Method for manufacturing resin-molded article

Abstract

This method for manufacturing a resin-molded article comprises a step (A) for preparing a resin composition containing a thermoplastic resin and cellulose nanofibers, and a step (B) for molding the resin composition into a resin-molded article. The method additionally comprises a step (C) for drying the resin composition before the step (B).

Description

Manufacturing method for resin molded products

[0001] The present invention relates to a method for producing a resin molded product.

[0002] In recent years, cellulose nanofibers, which are obtained by defibrating cellulose fibers to nano-size, have been attracting attention. Cellulose fibers are biomass made from plant pulp such as wood, and their effective use is expected to reduce environmental impact.

[0003] One proposed use of cellulose nanofibers is to improve the strength of resin materials by dispersing cellulose nanofibers in resins. A resin composition containing cellulose nanofibers is disclosed in, for example, Patent Document 1.

[0004] JP 2019-131774 A

[0005] According to the investigations of the present inventors, it has been found that when a resin molded article is molded from a resin composition containing cellulose nanofibers, the resulting resin molded article is prone to developing silvery white streaks known as silver streaks. Since silver streaks are a cause of poor appearance, measures to address this problem are required.

[0006] The embodiments of the present invention have been made in view of the above-mentioned problems, and their purpose is to suppress the occurrence of silver streaks in a method for producing a resin molded product using a resin composition containing cellulose nanofibers.

[0007] This specification discloses a method for producing a resin molded article as described in the following items.

[0008] [Item 1] A method for producing a resin molded product, comprising: a step (A) of preparing a resin composition containing a thermoplastic resin and cellulose nanofibers; and a step (B) of molding the resin composition into a resin molded product, the method further comprising a step (C) of drying the resin composition before the step (B).

[0009] The manufacturing method according to an embodiment of the present invention includes step (C) of drying the resin composition before step (B) of molding the resin composition into a resin molded article, so that step (B) can be performed after reducing the moisture content of the resin composition, thereby suppressing the occurrence of silver streaks caused by moisture contained in the resin composition.

[0010] [Item 2] The method for producing a resin molded article according to Item 1, wherein the step (C) is carried out so that the moisture content of the resin composition is 400 ppm by mass or less.

[0011] From the viewpoint of more reliably suppressing the occurrence of silver streaks, step (C) is preferably carried out so that the moisture content of the resin composition is 400 ppm by mass or less.

[0012] [Item 3] The method for producing a resin molded article according to Item 1, wherein the step (C) is carried out so that the moisture content of the resin composition is 240 ppm by mass or less.

[0013] From the viewpoint of more reliably suppressing the occurrence of silver streaks, it is more preferable that step (C) be carried out so that the moisture content of the resin composition is 240 ppm by mass or less.

[0014] [Item 4] The method for producing a resin molded article according to any one of Items 1 to 3, wherein step (C) is carried out at a temperature of 100°C or higher.

[0015] From the viewpoint of sufficiently reducing the moisture content of the resin composition, step (C) is preferably carried out at a certain temperature or higher, specifically at a temperature of 100°C or higher.

[0016] [Item 5] The method for producing a resin molded product according to any one of Items 1 to 4, wherein step (C) is carried out for one hour or more.

[0017] From the viewpoint of sufficiently reducing the moisture content of the resin composition, step (C) is preferably carried out for a certain period of time or more, specifically, for one hour or more.

[0018] [Item 6] The method for producing a resin molded product according to any one of Items 1 to 5, wherein the thermoplastic resin is polypropylene.

[0019] As the thermoplastic resin contained in the resin composition, for example, polypropylene can be suitably used.

[0020] [Item 7] The method for producing a resin molded article according to any one of Items 1 to 6, wherein the cellulose nanofibers have an average fiber diameter of 10 nm or more and 100 nm or less.

[0021] The average fiber diameter of the cellulose nanofibers contained in the resin composition is preferably 10 nm or more and 100 nm or less.

[0022] [Item 8] The method for producing a resin molded article according to any one of Items 1 to 7, wherein the cellulose nanofibers have an average fiber length of 10 μm or more and 100 μm or less.

[0023] The average fiber length of the cellulose nanofibers contained in the resin composition is preferably 10 μm or more and 100 μm or less.

[0024] According to an embodiment of the present invention, the occurrence of silver streaks can be suppressed in a method for producing a resin molded product using a resin composition containing cellulose nanofibers.

[0025] 1 is a flowchart showing an example of a method for manufacturing a resin molded product according to an embodiment of the present invention; FIG. 2 is a diagram showing the results of checking the presence or absence of silver streaks for samples #1-1, #1-2, #1-3, #5-1, #5-2, #5-3, #8-1, and #8-2, along with external photographs; FIG. 3 is a graph showing the relationship between drying time and moisture content; FIG. 4 is a diagram showing the results of checking the presence or absence of silver streaks for samples #12-1 to #12-10, along with external photographs; FIG. 5 is a graph showing the relationship between drying time and moisture content; FIG. 6 is a flowchart showing an example of a method for manufacturing a resin composition; FIG. 7 is a diagram showing an engine cover 1 and a crossbar member 10 for a jet ski;

[0026] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to the following embodiments.

[0027] A method for manufacturing a resin molded product according to an embodiment of the present invention will be described with reference to Fig. 1. Fig. 1 is a flowchart showing the method for manufacturing a resin molded product.

[0028] First, a resin composition containing a thermoplastic resin and cellulose nanofibers (CNF) is prepared (step s1). While polypropylene, for example, can be suitably used as the thermoplastic resin contained in the resin composition, a thermoplastic resin other than polypropylene (e.g., nylon) may also be used. The resin composition may also contain components other than the thermoplastic resin and cellulose nanofibers (e.g., a dispersant).

[0029] Cellulose nanofibers are fibers with nano-sized diameters obtained by defibrating cellulose fibers. The average fiber diameter of the cellulose nanofibers contained in the resin composition is preferably 10 nm or more and 100 nm or less. The average fiber length of the cellulose nanofibers contained in the resin composition is preferably 10 μm or more and 100 μm or less.

[0030] As used herein, the terms "average fiber diameter" and "average fiber length" of cellulose nanofibers refer to the "number average fiber diameter" and "number average fiber length," respectively. The number average fiber diameter and number average fiber length of cellulose nanofibers can be measured, for example, by observing the cellulose nanofibers with a microscope such as an electron microscope or an atomic force microscope.

[0031] There are no particular restrictions on the cellulose nanofiber content of the resin composition, but it can be, for example, from 5% by mass to 30% by mass.

[0032] The resin composition prepared in step s1 is, for example, in the form of pellets, but may be in other forms. The resin composition containing a thermoplastic resin and cellulose nanofibers can be produced, for example, by the method described below.

[0033] Next, the resin composition is dried (step s2: drying step). There are no particular limitations on the method for drying the resin composition, and the resin composition can be dried using, for example, a dryer. As the dryer, for example, a hot air dryer, a dehumidifying dryer (sometimes called a "dehumidifying hot air dryer"), or a vacuum dryer can be used.

[0034] The resin composition is then molded into a resin molded product (step s3: molding step). This step s3 can be suitably carried out by, for example, injection molding. The molding temperature and filling pressure when molding by injection molding can be appropriately set depending on the thermoplastic resin used, the cellulose nanofiber content, etc.

[0035] As described above, the manufacturing method according to the embodiment of the present invention includes step s2 (drying step) of drying the resin composition before step s3 (molding step) of molding the resin composition into a resin molded product. Therefore, the molding step can be performed after reducing the moisture content of the resin composition, thereby suppressing the occurrence of silver streaks caused by moisture contained in the resin composition.

[0036] From the viewpoint of more reliably suppressing the occurrence of silver streaks, the drying step is preferably carried out so that the moisture content of the resin composition is sufficiently reduced. Specifically, the drying step is preferably carried out so that the moisture content of the resin composition is 400 ppm by mass or less, and more preferably 240 ppm by mass or less.

[0037] From the viewpoint of sufficiently reducing the moisture content of the resin composition, the drying step is preferably carried out at a certain temperature or higher, specifically, at a temperature of 100° C. or higher. The drying step is also preferably carried out at a temperature lower than the melting point of the thermoplastic resin contained in the resin composition. For example, when the thermoplastic resin is polypropylene, the drying step is preferably carried out at 150° C. or lower.

[0038] Furthermore, from the viewpoint of sufficiently reducing the moisture content of the resin composition, it is preferable that the drying process be carried out for a certain period of time or more; specifically, it is preferable that it be carried out for one hour or more, and more preferably for two hours or more.

[0039] [Verification Results of the Relationship between the Moisture Content of Resin Composition and the Silver Streak Inhibition Effect] Here, the results of verifying the correlation between the moisture content of the resin composition and the inhibition of the occurrence of silver streaks by actually producing a resin molded product will be described. The verification was performed as follows.

[0040] First, three lots (referred to as "Lot A," "Lot B," and "Lot C") of pellet-shaped resin composition were prepared, each consisting of cellulose nanofibers dispersed in commercially available polypropylene (J-466HP manufactured by Prime Polymer Co., Ltd.). The cellulose nanofiber content of the resin composition was 5% by mass. The average fiber diameter and average fiber length of the cellulose nanofibers were approximately 100 nm and approximately 100 μm, respectively.

[0041] Next, the prepared resin compositions were dried under different drying conditions (temperature and time), or the moisture content was measured without drying. Drying was performed using a 50 kg capacity box-type hot air dryer. The moisture content was measured using a CHINO CZA3100 precision moisture meter, and the mass measurements required for moisture content measurement were performed using a precision mass analyzer. These measurements were performed on four samples extracted from Lot A (Samples #1-#4), three samples extracted from Lot B (Samples #5-#7), and four samples extracted from Lot C (Samples #8-#11). In Table 2, which will be referenced later, the samples without drying, dried at 80°C for 4 hours, and dried at 80°C for 12 hours are designated with sub-numbers as Samples #1-1, #1-2, and #1-3, respectively. The other samples #2, #3, and #5-#11 are designated with sub-numbers in the same manner.

[0042] Each sample was then injection molded to obtain a resin molded product. A flat mold measuring 150 mm in length, 120 mm in width, and 3 mm in thickness (with a gate cross-section size of 2 mm x 3 mm) was used in the molding process. A Kawaguchi Iron Works KB80B2 injection molding machine was used as the molding machine. The injection molding conditions were as shown in Table 1 below. The resulting resin molded products were visually inspected for the presence or absence of silver streaks.

[0043]

[0044] Tables 2, 3, and 4 below show the results of moisture content measurements for each sample. FIG. 2 also shows the results of checking for the presence or absence of silver streaks for eight samples (samples #1-1, #1-2, #1-3, #5-1, #5-2, #5-3, #8-1, and #8-2) along with photographs of their appearance. FIG. 3 shows a graph with drying time on the horizontal axis and moisture content on the vertical axis. For Lot A, FIG. 3 plots the average values ​​of samples #1-1, #2-1, and #3-1, the average values ​​of samples #1-2, #2-2, and #3-2, and the average values ​​of samples #1-3, #2-3, #3-3, and #4. For lot B, the average values ​​of samples #5-1, #6-1, and #7-1, and the average values ​​of samples #5-2, #6-2, and #7-2 are plotted, and for lot C, the average values ​​of samples #8-1, #9-1, #10-1, and #11-1, and the average values ​​of samples #8-2, #9-2, #10-2, and #11-2 are plotted. In Figure 3, the marker "◎" means that there was almost no silver streak, and the marker "×" means that there was silver streak.

[0045]

[0046]

[0047]

[0048] Tables 2, 3, and 4, as well as Figures 2 and 3, show that performing a drying process reduces the moisture content in the resin composition, thereby tending to suppress the occurrence of silver streaks. It can be seen that a moisture content of 240 mass ppm or less is particularly effective in suppressing the occurrence of silver streaks. Note that Figure 2 shows only some of the samples, but similar trends were observed for the other samples. Specifically, samples #2-1, #2-2, #3-1, #3-2, #6-1, #7-1, #9-1, #10-1, and #11-1 exhibited silver streaks, similar to samples #1-1, #1-2, #5-1, and #8-1. Furthermore, samples #2-3 and #3-3 exhibited slight silver streaks, similar to sample #1-3. In contrast, samples #6-2, #6-3, #7-2, #7-3, #9-2, #10-2, and #11-2 showed almost no silver streaks, similar to samples #5-2, #5-3, and #8-2. Furthermore, it was found that a drying temperature of 100°C or higher can sufficiently reduce the moisture content even when the moisture content of the resin composition in its wet state is relatively high (in other words, it reduces the need for strict moisture content control at the stage of preparing the resin composition). If the drying temperature is less than 100°C (e.g., 80°C), it may be difficult to sufficiently reduce the moisture content if the moisture content of the resin composition is relatively high, as in the case of Lot A.

[0049] Next, the results of similar verifications performed on other resin compositions will be described.

[0050] First, one lot (referred to as "Lot D") of a pellet-shaped resin composition (CellenpiaPlas (registered trademark) manufactured by Nippon Paper Industries Co., Ltd.) in which cellulose nanofibers were dispersed in polypropylene was prepared as the resin composition. The cellulose nanofiber content of the resin composition was 5% by mass. The average fiber diameter of the cellulose nanofibers was 100 nm or less.

[0051] Next, the prepared resin compositions were dried under different drying conditions (temperature and time), or without drying, and their moisture content was measured. Drying was performed using a 50 kg capacity box-type hot air dryer. The moisture content was measured using a VaporProXL precision moisture meter manufactured by Eiko Seiki Co., Ltd., and the mass required for the moisture content measurement was measured using a precision mass measuring instrument. These measurements were performed on one sample (sample #12) extracted from Lot D. In Table 6, which will be referred to later, Sample #12 that was not dried is designated Sample #12-1, and Samples that were dried at 110°C for 0.5 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, and 4.5 hours are designated Samples #12-2, #12-3, #12-4, #12-5, #12-6, #12-7, #12-8, #12-9, and #12-10, respectively.

[0052] Each sample was then injection molded to obtain a resin molded product. A flat mold measuring 150 mm in length, 120 mm in width, and 3 mm in thickness (gate cross-section size: 2 mm x 3 mm) was used as the mold in the molding process. An electric injection molding machine J80ADS-110U manufactured by The Japan Steel Works, Ltd. was used as the molding machine. The injection molding conditions were as shown in Table 5 below. The obtained resin molded products were visually observed to check for the presence or absence of silver streaks.

[0053]

[0054] Table 6 below shows the results of moisture content measurement for each sample. FIG. 4 also shows the results of checking the presence or absence of silver streaks for each sample, along with photographs of their appearance. FIG. 5 shows a graph in which the horizontal axis represents drying time and the vertical axis represents moisture content. In FIG. 5, the marker "◎" means that there were almost no silver streaks, the marker "◯" means that there were sufficiently few silver streaks, and the marker "×" means that there were silver streaks.

[0055]

[0056] 4 and 5, it can be seen that the moisture content in the resin composition decreases by performing a drying step, and the occurrence of silver streaks tends to be suppressed. It can also be seen that a moisture content of 400 ppm by mass or less is highly effective in suppressing the occurrence of silver streaks, and a moisture content of 240 ppm by mass or less is particularly effective in suppressing the occurrence of silver streaks.

[0057] [Method for Producing Resin Composition] There are no particular limitations on the method for producing the resin composition prepared in step s1, and various methods known for producing resin compositions containing cellulose nanofibers can be used.

[0058] An example of a method for producing a resin composition will be described with reference to Fig. 6. Fig. 6 is a flowchart showing an example of a method for producing a resin composition.

[0059] First, pulp is prepared (step s11). The pulp may be derived from wood, for example. The tree species is not particularly limited. Various known methods can be used to pulp wood.

[0060] Next, the pulp is subjected to a hydrophobic treatment (step s12). The hydrophobic treatment can increase the compatibility between the cellulose nanofibers and the resin. There are no particular limitations on the method of the hydrophobic treatment.

[0061] Next, the hydrophobized pulp and thermoplastic resin are kneaded to form a masterbatch (step s13). There are no particular limitations on the kneading method, and kneading can be performed using a kneader. In this step s13, the pulp is defibrated, and the cellulose nanofibers are dispersed in the resin.

[0062] The masterbatch and thermoplastic resin are then kneaded to form a resin composition (step s14). There are no particular limitations on the kneading method used in step s14, and kneading can be performed using a kneader. The cellulose nanofiber content of the masterbatch can be, for example, about 30% to 45% by mass, and the cellulose nanofiber content of the resin composition obtained in step s14 can be about 5% to 15% by mass.

[0063] According to the production method illustrated in Fig. 6, pulp defibration and dispersion of cellulose nanofibers in resin can be carried out simultaneously, thereby reducing production costs. Of course, the method for producing a resin composition is not limited to the method illustrated in Fig. 6.

[0064] [Resin Molded Article] The resin molded article obtained by the manufacturing method of the embodiment of the present invention can be used in a variety of applications because the occurrence of silver streaks is suppressed. Furthermore, since the resin molded article obtained by the manufacturing method of the embodiment of the present invention is formed from a resin composition containing cellulose nanofibers, it has excellent strength and can be suitably used as a component for various transportation equipment (particularly as a replacement for components conventionally formed from resin compositions containing reinforcing fillers such as talc or glass fiber).

[0065] A resin molded product obtained by the manufacturing method of an embodiment of the present invention may be, for example, a crossbar member 10 for a jet ski, as shown in Fig. 7. In addition to the crossbar member 10, Fig. 7 also shows an engine cover 1 that is placed on top of the engine. The crossbar member 10 is attached to the back side of the engine cover 1 and is a member that supports and reinforces the engine cover 1.

[0066] Other examples of resin molded products obtainable by the manufacturing method of the embodiment of the present invention include inner engine hatches and base seats for jet skis, assist grips and helmet boxes for motorcycles, lower covers and leg shields for scooter-type motorcycles, and engine covers for outboard motors.

[0067] As described above, the method for producing a resin molded article according to an embodiment of the present invention is a method for producing a resin molded article that includes step (A) of preparing a resin composition containing a thermoplastic resin and cellulose nanofibers, and step (B) of molding the resin composition into a resin molded article, and further includes step (C) of drying the resin composition before step (B).

[0068] The manufacturing method according to an embodiment of the present invention includes step (C) of drying the resin composition before step (B) of molding the resin composition into a resin molded article, so that step (B) can be performed after reducing the moisture content of the resin composition, thereby suppressing the occurrence of silver streaks caused by moisture contained in the resin composition.

[0069] In one embodiment, the step (C) is carried out so that the moisture content of the resin composition is 400 ppm by mass or less.

[0070] From the viewpoint of more reliably suppressing the occurrence of silver streaks, step (C) is preferably carried out so that the moisture content of the resin composition is 400 ppm by mass or less.

[0071] In one embodiment, the step (C) is performed so that the moisture content of the resin composition is 240 ppm by mass or less.

[0072] From the viewpoint of more reliably suppressing the occurrence of silver streaks, it is more preferable that step (C) be carried out so that the moisture content of the resin composition is 240 ppm by mass or less.

[0073] In one embodiment, step (C) is carried out at a temperature of 100° C. or higher.

[0074] From the viewpoint of sufficiently reducing the moisture content of the resin composition, step (C) is preferably carried out at a certain temperature or higher, specifically at a temperature of 100°C or higher.

[0075] In one embodiment, step (C) is carried out for 4 hours or more.

[0076] From the viewpoint of sufficiently reducing the moisture content of the resin composition, step (C) is preferably carried out for a certain period of time or more, specifically, for 4 hours or more.

[0077] In one embodiment, the thermoplastic resin is polypropylene.

[0078] As the thermoplastic resin contained in the resin composition, for example, polypropylene can be suitably used.

[0079] In one embodiment, the average fiber diameter of the cellulose nanofibers is 10 nm or more and 100 nm or less.

[0080] The average fiber diameter of the cellulose nanofibers contained in the resin composition is preferably 10 nm or more and 100 nm or less.

[0081] In one embodiment, the average fiber length of the cellulose nanofibers is 10 μm or more and 100 μm or less.

[0082] The average fiber length of the cellulose nanofibers contained in the resin composition is preferably 10 μm or more and 100 μm or less.

[0083] According to an embodiment of the present invention, the occurrence of silver streaks can be suppressed in a method for producing a resin molded article using a resin composition containing cellulose nanofibers. The resin molded article produced using the production method according to an embodiment of the present invention is suitable for use as a component for various types of transportation equipment, for example.

[0084] 1 engine cover 10 crossbar member

Claims

1. Step (A) of preparing a resin composition containing a thermoplastic resin and cellulose nanofibers; Step (B) of molding the resin composition into a resin molded product; A method for manufacturing a resin molded product, comprising: A method for manufacturing a resin molded product, further comprising step (C) of drying the resin composition before step (B).

2. The method for manufacturing a resin molded product according to claim 1, wherein step (C) is performed such that the moisture content of the resin composition is 400 ppm by mass or less.

3. The method for manufacturing a resin molded product according to claim 1, wherein step (C) is performed such that the moisture content of the resin composition is 240 ppm by mass or less.

4. The method for manufacturing a resin molded product according to any one of claims 1 to 3, wherein step (C) is performed at a temperature of 100°C or higher.

5. The method for manufacturing a resin molded product according to any one of claims 1 to 3, wherein step (C) is performed for 1 hour or more.

6. The method for manufacturing a resin molded product according to any one of claims 1 to 3, wherein the thermoplastic resin is polypropylene.

7. The method for manufacturing a resin molded product according to any one of claims 1 to 3, wherein the average fiber diameter of the cellulose nanofibers is 10 nm or more and 100 nm or less.

8. The method for manufacturing a resin molded product according to any one of claims 1 to 3, wherein the average fiber length of the cellulose nanofibers is 10 µm or more and 100 µm or less.