WO2024154769A1 - Foam sheet, foam molded body, and method for producing foam sheet - Google Patents

Abstract

The present invention addresses the problems of: providing a foam sheet that has a low apparent density and excellent heat deformation resistance and moldability despite containing an aliphatic polyester resin; and providing a foam molded body that is easy to produce, lightweight, cushioning, and has excellent dimensional accuracy.　In order to solve these problems, the present invention provides a foam sheet made of a resin composition containing one or more types of aliphatic polyester resin, wherein the apparent density is 30 kg/m³ to 100 kg/m³, the gel fraction measured using chloroform is 25 mass% or less, and the calorific value observed in the first temperature elevation process determined by heat flux differential scanning calorimetry at a heating rate of 10°C/min is 5.0 J/g or less.

Description

Foam sheet, foam molded product, and method for producing foam sheet CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Japanese Patent Application No. 2023-007392 and Japanese Patent Application No. 2023-007405, the disclosures of which are incorporated herein by reference.

The present invention relates to a foam sheet, a foam molded product, and a method for manufacturing a foam sheet, and more specifically to a foam sheet composed of a resin composition containing an aliphatic polyester resin, a foam molded product composed of such a foam sheet, and a method for manufacturing such a foam sheet.

Conventionally, foamed products made of a foamed resin composition have been widely used. The foamed products are lightweight yet have excellent strength, cushioning properties, and heat insulation properties. As this type of foamed product, foamed sheets and foamed molded bodies produced by processing foamed sheets into three-dimensional shapes using a mold are known. As foamed products, foamed beads and foamed molded bodies produced by processing foamed beads into three-dimensional shapes using a mold are also known. Foamed sheets and foamed beads are obtained, for example, by heating and foaming non-foamed resin sheets or non-foamed resin beads impregnated with a foaming agent such as acetone or butane. Foamed beads and foamed sheets can also be produced by an extrusion foaming method. Specifically, as for foamed sheets, extrusion foam sheets are known, which are obtained by melt-kneading a foaming resin composition and a foaming agent in an extruder, and extruding the resulting molten mixture into a sheet shape from a sheeting die (flat die or circular die) attached to the tip of the extruder and foaming the mixture.

This type of extruded foam sheet is used as a cushioning sheet in sheet form, or made into a bag for use as packaging material. This type of extruded foam sheet is also used as a base sheet for producing foam molded products by thermoforming. In addition to being used for the above-mentioned applications by itself, extruded foam sheets are also widely used in the form of laminated foam sheets with a film layer (non-foamed layer) laminated on one or both sides.

In recent years, there has been a demand for biodegradable resin products that can be decomposed in the natural environment, and resin compositions based on aliphatic polyester resins have come to be used as raw materials for foam sheets and foam beads. Foam products such as foam sheets are required to have a low apparent density from the viewpoint of cushioning properties, but aliphatic polyester resins generally have poor foamability. For this reason, when producing foam products containing aliphatic polyester resins, it has been considered to impart foaming suitability by imparting a crosslinked structure or a long-chain branched structure to the molecular structure of the aliphatic polyester resin or by increasing the molecular weight. For example, Patent Document 1 describes a foam sheet having a foam layer containing a biodegradable polyester resin with a mass average molecular weight of 150,000 to 400,000.

Japanese Patent Application Publication No. 2020-164686

Foam sheets containing aliphatic polyester resins and molded products thereof are required to have low density from the viewpoint of cushioning properties. In addition, foam sheets containing aliphatic polyester resins are required to have excellent moldability. If a foam sheet containing aliphatic polyester resins is prone to thermal deformation, the dimensional accuracy is likely to decrease when a foam molded product is manufactured by thermoforming, and it is difficult to ensure dimensional stability even when the sheet is used as is. For this reason, foam sheets are required to have heat deformation resistance. However, it is difficult to obtain a foam sheet containing aliphatic polyester resins that has low density but excellent heat deformation resistance and moldability, and the above-mentioned demands have not been met. Therefore, the present invention aims to provide a foam sheet that contains aliphatic polyester resins but has excellent heat deformation resistance and moldability and a low apparent density, and ultimately to provide a foam molded product that is easy to manufacture and has excellent light weight, cushioning properties, and dimensional accuracy.

In order to solve the above problems, the present invention provides:
A foamed sheet made of a resin composition containing one or more aliphatic polyester resins,
The apparent density is 30 kg/ ^m3 or more and 100 kg/ ^m3 or less,
The gel fraction measured using chloroform is 25% by mass or less,
The foamed sheet has a calorific value of 5.0 J/g or less observed during the first heating step as determined by heat flux differential scanning calorimetry at a heating rate of 10° C./min.

In order to solve the above problems, the present invention provides:
The present invention provides a foamed molded article, which is a thermoformed article made of the foamed sheet as described above.

Furthermore, the present invention provides
melt-kneading an aliphatic polyester resin with an organic peroxide to obtain a modified aliphatic polyester resin;
A method for producing a foamed sheet, comprising extruding a resin composition containing the modified aliphatic polyester resin into a sheet together with a foaming agent,
The foam sheet to be manufactured is
The apparent density is 30 kg/ ^m3 or more and 100 kg/ ^m3 or less,
The gel fraction measured using chloroform is 25% by mass or less,
The present invention provides a method for producing a foamed sheet in which the calorific value observed during the first heating step, as determined by heat flux differential scanning calorimetry at a heating rate of 10° C./min, is 5.0 J/g or less.

FIG. 1 is a schematic cross-sectional view showing a cross section of a foamed sheet of one embodiment cut along a plane parallel to the thickness direction. FIG. 2 is a schematic cross-sectional view showing a cross section of a laminated foam sheet cut along a plane parallel to the thickness direction. FIG. 3 is a graph showing the results (DSC curve) of differential scanning calorimetry (DSC) performed on the foamed sheet of Comparative Example 1. FIG. 4 is a graph showing the results (DSC curve) of differential scanning calorimetry (DSC) performed on the foamed sheet of Example 1. FIG. 5 is a graph showing the relationship between frequency and storage modulus (G′) in the dynamic viscoelasticity measurement of the foaming resin composition used in Example 1, with the graph being a double logarithmic axis and linearly approximated in the frequency range of 0.01 Hz to 0.1 Hz. FIG. 6 is a graph showing the results of measuring the complex viscosity (η ^* ) of the foaming resin composition used in Example 1. FIG. 7 is a graph showing the relationship between frequency and storage modulus (G′) in the dynamic viscoelasticity measurement of the foaming resin composition used in Comparative Example 1, with the graph being a double logarithmic axis and linearly approximated in the frequency range of 0.01 Hz to 0.1 Hz. FIG. 8 is a graph showing the results of measuring the complex viscosity (η ^* ) of the foaming resin composition used in Comparative Example 1.

One embodiment of the present invention will be described below. In the following, the foam sheet 1 of this embodiment will be described taking as an example a single-layer structure with only a single foam layer 10 as shown in FIG. 1. The foam sheet 1 of this embodiment may also constitute the foam layer 10 in a laminated foam sheet 2 in which a non-foamed layer 20 is laminated on one or both sides of the foam layer 10 as shown in FIG. 2. In addition, in the following, the embodiment of the present invention will be described taking as an example a case in which the foam sheet 1 is an extruded foam sheet obtained by an extrusion foaming method.

The foam sheet 1 and the laminated foam sheet 2 of this embodiment are not particularly limited in their applications, but may be used, for example, as a raw sheet when producing a foam molded body by thermoforming. The foam sheet 1 and the laminated foam sheet 2 may be used as flat sheets and may be used as materials for forming folding boxes, buffer sheets, packaging bags, seedling sheets, heat insulating materials, etc. The foam sheet 1 and the laminated foam sheet 2 of this embodiment may be used widely in applications other than these. In this embodiment, the foam sheet 1 (foam layer 10) has a low density, so that the foam sheet is not only excellent in lightness and cushioning properties, but also exhibits excellent lightness and cushioning properties for foam products such as foam molded bodies. In this embodiment, the foam sheet 1 (foam layer 10) has heat deformation resistance, so that it can exhibit heat deformation resistance for foam products and can exhibit high dimensional stability for foam products. The foam molded body may be used for packaging/buffer trays, seedling trays, food containers, heat insulating materials, etc. Packaging and cushioning trays can be used, for example, to hold agricultural products (fruits, vegetables, etc.) and industrial materials (cosmetic containers, etc.).

The foam sheet 1 of this embodiment is composed of a resin composition (aliphatic polyester resin composition) containing an aliphatic polyester resin such as polylactic acid (PLA) or polybutylene succinate (PBS). The thickness of the foam sheet 1 of this embodiment is not particularly limited, but can be, for example, 0.5 mm or more. The thickness of the foam sheet 1 may be 1 mm or more, or 1.5 mm or more. The thickness of the foam sheet 1 can be, for example, 8 mm or less. The thickness of the foam sheet 1 may be 6 mm or less, or 4 mm or less.

When the foamed sheet 1 of this embodiment constitutes the foamed layer 10 of the laminated foamed sheet 2, the thickness of the non-foamed layer 20 laminated on its surface can be, for example, 1 μm or more. The thickness of the non-foamed layer 20 can be 5 μm or more, or 10 μm or more. The thickness of the non-foamed layer 20 can be, for example, 500 μm or less. The thickness of the non-foamed layer 20 can be 400 μm or less, 300 μm or less, or 200 μm or less. When the laminated foamed sheet 2 has non-foamed layers 20 on both sides of the foamed layer 10, the first non-foamed layer 20 (first non-foamed layer 21) laminated on one side of the foamed layer 10 and the second non-foamed layer 20 (second non-foamed layer 22) laminated on the other side do not need to have the same thickness, and may have different thicknesses.

The thickness of the foam sheet 1 can be measured using a constant pressure thickness gauge, for example, the "Peacock Digital Linear Gauge PDN25" manufactured by Ozaki Manufacturing Co., Ltd. Specifically, the thickness of the foam sheet 1 can be obtained by measuring the thickness when a load of 100 g is applied to the foam sheet using a circular jig with a diameter of 35.7 mm using a constant pressure thickness gauge. The thickness of the foam sheet 1 can be obtained, for example, by measuring 10 or more points every 5 cm in the width direction (TD) except for 20 mm at both ends of the width direction (TD) perpendicular to the extrusion direction (MD) and taking the arithmetic mean of the measured values. In addition, if the width of the foam sheet 1 is narrow and it is not possible to secure 10 measurement points, the arithmetic mean of all the measured values can be obtained as the thickness of the foam sheet 1 after securing as many measurement points as possible.

The thickness of the non-foamed layer 20 in the laminated foam sheet 2 can be found by taking a microscopic photograph of the cross section of the non-foamed layer 20 (a cross section in a plane perpendicular to the planar direction of the foam sheet 1), measuring the thickness of the non-foamed layer 20 at multiple randomly selected points (e.g., 10 points) on the photograph, and calculating the arithmetic mean of the measured values. The thickness of the foamed layer 10 can be found by subtracting the thickness of the non-foamed layer 20 from the thickness of the foam sheet 1.

The foamed sheet 1 (foamed layer 10) of this embodiment has an apparent density of 30 kg/m ³ or more. In order to ensure high strength in a foamed product, it is advantageous for the apparent density to be a certain level or higher. The apparent density may be 40 kg/m ³ or more, or 50 kg/m ³ or more. On the other hand, in order to ensure light weight and cushioning properties, it is desirable for the apparent density to be a certain level or lower. The foamed sheet 1 (foamed layer 10) of this embodiment has an apparent density of 100 kg/m ³ or less. The apparent density may be 90 kg/m ³ or less, or 80 kg/m ³ or less.

The apparent density (kg/ ^m3 ) can be determined by dividing the mass (basis weight: g/ ^m2 ) of the foam sheet 1 (foam layer 10) per unit area by the thickness (mm) of the foam sheet 1 (foam layer 10). The basis weight can be determined by arithmetically averaging values measured for a plurality of samples cut out from the foam sheet. The basis weight of the foam sheet can be determined by cutting out six pieces of 10 cm x 10 cm at equal intervals in the width direction (TD) of the foam sheet 1, excluding 20 mm at both ends in the width direction (TD), and measuring the mass (g) of each piece. The basis weight can be determined by converting the average value of the mass (g) of each piece into a mass per ^m2 .
Apparent density (kg/m ³ ) = basis weight (g/m ² ) ÷ thickness (mm)
The apparent density of the foamed layer 10 in the laminated foamed sheet 2 can be calculated by measuring the overall apparent density and the density of the non-foamed layer 20. The density of the non-foamed layer 20 can be determined by an underwater displacement method (Archimedes method) or the like.

The foamed sheet 1 (foamed layer 10) of this embodiment has a gel fraction of 25% by mass or less, measured using chloroform. The resin composition constituting the foamed sheet 1 (foamed layer 10) has a low gel fraction, which allows it to exhibit good foamability and thermoformability during extrusion foaming, making it easy to produce a foamed sheet 1 (foamed layer 10) or a foamed molded article having the above-mentioned preferred apparent density. The gel fraction may be 20% by mass or less, or 15% by mass or less. The gel fraction may be, for example, 0.1% by mass or more, 0.2% by mass or more, or 0.3% by mass or more.

The gel fraction can be determined by taking a plurality of samples (e.g., 5 samples) each weighing about 0.5 g from the foamed sheet 1 (foamed layer 10) and arithmetically averaging the gel fractions of the respective samples. The gel fraction of a sample can be measured, for example, by the following procedure.
Accurately weigh the initial mass (Mo) of the sample.
Prepare an 80 mesh wire screen (wire diameter φ0.12 mm) for filtering the solution in which the sample is dissolved, and accurately weigh the initial mass (Ms) of this wire screen.
Put the sample, 50 cc of chloroform, and a stir bar into a beaker (volume: 100 cc), cover with aluminum foil, set on the stirrer and stir for 2 hours to dissolve the sample at room temperature.
After 2 hours, remove the lid, filter the soluble matter in the beaker through the wire mesh, and collect the insoluble resin matter on the wire mesh.
· After filtering the insoluble resin, let the wire mesh and other parts dry naturally in a draft chamber to evaporate the chloroform.
After filtering the insoluble resin, dry the wire mesh at 120°C for 2 hours in a thermostatic dryer, then leave to cool in a desiccator.
After cooling, the total mass (Mx) of the insoluble resin and the wire mesh is measured.
- Calculate the gel fraction (mass%) using the following formula.
Gel fraction (mass%)=(Mx−Ms)/Mo×100

The foamed sheet 1 (foamed layer 10) of this embodiment has a calorific value of 5.0 J/g or less observed during the first heating process as determined by heat flux differential scanning calorimetry (heat flux DSC) at a heating rate of 10°C/min. The calorific value observed by heat flux differential scanning calorimetry is the amount of heat generated with the crystallization of the aliphatic polyester resin, and is usually observed as an exothermic peak having an apex between 65°C and 130°C on the DSC curve.

　It is known that polyester resins, among crystalline resins, have a relatively slow crystallization rate compared to polyolefin resins and the like. If the foamed sheet 1 (foamed layer 10) is not sufficiently crystallized, it may undergo shape changes accompanied by warping and undulations under high-temperature storage conditions, resulting in loss of dimensional accuracy. Furthermore, if the foamed sheet 1 (foamed layer 10) contains an aliphatic polyester resin in a state that allows it to crystallize, crystallization will occur at the above-mentioned temperatures, which will result in a similar shape change in the foamed sheet 1 and loss of dimensional accuracy. Furthermore, during thermoforming, the foamed sheet is heated and supplied to a mold while both ends of the sheet are held by a jig called a clamp, but in such a case, if the foamed sheet deforms, problems such as it coming off the clamps may occur.

When manufacturing a low-density foam sheet by the extrusion foaming method, the foam layer has a particularly thin bubble membrane, so cooling air is applied to the foam sheet immediately after extrusion to prevent bubbles from breaking during foaming. This can result in a foam sheet with a large amount of heat generation (containing many components that have not yet crystallized), especially in low-density foam sheets. Therefore, as described below, in this embodiment, it is desirable to adjust the air-cooling temperature and air volume to ease the cooling conditions of the foam sheet after extrusion and sufficiently crystallize the aliphatic polyester resin.

In polylactic acid, it is known that α crystals are formed at temperatures of 120°C or higher during the cooling process from the molten state, and α' crystals are formed at temperatures below 90°C, and it is known that α crystals have better heat resistance than α' crystals. It is also known that β crystals of polybutylene succinate change to α crystals when heated. Therefore, the foamed sheet 1 (foamed layer 10) can be made less susceptible to thermal deformation by slowing down the cooling during the manufacturing process to make the crystals contained therein α crystals. The presence of α crystals can be confirmed by wide-angle X-ray diffraction (WAXD). And, it can be confirmed that the crystals contained in the foamed sheet 1 (foamed layer) are mostly α crystals by heating the foamed sheet under temperature conditions at which α crystals are formed, and comparing the intensities of the peaks derived from α crystals by performing wide-angle X-ray diffraction before and after heating. For example, when the foam sheet 1 is made of a resin composition containing polybutylene succinate, a peak derived from the (020) plane of α crystal appears near 2θ of 13° in wide-angle X-ray diffraction, and the foam sheet 1 is heated under certain conditions (e.g., 100°C x 20 minutes). If the change in peak intensity near 13° before and after heating (intensity after heating/intensity before heating) is, for example, about 1.2 times or less, it can be determined that the crystals of the foam sheet 1 are sufficiently α-crystallized. The foam sheet 1 is preferably produced so that the change in strength is 1.1 times or less, and preferably produced so that no change in strength is observed (intensity after heating/intensity before heating ≒ 1.0).

The amount of heat generated during the first heating process as determined by heat flux differential scanning calorimetry (heat flux DSC) may be 4 J/g or less, 3 J/g or less, 2 J/g or less, or 1 J/g or less.

In the foamed sheet 1 (foamed layer 10) of this embodiment, a peak showing endothermic heat is observed during the heating process in the heat flux differential scanning calorimetry. The difference between the absolute value of the endothermic heat at the endothermic peak observed during the heating process and the heat generation amount is preferably 30 J/g or more and 90 J/g or less. The endothermic heat observed in the heat flux differential scanning calorimetry is the amount of heat generated with the melting of the crystals of the aliphatic polyester resin, and is observed as an endothermic peak having an apex at a temperature several tens of degrees higher than the heat generation peak due to crystallization in the DSC curve.

The foamed sheet 1 (foamed layer 10) contains an appropriate amount of aliphatic polyester resin crystals, which can provide a foamed product with a good balance of heat deformation resistance and moldability. The difference between the absolute value of the amount of heat absorption and the amount of heat generation may be 40 J/g or more, or 50 J/g or more. The difference between the absolute value of the amount of heat absorption and the amount of heat generation may be 80 J/g or less, or 70 J/g or less.

The endothermic amount and the exothermic amount can be measured, for example, as follows.
The endothermic amount (a) (amount of heat of fusion) and the exothermic amount (b) (amount of heat of crystallization) can be measured by the methods described in JIS K7122: 1987 and JIS K7122: 2012. However, the sampling method and temperature conditions are as follows.
A sample cut out from the foam sheet 1 (foam layer 10) is packed into the bottom of an aluminum measurement container in an amount of 5.5±0.5 mg so as to leave no gaps, and then the aluminum lid is placed.
Differential scanning calorimetry is carried out using a differential scanning calorimeter (for example, "DSC7000X, AS-3" manufactured by Hitachi High-Tech Science Corporation).
In differential scanning calorimetry, a sample is heated and cooled in the following steps 1 and 2 under a nitrogen gas flow rate of 20 mL/min to obtain a DSC curve.
(Step 1) Decrease temperature from 30° C. to −40° C. at a rate of 10° C./min.
(Step 2) Heat the temperature from −40° C. to 200° C. at a rate of 10° C./min (first heating process).
In this case, alumina is used as the reference material.
The endothermic amount (a) and the calorific value (b) can be calculated using analysis software attached to the device. Specifically, as shown in Fig. 3, the endothermic amount (a) is calculated from the area enclosed by the line connecting the point where the DSC curve leaves the low-temperature baseline and the point where the DSC curve returns to the high-temperature baseline, and the DSC curve. The calorific value (b) is calculated from the area enclosed by the line connecting the point where the DSC curve leaves the low-temperature baseline and the point where the DSC curve returns to the high-temperature baseline, and the DSC curve.
4, if no crystallization (exothermic) peak is observed during the first heating process, the amount of heat generated (b) is set to 0 J/g. The same applies to the amount of heat endotherm (a). However, if multiple melting (endothermic) peaks or crystallization (exothermic) peaks are observed, the total amount of heat generated at each peak is set to the amount of heat endotherm (a) and the amount of heat generated (b).

If the foam sheet 1 (foam layer 10) of this embodiment has excessive secondary foaming properties, it may exhibit thermal deformation during thermoforming for reasons other than crystallization, which may impair moldability. The secondary foaming ratio of the foam sheet 1 (foam layer 10) of this embodiment is preferably 0.9 times or more and 1.5 times or less. The secondary foaming ratio may be 1.4 times or less, 1.3 times or less, or 1.2 times or less.

The secondary expansion ratio of the foam sheet 1 (foam layer 10) is preferably as described above when the following measurements are performed on the foam sheet 1 or the laminated foam sheet 2 immediately after extrusion (for example, within 24 hours).

The secondary expansion ratio of the foamed sheet 1 (foamed layer 10) can be determined as follows.
- Cut out three test pieces measuring 100 mm in length and 100 mm in width from the foam sheet.
The test pieces are conditioned for 168 hours under a standard atmosphere of grade 2 (temperature 23±2°C, relative humidity 50±5%) according to JIS K 7100:1999, and the thickness (T1) (mm) of the center of each test piece before heating is measured. The thickness (T1) of the center of each test piece before heating can be measured in the same manner as in the case of determining the thickness of the foamed sheet 1.
The test pieces are placed on a platform in an oven set to 70° C. without humidity control, heated for 150 seconds, removed from the oven and cooled at room temperature for 30 minutes, and the thickness (T2) (mm) of the central part of each test piece after heating is measured. The thickness (T2) of the central part of each test piece after heating can be measured in the same manner as in the case of determining the thickness of the foamed sheet 1.
The secondary expansion ratio (times) of each of the three test pieces is calculated using the following formula, and the arithmetic mean value is regarded as the secondary expansion ratio (times) of the foamed sheet.
Secondary expansion ratio (times) = T2/T1
When determining the secondary expansion ratio of the foam layer 10 in the state of the laminated foam sheet 2, the thickness of the laminated foam sheet is measured before and after heating in the same manner as described above, and the thickness of the non-foamed layer 20 is subtracted from each of them to determine the thickness (T1) of the foam layer before heating and the thickness (T2) of the foam layer after heating, and the secondary expansion ratio can be calculated.

The foamed sheet 1 (foamed layer 10) of this embodiment preferably has a heat loss of 0.1% by mass or more and 1.5% by mass or less. The heat loss may be 1.3% by mass or less, 1.1% by mass or less, 0.9% by mass or less, or 0.7% by mass or less.

The heat loss of the foamed sheet 1 (foamed layer 10) is preferably the value described above when the foamed sheet 1 or the laminated foamed sheet 2 is subjected to the following measurements immediately after extrusion (for example, within 24 hours).

The heat loss of the foamed sheet 1 (foamed layer 10) can be determined as follows.
Three samples are cut out from the foam sheet 1 (foam layer 10) so that each sample has a mass of about 10 g.
Each sample is conditioned for 168 hours under a standard atmosphere of grade 2 (temperature 23±2° C., relative humidity 50±5%), code “23/50” of JIS K 7100:1999.
After conditioning, each sample is wrapped in aluminum foil, and the mass (W1) (g) of each sample before heating is measured.
- Each sample is heated stationary on a platform in an unhumidified oven set at 180°C for 30 minutes.
After heating, each sample is removed from the oven and allowed to cool for 30 minutes under a standard atmosphere of grade 2 (temperature 23±2°C, relative humidity 50±5%), as specified in JIS K 7100:1999, and the mass (W2) (g) of each sample after heating is measured.
The heat loss (mass%) of each of the three samples is calculated using the following formula, and the arithmetic mean value is regarded as the heat loss (mass%) of the foamed sheet.
Heating loss (mass%) = (W1-W2)/W1×100
In cases where the sample adheres to the aluminum foil after heating, making accurate measurement difficult, the mass of the aluminum foil may be precisely weighed in advance, and the mass of each aluminum foil before and after heating may be measured. The mass of each sample before heating (W1) and after heating (W2) may be calculated by subtracting the mass of the aluminum foil from each measured value.

In the foam sheet 1 of this embodiment, it is advantageous for the foam layer 10 to have an open cell rate of a certain level or less in order to exhibit excellent moldability. In the foam sheet 1 (foam layer 10) of this embodiment, it is preferable for the open cell rate to be 60% or less. The open cell rate may be 50% or less, or may be 40% or less. In the foam sheet 1 of this embodiment, it is advantageous for the foam layer 10 to have an open cell rate of a certain level or more in order to exhibit excellent cushioning properties and excellent dimensional stability by suppressing excessive secondary foaming. The open cell rate may be 10% or more, 15% or more, or 20% or more.

The open cell ratio of the foamed sheet 1 (foamed layer 10) can be determined as follows.
- Two or more sheet samples measuring 25 mm in length and 25 mm in width are cut out from the foam sheet, and the cut samples are stacked together so that there are no gaps, to create five test pieces with a thickness of approximately 25 mm.
The length and width of the obtained test piece are measured using a vernier caliper (e.g., "Digimatic Caliper" manufactured by Mitutoyo Corporation). The thickness can be measured in the same manner as in measuring the thickness of the foamed sheet 1 using a constant pressure thickness measuring device (e.g., "Peacock Digital Linear Gauge PDN25" manufactured by Ozaki Seisakusho Co., Ltd.). The apparent volume (V1: ^cm3 ) is calculated from the measured dimensions.
Using an air comparison type specific gravity meter (for example, "1000 type" manufactured by Tokyo Science Co., Ltd.), the volume of the test piece (V2: cm ³ ) is determined by the 1-1/2-1 atmospheric pressure method.
The open cell rate (%) is calculated using the following formula, and the arithmetic mean value of the open cell rates of the five test pieces is obtained. The test pieces are conditioned in advance for at least 24 hours in a standard atmosphere of class 2, JIS K 7100:1999, "23/50" (temperature 23±2°C, relative humidity 50±5%). Measurements are also carried out in the same standard atmosphere. The air comparison specific gravity meter is used after correction using standard balls (large 28.96 ^cm3 , small 8.58 ^cm3 ).
Open cell ratio (%) = (V1 - V2) / V1 x 100
(V1: apparent volume, V2: volume measured by air comparison type specific gravity meter)

In order for the foamed sheet 1 of this embodiment to exhibit the above-mentioned properties, it is preferable that the resin composition (foamable resin composition) used to form the foamed layer 10 before extrusion foaming has a predetermined melting property. The resin composition that constitutes the foamed layer 10 is described below.

The resin composition in this embodiment may contain one or more aliphatic polyester resins. It is preferable that the foaming resin composition is prepared so that it exhibits good foaming properties when producing a foamed product such as a foam sheet, and that the characteristic values of the resulting foamed product are within a certain range in order to exhibit characteristics such as cushioning.

It is preferable that the foaming resin composition exhibits a suitable fluidity when heated in order to exhibit good foamability. Specifically, in this embodiment, the melt mass flow rate (MFR) of the foaming resin composition at 190°C before being used to form the foam sheet 1 (foam layer 10) can be, for example, 0.1 g/10 min or more and 5.0 g/10 min or less. The MFR of the resin composition may be, for example, 0.2 g/10 min or more, 0.3 g/10 min or more, or 0.4 g/10 min or more. The MFR of the resin composition may be, for example, 0.5 g/10 min or more, 0.6 g/10 min or more, 0.8 g/10 min or more, or 0.9 g/10 min or more. The MFR of the resin composition may be, for example, 4.5 g/10 min or less, 4.0 g/10 min or less, 3.5 g/10 min or less, or 3.0 g/10 min or less.

In this embodiment, it is preferable that the resin composition in the state in which the foamed sheet 1 (foamed layer 10) is formed (hereinafter also referred to as the "foamed resin composition") also has the above-mentioned MFR value.

The melt mass-flow rate (MFR) of the foaming resin composition before forming the foamed sheet 1 (foamed layer 10) or the foamed resin composition in the state of constituting the foamed sheet 1 (foamed layer 10) can be measured as follows.
The melt mass flow rate (MFR) of the resin composition can be measured using a commercially available measuring device (for example, "Melt Flow Index Tester (Automatic) 120-SAS" manufactured by Yasuda Seiki Seisakusho Co., Ltd.).
The MFR can be measured under the following conditions in accordance with JIS K 7210:1999. The sample to be measured is vacuum dried at 70°C for 5 hours or more, and after drying, is vacuum-packed in a nylon polybag for vacuum packing and stored in a desiccator until immediately before the measurement.

(Measurement condition)
Sample: 3 to 8 g
Preheat 1: 200 seconds Preheat 2: 30 seconds Test temperature: 190°C
Test load: 21.18N
Piston travel distance (interval): 25mm
Number of tests: 3 times. The arithmetic mean of the measured values obtained in each test is taken as the MFR (g/10 min).

The foaming resin composition preferably exhibits an appropriate melt tension when heated in order to exhibit good foamability. The melt tension at 190°C of the resin composition used in this embodiment can be, for example, 30 cN or more and 100 cN or less. The melt tension may be 35 cN or more, or 40 cN or more. The melt tension may be 95 cN or less, or 90 cN or less. The melt tension may be 80 cN or less, 70 cN or less, or 60 cN or less. The melt tension may be 55 cN or less, or 50 cN or less. It is preferable that the foamed resin composition also has the above-mentioned melt tension.

The melt tension of the resin composition (foamable resin composition, foamed resin composition) can be measured as follows.
The melt tension can be measured using a commercially available rheometer and extensional viscosity measuring device (for example, a capillary rheometer "Capillograph 1D" (special specification for heating furnace) manufactured by Toyo Seiki Seisakusho Co., Ltd., and "Rheotens 71.97" manufactured by Goettfert). The melt tension can be measured under the following conditions.
The samples should be vacuum-dried at 70°C for at least 5 hours in advance, and after drying, they should be vacuum-packed in nylon plastic bags for vacuum packing and stored in a desiccator until immediately before measurement.
- Install the "Rheotens 71.97" so that the distance from the die exit of the "Capilograph 1D" to the measurement part is 80 mm (if it is not possible to bring the Rheotens within 80 mm due to interference, take measures to avoid interference and set the Rheotens in the specified location).
First, the sample is filled into a barrel heated to a test temperature of 190° C., and then preheated for 5 minutes. The measurement time, including the preheating time after filling the barrel with the sample, should not exceed 10 minutes.
Next, a piston is inserted from the top of the barrel to extrude the molten resin into a string shape. At this time, the piston descending speed (20 mm/min) is kept constant, and the extruded string shape is passed through a Rheotens wheel and taken up. After that, the take-up speed is gradually increased to measure the melt tension of the sample.

The melt tension of the sample is determined as the average of the maximum and minimum tension values immediately before the point at which the string breaks. If there is only one maximum point on the tension chart, that maximum value is used as the melt tension. If the string becomes thinner and the winding becomes idling, this point is regarded as the break point, and the average of the maximum and minimum tension values immediately before that point is used as the melt tension of the sample.

(Measurement conditions for Capillograph 1D)
Die: diameter 2.095 mm, length 8 mm, inlet angle 90 degrees (conical)
Barrel diameter: 9.55mm
Piston speed: 20 mm/min
Measurement temperature: 190°C

(Conditions for measuring Rheotensibility)
Wheel spacing: Top 0.6mm, bottom 1.0mm
Acceleration: 10mm/s ²
Pulling speed: Initial speed 6.92 mm/s

The foamed sheet 1 (foamed layer 10) in this embodiment has a gel fraction of 25% by mass or less, measured using chloroform as described above. When the foamed sheet 1 is manufactured using the resin composition, a shear force is applied to the resin composition. Therefore, even if the foamable resin composition contains more than 25% by mass of gel before being used to manufacture the foamed sheet 1, the gel fraction in the foamed sheet 1 (foamed layer 10) (gel fraction of the foamed resin composition) can be 25% by mass or less. In order to make the gel fraction of the foamed resin composition 25% by mass or less, it is preferable that the gel fraction of the foamable resin composition be a certain amount or less.

The gel fraction of the foamable resin composition in this embodiment, measured using chloroform, can be, for example, 40% by mass or less in order to obtain a foamed sheet with good elongation and low basis weight when produced as an extruded sheet. The gel fraction of the foamable resin composition may be, for example, 30% by mass or less, 20% by mass or less, or 15% by mass or less. The gel fraction of the foamable resin composition can be measured in the same manner as the gel fraction in the foamed sheet 1 (foam layer 10) (gel fraction of the foamed resin composition).

The foaming resin composition can be prepared so that the complex viscosity (η ^* ) observed at a frequency of 0.01 Hz in dynamic viscoelasticity measurement is, for example, 10,000 Pa·s or more and 150,000 Pa·s or less in order to obtain a foamed sheet with good foamability. The foaming resin composition can also be prepared so that the slope of the storage modulus (G') in the frequency range of 0.01 Hz to 0.1 Hz in dynamic viscoelasticity measurement is 0.35 to 1.5.

The complex viscosity (η ^* ) may be 15,000 Pa·s or more, 20,000 Pa·s or more, or 25,000 Pa·s or more. The complex viscosity (η ^* ) may be 120,000 Pa·s or less, 100,000 Pa·s or less, 80,000 Pa·s or less, or 60,000 Pa·s or less.

The foaming resin composition exhibits good foaming properties and, in order to obtain a low-density foamed sheet, the slope of the storage modulus (G') at a frequency of 0.01 Hz to 0.1 Hz in dynamic viscoelasticity measurement may be 0.37 or more, 0.40 or more, or 0.45 or more. The slope of the storage modulus (G') may be 1.4 or less, 1.3 or less, 1.2 or less, or 1.1 or less. In this embodiment, the "slope of the storage modulus (G')" refers to the slope of a straight line obtained by linearly approximating the graph in the frequency range of 0.01 Hz to 0.1 Hz when dynamic viscoelasticity measurement is performed while changing the frequency, the storage modulus (G') is measured at each frequency, and the measurement results of the storage modulus (G') are represented on a double logarithmic axis graph with the horizontal axis being frequency (Hz) and the vertical axis being storage modulus (G') (Pa).

The slope of the storage modulus (G') and the complex viscosity (η ^* ) of the foamable resin composition can be determined by dynamic viscoelasticity measurement as described above. More specifically, the dynamic viscoelasticity can be measured using a commercially available viscoelasticity measuring device (e.g., manufactured by Anton Paar, product name "PHYSICA MCR301") and a temperature control system (e.g., manufactured by Anton Paar, product name "CTD450") according to the following procedure.
The samples should be vacuum-dried at 70°C for at least 5 hours in advance, and after drying, they should be vacuum-packed in nylon plastic bags for vacuum packing and stored in a desiccator until immediately before measurement.
The sample is placed on a 50 mm diameter parallel plate (lower side) of a viscoelasticity measuring device heated to 190° C., and heated for 5 minutes in a nitrogen atmosphere to melt the sample.
The molten sample is crushed between parallel plates (upper side) with a diameter of 25 mm until the distance between the parallel plates is 2 mm, any resin that protrudes from the plates is removed, and after the sample has reached the measurement temperature ±1°C and is heated for 5 minutes, dynamic viscoelasticity measurements are performed.
・Measurement conditions are as follows.
(Measurement condition)
Distortion: 5%
Frequency: 0.01 to 100 (Hz) (measurement starts from low frequency (0.01 Hz))
Number of measurement points: 21 (5 points/digit)
Measurement temperature: 190°C
Atmosphere gas: Nitrogen

The slope of the storage modulus (G') is obtained from the results of dynamic viscoelasticity measurement, and is obtained from the change curve of the storage modulus (G') with respect to frequency. Specifically, as shown in FIG. 5 and FIG. 7, for each measurement point of the storage modulus (G'), the value of the storage modulus (G') (y-axis) with respect to the frequency (x-axis) is plotted as a graph on both logarithmic axes, and a straight line is drawn with a power approximation formula (y=bx ^a ) for the measurement points at frequencies of 0.01 Hz to 0.1 Hz. Here, a is the slope of the straight line, and b is a constant. That is, the value of the exponent (a) of the power approximation formula is taken as the slope of the storage modulus (G'). In addition, the value of the complex viscosity (η ^* ) can be obtained by reading the value at a frequency of 0.01 Hz from the results of dynamic viscoelasticity measurement.

The various characteristic values of the foamable resin composition as described above can be adjusted by selecting the type of aliphatic polyester resin used or by modifying the aliphatic polyester resin to impart the desired molecular structure. In particular, the melting characteristics and gel fraction of the foamable resin composition can be easily adjusted by using a modified aliphatic polyester resin (hereinafter also referred to as "modified aliphatic polyester resin").

The foamable resin composition may be composed of only one or more aliphatic polyester-based resins, or may contain resins other than aliphatic polyester-based resins. The content of resins other than aliphatic polyester-based resins is preferably less than 20% by mass of all resins contained in the foamable resin composition. In other words, the proportion of aliphatic polyester-based resins in all resins contained in the foamable resin composition is preferably 80% by mass or more. The proportion of aliphatic polyester-based resins may be 90% by mass or more, or may be 95% by mass or more.

Resins other than aliphatic polyester resins, such as tackifiers and polymeric antistatic agents, can be introduced into the foaming resin composition.

The aliphatic polyester resin contained in the foaming resin composition may be a hydroxy acid polycondensate, a ring-opening polymer of lactone, or a polycondensate of a polyhydric alcohol component and a polycarboxylic acid component. Examples of hydroxy acid polycondensates include polylactic acid and polycondensates of hydroxybutyric acid. Examples of ring-opening polymers of lactone include polycaprolactone and polypropiolactone. Examples of polycondensates of a polyhydric alcohol component and a polycarboxylic acid component include polyethylene succinate, polybutylene succinate, polybutylene adipate, polybutylene succinate adipate, and polybutylene adipate terephthalate. Of these, the aliphatic polyester resin contained in the foaming resin composition is preferably either polybutylene succinate (PBS) or polybutylene succinate adipate (PBSA). The foaming resin composition preferably contains modified polybutylene succinate (hereinafter also referred to as "modified polybutylene succinate") or modified polybutylene succinate adipate (hereinafter also referred to as "modified polybutylene succinate adipate").

The unmodified aliphatic polyester resin (hereinafter also referred to as "unmodified aliphatic polyester resin") that is the starting material for modified aliphatic polyester resin preferably has diols and dicarbons, which are its constituent units, that are derived from plants. In other words, it is preferable that at least a portion of unmodified aliphatic polyester resins such as polylactic acid (PLA), polybutylene succinate (PBS), and polybutylene succinate adipate (PBSA) is derived from plants.

The unmodified aliphatic polyester resin preferably has a biomass degree of 20% or more as measured by ASTM D 6866 (2004). The biomass degree of the unmodified aliphatic polyester resin may be 30% or more, or 40% or more.

The unmodified aliphatic polyester resin preferably has a melt mass flow rate (MFR) at 190°C of 8 g/10 min or more and 40 g/10 min or less. The MFR of the unmodified aliphatic polyester resin may be 10 g/10 min or more, 12 g/10 min or more, 15 g/10 min or more, or 20 g/10 min or more. The MFR of the unmodified aliphatic polyester resin may be 35 g/10 min or less, or 30 g/10 min or less.

The MFR of unmodified aliphatic polyester resins can be measured using the same method as the MFR of resin compositions.

The number average molecular weight (Mn) of the unmodified aliphatic polyester resin is preferably 20,000 or more and 60,000 or less. The number average molecular weight (Mn) of the unmodified aliphatic polyester resin may be 25,000 or more, 30,000 or more, or 35,000 or more. The number average molecular weight (Mn) of the unmodified aliphatic polyester resin may be 55,000 or less, 50,000 or less, 45,000 or less, or 40,000 or less.

The unmodified aliphatic polyester resin preferably has a mass average molecular weight (Mw) of 100,000 or more and 200,000 or less. The mass average molecular weight (Mw) of the unmodified aliphatic polyester resin may be 110,000 or more, or 120,000 or more. The mass average molecular weight (Mw) of the unmodified aliphatic polyester resin may be 190,000 or less, 180,000 or less, 170,000 or less, or 160,000 or less.

The unmodified aliphatic polyester resin preferably has a dispersity (Mw/Mn), which is the ratio of the mass average molecular weight (Mw) to the number average molecular weight (Mn), of 2.2 or more and 5.0 or less. The dispersity (Mw/Mn) may be 2.5 or more, 2.8 or more, 3.1 or more, or 3.4 or more. The dispersity (Mw/Mn) may be 4.8 or less, 4.5 or less, 4.2 or less, or 3.9 or less.

The Z-average molecular weight (Mz) of the unmodified aliphatic polyester resin is preferably 100,000 or more and 500,000 or less. The Z-average molecular weight (Mz) of the unmodified aliphatic polyester resin may be 140,000 or more, 180,000 or more, or 220,000 or more. The Z-average molecular weight (Mz) of the unmodified aliphatic polyester resin may be 450,000 or less, 400,000 or less, 350,000 or less, or 300,000 or less. In particular, the Z-average weight molecular weight (Mz) is preferably 100,000 or more and 300,000 or less.

The average molecular weight of the unmodified aliphatic polyester resin can be measured by gel permeation chromatography (GPC) using the following method in terms of standard polystyrene (PS) average molecular weight.
Dissolve 15 mg of unmodified aliphatic polyester resin in 6 mL of chloroform (permeation time: 6.0±1.0 hours (complete dissolution)).
The chloroform solution is filtered through a filter (non-aqueous 0.45 μm syringe filter, manufactured by Shimadzu GLC) to prepare a measurement sample.
- Analyze standard polystyrene under the following measurement conditions and create a standard polystyrene calibration curve.
The unmodified aliphatic polyester resin is measured under the following conditions, and the average molecular weight is calculated using a previously prepared standard polystyrene calibration curve.

(Measurement condition)
Measurement device: Gel permeation chromatograph (with built-in RI detector and UV detector) (HLC-8320GPC EcoSEC, manufactured by Tosoh Corporation)
Column configuration: Sample side guard column TSKgel guardcolumn HXL-H (Tosoh Corporation; 6.0 mm I.D. x 4 cm) x 1 Measurement column TSKgel GMHXL (7.8 mm I.D. x 30 cm) x 2 in series Reference side resistance tube (inner diameter 0.1 mm x 2 m) x 2 in series Column temperature: 40°C
Mobile phase: chloroform Mobile phase flow rate: sample side 1.0 mL/min, reference side 0.5 mL/min Detector: differential refractive index (RI) detector Standard sample: standard polystyrene for calibration curve Sample injection volume: 50 μL
Measurement time: 26 minutes Sampling pitch: 500 ms

The standard polystyrene samples used for the calibration curve are STANDARD SM-105 (Showa Denko K.K.) and STANDARD SH-75 (Showa Denko K.K.), polystyrene (PS) with mass average molecular weights (Mw) of 5,620,000, 3,120,000, 1,250,000, 442,000, 151,000, 53,500, 17,000, 7,660, 2,900, and 1,320.

A calibration curve is prepared by the following method.
The standard polystyrene for the calibration curve is divided into group A (PS with Mw=5,620,000, PS with Mw=1,250,000, PS with Mw=151,000, PS with Mw=17,000, PS with Mw=2,900) and group B (PS with Mw=3,120,000, PS with Mw=442,000, PS with Mw=53,500, PS with Mw=7,660, PS with Mw=1,320).
From group A, weigh out 2 mg of PS with Mw = 5,620,000, 3 mg of PS with Mw = 1,250,000, 4 mg of PS with Mw = 151,000, 4 mg of PS with Mw = 17,000, and 4 mg of PS with Mw = 2,900, and dissolve the entire amount in 30 mL of chloroform.
From group B, weigh out 3 mg of PS with Mw = 3,120,000, 4 mg of PS with Mw = 442,000, 4 mg of PS with Mw = 53,500, 4 mg of PS with Mw = 7,660, and 4 mg of PS with Mw = 1,320, and dissolve the entire amount in 30 mL of chloroform.
The standard polystyrene calibration curve is obtained by injecting 50 μL of the solutions of groups A and B and creating a calibration curve (cubic equation) from the retention times obtained after measurement.

One method for modifying such unmodified polyester resins is to crosslink (partially crosslink) them with an organic peroxide.

Examples of the organic peroxides include peroxyesters, hydroperoxides, dialkyl peroxides, diacyl peroxides, peroxydicarbonates, peroxyketals, and ketone peroxides.

Examples of the peroxy ester include t-butylperoxy 2-ethylhexyl carbonate, t-hexylperoxy isopropyl monocarbonate, t-hexylperoxy benzoate, t-butylperoxy benzoate, t-butylperoxy laurate, t-butylperoxy-3,5,5-trimethylhexanoate, t-butylperoxy acetate, 2,5-dimethyl 2,5-di(benzoylperoxy)hexane, and t-butylperoxy isopropyl monocarbonate.

Examples of the hydroperoxide include permethane hydroperoxide, diisopropylbenzene hydroperoxide, cumene hydroperoxide, and t-butyl hydroperoxide.

Examples of the dialkyl peroxide include dicumyl peroxide, di-t-butyl peroxide, and 2,5-dimethyl-2,5-di(t-butylperoxy)-hexyne-3.

Examples of the diacyl peroxide include dibenzoyl peroxide, di(4-methylbenzoyl) peroxide, and di(3-methylbenzoyl) peroxide.

Examples of the peroxydicarbonate include di(2-ethylhexyl) peroxydicarbonate and diisopropyl peroxydicarbonate.

Examples of the peroxyketal include 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane, 1,1-di-t-butylperoxycyclohexane, 2,2-di(t-butylperoxy)-butane, n-butyl 4,4-di-(t-butylperoxy)valerate, and 2,2-bis(4,4-di-t-butylperoxycyclohexyl)propane.

Examples of the ketone peroxide include methyl ethyl ketone peroxide and acetylacetone peroxide.

In this embodiment, it is preferable to use an organic peroxide that is mildly reactive, and it is preferable to use an organic peroxide that has a high half-life temperature. The one-minute half-life temperature of the organic peroxide is preferably 150°C or higher and 200°C or lower.

The one-minute half-life temperature can be measured using a 0.1 mol/L solution in benzene as a solvent. Since the decomposition reaction in a dilute solution of an organic peroxide can be regarded as a first-order reaction, the decomposition reaction can be expressed as the following formulas (1) and (2) where C is the initial concentration of the organic peroxide, ΔC is the amount of decomposition, k is the decomposition rate constant, and t is the time.
Formula (1): dx/dt=k(C-ΔC)
Formula (2): ln C/(C-ΔC)=kt
If the half-life time is t _1/2 , then (C-ΔC) is C/2, and the above formula becomes as follows.
Formula (3): k・t _1/2 = ln2
Therefore, if an organic peroxide is thermally decomposed at a certain temperature (T) and the relationship between time (t) and "ln C/(C-ΔC)" is plotted on a graph, the decomposition rate constant (k) can be found from the slope of the approximate line, and from this the half-life time (t _1/2 ) at that temperature can be calculated.
The decomposition rate constant k is expressed by the Arrhenius equation as follows:
Formula (4): k=Aexp(-ΔE/RT)
Formula (5): lnk=lnA-ΔE/RT
(A: frequency factor, ΔE: activation energy, R: gas constant, T: absolute temperature)
The decomposition rate constant k at each temperature is calculated at several points, and the activation energy ΔE is determined from the slope of the straight line obtained by plotting "lnk" versus "1/T". The one-minute half-life temperature can be determined from the approximate straight line obtained by plotting the relationship between "lnt _1/2 " versus "1/T" instead of "lnk" using the determined activation energy ΔE.

A preferred organic peroxide having the half-life temperature described above is t-butylperoxyisopropyl carbonate (1-minute half-life temperature: 158.8°C).

The organic peroxide is preferably used in an amount of 0.02 parts by mass or more and 0.45 parts by mass or less when the amount of the unmodified aliphatic polyester resin to be modified is taken as 100 parts by mass. The amount of the organic peroxide may be 0.05 parts by mass or more, or may be 0.08 parts by mass or more. The amount of the organic peroxide may be 0.4 parts by mass or less, or may be 0.3 parts by mass or less.

The foamable resin composition of this embodiment may contain one or more modified aliphatic polyester resins modified as described above. For example, the foamable resin composition of this embodiment may contain modified polybutylene succinate and modified polybutylene succinate adipate. The foamable resin composition of this embodiment may also contain one or more modified aliphatic polyester resins and one or more unmodified aliphatic polyester resins.

The foamable resin composition of this embodiment exhibits good foamability when it is melt-kneaded and extruded in an extruder together with components for foaming the foamable resin composition. Examples of the components for foaming that are supplied to the extruder together with the foamable resin composition include a bubble regulator and a foaming agent. In addition to these, various additives may be added to the foamable resin composition during extrusion foaming. Examples of such additives include fillers, colorants, flame retardants, antibacterial agents, weather resistance agents, and surfactants. The proportion of additives other than resin in the foamable resin composition of this embodiment is usually 10% by mass or less. The proportion of additives may be 8% by mass or less, or may be 6% by mass or less.

The foamed sheet 1 in this embodiment can be produced by carrying out a modification step in which an aliphatic polyester resin and an organic peroxide are melt-kneaded to obtain a modified aliphatic polyester resin, and a sheet production step in which a resin composition containing the modified aliphatic polyester resin is extruded together with a foaming agent into a sheet to produce a foamed sheet. The foamable resin composition may be prepared using only the modified aliphatic polyester resin obtained in the modification step, or may be prepared by blending two or more types of modified aliphatic polyester resins, or blending one or more types of modified aliphatic polyester resins with one or more types of unmodified aliphatic polyester resins.

In this embodiment, the modification step, the resin composition preparation step of preparing a foamable resin composition containing a modified aliphatic polyester resin, and the sheet preparation step using the foamable resin composition obtained in the resin composition preparation step may each be carried out in a batch manner, or these steps may be carried out continuously. In this embodiment, a lamination step of laminating a non-foamed layer on one or both sides of a foam sheet may be carried out after the sheet preparation step to produce a laminated foam sheet. This lamination step may also be carried out in a batch manner, or may be carried out continuously.

When the above-mentioned steps are carried out continuously, all the steps may be carried out in one extrusion line. For example, the above-mentioned steps can be carried out continuously using a tandem line equipped with a first extruder provided upstream in the direction of material movement and a second extruder connected downstream of the first extruder, with a sheeting die (flat die, circular die) attached to the tip of the second extruder. In this case, for example, the modification step can be carried out by supplying the unmodified aliphatic polyester resin and the organic peroxide to the first extruder and melt-kneading the unmodified aliphatic polyester resin and the organic peroxide in the first extruder. In this case, by setting the unmodified aliphatic polyester resin and the organic peroxide to the above-mentioned ratio (for example, unmodified aliphatic polyester resin 100:organic peroxide 0.02 to 0.45 (mass ratio)), a modified aliphatic polyester resin with a low gel content (gel fraction 40 mass% or less) suitable for foaming can be obtained, and a foaming resin composition suitable for producing a foamed product in a good foaming state can be easily obtained.

As described above, the foaming resin composition is prepared so that the gel fraction measured using chloroform is 40% by mass or less, and thus the composition is suitable for obtaining a foamed sheet having good foamability, low density, and low basis weight. The foaming resin composition is prepared so that the melt mass flow rate (MFR) and melt tension at 190° C. are predetermined values, and thus the composition is suitable for obtaining a foamed sheet having good foamability, low density, and low basis weight. Furthermore, the foaming resin composition has a storage modulus (G') with a predetermined slope in the frequency range of 0.01 Hz to 0.1 Hz in dynamic viscoelasticity measurement, and a complex viscosity (η ^* ) at a frequency of 0.01 Hz is a predetermined value, and thus the composition is particularly suitable for obtaining a foamed sheet having good foamability, low density, and low basis weight.

The resin composition preparation step for obtaining such a foaming resin composition may be carried out in a first extruder or a second extruder. The resin composition preparation step may be carried out in parallel with the modification step by, for example, supplying other components such as a bubble control agent together with the unmodified aliphatic polyester resin from the middle of the first extruder. The foaming agent may also be supplied from the middle of the first extruder or the second extruder. The lamination step may be carried out by extruding a resin composition for forming a non-foamed layer (hereinafter also referred to as a "non-foamed layer resin composition") from the sheeting die by a co-extrusion method, and may be carried out in parallel with the sheet preparation step. The lamination step may be carried out by a method of extrusion laminating or dry laminating the non-foamed layer resin composition on a foamed sheet that has already been prepared. In addition to the above method, a foamed sheet or a laminated foamed sheet may be prepared by a conventionally known method.

In consideration of the material recyclability and biodegradability of the foamed product, it is preferable that the non-foamed layer is also composed of an aliphatic polyester-based resin composition. In that case, the aliphatic polyester-based resin contained in the non-foamed layer and the aliphatic polyester-based resin contained in the foamed layer may be the same or different.

In order to obtain a foamed sheet that is less likely to undergo thermal deformation, it is preferable to use a foaming agent that is less likely to remain in the foamed sheet and is less likely to cause secondary foaming. Carbon dioxide is preferable as the foaming agent used in this embodiment. In this embodiment, in addition to the carbon dioxide, or instead of carbon dioxide, a hydrocarbon or nitrogen gas may be used as the foaming agent. These foaming agents may be used alone or in combination of two or more types. As the bubble regulator, a common agent such as talc may be used.

As mentioned above, in the sheet production process, it is desirable to cool the foam sheet immediately after extrusion more slowly than in the past to allow the foam sheet to crystallize sufficiently. In terms of slowing down the cooling of the foam sheet (foam layer), it can be said that the co-extrusion method is more advantageous when producing a laminated foam sheet.

The foamed sheet thus produced has an apparent density of 30 kg/m ³ or more and 100 kg/m ³ or less, a gel fraction measured using chloroform of 25% by mass or less, and a calorific value observed during the first heating process measured by heat flux differential scanning calorimetry at a heating rate of 10 ° C./min of 5.0 J/g or less, and is excellent in lightness, cushioning properties, and dimensional accuracy. The foamed sheet can also be suitable as a raw sheet for producing a foamed molded body (thermoformed body) by thermoforming or the like. As a thermoforming method for obtaining a thermoformed body, a conventionally known method such as vacuum forming, pressure forming, and vacuum pressure forming can be adopted. A foamed molded body excellent in lightness, cushioning properties, and dimensional accuracy can be easily produced by using the foamed sheet of this embodiment.

In addition, although the above-mentioned examples are given in this embodiment, the present invention is in no way limited to the above-mentioned examples. As described above, the following inventions are disclosed in this embodiment.

(1)
A foamed sheet made of a resin composition containing one or more aliphatic polyester resins,
The apparent density is 30 kg/ ^m3 or more and 100 kg/ ^m3 or less,
The gel fraction measured using chloroform is 25% by mass or less,
A foamed sheet having a calorific value of 5.0 J/g or less observed during a first temperature rise as determined by heat flux differential scanning calorimetry at a heating rate of 10° C./min.

(2)
An endothermic peak is observed during the temperature rise process in the heat flux differential scanning calorimetry,
The foamed sheet according to (1), wherein the difference between the absolute value of the amount of heat absorbed at the endothermic peak and the amount of heat generated is 30 J/g or more and 90 J/g or less.

(3)
The foamed sheet according to (1) or (2), having a heat loss of 0.1% by mass or more and 1.5% by mass or less.

(4)
The foamed sheet according to any one of (1) to (3), having a secondary expansion ratio of 0.9 to 1.5 times.

(5)
5. The foamed sheet according to any one of (1) to (4), having an open cell ratio of 60% or less.

(6)
The foamed sheet according to any one of (1) to (5), wherein the aliphatic polyester-based resin is polybutylene succinate and/or polybutylene succinate adipate.

(7)
The foamed sheet according to any one of (1) to (6), wherein the aliphatic polyester-based resin is at least partially plant-derived polybutylene succinate and/or at least partially plant-derived polybutylene succinate adipate.

(8)
A foamed molded article, which is a thermoformed article composed of the foamed sheet according to any one of (1) to (7).

(9)
melt-kneading an aliphatic polyester resin with an organic peroxide to obtain a modified aliphatic polyester resin;
A method for producing a foamed sheet, comprising extruding a resin composition containing the modified aliphatic polyester resin into a sheet together with a foaming agent,
The foam sheet to be manufactured is
The apparent density is 30 kg/ ^m3 or more and 100 kg/ ^m3 or less,
The gel fraction measured using chloroform is 25% by mass or less,
A method for producing a foamed sheet, wherein the calorific value observed during a first heating step as determined by heat flux differential scanning calorimetry at a heating rate of 10° C./min is 5.0 J/g or less.

(10)
The method for producing a foamed sheet according to (9), wherein the foaming agent is carbon dioxide.

(11)
The resin composition comprises
The melt mass flow rate (MFR) at 190°C measured by the method specified in JIS K 7210 is 0.1 g/10 min or more and 5.0 g/10 min or less.
The melt tension at 190°C measured using a rheometer and an extensional viscosity measuring device is 30 cN or more and 100 cN or less,
The method for producing a foamed sheet according to (9) or (10), wherein the gel fraction measured using chloroform is 40 mass% or less.

(12)
The storage modulus (G') of the resin composition obtained by measuring the dynamic viscoelasticity under the following conditions is:
When the horizontal axis is frequency (Hz) and the vertical axis is storage modulus (G') on a double logarithmic scale,
The frequency varies linearly in the range from 0.01 Hz to 0.1 Hz.
The method for producing a foamed sheet according to any one of (9) to (11), wherein a slope of a straight line obtained by linearly approximating a frequency range of 0.01 Hz to 0.1 Hz on the graph is 0.35 or more and 1.50 or less.
<Measurement conditions>
Distortion: 5%
Frequency: 0.01 to 100 (Hz) (measurement starts from low frequency (0.01 Hz))
Number of measurement points: 21 (5 points/digit)
Measurement temperature: 190°C
Atmosphere gas: Nitrogen

(13)
The method for producing a foamed sheet according to any one of (9) to (12), wherein the complex viscosity (η ^* ) of the resin composition, which is determined by measuring the dynamic viscoelasticity under the following conditions, is 10,000 Pa·s or more and 150,000 Pa·s or less at a frequency of 0.01 Hz.
<Measurement conditions>
Distortion: 5%
Frequency: 0.01 to 100 (Hz) (measurement starts from low frequency (0.01 Hz))
Number of measurement points: 21 (5 points/digit)
Measurement temperature: 190°C
Atmosphere gas: Nitrogen

The above invention makes it possible to provide a foamed sheet that contains an aliphatic polyester resin but has excellent heat deformation resistance and moldability, and also has a low apparent density, and to provide a foamed molded product that is easy to manufacture, lightweight, has excellent cushioning properties, and is highly dimensionally accurate.

The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these.

<Evaluation Experiment 1>
The following aliphatic polyester resins were prepared for producing the foamed sheets of the Examples and Comparative Examples.
<Aliphatic polyester resin>
Aliphatic polyester resin (A): manufactured by PTT MCC BIOCHEM, product name "BioPBS FZ71PM", PBS in which part of the resin is derived from plants.
Aliphatic polyester resin (B): manufactured by PTT MCC BIOCHEM, product name "BioPBS FZ91PM", PBS in which part of the resin is derived from plants.
Aliphatic polyester resin (C): manufactured by PTT MCC BIOCHEM, product name "BioPBS FD92PM", PBSA in which part of the resin is derived from plants.
Aliphatic polyester resin (D): manufactured by Nature Works, product name "Biopolymer Ingeo 6202D", all resins are plant-derived PLA.
The physical properties of the above aliphatic polyester resin are shown in Table 1.

The organic peroxide used to modify the aliphatic polyester resin was as follows: The cell regulator and foaming agent used to prepare the foamed sheet were as follows.
<Organic Peroxide>
Organic peroxide (a): Chemical Nouryon Co., Ltd., trade name "Trigonox BPIC-C75", t-butylperoxyisopropyl monocarbonate, 1-minute half-life temperature 156°C
Organic peroxide (b): NOF Corp., trade name "Perbutyl P", α,α'-di-t-butylperoxydiisopropylbenzene, 1-minute half-life temperature 175°C
<Foam Regulator>
Talc: Matsumura Sangyo Co., Ltd., product name "Crown Talc PP"
<Foaming Agent>
Carbon dioxide Butane: a mixture of isobutane and normal butane. Isobutane: normal butane = 35:65 (mass ratio)

The properties of the foaming resin composition and foam sheet were evaluated as follows:

(Method of Evaluating the Physical Properties of the Foamable Resin Composition and the Foamed Sheet)
<Basance weight>
For each foam sheet, 6 pieces of 10 cm x 10 cm were cut out at equal intervals in the width direction (TD) of the foam sheet, excluding 20 mm at both ends, and the mass (g) of each piece was measured. The average value of the mass (g) of each piece was converted to the mass per ^m2 and used as the basis weight (g/ ^m2 ) of the foam sheet.

<Thickness>
The thickness of the foamed sheet in each example was measured using a constant pressure thickness gauge ("Peacock Digital Linear Gauge PDN25" manufactured by Ozaki Manufacturing Co., Ltd.). Specifically, the thickness was measured by the constant pressure thickness gauge when a load of 100 g was applied to the foamed sheet using a circular jig with a diameter of 35.7 mm. The thickness of the foamed sheet was measured at 10 or more points every 5 cm in the width direction (TD) except for 20 mm at both ends of the width direction (TD) perpendicular to the extrusion direction (MD) of the foamed sheet, and the arithmetic mean value of the measured values was used. In addition, when the width of the foamed sheet is narrow and it is not possible to secure 10 measurement points, the arithmetic mean value of all the measured values was used as the thickness of the foamed sheet after securing as many measurement points as possible.

<Apparent density>
The calculation was performed from the basis weight and thickness of the foamed sheet of each example using the following formula (s1).
Apparent density (kg/m ³ )=basis weight (g/m ² )÷thickness (mm) (s1)

<Open cell ratio>
From each foamed sheet, two or more sheet-like samples measuring 25 mm long x 25 mm wide were cut out, and the cut out samples were stacked so as not to leave any gaps to obtain a test piece having a thickness of 25 mm. The length and width of the obtained test piece were measured using a caliper ("Digimatic Caliper" manufactured by Mitutoyo Corporation). The thickness was measured to 1/100 mm using a constant pressure thickness measuring instrument ("Peacock Digital Linear Gauge PDN25" manufactured by Ozaki Seisakusho Co., Ltd.) in the same manner as in the case of determining the thickness of the foamed sheet 1, and the apparent volume (V1: cm ³ ) was obtained. Next, the volume (V2: cm 3 ) of the test piece was obtained by the 1-1/2-1 atmospheric pressure method using an air comparison specific gravity meter "1000 type" manufactured by Tokyo Science ^Co. , Ltd. The open cell ratio (%) was calculated using the following formula (s2), and the arithmetic mean value of the open cell ratios of the five test pieces was obtained. The test pieces were conditioned in advance for 24 hours or more in a standard atmosphere of class 2, "23/50" (temperature 23±2°C, relative humidity 50±5%) according to JIS K 7100:1999, and then measured in the same standard atmosphere. The air comparison specific gravity meter was calibrated using standard balls (large 28.96 ^cm3 , small 8.58 ^cm3 ).
Open cell ratio (%) = (V1 - V2) / V1 x 100 ... (s2)
(V1: apparent volume, V2: volume measured by air comparison type specific gravity meter)

<Melting point, crystallization temperature>
The melting point and crystallization temperature were measured by the methods described in JIS K7121: 1987 and JIS K7121: 2012. However, the sampling method and temperature conditions were as follows.
The foaming resin composition and the sample cut out from the foamed sheet of each example were packed in the bottom of an aluminum measuring container at 5.5±0.5 mg without leaving any gaps, and then the aluminum lid was placed on the container. Differential scanning calorimetry was then carried out using a Hitachi High-Tech Science DSC7000X, AS-3 differential scanning calorimeter. The sample was heated and cooled in the following steps 1 to 4 under a nitrogen gas flow rate of 20 mL/min to obtain a DSC curve.
(Step 1) The temperature is decreased from 30° C. to −40° C. at a rate of 10° C./min.
(Step 2) The temperature was increased from −40° C. to 200° C. at a rate of 10° C./min (first temperature increase process) and held for 10 minutes.
(Step 3) The temperature is decreased from 200° C. to −40° C. at a rate of 10° C./min (cooling process), and held for 10 minutes.
(Step 4) Heat the temperature from -40°C to 200°C at a rate of 10°C/min (second heating process).
Alumina was used as the reference material. Using the analysis software attached to the device, the top temperature of the melting peak observed during the second heating process was read as the melting point, as shown in Figures 3 and 4, and the top temperature of the crystallization peak during the cooling process was read as the crystallization temperature. However, when multiple melting peaks or crystallization peaks were observed, the higher temperatures were used as the melting point and crystallization temperature.

<Amount of heat absorbed (a) and amount of heat generated (b)>
The endothermic amount (a) (amount of heat of fusion) and the exothermic amount (b) (amount of heat of crystallization) were measured according to the methods described in JIS K7122: 1987 and JIS K7122: 2012. However, the sampling method and temperature conditions were as follows.
A sample cut out from the foamed sheet of each example was packed into the bottom of an aluminum measurement container at 5.5±0.5 mg without leaving any gaps, and then the container was covered with an aluminum lid. Differential scanning calorimetry was then carried out using a Hitachi High-Tech Science DSC7000X, AS-3 differential scanning calorimeter. The sample was heated and cooled in the following steps 1 and 2 under a nitrogen gas flow rate of 20 mL/min to obtain a DSC curve.
(Step 1) Decrease temperature from 30° C. to −40° C. at a rate of 10° C./min.
(Step 2) Heat the temperature from −40° C. to 200° C. at a rate of 10° C./min (first heating process).
Alumina was used as the reference material. The endothermic amount (a) and the calorific value (b) were calculated using the analysis software attached to the device. Specifically, as shown in FIG. 3, the endothermic amount (a) was calculated from the area of the part surrounded by the line connecting the point where the DSC curve leaves the low-temperature baseline and the point where the DSC curve returns to the high-temperature baseline, as well as the DSC curve. The calorific value (b) was calculated from the area of the part surrounded by the line connecting the point where the DSC curve leaves the low-temperature baseline and the point where the DSC curve returns to the high-temperature baseline, as well as the DSC curve. In addition, as shown in FIG. 4, when no crystallization (exothermic) peak was observed during the first heating process, the calorific value (b) was set to 0 J/g. However, when multiple melting (endothermic) peaks or crystallization (exothermic) peaks were observed, the total heat of each peak was set to the endothermic amount (a) and the calorific value (b).

<Loss on heating>
Three samples were cut out from the foamed sheet immediately after extrusion in each example so that the mass of each sample was about 10 g. Each sample was conditioned for 168 hours under the standard atmosphere of JIS K 7100:1999, "23/50" (temperature 23±2°C, relative humidity 50±5%), class 2. Then, each sample was wrapped in aluminum foil, and the mass (W1) (g) of each sample before heating was measured. Then, each sample was placed on a flat platform in an oven set to 180°C without humidity control and heated for 30 minutes. Each sample was removed from the oven and allowed to cool for 30 minutes under the standard atmosphere of JIS K 7100:1999, "23/50" (temperature 23±2°C, relative humidity 50±5%), class 2. Then, the mass (W2) (g) of each sample after heating was measured.
The heat loss (mass%) of each of the three samples was calculated by the following formula (s3), and the arithmetic mean value was determined as the heat loss (mass%) of the foamed sheet.
Heating loss (mass%) = (W1-W2)/W1×100...(s3)

<Secondary expansion ratio>
Three test pieces measuring 100 mm long x 100 mm wide were cut out from the foamed sheet immediately after extrusion in each example. The test pieces were conditioned for 168 hours under a standard atmosphere of grade 2, "23/50" (temperature 23±2°C, relative humidity 50±5%), as specified in JIS K 7100:1999, and the thickness (T1) (mm) of the center of each test piece before heating was measured. Thereafter, the test pieces were placed on a flat platform in an oven without humidity control set at 70°C and heated for 150 seconds, then removed from the oven and cooled at room temperature for 30 minutes, and the thickness (T2) (mm) of the center of each test piece after heating was measured. The thicknesses (T1 and T2) of the center of the test pieces were measured in the same manner as in the case of determining the thickness of the foamed sheet in each example, using a constant pressure thickness measuring instrument ("Peacock Digital Linear Gauge PDN25" manufactured by Ozaki Seisakusho Co., Ltd.).
The secondary expansion ratios (times) of the three test pieces were calculated by the following formula (s4), and the arithmetic mean value was regarded as the secondary expansion ratio (times) of the foamed sheet.
Secondary expansion ratio (times) = T2/T1 ... (s4)

<Gel Fraction>
About 0.5 g of sample was prepared from the modified aliphatic polyester resin and the foamed sheet of each example, and the initial mass (Mo) of the sample was precisely weighed. In addition, an 80-mesh wire net (wire diameter φ0.12 mm) was prepared for filtering the solution in which the sample was dissolved, and the initial mass (Ms) of this wire net was also precisely weighed. The sample, 50 cc of chloroform, and a stirrer bar were placed in a beaker (volume: 100 cc), covered with aluminum foil, set on a stirrer, and stirred for 2 hours to dissolve the sample at room temperature. After 2 hours, the lid was removed, the dissolved matter in the beaker was filtered through the wire net, and the resin insoluble matter was collected on the wire net. The resin insoluble matter was naturally dried together with the wire net after filtration in a draft chamber to evaporate the chloroform. Next, the resin insoluble matter was dried at a temperature of 120 ° C. for 2 hours using a thermostatic dryer together with the wire net after filtration, and allowed to cool in a desiccator after drying. The total mass (Mx) of the resin insoluble matter and the wire net after cooling was measured.
The gel fraction (mass%) was calculated by the following formula (s5) and was regarded as the gel fraction of the foamed sheet.
Gel fraction (mass%)=(Mx-Ms)/Mo×100 (s5)

(Method of Evaluating the Performance of Foam Sheet)
<Cushioning properties of foam sheet (25% compression stress)>
The 25% compressive stress was measured in accordance with JIS K6767: 1999. The 25% compressive stress was measured using a universal testing machine "Autograph AG-X plus 100kN" manufactured by Shimadzu Corporation and a universal testing machine data processing machine "TRAPEZIUM X" manufactured by Shimadzu Corporation.
A piece of 50 mm long x 50 mm wide was cut out from each foam sheet and stacked to give a thickness of 10 to 13 mm. The thickness of the test piece was measured using a digital linear gauge manufactured by Peacock Co., Ltd., with a circular jig having a diameter of 35.7 mm and a load of 100 g applied to the test piece. The number of test pieces was three or more. The test pieces were conditioned for 16 hours in a standard atmosphere of the JIS K 7100:1999 symbol "23/50" class 2 and used for the measurement. The measurement was performed under the same environment, using a φ100 mm compression plate, with an initial load of about 3.0 N and a compression speed of 5 mm/min. The point where the initial load was applied was set as the displacement origin.
From the graph obtained for each test piece, the stress when compressed by 25% from the thickness at the time of applying the initial load was calculated, and the arithmetic mean value was defined as the 25% compression stress (kPa) of the foamed sheet and used as the evaluation standard for the cushioning property.

<Cushioning evaluation criteria>
⊚: 25% compressive stress is less than 40 kPa.
◯: 25% compressive stress is 40 kPa or more and less than 80 kPa.
Δ: The 25% compressive stress is 80 kPa or more and less than 200 kPa.
×: 25% compressive stress is 200 kPa or more.

<Thermoformability (draw ratio) of foam sheet>
A flat rectangular test piece measuring 700 mm long x 1050 mm wide was cut out from each foamed sheet. A single shot molding machine (manufactured by Tosei Sangyo Co., Ltd., product name "Unic Automatic Molding Machine FM-3A") was prepared, and the average temperature of the upper heater of this single shot molding machine was set to 250°C, the average temperature of the lower heater was set to 222°C, the upper atmosphere temperature was set to 190°C, and the lower atmosphere temperature was set to 185°C.
Next, the test piece was introduced into a single shot molding machine, heated for a predetermined time, and then molded using a mold (mold surface temperature 50°C) in which 22 truncated cones of 10 mm diameter (top) x 35 mm diameter (bottom) and different heights were arranged. The heights and drawing ratios of the 22 truncated cones were as shown in Table 2. The heating time was 4 to 30 seconds, and the heating time was changed at 2 second intervals, and molding was performed for each. The molded bodies obtained at each heating time were visually observed, and the maximum drawing ratio at which the molded body was free of tears or holes and was molded into the shape of the mold was taken as the drawing ratio of the foamed sheet and was used as the evaluation criterion for thermoformability.

<Evaluation criteria for thermoformability>
◎: The drawing ratio is 2.00 or more.
◯: The drawing ratio is 1.60 or more and less than 2.00.
Δ: The drawing ratio is 1.30 or more and less than 1.60.
×: The drawing ratio is less than 1.30 or molding is not possible.
Here, "unmoldable" means that in the case of the No. 1 truncated cone in Table 2, the shape is significantly deviated from the shape of the mold, or tears or holes are found in the molded product.

<Evaluation of Heat Deformation Resistance of Foam Sheet (Heat Deformation Rate)>
Three flat square test pieces with sides of about 10 cm each were cut out from the foamed sheet of each example so that each side was parallel to the extrusion direction (MD) or width direction (TD) of the foamed sheet. Next, two straight lines were drawn on the foamed sheet of each test piece in the shape of a cross, connecting the centers of the opposing sides. At this time, the length of the straight line in the extrusion direction (MD1), the length of the straight line in the width direction (TD1), and the thickness of the foamed sheet at the intersection of the cross (VD1) were measured before heating. Next, each test piece was placed on a flat platform in an oven with no humidity control set at 70°C, heated for 150 seconds, and then removed from the oven and cooled at room temperature for 30 minutes. Thereafter, the straight line length (MD2, TD2) and thickness (VD2) in each direction of each test piece after heating were measured. The lengths of the straight lines (MD1 and MD2, TD1 and TD2) were measured using a vernier caliper ("Digimatic Caliper" manufactured by Mitutoyo Corporation), and the thicknesses (VD1 and VD2) were measured using a constant pressure thickness measuring instrument ("Peacock Digital Linear Gauge PDN25" manufactured by Ozaki Manufacturing Co., Ltd.) in the same manner as in the case of determining the thickness of the foamed sheet of each example.
The heat deformation rate (%) of each test piece was calculated by the following formula (s6), and the arithmetic mean value was regarded as the heat deformation rate (%) of the foamed sheet, which was used as the evaluation standard for heat deformation resistance.
Heat deformation rate (%) = (MDr + TDr + VDr) / 3 ... (s6)
MD deformation ratio (MDr) (%) = (|MD2-MD1|/MD1) x 100
TD deformation rate (TDr) (%) = (|TD2-TD1|/TD1) x 100
VD deformation rate (VDr) (%) = (|VD2-VD1|/VD1) x 100

<Evaluation criteria for heat deformation resistance>
⊚: Heat deformation rate is less than 3%.
Good: The heat deformation rate is 3% or more and less than 10%.
Δ: The heat deformation rate is 10% or more and less than 15%.
×: Heat deformation rate is 15% or more.

<Overall evaluation of foam sheets>
The foamed sheets of each example were comprehensively evaluated based on the various characteristics according to the following evaluation criteria.
: All items were rated as 'A'.
◯: All items were rated as either "◎" or "◯", with at least one being "◯".
Δ: There were no "x" marks in the evaluation of any item, and at least one "Δ" mark was given in the evaluation of any item.
×: Any of the items was rated as “×”.

<Preparation of modified aliphatic polyester resin>
(Production Example 1)
First, according to the formulation in Table 3, an aliphatic polyester resin that had been previously dried and dehumidified at 80° C. for 4 hours and an organic peroxide were dry-blended to obtain mixed pellets.
Next, the mixed pellets were fed to the hopper of a twin-screw extruder (diameter 57 mm, L/D=32), melt-kneaded under the extrusion conditions shown in Table 3, and strands of the modified aliphatic polyester resin were extruded from a strand die attached to the tip of the twin-screw extruder, cooled in a water tank, and cut into pellets with a pelletizer to obtain pellets of the foamable resin composition. The physical properties of the obtained foamable resin composition are shown in Table 3.

(Production Examples 2 to 9)
Pellets of a foamable resin composition were obtained using the blending and extrusion conditions shown in Table 3 in the same manner as in Production Example 1. The physical properties of the obtained foamable resin composition are shown in Table 3.

<Preparation of foam sheet>
Example 1
According to the formulation in Table 4, the modified aliphatic polyester resin and the cell regulator were dry blended to prepare a formulation.
A compound was supplied to the hopper of the first extruder of a tandem extruder in which two extruders were connected (the first extruder on the upstream side was a single screw extruder (diameter 55 mm), and the second extruder on the downstream side was a single screw extruder (diameter 65 mm)). The compound was melt-kneaded in the first extruder while a foaming agent was injected midway through the first extruder, to obtain a melt-kneaded compound.
The molten mixture was transferred to a second extruder, cooled, and extruded and foamed from a circular die (diameter 60 mm) attached to the tip of the second extruder to form a cylindrical body made of a foamed sheet. The resin temperature and discharge rate at this time were as shown in Table 4.
Cooling air was blown onto the inner and outer sides of the cylindrical foamed sheet, and then the inner surface of the cylinder was brought into sliding contact with the outer peripheral surface of a predetermined mandrel to cool the sheet from the inner surface of the cylinder. Thereafter, one portion of the cylinder was cut continuously along the extrusion direction to obtain a long strip-shaped foamed sheet, which was then wound into a roll.
The physical properties and evaluation of the resulting foamed sheet are shown in Table 4.

(Examples 2 to 9)
A foamed sheet was produced in the same manner as in Example 1, except that the extrusion conditions and the take-up speed of the take-up machine were adjusted according to the formulation in Table 4. The physical properties and evaluation of the obtained foamed sheet are shown in Table 4.

(Comparative Example 1)
A foamed sheet was produced in the same manner as in Example 1, except that an unmodified aliphatic polyester resin (B) (unmodified aliphatic polyester resin) was used instead of the modified aliphatic polyester resin, and the extrusion conditions and the take-up speed of the take-up machine were adjusted according to the formulation in Table 5. The physical properties and evaluation of the obtained foamed sheet are shown in Table 5.

(Comparative Examples 2 to 5)
A foamed sheet was produced in the same manner as in Example 1, except that the extrusion conditions and the take-up speed of the take-up machine were adjusted according to the formulation in Table 5. The physical properties and evaluation of the obtained foamed sheet are shown in Table 5.

The results of differential scanning calorimetry of the foamed sheet of Comparative Example 1 are shown in Fig. 3, and the results of differential scanning calorimetry of the foamed sheet of Example 1 are shown in Fig. 4. The relationship between frequency and storage modulus (G') in the dynamic viscoelasticity measurement of the foamable resin composition used in Example 1 is shown in a double logarithmic axis graph and linearly approximated in the frequency range of 0.01 Hz to 0.1 Hz in Fig. 5, and the measurement results of the complex viscosity (η ^* ) are shown in Fig. 6. The relationship between frequency and storage modulus (G') in the dynamic viscoelasticity measurement of the foamable resin composition used in Comparative Example 1 is shown in a double logarithmic axis graph and linearly approximated in the frequency range of 0.01 Hz to 0.1 Hz in Fig. 7, and the measurement results of the complex viscosity (η ^* ) are shown in Fig. 8.
The approximation line in FIG. 5 is expressed by the function "y = 28252x ^0.7015 ", and the approximation line in FIG. 7 is expressed by the function "y = 5283.5x ^1.7274 ", which show gradients of 0.7015 and 1.7274, respectively, on a double logarithmic graph.

The foamed sheets of Examples 1 to 9 to which the present invention was applied were evaluated as being "△" to "◎". On the other hand, the foamed sheets of Comparative Examples 1 to 5 were evaluated as being "×" in any of the cushioning property, thermoformability, and heat deformation resistance.
From the above results, it was confirmed that the foamed sheet according to the present invention is excellent in shock-absorbing properties, thermoformability and heat deformation resistance.

<Evaluation Experiment 2>
(Production examples A to F, K to M)
Pellets of the foamable resin composition of Production Examples A to F and K to M were prepared in the same manner as in Production Example 1 in Evaluation Experiment 1. The symbols indicating the type of aliphatic polyester resin and the type of organic peroxide are the same as those in Evaluation Experiment 1, and therefore will not be described again here. When preparing pellets of the foamable resin composition of Production Examples A to F and K to M, the amount of organic peroxide used, the resin temperature, and other conditions were changed as shown in Table 6 below. The physical properties of the prepared foamable resin compositions are also shown in Table 6.

(Production Examples G to J)
Except for using a twin-screw extruder with a bore of 51 mm and L/D=48, pellets of the foamable resin composition of Production Examples G to J were obtained in the same manner as in Production Examples A to F and K to M. When producing pellets of the foamable resin composition of Production Examples G to J, conditions such as the amount of organic peroxide used and the resin temperature were changed as shown in Table 6 below. The physical properties of the produced foamable resin compositions are also shown in Table 6.

In addition, in this evaluation experiment 2, in addition to the evaluation items in evaluation experiment 1, the dynamic viscoelasticity of the foamable resin composition was measured under the following conditions as described above, and the slope of the storage modulus (G') and the complex viscosity (η ^* ) were also measured.
<Measurement conditions>
Distortion: 5%
Frequency: 0.01 to 100 (Hz) (measurement starts from low frequency (0.01 Hz))
Number of measurement points: 21 (5 points/digit)
Measurement temperature: 190°C
Atmosphere gas: Nitrogen

The slope of the storage modulus (G') of the foamable resin composition, when plotted on a double logarithmic axis with frequency (Hz) on the horizontal axis and storage modulus (G') on the vertical axis, changes linearly in the frequency range of 0.01 Hz to 0.1 Hz, and was calculated from the slope of the straight line that approximated the frequency range of 0.01 Hz to 0.1 Hz on the graph.

In addition to the evaluation items in Evaluation Experiment 1, in Evaluation Experiment 2, the melt mass-flow rate (MFR) of the foamable resin composition was measured under the following conditions as described above.
<Melt Mass Flow Rate (MFR)>
The melt mass flow rate (MFR) was measured using a "Melt Flow Index Tester (Automatic) 120-SAS" manufactured by Yasuda Seiki Seisakusho Co., Ltd. The MFR was measured in accordance with JIS K 7210:1999 under the following measurement conditions. The measurement samples were vacuum dried at 70°C for 5 hours or more, and after drying, they were placed in nylon polybags for vacuum packing, vacuum packed, and stored in a desiccator until immediately before the measurement.

(Measurement condition)
Sample: 3 to 8 g
Preheat 1: 200 seconds Preheat 2: 30 seconds Test temperature: 190°C
Test load: 21.18N
Piston travel distance (interval): 25mm
Number of tests: 3 times. The arithmetic mean of the measured values obtained in each test was taken as the MFR (g/10 min).

In addition to the evaluation items in Evaluation Experiment 1, in Evaluation Experiment 2, the melt tension of the foamable resin composition was measured under the following conditions as described above.
<Melt tension>
The melt tension was measured using a capillary rheometer "Capillograph 1D" (special specification for heating furnace) manufactured by Toyo Seiki Seisakusho Co., Ltd. and "Rheotens 71.97" manufactured by Goettfert GmbH. The melt tension was measured under the following conditions.
The samples were vacuum-dried at 70° C. for 5 hours or more in advance, and after drying, they were vacuum-packed in nylon plastic bags for vacuum packing and stored in a desiccator until immediately before the measurement.
The Rheotens 71.97 was installed so that the distance from the die exit of the Capillograph 1D to the measurement part was 80 mm. (Note that, if the Rheotens could not be brought close to 80 mm due to interference, measures were taken to avoid interference and the Rheotens was set in a specified location.)
First, the sample was filled into a barrel heated to a test temperature of 190° C., and then preheated for 5 minutes.
The measurement time was not longer than 10 minutes, including the preheating time, after the barrel was filled.
Next, a piston was inserted from the top of the barrel and the molten resin was extruded into a string-like shape.
At this time, the piston descending speed (20 mm/min) was kept constant, and the extruded string-like material was passed through a Rheotens wheel. Thereafter, the take-up speed was gradually increased to measure the melt tension of the sample.

Regarding the measurement results, the average of the maximum and minimum tension values just before the point at which the string broke was taken as the melt tension of the sample.
When there was only one maximum point on the tension chart, the maximum value was taken as the melt tension.
When the string-like material became thinner and the winding started to rotate freely, that point was taken as the breaking point, and the average of the maximum and minimum tension values immediately before that point was taken as the melt tension of the sample.

(Measurement conditions for Capillograph 1D)
Die: diameter 2.095 mm, length 8 mm, inlet angle 90 degrees (conical)
Barrel diameter: 9.55mm
Piston speed: 20 mm/min Measurement temperature: 190° C.

(Conditions for measuring Rheotensibility)
Wheel spacing: Top 0.6mm, bottom 1.0mm
Acceleration: 10mm/s ²
Pulling speed: Initial speed 6.92 mm/s

(Preparation of foam sheet)
A compound was prepared by dry blending 100 parts by mass of a foamable resin composition that had been previously dried and dehumidified at 80° C. for 4 hours and 0.5 parts by mass of talc (manufactured by Matsumura Sangyo Co., Ltd., product name "Crown Talc PP") as a cell regulator.
The compound was supplied to the hopper of the first extruder of a tandem extruder in which two extruders were connected (the first extruder on the upstream side was a single-screw extruder (diameter 55 mm), and the second extruder on the downstream side was a single-screw extruder (diameter 65 mm)). The compound was melt-kneaded in the first extruder while 4.3 parts by mass of carbon dioxide was injected as a foaming agent midway through the first extruder to obtain a melt-kneaded compound.
The molten mixture was transferred to a second extruder, and cooled so that the resin temperature of the molten mixture became 140° C., and then extruded and foamed at a discharge rate of 30 kg/h from a circular die (diameter 60 mm) attached to the tip of the second extruder to form a cylindrical body composed of a foamed sheet.
Cooling air was blown onto the inner and outer sides of the cylindrical foamed sheet, and then the inner surface of the cylinder was brought into sliding contact with the outer peripheral surface of a predetermined mandrel to cool the sheet from the inner surface of the cylinder. Thereafter, one portion of the cylinder was cut continuously along the extrusion direction to obtain a long strip-shaped foamed sheet, which was then wound into a roll.
By changing the take-up speed of the winder, foamed sheets having basis weights of 100 g/m ² , 65 g/m ² and 40 g/m ² were produced.

In this case, when the foaming resin compositions of Production Examples C, D, E, and F were used, foam sheets with a basis weight of 100 g/ ^m2 and 65 g/ ^m2 could be produced by changing the take-up speed of the winder (the "production possibility" in Table 6 is "yes"). However, when an attempt was made to produce a foam sheet by further increasing the take-up speed of the winder, the resin did not stretch well and the foam sheet broke, making it impossible to produce a foam sheet with a basis weight of 40 g/ ^m2 (the "production possibility" in Table 6 is "no"). In addition, when the foaming resin compositions of Production Examples L and M were used, a foam sheet with a basis weight of 100 g/ ^m2 could be produced by changing the take-up speed of the winder (the "Production possibility" in Table 6 is "Yes"). However, when an attempt was made to produce a foam sheet by further increasing the take-up speed of the winder, the resin did not stretch well and the foam sheet broke, and foam sheets with a basis weight of 65 g/ ^m2 and 40 g/ ^m2 could not be produced (the "Production possibility" in Table 6 is "No").

<Evaluation criteria for basis weight>
The results of the preparation of the foamed sheet of each basis weight were evaluated according to the following criteria.
.circleincircle.: The feasibility of producing foamed sheets of all basis weights was "OK".
◯: Possibility of producing a foamed sheet having a basis weight of 65 g/ ^m2 or more is "OK".
×: Only the foamed sheet having a basis weight of 100 g/ ^m2 is capable of being produced.

<Evaluation criteria for apparent density>
The apparent density of the foamed sheet having a basis weight of 100 g/ ^m2 thus produced was evaluated according to the following criteria.
A: Apparent density is less than 60 kg/ ^m3 .
A: The apparent density is 60 kg/ ^m3 or more and less than 200 kg/ ^m3 .
×: Apparent density is 200 kg/ ^m3 or more

<Overall evaluation of foam sheets>
Based on the evaluation of the basis weight and apparent density of each foamed sheet, a comprehensive evaluation was made according to the following criteria.
: Only "A" was given in the evaluation of basis weight and apparent density.
◯: One each of "◎" and "◯" was given in the evaluations of basis weight and apparent density.
Δ: Only "◯" was given in the evaluation of basis weight and apparent density.
×: Either the basis weight or the apparent density was evaluated as "×".

The above evaluation results are shown in Table 6 below.

The results of Evaluation Experiment 2 show that when a resin composition containing an aliphatic polyester resin is foamed and used, if the properties of the resin composition before foaming, such as MFR, melt tension, gel fraction, and complex viscosity, are within the specified ranges, this can be advantageous for producing a foamed sheet that has excellent foamability and is lightweight (low density, low basis weight).

1: foamed sheet, 10: foamed layer, 2: laminated foamed sheet, 20: non-foamed layer

Claims (13)

1. A foamed sheet made of a resin composition containing one or more aliphatic polyester resins,
   The apparent density is 30 kg/ ^m3 or more and 100 kg/ ^m3 or less,
   The gel fraction measured using chloroform is 25% by mass or less,
   A foamed sheet having a calorific value of 5.0 J/g or less observed during a first temperature rise as determined by heat flux differential scanning calorimetry at a heating rate of 10° C./min.
2. An endothermic peak is observed during the temperature rise process in the heat flux differential scanning calorimetry,
   2. The foamed sheet according to claim 1, wherein the difference between the absolute value of the amount of heat absorbed at the endothermic peak and the amount of heat generated is 30 J/g or more and 90 J/g or less.
3. The foamed sheet according to claim 1 or 2, which has a heat loss of 0.1% to 1.5% by mass.
4. The foamed sheet according to any one of claims 1 to 3, wherein the secondary expansion ratio is 0.9 to 1.5 times.
5. The foamed sheet according to any one of claims 1 to 4, having an open cell rate of 60% or less.
6. The foamed sheet according to any one of claims 1 to 5, wherein the aliphatic polyester resin is polybutylene succinate and/or polybutylene succinate adipate.
7. The foam sheet according to any one of claims 1 to 6, wherein the aliphatic polyester resin is at least partially plant-derived polybutylene succinate and/or at least partially plant-derived polybutylene succinate adipate.
8. A foamed molded product, which is a thermoformed product made of the foamed sheet according to any one of claims 1 to 7.
9. melt-kneading an aliphatic polyester resin with an organic peroxide to obtain a modified aliphatic polyester resin;
   A method for producing a foamed sheet, comprising extruding a resin composition containing the modified aliphatic polyester resin into a sheet together with a foaming agent,
   The foam sheet to be manufactured is
   The apparent density is 30 kg/ ^m3 or more and 100 kg/ ^m3 or less,
   The gel fraction measured using chloroform is 25% by mass or less,
   A method for producing a foamed sheet, wherein the calorific value observed during a first heating step as determined by heat flux differential scanning calorimetry at a heating rate of 10° C./min is 5.0 J/g or less.
10. The method for producing a foamed sheet according to claim 9, wherein the foaming agent is carbon dioxide.
11. The resin composition comprises
    The melt mass flow rate (MFR) at 190°C measured by the method specified in JIS K 7210 is 0.1 g/10 min or more and 5.0 g/10 min or less.
    The melt tension at 190°C measured using a rheometer and an extensional viscosity measuring device is 30 cN or more and 100 cN or less,
    The method for producing a foamed sheet according to claim 9 or 10, wherein the gel fraction measured using chloroform is 40% by mass or less.
12. The storage modulus (G') of the resin composition obtained by measuring the dynamic viscoelasticity under the following conditions is:
    When the horizontal axis is frequency (Hz) and the vertical axis is storage modulus (G') on a double logarithmic scale,
    The frequency varies linearly in the range from 0.01 Hz to 0.1 Hz.
    12. The method for producing a foamed sheet according to claim 9, wherein a gradient of a straight line obtained by linearly approximating a frequency range of 0.01 Hz to 0.1 Hz on the graph is 0.35 or more and 1.50 or less.
    <Measurement conditions>
    Distortion: 5%
    Frequency: 0.01 to 100 (Hz) (measurement starts from low frequency (0.01 Hz))
    Number of measurement points: 21 (5 points/digit)
    Measurement temperature: 190°C
    Atmosphere gas: Nitrogen
13. 13. The method for producing a foamed sheet according to claim 9, wherein the complex viscosity (η ^* ) of the resin composition determined by measuring dynamic viscoelasticity under the following conditions is 10,000 Pa·s or more and 150,000 Pa·s or less at a frequency of 0.01 Hz.
    <Measurement conditions>
    Distortion: 5%
    Frequency: 0.01 to 100 (Hz) (measurement starts from low frequency (0.01 Hz))
    Number of measurement points: 21 (5 points/digit)
    Measurement temperature: 190°C
    Atmosphere gas: Nitrogen

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