ]=density of water [g/cm ³ ]×mass of test piece in air [g]/(mass of test piece in air [g]−mass of test piece in liquid [g]) Equation (1)

- Foam diameter (median diameter) -
The foam diameter of the foamed sheet is not particularly limited and can be appropriately selected depending on the purpose, but the median diameter is preferably 1,220 μm or less, more preferably 800 μm or less, and even more preferably 500 μm or less. By setting the foam diameter (median diameter) of the foamed sheet to 1,220 μm or less, convection within the bubbles is suppressed, heat conduction is reduced, and heat insulation is improved, and by setting it to 800 μm or less, heat insulation is further improved.

The method for measuring the foam diameter (median diameter) of the foamed sheet is not particularly limited and can be appropriately selected depending on the purpose.
For example, the foam sheet is cut into a cross section using a sharp razor (e.g., 76 razor manufactured by Nisshin EM Co., Ltd.), and the cross section of the foam sheet is observed using a scanning electron microscope (SEM) (e.g., 3D real surface view microscope VE-9800 manufactured by KEYENCE Co., Ltd.). The magnification is adjusted so that the number of bubbles in the observation range is several tens to several hundreds so that an image suitable for the image analysis described below can be obtained (for example, 50 times for a foam diameter of about 100 μm). If necessary, multiple fields of view may be photographed, and the images may be linked and subjected to image analysis. The obtained image is subjected to region division by the watershed method (morphological segmentation) using, for example, image analysis software (e.g., MorphoLibJ plug-in of ImageJ). At this time, the tolerance is adjusted for each image so that the division is appropriate (for example, 60, etc.). The dividing lines of the regions are output as a binary image, and the distribution of the bubble area is obtained using the particle size analysis function of the image analysis software. At this time, bubbles in contact with the edge of the image are excluded from the analysis. The cumulative distribution of the bubble area is created using a spreadsheet software or the like, the area where the cumulative distribution is 50% is obtained, and the circle equivalent diameter of that area is calculated and used as the bubble diameter (median diameter).

- Grammage -
The basis weight of the foamed sheet is not particularly limited and can be appropriately selected depending on the application, but when the foamed sheet is used as a food packaging container, the basis weight is preferably 100 g/m ² or more and 300 g/m ² or less, more preferably 140 g/m ² or more and 280 g/m ² or less, and even more preferably 250 g/m ² or more and 280 g/m ² or less. When the basis weight of the foamed sheet is 100 g/m ² or more and 300 g/m ² or less, a foamed sheet that is both lightweight and strong tends to be obtained.

The method for measuring the basis weight of the foamed sheet in the present invention is not particularly limited, but for example, it can be measured as follows.
The foamed sheet is left to stand for 24 hours or more in an environment adjusted to a temperature of 23° C. and a relative humidity of 50%, and a test piece of 50 mm×50 mm is cut out. The mass of the test piece is measured with a balance. The basis weight can be calculated from the measured mass by the following formula (2). The basis weight of the foamed sheet is determined by measuring three or more points in each of the extrusion direction (MD direction) of the foamed sheet and the direction perpendicular to the MD direction (TD direction), and the arithmetic average value of the three points is used.
Basis weight [g/m ² ] = Measured mass [g]/(0.05m x 0.05m) ... Formula (2)

- Surface shape of foam sheet -
As described above, the shape of at least one surface of the foamed sheet in the TD direction when viewed from a cross section in the thickness direction and TD direction of the foamed sheet greatly affects molding defects such as breakage during molding of the foamed sheet. In particular, when the foamed sheet has corrugated wrinkles on its surface, molding defects are likely to occur during molding of the foamed sheet. In contrast, the surface of the foamed sheet of the present invention has very few undulations of corrugated wrinkles.

--Ratio [Ra/RSm]--
The corrugated wrinkles on the surface of the foamed sheet can be expressed as a ratio [Ra/RSm] of an arithmetic mean roughness Ra calculated in accordance with JIS B 0601:2013 (Geometric Product Specifications (GPS)-Surface Quality: Profile Curve Method-Terminology, Definitions and Surface Quality Parameters) to an average length RSm of roughness curve elements calculated in accordance with JIS B 0601:2013, for a shape of at least one surface of the foamed sheet in a direction perpendicular to the extrusion direction of the foamed sheet (TD direction) as viewed from a cross section in the thickness direction of the foamed sheet and in the direction perpendicular to the extrusion direction of the foamed sheet (TD direction).

The shape of at least one surface of the foamed sheet in the TD direction as viewed from a cross section in the thickness direction and TD direction of the foamed sheet will be specifically described with reference to the drawings.
FIG. 1 is a schematic diagram (perspective view) of the shape in the TD direction of at least one surface of the foamed sheet of the present invention, as viewed from a cross section in the thickness direction and TD direction of the foamed sheet. In FIG. 1, the up-down direction indicates the thickness direction of the foamed sheet 200, the left-right direction indicates the TD direction of the foamed sheet 200, and the depth direction indicates the MD direction of the foamed sheet 200. The contour shape of one surface (outermost surface) 202 of the foamed sheet 200, as viewed from a cross section 201 in the thickness direction and TD direction of the foamed sheet 200, has corrugated wrinkles (wave-shaped unevenness, periodic wave-shaped undulations), as shown by the thick line. Here, one surface (outermost surface) 202 of the foamed sheet 200 has been described, but the other surface (outermost surface) of the foamed sheet 200 also has a similar shape.

Hereinafter, in this specification, the shape of at least one surface of the foam sheet in the TD direction as viewed from a cross section of the foam sheet in the thickness direction and TD direction may be referred to as the "outermost surface shape in the TD direction of the cross section of the foam sheet."

Regarding the outermost surface shape in the TD direction of the cross section of the foam sheet, the ratio [Ra/RSm] of the arithmetic mean roughness Ra calculated in accordance with JIS B 0601:2013 to the average length RSm of the roughness curve element calculated in accordance with JIS B 0601:2013 is 0.050 or less, preferably 0.030 or less, and more preferably 0.021 or less. The ratio [Ra/RSm] being 0.050 or less indicates that the outermost surface in the TD direction of the cross section of the foam sheet has very few corrugated wrinkles, and the number of parts that can become the starting point of tears during molding of the foam sheet is small, and molding defects can be suppressed. On the other hand, if the ratio [Ra/RSm] is more than 0.050, the outermost surface in the TD direction of the cross section of the foam sheet has very many corrugated wrinkles, and the number of parts that can become the starting point of tears during molding of the foam sheet is large, resulting in molding defects.

--Arithmetic mean roughness Ra--
The arithmetic mean roughness Ra (hereinafter sometimes abbreviated as "arithmetic mean roughness Ra") calculated in accordance with JIS B 0601: 2013 for the outermost surface shape in the TD direction of the cross section of the foamed sheet correlates with the magnitude of undulations of the corrugated wrinkles of the foamed sheet. When the arithmetic mean roughness Ra is a large value, the corrugated wrinkles of the foamed sheet are more pronounced, which is likely to become the starting point of breakage when the foamed sheet is molded, leading to molding defects.

The arithmetic mean roughness Ra is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 0.15 mm or less, and more preferably 0.08 mm or less. By setting the arithmetic mean roughness Ra to 0.15 mm or less, molding defects during molding of the foamed sheet can be suppressed.

--Average length of roughness curve element RSm--
The average length RSm of roughness curve elements (hereinafter, sometimes abbreviated as "average length RSm of roughness curve elements") calculated in accordance with JIS B 0601:2013 for the outermost surface shape in the TD direction of the cross section of the foamed sheet indicates the period of the corrugated wrinkles of the foamed sheet. When the average length RSm of the roughness curve elements is small, the period of the corrugated wrinkles of the foamed sheet is short, resulting in sharper unevenness, which is likely to become the starting point of breakage when the foamed sheet is molded, leading to molding defects.

The average length RSm of the roughness curve element is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 4.0 mm or more, and more preferably 5.0 mm or more. By setting the average length RSm of the roughness curve element to 4.0 mm or more, molding defects during molding of the foam sheet can be suppressed.

In the present invention, the outermost surface shape in the TD direction of the cross section of the foamed sheet can be measured using a laser microscope, a three-dimensional measuring device, or the like, and the arithmetic mean roughness Ra and the average length of the roughness curve element RSm can be calculated from the attached software of the laser microscope or three-dimensional measuring device that measures the outermost surface shape in the TD direction of the cross section of the foamed sheet. The measuring device and measuring conditions are not particularly limited, but examples thereof include the following measuring methods.

First, to prepare a measurement sample, cut out a 5 cm x 5 cm square from the center of the foam sheet in the TD direction and fix the foam sheet to the stage of a laser microscope or three-dimensional measuring device using double-sided tape or similar to prevent it from floating.

The response waviness, which has a short period and a large height difference in the outermost surface shape in the TD direction of the cross section of the foam sheet, particularly affects molding defects when the foam sheet is molded. On the other hand, long-period waviness in the outermost surface shape in the TD direction of the cross section of the foam sheet does not significantly affect molding defects when the foam sheet is molded. Therefore, it is preferable to perform measurements while eliminating the influence of large waviness on the arithmetic mean roughness Ra and the average length RSm of the roughness curve element. In the present invention, when a value of 10 mm or more is measured when evaluating the average length RSm of the roughness curve element, the waviness cutoff (λc) is set to 10 mm and measurements are performed.

In the present invention, the arithmetic mean roughness Ra and the average length RSm of the roughness curve element of the foamed sheet are values calculated in accordance with JIS B 0601:2013, and specifically, the outermost surface shape in the TD direction of the cross section of the foamed sheet is observed with the following measuring device and measuring conditions, and the arithmetic mean roughness Ra and the average length RSm of the roughness curve element can be calculated using the attached software. Note that, in the present invention, the arithmetic mean roughness Ra and the average length RSm of the roughness curve element are calculated by cutting three or more measurement samples at different positions from the foamed sheet when preparing the measurement sample, and using the average values of the measurement results of three or more times in total.
[Measurement equipment and conditions]
・ Equipment: 3D measuring device VR-3200 (Keyence)
Observation magnification: 12x Measurement mode: Standard Measurement direction: Both sides Measurement brightness adjustment: Auto (Set value: 80)
- Reference plane setting: Select and execute for each of the x and y directions of the screen

-Cold crystallization enthalpy-
The cold crystallization enthalpy of the foamed sheet is not particularly limited and may be appropriately selected depending on the purpose, but is preferably 20 J/g or more, more preferably 30 J/g or more. The cold crystallization enthalpy of the foamed sheet being 20 J/g or more and the recrystallization enthalpy of the foamed sheet being 20 J/g or more mean that the crystallization rate of the foamed sheet is fast but there is still sufficient room for crystallization.

In thermoforming, the foam sheet is stretched at a temperature equal to or higher than the glass transition temperature, and the breaking elongation of the foam sheet tends to decrease as the crystallization degree of the foam sheet increases. If the cold crystallization enthalpy of the foam sheet is 20 J/g or more, problems such as the foam sheet not stretching during thermoforming, causing the molded product to break, or the foam sheet being unable to follow the mold and resulting in poor shaping can be prevented. There is no particular limit to the upper limit of the cold crystallization enthalpy of the foam sheet, and it can be selected appropriately depending on the purpose, but it is generally 50 J/g or less.

The cold crystallization enthalpy of the foam sheet can be adjusted to within the preferred range by appropriately setting the temperature of the entire foam sheet manufacturing apparatus or by cooling the extruded foam sheet. Although it depends on the concentration of the foaming agent in the foam sheet and the composition containing the polylactic acid resin, the cold crystallization enthalpy of the foam sheet tends to be adjusted to within the preferred range by setting the minimum temperature of the foam sheet manufacturing apparatus within a range of -20°C from the melting point of the polylactic acid resin and rapidly cooling the extruded foam sheet.

The cooling method for the foamed sheet is not particularly limited, and any known method can be used. For example, in the foamed sheet manufacturing method described below, a method in which the cylindrical foam extruded from the die is pulled along a cooled mandrel and cooled, or a method in which cooling air is blown around the outer periphery of the cylindrical foam can be used.

In the present invention, the cold crystallization enthalpy of the foamed sheet can be determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7122:2012 (Method for measuring heat of transition of plastics).
Specifically, about 5 mg to 10 mg of the foamed sheet is taken and flattened on a hot plate heated to 65°C by pressing (with a load of about 500 gf) with a copper rod (diameter about 20 mm, the rod also heated to 65°C) for 1 to 3 seconds to prepare a sample. This flattening is performed to improve the thermal contact between the sample and the sample pan and to measure the cold crystallization enthalpy with high accuracy. This sample is measured with the measuring device and measuring conditions described below, and analyzed by the analytical method described below. In the present invention, the cold crystallization enthalpy is calculated by the arithmetic average of the results obtained by performing the process from sample preparation to analysis five times.
[Measurement equipment and conditions]
・ Apparatus: Q-2000 (manufactured by TA Instruments)
Temperature program: Scan from 10° C. to 200° C. at a heating rate of 10° C./min (1st heating).
Analysis of cold crystallization enthalpy: The cold crystallization enthalpy is determined by integrating the area of the exothermic peak associated with crystallization observed at 60° C. to 100° C. during the first heating. The baseline for the integration is a straight line connecting the front and back of the exothermic peak.

-Cold crystallization temperature-
The cold crystallization temperature of the foamed sheet is not particularly limited and may be appropriately selected depending on the purpose, but is preferably 70° C. to 100° C., and more preferably 75° C. to 85° C. A cold crystallization temperature of 70° C. or higher is advantageous in terms of moldability, and a cold crystallization temperature of 100° C. or lower is advantageous in terms of heat resistance.
The cold crystallization temperature of the foamed sheet can be adjusted by the molar ratio of either the D-form of lactic acid or the L-form of lactic acid, which are constituent monomer units of the polylactic acid resin contained in the foamed sheet.

The cold crystallization temperature of the foamed sheet can be determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (Method for measuring transition temperature of plastics).
Specifically, a sample of 5 mg to 10 mg cut out from the foamed sheet is placed in a container of a differential scanning calorimeter (e.g., Q-2000 model manufactured by TA Instruments) and heated from 10° C. to 200° C. at a heating rate of 10° C./min. In this case, the peak top temperature of an exothermic peak observed in a temperature range equal to or higher than the glass transition temperature can be measured as the cold crystallization temperature of the foamed sheet.

- Recrystallization enthalpy -
In the present invention, the foamed sheet has high crystallinity meaning that the recrystallization enthalpy measured by DSC is 20 J/g or more. When the recrystallization enthalpy of the foamed sheet is 20 J/g or more, the shape of the foamed sheet can be fixed by crystallization on a mold during thermoforming, and the molded product can be imparted with heat resistance within a practical molding time, so that the foamed sheet has excellent thermoformability.

The upper limit of the recrystallization enthalpy of the foamed sheet is not particularly limited, but in the foamed sheet of the present invention, it is approximately 40 J/g to 50 J/g. The recrystallization enthalpy of the foamed sheet can be adjusted by a method of increasing the optical purity of the polylactic acid resin, a method of adding a component that acts as a crystal nucleating agent (a method of promoting the generation of crystal nuclei), a method of adding a component that acts as a crystallization promoter (a method of promoting the growth of crystal nuclei), a method of using an epoxy-functional (meth)acrylic monomer and a styrene monomer (an epoxy-functional (meth)acrylic-styrene-based chain extender) as a chain extender (crosslinking agent), and the like. Among these, it is preferable to adjust the recrystallization enthalpy of the foamed sheet by combining a method of increasing the optical purity of the polylactic acid resin and a method of using the epoxy-functional (meth)acrylic-styrene-based chain extender without inhibiting the foamability of the polylactic acid resin composition, from the viewpoints of the recyclability, environmental compatibility, and cost of the foamed sheet.

On the other hand, in extrusion molding of such polylactic acid resin compositions with excellent crystallinity, it is essential to maintain the temperature from kneading to extrusion, excluding the material input area, at or above the melting point of the polylactic acid resin composition or at or above the recrystallization temperature of the polylactic acid resin composition. The recrystallization temperature of the polylactic acid resin composition cannot be generalized because it can vary depending on the thermal history, the composition of the polylactic acid resin composition, the degree of shear, etc., but in the present invention, it is preferable that the temperature from kneading to extrusion is at least 20°C below the melting point of the polylactic acid resin composition.

If the temperature from kneading to extrusion is set to the melting point of the polylactic acid resin composition minus 20°C or less, the polylactic acid resin composition is cooled and the viscosity can be easily adjusted to a level suitable for foaming, but the degree of supercooling (the difference between the melting point and the resin temperature) becomes high, and there is a risk of operation stoppage due to sudden crystallization in the device, and the crystallization of the resulting foamed sheet progresses, tending to result in poor thermoformability. If the temperature from kneading to extrusion is set to the melting point of the polylactic acid resin composition minus 20°C or more, the risk of operation stoppage due to crystallization can be reduced, the crystallization degree of the resulting foamed sheet can be kept low, and a foamed sheet with excellent thermoformability tends to be obtained. The crystallization degree of the foamed sheet can be evaluated by the cold crystallization enthalpy of the foamed sheet measured by DSC.

In the present invention, the recrystallization enthalpy of the foamed sheet can be determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7122:2012 (Method for measuring heat of transition of plastics).
Specifically, about 5 mg to 10 mg of the foamed sheet is taken and flattened on a hot plate heated to 65°C by pressing (with a load of about 500 gf) with a copper rod (diameter about 20 mm, the rod also heated to 65°C) for 1 to 3 seconds to prepare a sample. This flattening is performed to improve the thermal contact between the sample and the sample pan and to measure the recrystallization enthalpy with high accuracy. This sample is measured with the measuring device and measuring conditions described below, and analyzed by the analytical method described below. In the present invention, the recrystallization enthalpy is calculated by the arithmetic average of the results obtained by performing the process from sample preparation to analysis five times.
[Measurement equipment and conditions]
・ Apparatus: Q-2000 (manufactured by TA Instruments)
Temperature program: Scan from 10° C. to 200° C. at a heating rate of 10° C./min (1st heating), hold at 200° C. for 1 minute, and then scan from 200° C. to 25° C. at a cooling rate of 10° C./min (1st cooling).
Analysis of recrystallization enthalpy: The area of the exothermic peak associated with crystallization observed in the first cooling is determined by integration, and the recrystallization enthalpy is determined. In the present invention, the position of the exothermic peak associated with crystallization is in the range of approximately 100°C to 130°C. The baseline for integration is a straight line connecting the front and rear of the exothermic peak.

-Recrystallization temperature-
The recrystallization temperature of the foamed sheet is not particularly limited and may be appropriately selected depending on the purpose, but is preferably 120° C. to 150° C., and more preferably 130° C. to 145° C. When the recrystallization temperature of the foamed sheet is 120° C. or higher, the crystallization rate of the polylactic acid resin is high, and crystallization advances during the molding process, which is advantageous for heat resistance, while when the recrystallization temperature is 150° C. or lower, crystallization does not advance too much at the time of production of the foamed sheet, and tearing during molding is unlikely to occur, which is advantageous from the viewpoint of moldability.
The recrystallization temperature of the foamed sheet can be adjusted by the molar ratio of either the D-form of lactic acid or the L-form of lactic acid, which are constituent monomer units of the polylactic acid resin contained in the foamed sheet.

The recrystallization temperature of the foamed sheet can be determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (Method for measuring transition temperature of plastics).
Specifically, a sample of 5 mg to 10 mg cut out from the foamed sheet is placed in a container of a differential scanning calorimeter (e.g., Q-2000 type manufactured by TA Instruments), and the temperature is increased from 10° C. to 200° C. at a rate of 10° C./min, maintained at that temperature for 10 minutes, and then decreased from 200° C. to 10° C. at a rate of 10° C./min. In this case, the peak top temperature of the exothermic peak can be measured as the recrystallization temperature of the foamed sheet.

-Melting point-
The melting point of the foamed sheet can be determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (Method for measuring transition temperature of plastics).
Specifically, the melting point of the foamed sheet can be measured by DSC using, for example, a differential scanning calorimeter (e.g., Q-2000 type, manufactured by TA Instruments). A sample of 5 mg to 10 mg of the foamed sheet is placed in a container of the differential scanning calorimeter, and heated at a heating rate of 10° C./min up to 200° C. at a heating rate of 10° C./min.
The peak top temperature of the endothermic peak (melting peak temperature, Tpm) associated with the melting of crystals observed on the higher temperature side than the glass transition temperature is taken as the melting point of the foamed sheet. When multiple endothermic peaks are observed on the higher temperature side than the glass transition temperature, the peak top temperature of the peak with the largest area is taken as the melting point of the foamed sheet. The melting point of the foamed sheet is generally within the range of 160°C to 190°C.

- Glass transition temperature -
The glass transition temperature of the foamed sheet is determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (measurement method for glass transition temperature of plastics).
Specifically, the glass transition temperature of the foamed sheet can be measured by DSC using, for example, a differential scanning calorimeter (e.g., Q-2000 type, manufactured by TA Instruments). A sample of 5 mg to 10 mg cut out from the foamed sheet is placed in a container of the differential scanning calorimeter, and the temperature is increased from 10° C. to 200° C. at a heating rate of 10° C./min.
The glass transition temperature in the present invention refers to the extrapolated glass transition onset temperature (Tig) described in JIS K 7121: 2012. The glass transition temperature of the foamed sheet is generally within the range of 55°C to 70°C.

-Weight average molecular weight (Mw)-
The weight average molecular weight (Mw) of the foamed sheet is not particularly limited and may be appropriately selected depending on the purpose. It is preferably from 230,000 to 600,000, more preferably from 250,000 to 400,000. When the weight average molecular weight (Mw) of the foamed sheet is from 230,000 to 600,000, the sheet has a suitable melt viscosity during foaming, and the foaming ratio is improved, leading to improved heat insulating properties.

The weight average molecular weight (Mw) of the foam sheet can be measured using gel permeation chromatography (GPC). The weight average molecular weight (Mw) is calculated using a calibration curve prepared using polystyrene samples with known weight average molecular weights (for example, A-500 (weight average molecular weight 589), A-1000 (weight average molecular weight 1,010), A-2500 (weight average molecular weight 312), A-5000 (weight average molecular weight 5,430), F-1 (weight average molecular weight 9,490), F-2 (weight average molecular weight 15,700), F-4 (weight average molecular weight 37,200), F-10 (weight average molecular weight 98,900), F-20 (weight average molecular weight 189,000), F-40 (weight average molecular weight 397,000), F-80 (weight average molecular weight 707,000), and F-128 (weight average molecular weight 1,110,000) manufactured by Tosoh Corporation).

The sample to be subjected to the GPC is prepared by mixing the foam sheet with chloroform so that the concentration of the foam sheet is about 2 mg/mL, shaking the mixture with a tabletop shaker (e.g., MSI-60 manufactured by AS ONE CORPORATION) for about half a day, confirming that the foam sheet has dissolved, and then filtering the mixture with a 0.45 μm membrane filter. Those that are difficult to dissolve can also be dissolved by heating at a temperature equal to or lower than the boiling point of chloroform. There are no particular limitations on the measuring device and measuring conditions for the GPC, but the weight average molecular weight (Mw) of the foam sheet can be measured by measuring the sample thus prepared with GPC under, for example, the following conditions.
[Measurement equipment and conditions]
・ Apparatus: HLC-8320GPC (manufactured by Tosoh Technosystems Co., Ltd.)
Column: TSKgel (registered trademark) guard column SuperHZ-L and TSKgel SuperHZM-M x 4 Detector: RI
Measurement temperature: 40°C
Mobile phase: chloroform Flow rate: 0.45 mL/min Injection volume: 20 μL

-Melt Viscosity-
The melt viscosity of the foamed sheet is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 10,000 Pa·s or more and 40,000 Pa·s or less. When the melt viscosity of the polylactic acid resin composition is 10,000 Pa·s or more, a sheet having low bulk density and excellent heat insulation, strength, and surface properties tends to be obtained while maintaining a low crystallinity, and when it is 40,000 Pa·s or less, the load on the foaming device increases, and poor productivity can be prevented.

The melt viscosity of the foamed sheet can be measured, for example, by weighing 1.5 g of the foamed sheet, drying it in a dryer at 80° C. for 2 hours, and using the resulting sample as a flow tester under the following measurement conditions.
[Measurement equipment and conditions]
・ Equipment: CFT-100EX (manufactured by Shimadzu Corporation)
Test conditions:
Constant temperature method, test temperature: 190°C, test force: 40 kgf, preheat time: 180 seconds, die hole diameter: 1 mm, die length: 1 mm
Analysis: Using the attached software CFT-EX, calculate the melt viscosity using the following calculation parameters.
Calculation parameters:
Limiting method, calculation start position: 3.0 mm, calculation end position: 7.0 mm, sample density: 1 g/cm ³

As a method for adjusting the melt viscosity of the foamed sheet to the preferred range, a method can be mentioned in which a polylactic acid resin having the characteristics disclosed in the present invention is melt-kneaded (reactive extrusion) with a chain extender (crosslinking agent) to obtain a polylactic acid resin composition. The melt viscosity of the foamed sheet varies greatly depending on the temperature from kneading to extrusion, but it is preferable to provide a section in the kneading section set at 200°C to 240°C, more preferably 220°C to 240°C.

<Composition containing polylactic acid resin>
In the present invention, the "composition containing a polylactic acid resin" means the composition before being foamed.
The composition containing the polylactic acid resin (polylactic acid resin composition) contains at least a polylactic acid resin, preferably a crystalline polylactic acid resin, and more preferably further contains a chain extender (also referred to as a "crosslinking agent") and a foaming nucleating agent, and may further contain other components as necessary.

<<Polylactic acid resin>>
The polylactic acid resin is biodegradable by microorganisms, and therefore has been attracting attention as an environmentally friendly polymer material with low environmental impact (see "Structure, Properties, and Biodegradability of Aliphatic Polyesters," Yoshio Inoue, Polymer, 2001, Vol. 50, No. 6, pp. 374-377). Examples of the polylactic acid resin include a copolymer of D-lactic acid (D-lactic acid) and L-lactic acid (L-lactic acid) (DL-lactic acid); a homopolymer of either D-lactic acid or L-lactic acid; a ring-opening polymer of one or more lactides selected from the group consisting of D-lactide (D-lactide), L-lactide (L-lactide), and DL-lactide. These may be used alone or in combination of two or more. The polylactic acid resin may be appropriately synthesized or commercially available.

The polylactic acid resin can be synthesized by a known method. For example, a method of producing lactide using lactic acid as a raw material and subjecting the lactide to ring-opening polymerization using an initiator such as alcohol, or a method of directly dehydrating and condensing lactic acid can be used.

The synthesis of the polylactic acid resin may include, in addition to the monomer, an initiator, a catalyst, an antioxidant, an end-capping agent, etc.

The initiator may be water or an alcohol having one or more active hydrogen groups.
The alcohol having one or more active hydrogen groups is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include aliphatic alcohols, polyalkylene glycols, polyhydric alcohols, etc. These may be used alone or in combination of two or more.

The polyhydric alcohol is not particularly limited and can be appropriately selected depending on the purpose. Examples include glycerin, trimethylolpropane, 1,4-cyclohexanedimethanol, neopentyl glycol, erythritol, etc. These may be used alone or in combination of two or more kinds.

[Physical properties of polylactic acid resin]
- Optical isomers -
When a copolymer of D-lactic acid and L-lactic acid (DL-lactic acid) or a ring-opening polymer of one or more lactides selected from the group consisting of D-lactide, L-lactide, and DL-lactide is used as the polylactic acid resin, the crystallinity tends to increase and the melting point and crystallization rate tend to increase as the amount of the lesser optical isomer of the D-form and the L-form decreases, and the crystallinity tends to decrease and eventually become amorphous as the amount of the lesser optical isomer of the D-form and the L-form increases.

In the present invention, since it is necessary to impart sufficient heat resistance by crystallization accompanying bubble growth during foaming, the molar ratio of either the D-form of lactic acid or the L-form of lactic acid, which are the constituent monomer units of the polylactic acid resin contained in the polylactic acid resin composition, is 98 mol% or more in the polylactic acid resin, and preferably 99 mol% or more. For this reason, the polylactic acid resin may be a polylactic acid resin consisting of only one optical isomer of either the D-form of lactic acid or the L-form of lactic acid. If either the D-form of lactic acid or the L-form of lactic acid, which are the constituent monomer units of the polylactic acid resin in the polylactic acid resin, is less than 98 mol%, the molded product obtained by molding the foamed sheet made of the polylactic acid resin composition does not have good heat resistance. On the other hand, by making either the D-form of lactic acid or the L-form of lactic acid, which are the constituent monomer units of the polylactic acid resin in the polylactic acid resin, 98 mol% or more, the crystallization rate is increased, crystallization progresses during the molding process, and the heat resistance of the molded product is improved.

The ratio of either the D-lactic acid or the L-lactic acid, which are the constituent monomer units, can be confirmed by analysis using liquid chromatography using an optically active column.
Specifically, the foamed sheet is freeze-pulverized, 200 mg of the powder of the foamed sheet is placed in an Erlenmeyer flask, 30 mL of 1N aqueous sodium hydroxide is added, and the Erlenmeyer flask is heated to 65°C while shaking to completely dissolve the polylactic acid resin in the foamed sheet. Then, the pH is adjusted to 4-7 using 1N hydrochloric acid, and the solution is diluted to a predetermined volume using a measuring flask to obtain a polylactic acid resin solution. Next, the polylactic acid resin solution is filtered through a 0.45 μm membrane filter and then analyzed using a liquid chromatograph. Based on the obtained chart, the area ratio is calculated from the peaks derived from the D-form of lactic acid and the L-form of lactic acid, and the amount of the D-form of lactic acid and the amount of the L-form of lactic acid are calculated as the abundance ratio. The arithmetic average of the results obtained by performing the above operation three times is taken as the amount of the D-form and the amount of the L-form of lactic acid constituting the polylactic acid resin in the foamed sheet.

The measurement device and measurement conditions for the liquid chromatography are not particularly limited, but for example, the measurement can be performed using the following measurement device and measurement conditions.
[Measurement equipment and conditions]
HPLC device (liquid chromatograph): PU-2085Plus system (manufactured by JASCO Corporation)
Column: Chromolith (registered trademark) coated with SUMICHIRAL OA-5000 (inner diameter 4.6 mm, length 250 mm) (manufactured by Sumitomo Analysis Center Co., Ltd.)
Column temperature: 25°C
Mobile phase: a mixture of 2 mM _CuSO4 aqueous solution and 2-propanol ( _CuSO4 aqueous solution: 2-propanol (volume ratio) = 95:5)
Mobile phase flow rate: 1.0 mL/min Detector: UV 254 nm
Injection volume: 20 μL

When the foamed sheet is subjected to the above-mentioned liquid chromatography analysis, if the area of the larger peak of the peaks derived from the D- and L-forms of lactic acid is 98% or more of the total area of the peaks derived from the D- and L-forms of lactic acid, it can be said that either the D- or L-form of lactic acid, which is a constituent monomer unit of the polylactic acid resin in the foamed sheet, accounts for 98 mol% or more of the polylactic acid resin.

- Weight average molecular weight -
The weight average molecular weight (Mw) of the polylactic acid resin is not particularly limited and can be appropriately selected according to the purpose, but is preferably 180,000 to 320,000, and more preferably 210,000 to 310,000. When the weight average molecular weight (Mw) of the polylactic acid resin is 180,000 to 320,000, the viscosity of the polylactic acid resin composition can be controlled to a range suitable for foaming, and the coalescence and breakage of bubbles during the production of the foamed sheet can be suppressed, and a foamed sheet having excellent foaming ratio and surface properties can be stably obtained. On the other hand, when the weight average molecular weight (Mw) of the polylactic acid resin is 180,000 or less, the amount of chain extender required to adjust the viscosity of the polylactic acid resin composition to a range suitable for foaming becomes large, or it tends to be difficult to adjust the viscosity to a suitable range for foaming. The chain extender is generally a petroleum-derived compound and is non-biodegradable, so it is desirable to reduce the amount of addition as much as possible from the viewpoint of reducing the environmental load. Furthermore, when the weight average molecular weight (Mw) of the polylactic acid resin is 320,000 or more, the viscosity of the polylactic acid resin composition tends to change sharply depending on the amount of the chain extender added, which reduces controllability of the viscosity of the polylactic acid resin composition and may make it difficult to stably produce the foamed sheet.

The weight average molecular weight (Mw) of the polylactic acid resin can be measured using GPC. The weight average molecular weight (Mw) of the polylactic acid resin is calculated using a calibration curve prepared using polystyrene samples with known weight average molecular weights (e.g., A-500 (weight average molecular weight 589), A-1000 (weight average molecular weight 1,010), A-2500 (weight average molecular weight 312), A-5000 (weight average molecular weight 5,430), F-1 (weight average molecular weight 9,490), F-2 (weight average molecular weight 15,700), F-4 (weight average molecular weight 37,200), F-10 (weight average molecular weight 98,900), F-20 (weight average molecular weight 189,000), F-40 (weight average molecular weight 397,000), F-80 (weight average molecular weight 707,000), and F-128 (weight average molecular weight 1,110,000) manufactured by Tosoh Corporation).

The sample to be subjected to the GPC is prepared by mixing the polylactic acid resin and chloroform so that the concentration of the polylactic acid resin is about 2 mg/mL, shaking the mixture for about half a day with a tabletop shaker (e.g., MSI-60 manufactured by AS ONE Corporation), and after confirming that the polylactic acid resin has dissolved, filtering the mixture through a 0.45 μm membrane filter to obtain a filtrate. Materials that are difficult to dissolve can also be dissolved by heating at a temperature below the boiling point of chloroform. The sample thus prepared can be measured, for example, with the same measuring device and under the same measuring conditions as those used to measure the weight average molecular weight (Mw) of the foamed sheet.

-Acid value-
The acid value of the polylactic acid resin in the polylactic acid resin composition is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 5 mgKOH/g or less. When the acid value of the polylactic acid resin is 5 mgKOH/g or less, the viscosity of the polylactic acid resin composition can be easily adjusted to a range suitable for foaming, and further, the decomposition of the polylactic acid resin during storage tends to be suppressed.
The method for measuring the acid value of the polylactic acid resin is not particularly limited, but it can be determined, for example, by a titration method.

Specifically, about 1 g to 3 g of the polylactic acid resin is placed in an Erlenmeyer flask, 40 mL of dichloromethane is added, and the mixture is shaken at room temperature for about half a day, and the resulting dichloromethane solution of the polylactic acid resin is used as a sample for measurement. If the viscosity of the dichloromethane solution is high, the amount of polylactic acid resin is reduced or the amount of dichloromethane is appropriately increased to adjust the sample. Phenolphthalein is added to the sample as an indicator, and the sample and blank are titrated using a 0.01 N ethanol solution of potassium hydroxide, and the acid value is calculated using the following formula (3).
Acid value [mgKOH/g] = (titer value [mL] - blank [mL]) x factor x 0.01 [N] x 56.1 [g/mol] / (mass of sample [g]) ... formula (3)

- Moisture content -
It is preferable that the water content of the polylactic acid resin is reduced to 500 ppm or less before it is used in the production of the polylactic acid resin composition or the foamed sheet. When the water content of the polylactic acid resin is 500 ppm or less, the viscosity of the polylactic acid resin composition tends to be adjusted to an appropriate level for foaming.

The method for measuring the water content of the polylactic acid resin is not particularly limited, and any known method can be used, such as Karl Fischer titration.
The method for drying the polylactic acid resin is not particularly limited, and any known method can be used, such as a method using a hot air dryer or a vacuum dryer.
The drying temperature is not particularly limited, but is preferably 60° C. to 80° C. The method for measuring the water content is not particularly limited, and for example, Karl Fischer titration or the like can be applied.

- Content -
The content of the polylactic acid resin in the polylactic acid resin composition is not particularly limited and can be appropriately selected depending on the purpose, but from the viewpoint of biodegradability and recyclability (easy recycling), it is preferably 98% by mass or more and more preferably 99% by mass or more based on the total amount of organic matter in the polylactic acid resin composition. Note that the content of the polylactic acid resin based on the total amount of organic matter in the polylactic acid resin composition is synonymous with the content of the polylactic acid resin based on the total amount of organic matter in the foamed sheet, since the foamed sheet is made of the polylactic acid resin composition.

The organic matter in the polylactic acid resin composition is mainly the polylactic acid resin, but examples of organic matter other than the polylactic acid resin include organic nucleating agents and chain extenders as foam nucleating agents described below. When an inorganic nucleating agent is used as the foam nucleating agent for the foam sheet, the inorganic nucleating agent does not fall under the category of organic matter.

The content of the polylactic acid resin relative to the total amount of organic matter in the polylactic acid resin composition can be calculated from the mixing ratio (feed ratio) when preparing the polylactic acid resin composition. If the mixing ratio is unknown, it may be determined by nuclear magnetic resonance analysis (NMR) as follows.

The solvent used in the nuclear magnetic resonance analysis is approximately 100 mg of 1,3,5-trimethoxybenzene standard (for quantitative NMR, Fujifilm Wako Pure Chemical Industries, Ltd.) dissolved in deuterated chloroform (containing 0.3% by volume of tetramethylsilane (TMS)) in a 10 mL measuring flask as an internal standard.

For the sample to be subjected to the nuclear magnetic resonance analysis, the polylactic acid resin composition or the foamed sheet is mixed with the solvent so that the concentration of the polylactic acid resin composition or the foamed sheet becomes 10 mg/mL, and the mixture is dissolved by shaking for about half a day using a tabletop shaker (e.g., MSI-60 manufactured by AS ONE Corporation). At this time, in order to minimize the change in the sample concentration due to evaporation, the smallest possible container is selected. The sample prepared by the above method is sealed in a sample tube with a diameter of 5 mm and subjected to NMR.

Using the above sample, ¹ H NMR measurement ( ¹ H-NMR measurement) is performed in accordance with JIS K0138:2018 (General rules for quantitative nuclear magnetic resonance spectroscopy (qNMR general rules)) using the following measurement device and measurement conditions.
[Measurement equipment and conditions]
・Nuclear magnetic resonance (NMR) device: JNM-ECX-500 FT-NMR (manufactured by JEOL Ltd.)
Observed nucleus: 1H
Measurement temperature: 30°C
Spin: Off Digital resolution: 0.25Hz
Observation range: -0.5 to 15 ppm
Pulse angle: 90°
Relaxation time: 60 seconds. Number of scans: 16 (two dummy scans were performed before the actual measurement).
・13C decoupling: Yes

The obtained data is integrated with respect to the peaks of the following chemical shifts, and the integral ratio is calculated according to the following formula (4).
Integral 1 (derived from polylactic acid resin): 5.2 ppm
Integral 2 (from internal standard): 6.1 ppm
Integral ratio = integral 1 / (integral 2 × sample mass) ... formula (4)

A similar NMR measurement is performed on a polylactic acid resin of known purity using the same solvent as the sample, and the ratio of the integral ratio obtained from the formula (4) for the polylactic acid resin of known purity to the integral ratio for the sample is calculated, and the content of the polylactic acid resin relative to the total amount of organic matter in the polylactic acid resin composition or the foamed sheet is calculated using the following formula (5). Note that the steps from sample preparation to analysis are performed three times, and the arithmetic average of the obtained polylactic acid resin contents is taken as the content of the polylactic acid resin relative to the total amount of organic matter in the polylactic acid resin composition or the foamed sheet.
Content of polylactic acid resin [mass %]=100×purity of polylactic acid resin of known purity [mass %]×(integral ratio of sample)/(integral ratio of polylactic acid resin of known purity) (5)

-Melting point-
The melting point of the polylactic acid resin can be determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (Method for measuring transition temperature of plastics).
Specifically, the melting point of the polylactic acid resin can be measured by DSC using, for example, a differential scanning calorimeter (e.g., Q-2000 type, manufactured by TA Instruments). A sample of 5 mg to 10 mg of the polylactic acid resin is placed in a container of the differential scanning calorimeter, and the temperature is increased at a rate of 10° C./min up to 200° C. at a rate of 10° C./min.
The peak top temperature (melting peak temperature, Tpm) of the endothermic peak associated with the melting of crystals observed on the higher temperature side than the glass transition temperature is taken as the melting point of the polylactic acid resin. When multiple endothermic peaks are observed on the higher temperature side than the glass transition temperature, the peak top temperature of the peak with the largest area is taken as the melting point of the polylactic acid resin. The melting point of the polylactic acid resin is generally within the range of 150°C to 190°C.

- Glass transition temperature -
The glass transition temperature of the polylactic acid resin can be determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (measurement method for glass transition temperature of plastics).
Specifically, the glass transition temperature of the polylactic acid resin can be measured by DSC using, for example, a differential scanning calorimeter (e.g., Q-2000 type, manufactured by TA Instruments). A sample of 5 mg to 10 mg of the polylactic acid resin is placed in a container of the differential scanning calorimeter, and the temperature is increased from 10° C. to 200° C. at a heating rate of 10° C./min.
The glass transition temperature in the present invention refers to the extrapolated glass transition onset temperature (Tig) described in JIS K 7121: 2012. The glass transition temperature of the polylactic acid resin is generally within the range of 55°C to 70°C.

-Melt Viscosity-
The melt viscosity of the polylactic acid resin is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 500 Pa.s to 1,500 Pa.s. When the melt viscosity of the polylactic acid resin is 500 Pa.s to 1,500 Pa.s, the viscosity of the polylactic acid resin composition can be easily adjusted to a viscosity suitable for foaming, the load on the foaming device can be reduced, productivity can be improved, and a foamed sheet having a low bulk density and excellent surface properties can be obtained.

The melt viscosity of the polylactic acid resin can be measured, for example, by weighing 1.5 g of the polylactic acid resin, drying it in a dryer at 80° C. for 2 hours, and using the resultant as a sample, using a flow tester under the following measurement conditions.
[Measurement equipment and conditions]
・ Equipment: CFT-100EX (manufactured by Shimadzu Corporation)
Test conditions: constant temperature method, test temperature: 190°C, test force: 40 kgf, preheat time: 180 seconds, die hole diameter: 1 mm, die length: 1 mm
Analysis: Using the attached software CFT-EX, calculate the melt viscosity using the following calculation parameters.
Calculation parameters: limiting method, calculation start position: 3.0 mm, calculation end position: 7.0 mm, sample density: 1 g/cm ³

- Ratio of melt viscosity of foam sheet to that of polylactic acid resin -
The ratio of the melt viscosity of the foamed sheet to the melt viscosity of the polylactic acid resin is not particularly limited and may be appropriately selected depending on the purpose, but is preferably from 7 to 80. When the ratio of the melt viscosity of the foamed sheet to the melt viscosity of the polylactic acid resin is from 7 to 80, the load on the foaming device is reduced, resulting in excellent productivity, and a foamed sheet having a low bulk density and excellent heat insulation properties, strength, and surface properties can be obtained while maintaining a low degree of crystallinity.

The ratio of the melt viscosity of the foamed sheet to the melt viscosity of the polylactic acid resin is calculated by the following formula (6).
Ratio of melt viscosity of foamed sheet to melt viscosity of polylactic acid resin=melt viscosity of foamed sheet/melt viscosity of polylactic acid resin ... formula (6)

<<Chain extender (crosslinking agent)>>
The chain extender (crosslinking agent) is preferably contained in the polylactic acid resin composition in order to adjust the viscosity of the polylactic acid resin to a range suitable for foaming. The chain extender (crosslinking agent) also has the effect of improving the heat resistance and hydrolysis resistance of the polylactic acid resin composition and the foamed sheet.

The chain extender (crosslinking agent) is not particularly limited and can be appropriately selected depending on the purpose. For example, a compound reactive with a hydroxyl group and/or a carboxylic acid group, a peroxide, etc. can be used. In the present invention, a compound reactive with a hydroxyl group and/or a carboxylic acid group of the polylactic acid resin is preferred from the viewpoint of not inhibiting the crystallinity of the polylactic acid resin after reaction with the polylactic acid resin or being able to improve the crystallinity.

The compound reactive with the hydroxyl group and/or carboxylic acid group of the polylactic acid resin is not particularly limited and can be appropriately selected depending on the purpose, but a compound having an epoxy group, a compound having an isocyanate group, or a compound having a carbodiimide group is preferred. These may be used alone or in combination of two or more.

As the compound reactive with the hydroxyl group and/or carboxylic acid group of the polylactic acid resin, a compound having two or more reactive groups in the molecule is preferred from the viewpoint of introducing a branched structure into the polylactic acid resin, efficiently improving the viscosity of the polylactic acid resin composition, and reducing the release of unreacted chain extender (crosslinking agent), and a compound having two or more epoxy groups in the molecule or a compound having two or more isocyanate groups in the molecule is more preferred, and from the viewpoints of workability and safety, a compound having two or more epoxy groups in the molecule is even more preferred, and from the viewpoints of reactivity, workability, and safety, an epoxy-functional (meth)acrylic-styrene chain extender having two or more epoxy groups in the molecule is particularly preferred.

- Compounds having epoxy groups -
The compound having two or more epoxy groups in the molecule as the chain extender (crosslinking agent) is a compound obtained by copolymerizing a (meth)acrylic monomer having at least an epoxy group.
The (meth)acrylic monomer having an epoxy group is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include monomers containing a 1,2-epoxy group such as glycidyl acrylate, glycidyl methacrylate, etc. These may be used alone or in combination of two or more kinds.

The compound having two or more epoxy groups in the molecule may further contain, as a copolymerization component, a (meth)acrylic monomer having no epoxy group in addition to the monomer. The (meth)acrylic monomer having no epoxy group is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, cyclohexyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, and cyclohexyl methacrylate. These may be used alone or in combination of two or more.

The compound having two or more epoxy groups in the molecule may further contain a monomer having a double bond group in addition to the (meth)acrylic monomer having no epoxy group. The monomer having a double bond group is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include styrene monomer, α-methylstyrene monomer, vinyl acetate monomer, etc. These may be used alone or in combination of two or more. Examples of the styrene monomer include styrene, α-methylstyrene, etc.

The epoxy-functional (meth)acrylic-styrene chain extender having two or more epoxy groups in the molecule is a compound obtained by copolymerizing at least an epoxy-group-containing (meth)acrylic monomer and a styrene monomer.

The compound having an epoxy group may be appropriately synthesized, or a commercially available product may be used. Examples of commercially available products of the compound having an epoxy group include, by trade name, Marproof (registered trademark) G-01100 (manufactured by NOF Corporation), Marproof (registered trademark) G-0105SA (manufactured by NOF Corporation), Marproof (registered trademark) G-2050M (manufactured by NOF Corporation), Marproof (registered trademark) G-0130SP (manufactured by NOF Corporation), Marproof (registered trademark) G-0130SF (manufactured by NOF Corporation), Marproof (registered trademark) G-0250SP (manufactured by NOF Corporation), Marproof G-0250SF (manufactured by NOF Corporation), Metablen (registered trademark) P1901 (manufactured by Mitsui Chemical Co., Ltd.), Joncy (registered trademark) ADR4368 (manufactured by BASF), Joncy (registered trademark) ADR4370 (manufactured by BASF), and Joncy (registered trademark). Examples of such an adhesive include ADR4468 (manufactured by BASF), Bondfast (registered trademark) BF-2C (manufactured by Sumitomo Chemical Co., Ltd.), Bondfast (registered trademark) BF-E (manufactured by Sumitomo Chemical Co., Ltd.), Bondfast (registered trademark) BF-2B (manufactured by Sumitomo Chemical Co., Ltd.), Bondfast (registered trademark) BF-7B (manufactured by Sumitomo Chemical Co., Ltd.), Bondfast (registered trademark) BF-7M (manufactured by Sumitomo Chemical Co., Ltd.), CESA-Extend OMAN698493 (manufactured by Clariant), and ARUFON UG-4040 (manufactured by Toagosei Co., Ltd.). Among these, as commercially available products of the compound having the epoxy group, it is preferable to use Marproof (registered trademark) G-0250SP (manufactured by NOF Corporation), Marproof (registered trademark) G-0250SF (manufactured by NOF Corporation), and Joncy (registered trademark) ADR4468 (manufactured by BASF) from the viewpoints of heat insulation, biodegradability, and prevention of molding defects, because they can achieve a high expansion ratio at a low addition concentration under conditions for manufacturing a foam sheet in which corrugated wrinkles are unlikely to occur in the foam sheet.

The content of the compound having an epoxy group is not particularly limited and can be appropriately selected according to the purpose, but is preferably 0.2% by mass or more and 1.3% by mass or less, and more preferably 0.7% by mass or more and 0.9% by mass or less, relative to the total amount of organic matter in the polylactic acid resin composition. Since the compound having an epoxy group is a non-biodegradable material, the content is reduced as much as possible to increase the content of the polylactic acid resin in the polylactic acid resin composition, thereby improving biodegradability. On the other hand, when the content of the compound having an epoxy group is low, the melt viscosity during melt foaming is low and the foaming ratio does not increase, so that the insulation properties may be deteriorated. By setting the content of the compound having an epoxy group to 0.2% by mass or more and 1.3% by mass or less relative to the total amount of organic matter in the polylactic acid resin composition, it is possible to achieve both biodegradability and insulation properties. In addition, when a compound having an epoxy group with a weight average molecular weight (Mw) and epoxy equivalent within an appropriate range is used as a chain extender (crosslinking agent), high insulation properties can be obtained even when the content of the compound having an epoxy group is low, leading to improved biodegradability.

The epoxy equivalent of the compound having an epoxy group is not particularly limited and can be appropriately selected according to the purpose, but is preferably 250 to 350, more preferably 270 to 330. The epoxy equivalent of the compound having an epoxy group affects the distance between crosslinking points of the reaction product of the polylactic acid resin and the compound having an epoxy group. When the amount (mass) of the compound having an epoxy group is the same as the amount (mass) of the polylactic acid resin, the smaller the epoxy equivalent of the compound having an epoxy group, the more the amount of epoxy groups per molecule of the compound having an epoxy group increases, and the shorter the distance between crosslinking points of the reaction product of the polylactic acid resin and the compound having an epoxy group becomes, and the larger the epoxy equivalent of the compound having an epoxy group, the more the amount of epoxy groups per molecule of the compound having an epoxy group decreases, and the longer the distance between crosslinking points of the reaction product of the polylactic acid resin and the compound having an epoxy group becomes. If the distance between crosslinking points in the reaction product of the polylactic acid resin and the compound having an epoxy group is too short, the crosslinking structure will be too localized in the reaction product of the polylactic acid resin and the compound having an epoxy group, causing unevenness in the melt viscosity and preventing an increase in the expansion ratio. If the distance between crosslinking points in the reaction product of the polylactic acid resin and the compound having an epoxy group is too long, the effect of improving the melt viscosity will be small, and the expansion ratio will not increase. As a result, by setting the epoxy equivalent of the compound having an epoxy group to 250 or more and 350 or less, the melt viscosity can be increased without unevenness, leading to a high expansion ratio and improved insulation.

The method for measuring the epoxy equivalent of the compound having an epoxy group is not particularly limited, but it can be measured by a titration method in accordance with JIS K 7236:2001 (method for determining the epoxy equivalent of an epoxy resin).
Specifically, 10 mL of chloroform is added to 0.1 g to 0.3 g of the compound having an epoxy group, and the mixture is completely dissolved while stirring with a magnetic stirrer or the like. 20 mL of acetic acid and 10 mL of a chloroform solution of tetraethylammonium bromide (concentration: 0.25 g/mL) are added to prepare a sample. The epoxy equivalent of the sample thus prepared can be measured, for example, using the following measuring device and under the following measuring conditions.
[Measurement equipment and conditions]
・ Equipment: Automatic titration equipment COM-A-19 (manufactured by HIRANUMA Co., Ltd.)
・ Standard solution: 0.1 mol/L perchloric acid-acetic acid standard solution ・ Electrode: Glass electrode GTRS10B
Reference electrode GTPH1B (internal solution is saturated sodium perchlorate/acetic acid solution)
Measurement mode: Inflection point detection Differential judgment value: 100 mV/mL
・Calculation formula: 1,000×S/((A1-BL)×M×f)
Here, S is the mass (g) of the compound having an epoxy group, A1 is the drop amount (mL) at the inflection point, BL is the result of the blank measurement (mL), M is the concentration of the standard solution (mol/L), and f is the factor of the standard solution. The blank measurement is performed twice, and the average value of the two measurements is used.

The weight average molecular weight (Mw) of the compound having an epoxy group is not particularly limited and can be appropriately selected according to the purpose, but is preferably 10,000 to 20,000, and more preferably 12,500 to 17,500. The weight average molecular weight (Mw) of the compound having an epoxy group affects the number of crosslinking points of the reaction product of the polylactic acid resin and the compound having an epoxy group. It is also affected by the number of epoxy groups per molecule of the compound having an epoxy group, but when the number of epoxy groups per molecule is the same and the amount (mass) of the compound having an epoxy group is the same as the amount (mass) of the polylactic acid resin, the smaller the weight average molecular weight (Mw) of the compound having an epoxy group, the smaller the number of crosslinking points of the reaction product of the polylactic acid resin and the compound having an epoxy group, and the larger the weight average molecular weight (Mw) of the compound having an epoxy group, the larger the number of crosslinking points of the reaction product of the polylactic acid resin and the compound having an epoxy group. The greater the number of crosslinking points of the reaction product of the polylactic acid resin and the compound having an epoxy group, the more the molecular chains of the reaction product of the polylactic acid resin and the compound having an epoxy group become entangled with each other, and the melt viscosity of the polylactic acid resin composition during melt foaming increases, the foaming ratio increases, and the thermal insulation property is improved. However, if the weight average molecular weight (Mw) of the compound having an epoxy group is too large, the compound having an epoxy group loses fluidity at the reaction temperature, the reactivity between the polylactic acid resin and the compound having an epoxy group decreases, and the melt viscosity of the polylactic acid resin composition does not increase, and the foaming ratio does not increase. As a result, by setting the weight average molecular weight of the compound having an epoxy group to 10,000 or more and 20,000 or less, the melt viscosity of the polylactic acid resin composition increases, the foaming ratio increases, and the thermal insulation property is improved.

The weight average molecular weight (Mw) of the compound having the epoxy group can be measured using GPC. The weight average molecular weight (Mw) of the compound having an epoxy group is calculated based on a calibration curve prepared using polystyrene samples with known weight average molecular weights (e.g., A-500 (weight average molecular weight 589), A-1000 (weight average molecular weight 1,010), A-2500 (weight average molecular weight 312), A-5000 (weight average molecular weight 5,430), F-1 (weight average molecular weight 9,490), F-2 (weight average molecular weight 15,700), F-4 (weight average molecular weight 37,200), F-10 (weight average molecular weight 98,900), F-20 (weight average molecular weight 189,000), F-40 (weight average molecular weight 397,000), F-80 (weight average molecular weight 707,000), and F-128 (weight average molecular weight 1,110,000) manufactured by Tosoh Corporation).

The sample to be subjected to the GPC is prepared by mixing the compound having an epoxy group with chloroform so that the concentration of the compound having an epoxy group is about 2 mg/mL, shaking the mixture with a tabletop shaker (e.g., MSI-60 manufactured by AS ONE Corporation) for about half a day, and after confirming that the compound having an epoxy group has dissolved, filtering the mixture with a 0.45 μm membrane filter before use. Those that are difficult to dissolve can also be dissolved by heating at a temperature below the boiling point of chloroform. There are no particular limitations on the measuring device and measuring conditions for the GPC, but the sample prepared in this manner can be measured, for example, with the same measuring device and measuring conditions as those for measuring the weight average molecular weight (Mw) of the foamed sheet.

- Compounds having isocyanate groups -
The compound having two or more isocyanate groups in the molecule as the chain extender (crosslinking agent) is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include aliphatic diisocyanate compounds, alicyclic polyisocyanate compounds, aromatic diisocyanate compounds, triisocyanate compounds, modified polyisocyanate compounds, etc. These may be used alone or in combination of two or more.

Examples of the aliphatic diisocyanate compounds include 1,6-hexamethylene diisocyanate, 3-isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate), 1,4-tetramethylene diisocyanate, 2,4,4-trimethylhexamethylene diisocyanate, 2,2,4-trimethylhexamethylene diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, methylcyclohexyl-2,4-diisocyanate, methylcyclohexyl-2,6-diisocyanate, xylylene diisocyanate, 1,3-bis(isocyanato)methylcyclohexane, tetramethylxylylene diisocyanate, transcyclohexane-1,4-diisocyanate, and lysine diisocyanate.

Examples of the alicyclic polyisocyanate include isophorone diisocyanate, hydrogenated diphenylmethane diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated xylylene diisocyanate, hydrogenated tetramethylxylylene diisocyanate, and cyclohexane diisocyanate.

Examples of the aromatic diisocyanate include 2,4-toluylene diisocyanate, 2,6-toluylene diisocyanate, diphenylmethane-4,4'-isocyanate, 1,5'-naphthene diisocyanate, tolidine diisocyanate, diphenylmethylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, 4,4'-dibenzyl diisocyanate, and 1,3-phenylene diisocyanate.

Examples of the triisocyanate compound include lysine ester triisocyanate, triphenylmethane triisocyanate, 1,6,11-undecane triisocyanate, 1,8-isocyanate-4,4-isocyanate methyloctane, 1,3,6-hexamethylene triisocyanate, bicycloheptane triisocyanate, an adduct of trimethylolpropane and 2,4-toluylene diisocyanate, and an adduct of trimethylolpropane and a diisocyanate such as 1,6-hexamethylene diisocyanate.

Examples of the modified polyisocyanate compound include compounds obtained by reacting a polyhydric alcohol such as glycerin or pentaerythritol with the aliphatic diisocyanate compound, the aromatic diisocyanate compound, and/or the triisocyanate compound.

The content of the chain extender (crosslinker) in the polylactic acid resin composition varies depending on the molecular weight and molecular weight distribution of the polylactic acid resin used. For example, when the weight average molecular weight is small or when there is a large amount of low molecular weight polylactic acid resin, it tends to be necessary to add more of the chain extender (crosslinker) to adjust the viscosity of the polylactic acid resin composition to a suitable level for foaming. However, when the amount of the chain extender (crosslinker) added is increased, biodegradability tends to be poor, and the chain extender (crosslinker) is generally a petroleum-derived compound, which is undesirable from the perspective of contributing to a sustainable society.

The content of the chain extender other than the compound having an epoxy group in the polylactic acid resin composition is not particularly limited and can be appropriately selected depending on the purpose, but it is preferable that the sum of the compound having an epoxy group and the chain extender other than the compound having an epoxy group is less than 2 mass% of the total amount of organic matter in the polylactic acid resin composition.

Other viscosity adjustment methods besides adding the chain extender (crosslinking agent) include crosslinking the polylactic acid resin composition with an electron beam or blending it with another resin composition having a high melt tension or a small amount of a high molecular weight component.

<<Foaming nucleating agent>>
The foaming nucleating agent (hereinafter sometimes referred to as "filler") is preferably contained in the polylactic acid resin composition in order to adjust the foaming state of the foam sheet (bubble size, amount, arrangement, etc.).
The foaming nucleating agent is not particularly limited and may be appropriately selected depending on the purpose, and examples thereof include organic nucleating agents and inorganic nucleating agents. These may be used alone or in combination of two or more. Among these, inorganic nucleating agents are preferred in that the foaming diameter of the foamed sheet is reduced and the heat insulating properties are improved.

The organic nucleating agents include, for example, naturally occurring polymers such as starch, cellulose nanofibers, cellulose fine particles, wood flour, soybean pulp, rice husks, bran, and modified products thereof, as well as glycerin compounds, sorbitol compounds, metal salts of benzoic acid and its compounds, metal salts of phosphate esters, and rosin compounds. These may be used alone or in combination of two or more.

Examples of the inorganic nucleating agent include inorganic particles such as talc, kaolin, calcium carbonate, layered silicate, zinc carbonate, wollastonite, silica, alumina, magnesium oxide, calcium silicate, sodium aluminate, calcium aluminate, sodium aluminosilicate, magnesium silicate, glass balloons, carbon black, zinc oxide, antimony trioxide, zeolite, hydrotalcite, metal fibers, metal whiskers, ceramic whiskers, potassium titanate, boron nitride, graphite, glass fibers, and carbon fibers. These may be used alone or in combination of two or more. Among these, inorganic particles having active hydrogen groups are preferred as the inorganic nucleating agent because they are easy to achieve an average hydrophobicity and carbon content suitable for the foamed sheet by surface treatment, and layered silicate and silica are particularly preferred.

The average hydrophobicity and carbon content of the inorganic nucleating agent are not particularly limited and can be appropriately selected according to the purpose, but the average hydrophobicity is preferably 65% by volume or more, and the carbon content is preferably 4% by mass or more. When the average hydrophobicity and carbon content of the inorganic nucleating agent are within the above-mentioned preferred ranges, when a non-polar foaming agent such as carbon dioxide or nitrogen is used, the hydrophobic inorganic particle surface acts favorably as a foam nucleation field, and foam nuclei can be efficiently formed. In addition, the upper limit of the average hydrophobicity and carbon content of the inorganic nucleating agent is not particularly limited, but the average hydrophobicity is preferably 68% by volume or less, and the carbon content is preferably 8.9% by mass or less. When the average hydrophobicity of the inorganic nucleating agent is within the above-mentioned preferred ranges, the aggregation of inorganic particles tends to be suppressed. However, since the degree of aggregation is strongly dependent on the kneading conditions and kneading device used, the effect of the present invention is not limited to the above-mentioned preferred ranges. In addition, when the carbon content of the inorganic nucleating agent is within the above-mentioned preferred ranges, the amount of surface treatment agent-derived components released from the inorganic particles tends to be reduced.

According to classical nucleation theory, if the contact angle between the bubble nucleus and the inorganic particle surface is small, the activation energy of foam nucleation is reduced, and nucleation proceeds smoothly. Therefore, it is thought that only the hydrophobicity is important as a chemical property of the inorganic particle surface. However, as a result of the intensive research by the present inventors, it was found that the inorganic particles as the inorganic nucleating agent tend not to be effective as a foam nucleating agent simply because the hydrophobicity is high. Although the detailed reason is unclear, it was found that a remarkable effect as a foam nucleating agent can be obtained when the average hydrophobicity of the inorganic particles as the inorganic nucleating agent is 65% by volume or more and the carbon content of the inorganic particles is 4% by mass or more. It is believed that when the carbon content of the inorganic particles is 4% by mass or more, a certain volume with high affinity with the foaming agent is generated on the surface layer of the inorganic particles. It is conceivable that when the concentration of the foaming agent is low as in the present invention, the diffusion of the foaming agent becomes the rate-limiting factor for foam nucleation. It is believed that in a certain volume where the affinity with the blowing agent present on the inorganic particle surface is high, the blowing agent concentration is substantially higher than in other parts, which is advantageous for obtaining the blowing agent when forming the foam nuclei.

The hydrophobicity of the inorganic particles is determined by a methanol wettability method (MW method). A larger value indicates a higher hydrophobicity, and a smaller value indicates a higher hydrophilicity.
The hydrophobicity of the inorganic particles is obtained by adding the inorganic particles to V1 [mL] of pure water, dropping methanol thereinto while stirring, and letting the amount of methanol required for the inorganic particles to be wetted and dispersed in the liquid be V2 [mL], according to the following formula (7).
Hydrophobicity [volume %] = {V2/(V1+V2)} × 100 ... Formula (7)

The degree of hydrophobicity in the present invention refers to a value obtained by the measurement method described below.
50 mg of inorganic particles are weighed into a 50 mL screw tube (Labolan screw tube bottle 9-852-09, No. 7, manufactured by Labolantec), and 5 mL of pure water (V1 [mL]) is added to prepare a sample. A stir bar (diameter 6 mm, length 20 mm, elliptical) is gently placed in the tube, and the tube is gently stirred with a magnetic stirrer (MX-1 type, manufactured by Shibata Scientific Co., Ltd.) so that a vortex does not occur on the water surface. The mouth of the container is covered with a perforated parafilm, and methanol (special grade, >99.8%, manufactured by Kanto Chemical Co., Ltd.) is added at a rate of 0.3 mL/min along the wall surface using a 25 mL burette (tolerance ±0.03 mL, manufactured by AS ONE Co., Ltd.), and the amount of methanol (V2 [mL]) required until the inorganic particles are dispersed in the liquid is measured. The measurement is performed three times, and the hydrophobicity is calculated using the above formula (7), and the arithmetic average value is taken as the average hydrophobicity.

The carbon content of the inorganic particles can be measured in accordance with ISO 3262-20:2021 by the measurement method described below.
The inorganic particles are completely combusted at 800° C., and then carbon dioxide in the combustion gas components is detected by a thermal conductivity detector (TCD) gas chromatograph and quantified to calculate the carbon dioxide content.

-silica-
The silica is mainly composed of silicon dioxide represented by _SiO2 . The silica particles are roughly divided into two types, dry process silica and wet process silica, according to the method of producing the silica particles, and the silica produced by either method can be used in the present invention. The silica is preferably surface-treated with a reactive compound such as a silane coupling agent or a titanate coupling agent, and more preferably surface-treated with a reactive compound such as an organopolysiloxane (silicone oil) or an alkylsilane having 16 or more carbon atoms.

- Layered silicate -
The volume average particle diameter (Mv) of the layered silicate is not particularly limited and can be appropriately selected according to the purpose, but is preferably 10 μm or more and 200 μm or less. If the volume average particle diameter (Mv) of the layered silicate is 10 μm or more, the detour path becomes short, and the gas component cannot be sufficiently retained, so that the problem of not being able to improve the expansion ratio is unlikely to occur. In addition, if the volume average particle diameter (Mv) of the layered silicate is 200 μm or less, the bubble wall is unlikely to become brittle, which is unlikely to lead to bubble breakage, so that the expansion ratio improvement effect can be obtained.

The volume average particle size (Mv) of the layered silicate may be measured on the layered silicate before charging, or may be measured on a particle taken out of the foamed sheet by, for example, the following method.
A sample of the foamed sheet is cut out, placed in a crucible, and burned at 600° C. for 4 hours in a muffle furnace (e.g., FP-310, manufactured by Yamato Scientific Co., Ltd.) to burn off the organic components. The crucible is then cooled in a desiccator for 1 hour, and the resulting inorganic particles are used as a measurement sample. When two or more types of inorganic particles are contained, the layered silicate can be further separated by gravity separation.
The method for measuring the volume average particle size (Mv) of the layered silicate is not particularly limited, and can be determined, for example, by the following measuring device and measuring conditions. In the present invention, the volume average particle size (Mv) of the layered silicate is defined as the average particle size of the layered silicate.
[Measurement equipment and conditions]
・ Equipment: Microtrac MT3300EX (manufactured by Microtrac Bell Co., Ltd.)
Measurement conditions: Transmittance/transmitted, refractive index/1.53, shape/aspheric, solvent/AIR, solvent refractive index/1, measurement time/10 s, extension filter/invalid, distribution/volume

The aspect ratio of the layered silicate is not particularly limited and can be appropriately selected depending on the purpose, but is preferably 10 to 100, more preferably 25 to 75. When the aspect ratio of the layered silicate is 10 or more, the orientation is good and a sufficient curved path effect is obtained, so that the effect of improving the expansion ratio of the foamed sheet is obtained. In addition, when the aspect ratio of the layered silicate is 100 or less, when the polylactic acid resin composition and the layered silicate are kneaded in the kneading process, the layered silicate is crushed, and as a result, there is no problem that the aspect ratio becomes small, and therefore a sufficient effect can be obtained.

The aspect ratio of the layered silicate is calculated by the following formula (8).
Aspect ratio=volume average particle size (Mv) of layered silicate/average thickness of layered silicate Formula (8)
In the formula (8), the method for measuring the volume average particle diameter (Mv) of the layered silicate and the thickness of the layered silicate is not particularly limited, and for example, the volume average particle diameter (Mv) can be determined by the above-mentioned measurement method, and the thickness can be determined by the following method.

The thickness of the layered silicate can be measured, for example, by SEM observation of the layered silicate using a scanning electron microscope (SEM) (e.g., 3D Real Surface View Microscope VE-9800, manufactured by KEYENCE). The magnification is adjusted so that the number of particles in the observation range is several tens to several hundreds. If necessary, multiple fields of view can be photographed, and the images can be linked and subjected to image analysis. From the captured image, particles whose thickness plane of the layered silicate is horizontal to the observation plane are selected, and the thickness is measured. The average value of 50 particles is taken, which is the average thickness of the layered silicate.

The content of the foam nucleating agent in the polylactic acid resin composition is not particularly limited as long as it does not impair the physical properties of the foam sheet, and can be appropriately selected according to the purpose. However, it is preferably 0.1% by mass to 10% by mass, and more preferably 0.25% by mass to 2.5% by mass, based on the total mass of the polylactic acid resin composition. When the content of the foam nucleating agent is 0.1% by mass to 10% by mass, it is possible to prevent the foam nucleating agents from agglomerating together, and to prevent the specific gravity of the polylactic acid resin composition from increasing, thereby preventing the lightweight properties of the foam sheet from being impaired. In addition, when the content of the foam nucleating agent is 0.25% by mass to 2.5% by mass, the amount of the foam nucleating agent is small, reducing the environmental load, and preventing the foam sheet from becoming embrittled.

<<Other ingredients>>
The other components in the polylactic acid resin composition are not particularly limited as long as they do not impair the effects of the present invention and can be appropriately selected depending on the purpose, and examples thereof include resin components other than the polylactic acid resin, various additives, etc. These may be used alone or in combination of two or more.

- Resin components other than polylactic acid resin -
The resin component other than the polylactic acid resin is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include urethane resin, polyester resin, acrylic resin, vinyl acetate resin, styrene resin, butadiene resin, styrene-butadiene resin, vinyl chloride resin, acrylic styrene resin, acrylic silicone resin, etc. These may be used alone or in combination of two or more.

-Additives-
The additives are not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include heat stabilizers, antioxidants, plasticizers, lubricants, crystallization promoters, thickeners, etc. These may be used alone or in combination of two or more.
The thickener may be a high molecular weight component having a weight average molecular weight of 1,000,000 or more.

The content of the other components is not particularly limited and can be selected appropriately depending on the purpose, but it is preferably less than 2 mass% of the total amount of organic matter in the polylactic acid resin composition. In this case, biodegradability and recyclability will be improved.

The content of each component in the polylactic acid resin composition is the same as the content of each component in a foamed sheet made of the polylactic acid resin composition.

As described above, the present invention is characterized in that a highly crystalline polylactic acid resin composition is molded into a foamed sheet while maintaining a low degree of crystallinity in order to obtain a foamed sheet with excellent thermoformability. In order to achieve this characteristic, the temperature of the polylactic acid resin composition from kneading to extrusion must be maintained at a relatively high temperature of at least -20°C below the melting point of the polylactic acid resin composition, but this means that it is difficult to adjust the viscosity range of the polylactic acid resin composition to a range suitable for foaming by cooling it, as disclosed in prior art documents. Therefore, the polylactic acid resin composition of the present invention is characterized by having a high melt viscosity so that the foaming agent can be maintained at a relatively high temperature of at least -20°C below the melting point of the polylactic acid resin composition.

The foamed sheet of the present invention may be used as it is, or may be molded and used as a molded body (product). The foamed sheet of the present invention is excellent in moldability, heat resistance, heat insulation, and biodegradability, and is therefore suitable for use as food containers, tableware, and the like. It is also suitable as a heat-resistant food container, but is not limited to such uses. The foamed sheet of the present invention may also be used as it is after printing, etc.

(Method of manufacturing foam sheet)
The foamed sheet of the present invention is obtained by extrusion foaming the polylactic acid resin composition.
The process for producing the foamed sheet of the present invention will be described in more detail below.
The process for producing the foamed sheet in the present invention preferably includes at least a kneading step, an impregnation step, and a foaming step, and may further include other steps as necessary.

The extruder used for the extrusion foaming may be, for example, a single screw extruder, a twin screw extruder, or a tandem extruder that combines these. Among these, it is preferable to use a tandem extruder from the viewpoint of efficiently melt-kneading the raw materials of the foamed sheet, such as the polylactic acid resin and, if necessary, the chain extender, the foaming nucleating agent, the other components, and the foaming agent, and cooling the molten mixture to a predetermined temperature before extruding. From the viewpoint of efficiency of melt-kneading and cooling, a tandem extruder that combines a twin screw extruder and a single screw extruder is most preferable. In addition, a flow rate control mechanism such as a gear pump may be installed between the twin screw extruder and the single screw extruder or between the extruder and the die, if necessary.

A circular die called a T-die or circular die (sometimes called a "round die") is connected to the tip of the extruder to extrude the molten mixture and obtain a foamed sheet.

When attempting to obtain a foamed sheet with low bulk density, it is preferable to produce the foamed sheet using a circular die from the viewpoint of making it easier to alleviate corrugation. In this case, it is preferable to take the tubular foam extruded from the circular die along a cooled mandrel, take it up and cool it, and further blow air onto the outer periphery to cool it quickly. By cooling in this manner, crystallization of the foamed sheet after extrusion can be suppressed, and a foamed sheet with excellent thermoformability can be obtained.

If necessary, a flow rate control mechanism such as a gear pump may be installed between the twin-screw extruder and the single-screw extruder, or between the extruder and the die.

<<Foaming agent>>
The foamed sheet can be obtained by melt-kneading the polylactic acid resin composition and a foaming agent, followed by extrusion foaming. As the foaming agent, a known physical foaming agent can be used.
For example, physical blowing agents include hydrocarbons such as ethane, butane, pentane, hexane, heptane, ethylene, propylene, and petroleum ether; halogen-based blowing agents such as methyl chloride, monochlorotrifluoromethane, dichlorodifluoromethane, and dichlorotetrafluoroethane; air; carbon dioxide; and nitrogen.

In the present invention, carbon dioxide and nitrogen are preferred because they have a low environmental impact, are highly safe to work with, and are easy to handle. Carbon dioxide is more preferred than nitrogen in terms of its solubility in the polylactic acid resin composition.

On the other hand, carbon dioxide has a higher vapor pressure than hydrocarbon-based blowing agents, and is known to have a high diffusion rate in polylactic acid resin compositions. Therefore, when producing a foamed sheet using carbon dioxide as a blowing agent, at a high blowing agent concentration that would result in a finely foamed state, foaming tends to occur rapidly, resulting in streaky appearance defects known as corrugation, surface roughness due to broken cells, and a decrease in the foaming ratio.

A method of reducing the concentration of the foaming agent is known as a means of foaming without impairing the surface properties of the foamed sheet, but this reduces the degree of supersaturation and results in coarse foam diameters. The coarse bubbles break through the molten polylactic acid resin composition and are released outside the system, causing the bubbles to break, preventing an increase in the foaming ratio and worsening the insulation properties. Therefore, with conventional methods, there is a problem of a trade-off between the properties related to the surface properties and insulation properties of the foamed sheet.

In contrast, in the method for producing a foamed sheet of the present invention, the chain extender (crosslinker) used together with the polylactic acid resin is suitably selected to have an appropriate range of epoxy equivalent and weight average molecular weight (Mw), thereby increasing the viscosity of the molten polylactic acid resin composition and suppressing cell breakage even in the low foaming agent range, thereby making it possible to obtain a foamed sheet that has both surface properties and heat insulation properties. At the same time, since the chain extender (crosslinker) with an appropriate range of epoxy equivalent and weight average molecular weight (Mw) is effective even at a low addition concentration, the content of polylactic acid resin in the polylactic acid resin composition can be increased without impairing biodegradability.

When a tandem extruder is used, it is preferable to inject the foaming agent into the first-stage extruder. By injecting the foaming agent into the first-stage extruder, the time during which the foaming agent and the polylactic acid resin composition are in contact can be extended, and there is a tendency to suppress defects such as partial coarsening of bubbles and pinholes caused by the expansion of undissolved foaming agent.

In the present invention, the amount of the foaming agent added is 2 parts by mass or more and 5 parts by mass or less, and preferably 2 parts by mass or more and 4 parts by mass or less, relative to 100 parts by mass of the polylactic acid resin composition. If the amount of the foaming agent added is less than 2 parts by mass relative to 100 parts by mass of the polylactic acid resin composition, problems such as limited plasticization of the polylactic acid resin composition and inability to increase the foaming ratio occur. Furthermore, if the amount of the foaming agent added exceeds 5 parts by mass relative to 100 parts by mass of the polylactic acid resin composition, poor surface properties occur due to rapid foaming.

<Kneading process>
The kneading step is a step of melting and kneading a mixture containing the polylactic acid resin and, if necessary, the chain extender, the foaming nucleating agent, and the other components to obtain a polylactic acid resin composition having a viscosity suitable for foaming. The kneading step preferably further includes a compressive fluid. The compressive fluid is blended for the purpose of plasticizing the polylactic acid resin composition and reducing the load on the device, and is preferably the same as the foaming agent from the viewpoint of omitting the subsequent impregnation step.

When the chain extender is added to the polylactic acid resin, the viscosity of the polylactic acid resin composition is adjusted in the kneading process by the reaction between the polylactic acid resin and the chain extender.

In the present invention, since a compound having two or more epoxy groups in the molecule is preferably used as the chain extender, the temperature of the kneading process is preferably from the melting point of the polylactic acid resin composition to 240°C, more preferably from 220°C to 240°C. When the temperature of the kneading process is from the melting point of the polylactic acid resin composition to 240°C, the viscosity of the polylactic acid resin composition can be effectively improved, and furthermore, the elution of unreacted compounds having two or more epoxy groups in the molecule can be reduced.

<Impregnation process>
The impregnation step is a step of melting and kneading the polylactic acid resin composition and the foaming agent to obtain a foamable polylactic acid resin composition. The solubility (solubility and dissolution rate) of the foaming agent in the polylactic acid resin composition varies depending on the temperature and pressure of the impregnation step. The temperature and pressure of the impregnation step may be appropriately set while observing the state of the foamed sheet, but generally, the solubility of the foaming agent in the polylactic acid resin composition can be increased by increasing the pressure and decreasing the temperature of the polylactic acid resin composition.

In the present invention, the term "expandable polylactic acid resin composition" refers to a composition in which the foaming agent is dissolved and/or dispersed in the polylactic acid resin composition, and is a composition that foams when the pressure inside the extruder is released to atmospheric pressure in the foaming step described below.

The impregnation step is preferably carried out while slowly cooling the expandable polylactic acid resin composition. A tandem extruder combining a twin-screw extruder and a single-screw extruder can dissolve the foaming agent while slowly cooling the polylactic acid resin composition with the single-screw extruder, and is suitable for producing the foamed sheet of the present invention.

<Foaming process>
The foaming step is a step of discharging the expandable polylactic acid resin composition obtained in the impregnation step from an extruder to obtain a foam, and is preferably a step of vaporizing and removing the compressive fluid as a foaming agent dissolved in the expandable polylactic acid resin composition obtained in the impregnation step, generating bubbles in the polylactic acid resin composition to foam it, and discharging the polylactic acid resin composition from the extruder to mold it. In the foaming step, foaming occurs using the pressure difference between the pressure inside the extruder and the atmospheric pressure as a driving force.

The expandable polylactic acid resin composition is preferably adjusted to about 150°C to 170°C in the expansion process. Here, the temperature in the expansion process refers to the set temperature of the die. By setting the temperature in the expansion process in this range, the viscosity of the expandable polylactic acid resin composition can be adjusted to a range suitable for expansion while suppressing crystallization.

If the die is cooled to about 130°C, crystallization will progress, tending to impair the thermoformability of the foamed sheet and tending to cause the die to become clogged with crystals. By setting the temperature of the foaming step within the above range, it is possible to adjust the viscosity of the foamable polylactic acid composition to a range suitable for foaming while suppressing crystallization.

The kneading step and the impregnation step may be carried out simultaneously, or the kneading step alone may be carried out to obtain a polylactic acid resin composition, and then the impregnation step and the foaming step may be carried out to obtain a foam.

The non-foamable polylactic acid resin composition obtained by carrying out only the kneading process is sometimes called a master batch, or simply a polylactic acid resin composition.

Next, an example of an apparatus for carrying out the kneading step will be described with reference to the drawings, but the kneading step in the present invention is not limited to this.
FIG. 2 is a schematic diagram showing a twin-screw extrusion apparatus (continuous kneading apparatus) 100 as an example of a kneading means in the manufacturing apparatus for the foamed sheet of the present invention. For example, the twin-screw extrusion apparatus 100 has a screw diameter of 42 mm, and the ratio [L/D] of the extruder length (L) to the screw diameter (D) is 48. In this example, raw materials such as polylactic acid resin, foaming nucleating agent, and chain extender are supplied from the first supply section 1 and the second supply section 2 to the raw material mixing and melting section a, and mixed and melted. When the polylactic acid resin composition contains components other than the raw materials (inorganic particles, etc.), the components other than the raw materials are also supplied, melted and mixed. When the polylactic acid resin composition is composed of three or more components, the number of supply sections is appropriately increased, or the components are mixed with the polylactic acid resin in advance and supplied from the supply sections to the raw material mixing and melting section a.

The mixed and melted raw materials are supplied with carbon dioxide, preferably in a compressible fluid state, from compressible fluid storage section 3 in compressible fluid supply section b. The mixture containing the compressible fluid is then kneaded in kneading section c. The mixture is then subjected to compressible fluid removal section d to remove compressible fluid F, and then is pelletized in molding section e to become, for example, resin pellets P. In this manner, a polylactic acid resin composition (master batch) can be obtained.

The compressible fluid can be, for example, cooled and liquefied and then supplied by a metering pump, and solid raw materials such as resin pellets and foam nucleating agents can be supplied by, for example, a fixed-volume feeder.

When the kneading process, the impregnation process, and the foaming process are performed in a continuous manner, it is preferable to use a foaming agent as the compressible fluid, and the compressible fluid F is not removed in the compressible fluid removal section d.

Next, we will explain the processes carried out in each part of the kneading device shown in Figure 2.

-Raw material mixing and melting section a-
In the raw material mixing and melting section a, the resin pellets and the components other than the polylactic acid resin that are added as necessary are mixed and heated. The heating temperature is set to a temperature equal to or higher than the melting temperature of the resin, and the resin is made to be in a state where it can be uniformly mixed with the compressible fluid F in the next compressible fluid supply section b.

--Compressible fluid supply unit b--
In the compressible fluid supplying section b, compressible fluid F is supplied to the resin pellets which have been heated to a molten state, thereby plasticizing the molten resin.

--Kneading section c--
In the kneading section c, components other than the polylactic acid resin are uniformly dispersed in the polylactic acid resin. The set temperature may be appropriately changed depending on the specifications of the reaction device, the load condition, etc., but is preferably from the melting point of the polylactic acid resin composition to 240° C.

Next, FIG. 3 shows an example of a foam sheet manufacturing apparatus (continuous foam sheet manufacturing apparatus 110) in which the kneading and foaming are performed continuously.

The continuous foamed sheet manufacturing apparatus 110 may be a tandem extruder in which a kneading device 10 and a single screw extruder 20 are connected. In the continuous foamed sheet manufacturing apparatus 110, for example, raw materials such as polylactic acid resin, foam nucleating agent, and chain extender are supplied from a first supply section 1 and a second supply section 2 to the raw material mixing and melting section a, where they are mixed and melted.

The mixed and melted raw materials are supplied with a compressible fluid as a foaming agent from the compressible fluid storage section 3 in the compressible fluid supply section b. The mixture containing the compressible fluid as a foaming agent is then kneaded in the kneading section c to become a foamable polylactic acid resin composition.

The expandable polylactic acid resin composition is supplied to a temperature adjustment section f, where the temperature is adjusted to a temperature suitable for expansion and the foaming agent is further dissolved. Next, the composition is extruded from a die into the atmosphere and foamed, and the resulting cylindrical foam 4 is cooled while being placed on a cooling mandrel 5 and air-cooled from the outer periphery. A portion of the foam is then cut open with a rotary blade, flattened, and wound into a roll to obtain the foam sheet of the present invention.

In the present invention, the kneading section c has a temperature between the melting point of the polylactic acid resin composition and 240°C, and more preferably between 220°C and 240°C. In addition, the temperature control section f is preferably at least 20°C below the melting point of the polylactic acid resin composition.

In this example, the kneading process is performed by a kneading device 10, and the foaming process is performed by a single screw extruder 20. However, the present invention is not limited to this configuration. For example, the areas in which the kneading process and the foaming process are performed can be changed as appropriate.

(Molded body)
One embodiment of the molded article of the present invention contains the foamed sheet of the present invention. This may be a molded article made of the foamed sheet of the present invention, and may further contain other components as necessary.
Another embodiment of the molded article of the present invention is obtained by thermoforming the foamed sheet of the present invention, and may further contain other components, if necessary.
The other components are not particularly limited as long as they are used in ordinary resin products, and can be appropriately selected depending on the purpose.

The thermoforming of the foamed sheet is not particularly limited, and may be, for example, a process of thermoforming using a mold to obtain a product. The thermoforming method of the foamed sheet using a mold is not particularly limited, and any conventionally known method for thermoplastic resins may be used, such as vacuum molding, pressure molding, vacuum pressure molding, and press molding.

Examples of the molded body (also called a manufactured product, consumer product, etc.) include daily necessities such as containers, bags, packaging containers, trays, tableware, cutlery, stationery, and cushioning materials. The concept of molded body includes intermediates for processing molded bodies, such as rolled raw material of the foam sheet, and molded bodies as stand-alone bodies, as well as parts made of molded bodies such as tray handles, and products equipped with molded bodies such as trays with handles attached.

The form of the container can be selected without any particular limitations. For example, it may be a container without a lid, such as a tray, or a container in a form in which the opening is closed with a shrink film, a top seal, a fitting lid, or the like. The container of the present invention also includes a container lid, and can be used as a container lid.

Examples of such bags include plastic bags, shopping bags, and garbage bags.

Examples of stationery items include clear files and badges.

The molded body can also be used for purposes other than the above-mentioned household goods, such as industrial materials, daily necessities, agricultural products, food, pharmaceutical, cosmetic sheets, packaging materials, etc.

The foamed sheet may be processed, such as laminated or coated, as necessary. The processing may be performed before the foamed sheet is wound up during production, or after the foamed sheet has been wound up. There are no particular limitations on the type of laminating film or coating agent, and the processing method.

In such molded products, the properties of the foamed sheet may not be maintained, but as long as the foamed sheet is used as a raw material, it is within the scope of the present invention.

<Molded body manufacturing method and molded body manufacturing device>
The method for producing the molded body is not particularly limited and can be appropriately selected depending on the shape of the desired molded body, etc., but when the molded body is produced by a thermoforming method, it is preferable that the method includes a heating step and a thermoforming step, and further includes other steps as necessary.
The manufacturing apparatus for the molded body is not particularly limited and can be appropriately selected depending on the shape of the desired molded body, but it is preferable that the manufacturing apparatus has a heating means and a heat molding means, and further has other means as necessary.
The method for producing the molded body is suitably carried out by an apparatus for producing the molded body.

<<Heating step and heating means>>
The heating step is a step of heating and softening the foamed sheet before molding the foamed sheet of the present invention.
The heating means is a means for heating and softening the foamed sheet of the present invention before molding the foamed sheet.
The heating step is preferably carried out by the heating means.

In the heating step, the method of heating the foam sheet is not particularly limited and can be appropriately selected depending on the purpose. For example, the foam sheet may be heated by arranging the heating means above and below the foam sheet, or on either the upper or lower surface of the foam sheet.

The heating means is not particularly limited and can be appropriately selected from known heating elements according to the purpose, examples of which include an electric heater, a heating plate, and an IR (infrared) heater.

In the heating step, it is preferable from the viewpoint of improving heat resistance to not advance the crystallization of the polylactic acid resin before molding the acid foam sheet, but to advance the crystallization of the polylactic acid resin in the subsequent heat molding step. Therefore, in the heating step, a method capable of heating the foam sheet in a short time is preferable, and a method of heating the foam sheet by arranging IR (infrared) heaters above and below the foam sheet is particularly preferable.

The heating temperature of the foam sheet in the heating step is not particularly limited and can be appropriately selected depending on the purpose, but is preferably a temperature equal to or higher than the glass transition temperature of the polylactic acid resin, more preferably 60°C or higher, and even more preferably 80°C or higher. In addition, if the foam sheet is heated near the cold crystallization temperature of the polylactic acid resin, crystallization will progress during the heating step, so the heating temperature of the foam sheet in the heating step is preferably at most 110°C or lower. The lower and upper limits of the heating temperature can be appropriately combined, but the heating temperature of the foam sheet in the heating step is more preferably 60°C or higher and 100°C or lower, and particularly preferably 80°C or higher and 100°C or lower.

The heating temperature means the temperature of the foamed sheet itself. The heating time of the foamed sheet in the heating step is not particularly limited and can be appropriately selected depending on the purpose, but from the viewpoint of not promoting crystallization too much, it is preferably within 15 seconds, more preferably within 10 seconds, and even more preferably within 5 seconds.

<<Heat molding process>>
The heat molding step is a step of molding the foamed sheet softened by the heating step using a mold, preferably a metal die, and is preferably a step of forming the foamed sheet into the shape of a container.

There are no particular limitations on the molding method using the mold, and any conventionally known thermoplastic resin heat molding method can be used, such as vacuum molding, pressure molding, vacuum-pressure molding, and match mold molding. However, the match mold molding method is particularly preferred from the viewpoint of promoting crystallization of the polylactic acid resin of the foam sheet during the molding process and improving heat resistance.

The mold temperature in the heat molding process is not particularly limited and can be selected appropriately depending on the purpose, but it is preferable to perform the process at a temperature close to the cold crystallization temperature of the polylactic acid resin so that the crystallization of the polylactic acid resin in the foamed sheet proceeds.

In the present invention, "near the cold crystallization temperature of the polylactic acid resin" means a temperature 20°C or lower than the cold crystallization temperature of the polylactic acid resin. Specifically, the temperature of the mold in the heat molding process is preferably 100°C or higher and 120°C or lower, and more preferably 100°C or higher and 110°C or lower. By setting the temperature of the mold in the heat molding process near the cold crystallization temperature of the polylactic acid resin, a molded body with excellent heat resistance can be obtained.

The heat molding time in the heat molding step is not particularly limited and can be appropriately selected depending on the purpose, but it is preferable to ensure sufficient time for the foamed sheet to crystallize, more preferably 5 seconds or more, and even more preferably 7 seconds or more. There is no particular limit to the upper limit of the heat molding time, but it is preferably 10 seconds or less from the viewpoint of heat resistance. The lower limit and upper limit of the heat molding time can be appropriately combined, and the heat molding time is preferably 5 seconds or more and 10 seconds or less, and more preferably 7 seconds or more and 10 seconds or less.

<<Other steps and other means>>
The other steps are not particularly limited and may be appropriately selected depending on the purpose. Examples of the other steps include a demolding step of removing the molded body from the mold, a step of punching the molded body from the foamed sheet, and a step of cutting off excess portions of the foamed sheet other than the molded body.

The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to these examples. In the following examples and comparative examples, unless otherwise specified, "parts" refers to "parts by mass" and "%" refers to "% by mass" except in the evaluation criteria.

Example 1
<Preparation of foam sheet>
-Raw material mixing and melting process-
Using a tandem type continuous foamed sheet manufacturing apparatus 110 shown in FIG. 3, a polylactic acid resin composition containing a polylactic acid resin, inorganic particles as a foaming nucleating agent, and a chain extender was supplied to the raw material mixing and melting section a of the kneading device 10 at a rate of 20 kg/hour, in the following proportions: 98.1 parts of polylactic acid resin (REVODE190, manufactured by HISUN Corporation), 1 part of silylated silica as inorganic particles (AEROSIL (registered trademark) RY300, average hydrophobicity: 67.2% by volume, carbon content: 6.0% by mass to 8.5% by mass, manufactured by Nippon Aerosil Co., Ltd.), and 0.9 parts of a chain extender (Marproof (registered trademark) G-0250SP, manufactured by NOF Corporation).

-Compressive fluid supplying process and kneading/impregnation process-
Next, carbon dioxide, a compressible fluid serving as a foaming agent, was supplied to the compressible fluid supply section b of the kneading device 10 at 0.76 kg/hour (corresponding to 3.8 parts per 100 parts of the polylactic acid resin composition), and these were mixed, melted, and kneaded in the kneading section c, and supplied to the single-screw extruder 20.

- Foaming process -
Next, the polylactic acid resin composition was cooled in the temperature control section f of the single-screw extruder 20 until the resin temperature reached 160°C, and then discharged into the atmosphere from a circular die having a slit diameter of 70 mm and a gap of 0.5 mm attached to the tip of the single-screw extruder 20 to vaporize the carbon dioxide, thereby performing extrusion foaming.

- Molding process -
The obtained cylindrical foamed sheet was placed on a cooling mandrel 5 and forcedly cooled by blowing air onto its outer surface, and the sheet was cut with a rotary blade cutter to obtain a flat sheet-like foam (hereinafter, sometimes referred to as the "foamed sheet" of Example 1).

In Example 1, the temperatures of each part were as follows:
Raw material mixing and melting section a of the kneading device: 200°C
Compressive fluid supply section b of the kneading device: 240° C.
Kneading section c of the kneading device: 240° C.
- Temperature control section f of single screw extruder: Cooling from 180°C to 160°C - Circular die: 155°C

In Example 1, the pressures at each part were as follows:
Compressive fluid supply section b of the kneading device: 7 MPa to 10 MPa
Kneading section c of the kneading device: 8 MPa to 20 MPa
Temperature control section f of single screw extruder: 8MPa to 38MPa

(Examples 2 to 4, 7 to 9, and 13)
The foamed sheets of Examples 2 to 4, 7 to 9, and 13 were obtained in the same manner as in Example 1, except that the formulation of the polylactic acid resin composition in Example 1 was changed to the formulations shown in Tables 1 to 3 below.

(Examples 5, 6, 10, and 12)
Foam sheets of Examples 5, 6, 10, and 12 were obtained in the same manner as in Example 1, except that the conditions of the foam sheet preparation step in Example 1 were changed to those shown in Tables 2 and 3 below.

(Example 11)
A foamed sheet of Example 11 was obtained in the same manner as in Example 1, except that the formulation of the polylactic acid resin composition and the conditions of the foamed sheet preparation process in Example 1 were changed to those shown in Table 3 below.

(Comparative Examples 1 to 5)
The foamed sheets of Comparative Examples 1 to 5 were obtained in the same manner as in Example 1, except that the formulation of the polylactic acid resin composition and the conditions of the foamed sheet preparation process in Example 1 were changed to the formulation and conditions shown in Table 4 below.

<Measurement of physical properties>
For the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5, the following measurements were made using the following methods: "content of polylactic acid resin relative to the total amount of organic matter in the foamed sheet", "molar ratio of D-type lactic acid and L-type lactic acid constituting the polylactic acid resin in the foamed sheet", "melting point of polylactic acid resin in the foamed sheet", "recrystallization temperature of polylactic acid resin in the foamed sheet", "glass transition temperature of polylactic acid resin in the foamed sheet", "cold crystallization temperature of polylactic acid resin in the foamed sheet", "epoxy equivalent of chain extender (crosslinker)", "weight average molecular weight (Mw) of chain extender (crosslinker)", "outermost surface shape in the TD direction of the cross section of the foamed sheet", "bulk density of the foamed sheet", "foaming diameter (median diameter) of the foamed sheet" and "cold crystallization enthalpy of the foamed sheet". The results are shown in Tables 1 to 4 below.
In addition, for the polylactic acid resin compositions obtained in the foaming agent supplying step and the kneading step in Examples 1 to 13 and Comparative Examples 1 to 5, the "weight average molecular weight of the polylactic acid resin composition" and the "recrystallization enthalpy of the polylactic acid resin composition" were measured, and the results are shown in Tables 1 to 4 below.

<<Measurement of polylactic acid resin content relative to total amount of organic matter in foam sheet>>
The content of polylactic acid resin relative to the total amount of organic matter in the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 was measured by the following procedure.

- Preparation of solvent -
The solvent used in the measurement was prepared by weighing out about 100 mg of 1,3,5-trimethoxybenzene standard (for quantitative NMR, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as an internal standard substance and dissolving it in deuterated chloroform (containing 0.3 vol.% tetramethylsilane (TMS)) in a 10 mL measuring flask.

-Sample preparation-
The above-mentioned solvent was added to the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 so that the concentration of the foamed sheets was 10 mg/mL, and the foamed sheets were dissolved by shaking for about half a day using a tabletop shaker (MSI-60, AS ONE Corporation). In this case, the smallest possible container was selected in order to minimize the change in sample concentration due to evaporation. The samples prepared by the above method were sealed in sample tubes with a diameter of 5 mm and subjected to NMR.

-measurement-
¹ H NMR measurement ( ¹ H-NMR measurement) was carried out in accordance with JIS K 0138:2018 (General rules for quantitative nuclear magnetic resonance spectroscopy (qNMR general rules)) using the following measuring device and under the following measuring conditions.
[Measurement equipment and conditions]
・Nuclear magnetic resonance (NMR) device: JNM-ECX-500 FT-NMR (manufactured by JEOL Ltd.)
Observed nucleus: 1H
Measurement temperature: 30°C
Spin: Off Digital resolution: 0.25Hz
Observation range: -0.5 to 15 ppm
Pulse angle: 90°
Relaxation time: 60 seconds. Number of scans: 16 (two dummy scans were performed before the actual measurement).
・13C decoupling: Yes

-analysis-
The obtained data was integrated with respect to the peaks of the following chemical shifts, and the integral ratio was calculated according to the following formula (4).
Integral 1 (derived from polylactic acid resin): 5.2 ppm
Integral 2 (from internal standard): 6.1 ppm
Integral ratio = integral 1 / (integral 2 × sample mass) ... formula (4)

A similar NMR measurement was performed on a polylactic acid resin of known purity using the same solvent as the sample, and the ratio of the integral ratio obtained from the formula (4) for the polylactic acid resin of known purity to the integral ratio for the sample was calculated, and the content of the polylactic acid resin relative to the total amount of organic matter in the foamed sheet was calculated using the following formula (5).
Content of polylactic acid resin [mass %]=100×purity of polylactic acid resin of known purity [mass %]×(integral ratio of sample)/(integral ratio of polylactic acid resin of known purity) (5)

The process from sample preparation to analysis was carried out three times, and the arithmetic average of the obtained polylactic acid resin content was taken as the polylactic acid resin content relative to the total amount of organic matter in the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5.

<<Measurement of the Molar Ratio of D-Lactic Acid and L-Lactic Acid Constituting the Polylactic Acid Resin in the Foam Sheet>>
The foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were freeze-pulverized, and 200 mg of the freeze-pulverized powder of the foamed sheets was weighed out using a precision balance and placed in an Erlenmeyer flask, to which 30 mL of 1N aqueous sodium hydroxide solution was added. The Erlenmeyer flask was then heated to 65°C while being shaken to completely dissolve the polylactic acid resin. The pH was then adjusted to 7 using 1N hydrochloric acid, and the solution was diluted to a predetermined volume using a measuring flask to obtain a polylactic acid resin solution. The polylactic acid resin solution was then filtered through a 0.45 μm membrane filter, and analyzed by liquid chromatography under the following measurement conditions.
[Measurement equipment and conditions]
HPLC device (liquid chromatograph): PU-2085Plus system (manufactured by JASCO Corporation)
Column: Chromolith (registered trademark) coated with SUMICHIRAL OA-5000 (inner diameter 4.6 mm, length 250 mm) (manufactured by Sumitomo Analysis Center Co., Ltd.)
Column temperature: 25°C
Mobile phase: a mixture of 2 mM _CuSO4 aqueous solution and 2-propanol ( _CuSO4 aqueous solution: 2-propanol (volume ratio) = 95:5)
Mobile phase flow rate: 1.0 mL/min Detector: UV 254 nm
Injection volume: 20 μL

Based on the obtained chart, the peak area ratios of the D-form of lactic acid and the L-form of lactic acid were calculated from the peak areas of the D-form of lactic acid and the L-form of lactic acid, respectively, and the total area of these peak areas was used to calculate the D-form amount ratio and the L-form amount ratio. The above procedure was carried out three times, and the arithmetic mean of the results was taken as the amount of the D-form of lactic acid and the amount of the L-form of lactic acid constituting the polylactic acid resin in the foamed sheet. The results are shown in Tables 1 to 4 below as "molar ratio (L:D)".

<<Measurement of the melting point of polylactic acid resin in foam sheet>>
The melting points of the polylactic acid resins in the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (Method for measuring transition temperature of plastics).
Specifically, a sample of 5 mg to 10 mg cut out from the foamed sheet was placed in a container of a differential scanning calorimeter (Q-2000 model, manufactured by TA Instruments) and the temperature was increased from 10° C. to 200° C. at a rate of 10° C./min. for measurement.
The peak top temperature (melting peak temperature, Tpm) of the endothermic peak associated with the melting of crystals observed on the higher temperature side than the glass transition temperature was measured as the melting point of the polylactic acid resin in the foamed sheet. When multiple endothermic peaks were observed on the higher temperature side than the glass transition temperature, the peak top temperature of the peak with the largest area was determined as the melting point of the polylactic acid resin in the foamed sheet.

<<Measurement of recrystallization temperature of polylactic acid resin in foam sheet>>
The recrystallization temperatures of the polylactic acid resins in the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (Method for measuring transition temperature of plastics).
Specifically, a sample of 5 mg to 10 mg cut out from the foamed sheet was placed in the container of a differential scanning calorimeter (Q-2000 model, manufactured by TA Instruments), heated from 10° C. to 200° C. at a heating rate of 10° C./min, held for 10 minutes, and then cooled from 200° C. to 10° C. at a heating rate of 10° C./min. In this case, the peak top temperature of the exothermic peak was measured as the recrystallization temperature of the polylactic acid resin in the foamed sheet. In addition, samples in which the crystallization peak was not clearly visible were marked as "unclear" in Tables 1 to 4 below.

<<Measurement of glass transition temperature of polylactic acid resin in foam sheet>>
The glass transition temperatures of the polylactic acid resins in the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (Method for measuring glass transition temperatures of plastics).
Specifically, a sample of 5 mg to 10 mg cut out from the foamed sheet was placed in a container of a differential scanning calorimeter (Q-2000 model, manufactured by TA Instruments) and the temperature was increased from 10° C. to 200° C. at a rate of 10° C./min. for measurement.

<<Measurement of cold crystallization temperature of polylactic acid resin in foam sheet>>
The cold crystallization temperatures of the polylactic acid resins in the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7121:2012 (Method for measuring transition temperatures of plastics).
Specifically, a sample of 5 mg to 10 mg cut out from the foamed sheet was placed in a container of a differential scanning calorimeter (Q-2000 type, manufactured by TA Instruments) and heated from 10° C. to 200° C. at a heating rate of 10° C./min. In this case, the peak top temperature of the exothermic peak observed in a temperature range equal to or higher than the glass transition temperature was measured as the cold crystallization temperature of the polylactic acid resin in the foamed sheet.

<<Measurement of epoxy equivalent of chain extender (crosslinker)>>
The epoxy equivalent of the chain extender (crosslinking agent) was measured by the following method in accordance with JIS K 7236:2001 (method for determining epoxy equivalent of epoxy resin).

-Sample preparation-
10 mL of chloroform was added to 0.1 g to 0.3 g of a compound having an epoxy group as a chain extender (crosslinking agent), and the mixture was completely dissolved by stirring with a magnetic stirrer. 20 mL of acetic acid and 10 mL of a chloroform solution of tetraethylammonium bromide (concentration: 0.25 g/mL) were added to prepare a measurement sample.

-measurement-
The epoxy equivalent of the prepared measurement sample was measured using the following measurement device and under the following measurement conditions.
[Measurement equipment and conditions]
・ Equipment: Automatic titration equipment COM-A-19 (manufactured by HIRANUMA Co., Ltd.)
・ Standard solution: 0.1 mol/L perchloric acid-acetic acid standard solution ・ Electrode: Glass electrode GTRS10B
Reference electrode GTPH1B (internal solution is saturated sodium perchlorate/acetic acid solution)
Measurement mode: Inflection point detection Differential judgment value: 100 mV/mL
・ Calculation formula: 1,000×S/((A1-BL)×M×f)
Here, S represents the mass (g) of the compound having an epoxy group, A1 represents the amount of dripping at the inflection point (mL), BL represents the result of the blank measurement (mL), M represents the concentration of the standard solution (mol/L), and f represents the factor of the standard solution. The blank measurement was performed twice, and the average value of the two measurements was used.

<<Measurement of weight average molecular weight (Mw) of chain extender (crosslinker)>>
The weight average molecular weight (Mw) of the chain extender (crosslinker) was measured by the following method.

-Sample preparation-
A compound having an epoxy group as a chain extender (crosslinking agent) and chloroform were mixed so that the concentration of the compound having an epoxy group was about 2 mg/mL, and the mixture was shaken for about half a day using a tabletop shaker (MSI-60, AS ONE Corporation). After confirming that the compound having an epoxy group had dissolved, the mixture was filtered through a 0.45 μm membrane filter, and the filtrate was used as a measurement sample.

-measurement-
The weight average molecular weight (Mw) of the prepared measurement sample was measured using the following measurement device and under the following measurement conditions.
[Measurement equipment and conditions]
・ Apparatus: HLC-8320GPC (manufactured by Tosoh Technosystems Co., Ltd.)
Column: TSKgel (registered trademark) guard column SuperHZ-L and TSKgel SuperHZM-M x 4 Detector: RI
Measurement temperature: 40°C
Mobile phase: chloroform Flow rate: 0.45 mL/min Injection volume: 20 μL

<<Measurement of weight average molecular weight of polylactic acid resin composition>>
The weight average molecular weight (Mw) of the polylactic acid resin compositions obtained in the foaming agent supplying step and the kneading step in Examples 1 to 13 and Comparative Examples 1 to 5 was measured by the following method.

-Sample preparation-
The polylactic acid resin composition and chloroform were mixed so that the concentration of the polylactic acid resin composition was about 2 mg/mL, and the mixture was shaken for about half a day using a tabletop shaker (MSI-60, manufactured by AS ONE Corporation). After confirming that the polylactic acid resin composition had dissolved, the mixture was filtered through a 0.45 μm membrane filter, and the filtrate was used as a measurement sample.

-measurement-
The weight average molecular weight (Mw) of the prepared measurement sample was measured using the same measurement device and measurement conditions as those used in the measurement of the weight average molecular weight (Mw) of the chain extender (crosslinking agent) [[measurement device and measurement conditions]].

<<Measurement of recrystallization enthalpy of polylactic acid resin composition>>
The recrystallization enthalpy of the polylactic acid resin compositions obtained in the foaming agent supplying step and the kneading step in Examples 1 to 13 and Comparative Examples 1 to 5 was determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7122:2012 (Method for measuring heat of transition of plastics).
Specifically, 5 mg to 10 mg of the polylactic acid resin composition was taken and flattened on a hot plate heated to 65° C. by pressing (at a load of about 500 gf) for 1 to 3 seconds with a copper rod (diameter about 20 mm, the rod was also heated to 65° C.) to prepare a sample. This sample was measured using the following measuring device and measuring conditions, and analyzed by the following analytical method. The recrystallization enthalpy was calculated by the arithmetic average of the results obtained by performing the process from sample preparation to analysis five times.
[Measurement equipment and conditions]
・ Apparatus: Q-2000 (manufactured by TA Instruments)
Temperature program: Scanning from 10° C. to 200° C. at a heating rate of 10° C./min (1st heating), holding at 200° C. for 1 minute, and then scanning from 200° C. to 25° C. at a cooling rate of 10° C./min (1st cooling).
Analysis of recrystallization enthalpy: The area of the exothermic peak associated with crystallization observed in the first cooling was determined by integration to obtain the recrystallization enthalpy. The position of the exothermic peak associated with crystallization was in the range of approximately 100°C to 130°C. The baseline for integration was a straight line connecting the front and rear of the exothermic peak.

<<Outer surface shape in the TD direction of the cross section of the foam sheet>>
The outermost surface shape in the TD direction of the cross section of the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 was evaluated by the following method.

-sample-
The central portion of the foam sheet in the TD direction was cut into a 5 cm x 5 cm square, which was then fixed to the stage of the three-dimensional measuring device described below with double-sided tape so that the foam sheet would not float, to prepare a measurement sample.
Three measurement samples were prepared in the same manner by cutting out different locations from the foamed sheet.

- Treatment for swells -
When the average length RSm of the roughness curve elements was evaluated and a value of 10 mm or more was measured, the waviness cutoff (λc) was set to 10 mm and measurement was performed.

-measurement-
Using the following measuring device and measuring conditions, the TD shape of one surface of the three measuring samples in the thickness direction and TD direction was observed, and the arithmetic mean roughness Ra and the average length RSm of the roughness curve element were calculated using the attached software in accordance with JIS B 0601:2013, and then the average value was calculated. In addition, the ratio [Ra/RSm] of the arithmetic mean roughness Ra (average value) to the average length RSm of the roughness curve element (average value) was calculated.
[Measurement equipment and conditions]
・ Equipment: 3D measuring device VR-3200 (Keyence)
Observation magnification: 12x Measurement mode: Standard Measurement direction: Both sides Measurement brightness adjustment: Auto (Set value: 80)
- Reference plane setting: Select and execute for each of the x and y directions of the screen

<<Measurement of bulk density of foam sheet>>
The foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were left to stand for 24 hours or more in an environment adjusted to a temperature of 23° C. and a relative humidity of 50%, and test pieces of 50 mm×50 mm were cut out. The cut test pieces were subjected to a liquid weighing method to determine the bulk density using an automatic specific gravity meter (DSG-1, manufactured by Toyo Seiki Seisakusho Co., Ltd.). In the liquid weighing method, the mass (g) of the foamed sheet test piece in air was precisely weighed, and then the mass (g) of the foamed sheet test piece in water was precisely weighed, and the mass was calculated according to the following formula (1).
Bulk density [g/cm ³ ]=density of water [g/cm ³ ]×mass of test piece in air [g]/(mass of test piece in air [g]−mass of test piece in liquid [g]) Equation (1)

<<Measurement of foam diameter (median diameter) of foam sheet>>
The cross-sections of the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were cut using a sharp razor (76 razor, manufactured by Nisshin EM Co., Ltd.), and the cross-sections of the foamed sheets were observed with a scanning electron microscope (SEM) (3D real surface view microscope VE-9800, manufactured by KEYENCE Co., Ltd.) at a magnification of 20 to 50 times. The obtained images were subjected to area division by the watershed method (morphological segmentation) using the MorphoLibJ plug-in of the image analysis software ImageJ (free software). At this time, the tolerance was adjusted for each image so that the division was appropriate. The dividing lines of the regions were output as binary images, and the distribution of the bubble area was obtained by the particle size analysis function of the image analysis software while excluding bubbles in contact with the end of the image from the analysis. The cumulative distribution of the bubble area was created using a spreadsheet software (Excel, manufactured by Microsoft Corporation), and the area where the cumulative distribution was 50% was determined. The circle equivalent diameter of this area was calculated and used as the bubble diameter (median diameter).

<<Measurement of cold crystallization enthalpy of foam sheet>>
The cold crystallization enthalpy in the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 was determined by differential scanning calorimetry (DSC) measurement in accordance with JIS K 7122:2012 (Method for measuring heat of transition of plastics).
Specifically, a sample of 5 mg to 10 mg was cut out from the foamed sheet, and pressed with a copper rod (diameter about 20 mm, the rod was also heated to 65° C.) for 1 to 3 seconds under a load of 500 gf on a hot plate heated to 65° C. to prepare a sample. The sample was placed in a container of a differential scanning calorimeter (Q-2000 type, manufactured by TA Instruments) and measured with the following measuring device and under the following measuring conditions, and analyzed by the following analytical method. The cold crystallization enthalpy of the polylactic acid resin in the foamed sheet was taken as the arithmetic average of the results obtained by performing the process from sample preparation to analysis five times.
[Measurement equipment and conditions]
・ Apparatus: Q-2000 (manufactured by TA Instruments)
Temperature program: Scanning was performed from 10° C. to 200° C. at a heating rate of 10° C./min (1st heating).
Analysis of cold crystallization enthalpy: The cold crystallization enthalpy was determined by integrating the area of the exothermic peak associated with crystallization observed at 60° C. to 100° C. during the first heating. The baseline for integration was a straight line connecting the front and back of the exothermic peak.

<Evaluation>
The foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were evaluated for "moldability,""heatresistance,""thermalinsulation," and "biodegradability" by the following methods. The evaluation results are shown in Tables 1 to 4 below.

<<Moldability>>
The foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were preheated and then molded into the shape of a cup yakisoba container (opening diameter 180 mm, bottom diameter 110 mm, depth 60 mm) in a match mold type heating die at 110°C for 10 seconds. The obtained molded cup yakisoba containers were observed by a specialist evaluator, and the moldability was evaluated based on the following evaluation criteria. The evaluation result was A, which was the best, and A or B was within the acceptable range.
- Evaluation criteria for moldability -
A: There are no tears or thin areas, and the container is formed with a uniform thickness. B: There are no tears, but there are areas that have become thin and are close to being torn. C: A large tear has occurred.

<<Heat resistance>>
The foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were preheated and then molded into the shape of a cup yakisoba container (opening diameter 180 mm, bottom diameter 110 mm, depth 60 mm) in a match mold type heating die at 110° C. for 10 seconds. Water at 25° C. was poured into the opening of the molded product (cup yakisoba container) of the foamed sheet of Examples 1 to 13 and Comparative Examples 1 to 5 up to the top, the mass of the water poured into the molded product was measured, and the value converted into volume using the density of water at 25° C. was regarded as the "initial volume" of the molded product.
Next, the molded foam sheets of Examples 1 to 13 and Comparative Examples 1 to 5 were heated at 120° C. for 10 minutes, and then water at 25° C. was poured into the opening up to the top of the opening. The mass of the water poured into the molded body was measured, and the value converted into volume using the density of water at 25° C. was regarded as the "volume after heating" of the molded body.
The volume change rate of the molded body before and after heating was calculated using the following formula (9), and the heat resistance of the molded body was evaluated based on the following evaluation criteria, with A being the best result and A or B being the acceptable range.
Volume change rate (%)=(initial volume−volume after heating)/initial volume×100 Equation (9)
- Heat resistance evaluation criteria -
A: The volume change rate is less than 3%. B: The volume change rate is 3% or more and less than 10%. C: The volume change rate is 10% or more, or the deformation is so great that the original shape cannot be recognized.

<<Thermal insulation>>
Test pieces measuring 50 mm x 50 mm were cut out from the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5. The cut test pieces were placed on a hot plate heated to 100°C and left to stand for 3 minutes. After standing, a thermocouple was attached to the surface opposite to the heated surface to measure the surface temperature of the foamed sheet, and the foamed sheet surface temperature was used as an index of thermal insulation and evaluated based on the following evaluation criteria. The evaluation result was A, which was the best, and the acceptable range was A, B, or C.
-Insulation evaluation criteria-
A: The surface temperature of the foam sheet is less than 45° C. B: The surface temperature of the foam sheet is 45° C. or more and less than 55° C. C: The surface temperature of the foam sheet is 55° C. or more and less than 65° C. D: The surface temperature of the foam sheet is 65° C. or more

<<Biodegradability>>
The biodegradability of the foamed sheets of Examples 1 to 13 and Comparative Examples 1 to 5 was determined in accordance with JIS K 6953-2:2010 (Plastics - Determination of ultimate aerobic biodegradability under controlled compost conditions - Method by measurement of the amount of carbon dioxide generated - Part 2: Method for measuring the mass of carbon dioxide generated on a laboratory scale) and was evaluated based on the following evaluation criteria. The best evaluation result was A, and the acceptable range was A or B.
- Biodegradability evaluation criteria -
A: Biodegradability of 60% or more in 45 days B: Biodegradability of 60% or more in 6 months C: Biodegradability of less than 60% in 6 months

The aspects of the present invention include, for example, the following.
<1> A foamed sheet made of a composition containing a polylactic acid resin,
The polylactic acid resin contains 98 mol % or more of either D-lactic acid or L-lactic acid, which are constituent monomer units of the polylactic acid resin, in the polylactic acid resin;
The content of the polylactic acid resin relative to the total amount of organic matter in the foamed sheet is 98% by mass or more,
The bulk density of the foamed sheet is 0.063 g/ ^cm3 or more and 0.250 g/ ^cm3 or less,
The foamed sheet is characterized in that, with respect to a shape of at least one surface of the foamed sheet in a direction perpendicular to the extrusion direction of the foamed sheet as viewed from a cross section in the thickness direction of the foamed sheet and in a direction perpendicular to the extrusion direction of the foamed sheet, a ratio [Ra/RSm] of an arithmetic mean roughness Ra calculated in accordance with JIS B 0601:2013 to an average length RSm of roughness curve elements calculated in accordance with JIS B 0601:2013 is 0.050 or less.
<2> The foamed sheet according to <1>, wherein the ratio [Ra/RSm] is 0.030 or less.
<3> The foamed sheet according to <1> or <2>, wherein the foamed sheet has a bulk density of 0.063 g/ ^cm3 or more and 0.125 g/ ^cm3 or less.
<4> The composition containing the polylactic acid resin contains a compound having an epoxy group,
the epoxy equivalent of the compound having an epoxy group is 250 or more and 350 or less,
The foamed sheet according to any one of <1> to <3>, wherein the compound having an epoxy group has a weight average molecular weight (Mw) of 10,000 or more and 20,000 or less.
<5> The foamed sheet according to <4>, wherein the compound having an epoxy group is a compound having two or more epoxy groups in the molecule.
<6> The composition containing the polylactic acid resin contains inorganic particles,
The foamed sheet according to any one of <1> to <4>, wherein the inorganic particles have an average hydrophobicity of 65% by volume or more and a carbon content of the inorganic particles of 4% by mass or more.
<7> The foamed sheet according to <6>, wherein the inorganic particles are silica.
<8> The foamed sheet according to any one of <1> to <7>, wherein the foamed sheet has a median foam diameter of 800 μm or less.
<9> The foamed sheet according to any one of <1> to <8>, wherein the foamed sheet has a cold crystallization enthalpy of 20 J/g or more.
<10> A molded product obtained by thermoforming the foamed sheet according to any one of <1> to <9>.
<11> A molded article comprising the foamed sheet according to any one of <1> to <9>.
<12> A method for producing a foamed sheet according to any one of <1> to <9>,
kneading a composition containing a polylactic acid resin in the presence of 2 parts by mass or more and 5 parts by mass or less of a foaming agent relative to 100 parts by mass of the composition containing the polylactic acid resin;
a step of vaporizing the foaming agent from the composition containing the polylactic acid resin to foam the composition containing the polylactic acid resin;
The method for producing a foamed sheet is characterized by comprising the steps of:
<13> The method for producing a foamed sheet according to <12>, wherein the carbon dioxide is a compressible fluid.

The foamed sheet described in any one of <1> to <9>, the molded body described in <10> or <11>, and the method for producing the foamed sheet described in any one of <12> to <13> can solve the above-mentioned problems in the past and achieve the object of the present invention.

REFERENCE SIGNS LIST 1 First supply section 2 Second supply section 3 Compressible fluid storage section 4 Cylindrical foam 5 Cooling mandrel 10 Kneading device 20 Single screw extruder 100 Twin screw extruder (continuous kneading device)
110 Continuous foamed sheet manufacturing apparatus a Raw material mixing and melting section b Compressible fluid supply section c Kneading section d Compressible fluid removal section e Molding section f Temperature adjustment section F Compressible fluid P Resin pellets 200 Foamed sheet 201 Cross section of foamed sheet 200 in thickness direction and TD direction 202 One surface (outermost surface) of foamed sheet 200

JP 2007-46019 A Patent No. 5207277 Patent No. 5454137 JP 2006-328225 A Patent No. 4842745 JP 2020-158608 A

Claims (8)

A foamed sheet made of a composition containing a polylactic acid resin,
The polylactic acid resin contains 98 mol % or more of either D-lactic acid or L-lactic acid, which are constituent monomer units of the polylactic acid resin, in the polylactic acid resin;
The content of the polylactic acid resin relative to the total amount of organic matter in the foamed sheet is 98% by mass or more,
The bulk density of the foamed sheet is 0.063 g/ ^cm3 or more and 0.250 g/ ^cm3 or less,
A foamed sheet characterized in that, with respect to a shape of at least one surface of the foamed sheet in a direction perpendicular to the extrusion direction of the foamed sheet, as viewed from a cross section in the thickness direction of the foamed sheet and in a direction perpendicular to the extrusion direction of the foamed sheet, a ratio [Ra/RSm] of an arithmetic mean roughness Ra calculated in accordance with JIS B 0601:2013 to an average length RSm of roughness curve elements calculated in accordance with JIS B 0601:2013 is 0.050 or less. The foam sheet according to claim 1, wherein the ratio [Ra/RSm] is 0.030 or less. 3. The foamed sheet according to claim 1, wherein the foamed sheet has a bulk density of 0.063 g/ ^cm3 or more and 0.125 g/ ^cm3 or less. The foamed sheet contains a compound having an epoxy group,
the epoxy equivalent of the compound having an epoxy group is 250 or more and 350 or less,
2. The foamed sheet according to claim 1, wherein the compound having an epoxy group has a weight average molecular weight (Mw) of 10,000 or more and 20,000 or less. The foamed sheet according to claim 1, wherein the foam diameter of the foamed sheet is 800 μm or less in median diameter. A foamed sheet made of a composition containing a polylactic acid resin,
The polylactic acid resin contains 98 mol % or more of either D-lactic acid or L-lactic acid, which are constituent monomer units of the polylactic acid resin, in the polylactic acid resin;
The content of the polylactic acid resin relative to the total amount of organic matter in the foamed sheet is 98% by mass or more,
The bulk density of the foamed sheet is 0.063 g/cm3 ^or more and 0.250 g/cm3 ^or less,
With respect to a shape of at least one surface of the foamed sheet in a direction perpendicular to the extrusion direction of the foamed sheet as viewed from a cross section in the thickness direction of the foamed sheet and in a direction perpendicular to the extrusion direction of the foamed sheet, a ratio [Ra/RSm] of an arithmetic mean roughness Ra calculated in accordance with JIS B 0601:2013 to an average length RSm of roughness curve elements calculated in accordance with JIS B 0601:2013 is 0.050 or less,
The foamed sheet has a cold crystallization enthalpy of 20 J/g or more. A molded product obtained by thermoforming the foamed sheet according to claim 1 or 6 . A method for producing the foamed sheet according to claim 1 or 6 , comprising the steps of:
kneading a composition containing a polylactic acid resin in the presence of 2 parts by mass or more and 5 parts by mass or less of a foaming agent relative to 100 parts by mass of the composition containing the polylactic acid resin;
a step of vaporizing the foaming agent from the composition containing the polylactic acid resin to foam the composition containing the polylactic acid resin;
A method for producing a foamed sheet, comprising: