Method for manufacturing resin compositions

Abstract

To realize a method for manufacturing a composite resin molded body that has high paintability while maintaining high rigidity, i.e., high elastic modulus. [Solution] A method for producing a resin composition. When producing a resin composition containing a base resin, a fibrous filler, and a dispersant, the amount of hydroxyl groups present on the surface of a molded article made from the resin composition is made greater than the amount of hydroxyl groups in the base resin by using a fibrous filler that has not been hydrophobized in advance. This makes the contact angle of the molded article with a solvent contained in a paint that can be used to paint the molded article smaller than the contact angle with the solvent of the base resin that makes up the resin composition.

Description

Technical Field

[0001] The present invention relates to a method for producing a resin composition, and particularly to a method for producing a resin composition for molding a molded body having paintability.

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 and ceramics. Therefore, general-purpose plastics are often used as materials for various daily necessities such as bags, various packages, various containers, sheets, etc., as industrial parts such as automotive parts and electrical parts, and as materials for daily necessities and miscellaneous goods.

[0003] However, general-purpose plastics have drawbacks such as insufficient mechanical strength. Therefore, general-purpose plastics do not have sufficient properties required for materials used in various industrial products such as mechanical products for automobiles and various industrial products including electric, electronic, and information products, and their scope of application is currently limited.

[0004] On the other hand, so-called "engineering plastics" such as polycarbonate, fluororesin, acrylic resin, and polyamide are excellent in mechanical properties and are widely used in various industrial products such as mechanical products for automobiles and various industrial products including electric, electronic, and information products. However, engineering plastics have problems such as high cost, difficulty in monomer recycling, and high environmental load.

[0005] Therefore, there is a demand for a significant improvement in the material properties (mechanical strength, etc.) of general-purpose plastics. A technology is known that improves the mechanical strength of general-purpose plastics by dispersing fibrous fillers such as natural fibers, glass fibers, and carbon fibers into the resin of the general-purpose plastic for the purpose of strengthening it. Among these, organic fibrous fillers such as cellulose are attracting attention as reinforcing fibers because they are inexpensive and have excellent environmental impact when disposed of.

[0006] When fiber-reinforced resins are to be used as exterior components for mechanical products such as automobiles, or various industrial products including electrical, electronic, and information products, painting or film application is necessary to make the surface uniform and eliminate the unevenness caused by the fibers. In order to utilize fiber-reinforced resins as exterior components, strong adhesion to paints and films is required as a surface property of the fiber-reinforced resin.

[0007] Companies are conducting research to improve the paintability of molded articles made with fiber-reinforced resins. For example, Patent Document 1 describes how adding a chlorinated polyolefin resin and a hydroxyl group-containing vinyl copolymer to the base coat adjusts the material and composition of the base coat layer to improve paintability and enhance the adhesion of the base coat layer to the molded article. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2000-518 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] Patent Document 1 uses a base coat as described above to improve the adhesion between the polyolefin molded article and the paint. However, since the base coat has low rigidity, there is a problem that the overall rigidity of the molded article, including the base coat, decreases. In addition, the technology in Patent Document 1 has the problem of increasing costs due to the large number of materials and processes involved.

[0010] The present invention aims to solve the above-mentioned conventional problems and to realize a method for manufacturing a composite resin molded article that has high paintability while maintaining high rigidity, i.e., high modulus of elasticity. [Means for solving the problem]

[0011] To achieve the above objective, the present invention provides a method for producing a resin composition containing a main resin, a fibrous filler, and a dispersant, characterized in that, when producing the resin composition, the fibrous filler is not pre-treated for hydrophobicity, thereby increasing the amount of hydroxyl groups present on the surface of the molded article formed with the resin composition to more than the amount of hydroxyl groups in the main resin, and thereby making the contact angle of the molded article with respect to the solvent contained in a paint to which the molded article can be coated smaller than the contact angle of the main resin constituting the resin composition with respect to the solvent. [Effects of the Invention]

[0012] The resin composition obtained by the manufacturing method of the present invention contains fibrous fillers, which allows for the creation of molded articles that maintain a high modulus of elasticity while possessing high paintability. [Brief explanation of the drawing]

[0013] [Figure 1] This is a schematic diagram of a painted composite resin molded article according to an embodiment of the present invention. [Figure 2] This is a schematic diagram of a fibrous filler in an embodiment of the present invention. [Figure 3] This is a flowchart of the manufacturing process for a painted composite resin molded article according to an embodiment of the present invention. [Modes for carrying out the invention]

[0014] The composite resin composition obtained by the manufacturing method of the embodiment of the present invention and the molded article using the same composition will be described below with reference to the drawings. In the following description, the same reference numerals are used for the same components, and detailed explanations will be omitted as appropriate.

[0015] The composite resin composition obtained by the manufacturing method of the embodiment of the present invention is formed from a molten mixture containing a main resin, a fibrous filler, and a dispersant. As shown in the schematic diagram including a cross-section in Figure 1, the fibrous filler 2 is dispersed in the main resin 1. A coating film 3 is then formed on the surface of a molded body 5 made from this resin composition.

[0016] The main resin 1 is preferably a thermoplastic resin in order to ensure good moldability. Examples of thermoplastic resins include olefin resins (including cyclic olefin resins), styrene resins, (meth)acrylic resins, organic acid vinyl ester resins or their derivatives, vinyl ether resins, halogen-containing resins, polycarbonate resins, polyester resins, polyamide resins, thermoplastic polyurethane resins, polysulfone resins (polyethersulfone, polysulfone, etc.), polyphenylene ether resins (polymers of 2,6-xylenol, etc.), cellulose derivatives (cellulose esters, cellulose carbamates, cellulose ethers, etc.), silicone resins (polydimethylsiloxane, polymethylphenylsiloxane, etc.), rubber or elastomers (diene rubbers such as polybutadiene and polyisoprene, styrene-butadiene copolymers, acrylonitrile-butadiene copolymers, acrylic rubber, urethane rubber, silicone rubber, etc.). The above resins may be used individually or in combination of two or more types. Furthermore, the main resin 1 is not limited to the above-mentioned materials as long as it is thermoplastic.

[0017] The main resin 1 is preferably an olefin resin with a relatively low melting point among these thermoplastic resins. Olefin resins include homopolymers of olefin monomers, copolymers of olefin monomers, and copolymers of olefin monomers with other copolymerizable monomers. Examples of olefin monomers include linear olefins (such as ethylene, propylene, 1-butene, isobutene, 1-pentene, 4-methyl-1-pentene, 1-octene, and other α-C2-20 olefins) and cyclic olefins. These olefin monomers may be used individually or in combination of two or more. Among the above olefin monomers, linear olefins such as ethylene and propylene are preferred. Other copolymerizable monomers include, for example, vinyl fatty acid esters such as vinyl acetate and vinyl propionate; (meth)acrylic monomers such as (meth)acrylic acid, alkyl (meth)acrylate, and glycidyl (meth)acrylate; unsaturated dicarboxylic acids or their anhydrides such as maleic acid, fumaric acid, and maleic anhydride; vinyl esters of carboxylic acids (e.g., vinyl acetate, vinyl propionate); cyclic olefins such as norbornene and cyclopentadiene; and dienes such as butadiene and isoprene. These copolymerizable monomers may be used alone or in combination of two or more. Specific examples of olefin resins include copolymers of chain-like olefins (especially α-C2-4 olefins), such as polyethylene (low-density, medium-density, high-density, or linear low-density polyethylene), polypropylene, ethylene-propylene copolymers, and terpolymers such as ethylene-propylene-butene-1.

[0018] The fibrous filler 2 will now be described. The fibrous filler 2 (hereinafter sometimes simply referred to as "fibers") contained in the composite resin composition obtained by the manufacturing method of this embodiment is used primarily for improving the mechanical properties of resin molded articles formed using the composite resin composition and improving dimensional stability by reducing the coefficient of linear expansion. For this purpose, it is preferable that the fibrous filler 2 has a higher elastic modulus than the main resin 1. Examples of such fibrous fillers 2 include carbon fibers, carbon nanotubes, pulp, cellulose, cellulose nanofibers, lignocellulose, lignocellulose nanofibers, basic magnesium sulfate fibers (magnesium oxysulfate fibers), potassium titanate fibers, aluminum borate fibers, calcium silicate fibers, calcium carbonate fibers, silicon carbide fibers, wollastonite, xonotlite, various metal fibers, natural fibers such as cotton, silk, wool, or hemp, regenerated fibers such as jute, rayon, or cupro, semi-synthetic fibers such as acetate and promix, synthetic fibers such as polyester, polyacrylonitrile, polyamide, aramid, and polyolefin, and modified fibers with chemical modifications to their surfaces and ends. Among these, carbons and celluloses are particularly preferred from the viewpoint of availability, high elastic modulus, and low coefficient of thermal expansion. Furthermore, natural cellulose fibers are preferred from the viewpoint of environmental friendliness.

[0019] The shape of the fibrous filler 2 will be explained with reference to Figure 2. In Figure 2, the symbol L represents the length of the fibrous filler 2 (hereinafter sometimes referred to as "fiber length"), and the symbol d represents the width of the fibrous filler 2 (hereinafter sometimes referred to as "fiber diameter"). If the aspect ratio (L / d) of the fibrous filler 2 is high, the fibrous filler tends to orient in the flow direction during injection molding. While the strength in the orientation direction of the fibrous filler is high, the strength in the direction perpendicular to it becomes weak, resulting in a decrease in impact strength in drop tests, etc. Therefore, it is preferable for the fibrous filler as a whole to have a small aspect ratio (L / d), that is, a large fiber diameter d.

[0020] On the other hand, from the perspective of mechanical properties, since a larger area of the bonding interface between the fiber and the resin leads to an improvement in the elastic modulus, it is preferable that the specific surface area of the fiber is high, that is, the fiber diameter d is small. To achieve the two objectives of a small aspect ratio and a large specific surface area, as shown in FIG. 2, within one fiber, a structure in which the end portions in the fiber length direction are partially fibrillated is most preferable. Reference numeral 4 indicates the fibrillated portion. The optimal fiber shape is calculated as follows from experimental and simulation results. That is, the length of the fibrillated portion 4 is preferably 5% or more and 50% or less of the fiber length L of the entire fibrous filler 2. When the fibrillated portions 4 are formed at both the one end and the other end of the fiber 2, the sum of the two is taken as the "length of the fibrillated portion 4". If the length of the fibrillated portion 4 is less than 5% of the total fiber length L, it is difficult to observe an improvement in the elastic modulus because the specific surface area is small. Also, if this exceeds 50%, the fibrillated portion 4 with a large aspect ratio becomes dominant, so it becomes easy to orient during injection molding and the impact strength tends to decrease.

[0021] From the perspective of the paintability of the molded body composed of the resin composition, as shown in FIG. 2, within one fiber, a structure in which the end portions in the fiber length direction are partially fibrillated is most preferable. This is because this structure increases the surface area of the fibers existing near the surface of the molded body, improves the wettability of the paint, and improves the paintability.

[0022] The paintability is further improved by the exposure of the fibrillated portion 4 of the fiber, in which the end portions in the fiber length direction are partially fibrillated, on the surface of the molded body. By controlling the molding conditions during molding, the above structure can be created. The length exposed on the surface is preferably 0.1% or more and 50% or less of the fibrillated portion 4, that is, the fibrillated fiber portion. If it is less than 0.1%, the fibers are not sufficiently exposed and the paintability tends to deteriorate. On the other hand, if 50% or more of the fibers are exposed, the touch feeling deteriorates and it becomes difficult to use as a product.

[0023] In the above structure where the fiber fibrillation part 4 of the fiber is exposed on the surface of the molded body, since it is more exposed at the end part than at the central part of the molded body, the paint adhesion is improved. This is because when used as a product, the paint is likely to peel off from the end part. By having more fibers at the end part, the paint adhesion at the end part can be improved, and paint peeling can be suppressed. Aggregating more fibers from the central part to the end part is possible by controlling the molding conditions.

[0024] The properties required for the fibrous filler 2 will be described. Regarding the types of the main resin 1 and the fibrous filler 2, as described above, if the fibrous filler 2 is too soft with respect to the main resin 1, that is, if the elastic modulus is too small, the composite resin composition will have a small elastic modulus as a whole, and as a result, the strength will decrease. On the other hand, if the fibrous filler 2 is too hard with respect to the main resin 1, that is, if the elastic modulus is too large, the shock wave generated during impact will not be propagated and will be absorbed at the interface between the main resin 1 and the fibrous filler 2. Therefore, cracks and crazes are likely to occur near the interface, and as a result, the impact resistance strength will drop. Therefore, regarding the relationship between the elastic moduli of the main resin 1 and the fibrous filler 2, it is preferable that the elastic modulus of the fibrous filler 2 is higher and the difference therebetween is as small as possible. The optimal relationship is calculated from the simulation results, and the difference in the elastic moduli of the main resin 1 and the fibrous filler 2 is preferably within 20 GPa.

[0025] These fibrous fillers 2 are almost all hydrophilic except for artificial fibers derived from petroleum. In particular, the cellulose-based fibers described above have many hydroxyl groups in the molecule and are hydrophilic. When these hydrophilic fillers are combined with a resin, since the resin is hydrophobic, they are generally hydrophobized. However, in the present invention, for the purpose of improving the paintability, pre-hydrophobization is not performed. By not performing pre-hydrophobization, the hydrophilic groups of the fibers are likely to remain, and since the fibers are present near the surface of the molded body, the paintability is improved.

[0026] The fibrous filler 2 may be partially surface-treated with a titanate coupling agent; a silane coupling agent; an unsaturated carboxylic acid, maleic acid, maleic anhydride, or a modified polyolefin grafted with the same anhydride; a fatty acid; a fatty acid metal salt; or a fatty acid ester, for purposes such as improving adhesion to the main resin 1 or dispersibility in the composite resin composition. Alternatively, it may be partially surface-treated with a thermosetting or thermoplastic polymer component. In the present invention, the fibers are defibrated in the composite resin, but the inside of the defibrated fibers is hydrophilic. As a result, in the final molded article, the defibrated ends of the fibers in the molded article become highly hydrophilic, and the exposure of the defibrated ends to the surface of the molded article further improves paintability. At the same time, the hydrophobic groups in the center of the fibers disperse well with the resin, enabling high rigidity. By leaving some parts that are not hydrophobic as described above, it is possible to achieve both affinity with the resin and affinity with the paint.

[0027] As described above, by using fibers that are not pre-hydrophobized, the amount of hydroxyl groups present on the surface of the molded body becomes greater than the amount of hydroxyl groups in the main resin 1. As a result, the contact angle of the molded body with respect to the solvent contained in the paint used for coating becomes lower than the contact angle of the main resin 1 without fibers with respect to the solvent, making it easier for the paint to penetrate. Since the fibers are rooted within the molded body, the paint adhesion is good and the paint film is less likely to peel off. Furthermore, even in the case of partially hydrophobized fibers, as described above, the defibration portion 4 at the tip is highly hydrophilic and easily exposed to the surface, so hydrophilic paints penetrate easily and exhibit excellent paintability. Therefore, in the molded body obtained by the manufacturing method of the present invention, even with hydrophilic paints that do not easily adhere to the main resin 1, the contact angle with respect to the paint is reduced, wettability is increased, and paintability is improved.

[0028] The surface of a composite resin molded article is generally roughened to improve paintability. A roughened surface is an aggregate of multiple fine irregularities, but in this invention, the amount of hydroxyl groups in the recesses of these irregularities is greater than that in the protrusions. This is because the fibers present near the surface of the molded article are more exposed on the surface of the recesses. This structure increases the affinity of the surface layer for paint protection and decoration in the recesses, and also increases the adhesion area between the paint and the composite resin, thus further improving paintability.

[0029] The dispersant will now be described. The composite resin composition in the present invention contains a dispersant for purposes such as improving the adhesion between the fibrous filler 2 and the main resin 1, or improving the dispersibility of the fibrous filler 2 in the main resin 1. Examples of dispersants include various titanate coupling agents; silane coupling agents; unsaturated carboxylic acids, maleic acid, maleic anhydride, or modified polyolefins grafted with the same anhydride; fatty acids; fatty acid metal salts; and fatty acid esters. The silane coupling agent is preferably an unsaturated hydrocarbon or epoxy type. The surface of the dispersant may be modified with a thermosetting or thermoplastic polymer component. The content of the dispersant in the resin composition obtained by the manufacturing method of this embodiment is preferably 0.01% by mass or more and 20% by mass or less, more preferably 0.1% by mass or more and 10% by mass or less, and even more preferably 0.5% by mass or more and 5% by mass or less. If the dispersant content is less than 0.01% by mass, dispersion failure is likely to occur. On the other hand, if the dispersant content exceeds 20% by mass, the strength of the composite resin molded product made from the resin composition is likely to decrease. The dispersant is appropriately selected based on the combination of the main resin 1 and the fibrous filler 2.

[0030] The method for manufacturing the composite resin composition described above will now be explained. Figure 3 is a flow chart illustrating the manufacturing process of the composite resin composition in this embodiment. First, the main resin, fibrous filler, and dispersant are introduced into a melt-kneading apparatus and melt-kneaded within the apparatus. This melts the main resin, and the fibrous filler and dispersant are dispersed in the molten main resin. At the same time, the shearing action of the apparatus promotes the defibrillation of aggregates of the fibrous filler, allowing the fibrous filler to be finely dispersed in the main resin.

[0031] Conventionally, fibrous fillers have been used that have been pre-treated by methods such as wet dispersion to defibrate the fibers. However, when fibrous fillers are pre-defibrated in the solvent used in wet dispersion, they defibrate more easily than when they are defibrated in the molten main resin, making it difficult to defibrate only the ends, resulting in the entire fibrous filler being defibrated. In addition, the pre-treatment increases the number of steps, leading to problems such as decreased productivity.

[0032] In contrast, in the manufacturing process of the composite resin composition in this embodiment, the fibrous filler is melt-kneaded together with the main resin and dispersant (all-dry method) without performing defibration treatment of the fibrous filler or pretreatment by wet dispersion for the purpose of modification treatment. In this method, by not performing wet dispersion treatment of the fibrous filler, the fibrous filler can be partially defibrated only at the ends as described above, and the number of steps is reduced, improving productivity.

[0033] To produce a composite resin composition of the present invention using a completely dry process, it is preferable to apply high shear stress during mixing. Specific mixing equipment for this purpose includes single-screw mixers, twin-screw mixers, roll mixers, and Banbury mixers. Continuous twin-screw mixers and continuous roll mixers are particularly preferred from the viewpoint of easily applying high shear stress and having high mass production capabilities. Any mixing equipment capable of applying high shear stress other than those mentioned above may be used.

[0034] The composite resin composition extruded from the melting and mixing equipment is then cut into pellets using a pelletizer or similar device. Methods of pelletization include methods performed immediately after resin melting, such as air hot cutting, underwater hot cutting, and strand cutting. Alternatively, there are methods such as crushing and cutting after the material has been molded into a molded body or sheet.

[0035] By injection molding these pellets, injection-molded products can be produced as composite resin molded bodies. As described above, the fibrous fillers in the pellets, which are not pre-hydrophobic, have their ends partially defibrated, and these fibers are exposed on the surface of the molded body, resulting in injection-molded products with improved paintability. [Examples]

[0036] Examples and comparative examples of the present invention will be described below.

[0037] (Example 1) A pulp-dispersed polypropylene composite resin molded article was produced by the following manufacturing method.

[0038] Polypropylene (PP) (Prime Polymer Co., Ltd., product name: J108M) as the main resin, cotton-like softwood pulp (Mitsubishi Paper Mills, Ltd., product name: NBKP Celgar) as a fibrous filler, and maleic anhydride (Sanyo Chemical Industries, Ltd., product name: Yumex) as a dispersant were weighed in a mass ratio of 85:15:5 and dry blended. The cotton-like softwood pulp used as the fibrous filler was not hydrophobicized beforehand; instead, it was kneaded with a compatibilizer in the resin. Subsequently, the mixture was melt-kneaded in a twin-screw kneader (Kurimoto Iron Works Co., Ltd., KRC Kneader) to disperse each component. The shear force could be changed by changing the screw configuration of the twin-screw kneader, and in Example 1, a medium-shear type specification was used. The molten resin was hot-cut to produce pulp-dispersed polypropylene pellets.

[0039] Using the prepared pulp-dispersed polypropylene pellets, test specimens of composite resin molded articles were fabricated using an injection molding machine (Japan Steel Works, Ltd., 180AD). The conditions for fabricating the test specimens were a resin temperature of 190°C, a mold temperature of 60°C, an injection speed of 60 mm / s, and a holding pressure of 80 Pa. The pellets were fed into the molding machine's screw via a hopper, and the rate of penetration was measured by the amount of pellets lost per unit time, confirming that it was constant. The shape of the test specimens was changed according to the evaluation items described below. Specifically, a No. 1 size dumbbell was fabricated for measuring the elastic modulus, and a 60 mm square, 1.2 mm thick flat plate was fabricated for testing coating adhesion. The obtained pulp-dispersed polypropylene composite resin molded article test specimens were evaluated using the following method. The following evaluation methods were also applied to other examples and comparative examples.

[0040] [Presence or absence of fiber exposure, percentage of exposure, in-plane uniformity of exposed areas] The morphology of the fibers exposed on the surface of the obtained pulp-dispersed polypropylene composite resin molded body was observed by SEM. Since the above SEM is for planar observation, the exposure ratio in the depth direction was observed in three dimensions by repeatedly polishing and observing in increments of a few micrometers.

[0041] In Example 1, approximately 10 representative fibers were measured, and the results showed fiber exposure on the surface. The exposure rate, i.e., the length of the exposed portion, was approximately 0.5-10% of the fiber length, and the in-plane uniformity of the exposed portion was "uniformity in the center < uniformity at the edges".

[0042] [Percentage of fiber defibration by length] The resulting composite resin molded body was immersed in xylene solvent to dissolve the polypropylene, and the morphology of the remaining pulp fibers was observed using a scanning electron microscope (SEM).

[0043] In Example 1, defibration was observed at the ends in the length direction of the fiber, and the length of the defibration was approximately 20-30% of the total fiber length.

[0044] [Contact angle with respect to the main resin] The contact angle of the resulting composite resin molded body was measured using a solvent for paint. The contact angle was measured by the liquid application method, which involves applying a droplet to the surface of the molded body and measuring the angle it makes with the surface of the molded body when the droplet is in contact with the surface.

[0045] In Example 1, the contact angle was measured using toluene, a solvent contained in a synthetic resin paint, assuming coating with a synthetic resin paint. At the same time, the contact angle of polypropylene alone, which is the main resin of the composite resin molded body, was also measured and compared, and it was found that the contact angle of the composite resin molded body was smaller than that of the main resin.

[0046] [Relative to the amount of hydroxyl groups in the composite resin molded product relative to the main resin] The amount of hydroxyl groups in the main resin and the amount of hydroxyl groups in the uneven areas were measured by microscopic FT-IR on the obtained composite resin molded product.

[0047] In Example 1, the amount of hydroxyl groups in the resulting composite resin molded article was greater than the amount of hydroxyl groups relative to the main resin.

[0048] [Elastic modulus of composite resin molded articles] Tensile tests were conducted using the obtained dumbbell-shaped test specimen (No. 1). Here, the elastic modulus was evaluated as follows: specimens with an elastic modulus of less than 1.8 GPa were considered defective, while those with an elastic modulus of 2.0 GPa or higher were judged as good. In Example 1, the elastic modulus of the test specimen was 2.2 GPa, and its evaluation was "good quality".

[0049] [Texture] The resulting flat test pieces were used for sensory evaluation of their texture. ○ indicated no issues, △ indicated slight roughness, and × indicated significant roughness and a strong sense of discomfort.

[0050] The tactile feel of the composite resin molded product in Example 1 was evaluated as "○".

[0051] [Coating adhesion] The resulting composite resin molded bodies were painted and evaluated using a cross-section peel test (JIS K 5600). The results were evaluated as follows: ○ for no peeling of the paint film, △ for peeling of 30% or less of the surface area, and × for peeling of 50% or more of the surface area.

[0052] In Example 1, a synthetic resin paint containing toluene as a solvent was used for brush coating. The resulting painted composite resin molded product showed no peeling of the paint film and received a "○" rating.

[0053] (Example 2) Compared to Example 1, the screw configuration of the twin-screw kneader was changed to a low-shear type. Other material and process conditions were the same as in Example 1, and pulp-dispersed polypropylene pellets and coated molded articles were produced and evaluated.

[0054] (Example 3) Compared to Example 1, the screw configuration of the twin-screw kneader was changed to a high-shear type. Other material and process conditions were the same as in Example 1, and pulp-dispersed polypropylene pellets and coated molded articles were produced and evaluated.

[0055] (Example 4) Pulp-dispersed polypropylene pellets and molded articles were prepared under the same material and process conditions as in Example 1. Furthermore, the surface resin was slightly removed by etching with xylene as a solvent, and then painting was performed. The resulting painted molded articles were then evaluated.

[0056] (Example 5) Compared to Example 1, the screw configuration of the twin-screw kneader was changed to a conveying screw that is subjected to almost no shear. The other material and process conditions were the same as in Example 1, and pulp-dispersed polypropylene pellets and coated molded articles were produced and evaluated.

[0057] (Example 6) Compared to Example 1, the pulp fibers were pre-treated to defibrate them using a refiner, and the screw configuration of the twin-screw kneader was changed to a high-shear type. Other material and process conditions were the same as in Example 1, and pulp-dispersed polypropylene pellets and coated molded articles were produced and evaluated.

[0058] (Example 7) Compared to Example 1, the mold temperature during molding was reduced to 20°C or lower by water cooling, allowing a resin skin layer to form on the surface of the molded article. Other material and process conditions were the same as in Example 1. Pulp-dispersed polypropylene pellets and coated molded articles were prepared and evaluated.

[0059] (Example 8) Compared to Example 4, the etching time was significantly increased, resulting in a higher percentage of exposed fibers. Otherwise, the evaluation was conducted in the same manner as in Example 4.

[0060] (Example 9) Compared to Example 1, the only modification was to water-cool the sides of the molding die to keep the die temperature below 20°C, thereby allowing a resin skin layer to form on the surface of the edges of the molded product. All other material and process conditions were the same as in Example 1, and pulp-dispersed polypropylene pellets and coated molded products were prepared and evaluated.

[0061] (Comparative Example 1) Compared to Example 1, the only change was that the pulp fibers were pre-treated with a silane coupling agent for hydrophobicity. Other material and process conditions were the same as in Example 1. Pulp-dispersed polypropylene pellets and coated molded articles were then prepared and evaluated.

[0062] Table 1 shows the molding conditions and evaluation results for Examples 1-9 and Comparative Example 1.

[0063] [Table 1]

[0064] As is clear from Table 1, Example 1 obtained excellent evaluation results. In Example 2, where the screw configuration was changed to a low-shear type, the fibers were not defibrated much in the molten resin, and the length ratio of the defibrated portion was 5-10%. Conversely, in Example 3, where the screw configuration was changed to a high-shear type, the fibers were well defibrated in the molten resin, and the length ratio of the defibrated portion was 40-50%. In Example 4, where the surface of the molded body was etched, only the resin on the surface was dissolved, and the exposure ratio of the fibers on the surface of the molded body relative to the fiber length was 20-45%. Examples 2, 3, and 4 all had no problems with elastic modulus and paintability, similar to Example 1. Furthermore, it was confirmed that if the length ratio of the defibrated portion is 5-50%, and there is fiber exposure on the surface of the molded body, and the exposure ratio is in the range of 0.1-50%, paintability can be improved without reducing the elastic modulus.

[0065] In Example 5, where the conveying screw was changed to one that does not apply shear, and the fibers were not significantly defibrated in the molten resin, the length ratio of the defibrated portion was 0-4%. As a result, the specific surface area of ​​the fibers in the molded body was low, and the elastic modulus decreased slightly to 1.9 GPa, but there were no problems with paintability.

[0066] In Example 6, where the pulp fibers were pre-treated with a refiner to remove defibration, and the screw configuration during kneading was changed to a high-shear type, the length ratio of the defibrated portion became 80-100%. As a result, the number of thin and short fibers increased, making it difficult for the paint to penetrate the molded body. In the adhesion evaluation using a grid test, approximately 20% peeled off, resulting in a slight decrease in paintability.

[0067] In Example 7, where the mold temperature during molding was kept below 20°C by water cooling, a resin skin layer was formed on the surface of the molded body, resulting in no fiber exposure on the surface of the molded body. As a result, the paint did not easily wet the molded body, and approximately 20% of the paint peeled off in the adhesion evaluation using a grid test, resulting in a slight decrease in paintability.

[0068] In Example 8, where the surface of the molded body was etched for a long time, only the resin on the surface was dissolved, and the exposure ratio of the fibers relative to their length on the surface of the molded body was 55-70%. When paint was applied on top of this, the adhesion of the paint film was good, but some fibers were exposed on the surface of the painted molded body, resulting in a slightly poor tactile feel.

[0069] In Example 9, where only the sides of the mold were cooled to below 20°C by water cooling during molding, allowing a resin skin layer to form on the surface of the molded body's edges, the degree of fiber exposure on the edge surface, which is closer to the side of the painted molded body, was smaller than the degree of fiber exposure in the center. As a result, the paint adhesion at the edges deteriorated. Since paint tends to peel off from the edges, if the edges are weak, the entire paint film becomes more prone to peeling. In the adhesion evaluation using a grid test, approximately 20% peeled off, resulting in a slight decrease in paintability.

[0070] In Comparative Example 1, where pulp fibers were pre-treated hydrophobically with a silane coupling agent, the contact angle of the molded body with respect to the solvent contained in the paint used for coating the molded body was approximately the same as the contact angle of the main resin with respect to the same solvent. Furthermore, the amount of hydroxyl groups in the composite resin molded body and the main resin were also approximately the same. As a result, approximately 50% of the adhesion was peeled off in the cross-sectional test, resulting in poor paintability.

[0071] Based on the above evaluation, it was found that by using a resin material in which only the ends of the fibers are defibrated, a high modulus of elasticity can be achieved when manufacturing a molded body. Furthermore, because the contact angle of the molded body with respect to the solvent contained in the paint used to coat the molded body is smaller than the contact angle of the main resin with respect to the same solvent, a composite resin coated molded body with good paintability and a high modulus of elasticity can be provided without the use of a primer layer. Similarly, because the amount of hydroxyl groups in the composite resin molded body is greater than the amount of hydroxyl groups in the base resin, and because fibers are exposed on the surface of the molded body, a composite resin coated molded body with good paintability and a high modulus of elasticity can be provided without the use of a primer layer. [Industrial applicability]

[0072] The composite resin composition according to the present invention can provide molded articles with better coating adhesion and superior mechanical strength compared to conventional general-purpose resins. Since the resin composition in the present invention can improve the properties of the main resin, it can be used as a substitute for engineering plastics or metal materials. Therefore, it can significantly reduce the manufacturing costs associated with various industrial products or household goods made of engineering plastics or metals. For this reason, it can be used in home appliance casings, building materials, and automotive components. [Explanation of symbols]

[0073] 1. Main resin 2. Fibrous filler 3. Coating film 4 Defibration site 5 Molded body

Claims

1. When manufacturing a resin composition containing a main resin, a fibrous filler, and a dispersant, A process is carried out using a fibrous filler that has not been pre-treated for hydrophobicity, a main resin, and a dispersant, and using a twin-screw kneader which is a melt-kneading device with low to high shear specifications, the shear action due to the low to high shear specifications promotes the defibrillation of the aggregated mass of the fibrous filler, and the fibrous filler is partially defibrillated only at the ends, A step of obtaining a resin composition by discharging a kneaded mixture containing the fibrous filler, the main resin, and the dispersant from the twin-screw kneader, Includes, By using the aforementioned fibrous filler that has not been pre-hydrophobized, the amount of hydroxyl groups present on the surface of the molded body formed with the resin composition is made greater than the amount of hydroxyl groups in the main resin, thereby making the contact angle of the molded body with respect to the solvent contained in the paint to which the molded body can be coated smaller than the contact angle of the main resin constituting the resin composition with respect to the solvent, A method for producing a resin composition, comprising obtaining a resin composition containing a fibrous filler in which only the ends of the fibers are defibrated and the defibrated portion comprises 5% to 50% of the fiber length.

2. A method for producing a resin composition according to claim 1, characterized in that natural cellulose fibers are used as the fibrous filler that has not been pre-treated for hydrophobicity.

3. A method for producing the resin composition according to claim 1 or 2, characterized in that an olefin resin is used as the main resin component.

4. A method for manufacturing a molded article, characterized by molding it using a resin composition obtained by the method for manufacturing a resin composition described in any one of claims 1 to 3.

5. The method for manufacturing a molded article according to claim 4, characterized in that the defibrated portion of the fibrous filler, in which only the ends have been defibrated, is exposed from the surface of the molded article.

6. The method for manufacturing a molded article according to claim 5, characterized in that the length of the defibrated portion of the fibrous filler exposed from the surface of the molded article is 0.1% or more and less than 50% of the fiber length of the fibrous filler.

7. A method for coating a molded article, characterized in that a molded article obtained by a method for manufacturing a molded article according to any one of claims 4 to 6 is coated with a solvent-containing coating, wherein the contact angle of the molded article with respect to the solvent is smaller than the contact angle of the main resin in the resin composition forming the molded article with respect to the solvent.

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