JP2007182484A - Method for producing crosslinked molded article of polylactic acid, and crosslinked molded article of polylactic acid - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a biodegradable crosslinked molded article of a polylactic acid, hardly causing reduction of strength even at a high temperature over 60°C which is the glass transition temperature of the polylactic acid, and capable of maintaining the shape. <P>SOLUTION: The method for producing the crosslinked molded article of the polylactic acid comprises the first step for producing a primary crosslinked product by primarily crosslinking a mixture of a polylactic acid and a crosslinkable monomer (A), the second step for impregnating the primary crosslinked product of the polylactic acid produced at the first step with a crosslinkable monomer (B) by soaking the primarily crosslinked product of the polylactic acid in the crosslinkable monomer (B) at a temperature higher than the glass transition temperature of the polylactic acid and not higher than the melting point thereof, and the third step for crosslinking the primarily crosslinked product of the polylactic acid, impregnated with the crosslinkable monomer (B). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a polylactic acid crosslinked molded article having biodegradability and a polylactic acid crosslinked molded article produced by the method. The polylactic acid crosslinked molded article comprises a structure such as a film, a container, or a casing, In the field where plastic products such as parts are used, they are used as biodegradable products or parts useful for solving the problem of disposal treatment after use.

Petroleum synthetic polymer materials currently used for many films and containers are used for the global warming caused by heat and exhaust gas from heat treatment and the food and health caused by toxic substances in combustion gases and residues after combustion. Various social problems are concerned about the disposal process alone, such as adverse effects on the environment and securing of landfill sites.
Biodegradable polymer materials typified by starch and polylactic acid have been attracting attention as materials for solving such problems of disposal of petroleum synthetic polymer materials. Biodegradable polymer materials have a negative impact on the global environment, including ecosystems, such as the amount of heat generated by combustion is less than petroleum synthetic polymer materials and the cycle of decomposition and resynthesis in the natural environment is maintained. Don't give. Among biodegradable polymer materials, aliphatic polyester resins have characteristics comparable to petroleum synthetic polymer materials in terms of strength and processability, and are recently attracting particular attention. Among the aliphatic polyester resins, polylactic acid is especially made from starch supplied from plants, and it is becoming cheaper than other biodegradable polymer materials due to cost reduction due to mass production in recent years. Therefore, many studies are currently being made on its application.

However, polylactic acid becomes so soft that the shape cannot be maintained at a glass transition temperature of 60 ° C. or higher, which hinders practical use. The temperature of 60 ° C. is a temperature that cannot be easily reached as the air temperature and water temperature in nature, but is a temperature that can be reached, for example, in the interior of automobiles and window materials that are closed in midsummer. Therefore, a remarkable change in characteristics that the formed shape cannot be maintained when the temperature exceeds 60 ° C. is a fatal defect.
Such a significant change in properties is derived from the crystal structure of polylactic acid. That is, at a normal cooling speed after melt molding, the polylactic acid hardly crystallizes and is mostly amorphous. Polylactic acid has a high melting point of 160 ° C., and the crystalline portion does not melt easily, but the non-crystalline portion occupying the majority starts to move out of the restriction at around 60 ° C. of the glass transition temperature. Therefore, an extreme characteristic change occurs near the glass transition temperature of 60 ° C.

  In order to solve the problem that when the glass transition temperature is 60 ° C. or higher, the resin becomes too flexible and the strength decreases, it is possible to crosslink polylactic acid using ionizing radiation or a chemical initiator. No. 31314 (Patent Document 1).

However, the crosslinked polylactic acid also has a glass transition temperature, and the amorphous portion becomes flexible above the glass transition temperature, and the strength below the glass transition temperature cannot be maintained as it is.
In the study of the present inventors, even if the addition amount of the crosslinkable monomer for crosslinking polylactic acid is increased, or the irradiation dose for causing the crosslinking reaction to activate the crosslinkable monomer is increased, We have determined that there is a limit to the improvement in strength at temperatures above the glass transition temperature. That is, when the addition amount of the crosslinkable monomer is increased and added to several tens of percent or more with respect to polylactic acid, the mixed state cannot be maintained and precipitation of the crosslinkable monomer occurs. In addition, as the radiation dose increases, the radiation-disintegrating polylactic acid gradually decomposes and gradually decreases rather than improving the strength, and the problem cannot be solved.

JP 2003-313214 A

The present invention provides a biodegradable polylactic acid cross-linked molded article that can hardly maintain its shape even when the glass transition temperature of polylactic acid reaches 60 ° C. or higher, and can maintain the shape, and a method for producing the same. It is an issue.
More specifically, the present invention provides a biodegradable polylactic acid crosslinked molded product that does not have a glass transition temperature and can maintain excellent strength without being influenced by the temperature environment, and a method for producing the same. It is an issue.

In order to solve the above problems, as a first invention,
A first step of primary crosslinking a mixture of polylactic acid and a crosslinkable monomer (A) to produce a polylactic acid primary crosslinked product;
The polylactic acid primary cross-linked product obtained in the first step is immersed in the cross-linkable monomer (B) at a temperature not lower than the glass transition temperature of the polylactic acid and not higher than the melting point, and the cross-linkable monomer ( A second step of impregnating B);
And a third step of secondary cross-linking the polylactic acid primary cross-linked product impregnated with the cross-linkable monomer (B).

As a result of intensive studies on strength maintenance at a temperature of 60 ° C. or higher, which is the glass transition temperature of polylactic acid, the present inventors have increased the content and increased the crosslinking density without precipitating the crosslinkable monomer. Succeeded.
That is, in the first step, the polylactic acid molded product formed by mixing the crosslinkable monomer (A) is subjected to primary crosslinking to substantially completely crosslink the polylactic acid, and then the crosslinkable monomer (B) is formed in the second step. Soaked.
Normally, when the polylactic acid is above the glass transition temperature, the non-crystalline part moves to form a crystal structure, but when crystallized, it does not move until it reaches the melting point.
In the present invention, since the molecules are bound by crosslinking in the first step and the movement of the molecules is restrained, it is immersed in the crosslinkable monomer (B) in the second step and heated above the glass transition temperature of polylactic acid. However, crystallization of polylactic acid does not occur, and the immersed crosslinkable monomer (B) penetrates between the molecules of polylactic acid, and only swelling can be generated. In this way, by performing primary crosslinking in the first step and approximately 100% crosslinking, the crosslinkable monomer (B) can be swollen without causing molecular crystallization in the second step, and the weight is increased. Can be softened together.
In this state, when secondary crosslinking is performed in the third step, the crosslinkable monomers (B) impregnated in the second step and the crosslinkable monomer (B) and polylactic acid can be crosslinked.
That is, in the present invention, the primary crosslinking in the first step and the secondary crosslinking in the third step are performed twice, the polylactic acid is crosslinked by the primary crosslinking, and the crosslinkable monomers and the crosslinking are crosslinked by the secondary crosslinking. Crosslinkable monomers and polylactic acid are combined to form a crosslinked structure.
As a result, the strength of 60 ° C. or lower can be maintained even when the glass transition temperature is 60 ° C. or higher. In addition, since the impregnated crosslinkable monomer is restrained by crosslinking, precipitation of the crosslinkable monomer can be prevented.

The said process is demonstrated using FIG.
First, as shown in (a), in the first step, the crosslinkable monomer (A) is mixed with polylactic acid and then molded into a required shape, and this polylactic acid molded product is subjected to primary crosslinking, as shown in (b). In addition, the polylactic acid is approximately 100% crosslinked by gel fraction. When the polylactic acid primary crosslinked product 1 is viewed microscopically, the polylactic acid molecules are mutually constrained by the crosslinking 11 as shown in FIG. In this state, even when the temperature is higher than the glass transition temperature, the molecules are cross-linked, so that the movement is restricted and the deformation does not occur.

Next, when the polylactic acid primary crosslinked product 1 is immersed in the liquid crosslinkable monomer (B) at a temperature not lower than the glass transition temperature of the polylactic acid and not higher than the melting point in the second step, as shown in (d), In the meantime, the crosslinkable monomer (B) is impregnated.
In the second step, when the polylactic acid primary cross-linked product 1 is exposed to a temperature equal to or higher than the glass transition temperature, the above property that the restriction of the non-crystalline part is released and the film becomes flexible to some extent is used. That is, by bringing the polylactic acid primary crosslinked product 1 to a temperature not lower than the glass transition temperature in the liquid crosslinkable monomer (B), the non-crystalline portion of the polylactic acid is moved, and between the crosslinked polylactic acid molecules. The crosslinkable monomer (B) is infiltrated, and the polylactic acid primary crosslinked product 1 is swollen by the crosslinkable monomer (B).

Next, when the temperature is returned to room temperature, the state shown in (e) and (f) is obtained. In this state, the crosslinkable monomer (B) is only impregnated between the polylactic acid molecules and is not immobilized.
Thereafter, in the third step, when crosslinked by irradiating ionizing radiation or the like, the impregnated crosslinkable monomers (B) are cross-linked 12 and fixed, and the crosslinkable monomer (B) and polylactic acid are immobilized. The polylactic acid cross-linked molded article 10 having a composite cross-linked structure shown in (g) and (h) is also obtained by graft cross-linking.

  Thus, since the composite cross-linked structure is provided by two cross-linkings of the primary cross-linking and the secondary cross-linking, the strength of the polylactic acid cross-linked molded body 10 is increased, and the glass transition temperature becomes 60 ° C. or higher. Further, it is possible to impart strength that does not deform.

In the method for producing a polylactic acid cross-linked molded article of the present invention, it is important that a polylactic acid primary cross-linked product is prepared by first cross-linking nearly 100% of the polylactic acid molded product.
The phenomenon that occurs when the non-crosslinked polylactic acid molded product 4 is immersed in the crosslinkable monomer in the second step is shown in FIGS.
As shown in FIG. 2 (b), when the non-crosslinked polylactic acid molded product 4 is immersed in the crosslinkable monomer (B), there is no crosslinking that binds the polylactic acid molecules to each other, so that the crosslinkable monomer (B). As shown in FIG. 2 (c), the polylactic acid melts to cause deformation or collapse of the shape.
Further, as shown in FIG. 3B, when the non-crosslinked polylactic acid molded product 4 is placed at a temperature higher than the glass transition temperature, the non-crystalline portion gradually crystallizes (reference numeral 5 in FIG. 3B). ), Before the crosslinkable monomer (B) enters, it hardens and hardens as shown in FIG.
In the present invention, the polylactic acid primary crosslinked product 1 is impregnated with the crosslinkable monomer (B), and the polylactic acid molecules are constrained and integrated by the crosslinking 11, so that the non-crystalline portion begins to gradually crystallize. No recrystallization is seen.

In order to prevent the phenomenon shown in FIG. 2 and FIG. 3 from occurring, the polylactic acid primary crosslinked product prepared in the first step has a gel fraction of 95% or more, preferably 98% or more, and particularly substantially 100%. It is preferable that the resin is completely cross-linked.
Even in the range where the gel fraction substantially exceeds 100%, the amount of crosslinking points, that is, the crosslinking density is important. By increasing the crosslinking density, the amount of the crosslinking monomer (B) impregnated in the second step Can be controlled. This utilizes the fact that the cross-linked network structure becomes dense, making it difficult for structural changes and volume changes to occur. The amount of cross-linkable monomer (A) contained in the polylactic acid molded product, the amount of ionizing radiation to be cross-linked It is possible to increase / decrease the crosslinking density by increasing / decreasing etc. and to control the amount of impregnation of the crosslinkable monomer (B).

The polylactic acid used in the present invention includes polylactic acid composed of L-lactic acid, polylactic acid composed of D-lactic acid, polylactic acid obtained by polymerizing a mixture of L-lactic acid and D-lactic acid, or two or more of these. A mixture is mentioned. Note that L-lactic acid or D-lactic acid, which is a monomer constituting polylactic acid, may be chemically modified.
The polylactic acid used in the present invention is preferably a homopolymer as described above, but a polylactic acid copolymer in which a lactic acid monomer or lactide and other components copolymerizable therewith are copolymerized may be used. Examples of the “other components” forming the copolymer include hydroxycarboxylic acids such as glycolic acid, 3-hydroxybutyric acid, 5-hydroxyvaleric acid, and 6-hydroxycaproic acid; succinic acid, adipic acid, and sebacic acid Dicarboxylic acids typified by glutaric acid, decanedicarboxylic acid, terephthalic acid or isophthalic acid; polyhydric alcohols typified by ethylene glycol, propanediol, octanediol, dodecanediol, glycerin, sorbitan or polyethylene glycol; glycolide, Examples include lactones represented by ε-caprolactone or δ-butyrolactone.

In the first step, the crosslinkable monomer (A) to be blended with the polylactic acid that undergoes primary crosslinking is preferably an allylic crosslinkable monomer because a high degree of crosslinking can be obtained at a relatively low concentration.
Allyl monomers include triallyl isocyanurate, trimethallyl isocyanurate, triallyl cyanurate, trimethallyl cyanurate, diallylamine, triallylamine, diacrylic chlorate, 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, dimethylallyl Tallates, ethyl allyl malate, methyl allyl fumarate, methyl meta-allyl maleate, diallyl monoglycidyl isocyanurate.
Of these, triallyl isocyanurate (hereinafter referred to as TAIC) is particularly preferable because of its high crosslinking effect on polylactic acid. Further, even when triallyl cyanurate, which can be structurally converted by TAIC and heating, is used, the effect is substantially the same.

  It is preferable that the crosslinkable monomer (A) is contained in an amount of 4% by weight or more and 10% by weight or less with respect to the polylactic acid subjected to primary crosslinking. If it is less than 4% by weight, the gel fraction cannot be made close to 100%. On the other hand, if it exceeds 10% by weight, it becomes difficult to uniformly mix the entire amount of the crosslinkable polymer with polylactic acid, and the crosslinkage is substantially reduced. This is because there is no significant difference in effect. More preferably, it is 5 to 7 weight% with respect to polylactic acid.

In addition to the polylactic acid and the crosslinkable monomer (A), other components may be added to the composition constituting the polylactic acid molded product used in the present invention as long as the object of the present invention is not adversely affected.
For example, a biodegradable resin other than polylactic acid may be blended. As biodegradable resins other than polylactic acid, natural biodegradable resins such as lactone resins, synthetic biodegradable resins such as aliphatic polyester or polyvinyl alcohol, or natural linear polyester resins such as polyhydroxybutyrate / valerate, etc. Resins can be mentioned.
Moreover, you may mix the synthetic polymer and / or natural polymer which have biodegradability in the range which does not impair a melt characteristic. Examples of synthetic polymers having biodegradability include cellulose acetate, cellulose butyrate, cellulose propionate, cellulose nitrate such as cellulose nitrate, cellulose sulfate, cellulose acetate butyrate or cellulose nitrate acetate, or polyglutamic acid, polyaspartic acid or Examples include polypeptides such as polyleucine. Examples of the natural polymer include starch, raw starch such as corn starch, wheat starch or rice starch, or processed starch such as acetate esterified starch, methyl etherified starch or amylose.

  Further, the composition includes a resin component other than a biodegradable resin, a curable oligomer, various stabilizers, a flame retardant, an antistatic agent, an antifungal agent or a viscosity imparting agent, glass fiber, glass beads, Metal powders, inorganic and organic fillers such as talc, mica or silica, and colorants such as dyes or pigments can also be added.

  A composition containing the above-described polylactic acid, a crosslinkable monomer, and optionally other components is formed into a desired shape. A shaping | molding method is not specifically limited, You may use a well-known method. For example, known molding machines such as an extrusion molding machine, a compression molding machine, a vacuum molding machine, a blow molding machine, a T-die molding machine, an injection molding machine, and an inflation molding machine are used.

After the polylactic acid composition is molded into a required shape, the polylactic acid molded product is subjected to primary crosslinking, and the crosslinking method is not particularly limited, and a known method is used. In particular, irradiation with ionizing radiation is performed for crosslinking. Most preferably.
As ionizing radiation, γ-rays, X-rays, β-rays or α-rays can be used. However, for industrial production, γ-ray irradiation with cobalt-60 or electron beam irradiation with an electron beam accelerator is preferable.
The irradiation with ionizing radiation is preferably performed in an inert atmosphere or air except for air. This is because when the active species generated by the irradiation with ionizing radiation are combined with oxygen in the air and deactivated, the crosslinking efficiency is lowered.

The dose of ionizing radiation is preferably 50 kGy or more and 200 kGy or less.
Depending on the amount of crosslinkable monomer, polylactic acid can be crosslinked even if the irradiation dose of ionizing radiation is 1 kGy or more and 10 kGy or less. However, to crosslink almost 100% of polylactic acid molecules, the irradiation dose of ionizing radiation is It is preferably 50 kGy or more. Furthermore, in order to suppress a change in shape and swell uniformly when immersed in a liquid crosslinkable monomer in a later step, it is more preferable that the irradiation dose of ionizing radiation is 80 kGy or more.
On the other hand, the irradiation dose of ionizing radiation is 200 kGy or less because polylactic acid has the property of being disintegrated by radiation when the resin alone is used. Therefore, when the irradiation dose of ionizing radiation exceeds 200 kGy, the decomposition proceeds contrary to crosslinking. It is because it will make it. The upper limit of the ionizing radiation dose is preferably 150 kGy, and more preferably 100 kGy.

Instead of primary crosslinking by irradiating with ionizing radiation, the polylactic acid is mixed with the crosslinkable monomer (A) and the chemical initiator, then formed into a desired shape, and raised to a temperature at which the chemical initiator is thermally decomposed. Can also produce a polylactic acid primary cross-linked product.
As the crosslinkable monomer (A), the same substance as in the above embodiment can be used.
Chemical initiators include dicumyl peroxide that generates peroxide radicals by thermal decomposition, propionitrile peroxide, benzoyl peroxide, di-t-butyl peroxide, diacyl peroxide, pelargonyl peroxide, myristoyl peroxide, Any catalyst that initiates polymerization of a monomer such as a peroxide catalyst such as t-butyl benzoate or 2,2′-azobisisobutyronitrile may be used.
The temperature conditions for crosslinking can be appropriately selected depending on the type of chemical initiator. Crosslinking is preferably performed in an inert atmosphere or air except for air, as in the case of irradiation with radiation.
Specifically, in the first step, polylactic acid is blended with 5% by weight or more of an allylic monomer as a crosslinkable monomer (A), and subjected to primary crosslinking by irradiation with ionizing radiation at 50 kGy or more. Most preferably, the polylactic acid is crosslinked approximately 100% by primary crosslinking.

In the second step, the polylactic acid primary crosslinked product obtained in the first step is immersed in the crosslinkable monomer (B) at a temperature not lower than the glass transition temperature of the polylactic acid and not higher than the melting point.
The crosslinkable monomer (B) is not particularly limited as long as it is liquid at room temperature, or is solid at room temperature and melts at a temperature not lower than the glass transition temperature of the polylactic acid and not higher than the melting point to become a liquid. Can be used.
Examples of the crosslinkable monomer (B) include acrylic or methacrylic acid monomers, styrene monomers, allyl monomers, or lactone monomers.
The allyl crosslinking monomer is suitable for improving the crosslinking density of polylactic acid.
Acrylic or methacrylic crosslinkable monomers are suitable for the purpose of improving the strength of polylactic acid at high temperatures above the glass transition temperature. In particular, since acrylic is hard when it becomes a polymer, heat resistance at high temperatures can be improved. And since it is transparent even after compounding, it can be adopted as an optical system material.
For the purpose of imparting a graft chain as a base point for graft polymerization to polylactic acid and introduction of a functional group, a styrenic crosslinkable monomer is also effective.
Lactone-based crosslinkable monomers are suitable for the purpose of further enhancing the biodegradability of the polylactic acid crosslinked molded article.

  Examples of the acrylic or methacrylic crosslinkable monomer include (meth) acrylic acid, methyl (meth) acrylate, 1,6-hexanediol di (meth) acrylate, 1,4-butanediol di (meth) acrylate, Methylolpropane 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, dipenta Erythritol hexaacrylate, dipentaerythritol monohydroxypentaacrylate, caprolactone modified dipentaerythritol hexaacrylate, pentaerythritol Rutori (meth) acrylate, pentaerythritol tetra (meth) acrylate, polyethylene glycol di (meth) acrylate, tris (acryloyloxyethyl) isocyanurate, tris (methacryloxyethyl) isocyanurate.

  Examples of the styrenic crosslinkable monomer include styrene, p-methyltoluene and the like, which are mainly provided with a functional group at the para position, styrene sulfonate, chlorostyrene, α-methylstyrene, and the like.

  Examples of lactone crosslinkable monomers include ε-caprolactone, 4-methylcaprolactone, 3,5,5-trimethylcaprolactone, various methylated caprolactones such as 3,3,5-trimethylcaprolactone, β-propiolactone, and γ-butyrolactone. , Δ-valerolactone, enanthlactone and the like.

The temperature of the crosslinkable monomer (B) at the time of immersing the polylactic acid primary cross-linked product may be a temperature that is not lower than the glass transition temperature of the polylactic acid and not higher than the melting point and that the crosslinkable monomer (B) can maintain a liquid state. For example, it can be appropriately selected according to the type of the crosslinkable monomer (B). The cross-linking monomer (B) diffuses into the polylactic acid cross-linked structure at a higher temperature but is generally in the range of 80 to 120 ° C.
Although the immersion time is not particularly limited, since the diffusion phenomenon is generally proportional to the square of the thickness, the thickness within 1 mm is 5 to 120 minutes, more preferably 30 to 90 minutes, and the thickness is several mm or more. In the case, it is 10 to 20 hours.

  The polylactic acid primary cross-linked product is impregnated with the cross-linkable monomer (B), and the polylactic acid primary cross-linked product is swollen and cooled below the glass transition temperature of the polylactic acid, so that the polylactic acid and the cross-linkable monomer (B) are obtained. A complexed polylactic acid-crosslinkable monomer complex is obtained. The polylactic acid-crosslinkable monomer complex 3 produced in this way is impregnated with a crosslinkable monomer 2 in a polylactic acid crosslinking network 11 as shown in FIG.

In the third step, secondary cross-linking is performed, the cross-linkable polymers (B) impregnated in the second step are cross-linked, and the cross-linkable polymer (B) and polylactic acid are graft cross-linked. The secondary cross-linking in is different from the primary cross-linking in the first step, and the gel fraction by cross-linking is not necessarily 100%. Therefore, the blending amount of the crosslinkable monomer (B) impregnated in the second step also depends on the crosslink density of the primary crosslinked polylactic acid and the affinity between the crosslinkable monomer (B) and the polylactic acid.
The secondary crosslinking method is not particularly limited, and a known method is used, but a method of irradiating ionizing radiation is preferable.
The crosslinking method by irradiation with ionizing radiation is the same as in the first step, but the irradiation amount of ionizing radiation depends on the amount of the crosslinkable monomer impregnated, but the irradiation required for crosslinking in the first step. It may be less than the amount.
That is, the irradiation dose of ionizing radiation in the third step is 1 kGy or more and 200 kGy or less, preferably 10 kGy or more and 200 kGy or less, more preferably 30 kGy or more and 200 kGy or less.

As described above, the present invention provides a polylactic acid crosslinked molded article produced by being crosslinked twice in the first step, the second step, and the third step.
The polylactic acid cross-linked molded product is integrated by cross-linking polylactic acid by primary cross-linking, and the primary cross-linked polylactic acid molded product is impregnated with a crosslinkable monomer, followed by secondary cross-linking and impregnated cross-linkability. The monomers have a crosslinked structure in which the monomers and the crosslinkable monomer and polylactic acid are combined by graft crosslinking.
Thus, since it is compounded to form a dense cross-linked structure, it has heat resistance that does not cause deformation even at high temperatures of polylactic acid at a glass transition temperature of 60 ° C. or higher.

In the polylactic acid crosslinked molded article of the present invention, the content of the crosslinkable monomer is preferably 5% by mass or more and 50% by mass or less with respect to polylactic acid.
This is because the content of the crosslinkable monomer is set to 5% by mass or more. If the content of the crosslinkable monomer is less than 5% by mass, the crosslinking density is not sufficiently improved by blending the crosslinkable monomer. On the other hand, the reason why it is 50% by mass or less is to prevent the occurrence of bleeding due to the precipitation of the crosslinkable monomer.

Further, in the polylactic acid cross-linked molded article of the present invention, both the heat absorption at the glass transition temperature of polylactic acid and the heat absorption due to crystal melting near the melting point in the calorimetric analysis from 40 ° C. to 200 ° C. by a differential scanning calorimeter. Preferably not.
With such a polylactic acid cross-linked molded article, the restriction of the non-crystalline part is released at the glass transition temperature, as seen in conventional polylactic acid molded articles, and it begins to move at once, and an extreme strength change occurs around the glass transition temperature. The phenomenon that occurs is unlikely to occur.

  The polylactic acid cross-linked molded article of the present invention has a cross-linked structure in which cross-links between molecules of polylactic acid, cross-links between blended cross-linkable monomers, and cross-links between the cross-linkable monomers and polylactic acid are combined. Therefore, even at a high temperature exceeding 60 ° C., which is the glass transition temperature of polylactic acid, the shape can be reliably maintained by the crosslinked network of polylactic acid. In addition, transparency is maintained despite a high graft ratio. Thus, the polylactic acid cross-linked molded product of the present invention can improve its drawbacks while maintaining the advantages of polylactic acid, and can be used as a substitute for general-purpose plastics derived from petroleum, which is the original purpose of biodegradable resins. This greatly improves the performance.

  Since the polylactic acid cross-linked molded article of the present invention has biodegradability, it has very little influence on the ecosystem in nature, and can solve various problems related to disposal treatment that conventional plastics have. Therefore, it is expected to be used as an alternative material for plastic products and parts that are manufactured and discarded in large quantities. In particular, it can be expected to be applied to optical products such as optical fibers and optical disks because it maintains the unique transparency of polylactic acid, which is not found in other biodegradable resins, and has improved strength maintenance at high temperatures. In addition, since it does not affect the living body, it can be applied to medical instruments such as syringes and catheters used inside and outside the living body.

  Furthermore, the present invention can be combined with other materials such as styrene, acrylic acid, and methacrylic acid, and used in various fields to compound other materials and to make polylactic acid highly functional. The method is provided and can be applied to a wide range of technical fields.

Hereinafter, embodiments of the present invention will be described.
In the method for producing a polylactic acid crosslinked molded article of the present invention, first, a polylactic acid primary crosslinked product is first produced by the following procedure.
First, polylactic acid is softened by heating, or polylactic acid is dissolved or dispersed in a solvent in which polylactic acid such as chloroform and cresol can be dissolved.
Next, the crosslinkable monomer (A) is added. TAIC is particularly preferred as the crosslinkable monomer. The addition amount of the crosslinkable monomer is preferably 5% by weight or more and 7% by weight or less with respect to 100% by weight of polylactic acid.
After the addition, the mixture is stirred and mixed so that the crosslinkable monomer is uniform.
Subsequently, the solvent may be further removed by drying.
In this way, a polylactic acid composition constituting the polylactic acid molded product is prepared.

  The polylactic acid composition is softened again by heating or the like, and formed into a desired shape such as a sheet, film, fiber, tray, container or bag. This molding may be performed after preparing the polylactic acid composition, for example, in a state dissolved in a solvent, or may be performed after cooling or removing the solvent by drying.

Next, the resulting polylactic acid molded product is irradiated with ionizing radiation to crosslink the polylactic acid to obtain a polylactic acid primary crosslinked product.
The ionizing radiation is preferably electron beam irradiation by an electron beam accelerator.
The radiation irradiation amount is appropriately selected from the range of 80 kGy or more and 100 kGy or less in accordance with the blending amount of the crosslinkable monomer. In particular, it is selected based on the fact that the gel fraction of the polylactic acid primary crosslinked product obtained after ionizing radiation irradiation is substantially 100%.

The obtained polylactic acid primary crosslinked product is immersed in the crosslinking monomer (B).
Examples of the crosslinkable monomer (B) include methacrylic crosslinkable monomer methacrylic acid or methyl methacrylate, allyl crosslinkable monomer TAIC, styrene crosslinkable monomer styrene, and lactone crosslinkable monomer ε-caprolactone. Is used.
The temperature at the time of immersing in a crosslinkable monomer (B) is 65-100 degreeC, and the temperature which a crosslinkable monomer (B) can maintain a liquid state is required. Moreover, when the polylactic acid primary crosslinked product has a thickness of about 1 mm or less, the time for dipping in the crosslinkable monomer (B) is preferably 30 to 90 minutes, and more preferably 60 minutes.

  The polylactic acid primary cross-linked product is impregnated with the cross-linkable monomer (B), and the polylactic acid primary cross-linked product is swollen and cooled below the glass transition temperature of polylactic acid to obtain a polylactic acid-crosslinkable monomer complex. It is done. The cooling may be performed gradually by standing or may be rapidly cooled by water cooling.

Next, the resulting polylactic acid-crosslinkable monomer complex is irradiated with ionizing radiation for secondary crosslinking, and the crosslinkable monomer impregnated with polylactic acid is graft-crosslinked and the crosslinkable monomers (B) are crosslinked. Thus, the polylactic acid crosslinked molded article of the present invention is produced.
The irradiation dose during secondary crosslinking is appropriately selected from the range of 30 kGy or more and 200 kGy or less according to the type and blending amount of the crosslinking monomer.

The polylactic acid crosslinked molded article of the present invention produced by the above method contains a high concentration of a crosslinkable monomer. Specifically, the crosslinkable monomer is contained in an amount of 15% by mass to 100% by mass, preferably 5% by mass or more and 50% by mass or less with respect to polylactic acid.
And the crosslinkable monomer has a fixing ratio measured by the method described in the Examples of 5 to 95%, more preferably 8 to 85%.

In the polylactic acid crosslinked molded article of the present invention, even if the crosslinkable monomer (A) (B) is contained in a high concentration as described above, the crosslinkable monomer is crosslinked with polylactic acid or with the crosslinkable monomers. Therefore, it does not precipitate. The cross-linked structure becomes dense due to the high-concentration cross-linkable monomer, and the polylactic acid cross-linked molded article of the present invention has a temperature condition of 60 ° C. or lower even at a high temperature of 60 ° C. or higher which is the glass transition temperature of polylactic acid. The strength at can be maintained.
As the index, in the heat resistance deformation test described in the examples, it is preferable that the downward bending is less than 45 °.

  EXAMPLES Hereinafter, although an Example and a comparative example are given and this invention is demonstrated concretely, this invention is not limited only to these Examples.

(Examples 1-4)
As polylactic acid, pellet-shaped polylactic acid lacia (LACEA) H-400 manufactured by Mitsui Chemicals, Inc. was used. TAIC, which is a kind of allylic crosslinkable monomer, is prepared. When polylactic acid is melt-extruded at a cylinder temperature of 180 ° C. using an extruder (PCM30, manufactured by Ikekai Tekko Co., Ltd.), TAIC was added to polylactic acid by dropping TAIC at a constant speed with a peristaltic pump. At that time, the ratio of the dropping speed of TAIC and the extrusion speed of the extruder was adjusted so that the blending amount of TAIC was 7 parts by mass with respect to 100 parts by mass of polylactic acid. The extruded product was cooled with water and then pelletized with a pelletizer to obtain a pellet-like kneaded product of polylactic acid and the crosslinkable monomer (A).

The kneaded product was hot-pressed into a sheet at 160 ° C. and then rapidly cooled with water to prepare a sheet-like polylactic acid molded product having a thickness of 500 μm.
This sheet-like polylactic acid molded product was irradiated with an electron beam at 100 kGy by an electron accelerator (acceleration voltage 10 MeV, current amount 12 mA) in an inert atmosphere excluding air to obtain a polylactic acid primary crosslinked product.
The obtained polylactic acid primary crosslinked product was immersed in the crosslinkable monomer (B) at a temperature not lower than the glass transition temperature of the polylactic acid and not higher than the melting point. Methacrylic acid was used as a crosslinkable monomer. Specifically, the polylactic acid primary crosslinked product was immersed and swollen in methacrylic acid for 1 hour in a constant temperature bath at 80 ° C.
Thereafter, the temperature was returned to room temperature, and after surplus monomer was wiped off, the electron beam was irradiated again with an electron accelerator (acceleration voltage 10 MeV, current amount 12 mA) with an electron beam of 30 kGy, 60 kGy, 100 kGy, and 200 kGy. Then, the excess monomer which was not fixed by vacuum-drying for 24 hours was removed, and the polylactic acid crosslinked molding of this invention was obtained.

(Examples 5 to 10)
TAIC, styrene, ε-caprolactone, methyl methacrylate, trimethylolpropane methacrylate (hereinafter referred to as TMPTMA), trimethylolpropane acrylate (squid, squid) instead of methacrylic acid as a crosslinkable monomer (B) for immersing the polylactic acid primary crosslinked product Examples 5 to 10 were used in the same manner as Example 3, except that TMPTA was used.

(Comparative Examples 1-3)
It was set as the comparative example 1 like Example 1-4 except not having performed 2nd electron beam irradiation.
The polylactic acid primary crosslinked product obtained in the same manner as in Examples 1 to 4 was used as Comparative Example 2.
Comparative Example 3 was made in the same manner as in Examples 1 to 4 except that the first electron beam irradiation was not performed and the irradiation amount of the second electron beam irradiation was 90 kGy.

  In Examples and Comparative Examples, the gel fraction of the polylactic acid primary cross-linked product before impregnation into the cross-linkable monomer (B), the fixing rate of the cross-linkable monomer (B) of the polylactic acid cross-linked molded product as the final product, and heat resistance Deformability and transparency were evaluated by the following methods.

(1) Evaluation of gel fraction After accurately measuring the dry mass of each polylactic acid primary cross-linked product, wrapped in a 200 mesh stainless steel wire, boiled in chloroform solution for 48 hours, and then dissolved in chloroform. The remaining gel content was obtained. After drying at 50 ° C. for 24 hours, chloroform in the gel was removed, and the dry mass of the gel was measured. Based on the obtained value, the gel fraction was calculated based on the following formula.
Gel fraction (%) = (dry weight of gel fraction / dry weight of polylactic acid primary crosslinked product) × 100

(2) Evaluation of Crosslinkable Monomer Fixation Ratio The mass of the polylactic acid primary crosslinked product at room temperature before being immersed in the crosslinkable monomer (B) is measured in advance, and the mass of the finally obtained polylactic acid crosslinked molded product Was measured. Based on the obtained value, the crosslinkable monomer fixation rate was calculated based on the following formula.
Crosslinkable monomer fixation rate (%) = {(BA) / A} × 100
A: Mass of the polylactic acid primary crosslinked product before impregnation into the crosslinkable monomer (B) B: Mass of the polylactic acid crosslinked product

(3) Evaluation of heat-resistant deformation The polylactic acid cross-linked molded body was cut into a strip shape having a width of 1 cm and a length of 7 cm, and 2 cm from the end was fixed as shown in FIG. The sample was allowed to stand for 1 hour, and the downward deformability due to gravity was measured.
In FIG. 4, the solid line indicates the polylactic acid cross-linked molded body 10 before the test, and the dotted line indicates the polylactic acid cross-linked molded body 10 deformed downward due to gravity after the test.
“◎” indicates that the downward curve is 1 ° or less and no deformation is observed, “○” indicates that the downward curve is less than 5 °, and “◯” indicates that the downward curve is less than 5 °. Was evaluated as “C”, and “X” was evaluated when the downward bending was 45 ° or more.

(4) Transparency The case where the finally obtained polylactic acid crosslinked molded product maintained the transparency of the polylactic acid as a raw material was “◯”, and the case where a cloudy part was seen was “△”, The case of whitening was evaluated as “x”.

The result of the said evaluation was put together in the following table | surface with the difference of manufacturing conditions.

In each of the examples, a polylactic acid crosslinked molded article in which the crosslinkable monomer (B) was crosslinked and fixed was obtained. Since the unfixed crosslinkable monomer (B) is removed by vacuum drying for 24 hours, the crosslinkable monomer (B) may be graft-polymerized inside the polylactic acid or may form a crosslinked product. It could be confirmed.
A feature of the polylactic acid cross-linked molded article of the present invention is that there is no deformation at a temperature higher than the glass transition temperature of polylactic acid. Second, in Example 6, a slightly cloudy part was observed, but the transparency of polylactic acid and its cross-linked product was almost maintained.
In particular, a methacrylic crosslinkable monomer such as methacrylic acid or methyl methacrylate, or an acrylic monomer such as TMPTA has a high fixation rate of 45 to 86%, and has an excellent strength maintaining effect and transparency at high temperatures. It was confirmed that it was the best for the purpose.

In contrast to the examples, in Comparative Example 1 in which the second electron beam irradiation was not performed, the crosslinkable monomer could not be fixed with a fixing ratio of less than 1%, and the strength maintaining effect at high temperature was not observed.
Also, the effect of maintaining the strength at high temperature is not seen in Comparative Example 2 in which the second and third steps of impregnation with the crosslinkable monomer (B) and subsequent re-crosslinking are not performed and polylactic acid is only crosslinked. It was.
In Comparative Example 3 in which the polylactic acid molded product was not crosslinked and subjected to the second step and the third step, the crosslinking monomer (B) could not be impregnated, and a part was dissolved. In addition, since it was exposed to a temperature not lower than the glass transition temperature, crystallization occurred and became hard, and at the same time, light was not allowed to pass due to irregular reflection by the crystal, resulting in marked whitening.

It is a schematic diagram which shows the manufacturing process of the polylactic acid crosslinked molded object of this invention. It is the schematic diagram which showed the phenomenon which occurs when the crosslinkable monomer (B) was impregnated with the non-crosslinked polylactic acid molded product. It is the schematic diagram which showed the phenomenon which occurs when the crosslinkable monomer (B) was impregnated with the non-crosslinked polylactic acid molded product. It is the schematic of the test instrument used for a heat-resistant deformation test.

Explanation of symbols

1 Polylactic acid primary crosslinked product 3 Polylactic acid-crosslinkable monomer complex 4 Polylactic acid molded product 5 Crystallization 10 Polylactic acid crosslinked molded product (A) Crosslinkable monomer blended in the first step (B) Impregnation in the second step Crosslinkable monomer

Claims (7)

A first step of primary crosslinking a mixture of polylactic acid and a crosslinkable monomer (A) to produce a polylactic acid primary crosslinked product;
The polylactic acid primary cross-linked product obtained in the first step is immersed in the cross-linkable monomer (B) at a temperature not lower than the glass transition temperature of the polylactic acid and not higher than the melting point, and the cross-linkable monomer ( A second step of impregnating B);
And a third step of secondary crosslinking the polylactic acid primary crosslinked product impregnated with the crosslinkable monomer (B). As the crosslinkable monomer (A) mixed with the polylactic acid to be primarily crosslinked, an allyl monomer is used,
The methacrylic acid monomer, styrene monomer, allyl monomer, or lactone monomer is used as the crosslinkable monomer (B) impregnated in the polylactic acid primary crosslinked product in the second step. A method for producing a polylactic acid crosslinked molded article. The primary crosslinking of the first step and the secondary crosslinking of the third step are performed by irradiating with ionizing radiation,
In the secondary crosslinking, polylactic acid is crosslinked by the primary crosslinking, and in the secondary crosslinking, the crosslinking polymers (B) impregnated in the second step are crosslinked and the crosslinking polymer (B) and the polylactic acid are graft-crosslinked. The manufacturing method of the polylactic acid bridge | crosslinking molded object of Claim 1 or Claim 2.   The polylactic acid to be primarily crosslinked is blended with 4% by weight or more and 10% by weight or less of an allylic monomer as the crosslinking monomer (A), and the primary crosslinking is performed by irradiating ionizing radiation at 50 kGy or more, The method for producing a polylactic acid crosslinked molded article according to any one of claims 1 to 3, wherein the polylactic acid is 100% crosslinked by primary crosslinking.   A polylactic acid cross-linked molded article produced by the production method according to any one of claims 1 to 4.   Polylactic acid is 100% cross-linked by gel fraction and integrated, and the cross-linkable monomers impregnated in the second step are cross-linked to form a composite of cross-linking of the polylactic acid and cross-linking of the cross-linkable monomers. The polylactic acid cross-linked molded article according to claim 5, which has a cross-linked structure.   7. A calorimetric analysis from 40 ° C. to 200 ° C. using a differential scanning calorimeter has no heat absorption at the glass transition temperature of polylactic acid and no heat absorption due to crystal melting near the melting point. The polylactic acid cross-linked molded article described in 1.