JP6715149B2 - Method for producing resin foam sheet and resin foam molded article - Google Patents

Description

The present invention relates to a resin foam sheet and a method for producing a resin foam molded article, more specifically, a resin foam sheet having a resin foam layer formed of a polyamide resin composition and thermoforming such a resin foam sheet. The present invention relates to a method for producing a resin foam molded article for producing a resin foam molded article.

Conventionally, a resin foam sheet obtained by extruding and foaming a resin composition containing a general-purpose polystyrene resin (GPPS) as a main component is also called polystyrene paper (PSP) and is used as a resin foam molded article.
As this type of resin foam sheet, not only a sheet of a resin foam layer such as PSP but also a type in which a resin foam layer and a resin film layer are laminated and integrated is known. Since it also has a resin foam layer, it is lightweight, yet has high strength, and is excellent in moldability in thermoforming and the like.

A resin foam molded article obtained from such a resin foam sheet as a raw material is called a sheet molded article and is widely used as a food container or a cushioning material.
Conventionally, in the case of producing a resin foam molded article, what is called a raw roll in which a long strip-shaped resin foam sheet is wound into a roll is used, and a resin foam sheet fed from the raw roll is thermoformed. A method of punching after giving a predetermined shape is widely adopted.
In this thermoforming, a resin foam sheet is heated by a radiant heater or the like to be softened and foamed, and the resin foam sheet softened in the preheating step is deformed so as to follow the surface shape of the molding die. Then, a thermoforming method is adopted in which a molding step of sequentially forming a product shape on the resin foam sheet is performed.
The resin foam sheet is not limited to one having a resin foam layer formed of a polystyrene resin composition containing GPPS as a main component, but is also formed of a polyamide resin composition as shown in Patent Document 1 below. Those having a resin foam layer are known.

JP, 2013-185074, A

Foaming in the preheating step is called secondary foaming, whereas foaming when manufacturing a resin foam sheet is called primary foaming.
In thermoforming, secondary foaming occurs due to the foaming agent contained in the resin foam sheet.
At the time of secondary foaming, the internal pressure of the bubbles of the heated resin foam sheet is increased by the foaming agent, and the resin foam sheet is in a tensioned state.
If the temperature rise of the resin foam sheet due to this preheating and the increase of the internal pressure accompanying it are insufficient, the elongation of the resin foam sheet lacks uniformity when it is shaped by a molding die, and buckling or wrinkling occurs. As a result, there is a possibility that a good quality resin foam molded article cannot be obtained.
On the other hand, if excessive secondary foaming occurs in the resin foam sheet in the preheating step, wrinkles may occur in the resin foam sheet before being shaped by the molding die, and wrinkles may also occur in the resin foam molded product. is there.
In addition, if excessive preheating is performed to secondary-foam the resin foam sheet, the internal pressure in the bubbles increases, causing the bubbles to break and impairing the surface property of the resin foam molded product called heat discoloration. is there.

In this regard, it is difficult for a resin foam sheet having a resin foam layer formed of a polyamide resin composition to exhibit good secondary foamability as compared with PSP and the like.
INDUSTRIAL APPLICABILITY The present invention imparts good secondary foamability to a resin foam sheet having a resin foam layer formed of a polyamide resin composition, and can easily obtain a good resin foam molded product. It is an object to provide a method for producing a resin foam molded article.

The present inventors have conducted extensive studies to solve the above problems, and in a resin foam sheet having a resin foam layer formed of a polyamide resin composition, a predetermined amount of water as a foaming agent is added to the resin foam layer. The present invention has been found to exhibit good secondary foaming properties when contained, and has completed the present invention.

That is, the present invention is a resin foam sheet provided with a resin foam layer, which is used for forming a resin foam molded article by thermoforming,
The resin foam layer comprises a polyamide resin composition containing a foaming agent and provides a resin foam sheet containing water as the foaming agent in a ratio of 4.23 % by mass or more and 6% by mass or less.

A method for producing a resin foam molded article by thermoforming a resin foam sheet having a resin foam layer, the method comprising: a polyamide resin composition containing a foaming agent; Provided is a method for producing a resin foam-molded article, which carries out the thermoforming using a resin foam sheet provided with the resin foam layer containing water in a proportion of 4.23 % by mass or more and 6% by mass or less.

By using the resin foam sheet of the present invention, a good resin foam molded article can be easily produced.

It is a schematic diagram showing a procedure for measuring an average cell diameter of a polyamide resin foam sheet.

Embodiments of the present invention will be described below.
The resin foam sheet of the present embodiment is used as a raw material for a resin foam molded product having a three-dimensional shape such as a container, and is used so that the three-dimensional shape is given by thermoforming.
In the present embodiment, the embodiment of the present invention will be described by taking as an example the case where the resin foam sheet for thermoforming is composed of a single resin foam layer.
The resin foam sheet for thermoforming of the present embodiment (hereinafter, also simply referred to as “foam sheet”) is manufactured by the extrusion foaming method.
In the resin foam sheet according to the present embodiment, the resin foam layer is formed of a polyamide resin composition.
The resin foam sheet of the present embodiment contains a foaming agent in the resin foam layer in order to exhibit good moldability in thermoforming.

First, the polyamide resin composition used for forming the resin foam layer will be described.
The polyamide-based resin composition forming the resin foam layer may be a general one in which the polyamide-based resin as the main component is not modified, but exhibits good moldability in thermoforming. Above, it is preferable to include a modified polyamide resin which is a reaction product of the polyamide resin and a modifier.
That is, the polyamide-based resin composition uses the modified polyamide-based resin, which is a reaction product of the polyamide-based resin (A1) and the modifier (A2), as the base polymer (A), and further includes the foaming agent (B1) and Those containing the component (B) for foaming, such as a cell nucleating agent, and various additives (C) are preferable.

(A) Base Polymer The polyamide resin composition constituting the resin foam layer contains a modified polyamide resin and an unmodified polyamide resin (hereinafter also referred to as “unmodified polyamide resin”) as main components. Preferably.

The polyamide-based resin composition used for forming the resin foam layer preferably has a total amount of the modified polyamide-based resin and the non-modified polyamide-based resin of 90% by mass or more to form a foam-molded article. In the polyamide-based resin composition for achieving the above, the proportion of the modified polyamide-based resin is preferably 90% by mass or more.

(A1) Polyamide-based resin Polyamide-based resin that is a starting material for the modified polyamide-based resin, and unmodified polyamide-based resin that is contained in the polyamide-based resin composition together with the modified polyamide-based resin are homopolycondensation type resins. Or a copolycondensation type.
If the polyamide-based resin is a homopolycondensation type, for example, polyamide 6 obtained by ring-opening polymerization of ε-caprolactam, polyamide 11 obtained by ring-opening polymerization of undecane lactam, ring-opening polymerization of lauryl lactam The obtained polyamide 12 or the like can be used.

Further, if the polyamide resin is a copolycondensation type, polyamide 4,6 obtained by polycondensation of tetramethylenediamine and adipic acid, and polyamide obtained by polycondensation of hexamethylenediamine and adipic acid. 6,6, polyamide 6,10 obtained by condensation polymerization of hexamethylenediamine and sebacic acid, polyamide 6T obtained by condensation polymerization of hexamethylenediamine and terephthalic acid, obtained by condensation polymerization of hexamethylenediamine and isophthalic acid Polyamide 6I, polyamide 9T obtained by polycondensation of nonanediamine and terephthalic acid, polyamide 5MT obtained by polycondensation of methylpentadiamine and terephthalic acid, polyamide 6,12 obtained by polycondensation of caprolactam and lauryllactam, etc. Can be
Further, the polyamide-based resin may be a so-called aromatic polyamide as long as it is a copolycondensation type, and poly-p-phenylene terephthalamide and m-phenylene obtained by polycondensation of p-phenylenediamine and terephthalic acid. For example, poly-m-phenylene isophthalamide obtained by polycondensation of diamine and isophthalic acid can be used.

(A2) Modifier As a modifier for modifying the polyamide resin, it has a reactive group capable of chemically bonding to the polyamide resin, such as a carboxylic acid anhydride group, an epoxy group, a carboxyl group or a hydroxy group. There are things.
The modifier may be a monomer compound, but a polymer is preferable from the viewpoint of easy handling.
The modifier is preferably a copolymer having styrene and maleic anhydride as constituent units, or a copolymer having styrene, methyl methacrylate and maleic anhydride as constituent units.

Examples of the modifier for the compound of the monomer include a compound having two or more carboxylic acid anhydrides in one molecule or a polyfunctional epoxy compound, and specifically, pyromellitic acid anhydride and naphthalenetetracarboxylic acid. An anhydride, a benzophenone tetracarboxylic acid anhydride, a cyclopentane tetracarboxylic acid anhydride, etc. are mentioned.
As the polymer modifier, a copolymer of α,β-unsaturated carboxylic acid anhydride and aromatic vinyl, or α,β-unsaturated carboxylic acid anhydride, and (meth)acrylic acid ester And a copolymer of aromatic vinyl and polycarbonate.
Examples of the α,β-unsaturated carboxylic acid anhydride include maleic anhydride, methyl maleic anhydride, chloromaleic anhydride and the like.
Examples of the (meth)acrylic acid ester include (meth)acrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, lauryl (meth)acrylate, and (meth)acrylic acid. Examples thereof include 2-ethylhexyl, cyclohexyl (meth)acrylate, (meth)acrylamide, and benzyl (meth)acrylate.
Examples of the aromatic vinyl include styrene and methylstyrene.

The polyamide-based resin becomes a modified polyamide-based resin having an improved melt tension by grafting the modifier by being melt-kneaded with the modifier.
For modifying the polyamide resin, a polymer grafted onto the polyamide resin via a modifier may be further used.
That is, a polymer having a functional group that does not chemically bond directly to the polyamide-based resin but chemically bonds to the modifier may be used as a raw material of the modified polyamide-based resin together with the modifier.

By using the modifier as described above, in the present embodiment, the melt tension of the polyamide-based resin forming the resin foam layer is improved, and good secondary foamability is exhibited at the time of thermoforming. Therefore, a high-quality resin foam-molded product can be easily manufactured.

The ratio of the polyamide-based resin and the modifier used for producing the modified polyamide-based resin is not particularly limited.
The proportion of the polyamide-based resin as a starting material contained in the modified polyamide-based resin is usually 90% by mass or more and less than 100% by mass, and preferably 95% by mass or more and less than 100% by mass.
The proportion of the modifier contained in the modified polyamide resin is usually more than 0% by mass and 5% or less, preferably more than 0% by mass and 3% by mass or less.

The polyamide resin composition may contain a resin other than the above polyamide resin, if necessary.
Examples of the resin other than the polyamide resin that can be contained in the polyamide resin composition include, for example, polystyrene resin, polyethylene resin, polypropylene resin, polyethylene terephthalate resin, polybutylene terephthalate resin, polyethylene naphthalate resin, and polycarbonate resin. Are listed.
The other resin can be usually contained so that the proportion of the polyamide resin composition in all the resin components is more than 0% by mass and 50% by mass or less, and the ratio is 10% by mass or less. It is preferable that the content is 5% by mass or less.

(B) Foaming component As a component for foaming the above resin, a foaming agent (B1), a cell nucleating agent (B2) and the like can be mentioned.

(B1) Foaming Agent As the foaming agent, the same foaming agent as used in general extrusion foaming of resin can be adopted.
Examples of the foaming agent include hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane and hexane, ketones such as acetone, methyl ethyl ketone and acetylacetone, ethers such as dimethyl ether, methyl chloride, ethyl chloride and the like. Examples thereof include halogenated hydrocarbons, water, carbon dioxide, nitrogen, and inorganic gases such as air.
The foaming agent to be contained in the foamed sheet (resin foam layer) of the present embodiment may remain the foaming agent used in manufacturing the foamed sheet, and is necessary for the foamed sheet once prepared. It may be added later depending on the situation.
Moreover, when the content of the foaming agent in the produced foamed sheet is excessive, a treatment for appropriately reducing the amount of the foaming agent may be performed.

The foamed sheet of this embodiment contains at least water among the foaming agents.
The foamed sheet of this embodiment preferably contains hydrocarbons (hydrocarbon-based foaming agent) together with water as a foaming agent.
The hydrocarbon-based foaming agent is preferably normal butane or isobutane.

The foamed sheet of the present embodiment exhibits good secondary foamability by containing water.
The content of water in the foam sheet is usually 1.2% by mass or more.
The content of water in the foamed sheet is preferably 1.5% by mass or more, and more preferably 2.0% by mass or more.
If the water content is less than 1.2% by mass, the secondary foamability will be low, and the temperature of the foamed sheet will not drop due to the heat of vaporization of water during preheating, which may cause thermal burn. is there.
However, excessive water content causes excessive secondary foaming, which may cause wrinkles in the resin foam molded product such as waviness in the resin foam sheet before being shaped by the molding die.
Moreover, since the temperature of the foamed sheet is remarkably lowered by the heat of vaporization of water during preheating, the resin foamed sheet may not be sufficiently heated and the elongation may deteriorate.
Therefore, the content of water in the foamed sheet is usually 6% by mass or less.
The content of water in the foamed sheet is preferably 5% by mass or less, and more preferably 4% by mass or less.

The water content of the foamed sheet can be determined by evaluation by the Karl Fischer titration method (water vaporization method) of JIS K0068:2001 (method for measuring water content of chemical products).
Specifically, for example, 70 mg of a sample collected from a foamed sheet is set in a Karl Fischer moisture measuring device CA-200 and a moisture vaporization device VA-236S manufactured by Mitsubishi Chemical Analytech Co., Ltd. to measure the foamed sheet. The amount of water contained in can be calculated.
The water content of the foamed sheet can be obtained by converting the water content into a ratio per unit mass of the foamed sheet.
The measurement was carried out by using AQUAMICRON AX and AQUAMICRON CXU manufactured by API Corporation as the anolyte and the catholyte respectively, the measurement temperature was 230° C., nitrogen was used as the carrier gas, and the flow rate was 250 mL/ It will be done as min.
The sample was tested three times, and the water content of the blank container (average of two test times) measured simultaneously was subtracted from each water content, and that value was taken as the water content contained in the sample. Determine the content rate (mass %).
The water content of the foamed sheet can be obtained as an arithmetic mean value of the results of three tests.

(B2) Cell Nucleating Agent As the cell nucleating agent, a widely used one can be adopted, and examples thereof include talc, mica, silica, diatomaceous earth, alumina, titanium oxide, zinc oxide, magnesium oxide, and hydroxide. Inorganic compounds such as magnesium, aluminum hydroxide, calcium hydroxide, potassium carbonate, calcium carbonate, magnesium carbonate, potassium sulfate, barium sulfate, sodium hydrogen carbonate and glass beads; polytetrafluoroethylene, azodicarbonamide, sodium hydrogen carbonate and quench Examples thereof include organic compounds such as a mixture of acids, inert gases such as nitrogen, and talc is preferable.
The cell nucleating agents may be used alone or in combination of two or more.
The amount of the cell nucleating agent added to the polyamide resin composition is preferably 0.01 to 10 parts by mass with respect to 100 parts by mass of all resin components contained in the polyamide resin composition.

(C) Additives The additives may be those conventionally used for processing thermoplastic resins, if necessary, and include, for example, weathering agents, light stabilizers, ultraviolet absorbers, and oxidation agents. An inhibitor, a pigment, a dye, a flame retardant, a crystal nucleating agent, a crystallization retarder, a plasticizer, a lubricant, a surfactant, a dispersant, a filler, a reinforcing agent, an antistatic agent and the like can be contained.

The foamed sheet of the present embodiment is produced by extruding and foaming a polyamide resin composition containing these materials into a sheet form from an extruder or the like.
More specifically, for example, a compound prepared by dry blending all the materials except the foaming agent in a predetermined ratio, or a compound prepared as a full compound is prepared, and the compound is attached to a circular die or a flat die. Was supplied to the extruder, melt-kneading the compound in the extruder and press-fitting the foaming agent in the middle of the extruder, adding the foaming agent to the melt-kneaded compound, and further melt-kneading. The foamed sheet can be obtained by carrying out extrusion-foaming of the obtained melt-kneaded product into a sheet form through a die slit of a circular die or a flat die.

Incidentally, the foam sheet is the thickness and basis weight not particularly limited, usually a thickness 0.5 mm to 5.0 mm, it is basis weight ^{100g / m 2 ~1000g / m 2} .
Further, in the foamed sheet, the lower the open cell ratio, the better the secondary foaming property at the time of molding and the easier it is to obtain a good resin foam molded article.
Therefore, the open cell rate of the foamed sheet is preferably 60% or less, more preferably 50% or less, and particularly preferably 40% or less.
However, since a foamed sheet having an excessively low open cell ratio requires a great deal of time to manufacture, and the effect of improving the secondary foamability is small in comparison with the time and labor required, the open cell ratio of the foamed sheet is small. Is preferably 1% or more, more preferably 3% or more, particularly preferably 5% or more.

Here, the open cell rate can be measured as follows.
From the foamed sheet, a plurality of sheet-shaped samples of 25 mm length×25 mm width are cut out. Test pieces are obtained by stacking the cut-out samples so that there is no space and making the thickness 25 mm. The outer size of the obtained test piece is measured to 1/100 mm using a "Digimatic Caliper" manufactured by Mitutoyo Co., Ltd., and an apparent volume (cm ³ ) is obtained.
Next, the volume (cm ³ ) of the test piece is determined by the 1-1/2-1 atmospheric pressure method using an air comparison type hydrometer 1000 (manufactured by Tokyo Science Co., Ltd.).
The open cell rate (%) is calculated by the following formula, and the average value of the open cell rates of the five test pieces is calculated.
The test piece is stored in a JIS K7100-1999 symbol 23/50, second class environment for 16 hours and then measured in JIS K7100-1999 symbol 23/50, second class environment. In addition, as the air-comparison type hydrometer, a standard sphere (large 28.96 cc small 8.58 cc) is used.

Open cell rate (%) =
100×(apparent volume-air comparison type hydrometer measurement volume)/apparent volume

When the average bubble diameter of the foam sheet is too large, the number of bubbles existing in the foam sheet decreases.
A foamed sheet having a small number of bubbles has a thicker bubble film as compared with a sheet having a large number of bubbles, and thus is inferior in heat insulation and buffering properties.
On the other hand, when the average bubble diameter of the foam sheet is too small, the number of bubbles existing in the foam sheet becomes large.
If the number of bubbles contained in the foamed sheet is too large, the thickness of the foamed film becomes too thin, and the foamed film easily breaks, so that the expansion of the foamed sheet during thermoforming becomes poor.
From such a viewpoint, the average cell diameter of the foamed sheet is preferably 10 μm or more and 1000 μm or less, more preferably 100 μm or more and 800 μm or less, and further preferably 150 μm or more and 500 μm or less.

The average cell diameter of the foamed sheet can be measured by the following test method. As shown in FIG. 1, the foamed sheet 1 is cut at a central portion in the width direction along a direction of extrusion (MD: Machine Direction) and a surface α perpendicular to the surface of the foamed sheet, and the cut surface is a scanning electron microscope. And magnify the image with 18 to 30 times (200 times in some cases) to obtain an enlarged photograph A. The size of the enlarged photograph A is about 1/4 of the size of A4 paper.

The foam sheet 1 is cut along a width direction (TD: Transverse Direction) and at a plane β perpendicular to the foam sheet surface, and the cut surface is magnified 18 to 30 times (200 times in some cases) with a scanning electron microscope. Then, the photograph is taken to obtain an enlarged photograph B. The size of the enlarged photograph B is about 1/4 of the size of A4 paper.

The resin foam sheet 1 is cut along a plane γ orthogonal to MD and TD, and the cut surface is magnified 18 to 30 times (200 times in some cases) with a scanning electron microscope to obtain a magnified photograph C. The size of the enlarged photograph C is about 1/4 of the size of A4 paper.

The magnification of the scanning electron microscope is adjusted so that when a straight line having a length of 60 mm is drawn on a printed photograph, the number of bubbles existing on this straight line is about 10 to 20.

The magnified photographs A to C are taken at arbitrary two points on the foamed sheet 1, and two magnified photographs A to C are obtained.

As the scanning electron microscope, a scanning electron microscope marketed by Hitachi, Ltd. under the trade name "S-3000N", a scanning electron microscope marketed by Hitachi High-Technologies under the trade name "S-3400N" Type electron microscope can be used.

In the enlarged photograph A, a straight line having a length of 60 mm parallel to each of MD and VD (the sheet thickness direction orthogonal to MD on the photograph) is drawn at an arbitrary position, and the number of bubbles is determined from the number of bubbles on this straight line. The average chord length (t) in the direction is calculated by the following formula (1). However, when the thickness of the test piece is thin and the number of bubbles for the length of 60 mm cannot be counted in VD, the number of bubbles for 30 mm or 20 mm is counted and converted to the number of bubbles for 60 mm. Draw a straight line so that the entire bubble lies on the straight line as much as possible. A bubble that only partially occupies a straight line is also included in the bubble count.

Average chord length t (mm)=60/(number of bubbles x magnification of photograph) (1)

In the enlarged photograph B, a straight line having a length of 60 mm parallel to each of TD and VD (the sheet thickness direction orthogonal to TD in the photograph) is drawn at an arbitrary position, and the number of bubbles is determined from the number of bubbles on this straight line. The average chord length (t) in the direction is calculated by the above equation (1).

In the enlarged photograph C, a straight line having a length of 60 mm parallel to each of MD and TD is drawn at an arbitrary position, and the average chord length (t) in each direction of the bubbles is calculated from the number of bubbles on this straight line by the above formula (1). ).

The magnification of a photograph is obtained by measuring the scale bar on the photograph to 1/100 mm, and using the following formula (2). The scale bar may be, for example, one commercially available from Mitutoyo under the trade name “Digimatic Caliper”.

Image magnification =
Scale bar actual measurement value (mm)/scale bar display value (mm) (2)

Then, the bubble diameter D in each direction of MD, TD, and VD is calculated by the following equation (3).

D (mm) = t/0.616 (3)

Based on the obtained MD cell diameter (D _MD ), TD cell diameter (D _TD ), and VD cell diameter (D _VD ), the average cell diameter of the foam sheet 1 is calculated by the following formula (4). The MD bubble diameter D _MD is the arithmetic mean value of the two MD bubble diameters calculated above. The bubble diameter DTD of _TD is an arithmetic mean value of the bubble diameters of the two TDs calculated above. The bubble diameter D _VD of _VD is the arithmetic mean value of the bubble diameters of the two VDs calculated above.

Average bubble diameter (mm)=(D _MD ×D _TD ×D _VD ) ^⅓ (4)

D _MD : MD bubble diameter (mm)
D _TD : TD bubble diameter (mm)
_DVD : VD bubble diameter (mm)

Even if the resin foam sheet has a resin film layer in addition to the resin foam layer, it exhibits good thermoformability as long as the resin foam layer has the foaming agent content as described above.
Even if the resin foam sheet has a resin film layer in addition to the resin foam layer, the resin foam layer exhibits excellent thermoformability as long as the resin foam layer has the above-mentioned open cell ratio.
Even if the resin foam sheet has a resin film layer in addition to the resin foam layer, it exhibits good thermoformability as long as the resin foam layer has the above-mentioned average cell diameter.

The foamed sheet is continuously produced by an extruder and is produced as a long strip.
Then, the long strip-shaped foamed sheet extruded and foamed from the die slit is wound into a roll and used as a raw roll for thermoforming.

The raw roll is used for producing a resin foam molded article by laminating a film on one side or both sides of a foamed sheet, or by subjecting it to thermoforming as it is.
The thermoforming is usually carried out by any one of vacuum forming, pressure forming, vacuum/pressure forming, press forming, match mold forming and the like.
After the thermoforming, punching using a Thomson blade die or a punching press is performed on the foamed sheet having the product shape, and the foamed sheet is separated into a product (resin foam molded article) and a blanking bar. To be done.
That is, the foamed sheet according to the present embodiment is provided with a three-dimensional shape by thermoforming and an outline shape by punching as outer shape processing.

More specifically, in the method for producing a resin foam molded article involving thermoforming, a preheating step of preheating a foam sheet to soften the foam sheet to an appropriate degree and secondary foaming, and molding the preheated foam sheet. Molding step of deforming along the molding surface of the mold, demolding step of taking out the foamed sheet having a three-dimensional shape given in the molding step from the molding die, product part of the foamed sheet taken out of the molding die (forming A trimming process is performed in which the periphery of the (part) is cut and the product (resin foam molded product) is taken out from the foam sheet.

The foamed sheet of the present embodiment exhibits good secondary foamability by containing a proper amount of water as a foaming agent.
Water has a small molecular size, and the permeability of a cell membrane forming a foamed sheet is higher than that of hydrocarbons.
On the other hand, water has a lower vapor pressure than hydrocarbons generally used as a blowing agent when compared at the same temperature.
In other words, water has a higher foaming power than hydrocarbons.
In addition, in the foamed sheet formed of the polyamide resin composition, the surface layer portion has a higher apparent density than the central portion in the thickness direction.
That is, the foamed sheet of this embodiment has a surplus capacity for further foaming in the surface layer portion.
Then, the temperature of the surface layer portion is quickly raised in the preheating.
Therefore, in the preheating step, the water contained in the foamed sheet mainly functions for foaming of the surface layer portion in which the temperature rise is relatively steep and there is a reserve capacity for foaming, and thereafter it is quickly diffused.
Therefore, in the present embodiment, although the secondary foaming occurs due to the foamed sheet containing water, excessive secondary foaming does not occur, and good thermoforming can be performed.
In order to exert such an effect more remarkably, when the foamed sheet is divided into three in the thickness direction and divided into two surface layer portions and one central portion, compared with the water content (Xmid (mg)) in the central portion. It is preferable that the amount of water (Xsuf (mg)) in one surface layer is larger.
The water content (Xsuf (mg)) contained in each surface layer part is preferably 1.1 times or more, and more preferably 1.3 times or more the water content (Xmid (mg)) in the central part. ..
However, since it is not always preferable that there is an excess water content gap between them, the water content (Xsuf (mg)) contained in each surface layer part is the water content in the central part (Xmid (mg) It is preferably 30 times or less, and more preferably 20 times or less.
The water content of the central portion and the surface layer portion can be obtained by performing the measurement based on the Karl Fischer method on the divided pieces in the thickness direction.

When the foamed sheet produced by extrusion foaming does not have the appropriate water content as described above, a humidity control step of adjusting the water content of the foamed sheet may be performed before the preliminary foaming step.

In the humidity control step, for example, a method of simply storing the original roll for a certain period of time may be adopted.
However, in that case, it may take a long time until the foamed sheet contains a predetermined amount of water.
Therefore, in the humidity control step, instead of such a method, the raw fabric roll is stored in a heating/humidifying environment (for example, temperature 30° C. to 50° C., relative humidity 50% to 95%), and You may implement so that water absorption may be accelerated.
In addition, the wider and larger the diameter of the original roll is, the more the degree of water absorption between the portion closer to the core and the outer peripheral portion becomes different.
Therefore, it is preferable that the humidity control step includes a step of rewinding the original fabric roll and a step of loosening the original fabric roll to provide a gap between the foamed sheets.
In the thermoforming, it is preferable to increase the thickness of the resin foam sheet 1.1 times or more by utilizing the secondary foamability of the resin foam sheet.
The extent to which the resin foam sheet has increased in thickness due to the secondary foaming may be determined by measuring the sheet thickness after preheating, but it can also be confirmed by the finally formed resin molded product.
That is, in thermoforming, there is a portion where the thickness becomes thinner than the resin foam sheet due to the elongation in the molding process. However, whether or not the resin foam sheet has increased the thickness by 1.1 times or more in thermoforming is Among the products, the thickness of the above-mentioned non-expanded portion is measured at several places (for example, 5 places or more) to obtain an average value, and the average value is 1. of the thickness of the resin foam sheet before thermoforming. It can be confirmed that it is more than 1 time.
The increase rate of the thickness of the resin foam sheet is more preferably 1.2 times or more.
The increase rate of the thickness of the resin foam sheet is usually 2 times or less, preferably 1.8 times or less.

The thickness of the resin foam sheet or the resin foam molded article is an arithmetic average value of the thicknesses measured at 10 arbitrary locations using a thickness gauge. As the thickness gauge, for example, one commercially available from Mitutoyo under the trade name "Six gauge 547-301" can be used.

The resin foam-molded article formed of the foam sheet is not particularly limited, and various kinds can be considered.
Specifically, the resin foam-molded article produced by the foamed sheet of the present embodiment can be used as daily commodities such as containers, casings (body) of mobile bodies such as automobiles and industrial machines, and heat insulating materials. ..

In the present embodiment, an extruded foam sheet is exemplified as the resin foam sheet for thermoforming in that the resin foam molded article as described above can be efficiently produced by continuous thermoforming, but a thickness suitable for thermoforming. A sheet-shaped bead foam molded article or a board foam molded article having, or a sheet-shaped foam molded as a sliced piece from a thick bead foam molded article or a board foam molded article, as long as it is used for thermoforming In the range intended as the resin foam sheet for thermoforming of the present invention.

Further, in the present embodiment, the case where the resin foam sheet for thermoforming is composed of a single resin foam layer is taken as an example, but a resin foam sheet having a plurality of resin foam layers or one or more resin foam layers is used. A type in which one or more resin film layers are laminated on the layer is also within the range intended as the resin foam sheet of the present invention.
When the resin foam sheet of the present embodiment is of a type having two or more resin foam layers made of a polyamide resin composition, all the resin foam layers are 1.2% by mass or more and 6% by mass or more. The water content does not have to be the following, and at least one layer having such a water content may be provided.

Further, regarding the method for producing a resin foam sheet for thermoforming or a resin foam molded article according to the present embodiment, various modifications can be made to the above-described exemplary embodiment within a range in which the effects of the present invention are not significantly impaired Is.

Next, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.
( Reference example 1)
As a polyamide-based resin, polyamide 6 (manufactured by Unitika Ltd., trade name “A1030BRT”; density 1.13 g/cm ³ ) was prepared.
The polyamide 6 was used after being dehumidified and dried at 100° C. for 4 hours or more in advance.
To 100 parts by mass of this resin (polyamide 6), 0.4 parts by mass of a styrene/maleic anhydride copolymer (manufactured by CRAY VALLEY, trade name "SMA1000P"), a master batch of talc powder polyethylene terephthalate (PET) (Talc 40%, PET 60%; trade name "PET-F40-1" manufactured by Telabo Co., Ltd.) were mixed at a ratio of 2.5 parts by mass, and this mixture was mixed with a tumbler mixer.
A circular die with an annular slit of 70 mm in diameter and a slit width of 0.6 mm was set at the tip of a single-screw extruder (caliber 65 mm, L/D=34), and a cylinder was placed in front of this circular die in the extrusion direction. A cooling mandrel (diameter 206 mm, length 300 mm) was placed.
Cooling was circulated in the cooling mandrel, the extruder was set to a predetermined temperature, and the compound was supplied to the hopper of the extruder and melt-kneaded in the extruder.
Further, butane as a foaming agent was pressed in from the middle of the extruder barrel, added to the melt-kneaded product, and melt-kneaded.
The resin temperature during extrusion was set to 240° C., and the molten polyamide resin composition was extruded and foamed from the die slit of the circular die at a discharge rate of 40 kg/h to form a cylindrical foam.
The diameter of the cylindrical foam was expanded by the cooling mandrel, and the expanded foam was collected by a taker arranged further downstream than the cooling mandrel.
Then, the outer peripheral surface of the cooling mandrel was brought into sliding contact with the inner peripheral surface of the foam to cool the foam, and the cylindrical foam was cut along the extrusion direction on the downstream side of the cooling mandrel.
Then, the cylindrical foam was formed into a flat band and wound into a roll by the take-up machine.
The obtained foamed sheet had a basis weight of 600 g/m ² and a thickness of 1.95 mm.
Further, the width of the obtained foamed sheet was 640 mm and the density was 0.31 g/cm ³ .
Furthermore, the obtained foamed sheet had an average cell diameter of 430 μm and an open cell ratio of 7.0%.
The water content of the foamed sheet was 0.3% by mass.

The foamed sheet obtained was cut into 350 mm×350 mm.
The cut foamed sheet is left standing at 23° C. and a relative humidity of 50% for 168 hours, and then conditioned with a humidity control device (dryer, constant temperature/humidity machine) to have a moisture content of 1.25 mass. The humidity was adjusted to be %.

Here, thermoforming was performed as follows.
That is, the four corners of the foamed sheet were clamped and introduced into a preheating heating furnace in which the upper and lower heater temperatures were set to 330°C.
The foamed sheet was heated for 18 seconds to 26 seconds in this heating furnace, and then this foamed sheet was introduced into a press molding machine to carry out molding.
At this time, the molding die is an aluminum die surface-treated with a polytetrafluoroethylene resin coat, and the opening has a diameter of 120 mm, the bottom has a diameter of 100 mm, and the depth is 40 mm. A cup container having a cylindrical convex portion with a height of 50 mm and a diameter of 30 mm was produced.

(Evaluation criteria)
The obtained resin foam molded article was evaluated for the following items (elongation, heat discoloration, wrinkles, secondary foamability).

(Stretch)
◯: No breakage is observed in the molded product.
X: Breakage is observed in the molded product.

(Heat burn)
◯: No thermal discoloration was observed on the molded product.
X: Heat discoloration is observed in the molded product.

(Wrinkle)
◯: No wrinkles are found on the molded product.
X: Wrinkles are observed on the molded product.

(Secondary foamability)
◯: T2/T1≧1.1
X: T2/T1<1.1
(However, T1 is the thickness of the foamed sheet before thermoforming, and T2 is the thickness of the foamed sheet after heating the foamed sheet in a heating furnace.)

( Reference example 2)
The foamed sheet obtained in the same manner as in Reference Example 1 was cut into 350 mm×350 mm, and the humidity was adjusted in the same manner as in Reference Example 1 to obtain a foamed sheet having a water content of 1.76% by mass.

( Reference example 3)
The foamed sheet obtained in the same manner as in Reference Example 1 was cut into 350 mm×350 mm, and the humidity was adjusted in the same manner as in Reference Example 1 to obtain a foamed sheet having a water content of 3.56% by mass.

(Example 4)
The foamed sheet obtained in the same manner as in Reference Example 1 was cut into 350 mm×350 mm, and the humidity was adjusted in the same manner as in Reference Example 1 to obtain a foamed sheet having a water content of 4.23% by mass.

(Example 5)
The foamed sheet obtained in the same manner as in Reference Example 1 was cut into 350 mm×350 mm, and the humidity was adjusted in the same manner as in Reference Example 1 to obtain a foamed sheet having a water content of 5.11% by mass.

(Comparative Example 1)
The foamed sheet obtained in the same manner as in Reference Example 1 was cut into 350 mm×350 mm, and the humidity was adjusted in the same manner as in Reference Example 1 to obtain a foamed sheet having a water content of 0.40% by mass.

(Comparative example 2)
The foamed sheet obtained in the same manner as in Reference Example 1 was cut into 350 mm×350 mm, and the humidity was adjusted in the same manner as in Reference Example 1 to obtain a foamed sheet having a water content of 1.10% by mass.

(Comparative example 3)
The foamed sheet obtained in the same manner as in Reference Example 1 was cut into 350 mm×350 mm, and the humidity was adjusted in the same manner as in Reference Example 1 to obtain a foamed sheet having a water content of 6.37% by mass.

The results of evaluation of each Example, Reference Example and Comparative Example are shown in the table.

From the above results, it is understood that according to the present invention, a foamed sheet suitable for thermoforming is provided and a method for easily producing a high-quality resin foam-molded article is provided.

Claims (5)

A resin foam sheet provided with a resin foam layer and used for forming a resin foam molded article by thermoforming,
The resin foam layer is a resin foam sheet which is made of a polyamide resin composition containing a foaming agent and which contains water as the foaming agent in a ratio of 4.23 % by mass or more and 6% by mass or less. The resin foam sheet according to claim 1, wherein the resin foam layer further contains a hydrocarbon-based foaming agent as the foaming agent. The resin foam sheet according to claim 1, wherein the resin foam layer has an open cell ratio of 5% or more and 60% or less. A method for producing a resin foam molded article, comprising producing a resin foam molded article by thermoforming a resin foam sheet having a resin foam layer,
A resin foam sheet formed of a polyamide resin composition containing a foaming agent and provided with the resin foam layer containing water as the foaming agent in a ratio of 4.23 % by mass or more and 6% by mass or less is used. And a method for producing a resin foam-molded article, wherein the thermoforming is performed. The method for producing a resin foam molded article according to claim 4, wherein in the thermoforming, the thickness of the resin foam sheet is increased by 1.1 times or more.

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