JP7492893B2 - Polylactic acid resin foam sheet and sheet molded product - Google Patents

Description

The present invention relates to a polylactic acid-based resin foam sheet and a sheet molded product made of a polylactic acid-based resin foam sheet.

2. Description of the Related Art Conventionally, a resin foam sheet has been produced by melt-kneading a resin composition containing a foaming agent in an extruder and then extruding the resulting mixture from a circular die attached to the tip of the extruder.
Known types of resin foam sheets of this kind include those having a single foamed layer and those having a foamed layer and a non-foamed layer laminated thereto.
This type of resin foam sheet is used as a raw material sheet for producing sheet molded products such as trays, cups, and bowls.

2. Description of the Related Art With the recent increase in environmental awareness, attempts have been made to use polylactic acid-based resins, which can be polymerized using naturally occurring monomers, as the raw material for various resin products.
However, it is difficult to say that polylactic acid resins have sufficient impact resistance.
For this reason, methods for improving the impact resistance of polylactic acid-based resin foam sheets having a foam layer formed of a polylactic acid-based resin composition have been investigated (see Patent Document 1 below).
One such improvement is the use of an impact resistance improver, as described in Patent Document 2 below (see paragraph 0068, etc.).
As an impact resistance improver to be contained in a polylactic acid-based resin composition, a core-shell type elastomer is known, as disclosed in Patent Document 3 below.

JP 2011-093982 A JP 2005-60689 A JP 2016-501741 A

As described above, there is a demand for improving the impact resistance of polylactic acid-based resin foam sheets, and although efforts have been made to satisfy such demand, the demand has not yet been fully met.
In order to improve impact resistance, it is conceivable to increase the amount of elastomer contained in the polylactic acid resin composition compared to conventional methods, but increasing the amount of elastomer may result in a decrease in heat resistance.
Therefore, an object of the present invention is to provide a polylactic acid resin foamed sheet having both impact resistance and heat resistance, and to provide a sheet molded product having excellent impact resistance and heat resistance.

The inventors conducted extensive research to solve the above problems and discovered that, among elastomers, diene-based elastomers do not reduce the heat resistance of polylactic acid-based resin foam sheets even when a relatively large amount of them is included, which led to the completion of the present invention.

To solve the above problems, the present invention provides:
A polylactic acid-based resin foam sheet having a foam layer made of a polylactic acid-based resin composition containing a polylactic acid-based resin,
The polylactic acid-based resin composition further contains a butadiene-based elastomer,
The polylactic acid-based resin composition contains the butadiene-based elastomer in an amount of 8 parts by mass or more and 28 parts by mass or less per 100 parts by mass of the polylactic acid-based resin.

To solve the above problems, the present invention provides:
A sheet molded product made of a polylactic acid resin foam sheet,
The polylactic acid-based resin foam sheet has a foam layer made of a polylactic acid-based resin composition containing a polylactic acid-based resin,
The polylactic acid-based resin composition further contains a butadiene-based elastomer,
and a sheet molded article in which the butadiene elastomer is contained in the polylactic acid resin composition in an amount of 8 parts by mass or more and 28 parts by mass or less per 100 parts by mass of the polylactic acid resin.

The present invention achieves both impact resistance and heat resistance in a polylactic acid resin foam sheet, and can provide a sheet molded product with excellent impact resistance and heat resistance.

FIG. 2 is a diagram showing the dispersion state of the butadiene-based elastomer in the pellet state of Example 1 and Example 2 (transmission electron microscope photographs, left: Example 1, right: Example 2). FIG. 2 is a diagram showing the dispersion state of the butadiene-based elastomer in the polylactic acid-based resin foam sheets of Examples 1 and 2 (transmission electron microscope photographs, left: Example 1, right: Example 2). FIG. 2 is a diagram showing the dispersion state of the butadiene-based elastomer in the sheet molded products of Examples 1 and 2 (transmission electron microscope photographs, left: Example 1, right: Example 2).

Hereinafter, an embodiment of the present invention will be described.
The polylactic acid based resin foam sheet of the present embodiment includes a foam layer.
The polylactic acid based resin foam sheet of the present embodiment may have a single-layer structure consisting of only a foamed layer, or may have a laminated structure consisting of a foamed layer and another layer (for example, a non-foamed layer).
The polylactic acid-based resin foam sheet of the present embodiment includes a foam layer made of a polylactic acid-based resin composition containing a polylactic acid-based resin.

First, the polylactic acid resin composition that is the main raw material of the polylactic acid resin foam sheet will be described below.
The polylactic acid-based resin composition of the present embodiment contains a polylactic acid-based resin and a butadiene-based elastomer, and the butadiene-based elastomer is contained in a ratio of 8 parts by mass or more to 28 parts by mass or less when the polylactic acid-based resin is 100 parts by mass.
In this embodiment, the polylactic acid-based resin composition constituting the foam layer of the polylactic acid-based resin foam sheet contains a butadiene-based elastomer in the above-mentioned ratio, thereby allowing the polylactic acid-based resin foam sheet to exhibit excellent impact resistance and also good heat resistance.

The ratio of the butadiene-based elastomer to 100 parts by mass of the polylactic acid-based resin is preferably 10 parts by mass or more.
The proportion may be 12 parts by mass or more, 14 parts by mass or more, or 16 parts by mass or more.
The ratio of the butadiene-based elastomer to 100 parts by mass of the polylactic acid-based resin is preferably 26 parts by mass or less.
The ratio may be 24 parts by mass or more, or 22 parts by mass or less.

The polylactic acid resin composition does not necessarily contain a single type of polylactic acid resin, but may contain a plurality of types of polylactic acid resins.
The polylactic acid resin composition does not necessarily contain one type of butadiene elastomer, but may contain a plurality of types of butadiene elastomers.
When the polylactic acid-based resin composition contains multiple types of either one or both of the polylactic acid-based resin and the butadiene-based elastomer, the above mass ratio is calculated based on the total content of the polylactic acid-based resin and the butadiene-based elastomer in each polylactic acid-based resin composition.

The polylactic acid resin contained in the polylactic acid resin composition may be a homopolymer of lactic acid or a copolymer of lactic acid and another monomer.
Examples of other monomers in the copolymer include aliphatic hydroxycarboxylic acids other than lactic acid, aliphatic polyhydric alcohols, and aliphatic polycarboxylic acids.
The monomer may be, for example, a polyfunctional polysaccharide.

The lactic acid constituting the polylactic acid resin may be either or both of the L-form and the D-form.
The polylactic acid-based resin which is a homopolymer may be any one of poly(L-lactic acid) resin, poly(D-lactic acid) resin, and poly(DL-lactic acid) resin.
It is preferable that the polylactic acid resin contains a larger amount of L-isomer than D-isomer in order to provide a foamed layer with excellent strength.
However, poly(L-lactic acid) resin containing substantially 100% by mass of L-form has excellent mechanical strength, but tends to cause the foam layer to become brittle.
Therefore, the polylactic acid resin contained in the polylactic acid resin composition of this embodiment preferably has a proportion (mass proportion) of the D-form in the total of the D-form and the L-form of 0.1 mass % or more.
The mass proportion is more preferably 0.2 mass % or more, and particularly preferably 0.3 mass % or more.
If the proportion of the D-form is high, the foamed layer may not exhibit excellent strength.
Therefore, the proportion of the D-isomer in the total of the D- and L-isomers is preferably 4% by mass or less, more preferably 3.5% by mass or less, and particularly preferably 3% by mass or less.
That is, the mass ratio of the D-form to the L-form (D-form/L-form) is preferably within the range of 0.1/99.9 to 4/96.
The same is true for copolymers, in which such a mass ratio of the D- to L-configuration is preferable.

Examples of the aliphatic polycarboxylic acid constituting the copolymer include oxalic acid, succinic acid, malonic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, undecanedioic acid, and dodecanedioic acid.
The aliphatic polycarboxylic acid may be an anhydride.

Examples of the aliphatic polyhydric alcohol include ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, dipropylene glycol, 1,3-butanediol, 1,4-butanediol, 3-methyl-1,5-pentanediol, 1,6-hexanediol, 1,9-nonanediol, neopentyl glycol, tetramethylene glycol, and 1,4-cyclohexanedimethanol.

Examples of the aliphatic hydroxycarboxylic acid other than lactic acid that constitutes the copolymer include glycolic acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 4-hydroxyvaleric acid, 5-hydroxyvaleric acid, and 6-hydroxycaproic acid.

Examples of the polyfunctional polysaccharides include cellulose, cellulose nitrate, methylcellulose, ethylcellulose, celluloid, viscose rayon, regenerated cellulose, cellophane, cupra, cuprammonium rayon, cuprophane, Bemberg, hemicellulose, starch, acropectin, dextrin, dextran, glycogen, pectin, chitin, chitosan, gum arabic, guar gum, locust bean gum, and acacia gum.

In the copolymer, the structural portion derived from lactic acid (L- and D-form) is preferably contained in the molecule in a proportion of 60% by mass or more.
The content of the structural moieties (L- and D-configuration) is more preferably 70% by mass or more, further preferably 80% by mass or more, and particularly preferably 90% by mass or more.

The polylactic acid resin composition preferably contains a polylactic acid resin in a crosslinked state (crosslinked polylactic acid resin), and preferably contains the homopolymer as a crosslinked polylactic acid resin in a crosslinked state.
In particular, it is preferable that the polylactic acid resin composition contains a crosslinked polylactic acid resin crosslinked with an organic peroxide.
By using at least a part of the polylactic acid resin contained in the polylactic acid resin composition as a crosslinked polylactic acid resin crosslinked with an organic peroxide, it is possible to impart melting properties suitable for foaming to the polylactic acid resin composition.

Examples of organic peroxides for crosslinking the polylactic acid resin 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 peroxyketals 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.

Crosslinking with the organic peroxide may cause the crosslinked polylactic acid resin to contain components with excessively large molecular weights that will gel when thermally melted, or may cause odor problems due to the decomposition residues of the organic peroxide.
The organic peroxide is preferably a peroxyester, since this can prevent the occurrence of such problems and makes it easy to bring the polylactic acid-based resin composition into a state suitable for foaming.
Among peroxyesters, the organic peroxide used for crosslinking the polylactic acid resin is preferably a peroxycarbonate-based organic peroxide such as peroxymonocarbonate or peroxydicarbonate.
In the present embodiment, the organic peroxide used for crosslinking the polylactic acid resin is preferably a peroxymonocarbonate organic peroxide among peroxycarbonate organic peroxides, and particularly preferably t-butylperoxyisopropylmonocarbonate.

The organic peroxides as described above are usually used in an amount of 0.1 part by mass or more per 100 parts by mass of the polylactic acid resin to be crosslinked.
The organic peroxide as described above can more reliably exert the crosslinking effect on the crosslinked polylactic acid resin by using an amount of 0.1 part by mass or more.
The amount of the organic peroxide used is preferably 0.2 parts by mass or more, and particularly preferably 0.3 parts by mass or more.
The amount of the organic peroxide used is preferably 2.0 parts by mass or less.
By setting the amount of the organic peroxide to 2.0 parts by mass or less, the presence of gel in the polylactic acid resin after crosslinking can be suppressed.
The amount of the organic peroxide used is more preferably 1.5 parts by mass or less, and particularly preferably 1.0 part by mass or less, per 100 parts by mass of the polylactic acid resin.
By crosslinking the polylactic acid resin using the organic peroxide in such a ratio, the crosslinked polylactic acid resin can be made suitable for foaming.

The polylactic acid resin contained in the polylactic acid resin composition preferably has a melt mass flow rate (MFR) of 0.1 g/10 min or more.
The melt mass flow rate of the polylactic acid resin is more preferably 0.5 g/10 min or more, and further preferably 1 g/10 min or more.
The melt mass flow rate of the polylactic acid resin is preferably 10 g/10 min or less, more preferably 8 g/10 min or less, and even more preferably 5 g/10 min or less.

The melt mass flow rate (MFR) of a polylactic acid resin or a polylactic acid resin composition can be determined, for example, as follows:

(Method of determining the melt mass-flow rate (MFR) of polylactic acid resin and polylactic acid resin composition)
The melt mass flow rate of the polylactic acid resin or the polylactic acid resin composition can be measured, for example, using a "Semi-Auto Melt Indexer 2A" manufactured by Toyo Seiki Seisakusho, Ltd.
The MFR can be measured by a method for measuring the time it takes for a piston to move a predetermined distance, as described in JIS K7210-1:2014 “Plastics-Method of determining melt mass flow rate (MFR) and melt volume flow rate (MVR) of thermoplastics-Part 1” Method B.
In principle, the measurement conditions are as follows:
Sample: 3 to 8 g
The measurement samples are vacuum dried at 70° C. for 4 hours, and after drying, are vacuum-packed in nylon polybags for vacuum packing and stored in a desiccator until immediately before measurement.
Preheat: 270 seconds Load hold: 30 seconds Test temperature: 190°C
Test load: 2.16 kg (21.18 N)
Piston travel distance (interval): 25mm
The test is carried out for each sample three times, and the average of the three tests is regarded as the melt mass-flow rate (g/10 min).

The polylactic acid resin preferably has a mass average molecular weight (Mw) of 100,000 or more and 300,000 or less, and more preferably 150,000 or more and 250,000 or less.
The molecular weight of the polylactic acid resin can be determined, for example, as follows.

(How to determine the average molecular weight of polylactic acid resin)
The number average molecular weight (Mn), mass average molecular weight (Mw), and Z average molecular weight (Mz) of the polylactic acid resin can be determined as follows.
20 mg of the sample to be measured for molecular weight was dissolved in 6 mL of chloroform (immersion time: 6±1.0 h), filtered through a non-aqueous 0.45 μm syringe filter manufactured by Shimadzu GLC Corporation, and then measured using a chromatograph under the following measurement conditions. The average molecular weight of the sample was calculated from a standard polystyrene calibration curve prepared in advance.

Equipment used: Tosoh Corporation's "HLC-8320GPC EcoSEC" gel permeation chromatograph (with built-in RI and UV detectors)
<GPC measurement conditions>
Sample side guard column = TSK guard column HXL-H (6.0 mm x 4.0 cm) x 1, manufactured by Tosoh Corporation Measurement column = TSKgel GMHXL (7.8 mm I.D. x 30 cm) x 2, manufactured by Tosoh Corporation 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 Reference pump = 0.5 mL/min
Sample side pump = 1.0 mL/min
Detector = RI detector Injection volume = 50 μL
Measurement time = 26 min
Sampling pitch = 500 msec

The standard polystyrene samples for the calibration curve are manufactured by Showa Denko K.K. and have the product names "STANDARD SM-105" and "STANDARD SH-75" and have mass average molecular weights 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.
The above-mentioned standard polystyrene for the calibration curve was divided into groups A (5,620,000, 1,250,000, 151,000, 17,000, 2,900) and B (3,120,000, 442,000, 53,500, 7,660, 1,320), and then 2 to 10 mg of each of the A's were weighed out and dissolved in 30 mL of chloroform, and 3 to 10 mg of each of the B's were weighed out and dissolved in 30 mL of chloroform.
The standard polystyrene calibration curve is obtained by injecting 50 μL of each of the prepared A and B solutions, measuring the retention times, and creating a calibration curve (cubic equation) from the retention times. The average molecular weight is calculated using the calibration curve.

The proportion of the homopolymer in the entire polylactic acid resin contained in the polylactic acid resin composition is preferably 50% by mass or more.
The proportion of the homopolymer may be 60% by mass or more, 70% by mass or more, or 80% by mass or more.

The polylactic acid resin preferably exhibits a melt tension of 5 cN or more and 40 cN or less in melt tension measurement at 190°C.
The polylactic acid resin of the present embodiment has a melt tension of 5 cN or more, which makes it possible to prevent excessive cell breakage in the foam layer during production of a polylactic acid resin foam sheet.
Furthermore, since the polylactic acid resin of this embodiment has a melt tension of 40 cN or less, the cell membrane exhibits good elongation during foaming, and the occurrence of cell breakage can be suppressed.
The melt tension is preferably 39 cN or less, more preferably 37 cN or less, and particularly preferably 35 cN or less.
The melt tension is preferably 8 cN or more, and more preferably 10 cN or more.

In order to exert the above-mentioned melt tension, a crosslinked polylactic acid resin is more advantageous than a non-crosslinked polylactic acid resin.
In this embodiment, when determining the melt tension at 190°C for the polylactic acid resin before crosslinking, the crosslinked polylactic acid resin, and the polylactic acid resin composition, the melt tension can be measured, for example, using a Capillograph 1D (manufactured by Toyo Seiki Seisakusho, Ltd.) as follows.

(Melt tension measurement method)
The sample is vacuum dried at 70° C. for 4 hours, and then vacuum-packed in a nylon polyethylene bag and stored in a desiccator until immediately before measurement.
A 9.55 mm diameter barrel heated to a test temperature of 190° C. is filled with a measurement sample resin, and the barrel is preheated for 5 minutes. The measurement sample resin is then extruded into a string-like shape from a capillary die (diameter 2.095 mm, length 8 mm, inlet angle 90° (conical)) of the measurement device while maintaining a constant piston descending speed (20 mm/min). This string-like material is then passed through a pulley for tension detection (manufactured by Rheotens 71.97 (Goettfert)) installed so that the distance from the die outlet of the capillary die to the measurement section is 80 mm. The string-like material is then wound up using a winding roll, with the winding speed gradually increased at an initial speed of 6.92 mm/s and an acceleration of 10 mm/ ^s2 . The average of the maximum and minimum values just before the string-like material is broken is taken as the melt tension of the sample.
If interference occurs and Rheotens cannot be brought within 80 mm, measures to avoid interference are taken and Rheotens is set in a specified location.
When the string-like material becomes thin and the winding starts to spin freely, that point is regarded as the breaking point, and the average of the maximum and minimum tension values immediately before that point is regarded as the melt tension of the sample resin.
When there is only one maximum point on the tension chart, the maximum value is regarded as the melt tension.
The measurement time is adjusted so as not to exceed 10 minutes, including the preheating time, after the barrel is filled.

The crosslinking of the polylactic acid resin can be carried out by melt-kneading the polylactic acid resin and an organic peroxide using an extruder, kneader or the like.
The crosslinking of the polylactic acid resin is preferably carried out before mixing the polylactic acid resin with the butadiene elastomer.
This can result in a favorable dispersion state of the butadiene-based elastomer in the polylactic acid-based resin composition.

The butadiene-based elastomer may be, for example, a homopolymer of butadiene (polybutadiene rubber) or a copolymer with styrene or the like (styrene-butadiene rubber).
The butadiene-based elastomer may be a multicomponent copolymer composed of three or more kinds of monomers.
Examples of copolymerizable monomers other than styrene include acrylic monomers such as methyl acrylate, methyl methacrylate, butyl acrylate, and butyl methacrylate.

The butadiene-based elastomer is preferably a core-shell type elastomer.
It is preferable that the core-shell elastomer comprises a core layer and a shell layer covering the core layer, the core layer being composed of a polymer having butadiene as a structural unit, and the shell layer being composed of a polymer having methyl methacrylate as a structural unit.

When the butadiene-based elastomer is a copolymer, it may be a block copolymer, a random copolymer, or a graft copolymer.
When the butadiene-based elastomer is a copolymer, it is preferably a graft copolymer in order to form the above-mentioned core-shell structure.

The butadiene-based elastomer preferably contains 50% by mass or more of butadiene units.
The butadiene unit content in the butadiene-based elastomer is more preferably 55% by mass or more, and further preferably 60% by mass or more.

As the butadiene-based elastomer, it is preferable to adopt a multicomponent copolymer containing styrene and an acrylic monomer in addition to butadiene.
It is preferable that styrene (St) and the acrylic monomer (Aa) are contained in the multi-component copolymer in a molar ratio of 0.8:1 to 1:0.8 (St:Aa).
As the acrylic monomer, methyl methacrylate is preferred.
That is, as the butadiene-based elastomer, MBS polymer (methyl methacrylate-butadiene-styrene copolymer) is preferable.
The MBS polymer may further contain a small amount (eg, 0.5% to 2% by weight) of butyl acrylate.

The polylactic acid resin composition preferably contains a core-shell elastomer in an amount of 80% by mass or more, and more preferably 90% by mass or more, of the butadiene elastomer.
The polylactic acid resin composition preferably contains 80% by mass or more of the butadiene elastomer as described above, and more preferably 90% by mass or more of the butadiene elastomer as described above.

By incorporating the above-mentioned core-shell type elastomer, it is possible to achieve both higher impact resistance and heat resistance for the polylactic acid resin foam sheet.

The polylactic acid resin composition preferably contains the butadiene elastomer in particulate form.
That is, the polylactic acid-based resin composition is formed of a matrix phase containing the polylactic acid-based resin and a dispersed phase containing the butadiene-based elastomer, and it is preferable that the particle size of the dispersed phase is 0.1 μm or more and 1 μm or less.
The particle size of the dispersed phase can be determined by the method described in the Examples.

The above-mentioned state of the dispersed phase is preferably maintained even when it constitutes the foam layer of the polylactic acid-based resin foam sheet, and is also preferably maintained after it is made into a sheet molded product.

Various additives can be added to the polylactic acid resin composition of the present embodiment as necessary.
Examples of the additives include polymers other than polylactic acid-based resins and butadiene-based elastomers, inorganic fillers, and various chemicals.
Examples of the chemicals include lubricants, antioxidants, antistatic agents, flame retardants, ultraviolet absorbers, light stabilizers, colorants, and antibacterial agents.
The proportion of these additives is preferably 25 parts by mass or less, more preferably 15 parts by mass or less, and even more preferably 10 parts by mass or less, per 100 parts by mass of the polylactic acid resin.

The polylactic acid based resin foam sheet of the present embodiment can be produced by extrusion foaming such a polylactic acid based resin composition.
When an extruded foam sheet is used as the polylactic acid-based resin foam sheet, it can be produced by melt-kneading a polylactic acid-based resin composition together with a foaming agent in an extruder and extruding and foaming the composition through a die attached to the tip of the extruder.

The foaming agent used in this case may be a commonly used one, and may be a volatile foaming agent that turns into a gas at room temperature (23° C.) and normal pressure (1 atm), or a decomposition type foaming agent that generates a gas by thermal decomposition.
As the volatile foaming agent, for example, an inert gas, an aliphatic hydrocarbon, an alicyclic hydrocarbon, etc. can be used.

Examples of the inert gas include carbon dioxide and nitrogen.

Examples of the aliphatic hydrocarbons include propane, normal butane, isobutane, normal pentane, and isopentane, and examples of the alicyclic hydrocarbons include cyclopentane and cyclohexane.
In this embodiment, among the above, normal butane and isobutane can be preferably used.

Examples of the decomposition type foaming agent include azodicarbonamide, dinitrosopentamethylenetetramine, sodium bicarbonate, or a mixture of an organic acid such as citric acid or its salt with a bicarbonate.

In order to improve the foaming state of the polylactic acid resin foam sheet to be produced, the polylactic acid resin composition may contain a foam regulator.
Examples of the cell regulator include polytetrafluoroethylene, talc, aluminum hydroxide, and silica.
The decomposition type foaming agent can adjust the foaming state by using it in combination with a volatile foaming agent, and can also be used as a foam adjusting agent.

The polylactic acid-based resin foam sheet of the present embodiment may be a single-layer sheet having only a foam layer composed of a polylactic acid-based resin composition, or it may be a laminate sheet having, for example, a non-foamed layer laminated on one or both sides of the foam layer.
In the polylactic acid based resin foam sheet of the present embodiment, in order to achieve both strength and light weight, the foam layer preferably has a thickness of 0.5 mm or more.
The thickness of the foam layer is more preferably 0.8 mm or more, and further preferably 1 mm or more.
In the polylactic acid based resin foam sheet of the present embodiment, the foam layer preferably has a thickness of 7 mm or less in order to provide easy moldability.
The thickness of the foam layer is more preferably 6 mm or less, and further preferably 5 mm or less.
When the polylactic acid based resin foam sheet of the present embodiment has a non-foamed layer, it is preferable that the thickness including the non-foamed layer is as described above.

The thickness of the foam layer can be determined by, for example, taking an enlarged photograph of a cross section of the polylactic acid-based resin foam sheet cut along a plane perpendicular to the polylactic acid-based resin foam sheet, and is calculated as the average value of multiple randomly selected locations (e.g., 20 locations).

In the polylactic acid based resin foam sheet of the present embodiment, the apparent density of the foam layer is preferably 63 kg/m ³ or more in terms of strength.
The apparent density of the foamed layer is more preferably 80 kg/m ³ or more, and further preferably 100 kg/m ³ or more.
In the polylactic acid based resin foam sheet of the present embodiment, in order to provide light weight, it is preferable that the apparent density of the foam layer is 500 kg/ ^m3 or less.
The apparent density of the foamed layer is more preferably 400 kg/m ³ or less, and further preferably 300 kg/m ³ or less.

The apparent density of the foam layer can be measured by the method described in JIS K7222:2005 "Foamed plastics and rubber - Measurement of apparent density", specifically, by the following method.

(Method of measuring apparent density)
The apparent density of the foam layer can be measured by the method described in JIS K7222:2005 “Foamed plastics and rubber—Determination of apparent density”.
Specifically, a test piece having an apparent volume of 100 cm ³ or more is prepared, and its mass is measured.
When cutting out test pieces from the polylactic acid resin foam sheet, care is taken not to change the original cell structure as much as possible.
If a test piece of ¹⁰⁰ cm3 or more cannot be prepared, a test piece of as large a volume as possible is prepared.
The apparent density is calculated by the following formula:

Apparent density (kg/m ³ )=test piece mass (g)/test piece volume (mm ³ )×10 ⁶

In principle, the test pieces are taken 72 hours or more after the preparation of the polylactic acid resin foam sheet, and are left to stand for 16 hours or more under atmospheric conditions of a temperature of 23±2°C and a relative humidity of 50±10% to condition the sheet, after which the mass and volume are measured under the same conditions.

In the polylactic acid-based resin foam sheet of the present embodiment, in order to provide strength, it is preferable that the crystallinity of the polylactic acid-based resin composition constituting the foam layer is 5% or more.
The degree of crystallinity of the polylactic acid resin composition is more preferably 6% or more, and further preferably 7% or more.
In order to provide the polylactic acid-based resin foam sheet of the present embodiment with easy formability in thermoforming or the like, it is preferable that the crystallinity of the polylactic acid-based resin composition constituting the foam layer is 15% or less.
The degree of crystallinity of the polylactic acid resin composition is more preferably 14% or less, and further preferably 13% or less.

The crystallinity of the polylactic acid resin composition can be measured by the method described in JIS K7122:1987 and JIS K7122:2012 "Method for measuring heat of transition of plastics."
However, the sampling method and temperature conditions will be as follows.
Fill the bottom of an aluminum measurement container with 5.5±0.5 mg of sample so that there are no gaps, and then cover with an aluminum lid.
Next, differential scanning calorimetry is performed using a Hitachi High-Tech Science DSC7000X, AS-3 differential scanning calorimeter.
The DSC curve is obtained by heating in the following steps under a nitrogen gas flow rate of 20 mL/min.
(Step 1) Hold at 30°C for 2 minutes.
(Step 2) Raise the temperature from 30° C. to 210° C. at a rate of 5° C./min.
In this case, alumina is used as the reference material.
The difference between the heat of fusion (J/g) determined from the area of the melting peak and the heat of crystallization (J/g) determined from the area of the crystallization peak is calculated.
The ratio calculated by dividing this difference by the theoretical heat of fusion of a perfectly crystalline polylactic acid, 93 J/g, is taken as the degree of crystallinity.
The heat of fusion and the heat of crystallization are calculated using the analysis software provided with the device.
Specifically, the heat of fusion is calculated from the area enclosed by the DSC curve and a straight line connecting the point where the DSC curve departs from the low-temperature baseline and the point where the DSC curve returns to the high-temperature baseline.
The heat of crystallization is calculated from the area enclosed by a straight line connecting the point where the DSC curve departs from the low-temperature baseline and the point where the DSC curve returns to the high-temperature side, and the DSC curve.
That is, the crystallinity can be calculated from the following formula:

Crystallinity (%)=(heat of fusion (J/g)−heat of crystallization (J/g))/93 (J/g)×100 (%)

When providing the polylactic acid-based resin foam sheet of this embodiment with a non-foamed layer, it can be formed by a co-extrusion method in which the above-mentioned polylactic acid-based resin composition is extruded and foamed from a circular die or a flat die while a non-foamed layer is extruded from the same die; an extrusion lamination method in which a polylactic acid-based resin foam sheet having only a foamed layer is produced and then a non-foamed layer is extruded in a sheet form and laminated on the surface of the polylactic acid-based resin foam sheet; or a method in which a polylactic acid-based resin foam sheet having only a foamed layer and a sheet having only a non-foamed layer are produced and then bonded together by a thermal lamination method.

The polylactic acid based resin foam sheet of the present embodiment can be used as a raw material for various sheet molded products.
The sheet molded product may be, for example, a thermoformed product in which a three-dimensional shape is imparted by thermoforming a polylactic acid-based resin foam sheet, or it may be a flat plate-like product in which a polylactic acid-based resin foam sheet has simply been cut, or it may be something like a folding box made by combining such flat plate-like components.

Various methods can be used for the thermoforming, including match mold forming, plug-assisted vacuum forming, vacuum forming, pressure forming, and vacuum-pressure forming.

The sheet molded product can be produced by subjecting a thermoformed polylactic acid resin foam sheet to a trimming process.
The trimming process can be carried out by a conventionally known method, such as a method of punching out only the product portion using a punching press having holes shaped to correspond to the contour shape of the product (sheet molded product) to be produced, or a method of cutting out the parts other than the product using a Thomson blade having a cutting blade erected along the contour shape of the product.

The sheet molded product of the present embodiment is preferably subjected to a heat treatment in the manufacturing process thereof to promote crystallization of the polylactic acid-based resin composition.
This allows the sheet molded product to exhibit high strength and heat resistance.
The degree of crystallinity of the polylactic acid resin composition in the sheet molded product is preferably 35% or more.
The degree of crystallinity of the polylactic acid resin composition in the sheet molded product is more preferably 36% or more, further preferably 37% or more, and particularly preferably 38% or more.
The degree of crystallinity of the polylactic acid resin composition in the sheet molded product is usually 60% or less.

In the sheet molded product of this embodiment, the polylactic acid-based resin foam sheet constituting the sheet molded product has a foam layer composed of a polylactic acid-based resin composition containing a polylactic acid-based resin, and the polylactic acid-based resin composition further contains a butadiene-based elastomer, and the butadiene-based elastomer is contained in the polylactic acid-based resin composition in a ratio of 8 parts by mass to 28 parts by mass when the polylactic acid-based resin is taken as 100 parts by mass, so that the sheet molded product has both excellent impact resistance and heat resistance.

The foam layer of the sheet molded product of this embodiment is composed of the foam layer of the polylactic acid-based resin foam sheet, and since the butadiene-based elastomer is a core-shell type elastomer as described above, similar to the polylactic acid-based resin foam sheet, it achieves both higher impact resistance and heat resistance.

In the foam layer of the sheet molded product of this embodiment, similarly to the polylactic acid-based resin foam sheet, the butadiene-based elastomer is in particulate form and is included in the polylactic acid-based resin composition, forming a matrix phase containing the polylactic acid-based resin and a dispersed phase containing the butadiene-based elastomer, and it is preferable that the particle size of the dispersed phase is 0.1 μm or more and 1 μm or less.

Examples of the sheet molded product of this embodiment include food containers such as lunch boxes, instant noodle containers, gratin containers, fruit containers, and vegetable containers, precision instruments, and cushioning packaging containers for electrical products.
The sheet molded product of this embodiment may be one other than the above-mentioned examples.
Furthermore, the above examples are merely specific cases, and the present invention is not limited to the above examples in any way.

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

A crosslinked polylactic acid resin (hereinafter, also referred to as "XPLA1") was prepared as a base polymer for the polylactic acid resin composition.
The crosslinked polylactic acid resin was prepared by supplying a polylactic acid resin (hereinafter also referred to as "PLA1"), which is a homopolymer of lactic acid (Nature Works'"BiopolymerIngeo4032D", density = 1240 kg/ ^m3 , D-isomer content about 1.5%, MFR = 3.4 g/10 min), and t-butylperoxyisopropyl monocarbonate (Kayaku Akzo's "Kayacarvon BIC-75", 1-minute half-life temperature T1: 158.8°C) (PO) in a mass ratio of 100:0.5 to an extruder (φ57 mm twin-screw extruder) and melt-kneading them in the extruder.

Next, the impact modifier was prepared.
As the impact resistance improver, two types were prepared: a butadiene-based elastomer ("Metablen C-223A" manufactured by Mitsubishi Chemical Corporation) (MBS polymer, hereinafter also referred to as "MBS1") which has a core layer and a shell layer covering the core layer, the core layer being composed of a polymer whose constituent units are butadiene, and the shell layer being composed of a polymer whose constituent units are methyl methacrylate; and a hydrogenated styrene-based thermoplastic elastomer ("Tuftec MP10" manufactured by Asahi Kasei Corporation) (SEBS, hereinafter also referred to as "TPS1").

Example 1
A mixture in which 100 parts by mass of a crosslinked polylactic acid resin (XPLA1) and 10 parts by mass of an impact resistance improver (MBS1) were blended was melt-kneaded in a φ30 mm twin-screw extruder to obtain a melt-kneaded product.
The resulting molten mixture was extruded into a strand shape, cooled in a water tank, and then cut with a pelletizer to obtain pellets of a polylactic acid-based resin composition.
Next, a tandem extruder was prepared, in which a second extruder having a diameter of 65 mm was connected to the tip of a first extruder having a diameter of 50 mm.
Then, the pellets of the polylactic acid resin composition obtained above and a cell regulator master batch containing talc were supplied to the first extruder of the tandem extruder.
The amounts of these materials fed to the first extruder were adjusted so that talc was 1 part by mass per 100 parts by mass of the polylactic acid resin (XPLA1).
These were melt-kneaded in a first extruder, while butane was injected as a foaming agent midway through the first extruder, and further melt-kneaded. The resulting molten and kneaded material was then continuously fed to a second extruder, where it was continued to be melt-kneaded while being cooled to a resin temperature suitable for foaming.
Thereafter, extrusion foaming was carried out through the die slit of a circular die having a slit aperture of 70 mm attached to the tip of the second extruder under conditions of a discharge rate of 28 kg/h and a resin temperature of 168° C., and the cylindrical foam extruded through the die slit was placed on a cooling mandrel and cooled from the inner peripheral surface side, and its outer surface was cooled by blowing air from an air ring, and the foam was cut with a cutter at one point on the mandrel to obtain a band-shaped polylactic acid-based resin foamed sheet.

Next, a match mold type molding machine was prepared, equipped with upper and lower heating plates, heating molding molds in the shape of male and female gratin containers (container opening (including ribs) external dimensions: 114 mm x 175 mm, bottom external dimensions: 80 mm x 125 mm, container depth external dimensions: 26 mm), and cooling molds in the shape of male and female gratin containers of similar shape and size.
The temperature of the heating plate was set to 140°C, and the temperature of the hot molding mold was set to 120°C. The polylactic acid-based resin foam sheet obtained above was set in the molding machine and heated by clamping it between the heating plates for 5 seconds. It was then immediately pressed in the hot molding mold for 60 seconds to mold it and promote crystallization, thereby producing a sheet molded product in the shape of a gratin container.

Example 2
A polylactic acid-based resin foamed sheet was prepared in the same manner as in Example 1, except that the amount of the impact resistance improver (MBS1) was changed from 10 parts by mass to 20 parts by mass, and a sheet molded product in the shape of a gratin container was produced.

Example 3
A polylactic acid-based resin foamed sheet was produced in the same manner as in Example 2 except that cooling during extrusion foaming was strengthened, and a sheet molded product having a shape of a gratin container was produced.

(Comparative Example 1)
A polylactic acid resin foamed sheet was prepared in the same manner as in Example 1 except that no impact resistance improver was used, and a sheet molded product in the shape of a gratin container was produced.

(Comparative Example 2)
A polylactic acid-based resin foamed sheet was prepared in the same manner as in Example 1, except that the amount of the impact resistance improver (MBS1) was changed from 10 parts by mass to 5 parts by mass, and a sheet molded product having a gratin container shape was produced.

(Comparative Example 3)
A polylactic acid-based resin foamed sheet was prepared in the same manner as in Example 1, except that the amount of the impact resistance improver (MBS1) was changed from 10 parts by mass to 30 parts by mass, and a sheet molded product having a gratin container shape was produced.

(Comparative Example 4)
A polylactic acid-based resin foamed sheet was prepared in the same manner as in Example 2, except that the impact resistance improver was changed from the butadiene-based elastomer (MBS1) to a hydrogenated styrene-based elastomer (TPS1), and a sheet molded product in the shape of a gratin container was produced.

(evaluation)
The polylactic acid resin foam sheets were evaluated for the following items.
Evaluation items: "Thickness", "Expansion ratio (expansion ratio)", "Interconnected cell ratio", "Degree of crystallinity", "Maximum load", "Puncture energy", "Heat resistance"

Among the above evaluation items, the "thickness" and "crystallinity" were determined as described above.
The "magnification factor" was determined by first determining the apparent density, then determining the density of the polylactic acid resin, and dividing the density of the polylactic acid resin by the apparent density.

The "open cell ratio" was determined as follows.
(How to determine the open cell ratio)
A number of sheet-like samples measuring 25 mm long x 25 mm wide were cut out from the polylactic acid resin foam sheet.
The cut samples were stacked together so as to leave no gap and a test piece having a thickness of 25 mm was obtained.
The apparent volume (cm ³ ) of the obtained test piece was determined.
The apparent volume (cm ³ ) was determined by measuring the outer dimensions of the test piece to the nearest 1/100 mm.
The measurements were performed using a "Digimatic Caliper" caliper manufactured by Mitutoyo Corporation.
Next, the volume (cm ³ ) of the test piece was determined by the 1-1/2-1 atmospheric pressure method using a Tokyo Science Co., Ltd. "1000 Model" air comparison specific gravity meter.
The open cell ratio (%) was calculated using the following formula.
The open cell ratio was calculated as the average value of five test pieces.
The test pieces were stored in advance for 16 hours under a JIS K7100-1999, symbol 23/50, grade 2 environment, and then used for the measurement.
The measurements were carried out under the same environment.
The air comparison type specific gravity meter was calibrated using standard balls (large 28.96 cm ^{3 ,} small 8.58 cm ³ ).

Open cell rate (%) =
(Apparent volume - Volume measured using an air comparison type specific gravity meter) / Apparent volume x 100 (%)

The "maximum load" and "puncture energy" were measured in accordance with ASTM D-3763-15.
That is, using a drop weight impact tester "CEAST9350" manufactured by CEAST Corporation and measurement software "CEAST VIEW", the measurements were performed with a test specimen size of 100 mm in length × 100 mm in width (the thickness is the thickness of the foamed sheet or the sheet molded product), a test speed of 1.76 m/sec, a drop weight load of 1.9265 kg, a test specimen support span of φ76 mm, and a 4.5 kN instrumented tap (hemispherical tip φ12.7 mm).
The number of test pieces was a minimum of 5, and the test pieces were conditioned for 40 hours in an environment according to Procedure A of ASTM D618-13 (23±2° C., relative humidity 50±10%), and then measurements were performed in the same temperature environment.
The "maximum impact force (F _M )" obtained by the measurement was taken as the "maximum load."
The puncture energy was calculated by automatically calculating the integral value of the graph obtained by the measurement using the measurement software.
The impact strength was evaluated based on the puncture energy value (J) as follows:
(Judgment criteria)
A: 0.17J or more B: 0.11J or more but less than 0.17J C: 0.06J or more but less than 0.11J D: Less than 0.06J

"Heat-resistant"
Regarding the "heat resistance" of the polylactic acid-based resin foam sheet, since crystallization is not sufficient and it is practically difficult to expect high heat resistance, the sheet molded product was rated as having the lowest rating (D rating) as described below.

The sheet molded products were evaluated for the following items:
Evaluation items: "Degree of crystallinity", "Maximum load", "Puncture energy", "Heat resistance (rate of dimensional change due to heating)"

Of the above evaluation items, "degree of crystallinity," "maximum load," and "puncture energy" were determined in the same manner as for polylactic acid resin foam sheets.

The impact strength of the sheet molded product was also judged from the puncture energy value.
However, the evaluation criteria for the sheet molded products were as follows:
(Judgment criteria)
A: 0.30J or more B: 0.15J or more but less than 0.30J C: 0.08J or more but less than 0.15J D: Less than 0.08J

The "heat resistance (rate of dimensional change upon heating)" was determined as follows.
First, a horizontal table was prepared, and a sheet molded product having a gratin container shape was placed on the table to measure the initial height of the molded product.
The initial height (H0) of the molded product (container) was determined by placing the container upside down on a table with the contact surface facing upwards and measuring the height from the table to the contact surface.
The sheet was then placed in a hot air circulating oven at 140° C. and heated for 10 minutes.
After heating for 10 minutes, the sheet molded product was removed from the hot air circulating oven and allowed to cool naturally to room temperature.
The height (H) of the naturally cooled sheet molded product (container) was determined in the same manner as the initial height (H0), and the rate of dimensional change upon heating was calculated as follows.

Heat dimensional change rate (%) = (H (mm) - H0 (mm)) / H0 (mm) x 100 (%)

The heat resistance of the sheet molded product was judged based on the rate of dimensional change upon heating.
The evaluation was based on the following criteria.
(Judgment criteria)
A: The absolute value of the thermal dimensional change rate is 2.0 or less (-2.0% or more, +2.0% or less)
B: The absolute value of the thermal dimensional change rate is greater than 2.0 and less than 5.0 (-5.0% or more but less than -2.0% or greater than +2.0% but less than +5.0%)
C: The absolute value of the thermal dimensional change rate is greater than 5.0 and less than 8.0 (-8.0% or more but less than -5.0% or greater than +5.0% but less than +8.0%)
D: The absolute value of the thermal dimensional change rate exceeds 8.0 (less than -8.0% or more than +8.0%)

The above evaluation results are shown in the table below.

(Confirmation of the dispersion state of butadiene-based elastomer)
The microphase separation structure (dispersion state of the butadiene-based elastomer, which is a core-shell type elastomer) at the pellet stage in Example 1 and Example 2 was confirmed.
In addition, the microphase-separated structure in the polylactic acid resin foam sheet stage and the microphase-separated structure in the sheet molded product were also confirmed.
Here's how to check.

(Dispersion state of polylactic acid-based resin composition)
The resin pellet was fixed to a plastic sample support and then cut in a direction perpendicular to the extrusion direction using a Leica EM UC7 ultramicrotome manufactured by Leica Microsystems and an UltraSonic manufactured by DiATOME to prepare ultrathin sections (70 nm thick).
Next, the ultrathin sections were photographed using a Hitachi High-Technologies Corporation "H-7600" transmission electron microscope and an AMT "ER-B" CCD camera system.
The staining agents used for preparing the ultrathin sections were osmium tetroxide and ruthenium tetroxide.

(Dispersion state of polylactic acid resin foam sheet and sheet molded product)
The polylactic acid resin foam sheet and the molded sheet were sliced in a direction perpendicular to the extrusion direction, and a piece was cut out from near the center in the thickness direction.
The sections were embedded in epoxy resin.
The epoxy resin was cured at 60° C. for 24 hours to produce a cured product.
The cured body was sliced using an ultramicrotome "Leica EM UC7" manufactured by Leica Microsystems and "UltraSonic" manufactured by DiATOME to prepare ultrathin sections (thickness 70 nm).
Next, the ultrathin sections were photographed using a Hitachi High-Technologies Corporation "H-7600" transmission electron microscope and an AMT "ER-B" CCD camera system.
The staining agents used for preparing the ultrathin sections were osmium tetroxide and ruthenium tetroxide.

(How to determine the average particle size of the dispersed phase)
The average particle size of the dispersed phase was determined by analyzing images taken with a transmission electron microscope at a magnification of 5000 times using general-purpose image processing software (NanoHunter NS2K-Pro/Lt, manufactured by NanoSystems Co., Ltd.).
The average particle size was determined by averaging the particles having a particle size of 10 nm or more in the dispersed phase.
The results are shown in the table below.

The above results show that the present invention achieves both impact resistance and heat resistance in a polylactic acid resin foam sheet, providing a sheet molded product with excellent impact resistance and heat resistance.

Claims (11)

A polylactic acid-based resin foam sheet having a foam layer made of a polylactic acid-based resin composition containing a polylactic acid-based resin,
The polylactic acid-based resin composition further contains a butadiene-based elastomer,
The polylactic acid-based resin composition contains the butadiene-based elastomer in a particulate form,
A matrix phase containing the polylactic acid resin;
and a dispersed phase including the butadiene-based elastomer is formed.
The particle size of the dispersed phase is 0.1 μm or more and 1 μm or less,
The polylactic acid-based resin foam sheet comprises a polylactic acid-based resin composition containing the butadiene-based elastomer in an amount of 8 parts by mass or more and 28 parts by mass or less per 100 parts by mass of the polylactic acid-based resin. The polylactic acid resin foam sheet according to claim 1, wherein the butadiene elastomer is a core-shell type elastomer. The core-shell elastomer comprises a core layer and a shell layer covering the core layer,
The core layer is composed of a polymer having butadiene as a structural unit,
3. The polylactic acid resin foam sheet according to claim 2, wherein the shell layer is composed of a polymer having methyl methacrylate as a structural unit. 4. The polylactic acid resin foam sheet according to claim 1, wherein the polylactic acid resin composition has a crystallinity of 5% or more and 15% or less. 5. The polylactic acid resin foam sheet according to claim 1, wherein the foam layer has an apparent density of 63 kg/ ^m3 or more and 500 kg/ ^m3 or less. 6. The polylactic acid resin foam sheet according to claim 1, wherein the foam layer has a thickness of 0.5 mm or more and 7 mm or less. A sheet molded product made of a polylactic acid resin foam sheet,
The polylactic acid-based resin foam sheet has a foam layer made of a polylactic acid-based resin composition containing a polylactic acid-based resin,
The polylactic acid-based resin composition further contains a butadiene-based elastomer,
The polylactic acid-based resin composition contains the butadiene-based elastomer in a particulate form,
A matrix phase containing the polylactic acid resin;
and a dispersed phase including the butadiene-based elastomer is formed.
The particle size of the dispersed phase is 0.1 μm or more and 1 μm or less,
A sheet molded article in which the polylactic acid-based resin composition contains the butadiene-based elastomer in an amount of 8 parts by mass or more and 28 parts by mass or less per 100 parts by mass of the polylactic acid-based resin. 8. The sheet molded product according to claim 7 , wherein the butadiene-based elastomer is a core-shell type elastomer. The core-shell elastomer comprises a core layer and a shell layer covering the core layer,
The core layer is composed of a polymer having butadiene as a structural unit,
9. The sheet molded product according to claim 8 , wherein the shell layer is composed of a polymer having methyl methacrylate as a structural unit. 10. The sheet molded article according to claim 7 , wherein the polylactic acid-based resin composition has a crystallinity of 35% or more. The sheet molded product according to any one of claims 7 to 10 , which is a food container.