JP7688517B2 - Resin composition containing crosslinked polyhydroxyalkanoic acid - Google Patents

Description

The present invention relates to at least an article containing a crosslinked polyhydroxyalkanoic acid obtained from a resin composition containing polyhydroxyalkanoic acid, the resin composition, and the crosslinked polyhydroxyalkanoic acid, which has high heat resistance.

Biodegradable polymers retain their practical physical properties during normal use and are digested and decomposed by microorganisms in the soil and ocean after use, so they are attracting attention as materials that do not put a strain on the environment, and it is expected that their use will expand in various fields in the future.
Among these, from the viewpoints of biodegradability and carbon neutrality, aliphatic polyester-based resins have attracted attention as plant-derived plastics, and in particular, polyhydroxyalkanoate (polyhydroxyalkanoic acid, hereinafter sometimes referred to as PHA)-based resins such as poly(3-hydroxybutyrate) homopolymer resin (hereinafter sometimes referred to as PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resin (hereinafter sometimes referred to as PHBV), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resin (hereinafter sometimes referred to as PHBH), poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer resin, and polylactic acid have attracted attention.
However, Non-Patent Document 1 points out that although polylactic acid decomposes in the hot and humid environment of compost, it is difficult to decompose in normal soil or water environments, and therefore PHA is more desirable for uses other than composting.

Since PHA resins have a low glass transition temperature of -7 to 4°C, if they are used as tableware, the molded products will deform when exposed to hot water from a pot, hot air from a dish dryer, or the heat used to sterilize medical parts. Therefore, cross-linking the resin with radiation is thought to be the most effective way to improve the heat resistance of such materials.

In order to meet the above-mentioned heat resistance requirement, for example, Patent Document 1 proposes a crosslinkable resin composition in which polylactic acid, polyhydroxybutyric acid, and polybutylene succinate are mixed with a polyfunctional acrylic monomer or a polyfunctional allylic monomer, and then crosslinked by irradiating with active energy rays.

Moreover, Patent Document 2 proposes a production method in which polybutylene succinate is mixed with polylactic acid and an allyl monomer, and then crosslinked.
Moreover, Patent Document 3 proposes a production method in which polybutylene succinate is mixed with an inorganic substance and then crosslinked.

Japanese Patent Application Publication No. 10-1447720 Patent No. 3759067 Patent No. 3581138

"Challenges and Future Prospects of Biodegradable Plastics" Mitsubishi Research Institute Report, April 2019

However, the inventors have clarified through experiments that, contrary to conventional knowledge, aliphatic polyester resins such as polybutylene succinate and PHA resins are difficult to crosslink by radiation. The inventors have considered that the applications of articles made of the above resins are limited due to the difficulty in increasing the molecular weight and limitations in physical properties.
In this context, an object of the present invention is to provide an article made of, for example, an aliphatic polyester-based resin having a structure different from that of conventional ester-based resins.

The inventors conducted extensive research to achieve the above objective, and discovered that it may be possible to control the structure of aliphatic polyesters by using a certain type of monomer together with PHA, which is a type of aliphatic polyester resin. After further research, they completed the present invention. That is, the present invention is as follows.

[1]
An article comprising a crosslinked polyhydroxyalkanoic acid, the average molecular weight of which measured by size exclusion chromatography is 320,000 or more, or the weight loss rate after refluxing in chloroform for 4 hours is less than 30%.
[2]
The article according to [1], which has a storage modulus (E') at 200°C of 1.00 x ¹⁰⁵ Pa or more.
[3]
The article according to [1] or [2], wherein the polyhydroxyalkanoic acid comprises 3-hydroxybutyrate, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), or a copolymer thereof.
[4]
An article obtained by blending 0.1 to 20 parts by weight of a crosslinkable monomer containing an allyl group or an acryl group with 100 parts by weight of a polyhydroxyalkanoic acid or a copolymer of a hydroxyalkanoic acid, and irradiating the blend with ionizing radiation.
[5]
The article according to [4], wherein the ionizing radiation is an electron beam and the exposure dose is 1 kGy or more and 200 kGy or less.
[6]
A method for sterilizing the article according to any one of [1] to [5], or the crosslinked polymer-containing resin composition constituting the article, using an electron beam, wherein the average molecular weight of the crosslinked polymer containing the polyhydroxyalkanoic acid after sterilization is 320,000 or more as measured by size exclusion chromatography, or the article after sterilization or the crosslinked polymer-containing resin composition constituting the article has a weight loss rate of less than 30% after refluxing in chloroform for 4 hours.
[7]
Crosslinked polymers comprising polyhydroxyalkanoic acids or copolymers of hydroxyalkanoic acids crosslinked with crosslinkable monomers containing allyl or acrylic groups.
[8]
The crosslinked polymer according to [7], having an average molecular weight of 320,000 or more as measured by size exclusion chromatography.
[9]
The crosslinked polymer according to [7] or [8], wherein the hydroxyalkanoic acid includes 3-hydroxybutyrate, 3-hydroxybutyrate-co-3-hydroxyvalerate, or 3-hydroxybutyrate-co-3-hydroxyhexanoate.
[10]
A resin composition comprising one or more of polyhydroxyalkanoic acids or copolymers of hydroxyalkanoic acids and mixtures thereof, and one or more crosslinking monomers, wherein the polyhydroxyalkanoic acid or copolymer of hydroxyalkanoic acids is poly 3-hydroxybutyrate, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate).
[11]
A method for producing a crosslinked polymer containing a polyhydroxyalkanoic acid, comprising irradiating the resin composition according to [10] with ionizing radiation to obtain a crosslinked polymer containing a crosslinked polyhydroxyalkanoic acid.
[12]
The method according to [11], comprising blending 0.1 to 20 parts by weight of a crosslinkable monomer containing an allyl group or an acryl group with respect to 100 parts by weight of a polyhydroxyalkanoic acid or a copolymer of a hydroxyalkanoic acid.

According to the present invention, a biodegradable crosslinked polymer containing polyhydroxyalkanoic acid is provided. By using such a crosslinked polymer, molded articles, films/sheets, threads, particles, and the like containing polyhydroxyalkanoic acid and having a high storage modulus can be provided.
The article or crosslinked polymer of the present invention has excellent storage modulus and heat resistance. In addition, the article or crosslinked polymer of the present invention can be suitably sterilized using electron beams. Therefore, the article or crosslinked polymer of the present invention is suitable for use in the manufacture or application of instruments that require sterilization and require importance of cleanliness, and that are sterilized by electron beams.
Without being bound by theory, it is speculated that the present invention provides a crosslinked polymer as follows. When irradiated with radiation, generally, reactive species such as radicals are generated in polymeric materials, which become the starting point of chemical reactions, causing crosslinking, molecular chain scission, etc. Hydroxyalkanoic acid easily generates reactive species in the main chain and also has excellent compatibility with crosslinkable monomers, so crosslinking proceeded.
In contrast, in the case of PBS, which has characteristics such as low crystallinity and a large amount of secondary carbon and is theoretically more likely to crosslink, crosslinking of the resin by radiation is unlikely to occur as described above.
For these reasons, even a person skilled in the art would never have imagined the crosslinked polymer of the present invention or its production method.

The four photographs show examples of the appearance of the resin composition and article of the present invention according to the manufacturing process. From the left, the four photographs show PHA ("PHA only"), a crosslinkable resin composition ("PHA + crosslinkable monomer") obtained by blending and kneading various predetermined materials, an article precursor ("press film") obtained by pressing the crosslinkable resin composition, and an article obtained by irradiating the article precursor with an electron beam ("after electron crosslinking"). 1 is a graph showing the results of producing a crosslinked polymer in the examples by changing the amount of crosslinking monomer added and the intensity of the electron beam. When no crosslinking monomer was used, PHBV was decomposed by irradiation with the electron beam and the molecular weight was reduced. In contrast, when the crosslinking monomer was used, a product with a higher molecular weight was produced, indicating that the PHBV was crosslinked.

Polyhydroxyalkanoates or polyhydroxyalkanoic acids (sometimes abbreviated as "PHA" in this specification) are polymers containing hydroxyalkanoic acids as monomers, which are linearly polymerized. There are no limitations on the type of monomer, and the monomer may be a single monomer or a copolymer containing two or more types of monomers.

The "crosslinked polyhydroxyalkanoic acid" in the present invention is a crosslinked polymer in which polyhydroxyalkanoic acid is crosslinked by a crosslinkable monomer. The type of polymer is not limited, and may be the same polyhydroxyalkanoic acid as the polyhydroxyalkanoic acid, may be a polyhydroxyalkanoic acid different from the polyhydroxyalkanoic acid, or may be a polymer such as polybutylene succinate (PBS) or polylactic acid. That is, the "crosslinked polyhydroxyalkanoic acid" in the present invention includes molecules in which PBS or lactic acid is crosslinked with polyhydroxyalkanoic acid. The "resin composition containing one or more of polyhydroxyalkanoic acid or copolymers of hydroxyalkanoic acid and their mixtures, and a crosslinkable monomer" in the present invention is a crosslinkable resin composition in which the above-mentioned components are blended and before irradiation with ionizing radiation is applied. The crosslinkable resin composition becomes a resin composition containing crosslinked polyhydroxyalkanoic acid by irradiation with ionizing radiation.
In this specification, a resin composition containing a crosslinked polymer that includes (contains) a crosslinked polyhydroxyalkanoic acid may be referred to as a "crosslinked polymer-containing resin composition."
The crosslinkable resin composition before irradiation with ionizing radiation and the crosslinked polymer-containing resin composition may also contain a polymer such as polybutylene succinate (PBS) or polylactic acid. The composition containing a crosslinked polyhydroxyalkanoic acid constituting the article of the present invention does not include a composition consisting of only a polymer in which only polybutylene succinate (PBS) is crosslinked or a polymer in which only polylactic acid is crosslinked.
The term "article" in the present invention includes any object that can be thought of as having a certain shape, such as a molded object, a film/sheet, thread, and particles, but is not limited to these.

[1. Film containing crosslinked polyhydroxyalkanoic acid]
Taking a film as an example of the article containing the polyhydroxyalkanoic acid of the present invention, the storage modulus (E') of the film at 200° C. measured under the following measurement conditions is set in the range of 1.00×10 ⁵ Pa or more.
Measurement conditions for storage modulus (E'): According to JIS K7244-1 and 7244-4, a sheet of a crosslinked polymer-containing resin composition having a width of 5 mm and a thickness of 150 μm is measured at a clamp distance of 10 mm, a starting temperature of -25°C, an end temperature of 200°C, a heating rate of 5°C/min, and a measurement frequency of 1 Hz.

The storage modulus (E') at 200° C. is preferably at least 1.0×10 ⁵ Pa. By making the storage modulus at or above the lower limit, the dimensional stability of the crosslinked polymer-containing resin composition (film) can be made favorable.
Among the articles containing the polyhydroxyalkanoic acid of the present invention, those containing the polyhydroxyalkanoic acid as a film are also preferred, which have an average molecular weight of 320,000 or more as measured by size exclusion chromatography, or have a weight loss rate of less than 30% after refluxing with chloroform for 4 hours. It is clear that crosslinking has occurred in the polyhydroxyalkanoic acid having these characteristics.

An example of the method for measuring the weight loss rate is as follows: The method for measuring the weight loss rate can be modified as long as the same weight loss rate is obtained.
(Example of method for measuring weight loss rate)
The test piece (weighing value is A(g)) is refluxed in chloroform for 4 hours. After completion, the test piece is taken out, dried at 50°C for 24 hours, and the mass is measured (weighing value is B(g)). The weight loss rate (%) is calculated using the following formula.

Weight reduction rate (%) = 100 x (AB)/A

A low weight loss rate after crosslinking treatment, that is, reduced solubility in chloroform, indicates that the crosslink density of the resin is high and the molecular weight is increased.
In the film containing the polyhydroxyalkanoic acid of the present invention, the weight loss rate is 30% or less, preferably 20% or less, and more preferably 10% or less.

<Polyhydroxyalkanoic acid>
The polyhydroxyalkanoic acid that can be used in the article containing the polyhydroxyalkanoic acid of the present invention is not particularly limited, and examples thereof include poly(3-hydroxybutyrate) homopolymer resin, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resin, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resin, poly(3-hydroxybutyrate-co-4-hydroxybutyrate) copolymer resin, and polylactic acid.
Among these, poly(3-hydroxybutyrate) homopolymer resin, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resin, and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resin are particularly excellent in biodegradability, and are therefore suitable for applications other than compost bags, such as disposable tableware, medical parts, agricultural and horticultural parts, forestry parts, and fishing parts, and materials using these parts.

It has been confirmed that even when poly(3-hydroxybutyrate) homopolymer resin (sometimes referred to as "PHB" in this specification), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resin (sometimes referred to as "PHBV" in this specification), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resin (sometimes referred to as "PHBH" in this specification), and polybutylene succinate (sometimes referred to as "PBS" in this specification) are irradiated with an electron beam (for example, absorbed dose: 100 kGy), the molecular weight is reduced due to the scission of molecular chains that occurs simultaneously with crosslinking. However, the present inventors have found that poly(3-hydroxybutyrate) homopolymer resin, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resin, and poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resin contain the crosslinkable monomer according to the present invention, and thus crosslinking reaction occurs effectively when irradiated with an electron beam. The crosslinked polymer obtained by this crosslinking reaction has excellent heat resistance, can withstand boiling water, and prevents the resin from deforming.

It was confirmed that the molecular weight of polybutylene succinate, either alone or in a mixture with a crosslinkable monomer, was reduced by decomposition rather than by crosslinking. Patent Document 2 and Non-Patent Document 1 are examples of previous cases in which crosslinking was successful using electron beams, but both were evaluated by gel fraction measurement using a 200 mesh, and it is presumed that the mesh was clogged with resin decomposition products and inorganic substances, which inhibited elution and led to the erroneous recognition that crosslinking had occurred.
In contrast, in the present invention, size exclusion chromatography was performed, and the phenomenon of molecular weight reduction could be confirmed by measuring the change in molecular weight in more detail. The storage modulus also decreased, and the result correlated with the decrease in molecular weight. However, it was confirmed that even polybutylene succinate can effectively cause a crosslinking reaction by electron beam irradiation by mixing it with poly(3-hydroxybutyrate) homopolymer resin, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resin, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resin and a crosslinkable monomer. That is, the article of the present invention may contain a biodegradable polymer other than polyhydroxyalkanoic acid, such as polybutylene succinate or polylactic acid. From the above points, it can be said that PHA is extremely effective in the present invention.
Among the articles of the present invention, those containing a crosslinked polymer consisting only of polyhydroxyalkanoic acid or a copolymer of hydroxyalkanoic acid as the crosslinked polymer are preferred, and those in which the polyhydroxyalkanoic acid or the copolymer of hydroxyalkanoic acid constituting the crosslinked polymer is 3-hydroxybutyrate, 3-hydroxybutyrate-co-3-hydroxyvalerate, and/or 3-hydroxybutyrate-co-3-hydroxyhexanoate are even more preferred.

The molecular weight of the polyhydroxyalkanoic acid before electron beam irradiation is preferably 100,000 or more and 5,000,000 or less in terms of polystyrene equivalent weight average molecular weight Mw measured by size exclusion chromatography, from the viewpoints of flowability and processability.
The crosslinked polymers used in the articles of the present invention are preferably those which are insoluble in chloroform and have a high molecular weight, such as a molecular weight of more than 5 million.

<Crosslinking Monomer>
The crosslinkable monomer according to the present invention is not particularly limited as long as it is a monomer that can be crosslinked by irradiation with ionizing radiation, and examples thereof include acrylic or methacrylic crosslinkable monomers, and allyl monomers.

The molecular weight of the crosslinkable monomer according to the present invention is preferably 1000 or less, more preferably 500 or less, and even more preferably 300 or less. By having a molecular weight of 1000 or less, it is possible to prevent the dispersibility in the resin composition from decreasing, and to more efficiently cause an effective crosslinking reaction by electron beam irradiation.

The melting point of the crosslinkable monomer is preferably equal to or lower than the melting point of the polyhydroxyalkanoic acid used, for example, 120° C. or lower.
The above-mentioned crosslinkable monomers have excellent fluidity during processing, and therefore can lower the processing temperature of the thermoplastic resin, thereby reducing the thermal load, or reduce friction during processing.

Specifically, examples of the acrylic or methacrylic crosslinkable monomer include acrylate derivatives and methacrylate derivatives. Specific examples of the di(meth)acrylate include urethane acrylate, rosin acrylate, 1,6-hexanediol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, trimethylolpropane tri(meth)acrylate, ethylene oxide modified trimethylolpropane tri(meth)acrylate, propylene oxide modified trimethylolpropane tri(meth)acrylate, ethylene oxide modified bisphenol A di(meth)acrylate, diethylene glycol di(meth)acrylate, dipentaerythritol hexaacrylate, dipentaerythritol monohydroxypentaacrylate, caprolactone modified dipentaerythritol hexaacrylate, pentaerythritol tri(meth)acrylate, pentaerythritol tetra(meth)acrylate, polyethylene glycol di(meth)acrylate, tris(acryloxyethyl)isocyanurate, and tris(methacryloxyethyl)isocyanurate.
As the acrylic or methacrylic crosslinkable monomer, urethane acrylate and rosin acrylate are preferred.

Examples of allyl-based crosslinkable monomers containing an allyl group include triallyl isocyanurate, trimethallyl isocyanurate, triallyl cyanurate, trimethallyl cyanurate, diallylamine, triallylamine, diacrylchlorentate, allyl acetate, allyl benzoate, allyl dipropyl isocyanurate, allyl octyl oxalate, allyl propyl phthalate, bityl allyl malate, diallyl adipate, diallyl carbonate, diallyl dimethyl ammonium chloride, diallyl fumarate, diallyl isophthalate, diallyl malonate, diallyl oxalate, diallyl phthalate, diallyl propyl isocyanurate, diallyl sebacate, diallyl succinate, diallyl terephthalate, diallyl tartrate, dimethyl allyl phthalate, ethyl allyl malate, methyl allyl fumarate, methyl methallyl malate, and diallyl monoglycidyl isocyanurate.
Triallyl isocyanurate and trimethallyl isocyanurate are preferred, with triallyl isocyanurate being more preferred.

The crosslinking monomer according to the present invention is blended in an amount of more than 0.1 part by weight and not more than 20 parts by weight, preferably 2 to 15 parts by weight, and more preferably 4 to 12 parts by weight, per 100 parts by weight of polyhydroxyalkanoic acid. By blending in such an amount, the crosslinking reaction can proceed effectively without bleeding out.

<Method of manufacturing the article of the present invention>
The article of the present invention can be produced by preparing a crosslinked polymer from a crosslinkable resin composition obtained by mixing at least the above-mentioned polyhydroxyalkanoic acid and crosslinkable monomer with a rosin derivative.

The rosin derivative acts as a crystal nucleating agent in the preparation of the crosslinked polymer of the present invention. Examples of the rosin derivative include raw rosins such as gum rosin, wood rosin, or tall oil rosin, stabilized rosins and polymerized rosins obtained by disproportionating or hydrogenating the raw rosins, and other rosin esters, reinforced rosin esters, rosin phenols, and rosin-modified phenolic resins. These may be used alone or in combination of two or more.
Rosin esters refer to products obtained by esterification reaction of raw rosins with alcohols. Rosin phenols refer to products obtained by adding phenols to raw rosins and thermally polymerizing them, or by subsequently esterifying them. Compounds obtained by addition reaction of alkylene oxides such as ethylene oxide and propylene oxide to the above-mentioned rosins can also be used in the same manner as the above-mentioned rosin esters.
As the rosin derivative, a disproportionated rosin partial metal salt alone or a mixture of a disproportionated rosin partial metal salt and magnesium oxide is preferred.

To mix the above components to obtain a crosslinkable resin composition, known means can be used, such as a mixer such as a two-roll or three-roll, homogenizer, or planetary mixer, or a melt kneader such as a Polylab System or Labo Plastomill, or a single-screw or twin-screw kneader. These may be performed at room temperature, in a cooled state, in a heated state, or under normal pressure, reduced pressure, or pressurized state.

In addition, various additives can be added to the crosslinkable resin composition of the present invention as long as the effect of the present invention is not impaired. For example, in order to improve the properties of the crosslinkable resin composition, additives such as various whiskers, silicone powder, thermoplastic elastomers, organic synthetic rubbers, fatty acid esters, glyceric acid esters, zinc stearate, stearate esters, calcium stearate, etc., internal mold release agents, antioxidants such as benzophenones, salicylic acid, cyanoacrylates, isocyanurates, anilide oxalates, benzoates, hindered amines, benzotriazoles, phenols, etc., light stabilizers such as hindered amines and benzoates, and dispersants such as silane coupling agents can be added.
The crosslinkable resin composition of the present invention may contain a polymer other than polyhydroxyalkanoic acid, such as polybutylene succinate or polylactic acid. Of the crosslinkable resin compositions of the present invention, compositions containing only polyhydroxyalkanoic acid or a copolymer of hydroxyalkanoic acid as the polymer are preferred. Of the crosslinkable resin compositions of the present invention, compositions in which the polyhydroxyalkanoic acid or the copolymer of hydroxyalkanoic acid is 3-hydroxybutyrate, 3-hydroxybutyrate-co-3-hydroxyvalerate, and/or 3-hydroxybutyrate-co-3-hydroxyhexanoate are even more preferred.

By using the crosslinkable resin composition of the present invention, various articles can be produced, and even thinner articles can be produced. That is, the crosslinkable resin composition of the present invention is irradiated with electron beams to cause a crosslinking reaction with a crosslinkable monomer, and polymers containing polyhydroxyalkanoic acid are crosslinked to each other, thereby producing an article made of a cured resin composition (a crosslinked polymer-containing resin composition).
Such an article is preferably produced by the method of the present invention. That is, as the article of the present invention, a film is preferably produced by a method including a step of forming the crosslinkable resin composition of the present invention into a film at a cylinder temperature of 150 to 200° C. and a mold temperature of 40 to 80° C., and an electron beam irradiation step of carrying out an electron beam irradiation treatment before or after the step.
Alternatively, the film of the present invention may be obtained by subjecting a precursor obtained by pressing the crosslinkable resin composition of the present invention to electron beam irradiation treatment.
As long as the moldability of the crosslinkable resin composition of the present invention is not impaired, the crosslinking reaction by electron beam irradiation to obtain the crosslinked polymer or article of the present invention can be carried out before a molding process such as press working, such as a film formation process.

The acceleration voltage of the electron beam can be appropriately selected depending on the resin used and the thickness of the layer. For example, in the case of an article having a thickness of 0.1 to 10 mm, it is preferable to cure the uncured resin layer at an acceleration voltage of about 250 to 3000 kV. In addition, in the case of irradiation with electron beams, the higher the acceleration voltage, the higher the penetration ability. Therefore, when a substrate that deteriorates due to electron beams is used as the substrate, the acceleration voltage is selected so that the penetration depth of the electron beam and the thickness of the resin layer are substantially equal, thereby suppressing the irradiation of excess electron beams on the substrate and minimizing the deterioration of the substrate due to excess electron beams.
In addition, the absorbed dose when irradiating with electron beams is appropriately set depending on the composition of the crosslinkable resin composition. There is no particular limit to the increase in the irradiation dose as long as the physical properties are not impaired. However, in consideration of practicality, an amount at which the crosslink density of the resin layer is saturated is preferable. A specific irradiation dose is preferably 0.1 to 200 kGy (0.1 kGy or more and 200 kGy or less; the meaning of "to" is the same above and below), more preferably 1 to 200 kGy, and even more preferably 10 to 100 kGy.
Furthermore, the electron beam source is not particularly limited, and various electron beam accelerators such as Cockcroft-Walton type, Van de Graft type, resonant transformer type, insulating core transformer type, linear type, dynamitron type, and high frequency type can be used.
By applying an ionizing radiation curable functional coating to the surface of the crosslinkable resin composition of the present invention and then irradiating it with electron beams, the crosslinkable resin composition and the coating can be cured simultaneously, and thus desired functionality can be imparted in addition to heat resistance. Examples of such functionality include imparting scratch resistance, improving chemical resistance and contamination resistance, and improving weather resistance.

2. Crosslinked Polymers
The present invention also provides a crosslinked polymer comprising a polyhydroxyalkanoic acid or a copolymer of hydroxyalkanoic acid crosslinked with a crosslinkable monomer containing an allyl group or an acrylic group. Among the crosslinked polymers of the present invention, a crosslinked polymer in which a polyhydroxyalkanoic acid or a copolymer of hydroxyalkanoic acid is crosslinked with a crosslinkable monomer containing an allyl group or an acrylic group is preferred.
Among such crosslinked polymers, a crosslinked polymer having an average molecular weight of 320,000 or more as measured by size exclusion chromatography is preferred. Among such crosslinked polymers, a crosslinked polymer having a weight loss rate of less than 30% after refluxing in chloroform for 4 hours is also preferred.
Among such crosslinked polymers, crosslinked polymers in which the hydroxyalkanoic acid is 3-hydroxybutyrate, 3-hydroxybutyrate-co-3-hydroxyvalerate, or 3-hydroxybutyrate-co-3-hydroxyhexanoate are also preferred.
Among the crosslinked polymers of the present invention, poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer resin (or poly(3-hydroxybutyrate-co-3-hydroxyvalerate) copolymer: PHBV) is preferred. The molar ratio of 3-hydroxybutanoic acid and 3-hydroxyvaleric acid constituting the PHBV of the present invention is not limited. PHBV of the present invention having a 3HV fraction (the ratio of the number of moles of 3-hydroxyvaleric acid to the sum of the number of moles of 3-hydroxybutanoic acid and the number of moles of 3-hydroxyvaleric acid constituting the copolymer) of about 0.1% to about 28.0% is preferred, PHBV of the present invention having a 3HV fraction of about 0.5% to 18.0% is more preferred, and PHBV of the present invention having a 3HV fraction of about 1.0% to about 15.0% is even more preferred.
Among the crosslinked polymers of the present invention, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer resin (or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) copolymer: PHBH) is preferred. The molar ratio of 3-hydroxybutanoic acid and 3-hydroxyhexanoic acid constituting the PHBH of the present invention is not limited. PHBH of the present invention having a 3HH fraction (the ratio of the number of moles of 3-hydroxyhexanoic acid to the sum of the number of moles of 3-hydroxybutanoic acid and the number of moles of 3-hydroxyhexanoic acid constituting the copolymer) of about 0.1% to about 28.0% is preferred, PHBH of the present invention having a 3HH fraction of about 0.5% to 18.0% is more preferred, and PHBH of the present invention having a 3HH fraction of about 1.0% to about 15.0% is even more preferred.

The crosslinked polymer of the present invention may be prepared from a crosslinkable resin composition obtained by mixing a rosin derivative with a polyhydroxyalkanoic acid and a crosslinkable monomer. The above-mentioned polyhydroxyalkanoic acid, crosslinkable monomer, and rosin derivative may be used for the preparation.

An example of the method for measuring the weight loss rate of the crosslinked polymer of the present invention is as follows: The method for measuring the weight loss rate can be modified within a range in which the same weight loss rate can be obtained.

(Example of method for measuring weight loss rate)
The test piece (weighing value is A(g)) is refluxed in chloroform for 4 hours. After completion, the test piece is taken out, dried at 50°C for 24 hours, and the mass is measured (weighing value is B(g)). The weight loss rate (%) is calculated using the following formula.

Weight reduction rate (%) = 100 x (AB)/A

A low weight loss rate after crosslinking treatment, that is, reduced solubility in chloroform, indicates that the crosslink density of the resin is high and the molecular weight is increased.
In the crosslinked polymer of the present invention, the weight loss rate is 30% or less, preferably 20% or less, and more preferably 10% or less.

[3. Uses of the Article of the Present Invention]
The article or crosslinked polymer of the present invention has excellent storage modulus and heat resistance. In addition, the article or crosslinked polymer of the present invention can be suitably sterilized using electron beams. Therefore, the resin composition containing the article or crosslinked polymer of the present invention is suitable for use in the manufacture or application of instruments in which cleanliness is important and sterilization by electron beams is performed.
Examples of such implements and uses include uses other than compost bags, such as disposable tableware, medical parts, agricultural and horticultural parts, forestry parts, and fishing parts, and materials using these parts.

The method of manufacturing disposable tableware, medical parts, agricultural and horticultural parts, forestry parts, and fishing parts using the article of the present invention, and materials using these parts, is not limited, and a method that is common in this technical field may be used. For example, when the article of the present invention is a film, the film may be laminated with layers according to the application to obtain a material, and the material may be processed into the desired shape to manufacture each of the above-mentioned materials.

The present invention will now be described in more detail with reference to examples, but the present invention is not limited to these examples in any way.
The materials used in Examples 1 to 13 and Comparative Examples 1 to 10 are as follows.

(A) Resin/PHA-1
PHBV: ENMAT Y1000P (product name, manufactured by TiaAn Biologic Materials)
・PHA-2
PHB: ENMAT Y3000P (product name, manufactured by TiaAn Biologic Materials)
・PHA-3
PHBH: 3-hydroxybutanoic acid-3-hydroxyhexanoic acid polymer With reference to Biomacromolecules 2002, 3, 208-213, Cupriavidus necator NCIMB11599 was cultured in a 5 L jar fermenter. After the culture, the cells were dissolved in chloroform, filtered to remove the cells, and then reprecipitated in methanol, recovered, and vacuum dried for use.
PBS: BioPBS FZ91PM (product name, manufactured by Mitsubishi Chemical Corporation)

(B) Crosslinkable Monomer Crosslink-1: Triallyl isocyanurate: TAIC (product name, manufactured by Nippon Kasei Chemical Co., Ltd.)
Crosslinking-2: Urethane acrylate: Beamset 550B (product name, manufactured by Arakawa Chemical Industries, Ltd.)
Crosslinking-3: Rosin acrylate: Beamset 101 (product name, manufactured by Arakawa Chemical Industries, Ltd.)

(C) Rosin ester derivative: KM-1500 (product name, manufactured by Arakawa Chemical Industries, Ltd.)
・KR-50M (product name, manufactured by Arakawa Chemical Industries, Ltd.)

[Examples 1 to 13, Comparative Examples 1 to 10]
Materials and Methods
Various materials were blended and kneaded as shown in Tables 1 and 2 below to obtain a resin composition (which may be a crosslinkable resin composition). The composition was obtained by blending various materials and using a resin kneader (product name: Labo Plastomill 3S150, manufactured by Toyo Seiki Seisakusho Co., Ltd.). The "parts" in the tables indicate parts by weight.
These compositions were pressed at 150-180°C, 30 seconds, and 20 MPa to produce article precursors (which may be crosslinkable resin compositions) of 100 (±1) mm x 100 (±0.1) mm x 0.15 (±0.1) mm thick before electron beam irradiation. The article precursors were irradiated with electron beams in an electron beam irradiation device at an accelerating voltage of 250 kV and an absorbed dose of 10-200 kGy to obtain films (Figure 1).
For the sample of each example article, the following characteristics were evaluated according to the methods and indices described for each.
The results are shown in the following Tables 1 and 2. The absorbed dose of the electron beam irradiation conditions used in obtaining the sample of each example article is also shown in the following Tables 1 and 2.

(Rating 1)
Storage modulus The sample of the article was measured using RSA3 (trade name, manufactured by TA INSTRUMENTS) under the conditions of a measurement temperature of 25 to 200° C., a temperature rise rate of 5° C./min, a measurement frequency of 1 Hz, and a strain of 0.1%. The storage modulus at 200° C. is shown in Tables 1 and 2 below.

(Rating 2)
Molecular weight The molecular weight of the mixture (resin composition) or the sample after electron beam irradiation was measured by size exclusion chromatography (GPC gel filtration chromatography) using polystyrene standards. For the measurement, EcoSEC HLC-8320GPC (trade name) manufactured by Tosoh Corporation was used. The guard column was TSKgel guardcolumn SuperHZ-H (trade name), and the column was TSKgel Super HZM-H (trade name) in series. Chloroform (0.6 mL/min) was used as the mobile phase, and the column temperature was 40°C. The sample concentration was about 0.5 mg/mL, and the sample injection amount was 10 μL. A polystyrene standard was used to create the calibration curve. The results are shown in Tables 1-2 below.

(Rating 3)
Weight loss rate: A test piece (weighing value: A(g)) was refluxed in chloroform for 4 hours. After completion, the test piece was taken out, dried at 50°C for 24 hours, and the mass was measured (weighing value: B(g)). The weight loss rate (%) was calculated using the following formula.

Weight reduction rate (%) = 100 x (AB)/A

The results are shown in Tables 1 and 2 below.

(Rating 4)
Dimensional stability: A sample was cut into a 100 mm square, immersed in a 200°C talc bath for 10 minutes, and then left to cool to room temperature (25°C). The appearance was checked to see whether the sample had changed shape before and after heating. The results are shown in Tables 1 and 2.
〇: No change in appearance ×: Change in appearance

<Results>
As is clear from the results of the above-mentioned Examples and Comparative Examples, the crosslinkable resin composition containing the polyhydroxyalkanoic acid of the present invention and a specific crosslinkable monomer can be used to produce an article having a storage modulus (E') at 200°C of 1.0 x ¹⁰⁵ Pa or more in the article precursor. In other words, by using the above-mentioned composition for the crosslinkable resin composition, a crosslinked polymer-containing resin composition can be obtained that exhibits excellent heat distortion resistance when made into an article.
In addition, the molecular weight of each of the crosslinked polymer-containing resin compositions after electron beam irradiation was 5 million or more, and the weight loss rate was 10% or less (Table 1), and the molecular weight reduction was clearly suppressed compared to the corresponding comparative example (Table 2). This suggests that the article can withstand sterilization by electron beam irradiation, and that the crosslinked polymer-containing resin composition can be a material for producing such a resin composition that can also withstand sterilization by electron beam irradiation.
From the above, it can be said that the resin composition of the present invention is particularly useful for applications requiring sterilization and heat resistance, such as tableware and medical parts.

[Preparation of Crosslinked Polymer]
Further crosslinked polymers were prepared by varying the amount of crosslinking monomer added and the intensity of the electron beam.

Materials and Methods
The following materials were used:
(A) Resin PHBV: ENMAT Y1000P (trade name, manufactured by TiaAn Biologic Materials), molecular weight 310,000 (B) Crosslinkable monomer triallyl isocyanurate: TAIC (trade name, manufactured by Nippon Kasei Co., Ltd.)
(C) Rosin ester derivative KM-1500 (trade name, manufactured by Arakawa Chemical Industries, Ltd.)

After mixing the predetermined materials, the mixture was kneaded using a resin kneader (Labo Plastomill 3S150, manufactured by Toyo Seiki Seisakusho Co., Ltd.) to obtain a resin composition (which may be a crosslinkable resin composition).
The obtained composition was pressed to a size of 100 (±1) mm × 100 (±0.1) mm × thickness of 0.15 (±0.1) mm under conditions of 150 to 180°C, 30 seconds, and 20 MPa to prepare an article precursor (which may be a crosslinkable resin composition) before irradiation with electron beams. The article precursor was irradiated with electron beams using an electron beam irradiation device at an acceleration voltage of 250 kV and an absorbed dose of 0, 2, 10, 20, 50, 100, or 200 kGy to obtain a preparation.
The molecular weight of the obtained preparation was measured by the same method as described in the above section "(Evaluation 2)".

<Results>
The results are shown in Table 3.
When no crosslinking monomer was used, PHBV was decomposed by electron beam irradiation, resulting in a decrease in molecular weight.
By using a crosslinking monomer, PHBV was crosslinked, and a product with a higher molecular weight was produced. It was confirmed that the molecular weight increased with increasing electron beam irradiation dose up to 10 kGy. When the molecular weight exceeded 450,000, the product became insoluble in the solvent and the molecular weight could not be measured. However, it was estimated that the molecular weight exceeded 5 million (Figure 2).
From these results, it was revealed that the method of the present invention can obtain a crosslinked polymer of polyhydroxyalkanoic acid, and the degree of crosslinking in the crosslinked polymer can be controlled by the amount of crosslinking monomer and the irradiation intensity of the electron beam. It was also revealed that a crosslinked polymer with a higher molecular weight can be obtained by increasing the amount of crosslinking monomer and increasing the irradiation intensity of the electron beam.

According to the present invention, a biodegradable crosslinked polymer containing polyhydroxyalkanoic acid is provided as an article using an aliphatic polyester resin having a structure different from that of conventional ester resins, and an article containing polyhydroxyalkanoic acid and having a high storage modulus is provided by using the crosslinked polymer. Therefore, the present invention greatly contributes to the development of the manufacturing industry related to articles using polymers.

Claims (10)

The average molecular weight as measured by size exclusion chromatography is 320,000 or more, or the weight loss rate after refluxing in chloroform for 4 hours is less than 30%;
The storage modulus (E') at 200°C is 2.00 x ¹⁰⁵ Pa or more.
An article comprising a crosslinked polyhydroxyalkanoic acid;
However, the polyhydroxyalkanoic acid includes 3-hydroxybutyrate, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), or a copolymer thereof . A method for producing an article containing a crosslinked polyhydroxyalkanoic acid, comprising the steps of blending 0.1 to 20 parts by weight of a crosslinkable monomer containing an allyl group with 100 parts by weight of a polyhydroxyalkanoic acid or a copolymer of a hydroxyalkanoic acid, and irradiating the blend with ionizing radiation to obtain an article containing a crosslinked polyhydroxyalkanoic acid;
However, the article containing the crosslinked polyhydroxyalkanoic acid is
The average molecular weight as measured by size exclusion chromatography is 320,000 or more, or the weight loss rate after refluxing in chloroform for 4 hours is less than 30%;
The storage modulus (E') at 200°C is 2.00 x 105 ^Pa or more,
The polyhydroxyalkanoic acid includes 3-hydroxybutyrate, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate), or copolymers thereof . 3. The method according to claim 2 , wherein the ionizing radiation is an electron beam and the exposure dose is 20 kGy or more and 200 kGy or less. A method for sterilizing the article according to claim 1 or the crosslinked polymer-containing resin composition constituting said article using electron beams, wherein the average molecular weight of the crosslinked polymer containing said polyhydroxyalkanoic acid after sterilization is 320,000 or more as measured by size exclusion chromatography, or the weight loss rate of the article after sterilization or the crosslinked polymer-containing resin composition constituting said article after sterilization is less than 30% after refluxing in chloroform for 4 hours. A crosslinked polymer for producing the article of claim 1 comprising a polyhydroxyalkanoic acid or copolymer of hydroxyalkanoic acid crosslinked with a crosslinking monomer containing an allyl group. The crosslinked polymer according to claim 5 , having an average molecular weight of 320,000 or more as measured by size exclusion chromatography. The crosslinked polymer of claim 5 or 6 , wherein the hydroxyalkanoic acid comprises 3-hydroxybutyrate, 3-hydroxybutyrate-co-3-hydroxyvalerate, 3-hydroxybutyrate-co-3-hydroxyhexanoate. A resin composition for producing the crosslinked polymer according to any one of claims 5 to 7 , comprising one or more of polyhydroxyalkanoic acids or copolymers of hydroxyalkanoic acids and mixtures thereof, and one or more crosslinking monomers, wherein the polyhydroxyalkanoic acid or copolymer of hydroxyalkanoic acids is poly 3-hydroxybutyrate, poly(3-hydroxybutyrate-co-3-hydroxyvalerate), or poly(3-hydroxybutyrate-co-3-hydroxyhexanoate). A method for producing a crosslinked polymer comprising a polyhydroxyalkanoic acid, comprising irradiating the resin composition according to claim 8 with ionizing radiation to obtain a crosslinked polymer comprising a crosslinked polyhydroxyalkanoic acid. The method according to claim 9 , comprising mixing 0.1 to 20 parts by weight of a crosslinkable monomer containing an allyl group with respect to 100 parts by weight of polyhydroxyalkanoic acid or a copolymer of hydroxyalkanoic acid.