Marine biodecomposition accelerator

Abstract

To provide new technique for promoting marine biodecomposition of a polylactic acid.SOLUTION: A marine biodecomposition accelerator for promoting marine biodecomposition of a polylactic acid contains a copolymer polyester of a lactic acid and another hydroxy carboxylic acid. Another hydroxy carboxylic acid is preferably a 3-hydroxyalkanoic acid. The copolymer polyester is preferably a random copolymer of a lactic acid and another hydroxy carboxylic acid. A resin composition contains a polylactic acid and the marine biodecomposition accelerator.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a marine biodegradation promoter that promotes the marine biodegradation of polylactic acid, and use thereof. [Background technology]

[0002] Because plastics are difficult to decompose in the natural environment, environmental pollution caused by their large-scale disposal has become a serious problem. Furthermore, microplastics, tiny plastic particles with a diameter of 5 mm or less, are becoming a major problem in the marine environment. In order to address these problems, efforts are being made to put biodegradable plastics, which are plastics that can be decomposed into water and carbon dioxide by the action of microorganisms, into practical use.

[0003] Polylactic acid, a representative biodegradable plastic, is a type of biopolymer that can be synthesized from biomass-derived raw materials. Because it has relatively high rigidity, strength, and transparency, it is being used in a wide range of applications as an alternative to petroleum-derived plastics. However, it is known that polylactic acid exhibits almost no marine biodegradability (see, for example, Non-Patent Documents 1 and 2).

[0004] For this reason, technology to improve marine biodegradation of resin materials containing polylactic acid has been studied. For example, Patent Document 1 discloses that regenerated cellulose is used, Patent Document 2 discloses that nitrogen compounds such as ammonium sulfate and phosphorus compounds such as potassium dihydrogen phosphate are used, and Patent Document 3 discloses that agarose, sodium alginate, casein, or keratin are used to improve marine biodegradation of polylactic acid-containing materials.

[0005] On the other hand, Patent Document 4 does not mention marine biodegradation of polylactic acid, but discloses that polylactic acid can be plasticized without substantially decreasing the transparency of the polylactic acid by blending it with a copolymer polyester of lactic acid and another hydroxycarboxylic acid. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2022 / 085725 [Patent Document 2] JP 2022-142016 A [Patent Document 3] Patent Publication No. 2022-172526 [Patent Document 4] International Publication No. 2020 / 066679 [Non-patent literature]

[0007] [Non-Patent Document 1] Advanced Science,2023,10,2205554 [Non-Patent Document 2] Materials Chemistry and Physics,2023,303,127813 Summary of the Invention [Problem to be solved by the invention]

[0008] Although Patent Documents 1 to 3 disclose technologies for improving the marine biodegradation of polylactic acid-containing materials, the effects are insufficient, and there are concerns that the transparency and mechanical properties of polylactic acid may be impaired because substances with significantly different chemical structures from polylactic acid are mixed into the polylactic acid.

[0009] An object of the present invention is to provide a novel technology for promoting marine biodegradation of polylactic acid. [Means for solving the problem]

[0010] The present inventors have found that a copolymer polyester of lactic acid with another hydroxycarboxylic acid can promote marine biodegradation of polylactic acid, and have arrived at the present invention.

[0011] That is, the present invention relates to a marine biodegradation promoter that promotes the marine biodegradation of polylactic acid, which contains a copolymer polyester of lactic acid and another hydroxycarboxylic acid. The present invention also relates to a resin composition containing polylactic acid and the marine biodegradation promoter. The present invention further relates to a molded article obtained by molding the resin composition. Furthermore, the present invention also relates to a method for promoting marine biodegradation of polylactic acid, which comprises contacting polylactic acid with a copolymer polyester of lactic acid and another hydroxycarboxylic acid. Furthermore, the present invention also relates to the use of a copolymerized polyester of lactic acid and another hydroxycarboxylic acid as a marine biodegradation promoter for promoting the marine biodegradation of polylactic acid. Effect of the Invention

[0012] The present invention can provide a novel technology for promoting marine biodegradation of polylactic acid. According to a preferred embodiment of the present invention, it is possible to promote marine biodegradation of polylactic acid without substantially decreasing the transparency of the polylactic acid. Furthermore, it is also possible to plasticize the polylactic acid and improve its elongation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, an embodiment of the present invention will be described in detail. The marine biodegradation promoter according to the present embodiment is an agent for promoting marine biodegradation of polylactic acid by being used for polylactic acid. The marine biodegradation promoter contains at least a copolymer polyester of lactic acid and another hydroxycarboxylic acid.

[0014] (Polylactic acid) Polylactic acid is a polyester containing lactic acid as a constituent monomer. As described above, polylactic acid is known to have extremely low biodegradability in the ocean. However, by using the marine biodegradation promoter according to the present embodiment, the biodegradation of polylactic acid in the ocean can be promoted. Here, biodegradation means that the target resin is decomposed into water and carbon dioxide by the action of microorganisms, and marine biodegradation means that the target resin is biodegraded in the ocean. The marine biodegradation accelerator according to this embodiment is expected to be a means of solving the problems of marine pollution and microplastics caused by the disposal of plastic materials containing polylactic acid as a main component.

[0015] The polylactic acid may be any conventionally known polylactic acid, and may be either crystalline or amorphous. The polylactic acid is preferably a homopolymer of lactic acid, but may contain a small amount of other monomers in addition to lactic acid.

[0016] The lactic acid constituting the polylactic acid may be either the L-form or the D-form, or may contain both. In the latter case, the ratio of the L-form to the D-form is not particularly limited. The polylactic acid may be any one of poly(L-lactic acid) resin, poly(D-lactic acid) resin, and poly(DL-lactic acid) resin, or may be a blend of these.

[0017] Examples of the other monomers that may be contained in the polylactic acid include aliphatic hydroxycarboxylic acids other than lactic acid, aliphatic polyhydric alcohols, aliphatic polycarboxylic acids, and polyfunctional polysaccharides. When polylactic acid is a copolymer of lactic acid and other monomers, from the viewpoint of the crystallinity of polylactic acid, the content of the other monomers is preferably about 0 to 3 mol %, more preferably 0 to 2 mol %, based on the total monomers contained in polylactic acid.

[0018] The lactic acid raw material for producing polylactic acid is not particularly limited, and can be L-lactic acid, D-lactic acid, DL-lactic acid, or a mixture thereof, or L-lactide, D-lactide, meso-lactide, or a mixture thereof, etc. Lactic acid obtained by microbial fermentation from renewable raw materials derived from plants such as starch can be suitably used. The method for producing polylactic acid is not particularly limited, and any known method such as dehydration condensation polymerization or ring-opening polymerization can be used.

[0019] The molecular weight of the polylactic acid is not particularly limited and may be appropriately set depending on the application, but the number average molecular weight is preferably 1,000 to 700,000, and more preferably 10,000 to 300,000.

[0020] (copolyester) The marine biodegradation promoter according to the present embodiment contains a copolymer polyester of lactic acid and another hydroxycarboxylic acid as an active ingredient for promoting marine biodegradation. When the copolymer polyester comes into contact with polylactic acid, it is possible to promote the marine biodegradation of the polylactic acid itself.

[0021] In addition, since the copolyester is a polyester containing lactic acid as one of the constituent monomers, it has good compatibility with polylactic acid and can form a homogeneous mixture with polylactic acid. As a result, it is possible to avoid a substantial decrease in the transparency of polylactic acid. In addition, it is possible to plasticize polylactic acid and improve the elongation of polylactic acid.

[0022] The copolyester itself is a polymer material that is biodegradable. Bacteria that degrade copolymers of lactic acid and 3-hydroxybutyric acid have been isolated from the environment (see Polymer Degradation and Stability, 2014, 110, 44 and Applied Microbiology and Biotechnology, 2015, 99, 9555), so it is expected that the copolyester will be highly biodegradable in environments including seawater. In addition, when an enzyme secreted by the isolated copolyester biodegrading bacteria was isolated, it was found that it degrades poly-D-lactic acid oligomers of 31 or less.

[0023] The copolyester may be any of a random copolymer, an alternating copolymer, a block copolymer, and a graft copolymer. However, from the viewpoints of marine biodegradation promoting effect, transparency, mechanical properties, and availability, the copolyester is preferably a random copolymer. A random copolymer refers to a copolymer in which two or more types of monomer units are arranged in a random order. When the copolyester is produced by a microorganism, it is usually a random copolymer.

[0024] The lactic acid monomer unit in the polyester copolymer may be either an L-lactic acid monomer unit or a D-lactic acid monomer unit, or may contain either one of them or both.

[0025] When the copolyester is produced by a microorganism, the lactic acid monomer units in the copolyester are substantially composed of D-lactic acid monomer units. "Substantially composed of D-lactic acid monomer units" means that the proportion of D-lactic acid monomer units in the total amount of lactic acid monomer units is usually 90% or more, preferably 95% or more, more preferably 99% or more.

[0026] The hydroxycarboxylic acid other than lactic acid contained in the polyester copolymer is not particularly limited as long as it is a hydroxyalkanoic acid that can be copolymerized with lactic acid. The number of carbon atoms in the hydroxyalkanoic acid is preferably 3 or more. The upper limit of the number of carbon atoms is preferably 15 or less, more preferably 10 or less, even more preferably 8 or less, even more preferably 6 or less, and particularly preferably 5 or less.

[0027] Examples of hydroxyalkanoic acids include 2-hydroxyalkanoic acids, 3-hydroxyalkanoic acids, and 4-hydroxyalkanoic acids, with 3-hydroxyalkanoic acids being particularly preferred. Specific examples of 3-hydroxyalkanoic acid include 3-hydroxybutanoic acid (hereinafter sometimes abbreviated as 3HB), 3-hydroxypentanoic acid, 3-hydroxyhexanoic acid, 3-hydroxyheptanoic acid, 3-hydroxyoctanoic acid, 3-hydroxynonanoic acid, 3-hydroxydecanoic acid, 3-hydroxydodecanoic acid, 3-hydroxytetradecanoic acid, 3-hydroxypentadecanoic acid, and 3-hydroxyhexadecanoic acid. Only one of these may be contained in the copolymer polyester, or two or more may be contained. Among these, it is preferable that at least 3HB is contained. In particular, P(LA-co-3HB) (hereinafter sometimes abbreviated as LAHB), which is a copolymer polyester of lactic acid and 3HB, is the most preferable as the copolymer polyester.

[0028] The ratio of lactic acid units contained in the copolymer polyester is not particularly limited. However, since the effect of promoting marine biodegradation of polylactic acid is high, the molar fraction of lactic acid units relative to all monomer units constituting the copolymer polyester is preferably 10 to 70 mol%, more preferably 15 to 60 mol%, and even more preferably 15 to 50 mol%. The value of the molar fraction of lactic acid units can be determined using HPLC. Alternatively, it can be determined using NMR or GC.

[0029] The molecular weight of the copolymer polyester is not particularly limited, but the weight average molecular weight Mw may be, for example, 10,000 to 1,000,000, and preferably 10,000 to 500,000. The value of the weight average molecular weight can be determined based on standard polystyrene using gel permeation chromatography (GPC) (manufactured by Shimadzu Corporation) equipped with a tandem TSKgel Super HZM-H column (manufactured by Tosoh Corporation).

[0030] The method for producing the copolymer polyester of lactic acid and other hydroxycarboxylic acid is not particularly limited and may be a conventionally known method. It may be biosynthesized by a microorganism or may be produced by chemical synthesis. In particular, an example of a method for producing P(LA-co-3HB) is a production method using a recombinant microorganism as described in WO 2009 / 131186 and WO 2006 / 126796.

[0031] The marine biodegradation accelerator according to this embodiment may be composed only of a copolymerized polyester of lactic acid and another hydroxycarboxylic acid, or may contain components other than the copolymerized polyester. Examples of such other components include known resin additives, specifically, plasticizers, hydrolysis inhibitors, compatibilizers, antioxidants, UV absorbers, processing aids, antistatic agents, colorants, crystal nucleating agents, inorganic or organic particles, lubricants, release agents, water repellents, inorganic fillers, antifungal agents, antibacterial agents, foaming agents, flame retardants, etc.

[0032] The content of the copolymerized polyester in the marine biodegradation promoter is not particularly limited, and may be, for example, 10 to 100% by weight. The lower limit may be 20% by weight or more, 30% by weight or more, 50% by weight or more, 70% by weight or more, 90% by weight or more, or 99% by weight or more.

[0033] (Usage) By using the marine biodegradation promoter according to this embodiment so as to contact with polylactic acid, it is possible to obtain an effect of promoting marine biodegradation of polylactic acid. Specifically, it is preferable to mix the marine biodegradation promoter and polylactic acid so that they come into contact with each other, and it is particularly preferable to mix the two components uniformly, for example, by melt kneading, mixing in an organic solvent and then removing the solvent, etc.

[0034] (Amount used) The amount of the marine biodegradation promoter according to the present embodiment may be an amount that can promote marine biodegradation of polylactic acid by using the marine biodegradation promoter. From the viewpoint of the balance between the marine biodegradation promotion effect and the transparency and mechanical properties of polylactic acid, it is preferable to use the marine biodegradation promoter in an amount such that the amount of copolymerized polyester, which is an active ingredient of the marine biodegradation promoter, is 1 part by weight or more and 200 parts by weight or less per 100 parts by weight of polylactic acid. More preferably, it is 5 parts by weight or more and 100 parts by weight or less, and even more preferably, it is 10 parts by weight or more and 80 parts by weight or less. The lower limit of the amount of the copolymerized polyester may be 20 parts by weight or more, or may be 30 parts by weight or more. The upper limit may be 60 parts by weight or less, or may be 50 parts by weight or less.

[0035] (Resin composition) One aspect of the present invention may be a resin composition containing polylactic acid and the marine biodegradation promoter. Since the incorporation of the marine biodegradation promoter promotes marine biodegradation of the polylactic acid, the resin composition may exhibit good marine biodegradation.

[0036] (Other ingredients) The resin composition may contain a thermoplastic resin other than polylactic acid and the copolymer polyester. Such other thermoplastic resin is not particularly limited, and a conventionally known resin may be used. Specifically, biodegradable aliphatic polyesters other than polylactic acid and the copolymer polyester, aromatic polyesters, etc. may be used.

[0037] The amount of the other thermoplastic resin is not particularly limited, but may be, for example, 0 to 200 parts by weight relative to 100 parts by weight of polylactic acid. The upper limit may be 100 parts by weight or less, 50 parts by weight or less, 30 parts by weight or less, or 10 parts by weight or less.

[0038] In addition, resins that are poorly compatible with polylactic acid may deteriorate the transparency of polylactic acid, so it is preferable not to add them or to add them in a small amount when they are added. Although not particularly limited, for example, the amount of polyhydroxyalkanoate resin, which is one of the resins that are poorly compatible with polylactic acid, is preferably about 0 to 100 parts by weight, more preferably about 0 to 50 parts by weight, relative to 100 parts by weight of polylactic acid.

[0039] The resin composition may contain other additives as appropriate within the scope of not impairing the effects of the invention. Such additives are not particularly limited, but examples thereof include plasticizers, hydrolysis inhibitors, compatibilizers, antioxidants, UV absorbers, processing aids, antistatic agents, colorants, crystal nucleating agents, inorganic or organic particles, lubricants, release agents, water repellents, inorganic fillers, antifungal agents, antibacterial agents, foaming agents, and flame retardants. The content of each additive can be appropriately determined depending on the purpose. In addition, only one type of additive may be blended, or two or more types may be blended.

[0040] As the plasticizer, any plasticizer generally used as a plasticizer for polymers can be used. Specific examples of the plasticizer include polyester-based plasticizers, glycerin-based plasticizers, polycarboxylic acid ester-based plasticizers, polyalkylene glycol-based plasticizers, and epoxy-based plasticizers.

[0041] (Application) The resin composition can be prepared by melt-kneading each component, extruding the molten resin into a strand shape, cutting it, and forming it into pellets. The pellets obtained can be dried to remove moisture, and then molded by a known molding method to obtain any molded product. Such a molded product also constitutes one aspect of the present invention. Examples of the molding method include film molding, sheet molding, injection molding, blow molding, fiber spinning, extrusion foaming, and bead foaming.

[0042] The method for producing the film molded product is not particularly limited, but examples thereof include T-die extrusion molding, calendar molding, roll molding, and inflation molding. The obtained film can also be thermoformed by heating, vacuum molded, and press molded.

[0043] As a method for producing an injection molded article, for example, injection molding methods generally used when molding a thermoplastic resin, such as injection molding, gas-assisted molding, and injection compression molding, can be used. In addition, in addition to the above-mentioned methods, in-mold molding, gas press molding, two-color molding, sandwich molding, PUSH-PULL, SCORIM, and the like can also be used according to other purposes. However, the injection molding method is not limited to these.

[0044] The resin composition may be processed into pellets, or into a molded article in the form of a film, sheet, fiber, or the like, using an extrusion molding machine, or it can also be processed into a molded article of a predetermined shape by injection molding.

[0045] When the resin composition contains a foaming agent, the molded article may be a foamable molded article, or may be a molded foam obtained by foaming the foamable molded article.

[0046] The resin composition can be processed into molded articles of various shapes. Examples of the molded articles include paper, film, sheet, tube, plate, rod, container, bag, parts, etc. The molded article can also be compounded with other molded articles (e.g., fiber, thread, rope, woven fabric, knitted fabric, nonwoven fabric, paper, film, sheet, tube, plate, rod, container, bag, part, foam, etc.) made of a material different from the resin composition according to the present embodiment.

[0047] The applications of the molded articles are not particularly limited, and the molded articles can be suitably used in the fields of agriculture, fisheries, forestry, horticulture, medicine, sanitary products, clothing, non-clothing, packaging, automobiles, building materials, and other fields. The molded articles exhibit good marine biodegradability, and therefore can be suitably used in applications where they may be dumped into the ocean. Specific examples include, but are not limited to, packaging containers used in restaurants in seaside areas and bottles for storing sunscreens.

[0048] The following items enumerate preferred aspects of the present disclosure, but the present invention is not limited to the following items. [Item 1] A marine biodegradation promoter that promotes the marine biodegradation of polylactic acid, the marine biodegradation promoter comprising a copolymer polyester of lactic acid and another hydroxycarboxylic acid. [Item 2] The marine biodegradation promoter according to item 1, wherein the other hydroxycarboxylic acid is a 3-hydroxyalkanoic acid. [Item 3] The marine biodegradation promoter according to item 1 or 2, wherein the other hydroxycarboxylic acid is at least one selected from the group consisting of 3-hydroxybutanoic acid, 3-hydroxypentanoic acid, 3-hydroxyhexanoic acid, 3-hydroxyheptanoic acid, 3-hydroxyoctanoic acid, 3-hydroxynonanoic acid, 3-hydroxydecanoic acid, 3-hydroxydodecanoic acid, 3-hydroxytetradecanoic acid, 3-hydroxypentadecanoic acid, and 3-hydroxyhexadecanoic acid. [Item 4] 4. The marine biodegradation promoter according to any one of items 1 to 3, wherein the other hydroxycarboxylic acid is 3-hydroxybutanoic acid. [Item 5] 5. The marine biodegradation promoter according to any one of items 1 to 4, wherein the copolyester is a random copolymer of lactic acid and another hydroxycarboxylic acid. [Item 6] 6. The marine biodegradation promoter according to any one of items 1 to 5, wherein the lactic acid monomer of the copolyester is a D-lactic acid monomer. [Item 7] 7. The marine biodegradation promoter according to any one of items 1 to 6, wherein the molar fraction of lactic acid units in the copolymer polyester is 10 to 70 mol %. [Item 8] 8. The marine biodegradation promoter according to any one of items 1 to 7, wherein the copolymer polyester has a weight average molecular weight of 10,000 to 1,000,000. [Item 9] A resin composition comprising polylactic acid and the marine biodegradation promoter according to any one of items 1 to 8. [Item 10] Item 10. The resin composition according to item 9, wherein the amount of the copolyester is 1 to 200 parts by weight based on 100 parts by weight of the polylactic acid. [Item 11] 11. A molded article obtained by molding the resin composition according to item 9 or 10. [Item 12] A method for promoting marine biodegradation of polylactic acid, comprising contacting polylactic acid with a copolymer polyester of lactic acid and another hydroxycarboxylic acid. [Item 13] Use of a copolymerized polyester of lactic acid and another hydroxycarboxylic acid as a marine biodegradation promoter for promoting the marine biodegradation of polylactic acid. EXAMPLES

[0049] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0050] [Biosynthesis of copolymer polyesters] A copolymer polyester of lactic acid and 3-hydroxybutanoic acid was biosynthesized by recombinant Escherichia coli according to the description in the Journal of Biotechnology, 154 (2011), pp. 255-260. The biosynthesized copolymer polyester was extracted from the cells with chloroform. The resulting copolymerized polyester had a molar fraction of lactic acid of 40 mol % and a weight average molecular weight Mw of 99,000.

[0051] [Preparation of test samples] Polylactic acid (Ingeo 10361D, NatureWorks) and the copolymer polyester were mixed in a weight ratio of 8:2 (blend a) or 7:3 (blend b) in chloroform, and the solvent was removed and the mixture was dried to obtain a cast film. The film was freeze-crushed to obtain a powder of a blend consisting of polylactic acid and the copolymer polyester.

[0052] Similarly, a cast film composed only of the polylactic acid and a cast film composed only of the copolymer polyester were also produced. In addition, instead of the copolymer polyester, poly(hydroxybutyrate-co-hydroxyhexanoate) [PHBH, manufactured by Kaneka, 3-hydroxyhexanoic acid ratio: 11 mol%], which is a copolymer of 3-hydroxybutyric acid and 3-hydroxyhexanoic acid, was used to obtain a cast film containing the polylactic acid and PHBH in a weight ratio of 8:2 or 7:3 in the same manner as described above. A cast film consisting of only PHBH was also produced.

[0053] [Transmittance measurement] A film sample with a smooth surface was prepared as a sample for measuring transmittance using a hydraulic vacuum heating press (IMC-11FD type manufactured by Imoto Manufacturing Co., Ltd.). First, the cast film was melted at 180°C under vacuum conditions and pressure molded, and then the sample was removed and rapidly cooled between cold plates at room temperature to obtain a smooth film sample with a thickness of about 0.16 mm. The transmittance of each of the obtained smooth films was measured at a wavelength of 560 nm using a UV-1850 ultraviolet-visible spectrophotometer manufactured by Shimadzu Corporation. The results are shown in Table 1.

[0054] [Table 1]

[0055] From Table 1, it can be seen that the smooth film containing polylactic acid and copolymer polyester exhibits high transparency that is comparable to that of films made of polylactic acid alone or copolymer polyester alone, confirming that polylactic acid and copolymer polyester have a high affinity. On the other hand, it can be seen that the transparency of the smooth films containing polylactic acid and poly(hydroxybutyrate-co-hydroxyhexanoate) was significantly decreased.

[0056] [Marine biodegradability test] Foreign matter was removed from seawater collected from the port area of ​​Takasago City, Hyogo Prefecture, using a mesh with an opening of 80 μm. The seawater after removing foreign matter was placed in a plastic container, and 0.5 g of ammonium chloride and 0.1 g of dipotassium phosphate were added per 1 L of seawater in accordance with ASTM D-7081.

[0057] A sample of cellulose (Reference Example 1), PHBH (Reference Example 2), or blend a or b (Example 1 or 2) powdered to an average particle size of about 500 μm was added to 250 mL of seawater, and a BOD test was performed according to ASTM D6691. The test temperature was 30° C., and the amount of oxygen consumed was monitored using OxiTop IDS (WTW).

[0058] The degree of biodegradation was calculated by dividing the measured amount of oxygen consumed by the total amount of oxygen that would be consumed if all of the cellulose, PHBH, or blend were converted to carbon dioxide. Table 2 shows the average values ​​of the degree of biodegradation obtained from the three samples.

[0059] In addition, all solid components contained in the container after the test was completed on the 154th day were collected and dried in a vacuum dryer. The dried components were heated at 100°C with methanol and sulfuric acid to convert the polymer contained inside into methyl esters through a methanolysis reaction, and the amount of methyl esters was quantified by GC to quantify the total amount of polymer remaining in the solids collected from the seawater after the test. Based on this, the amount of polymer consumed was calculated. The results are shown in Table 2.

[0060] [Table 2]

[0061] As a result of the measurement, biodegradation proceeded smoothly in the cellulose or PHBH of Reference Example 1 or 2, and an increase in the amount of oxygen consumed was observed as the days passed. In addition, the amount of oxygen consumed was measured under the same conditions for seawater alone without the addition of organic matter such as cellulose, but no oxygen consumption was observed.

[0062] In blends a and b, if it is assumed that only the copolyester is completely biodegraded and converted to carbon dioxide, the degree of biodegradation would be 21% for blend a and 31% for blend b. If blends a and b showed a degree of biodegradation equal to or higher than these, it would mean that in addition to the copolyester, the biodegradation of the components other than the copolyester, i.e., polylactic acid, had also progressed.

[0063] As a result of the measurements, the biodegradability reached the level assumed to be achieved when only the copolymer polyester was completely biodegraded on the 121st day in Example 1, in which blend product a was used, and on the 119th day in Example 2, in which blend product b was used. Furthermore, in all of the Examples, by the 154th day, the biodegradation degree reached a level far exceeding that of the copolymer polyester alone, and it was confirmed that the biodegradation of the polylactic acid proceeded in addition to the copolymer polyester. This result indicates that the copolymer polyester promotes the biodegradation of the polylactic acid in seawater.

[0064] In Example 1, the polymer content in the solids recovered from the seawater after the test was 7.0% by weight on average for the three samples, and the weight of the residual polymer calculated from this was 2.6 mg on average for the three samples. This indicates that 89.6% of the 25 mg of polymer initially added was consumed and converted into carbon dioxide, bacterial components, or water-soluble components. A similar calculation revealed that 90.7% of the polymer was consumed in Example 2.

[0065] Furthermore, the ratio of 3HB in the residual polymer was analyzed and found to be 5 mol % in Example 1. From this value, the amount of the copolyester contained in the residual polymer was calculated to be 8.7 wt %. This indicates that both the copolymer polyester and polylactic acid are biodegraded and consumed.

[0066] <Consideration> As shown in Table 1, the blend of the polyester copolymer and polylactic acid exhibits high transparency. This suggests that the two components are either mutually compatible or form a morphology in which islands of the polyester copolymer are finely dispersed in the polylactic acid on a scale smaller than the wavelength of light.

[0067] In the former case, the plasticizing effect of the polyester copolymer may have made the molecular chains of polylactic acid more mobile, making it easier for decomposing enzymes to bind and for water molecules to penetrate, which may have resulted in a phenomenon in which the biodegradation of polylactic acid proceeds more easily.

[0068] In the latter case, the action of the polyester copolymer decomposition enzyme decomposes the islands of polyester copolymer exposed on the surface of the blend, generating voids, forming very fine irregularities on the polylactic acid surface, and the surface area can increase dramatically. As a result, chemical hydrolysis of polylactic acid may progress, producing oligomeric polylactic acid that can be decomposed by enzymes, and promoting the biodegradation of polylactic acid. In this case, the low molecular weight polyester copolymer that remains in the voids of polylactic acid may act as an acid, promoting the hydrolysis of the polylactic acid surface.

[0069] In addition, when the polyester copolymer is present on the surface of the blend, microorganisms that secrete enzymes that hydrolyze the polyester copolymer and can assimilate the decomposition products grow preferentially, and the high concentration of hydrolytic enzymes produced by these microorganisms may also promote the hydrolysis of polylactic acid.

[0070] It is speculated that the effect of promoting marine biodegradation of polylactic acid is achieved through one or a combination of two or more of the mechanisms mentioned above.

Claims

1. A marine biodegradation promoter that promotes the marine biodegradation of polylactic acid, the marine biodegradation promoter comprising a copolymer polyester of lactic acid and another hydroxycarboxylic acid.

2. The marine biodegradation promoter according to claim 1, wherein the other hydroxycarboxylic acid is a 3-hydroxyalkanoic acid.

3. The other hydroxycarboxylic acid is at least one selected from the group consisting of 3-hydroxybutanoic acid, 3-hydroxypentanoic acid, 3-hydroxyhexanoic acid, 3-hydroxyheptanoic acid, 3-hydroxyoctanoic acid, 3-hydroxynonanoic acid, 3-hydroxydecanoic acid, 3-hydroxydodecanoic acid, 3-hydroxytetradecanoic acid, 3-hydroxypentadecanoic acid, and 3-hydroxyhexadecanoic acid. The marine biodegradation promoter according to claim 1,

4. The marine biodegradation promoter according to claim 1, wherein the other hydroxycarboxylic acid is 3-hydroxybutanoic acid.

5. The marine biodegradation promoter according to any one of claims 1 to 4, wherein the copolyester is a random copolymer of lactic acid and another hydroxycarboxylic acid.

6. The marine biodegradation promoter according to any one of claims 1 to 4, wherein the lactic acid monomer of the copolymerized polyester is a D-lactic acid monomer.

7. The marine biodegradation promoter according to any one of claims 1 to 4, wherein the molar fraction of lactic acid units in the copolymer polyester is 10 to 70 mol %.

8. The marine biodegradation promoter according to any one of claims 1 to 4, wherein the weight average molecular weight of the copolymer polyester is 10,000 to 1,000,000.

9. A resin composition comprising polylactic acid and the marine biodegradation promoter according to any one of claims 1 to 4.

10. The resin composition according to claim 9, wherein the amount of the copolyester is 1 to 200 parts by weight based on 100 parts by weight of the polylactic acid.

11. A molded article obtained by molding the resin composition according to claim 9.

12. A method for promoting marine biodegradation of polylactic acid, comprising contacting polylactic acid with a copolymer polyester of lactic acid and another hydroxycarboxylic acid.

13. Use of a copolymerized polyester of lactic acid and another hydroxycarboxylic acid as a marine biodegradation promoter for promoting the marine biodegradation of polylactic acid.

Images