JP2016216546A - Polystyrene-based resin foam sheet for thermoforming - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a monolayer polystyrene-based resin foam sheet for thermoforming excellent in thermoforming property while having high open cell percentage.SOLUTION: The polystyrene-based resin foam sheet for thermoforming has thickness of 0.5 to 3 mm, apparent total density of 0.05 to 0.3 g/cm, open cell percentage of 70% or more, the foam sheet contains cell film aperture agent consisting of calcium inorganic salt of 2 to 7 wt.% and a saturated hydrocarbon compound having 3 to 6 carbon atoms of 1 to 3 wt.%, a ratio of apparent density of a surface layer part from a surface to 200 μm in a thickness direction to apparent total density of the foam sheet is 1.5 to 3.0, and secondary foam ratio of the foam sheet when the foam sheet is heated at 145°C for 27 seconds and cooled to 23°C is 1.4 times or more.SELECTED DRAWING: None

Description

  The present invention relates to a polystyrene-based resin foam sheet for thermoforming, and more particularly to a polystyrene-based resin foam sheet having excellent thermoformability despite being a single-layer polystyrene resin foam sheet having an open cell structure.

  Thin polystyrene resin foam sheets with a thickness of about 0.5 to 5 mm are excellent in light weight, heat insulation, thermoformability and cutting, so tray containers, instant noodle containers, bento containers, natto containers It is widely used as a polystyrene resin foam sheet for thermoforming for processing food distribution packaging materials such as folded box containers.

  Many of the foam sheets used for these thermoforming applications have a closed cell structure. However, on the other hand, a polystyrene resin foam sheet having an open cell structure has also been developed. Water absorption is imparted to a molded body obtained by thermoforming a foam sheet having an open cell structure by providing holes on the surface thereof. When the molded body with these holes is used as a packaging container for fresh food such as meat and fish, a container for warm food such as cooked rice, and its lid, it absorbs moisture such as steam from meat juice and cooked rice. It becomes possible.

  As a means to make the polystyrene resin foam sheet into an open-cell structure, a method using an inorganic gas having low solubility in polystyrene such as nitrogen and carbon dioxide as a foaming agent, and a large amount of inorganic additives such as talc are blended And a method of blending a copolymer resin of styrene and a rubber component into a polystyrene resin, and the like, and combining these methods to produce a foam sheet having an open cell structure has been performed. Specifically, Patent Documents 1 to 4 disclose a foam sheet having an open cell structure and a method for producing the same.

JP-A-2-76715 JP 2004-35297 A Japanese Patent No. 2004-35578 JP 9-254294 A

  However, the foamed sheet having an open cell structure disclosed in Patent Documents 1 to 3 has poor thermoformability, and the formable container shape is limited. Moreover, the rigidity was low compared with the foam sheet of polystyrene-type resin alone, and the container obtained by thermoforming was too soft.

On the other hand, Patent Document 4 discloses that a continuous cell is formed by laminating a styrene resin closed cell foam layer having a closed cell ratio of 60% or more on a styrene resin cell foam cell layer having an open cell ratio of 50 to 95%. A styrenic resin foam laminate sheet is disclosed in which the poor thermoformability of the foam layer is compensated by the good thermoformability of the closed cell foam layer.
This is excellent in thermoformability, but the improvement in thermoformability is due to the effect of laminating a styrene resin foam layer with a closed cell ratio of 60% or more. It does not improve the thermoformability of the sheet itself. In addition, such a foam laminated sheet is produced by a method in which a foam sheet having an open cell structure and a foam sheet having a closed cell structure are bonded together, or a method in which both are coextruded and laminated. Was complicated.

  The present invention has been made in order to solve such a problem, and is a single-layer polystyrene foam sheet, and has a high open cell ratio and excellent thermoformability, and is a polystyrene-based resin for thermoforming. The object is to provide a foam sheet.

According to this invention, the novel polystyrene-type resin foam sheet for thermoforming as described in the following <1>-<5> and a molded object are provided.
<1> In a polystyrene-based resin foam sheet for thermoforming having a thickness of 0.5 to ³ mm, an apparent total density of 0.05 to 0.3 g / cm ³ , and an open cell ratio of 70% or more,
The foamed sheet contains 2 to 7% by weight of a cellular membrane pore-forming agent made of a calcium inorganic salt, and contains 1 to 3% by weight of a saturated hydrocarbon compound having 3 to 6 carbon atoms.
The ratio of the apparent density of the surface layer part up to 200 μm in the thickness direction from the surface to the apparent overall density of the foamed sheet is 1.5 to 3.0,
A polystyrene-based resin foam sheet for thermoforming, wherein the foam sheet has a secondary foaming ratio of 1.4 times or more when the foam sheet is heated at 145 ° C. for 27 seconds and cooled to 23 ° C.
<2> The thermoforming as described in 1 above, wherein the base resin constituting the foamed sheet is general-purpose polystyrene (GPPS), and the general-purpose polystyrene (GPPS) has a melt tension at 200 ° C. of 200 mN or less. Polystyrene resin foam sheet.
<3> The polystyrene-based resin foam sheet for thermoforming as described in 1 or 2 above, wherein the cell membrane pore-opening agent is calcium carbonate having an average particle diameter of 1 to 6 μm.
<4> The polystyrene-based resin foam sheet for thermoforming as described in any one of 1 to 3, wherein an average cell diameter in the thickness direction of the foam sheet is 30 to 200 μm.
<5> A polystyrene-based resin foam molded article obtained by thermoforming the polystyrene-based resin foam sheet for thermoforming described in any one of 1 to 4 above.

  The polystyrene-based resin foam sheet for thermoforming of the present invention (hereinafter, also simply referred to as “foam sheet”) is produced using a cell membrane pore-opening agent made of a calcium inorganic salt, and the entire appearance of the foam sheet is The ratio of the apparent density of the surface layer part up to 200 μm in the thickness direction from the surface to the density is within a specific range, contains a specific amount of a saturated hydrocarbon compound having 3 to 6 carbon atoms, and has a secondary foaming ratio of 1 under specific conditions When it is 4 times or more, it is excellent in thermoformability despite being a single-layer foam sheet having an open cell ratio of 70% or more.

FIG. 1 is an electron micrograph of the longitudinal section of the foam sheet surface layer part obtained in Example 2 (magnification factor 150 times).

Hereinafter, the polystyrene-based resin foam sheet for thermoforming of the present invention will be described in detail.
The thickness of the foamed sheet of the present invention is 0.5 to 3 mm, preferably 0.7 mm for the lower limit, more preferably 1 mm, and preferably 2.5 mm for the upper limit. If the thickness is too small, it may not be possible to obtain a molded article having a desired strength and water absorption. If the thickness is too large, wrinkles may occur when the foam sheet is bent with a roll of a take-up machine or a winder, resulting in poor appearance.

The apparent total density ρa of the foamed sheet is 0.05 to 0.3 g / cm ³ , preferably 0.07 to 0.25 g / cm ³ . If the apparent overall density is too small, the strength of the resulting molded article may be reduced, making it difficult to store and hold the object. If the apparent overall density is too high, the foam characteristics of light weight and excellent heat insulation may be impaired, and the film thickness of the bubbles also increases, making it difficult to break bubbles during extrusion foaming. It becomes difficult to get.
Moreover, in the foamed sheet of the present invention, it is important that the ratio of the apparent density of the surface layer portion up to 200 μm in the thickness direction from the surface to the apparent total density ρa is within a specific range, which will be described later.

  The open cell ratio of the foamed sheet of the present invention is 70% or more, preferably 80% or more. If it is less than 70%, there is a possibility that desired water absorption cannot be expressed when the molded body is used as a water absorption container. The upper limit is 100%, preferably 90%.

  The apparent overall density in this specification means the apparent overall density described in JIS K7222: 2005.

The open cell ratio in this specification is the ratio of the volume of open cells to the total volume of bubbles in the foamed sheet (the sum of the volume of closed cells and the volume of open cells). The measurement of the open cell ratio is performed as follows according to ASTM D-2856-70 (Procedure C).
The true volume Vx (cm ³ ) of the measurement sample is obtained using an air pycnometer, the apparent volume Va (cm ³ ) is obtained from the outer dimension of the measurement sample, and the open cell ratio (%) is calculated by the equation (1). calculate. The true volume Vx is the sum of the volume of the resin in the measurement sample and the volume of the closed cell portion.
Open cell ratio (%) = {(Va−Vx) / (Va−W / ρ)} × 100 (1)
W is the weight (g) of the measurement sample, and ρ is the density (g / cm ³ ) of the base material constituting the foam.
As a measurement sample, a plurality of sheet-like samples having a length of 40 mm and a width of 25 mm are cut out, and a cut sample having a total thickness of about 25 mm is used as a measurement sample.

  As described above, the foam sheet of the present invention has a large open cell ratio and has an open cell structure. However, the conventional foamed sheet having a large open cell ratio hardly undergoes secondary foaming even when heated, whereas the foamed sheet of the present invention is secondary foamed and is characterized by excellent thermoformability. Specifically, the secondary foaming ratio of the foamed sheet when heated to 145 ° C. for 27 seconds and cooled to 23 ° C. is 1.4 times or more, preferably 1.6 times or more, more preferably 1.7 times or more. Foamed sheets with a cell structure that has a secondary foaming ratio of less than 1.4 times are poorly stretched during thermoforming and cracked during shaping, and the reproducibility of molds during molding is reduced. Since the desired shape of the mold cannot be formed on the foamed sheet, a good molded body cannot be obtained depending on the shape of the mold. The upper limit of the secondary expansion ratio is preferably about 2.5 times.

  A possible reason for secondary foaming despite having an open-cell structure is that the diameter of the holes opened in the open-cell bubble film is smaller than that of a conventional open-cell foam sheet. In the foam sheet having the conventional open cell structure, most of the foaming agent composed of saturated hydrocarbons such as butane that existed in the open cells at the time of foaming quickly escapes from the foam sheet. On the other hand, the foaming agent that has not contributed to foaming or the foaming agent that has moved into the resin from the bubbles is dissolved in the resin constituting the foamed sheet. A part of the dissolved foaming agent moves from the bubble film into the bubbles during heating during thermoforming. However, in a foam sheet having a conventional open-cell structure, the foaming agent that has moved from the bubble film into the bubbles during heating is diffused into the atmosphere through the pores of the bubble film, and hardly contributes to secondary foaming. On the other hand, in the foamed sheet of the present invention, due to the high surface layer density, the saturated hydrocarbon added as a physical foaming agent at the time of production remains sufficiently in the cell membrane, and further opens into the cell membrane. It is considered that secondary foamability is exhibited because saturated hydrocarbons are less likely to escape from the foamed sheet due to the small diameter of the holes.

The secondary expansion ratio means the rate of change in the volume of the foamed sheet before and after heating. Usually, the thermoformability of a foam sheet is performed in a state where both ends of a long foam sheet are fixed. Therefore, since there is almost no dimensional change in the plane direction during thermoforming, in this specification, the four sides of the foamed sheet are fixed and heated, and the secondary foaming ratio is obtained from the change in sheet thickness before and after heating.
In the above measurement, the heating conditions were set to 145 ° C. for 27 seconds, which is a preferable condition for reproducing the behavior during thermoforming in the continuous production of a molded body using a heating furnace for the foamed sheet in an oven. Because.

  Furthermore, the foamed sheet of the present invention contains 1 to 3% by weight of a saturated hydrocarbon having 3 to 6 carbon atoms, preferably 1 to 2% by weight. These hydrocarbons are added as foaming agents, and some of the added hydrocarbons remain in the foam and foam film even after extrusion foaming. This can contribute to improvement of moldability. If the amount of hydrocarbon is too small, the thermoformability will deteriorate. On the other hand, in the foam sheet having a high open cell ratio as in the present invention, it is extremely rare that the content of the saturated hydrocarbon exceeds 3% by weight. Sometimes there is a risk that sufficient heating will not be possible.

  Examples of the saturated hydrocarbon having 3 to 6 carbon atoms include propane, normal butane, isobutane, normal pentane, isopentane, normal hexane, cyclohexane and the like, and mixtures thereof. Isobutane, normal pentane and isopentane are preferred in terms of the balance between foaming efficiency and remaining in the cell membrane, and isobutane is particularly preferred.

  The saturated hydrocarbon content in the present specification can be measured by an internal standard method by gas chromatography analysis. Specifically, by dissolving a measurement sample collected from a foam sheet in an organic solvent such as toluene containing an internal standard in a closed system, the saturated hydrocarbon in the foam sheet is dissolved in the organic solvent. By collecting a part and subjecting it to gas chromatography analysis, the content of saturated hydrocarbons in the foamed sheet can be measured.

  As described above, since the foamed sheet of the present invention contains a specific amount of saturated hydrocarbons, the foamed sheet is sufficiently plasticized by saturated hydrocarbons during thermoforming and has a unique cellular structure as described above. By having, it becomes difficult to produce a crack in a bubble film at the time of thermoforming, so that thermoformability is not hindered, and it is considered that excellent thermal formation is exhibited.

  The foamed sheet of the present invention contains a calcium inorganic salt. When these salts are used as a cell membrane pore-opening agent, a foam sheet capable of secondary foaming can be obtained even with an open cell structure. The reason is considered to be that pores having a smaller diameter are formed when these salts are used than the pores formed in the foamed sheet by talc that has been used as a conventional cell membrane pore-opening agent.

Calcium chloride, calcium sulfate, calcium hydrogen carbonate, calcium hydrogen carbonate, calcium nitrate, calcium sulfite, calcium silicate, calcium phosphate, calcium pyrophosphate, calcium hypochlorite, calcium chlorate, calcium perchlorate, bromic acid Examples include calcium, calcium iodate, calcium arsenite, calcium chromate, calcium tungstate, and calcium molybdate.
Of these, calcium oxoacid salts such as calcium sulfate and calcium carbonate are preferred, and calcium carbonate is more preferred from the viewpoint of availability.

  The content of the calcium inorganic salt is 2 to 7% by weight, preferably 2.5 to 5% by weight. If the content is too small, it is difficult to obtain a foam sheet having a high open cell ratio, and if it is too large, the thermoformability of the foam sheet may be reduced.

  In addition, it is preferable that the average particle diameter of the calcium inorganic salt used for this invention is 1-6 micrometers, More preferably, it is 3-5 micrometers. When the average particle size is too small, there is a possibility that a predetermined open cell ratio cannot be obtained. If the average particle size is too large, the bubble openness is prioritized over the bubble growth during the foam sheet production, and it may be difficult to achieve a predetermined apparent density, or the secondary foaming property is reduced, There is a possibility that the thermoformability is deteriorated. In addition, an average particle diameter means a volume average particle diameter (d50). For example, the volume average particle diameter (d50)) can be measured as a particle diameter when the particles are dispersed in water, the particle size distribution is measured by a laser diffraction method, and the cumulative volume with respect to the volume of all particles is 50%. Yes (JIS Z8901: 2006).

  In the present invention, resins or inorganic substances other than the above components can be added within a range that does not deteriorate the performance of the foamed sheet. For example, when calcium mineral salt is supplied in advance as a high-content masterbatch, the performance of the foamed sheet is also reduced by the masterbatch using a polyethylene resin such as low-density polyethylene, which is easy to produce a high-content masterbatch. It can be used within a range that does not. Further, together with the calcium inorganic salt, an inorganic substance other than the calcium inorganic salt such as talc, a chemical foaming agent, or the like can be used in combination with the calcium inorganic salt as a bubble regulator.

In the present invention, the ratio ρs / ρa of the apparent density of the surface layer portion (hereinafter also simply referred to as “surface layer density”) ρs from the surface to the thickness direction of 200 μm with respect to the apparent total density ρa is 1.5 to 3.0. It needs to be, and preferably 1.8 to 2.4. Since the foam sheet has front and back surfaces, it is preferable that each surface layer density satisfies this range. When ρs / ρa is less than 1.5, the foaming agent is easily removed from the surface of the foamed sheet, and it may be difficult to obtain a predetermined secondary foaming ratio. When ρs / ρa exceeds 3, the density difference between the inside and outside of the foamed sheet is too large, and the foamed sheet cannot withstand the difference between the inside and outside of the shear stress applied in the mold at the time of thermoforming, and molding defects are likely to occur.
From the same viewpoint, the surface layer density ρs is preferably 0.10~0.45g / cm ^3, more preferably 0.15~0.40g / cm ^3.

  The ratio ρs / ρa can be adjusted by changing the cooling condition of the surface of the foam sheet during the manufacture of the foam sheet. Specifically, the ratio ρs / ρa can be increased by increasing the amount of air blown onto the foam surface immediately after extrusion, and the ratio ρs / ρa can be decreased by decreasing the amount of air. .

The surface density is measured as follows.
A 200 μm portion is sliced from the surface of the foam sheet to obtain a test piece, and the weight of the test piece is measured. The weight of the test piece is divided by the volume of the test piece (width × length × thickness), and converted into units to obtain the surface layer density.

  In the foam sheet of this invention, it is preferable that the average cell diameter of the thickness direction is 30-200 micrometers, More preferably, it is 50-150 micrometers. When the average cell diameter is within this range, the foam sheet is particularly excellent in thermal formation.

In the present specification, the average cell diameter in the thickness direction of the foamed sheet is a value measured as follows.
In a cross section perpendicular to the extrusion direction of the foam sheet, a line segment l is drawn from one surface to the other surface in the thickness direction of the foam sheet, and the number n of all bubbles intersecting with the line segment l is counted. A value obtained by dividing the thickness t by the number of bubbles n is defined as an average cell diameter in the thickness direction of the foam sheet.

  As the polystyrene resin constituting the foamed sheet of the present invention, it can be used without particular limitation as long as it is a polystyrene resin usually used for polystyrene-based foamed sheets. For example, general-purpose polystyrene (GPPS), impact-resistant polystyrene ( HIPS), styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, styrene-maleic anhydride copolymer, styrene-methyl methacrylate copolymer, styrene-α-methylstyrene copolymer, etc. . A mixture of general-purpose polystyrene and polyphenylene ether resin can also be used. In addition, in the range which does not inhibit the intended purpose of the present invention, the foamed sheet can also contain other polymers other than the polystyrene-based resin. In that case, the content of the other polymer is the foamed sheet. The content is preferably 5% by weight or less, more preferably 3% by weight or less, and still more preferably 1% by weight or less.

  Among these, general-purpose polystyrene (GPPS) is preferable, and general-purpose polystyrene (GPPS) having a melt tension at 200 ° C. of 200 mN or less is more preferable. By using general-purpose polystyrene (GPPS) as the base resin, it is possible to obtain a foamed sheet that is particularly excellent in thermoformability, and furthermore, by having the melt tension within the above range, it has a predetermined open cell ratio. A foam sheet can be obtained easily. The melt tension is desirably 50 mN or more, preferably 70 mN or more in consideration of foamability.

The melt tension can be measured by using a Capillograph 1D manufactured by Toyo Seiki Seisakusho Co., Ltd. as a measuring device. Specifically, a cylinder with a cylinder diameter of 9.55 mm and a length of 350 mm and an orifice with a nozzle diameter of 2.095 mm and a length of 8.0 mm are used. The amount is put in the cylinder and left for 4 minutes, and then the molten resin is extruded from the orifice into a string with a piston speed of 10 mm / min, and this string is hung on a tension detection pulley having a diameter of 45 mm for 4 minutes. The maximum tension immediately before the string-like material breaks when the string-like material is taken up by the take-up roller while increasing the take-up speed at a constant speed so that the take-up speed reaches from 0 m / min to 200 m / min. Get the value. A total of 10 measurements were performed, and the three maximum values obtained in 10 times were removed in order from the largest value, and the three values in order from the smallest maximum value were removed. The value obtained by arithmetic averaging is taken as the melt tension (mN) in the method of the present invention.
If the string-like material does not break even when the take-up speed reaches 200 m / min, the value of melt tension (mN) obtained by setting the take-up speed to a constant speed of 200 m / min is adopted. Specifically, the acquisition of melt tension data is started after the rotational speed reaches 200 m / min, and the data acquisition is finished after 30 seconds. The average value (Tave) of the tension maximum value (Tmax) and the tension minimum value (Tmin) obtained from the tension load curve obtained during this 30 seconds is taken as the melt tension in the method of the present invention. Here, the Tmax is a value obtained by dividing the total value of detected peak (peak) values in the tension load curve by the detected number, and the Tmin is detected in the tension load curve. It is a value obtained by dividing the total value of the dip (valley) values by the detected number.

  The polystyrene resin foam sheet of the present invention is excellent in thermoformability, and can be processed into a polystyrene resin foam molded body (hereinafter also simply referred to as a foam molded body or a molded body) by a conventionally known thermoforming method. it can. The obtained foamed molded article has a sufficiently shaped mold.

  Conventionally known thermoforming methods include vacuum forming, pressure forming, and their applications include free drawing forming, plug and ridge forming, ridge forming, matched mold forming, straight forming, drape forming, reverse draw forming, Air slip molding, plug-assist molding, plug-assist reverse load molding, or a combination of these is employed.

  In the foamed molded product of the present invention, a hole that does not penetrate from one surface side toward the other surface is provided, and the foamed molded product is provided with water absorption by being connected to the open cell structure formed at the time of foaming. Can do. The foamed molded article to which water absorption is imparted can be suitably used as a packaging container for fresh food such as meat and fish, a container for warm food such as cooked rice, and a lid thereof.

  The method for forming the hole in the foam molded body is not particularly limited, and the foam molded body may be provided with holes after thermoforming, or the foamed sheet may be thermoformed after the holes are formed. From the viewpoint of securing the sheet thickness by foaming, it is preferable to form the holes after molding or immediately before entering the heating furnace of the molding machine.

  In the foam sheet, the hole can be formed by passing along a roll or the like provided with a large number of needles, and in the molded body, a plate or the like provided with a large number of needles is used. It can be formed by pressing the target position. Further, water removal can be imparted to the molded body by removing a part of the surface, but the method of forming holes with a needle or the like is preferable because the water absorption speed is easily controlled.

  The water absorption (water absorption speed) can be controlled by appropriately selecting the shape, depth, position, and interval of the holes according to the purpose of use, and the opening area of the holes is generally large, the depth is deep, and the hole interval is narrow. Tend to be faster.

From the balance between the water absorption and the strength of the molded body, the opening area per hole is preferably 0.04 to ³ mm ² and the surface area per unit area is 2 to 8%. The depth of the holes is preferably 10 to 90% of the thickness of the perforated portion of the foamed molded article, and the interval between the holes is preferably in the range of 1 to 20 mm.

In particular, when the foam sheet of the present invention is used as a water-absorbing lid, in order to prevent water vapor from cooling and falling as water droplets, the opening area, depth, and hole interval should be set within a range where the strength of the lid is acceptable. Is important, the opening area per hole is 0.1 to ^2.5 mm ² , the opening rate is 5% or more, the hole depth is 30 to 70% of the thickness of the hole, and the hole spacing is It is preferable to set it as the range of 1-5 mm.

  Next, the polystyrene-based resin foam sheet for thermoforming of the present invention will be described in detail with reference to examples. However, the present invention is not limited to the examples.

  Table 1 shows polystyrene resins used in Examples and Comparative Examples.

  The cell membrane pore-opening agent is shown in Table 2.

Examples 1-5, Comparative Examples 1-4
A first extruder having an inner diameter of 115 mm and a second extruder having an inner diameter of 150 mm are prepared by blending the polystyrene-based resin shown in Table 3 and calcium carbonate as the cell membrane pore-opening agent shown in Table 3 into Table 3. After supplying to the first extruder of the connected tandem extruder and melt-kneading, the amount of isobutane or mixed butane shown in Table 3 (isobutane 65%, normal butane 35%) as a foaming agent is injected and mixed further. It was smelted to obtain a foamable resin melt. In Example 4, it was supplied to the extruder in the form of a calcium carbonate masterbatch containing 80% by weight of calcium carbonate and using low density polyethylene as the base resin. In Comparative Example 1, in addition to calcium carbonate, talc was used as a bubble adjusting agent in order to match the bubble diameter with other examples.
Subsequently, the foamable resin melt was cooled by a second extruder, and after adjusting the resin temperature to the temperature shown in Table 3, the cylinder was discharged from an annular slit having a diameter of 180 mm and a thickness of 0.45 mm at a discharge rate of 300 kg / hr. This cylindrical foam is taken out along a cooling cylinder having a diameter of 670 mm, cut into the cylindrical foam with two cutters attached to the rear part of the cooling cylinder, and two upper and lower foam sheets. Got. The foam surface was cooled by blowing air at 25 ° C. with the airflow shown in Table 3 inside and outside the cylindrical foam immediately after coming out of the slit so as to have the surface layer density shown in Table 1.

  Table 4 shows the physical properties of the obtained foamed sheet.

  Although the foamed sheets obtained in Examples 1 to 5 had a large open cell ratio, the secondary foaming ratio was large and the thermoformability was excellent.

  Comparative Example 1 is an example in which the blending amount of calcium carbonate is reduced with respect to Example 1. Although the foamed sheet of Comparative Example 1 was excellent in thermoformability, the open cell ratio was much smaller than the desired value.

  Comparative Example 2 is an example in which the blending amount of calcium carbonate is increased with respect to Example 1. The foamed sheet of Comparative Example 2 had a low secondary foaming ratio and was greatly inferior in thermoformability.

  Comparative Example 3 is an example in which the apparent density ratio ρs / ρa is reduced. The foamed sheet of Comparative Example 3 had a low secondary foaming ratio and was inferior in thermoformability.

  Comparative Example 4 is an example in which the apparent density ratio ρs / ρa is increased. Although the foamed sheet of Comparative Example 4 sufficiently secondary foamed, it was greatly inferior in thermoformability.

In Table 4, each physical property was measured as follows.
(Saturated hydrocarbon content)
The content of saturated hydrocarbon in the foam sheet was measured as follows. Immediately before evaluating the thermoformability of the foam sheet, put the sample cut out from the foam sheet into a sample bottle with a lid containing toluene, add cyclopentane as an internal standard, close the lid and stir well. Then, the saturated hydrocarbon in the foam sheet was dissolved in toluene, and gas chromatographic analysis was performed. The concentration of saturated hydrocarbons in the sample was calculated from the peak area of the gas chromatogram obtained by gas chromatographic analysis using the following equation (2) to determine the content of saturated hydrocarbons in the foam sheet.
xi = (Fi × Ai × Ws × 100) ÷ (As × Wsm) (2)
xi: Weight% concentration of each saturated hydrocarbon in the sample Fi: Correction coefficient As: Peak area of the standard material Ai: Peak area of each saturated hydrocarbon Ws: Weight of the standard material Wsm: Sample weight ) GC-14B manufactured by Shimadzu Corporation was used, and the measurement conditions for gas chromatographic analysis were as follows.
column:
Manufacturer: Shinwa Kako Co., Ltd. Carrier: Chromosorb W, mesh 60-80, AW-DMCS treated liquid phase: Silicone DC550 (liquid phase amount 20%)
Column dimensions: Column length 4.1m, column inner diameter 3.2mm
Column material: Glass-filled column Empty conditions: 220 ° C, 40 hours Column temperature: 40 ° C
Inlet temperature: 200 ° C
Carrier gas: Nitrogen Carrier gas speed: 3.5ml / min
Detector: FID

Detector temperature: 200 ° C (apparent total density)
Measured based on JIS K7222: 2005 (n = 5).

(Surface density)
A test piece was prepared by cutting a 200 μm portion from the surface of the foamed sheet into a size of 5 mm wide × 20 mm long. The weight of the test piece was measured, the weight of the test piece was divided by the volume of the test piece (width × length × thickness), and converted into units to obtain the surface layer density. This measurement was performed on five randomly selected portions of the foam sheet, and the arithmetic average value (n = 5) of the measured values was defined as the surface layer density of the foam sheet.

(Average bubble diameter)
The foamed sheet was divided into 10 equal parts in the width direction, and a cross section perpendicular to the extrusion direction was taken with a microscope at the center position in the width direction. For each cross-sectional photograph, a line segment l was drawn from one surface to the other surface in the thickness direction of the sheet, and the number n of all bubbles intersecting with the line segment l was counted. The bubble diameter (t / n) is calculated for each cross-sectional photograph from the n thus obtained and the length t of the line segment l, and the arithmetic average value of 10 (t / n) is calculated as the thickness of the foam sheet. The average bubble diameter in the direction was taken.

(Secondary foaming ratio)
From five randomly selected foam sheets, a square measurement sample with a side of 260 mm is prepared so that the vertical and horizontal sides coincide with the extrusion direction (MD) and the width direction (TD) of the foam sheet. Cut out (5 pieces). About each sample, the thickness of five points | pieces selected at random from the area | region of the center part 150 mm x 150 mm was measured, and the arithmetic mean value was made into the thickness of the foamed sheet before a heating.
Next, the two sides of each sample were sandwiched from both sides and fixed using two wooden frame members each having a square through-hole of 200 mm in length and 200 mm in width in the center. The foam sheet sample fixed with a wooden frame was placed in an air circulation oven (product number PERFECT OVEN PH-200 manufactured by Tabai Espec Co., Ltd.) whose temperature was adjusted to 145 ° C. and heated for 27 seconds, and then 23 ° C. from the oven. It was taken out in the environment of and left to cool.
And about the foamed sheet sample after cooling, the thickness of the same location as the measurement of the thickness before the said heating is measured, and the foamed sheet thickness after heating (after secondary foaming) is calculated | required, (foamed sheet thickness after heating) / By calculating (foamed sheet thickness before heating), the secondary foaming ratio in each test piece was obtained. This measurement was performed on each sample, and the arithmetic average value thereof was defined as the secondary foaming ratio of the foamed sheet.

(Thermoformability)
The foamed sheet is thermoformed by matched mold vacuum forming (lower heater temperature: 320 ° C., upper heater temperature: 340 ° C., heating time 10 seconds) using the following three types of cup molds with different depths. The thermoformability of the foamed sheet was evaluated according to the above criteria. As a molding apparatus, an FKS-0631-10 molding machine manufactured by Asano Laboratory was used.
Mold 1: Cup shape with opening diameter 120mm, bottom diameter 80mm, depth 35mm Mold 2: Cup shape with opening diameter 120mm, bottom diameter 90mm, depth 26mm Mold 3: Opening diameter 120mm, bottom diameter 100mm , 17mm deep cup shape (evaluation criteria)
A: Good molded bodies were obtained in all of the molds 1 to 3.
Δ: Good molds were obtained in the molds 2 and 3, but in the mold 1, unevenness in elongation occurred and a good mold was not obtained.
X: A good molded product was obtained in the mold 3, but in the molds 1 and 2, unevenness in elongation occurred and a good molded product could not be obtained.

Claims (5)

In a polystyrene-based resin foam sheet for thermoforming having a thickness of 0.5 to ³ mm, an apparent overall density of 0.05 to 0.3 g / cm ³ , and an open cell ratio of 70% or more,
The foamed sheet contains 2 to 7% by weight of a cellular membrane pore-forming agent made of a calcium inorganic salt, and contains 1 to 3% by weight of a saturated hydrocarbon compound having 3 to 6 carbon atoms.
The ratio of the apparent density of the surface layer part up to 200 μm in the thickness direction from the surface to the apparent overall density of the foamed sheet is 1.5 to 3.0,
A polystyrene-based resin foam sheet for thermoforming, wherein the foam sheet has a secondary foaming ratio of 1.4 times or more when the foam sheet is heated at 145 ° C. for 27 seconds and cooled to 23 ° C.
The base resin constituting the foamed sheet is general-purpose polystyrene (GPPS), and the general-purpose polystyrene (GPPS) has a melt tension of 200 mN or less at 200 ° C. Resin foam sheet.
  The polystyrene-type resin foam sheet for thermoforming according to claim 1 or 2, wherein the cell membrane pore-opening agent is calcium carbonate having an average particle diameter of 1 to 6 µm. The polystyrene-based resin foam sheet for thermoforming according to any one of claims 1 to 3, wherein an average cell diameter in the thickness direction of the foam sheet is 30 to 200 µm.
A polystyrene-based resin foam molded article obtained by thermoforming the polystyrene-based resin foam sheet for thermoforming according to any one of claims 1 to 4.