Composite resin molded article and method for manufacturing the same

Abstract

The composite resin molded article contains a base resin and a plurality of natural fibers dispersed in the base resin. When the composite resin molded article is taken to be 100 mass%, the content of the plurality of natural fibers is 10-99 mass% (inclusive). At least one of the plurality of natural fibers has a defibrated site formed at the end in the fiber length direction. At least one of the plurality of natural fibers has a portion exposed on the surface of the composite resin molded article. The base resin is a biodegradable plastic including any selected from the group consisting of polyhydroxy acids, polyhydroxyalkanoates, polyalkylene dicarboxylates, and modified starches.

Description

Technical Field

[0001] The present invention relates to a composite resin molded body having excellent mechanical properties and excellent biodegradability in a humid environment.

Background Art

[0002] So-called "general-purpose plastics" such as polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC) are not only very inexpensive, but also easy to mold and are several times lighter than metals or ceramics. Therefore, general-purpose plastics are often used as materials for various daily necessities such as bags, various packages, various containers, and sheets, and also as materials for industrial parts such as automotive parts and electrical parts, and daily necessities and miscellaneous goods.

[0003] Under such circumstances, the amount of plastic waste after use has been increasing year by year. Since plastic waste is a substance with characteristics that are difficult to decompose, it accumulates in the natural environment, causing environmental problems such as destruction and pollution of the natural environment. In recent years, as one of the measures to solve such problems, biodegradable plastics that are decomposed into water and carbon dioxide under natural environmental conditions have been proposed, and it is expected that their use will expand in place of general-purpose plastics using petroleum-based raw materials.

[0004] However, biodegradable plastics have drawbacks such as insufficient mechanical strength compared to general-purpose plastics. Therefore, biodegradable plastics do not have sufficient properties required for materials used in various industrial products including mechanical products such as automobiles and electrical, electronic, and information products, and their current application range is limited.

[0005] In addition, the biodegradation rate of biodegradable plastics is greatly affected by the environment. In an environment with few microorganisms such as the ocean, it takes a significantly long time to be completely decomposed, and the characteristic of biodegradability cannot be fully utilized.

[0006] To solve these problems, methods have been disclosed in which a decomposition accelerator containing enzymes or superabsorbent polymers is applied to biodegradable plastics before disposal (see, for example, Patent Document 1), or in which an inorganic filler such as talc, which has a decomposition-promoting effect, is compounded with biodegradable plastics (see, for example, Patent Document 2). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2013-23643 [Patent Document 2] Japanese Patent Publication No. 2017-132967 [Overview of the Initiative]

[0008] A composite resin molded article according to one aspect of the present invention is characterized in that it contains a main resin and a plurality of natural fibers dispersed in the main resin, wherein when the composite resin molded article is considered as 100% by mass, the content of the plurality of natural fibers is 10% by mass or more and 99% by mass or less, at least one of the plurality of natural fibers has a defibration portion formed at the end in the fiber length direction, at least one of the plurality of natural fibers has a portion exposed on the surface of the composite resin molded article, and the main resin is a biodegradable plastic containing any of the following selected from the group: polyhydroxy acid, polyhydroxyalkanoate, polyalkylene carboxylate, and modified starch.

[0009] A method for producing a composite resin molded article according to one aspect of the present invention includes the steps of preparing a main resin and natural fibers, and melt-kneading the natural fibers together with the main resin, wherein the defibrillation process is advanced from the ends in the fiber length direction of the natural fibers, thereby increasing the specific surface area of ​​the ends. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic cross-sectional view of a composite resin molded body according to Embodiment 1. [Figure 2] This is a schematic diagram of natural fibers, which are constituent components of a composite resin molded body according to Embodiment 1. [Figure 3] This is a schematic diagram of the manufacturing process for a composite resin molded article according to Embodiment 1. [Figure 4] This is a scanning electron microscope image of the surface of a composite resin molded article according to Embodiment 1. [Figure 5] This figure shows the structure and measurement results of the composite resin molded articles in the examples and comparative examples of the embodiment. [Modes for carrying out the invention]

[0011] Patent Document 1 presents the problem that a processing step is required before disposing of biodegradable plastics, and that the decomposition rate depends on the shape of the molded product.

[0012] Patent Document 2 presented a problem in that, because an inorganic filler with a higher specific gravity compared to the resin alone was compounded with the resin, the specific gravity of the composite resin molded product became high.

[0013] One aspect of the present invention aims to solve the above-mentioned conventional problems and to provide a composite resin molded article that does not require pretreatment at the time of disposal and whose biodegradation is promoted in the ocean or soil.

[0014] The composite resin molded article according to the first embodiment contains a main resin and a plurality of natural fibers dispersed in the main resin, wherein when the composite resin molded article is considered as 100% by mass, the content of the plurality of natural fibers is 10% by mass or more and 99% by mass or less, at least one of the plurality of natural fibers has a defibration portion formed at the end in the fiber length direction, and at least one of the natural fibers has a portion exposed on the surface of the composite resin molded article, and the main resin is a biodegradable plastic containing one selected from the group consisting of polyhydroxy acid, polyhydroxyalkanoate, polyalkylene carboxylate, and modified starch.

[0015] In the second aspect, the composite resin molded body may have a water absorption rate of 5% or more in the measurement method defined in JIS K7209:2000 in the first aspect.

[0016] In the third aspect, the composite resin molded body may not have the plurality of natural fibers in the composite resin molded body hydrophobized in the first or second aspect.

[0017] In the fourth aspect, the composite resin molded body according to any one of the first to third aspects includes a surface layer and an inner layer located deeper than the surface layer, and the concentration of the plurality of natural fibers in the surface layer may be higher than the concentration of the plurality of natural fibers in the inner layer.

[0018] In the fifth aspect, the plurality of natural fibers in the composite resin molded body according to any one of the first to fourth aspects may be celluloses.

[0019] The method for manufacturing a composite resin molded body according to the sixth aspect includes the steps of preparing a main resin and natural fibers, and melt-kneading the natural fibers together with the main resin, the step of expanding the specific surface area of the end portions by advancing defibration from the end portions in the fiber length direction of the natural fibers.

[0020] In the seventh aspect, the method for manufacturing a composite resin molded body may increase the water absorption amount of the composite resin molded body in a humid environment by subjecting the natural fibers to a drying treatment to reduce the moisture content to 5% or less and kneading them with the main resin in the melt-kneading step in the sixth aspect.

[0021] Compared with a resin alone, the composite resin molded body according to one aspect of the present invention can achieve a composite resin molded body with a high elastic modulus, high biodegradability, and a high biomass ratio of raw materials.

[0022] Hereinafter, the composite resin molded body and its manufacturing method according to the embodiment will be described with reference to the accompanying drawings. In the following description, the same reference numerals are given to the same components, and the description is appropriately omitted.

[0023] (Embodiment 1) FIG. 1 is a schematic cross-sectional view of the composite resin molded body 10 according to Embodiment 1. FIG. 2 is a schematic view of the natural fiber 2 which is a constituent member of the composite resin molded body 10 according to Embodiment 1.

[0024] The composite resin molded body 10 according to Embodiment 1 is composed of a melt-kneaded product containing a main resin 1, natural fibers 2, and an additive 3 as needed. As shown in the schematic cross-sectional view of FIG. 1, in the main resin 1 of the composite resin molded body 10, the natural fibers 2 and the additive 3 are dispersed.

[0025] Also, at least one of the natural fibers 2 is exposed on the surface of this composite resin molded body 10.

[0026] Furthermore, as shown in the schematic view of the natural fiber 2 in FIG. 2, by having a fibrillation site 4 at the end of the natural fiber 2, the specific surface area of the fibrillation site 4 increases, and the contact points between the natural fibers 2 increase. Thereby, it becomes possible to absorb water up to the inside of the composite resin molded body 10 through the contact points between the natural fibers 2 in a humid environment.

[0027] According to this composite resin molded body 10, at least one natural fiber 2 is exposed on the surface of the composite resin molded body, and the natural fibers 2 have contact points with each other. Therefore, in addition to an increase in the elastic modulus, it has high water absorption and can realize a composite resin molded body excellent in biodegradability in a humid environment.

[0028] Hereinafter, each member constituting this composite resin molded body will be described.

[0029] <Main Resin> In this embodiment, the main resin 1 is preferably a biodegradable plastic containing one of the following: a polyhydroxy acid, a polyhydroxyalkanoate, a polyalkylene carboxylate, or a modified starch. Furthermore, to ensure good moldability, it is preferably a thermoplastic resin, and the above resins may be used alone or in combination of two or more. Note that the main resin 1 is not limited to the above materials as long as it is biodegradable.

[0030] In this embodiment, "biodegradable plastic" refers to "a resin that has the same function as conventional petroleum-derived resins during use, and after use is ultimately decomposed into water and carbon dioxide by microorganisms in the soil and oceans in nature." Specifically, examples include polyester resins such as polyhydroxyalkanoates like polyhydroxybutyrate and polyhydroxyvalate, polyhydroxy acids such as polylactic acid, polyglycolic acid, and polycaprolactone, and polyalkylene carboxylates such as polybutylene adipate terephthalate, polyethylene succinate, and polybutylene succinate, as well as modified starch. Polyester resins include homopolymers of polyester monomers, copolymers of polyester monomers such as poly(3-hydroxybutyrate-co-3-hydroxyvalate), and copolymers of polyester monomers with other copolymerizable monomers. These polyester resins may be used individually or in combination of two or more types.

[0031] <Additives> Next, additive 3 will be described. Additive 3 may be used as needed for purposes such as improving the affinity between the main resin 1 and the natural fiber 2.

[0032] <Natural Fibers> Next, the natural fiber 2 will be described. The natural fiber 2 (hereinafter sometimes simply referred to as "fiber") contained in the composite resin molded article in this embodiment is used primarily for the purpose of promoting biodegradation of the composite resin molded article by increasing the surface area that can come into contact with microorganisms of the main resin 1 through water absorption and expansion in soil and ocean, without burdening the environment when discarded after use. For this purpose, the natural fiber 2 is preferably highly absorbent, and pulp, cellulose, cellulose nanofiber, lignocellulose, lignocellulose nanofiber, cotton, silk, or hemp are preferred.

[0033] The second purpose of adding natural fiber 2 is to improve mechanical properties and dimensional stability by lowering the coefficient of thermal expansion. For this purpose, it is preferable that natural fiber 2 has a higher elastic modulus than the main resin 1. Specifically, examples include pulp, cellulose, cellulose nanofiber, lignocellulose, lignocellulose nanofiber, cotton, silk, wool, or hemp. Among these, celluloses are particularly preferred from the viewpoint of availability, high elastic modulus, and low coefficient of thermal expansion. It should be noted that natural fiber 2 is not limited to the above materials as long as it can improve mechanical properties and has water absorption properties.

[0034] The content of natural fiber 2 is preferably 10% by mass or more and 99% by mass or less, when the composite resin molded body is considered to be 100% by mass. If the content of natural fiber 2 is less than 10% by mass, the natural fibers 2 will not easily make contact with each other inside the composite resin molded body, and it will not have sufficient water absorption. On the other hand, if the content of natural fiber 2 is greater than 99% by mass, the proportion of the main resin 1 will be small, so the effect of bonding the natural fibers 2 together will be lost and the moldability will be poor.

[0035] The morphology of the natural fibers 2 in the composite resin molded body will be described. A larger bonding interface between the natural fibers 2 and the main resin 1 increases the surface area that can come into contact with microorganisms in the main resin 1 when the natural fibers 2 absorb water and expand. Therefore, a high specific surface area of ​​the natural fibers 2 is preferable. On the other hand, to improve the water absorption of the composite resin molded body, it is preferable that the natural fibers 2 are exposed on the surface of the composite resin molded body. When the natural fibers 2 are exposed on the surface of the composite resin molded body, they absorb water from the exposed parts and absorb it into the interior of the composite resin molded body through capillary action of the fibers. The smaller the specific surface area of ​​the natural fibers 2 exposed on the surface of the composite resin molded body, the higher the water absorption. This is because when the specific surface area of ​​the natural fibers 2 exposed on the surface is large, the water repellency is increased due to the effect of fine irregularities.

[0036] A structure that satisfies the above can be obtained by adjusting the molding conditions to increase the shrinkage rate during molding of the composite resin molded body, and by having defibration portions 4 at the ends of the natural fibers 2 as shown in Figure 2. In other words, by increasing the shrinkage rate during molding, the natural fibers 2 are more likely to be exposed on the surface of the composite resin molded body. Furthermore, the central portion of the undefibrated natural fibers 2, which has a small specific surface area, has less entanglement with the main resin 1 and is more likely to be exposed on the surface of the composite resin molded body depending on the molding conditions. Conversely, the tip portion of the defibrated natural fibers 2 has more entanglement with the main resin 1 and penetrates into the interior together with the main resin 1. As a result, a composite resin molded body can be obtained in which the central portion of the natural fibers 2, excluding both ends, is exposed on the surface.

[0037] Figure 4 shows a scanning electron microscope (SEM) image of the surface of the composite resin molded body in the embodiment. As can be seen from Figure 4, only the central portion of the natural fiber 2 is exposed on the surface. The tip defibration portion 4 is preferably 5% or more and 50% or less of the total fiber length L of the natural fiber 2. If the defibration portion 4 is less than 5% of the total fiber length L, no improvement in elastic modulus is observed due to the small specific surface area, and if it is longer than 50%, the defibration portion 4 with a large aspect ratio is exposed on the surface of the composite resin molded body, and the water absorption deteriorates.

[0038] Next, we will explain the state of natural fibers 2 in the composite resin molded body. The composite resin molded body includes a surface layer and an internal layer located inside the surface layer. As mentioned above, depending on the molding conditions, it is possible to segregate the natural fibers 2 near the surface of the composite resin molded body. As a result, natural fibers 2 are more abundant in the surface layer than in the internal layer of the composite resin molded body. Furthermore, when considering the molded body as a whole, having more natural fibers 2 on the surface layer increases the elastic modulus of the outer layer, thus increasing the overall rigidity of the molded body. Therefore, a structure that segregates natural fibers 2 near the surface of the molded body also leads to improved rigidity. The segregation of natural fibers 2 near the surface can be evaluated by SEM observation of the cross-section of the composite resin molded body, etc.

[0039] Next, the properties of natural fiber 2 will be explained. As described above, the types of main resin 1 and natural fiber 2 are as follows. If natural fiber 2 is too soft relative to main resin 1, i.e., has a low modulus of elasticity, the composite resin molded product will have a low modulus of elasticity overall, resulting in reduced strength. On the other hand, if natural fiber 2 is too hard relative to main resin 1, i.e., has a high modulus of elasticity, the shock wave generated during impact will not propagate, and the impact will be absorbed at the interface between main resin 1 and natural fiber 2. As a result, cracks and crazing are more likely to occur near that interface, resulting in reduced impact strength. Therefore, it is preferable that the modulus of elasticity of natural fiber 2 is higher than that of main resin 1, and that the difference is as small as possible. The optimal relationship can be calculated from simulation results, and it is preferable that the difference in modulus of elasticity between main resin 1 and natural fiber 2 is within 20 GPa.

[0040] Furthermore, these natural fibers 2 may be surface-treated to improve their adhesion to the main resin 1 or their dispersibility in the composite resin molded body. However, if the surface treatment impairs the water absorption of the natural fibers 2, it is preferable not to surface-treat them beforehand.

[0041] <Method for manufacturing a composite resin molded product> Next, the method for manufacturing the composite resin molded article will be described. Figure 3 is a flowchart illustrating the manufacturing process of the composite resin molded article in this embodiment.

[0042] (1) The main resin 1, natural fibers 2, and additive 3 are introduced into the melt-kneading apparatus and melt-kneaded within the apparatus. As a result, the main resin 1 melts, and the natural fibers 2 and additive 3 are dispersed in the molten main resin 1. At the same time, the shearing action of the apparatus promotes the defibration of aggregates of natural fibers 2, allowing the natural fibers 2 to be finely dispersed in the main resin 1. At this time, the ends of the natural fibers 2 are defibrated, creating defibrated portions 4.

[0043] Traditionally, when fibers were compounded with resin, pre-processing such as wet dispersion was used to defibrate the fibers beforehand. However, when natural fibers are defibrated in the solvent used in wet dispersion, the fibers swell due to the solvent. Therefore, in order for the natural fibers to absorb water and swell sufficiently in the composite resin molded product, the solvent in the natural fibers must be dried before mixing with the main resin. Furthermore, defibration by wet dispersion is easier than defibration in the molten main resin, making it difficult to defibrate only the ends, resulting in the entire natural fiber being defibrated. In addition, the addition of pre-processing increases the number of steps and reduces productivity.

[0044] In contrast, in the manufacturing process of the composite resin molded article in this embodiment, pretreatment by wet dispersion for the purpose of defibration of the natural fibers 2 is not performed, and a melt-kneading process (all-dry method) is carried out together with the main resin 1 and additives 3 that function as dispersants. In this method, by not performing wet dispersion treatment of the natural fibers, swelling of the natural fibers 2 during the manufacturing process can be suppressed, and the water absorption expansion rate of the natural fibers 2 in the main resin 1 of the composite resin molded article can be improved. Furthermore, by drying the natural fibers 2 in advance or during kneading and adjusting the moisture content to 5% or less, the expansion rate when absorbing water in the main resin 1 can be further improved. In addition, since the natural fibers 2 have defibration sites 4 as described above, the fibers have many contact points with each other inside the composite resin molded article, and water can be absorbed into the inside of the composite resin molded article through the contact points between the fibers.

[0045] To produce the natural fiber 2 of this embodiment using an all-dry process, it is preferable to apply high shear stress during mixing. Specific mixing methods include single-screw mixers, twin-screw mixers, roll mixers, Banbury mixers, and combinations thereof. From the viewpoint of easily applying high shear stress and having high mass production potential, continuous twin-screw mixers and continuous roll mixers are particularly preferred. Any mixing method that can apply high shear stress is acceptable, other than those mentioned above.

[0046] (2) The composite resin composition extruded from the melting and mixing apparatus is formed into pellets through a cutting process such as a pelletizer. Methods of pelletization include methods performed immediately after the resin melting, such as the air hot cutting method, the underwater hot cutting method, and the strand cutting method. Alternatively, there are also crushing methods, which involve crushing and cutting after the molded body or sheet has been formed.

[0047] (3) By injection molding these pellets, injection molded products can be produced as composite resin molded bodies. As described above, the natural fibers 2 in the pellets are mixed with the main resin 1, which makes it possible to obtain injection molded products with excellent elastic modulus, impact resistance, and appearance.

[0048] The following describes each example and each comparative example from the experiments conducted by the inventors.

[0049] (Example 1) In Example 1, a cellulose-composite polylactic acid resin molded article was produced by the following manufacturing method.

[0050] Coniferous pulp (manufactured by Mitsubishi Paper Mills Ltd., product name: NBKP Celgar) was used as the starting material for the natural fibers. Polylactic acid (manufactured by Unitika Ltd., product name: TE-2000) was used as the main resin, and the above coniferous pulp, which had been pre-dried and adjusted to a moisture content of 5% or less, was weighed in a weight ratio of 50:50 and dry-blended.

[0051] Subsequently, the mixture was melt-mixed using a twin-screw mixer (KRC kneader manufactured by Kurimoto Iron Works Co., Ltd.). The screw was of the medium-shear type. The melt-mixing conditions were: main resin temperature 200°C, rotation speed 50 min / second. -1 The composite resin composition discharged from the twin-screw kneader was hot-cut to produce cellulose-composite polylactic acid resin pellets.

[0052] Test specimens of cellulose-composite polylactic acid resin molded products were prepared using the fabricated cellulose-composite polylactic acid pellets and an injection molding machine (Japan Steel Works 180AD). The conditions for preparing the test specimens were: main resin temperature 200°C, mold temperature 30°C, injection speed 100 mm / s, and holding pressure 100 Pa. The shape of the test specimens was changed according to the evaluation items described below, and a No. 1 size dumbbell was prepared for elastic modulus measurement. The obtained cellulose-composite polylactic acid resin molded product test specimens were evaluated by the following method.

[0053] (Fiber end defidulation) The obtained cellulose-composite polylactic acid resin pellets were immersed in xylene solvent to dissolve the polylactic acid, and the shape of the remaining cellulose fibers was observed using a scanning electron microscope (SEM). As a result, the ends of the fibers were found to be defibrillated.

[0054] (Elastic modulus of composite resin molded articles) Tensile tests were conducted using the obtained dumbbell-shaped specimen No. 1. Here, the elastic modulus was evaluated as follows: specimens with a value of less than 1.8 GPa were designated as C, specimens with a value of 1.8 GPa or more but less than 2.1 GPa were designated as B, and specimens with a value of 2.1 GPa or more were designated as A.

[0055] The elastic modulus of the test specimen was 2.1 GPa or higher, and its evaluation was A.

[0056] (Evaluation of water absorption rate of composite resin molded products) The water absorption rate was measured using the obtained dumbbell-shaped test specimen No. 1 in accordance with JIS K7209:2000. Specifically, the test specimen was dried in a 50°C dryer for 24 hours, its weight was measured, and then it was immersed in distilled water at 23°C for 168 hours. After wiping off the surface moisture, its weight was measured again. As an evaluation method for water absorption rate, specimens with a value of 5% or more were classified as A, and those with a value of less than 5% were classified as C.

[0057] The water absorption rate of the test specimen was 5.2%, and its evaluation was A.

[0058] (Evaluation of the biodegradability of composite resin molded products in seawater) A seawater biodegradation test was conducted using bar-shaped test pieces made from the obtained cellulose composite resin molded body. To assess seawater biodegradability, 50 mL of seawater was placed in a plastic container, and a bar-shaped test piece measuring 20 mm in height, 10 mm in width, and 4 mm in thickness, whose weight had been measured beforehand, was submerged in the seawater. The water temperature was maintained at 27°C, and the weight loss after 4 months was evaluated. The seawater was collected from the coastal area of ​​Nankoku City, Kochi Prefecture. As a method of evaluating the biodegradation rate, a value of 5% or more was designated as A, and a value of less than 5% was designated as C.

[0059] The biodegradability of the test specimen was 5.5%, and its evaluation was A.

[0060] (Comparative Example 1) In Comparative Example 1, the main resin was changed to polypropylene (product name: BC03C, manufactured by Nippon Polypropylene Co., Ltd.), and cellulose-composite polypropylene resin pellets and composite resin molded articles were produced using the same material and process conditions as in Example 1. The evaluation was also performed in the same manner as in Example 1.

[0061] (Comparative Example 2) In Comparative Example 2, the main resin was changed to polypropylene, and the weight ratio of polypropylene to softwood pulp was changed to 85:15. Cellulose-compound polypropylene resin pellets and composite resin molded articles were produced under the same material and process conditions as in Example 1. The evaluation was also carried out in the same manner as in Example 1.

[0062] (Comparative Example 3) In Comparative Example 3, polylactic acid resin was used as the raw material without compounding natural fibers, and the process conditions were the same as in Example 1 to produce a polylactic acid resin molded article. The evaluation was also performed in the same way as in Example 1.

[0063] (Comparative Example 4) In Comparative Example 4, polypropylene resin was used as the raw material without compounding natural fibers, and a polypropylene resin molded article was produced under the same process conditions as in Example 1. The evaluation was also carried out in the same manner as in Example 1.

[0064] (Comparative Example 5) In Comparative Example 5, the main resin was changed to polypropylene, and the weight ratio of polypropylene to softwood pulp was changed to 85:15. A layer of resin only was formed on the outer layer of the composite resin molded body by two-layer molding. Cellulose-composite polypropylene pellets and composite resin molded bodies were produced under the same material and process conditions as in Example 1. The evaluation was also performed in the same manner as in Example 1.

[0065] (Comparative Example 6) In Comparative Example 6, the main resin was changed to polypropylene, and the weight ratio of polypropylene to softwood pulp was changed to 85:15. The mold temperature during molding was set to 120°C, and the resin was slowly cooled to allow the fibers and additives to flow inward. Cellulose-compound polypropylene pellets and composite resin molded articles were produced using the same material and process conditions as in Example 1. The evaluation was also performed in the same manner as in Example 1.

[0066] (Comparative Example 7) In Comparative Example 7, PET fibers were used instead of natural fibers, and the main resin was changed to polypropylene. The weight ratio of PET fibers to polypropylene was 15:85, and a PET fiber composite polypropylene resin molded article was produced under the same process conditions as in Example 1. The evaluation was also performed in the same manner as in Example 1.

[0067] Figure 5 shows the composition and measurement results of the composite resin molded articles in Example 1 and each of Comparative Examples 1 to 7.

[0068] As is clear from Figure 5, in Example 1, in which polylactic acid was compounded with cellulose fibers, the elastic modulus was high at 2.1 GPa or higher, and the water absorption rate and biodegradability were also improved compared to the polylactic acid resin molded article of Comparative Example 3 due to the water absorption of the natural fibers. It was confirmed that if natural fibers are compounded, the fibers are defibrated at the ends, no prior hydrophobic treatment is performed, the fibers are exposed on the surface of the composite resin molded article, the concentration distribution of natural fibers is higher on the surface side, and the water absorption rate of the molded article is high, a composite resin with a high elastic modulus and high biodegradability can be obtained.

[0069] Comparative Example 1, which used polypropylene as the main resin, had a low water absorption rate for the polypropylene resin, resulting in a water absorption rate of less than 5% for the composite resin molded product. Furthermore, since polypropylene is not biodegradable, its biodegradation rate was also C.

[0070] In Comparative Example 2, where the weight ratio of polypropylene resin to coniferous pulp was 85:15, the fiber content decreased, resulting in lower elastic modulus and water absorption compared to Comparative Example 1.

[0071] In Comparative Example 3, which was molded using only polylactic acid resin, the elastic modulus was lower compared to Example 1 because it did not contain natural fibers. The water absorption rate was also low at 0.6%, and the biodegradability was rated C.

[0072] In Comparative Example 4, which was molded using only polypropylene resin, the elastic modulus was lower than that of Comparative Examples 1 and 2, at less than 2.1 GPa (rated B), due to the absence of natural fibers. Furthermore, the water absorption rate was also lower at 0.0%.

[0073] In Comparative Example 5, where the mold temperature during molding was set to 120°C and the resin was slowly cooled, allowing the fibers and additives to flow inward, the ratio of fibers in the molded body was greater on the inside than on the surface. As a result, the elastic modulus decreased to less than 2.1 GPa compared to Comparative Example 2, resulting in a rating of B. Furthermore, the water absorption rate also decreased to 0.3%.

[0074] In Comparative Example 6, where a layer of resin only was molded on the outer layer of the composite resin molded body using a two-layer molding process, no fibers were exposed on the surface of the molded body. As a result, the water absorption rate of the surface of the composite resin molded body decreased, and the water absorption rate of the composite resin molded body decreased to 0.1%.

[0075] In Comparative Example 7, where PET fibers were used instead of natural fibers, the water absorption rate of the fibers themselves was low, and the water absorption rate of the molded product decreased to 0.1%.

[0076] Based on the above evaluation, it was confirmed that a composite resin molded body with high elastic modulus and high biodegradability can be obtained if the fibers are defibrated at the ends, are not pre-treated for hydrophobicity, the central part of the fibers is exposed on the surface of the composite resin molded body, the concentration distribution of natural fibers is higher on the surface side, and the water absorption rate of the composite resin molded body is high.

[0077] Furthermore, this disclosure includes appropriately combining any of the various embodiments and / or examples described above, and the effects of each embodiment and / or example can be achieved. [Industrial applicability]

[0078] The composite resin molded article according to the present invention provides a molded article with superior mechanical strength and biodegradability compared to conventional biodegradable plastics. Because the properties of the main resin can be improved by the present invention, it can be used as a substitute for general-purpose plastics derived from petroleum. Therefore, the environmental impact of various industrial products and household goods made from general-purpose plastics derived from petroleum can be significantly reduced. Furthermore, it can be used in packaging materials, daily necessities, home appliance casings, building materials, and the like. [Explanation of Symbols]

[0079] 1. Main resin 2 Natural Fibers 3 Additives 4 Defibration site 10 Composite resin molding

Claims

1. A composite resin molded article containing a main resin and natural fibers consisting of a plurality of fibers dispersed in the main resin, When the composite resin molded body is considered to be 100% by mass, the content of the natural fibers consisting of the plurality of fibers is 10% by mass or more and 99% by mass or less. At least one of the natural fibers, which consists of the plurality of fibers, has a defibration site formed at its end in the direction of the fiber length. At least one of the natural fibers consisting of the plurality of fibers has a portion exposed on the surface of the composite resin molded body. The main resin is a biodegradable plastic containing one selected from the group consisting of polyhydroxyalkanoates and polyalkylenedicarboxylates, The natural fibers are a composite resin molded body having contact points between the natural fibers.

2. The composite resin molded article according to claim 1, wherein the water absorption rate of the composite resin molded article is 5% or more, as measured by the method specified in JIS K7209:2000.

3. The composite resin molded article according to claim 1 or 2, wherein the natural fibers consisting of the plurality of fibers in the composite resin molded article have not undergone hydrophobic treatment.

4. The composite resin molded body includes a surface layer and an internal layer located inside the surface layer. The composite resin molded article according to any one of claims 1 to 3, wherein the concentration of natural fibers consisting of the plurality of fibers in the surface layer is higher than the concentration of natural fibers in the inner layer.

5. The composite resin molded article according to any one of claims 1 to 4, wherein the natural fibers comprising the plurality of fibers are cellulose.

6. The steps include preparing the main resin and natural fibers, A step of melt-kneading the natural fibers together with the main resin, wherein the defibration process is advanced from the ends in the fiber length direction of the natural fibers, thereby increasing the specific surface area of ​​the ends. Includes, The method for producing a composite resin molded article according to claim 1, wherein the main resin is a biodegradable plastic containing one selected from the group consisting of polyhydroxyalkanoate and polyalkylene carboxylate.

7. The method for producing a composite resin molded article according to claim 6, wherein in the step of melt-kneading, the moisture content of the natural fibers is reduced to 5% or less by drying and then kneaded with the main resin, thereby increasing the water absorption capacity of the composite resin molded article in a humid environment.

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