Method for manufacturing a composite of naturally derived nanofibers and synthetic resins

Abstract

To provide a method for producing a composite sheet of naturally-derived nanofiber such as chitin nanofiber and cellulose nanofiber and synthetic resin, and a method for producing a composite of naturally-derived nanofiber and synthetic resin by using the composite sheet.SOLUTION: A method for producing a composite sheet of naturally-derived nanofiber and synthetic resin comprises: a process of mixing dispersion aqueous solution of the nanofiber and alcohols to obtain nanofiber-dispersed preparation solution; a process of immersing synthetic-resin fiber body into the nanofiber-dispersed preparation solution to allow the fiber body to impregnate the nanofiber-dispersed preparation solution; and a process of drying the fiber body into which the nanofiber-dispersed preparation solution is impregnated. Further, a method for producing a composite of naturally-derived nanofiber and synthetic resin comprises: a process of performing extrusion molding after crushing the composite sheet.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present invention relates to a method for producing a composite sheet of a natural-derived nanofiber such as chitin nanofiber or cellulose nanofiber and a synthetic resin, and a method for producing a composite of a natural-derived nanofiber and a synthetic resin using this composite sheet.

Background Art

[0002] In recent years, various technologies have been developed towards achieving carbon neutrality, and the reduction of petroleum-derived plastics through the use of renewable biomass plastics and natural resources has also been promoted. As one of such efforts, chitin nanofibers derived from crustaceans (hereinafter sometimes referred to as "chitin NF") or cellulose nanofibers derived from plants (hereinafter sometimes referred to as "CNF") are added to resins to reduce petroleum-derived materials, suppress dimensional changes of the resin due to heat, and improve strength. In addition, chitosan nanofibers (hereinafter sometimes referred to as "chitosan NF") obtained by deacetylating chitin NF are also known, and since chitin NF and chitosan NF have high biocompatibility, great expectations are placed on them in the medical and pharmaceutical industries.

[0003] Generally, when compounding a resin and an additive, melt kneading is performed, and this melt kneading has advantages such as a small number of steps and the ability to perform continuous processing. However, if NF containing water is used as it is, aggregates are generated during kneading, resulting in problems with dispersibility. Since various NFs are sold in a state of being dispersed in water, drying is required before melt kneading, but the NF also aggregates during this drying.

[0004] Various technologies have been developed to improve the dispersibility of these various NFs in resin components. For example, since CNF is an extremely water-absorbent material with many hydroxyl groups on its surface, the Kyoto process disclosed in Non-Patent Document 1 involves hydrophobizing and modifying CNF by acetylation, and then kneading this hydrophobically modified CNF in a resin that has been heated and melted using a twin-screw extruder. According to this Kyoto process, the hydrophobically modified CNF is defibrated during melt-kneading with the resin, resulting in a composite material in which the material is uniformly dispersed in the resin. Non-Patent Document 2 also discloses a method to obtain dried NF powder by adding t-butanol to an aqueous solution in which NF is dispersed, mixing it, subjecting it to a centrifuge, and recovering the precipitate, repeating this process about 10 times, and then freeze-drying it, in order to prevent aggregation of NF containing moisture during drying. In this method, it is preferable to reduce the pressure for more than a week for sufficient drying, which presents the problem of requiring a long time for each process.

[0005] Furthermore, Non-Patent Document 3 describes an example in which a chitosan NF-epoxy resin composite material was produced by utilizing the amino group of chitosan NF, adding ethylene glycol diglycidyl ether (EGDG) to an aqueous solution of chitosan NF, performing defibrillation by rotary disc milling, freeze-drying, and then adding the curing agent triethylenetetramine and heating. Furthermore, Non-Patent Document 4 describes an example in which a test piece made of PP / CNF composite resin was produced by adding polyglycerin fatty acid ester as a surfactant to an aqueous dispersion of CNF to partially hydrophobicize the surface of the CNF, then melt-kneading it together with polypropylene (hereinafter sometimes referred to as "PP"), and then injection molding. Furthermore, Non-Patent Document 5 describes an example in which an aqueous dispersion of CNF was spray-dried using a spray dryer, and then kneaded together with polylactic acid (hereinafter sometimes referred to as "PLA") using a lab trust mill and a twin-screw extruder to produce a composite material in which CNF is dispersed in PLA.

[0006] Furthermore, Patent Document 1 discloses a method in which an aqueous dispersion of fine cellulose (CNF) is subjected to defibration treatment in an organic solvent to obtain an organic solvent dispersion of CNF, the organic solvent in this CNF organic solvent dispersion is replaced with water to obtain an aqueous dispersion of CNF, the aqueous dispersion of CNF and thermoplastic resin particles are mixed, and then dried to recover the finely powdered composite particles. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2021-155750 [Patent Document 2] Patent No. 4370416 [Non-patent literature]

[0008] [Non-Patent Document 1] Introduction to the Kyoto Process [Accessed January 19, 2022], <http: / / www.rish.kyoto-u.ac.jp / labm / wp-content / uploads / 2012 / 07 / cf1b30a5725ce3fd0d110d3376a0d314.pdf> [Non-Patent Document 2] Cellulose Materials Group: Freeze-drying method for nanocellulose slurry by alcohol substitution [Retrieved January 19, 2022], <https: / / unit.aist.go.jp / ischem / ischem-clm / technical_point / technical_point3.html> [Non-Patent Document 3] Proceedings of the 32nd Annual Meeting of the Japan Society of Polymer Processing, C-212 [Non-Patent Document 4] Proceedings of the 32nd Annual Meeting of the Japan Society of Polymer Processing, F-205 [Non-Patent Document 5] Journal of the Japan Society for Materials Life Studies, Vol. 28, No. 1, pp. 12-21 [Overview of the project] [Problems that the invention aims to solve]

[0009] According to the composite formation methods for various NFs and resins disclosed in Non-Patent Documents 1 to 5 and Patent Document 1, an NF composite resin in which various NFs are well dispersed in the resin can be obtained. However, each requires chemical modification or separation and drying treatment of the various NFs, making the manufacturing process complicated. Non-Patent Document 1 also discloses a method for producing a CNF-resin composite material by directly preparing a CNF sheet from an aqueous suspension of CNF, impregnating this CNF sheet with resin, and compression molding it. This method has the advantages of being able to use thermosetting resins and being able to form transparent films, but it has the drawbacks of being limited to liquid resins and being difficult to perform continuous molding.

[0010] Furthermore, Patent Document 2 discloses an invention of a bagasse fiber-reinforced resin material that can be injection molded using bagasse fibers, which are the residue left after extracting sugarcane juice, and polypropylene (PP). However, the bagasse fibers used here have a fiber length in the range of 0.5 mm to 50 mm and do not fall under the category of NF, so the above-mentioned problem of condensation of naturally derived NF during drying does not exist.

[0011] The inventors, after conducting various studies to solve the problems related to naturally derived NF as described above, discovered that by preparing a solution by mixing a hydrophilic organic solvent, particularly a water-soluble alcohol, with a solution in which commercially available natural NF is dispersed in water, and then immersing a nonwoven fabric in this solution to form a composite sheet, the aggregation of NF during drying, which had been a problem in the past, can be prevented. Furthermore, they discovered that by crushing this composite sheet and then extruding or injection molding it, a composite material in which naturally derived NF is homogeneously dispersed can be obtained, thus completing the present invention.

[0012] In other words, the present invention aims to provide a method for producing a composite sheet of NF-resin in which naturally derived NF is homogeneously dispersed in a resin using a method simpler than conventional methods, and a method for producing a composite using this composite sheet. [Means for solving the problem]

[0013] A first aspect of the present invention provides a method for producing a composite sheet of naturally derived NF and synthetic resin, comprising the steps of: mixing an aqueous dispersion of NF with alcohols to obtain an NF dispersion preparation solution; immersing a synthetic resin fiber in the NF dispersion preparation solution to impregnate the fiber with the nanofiber dispersion preparation solution; and drying the fiber impregnated with the nanofiber dispersion preparation solution.

[0014] Naturally derived NF has many hydroxyl groups on its surface, making it highly hydrophilic. While it disperses in water, these hydroxyl groups tend to aggregate when drying. Furthermore, immersing synthetic resin fibers in an aqueous dispersion of NF results in poor wettability, with the fibers repelling the solution. However, mixing alcohols with the aqueous dispersion of NF improves wettability with the fibers while maintaining the dispersibility of the NF. Therefore, according to the method for producing a composite sheet of naturally derived NF and synthetic resin in the first aspect of the present invention, it is possible to produce a composite sheet of naturally derived NF and synthetic resin in which naturally derived NF is homogeneously dispersed and attached within the fibers.

[0015] While such composite sheets can certainly be used for various applications as sheet products, they are particularly well-suited as materials for manufacturing composites of naturally derived nanofibers and synthetic resins, as will be described later. Furthermore, any fibrous material consisting of multiple fibers is acceptable; specifically, nonwoven fabrics, woven fabrics, and knitted fabrics can be used. The inventors conducted tests using these fibrous materials and found that nonwoven fabrics were the most suitable for homogeneously dispersing and adhering naturally derived NF.

[0016] In the method for producing a composite sheet of naturally derived NF and synthetic resin according to the present embodiment, chitin NF, cellulose NF, or chitosan NF can be used as the naturally derived NF. All of these NFs are commercially available in a dispersed state in water, and commercially available products can be used as is.

[0017] Furthermore, in the method for producing a composite sheet of naturally derived NF and synthetic resin according to this embodiment, at least one selected from methanol, ethanol, 1-propanol, and isopropyl alcohol can be used as the alcohol. Since all of these organic solvents are water-soluble alcohols, they can be mixed with the dispersed aqueous solution while maintaining the dispersibility of NF, and a composite sheet of naturally derived NF and synthetic resin can be produced in which the naturally derived NF is more homogeneously dispersed and attached to the fiber.

[0018] More preferable alcohols are ethanol or isopropyl alcohol, with isopropyl alcohol being the most preferred. Furthermore, there is no advantage to using any mixture of the above-mentioned alcohols, and alcohols with a molecular weight greater than butanol, etc., have low water solubility, making it difficult to achieve the desired effect.

[0019] In the method for manufacturing a composite sheet of a natural-derived NF and a synthetic resin according to such an aspect, it is preferable that the fibrous body made of the synthetic resin is made of a thermoplastic resin. A thermoplastic resin is a plastic material that has plasticity or can be molded at a specific high temperature and solidifies when cooled, and is a widely used plastic material. As thermoplastic resins, various ones such as polyethylene, acrylic resin, polyamide (nylon), polycarbonate, polylactic acid, polypropylene, polystyrene, and polyvinyl chloride are known. In the method for manufacturing a composite sheet according to one aspect of the present invention, it can be used for all these well-known thermoplastic resins. In particular, polypropylene (PP) has properties such as good heat resistance, a small specific gravity, being light, having strong strength, and excellent chemical resistance among general-purpose resins. Moreover, since it is produced in large quantities in the form of a fibrous body, it is most preferable.

[0020] Also, in the method for manufacturing a composite sheet of a natural-derived NF and a synthetic resin according to such an aspect, the step of immersing a fibrous body made of a synthetic resin in the NF dispersion preparation solution and impregnating the fibrous body with the NF dispersion preparation solution, and the step of drying the fibrous body impregnated with the NF dispersion preparation solution may be repeated a plurality of times. By repeating these steps a plurality of times, the amount of NF adhering to the fibrous body made of the synthetic resin can be increased, and moreover, the portion where NF is not adhered can be reduced. Thus, a composite sheet in which NF is more homogeneously impregnated or adhered can be obtained.

[0021] Furthermore, the method for manufacturing a composite of a natural-derived nanofiber and a synthetic resin according to the second aspect of the present invention is characterized by including a step of extruding after pulverizing the composite sheet manufactured by the method for manufacturing a composite sheet of a natural-derived NF and a synthetic resin according to any of the above aspects.

[0022] According to the method for producing a composite of a naturally-derived nanofiber and a synthetic resin in such an embodiment, since the naturally-derived NF is uniformly dispersed and adhered in the composite sheet, the naturally-derived NF is also uniformly dispersed in the powder obtained by pulverizing this composite sheet, and a composite in which the naturally-derived NF is uniformly dispersed in the synthetic resin can be obtained by extrusion molding this powder.

Advantages of the Invention

[0023] As described above, according to the method for producing a composite sheet of a naturally-derived NF and a synthetic resin in the first aspect of the present invention, a composite sheet of a naturally-derived NF and a synthetic resin in which the naturally-derived NF is uniformly dispersed and adhered in the fibrous body can be produced. Further, according to the method for producing a composite of a naturally-derived nanofiber and a synthetic resin in the second aspect of the present invention, a composite in which the naturally-derived NF is uniformly dispersed in the synthetic resin can be obtained.

Brief Description of the Drawings

[0024] [Figure 1] It is a photograph showing the surface state of the chitin NF-attached PP nonwoven fabric of Experimental Examples 1 and 2. [Figure 2] It is a diagram showing the preparation process of the chitin NF-attached PP nonwoven fabric. [Figure 3] It is a photograph showing the state of applying ninhydrin spray to various samples. [Figure 4] (a) is a digital microscope image of the PP nonwoven fabric itself, (b) is that of the chitin NF-attached PP nonwoven fabric of Sample No. 9, and (c) is that of the chitin NF-attached PP nonwoven fabric of Sample No. 12. [Figure 5] (a) is an electron microscope photograph of the PP nonwoven fabric itself, and (b) is that of the chitin NF-attached PP nonwoven fabric obtained by repeating the immersion and drying of the PP nonwoven fabric in the NF dispersion preparation solution of Sample No. 9 twice. [Figure 6] It is a photograph of the finely pulverized PP nonwoven fabric. [Figure 7] It is a diagram showing the measurement results of various samples by a thermal analyzer. [Figure 8]This is a photograph of sample number 9, a chitin-containing NF-adhered PP nonwoven fabric, which was finely ground, extruded, and then finely ground again. [Figure 9] These are photographs of various test specimens for tensile testing that were injection-molded. [Figure 10] These are electron microscope images of samples A and F. [Figure 11] This graph shows the results of tensile tests on dumbbell-shaped test specimens of various samples. [Figure 12] This graph shows the dimensional changes of samples A and C as determined by thermomechanical analysis. [Modes for carrying out the invention]

[0025] [Experimental Example 1] First, the adhesion state of chitin NF when polypropylene (PP) nonwoven fabric was immersed in a commercially available aqueous solution of chitin NF was investigated. As the PP nonwoven fabric (hereinafter referred to as "PP nonwoven fabric"), a roll of nonwoven fabric manufactured by TRUSCO Nakayama Co., Ltd. was cut into 15cm x 15cm pieces. As the chitin NF raw material, SFo-20010 (product name) (chitin NF concentration = 10 wt%) manufactured by Sugino Machine Co., Ltd. was used. In Experimental Example 1, the above PP nonwoven fabric was directly immersed in a solution of chitin NF raw material diluted to 1 / 10 with water (chitin NF concentration = 1 wt%), removed after 1 minute, and dried at room temperature for 24 hours or more to produce the chitin NF-adhered PP nonwoven fabric of Experimental Example 1. The surface state of the obtained chitin NF-adhered PP nonwoven fabric of Experimental Example 1 is shown in Figure 1(a).

[0026] In the embodiments described in this model, nonwoven fabric is used as the fibrous material of the present invention. However, the sheet-like fibrous material can consist of multiple fibers, and in addition to nonwoven fabric, woven fabrics and knitted fabrics can also be used. On the other hand, the inventors investigated the adhesion state of chitin NF by immersing woven fabric in an aqueous solution of chitin NF, and found that, in their tests, nonwoven fabric was superior in terms of uniform adhesion.

[0027] [Experimental Example 2] Using the same PP nonwoven fabric and chitin NF raw material as in Experimental Example 1, isopropyl alcohol (IPA) (manufactured by Fujifilm Wako Pure Chemical Industries) was first added to the chitin NF raw material in a ratio of water:IPA = 17:18, and then stirred to prepare an NF dispersion preparation solution (chitin NF concentration = 1.25 wt%) in which chitin NF, water, and IPA were mixed. Next, the PP nonwoven fabric was immersed for 1 minute in the same manner as in Experimental Example 1, removed, and dried at room temperature for 24 hours or more to produce the chitin NF-adhered PP nonwoven fabric of Experimental Example 2. The surface condition of the chitin NF-adhered PP nonwoven fabric of Experimental Example 2 is shown in Figure 1(b). A schematic of the preparation process for the chitin NF-adhered PP nonwoven fabric of Experimental Example 2 is shown in Figure 2. In this specification, "room temperature" refers to the generally accepted range of 15°C to 25°C.

[0028] In Experimental Example 1, a PP nonwoven fabric with chitin NF attached was produced by directly immersing the PP nonwoven fabric in an aqueous solution of commercially available chitin NF diluted with water. In this case, peeling of the chitin NF was observed after drying (Figure 1(a)). However, in Experimental Example 2, a PP nonwoven fabric with chitin NF attached was produced by immersing the PP nonwoven fabric in an NF dispersion preparation solution prepared by mixing chitin NF, water, and IPA to achieve the same chitin NF concentration as in Experimental Example 1. In this case, almost no peeling of the chitin NF from the nonwoven fabric was observed even after drying (Figure 1(b)).

[0029] [Experimental Example 3] Therefore, in Experimental Example 3, Sugino Machine's SFo-2002 (product name, raw material concentration 2 wt%) or SFo-20010 (product name, raw material concentration 10 wt%) was used as the chitin NF raw material, and 16 types of NF dispersion preparation solutions were prepared by adding water or IPA as appropriate to disperse chitin NF at various concentrations. Then, using PP nonwoven fabric of the same size as in Experimental Examples 1 and 2, the weight increase was calculated from the weight before immersion in each NF dispersion preparation solution and the weight after immersion for 1 minute, removal, and drying at room temperature for 24 hours or more. The NF content (wt%) attached to the PP nonwoven fabric was calculated from this weight increase. Furthermore, the obtained chitin NF-attached PP nonwoven fabric was observed with a digital microscope, and those showing voids between the fibers of the nonwoven fabric as in Figure 4(b) were marked with "○", those showing no voids as in Figure 4(c) were marked with "△", and those showing peeling during drying were marked with "×". The results are summarized in Table 1.

[0030] [Table 1]

[0031] Furthermore, Figure 3 shows the results of applying ninhydrin spray to the prepared nonwoven fabric to confirm the adhesion of chitin NF. Since ninhydrin reacts with amino groups to produce a blue-violet to reddish-purple color, the presence of chitin NF adhering to the surface of the PP nonwoven fabric can be confirmed. In Figure 3, (a) shows the color development of the PP nonwoven fabric alone without ninhydrin spray, (b) shows the color development of the PP nonwoven fabric alone with ninhydrin spray applied, (c) shows the color development of sample number 1, (d) shows the color development of sample number 16, and (e) to (h) show the color development of the chitin NF-adhered PP nonwoven fabrics of sample numbers 9 to 12, respectively.

[0032] According to the results shown in Figure 3, in the case of PP nonwoven fabric alone (Figure 3(b)) and when the amount of chitin NF attached was small (1.0 wt%) (Figure 3(C)), there was almost no discoloration due to the ninhydrin reaction. On the other hand, in the case of PP nonwoven fabric with chitin NF attached (Figures 3(d)-(h)) where the amount of chitin NF attached was 9.3-13 wt%, the ninhydrin reaction caused a bluish-purple discoloration, confirming that chitin NF was attached to the PP nonwoven fabric. Comparing the appearance of this discoloration, it was found that even with the same concentration of chitin NF dispersed in the solution, the degree of attachment differed depending on the concentration of IPA.

[0033] Here, the results of observing the PP nonwoven fabric itself and the chitin NF-attached PP nonwoven fabric with a digital microscope are shown in Figure 4. In Figure 4, (a) is a digital microscope image of the PP nonwoven fabric itself, (b) is a digital microscope image of sample number 9, and (c) is a digital microscope image of sample number 12 of the chitin NF-attached PP nonwoven fabric. In the chitin NF-attached PP nonwoven fabric of sample number 9 in Figure 4(b), spaces where chitin NF is not attached between the fibers can be seen, similar to the untreated PP nonwoven fabric shown in Figure 4(a). However, in the chitin NF-attached PP nonwoven fabric of sample number 12 in Figure 4(c), it was found that the chitin NF was attached between the fibers as if forming a film. From the results of the ninhydrin reaction shown in Figure 3 and the results of the digital microscope observation in Figure 4, it was found that by changing the concentrations of chitin NF and IPA, it is possible to attach chitin NF to the entire PP nonwoven fabric without damaging the morphology of the nonwoven fabric.

[0034] Here, Figure 5 shows the results of electron microscope observation of the surface of the nonwoven fabric. In Figure 5, (a) is an electron microscope image of the PP nonwoven fabric itself, and (b) is a microscope image of the PP nonwoven fabric after being immersed and dried twice in the NF dispersion preparation solution of sample number 9. As is clear from comparing the microscope images of (a) and (b) in Figure 5, fine fibers were observed on the surface of the nonwoven fabric in (b) that were not seen on the surface of the PP nonwoven fabric itself in (a). Therefore, it can be seen that the PP nonwoven fabric shown in Figure 5(b) has the surface of the fibers constituting the PP nonwoven fabric covered with chitin NF. Accordingly, in the chitin NF-coated PP nonwoven fabric of sample number 9 in Figure 4(b), the fiber surface is also covered with chitin NF.

[0035] [Experimental Example 4] In Experimental Example 4, the chitin NF-adhered PP nonwoven fabrics of samples 7, 8, 13, 14, and 15, prepared in Experimental Example 3, were subjected to repeated immersion and drying in the same NF dispersion preparation solution used initially. The results are summarized in Table 2. From the results shown in Table 2, it was found that repeating the process of immersing the PP nonwoven fabric in the NF dispersion preparation solution and drying it increased the chitin NF content in the total weight after drying. Furthermore, in sample number 7, which was prepared without mixing IPA, even after two immersions, areas where chitin nanofibers peeled off from the nonwoven fabric during drying were observed.

[0036] [Table 2]

[0037] [Experimental Examples 5 and 6] In Experimental Examples 5 and 6, the same multiple immersion and drying process as in Experimental Example 4 was performed on chitosan NF (Experimental Example 5) and CNF (Experimental Example 6). The results are shown in Tables 3 and 4, respectively. In Tables 3 and 4, the raw material concentration is the concentration of the stock solution of commercially available chitosan NF or CNF aqueous solution, and the NF concentration is the concentration in the NF dispersion preparation solution prepared by hand by mixing the stock solution with water and IPA, respectively. From the results shown in Tables 3 and 4, it was found that the same trend as in the case of chitosan NF shown in Experimental Example 4 was observed even in the case of chitosan NF and CNF.

[0038] [Table 3]

[0039] [Table 4]

[0040] [Experimental Example 7] In sample number 9 shown in Table 1, when an NF dispersion preparation solution using the same amount of ethanol instead of IPA was used, the chitin NF content after one immersion and drying was 9.1 wt%, and after two immersions and drying was 15.7 wt%. In this case as well, it was found that the chitin NF content in the PP nonwoven fabric increased with increasing immersion and drying steps. Therefore, it was found that similar effects and behaviors occur when ethanol is used instead of IPA. Since both IPA and ethanol are water-soluble alcohols, it is presumed that similar effects and behaviors will occur when methanol and 1-propanol, which are known to be water-soluble alcohols, are used, or even when a combination of multiple water-soluble alcohols is used.

[0041] [Experimental Example 8] In order to use the PP nonwoven fabric with various NFs attached, manufactured as described above (hereinafter sometimes referred to as "composite nonwoven fabric"), in products of various shapes, it is necessary that it be capable of molding processes such as extrusion molding and injection molding. Therefore, first, the nonwoven fabric was finely pulverized using a pulverizer (MGL2-100-J (product name), manufactured by Matsui Manufacturing Co., Ltd.) to a size that could be fed into a single-screw extruder (FRP-V32 (product name), manufactured by Myojo Metal Industry Co., Ltd.). The state of the nonwoven fabric after fine pulverization is shown in Figure 6.

[0042] Next, in order to investigate the molding temperature, the temperature at which the weight significantly decreased (thermal decomposition initiation temperature) was measured for various samples using a thermal analyzer (STA200RV (product name), manufactured by Hitachi High-Tech Science, Ltd.). The six samples measured were: (a) finely pulverized PP nonwoven fabric only, (b) chitin NF powder, (c) a commercially available chitin NF aqueous solution cast into a film, with the chitin NF film peeled off and dried, (d) finely pulverized chitin NF-adhered PP nonwoven fabric of sample number 9, (d') finely pulverized chitin NF-adhered PP nonwoven fabric of sample 9, extruded, and then further pulverized, and (e) a finely pulverized mixture of PP pellets (J-750HP (product name), manufactured by Prime Polymer Co., Ltd.) and chitin NF powder that was extruded. The chitin NF powder was obtained by dissolving a commercially available chitin NF dispersion (SFo-2002 (trade name), manufactured by Sugino Machine Co., Ltd.) with ethanol, removing the supernatant by centrifugation and vacuum drying, and then grinding the powder. The amount of ethanol added was 10 times the weight of the chitin NF dispersion. The results are summarized in Figure 7.

[0043] The results shown in Figure 7 indicate the following: Sample (d), obtained by finely grinding the chitin NF-adhered PP nonwoven fabric of sample 9 using a pulverizer, did not show a rapid weight loss up to approximately 300°C. Furthermore, when the NF-adhered PP nonwoven fabric of sample 9 was finely ground, extruded, and then finely ground again (d'), no rapid weight decrease was observed up to around 300°C. On the other hand, in sample (c), where chitin NF was cast onto a film and dried, a rapid weight loss, likely due to thermal decomposition, was observed at 220°C, and further discoloration of the chitin NF was observed around 200°C, where no significant weight loss was observed. Therefore, it was found that a molding temperature of around 190°C is preferable for extruding the chitin NF-adhered PP nonwoven fabric after fine grinding, as this minimizes the discoloration of the chitin NF.

[0044] [Experimental Example 9] In Experimental Example 9, dumbbell-shaped tensile test specimens with the shape shown in Figure 9 were manufactured using pellets (Figure 8) produced by extruding various samples. The injection molding conditions were an injection temperature of 190°C, a mold temperature of 40°C, and injection speeds of 30 mm / min and 60 mm / min. Sample A was produced by finely grinding a commercially available PP nonwoven fabric, similar to that used in Experimental Example 1, and then injection molding it. Sample B was produced by finely grinding a chitin NF-adhered PP nonwoven fabric, which was made using a commercially available PP nonwoven fabric, similar to that used in Experimental Example 1, an aqueous solution in which chitin NF was dispersed, and IPA, and then injection molding it. Samples C and D were produced by adding maleic anhydride-modified polypropylene (MAHPP) to improve the compatibility between chitin NF and polypropylene, and by further changing the molding conditions.

[0045] Furthermore, sample E, shown for comparison, was manufactured by injection molding polypropylene pellets (J-750HP (product name), manufactured by Prime Polymer Co., Ltd.) themselves, and sample F was manufactured by injection molding a mixture of polypropylene pellets and chitin NF (powder). The compositions of samples A to F subjected to injection molding are shown in Table 5. In addition, the results of electron microscopy observation of the internal structure of injection-molded samples D and F are shown in Figure 10. In Figure 10, (a) is an electron micrograph of sample D, and (b) is an electron micrograph of sample F. In sample D (Figure 10(a)), the chitin NF was dispersed throughout the cross-section in fibrous form of several tens of μm to 100 μm, unlike the particulate form seen in sample F (Figure 10(b)) to which powdered chitin NF was added. Therefore, it is considered that the sample using chitin NF-adhered PP has better dispersibility.

[0046] [Table 5]

[0047] Next, tensile tests were performed using the molded test pieces A to F. Figure 11 shows the results of the tensile tests on the dumbbell-shaped test pieces of each sample. The test piece of sample A, which was made by crushing the PP nonwoven fabric itself and then injection molding it, had a tensile strength of 36.7 MPa and a tensile modulus of 1523 MPa. On the other hand, test piece B, which contained 5 wt% chitin NF relative to its total weight, had a tensile strength of 31.0 MPa and a tensile modulus of 1408 MPa, showing a decrease in tensile strength and tensile modulus compared to the PP nonwoven fabric.

[0048] On the other hand, in test specimens C and D, where MAHPP was added to improve the compatibility of chitin NF and PP, and the molding conditions were further modified, the tensile strength remained at 95% of that of the PP nonwoven fabric even when the amount of chitin NF in the molded product increased to 7.2 wt% (test specimen C) and 20.0 wt% (test specimen D), respectively, and no significant decrease was observed. Furthermore, the tensile modulus of test specimen D was 1696 MPa, showing an 11% increase compared to the PP nonwoven fabric. For comparison, test specimen F was compounded in an extruder in the same way as test specimens B to D during injection molding. Although a slight improvement in tensile modulus was observed in test specimen F compared to test specimen E, it was within the range of deviation of test specimen E and was estimated to be substantially unchanged.

[0049] Furthermore, the expansion due to temperature changes was measured for test specimens A and C using a thermomechanical analyzer (TMA7100C (product name), manufactured by Hitachi High-Tech Science Co., Ltd.). The results are summarized in Figure 12. According to the results shown in Figure 12, sample C showed less dimensional change (expansion) and less change due to heat compared to sample A.

[0050] From the above, it can be seen that the molded products (test pieces B, C, and D) obtained by extruding PP nonwoven fabric with chitin NF attached after fine grinding have properties comparable to those of test pieces A and E, which do not contain chitin NF. Furthermore, it can be seen that test pieces B, C, and D have superior properties compared to test piece F, which simply contains powdered chitin NF mixed in. In particular, when comparing with test piece F, the electron microscope image of test piece F in Figure 10(b) shows large clumps of chitin NF, and it is thought that these clumps act as a starting point for rupture in test piece F, resulting in a significant decrease in tensile strength. On the other hand, since no large clumps of chitin NF are seen in the electron microscope image of test piece D in Figure 10(a), it is thought that no decrease in tensile strength occurred in test piece D.

Claims

1. A method for producing a composite of naturally derived nanofibers and synthetic resin, A step of mixing the aforementioned aqueous solution of dispersed nanofibers with alcohols to obtain a nanofiber dispersion preparation solution having a nanofiber concentration of 0.5 wt% or more, The process involves immersing a synthetic resin fiber in the nanofiber dispersion preparation solution to impregnate the fiber with the nanofiber dispersion preparation solution, A step of drying the fiber impregnated with the nanofiber dispersion preparation solution, By manufacturing composite sheets, A method for producing a composite of naturally derived nanofibers and synthetic resin, characterized by comprising the step of crushing the composite sheet and then extruding it.

2. The method for producing a composite of naturally derived nanofibers and synthetic resin according to claim 1, characterized in that the naturally derived nanofibers are chitin nanofibers, cellulose nanofibers, or chitosan nanofibers.

3. The method for producing a composite of naturally derived nanofibers and synthetic resin according to claim 1 or 2, characterized in that the alcohols are at least one selected from methanol, ethanol, 1-propanol, and isopropyl alcohol.

4. The method for producing a composite of naturally derived nanofibers and synthetic resin according to any one of claims 1 to 3, characterized in that the synthetic resin fiber body is made of thermoplastic resin.

5. A method for producing a composite of naturally derived nanofibers and synthetic resin according to any one of claims 1 to 4, characterized by repeating the steps of several times: immersing a synthetic resin fiber in the nanofiber dispersion preparation solution to impregnate the fiber with the nanofiber dispersion preparation solution, and drying the fiber impregnated with the nanofiber dispersion preparation solution.

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