JP7490182B2 - Biodegradable composite material containing cellulose nanofibers and biodegradable resin - Google Patents

Description

The present invention relates to a biodegradable composite material in which strength and biodegradability are enhanced by blending cellulose nanofibers with a biodegradable resin.

In order to prevent environmental pollution caused by plastic waste, the "3R Movement" of reducing, reusing, and recycling plastics is being promoted all over the world. Plastics are being collected separately for reuse and recycling. Collected plastic waste was exported to China and Southeast Asia, but these countries have banned the import of plastic waste, and plastic waste is now stuck in Japan. In recent years, interest in marine litter has been particularly high. When plastic waste flows into the ocean, it does not just drift as it is, but after drifting for a long distance and for a long time, it becomes finer and becomes microplastics less than 1 mm in size, which causes a major problem of destroying the ecosystem. Although collected plastic waste is disposed of in landfills, it sometimes flows into rivers due to floods and is carried to the sea. In 2010, it was estimated that the amount of plastic waste flowing from land to the ocean in Japan was 20,000 to 60,000 tons per year (Non-Patent Document 1).
Reducing plastic usage is a powerful solution, but reducing it to zero is not possible in modern society, so it is not ideal.

As a result, a variety of biodegradable plastics have been developed. Biodegradable plastics have the property of being decomposed by microorganisms and ultimately turning into carbon dioxide and water. Examples of biodegradable plastics include polylactic acid, polycaprolactone, polyhydroxyalkanoate, polybutylene succinate, copolymers of polybutylene adipate and terephthalate, polyglycolic acid, modified polyvinyl alcohol, and casein, and starch-derived biodegradable plastics have also been developed.

Biodegradable plastics are expected to reduce environmental pollution, but they have the drawback of low strength. Therefore, research is being conducted on composite materials that contain plant-derived biomass in order to increase strength without impairing biodegradability (Patent Documents 1 and 2, etc.). Furthermore, in order to improve the strength of biomass/biodegradable resin composite materials, attempts are being made to improve the compatibility at the interface between different phases at the interface between biomass and biodegradable resin (Patent Document 3).

JP 2000-160034 A JP 2002-069303 A JP 2018-100312 A

J. R. Jambeck et al., "Plastic waste inputs from land into the ocean; Science 13 Feb. 2015, p.768

Research into biodegradable composite materials such as those mentioned above has focused on improving strength, but little is known about the effect of composite formation on biodegradability.
Therefore, the present inventors have investigated new biodegradable composite materials with the aim of increasing the strength of biodegradable resins and improving their biodegradability.

By blending a specific amount of cellulose nanofibers (CNF) into a biodegradable composite material containing cellulose nanofibers and a biodegradable resin, the inventors have succeeded in increasing the tensile strength of the biodegradable resin by 15% or more and promoting its decomposition rate by 7.5% or more.

More specifically, the present invention provides a biodegradable composite material containing cellulose nanofibers and a biodegradable resin, in which the content of cellulose nanofibers is 2% by weight or more, preferably 5% by weight, and more specifically, 5 to 30% by weight, based on a total weight of 100% by weight of the cellulose nanofibers and the biodegradable resin.

Cellulose nanofibers that can be suitably used in the present invention can be obtained by subjecting wood chips to solvothermal treatment and then chemically treating the crushed wood pulverized material.

According to the present invention, a biodegradable composite material is prepared by blending a specific CNF with a biodegradable resin, which enhances the tensile strength and improves the biodegradability.

Cellulose nanofibers (CNF) were blended with biodegradable resin to prepare CNF-containing biodegradable composite materials, and the tensile strength and biodegradability of these biodegradable composite materials were improved.

Examples of biodegradable resins that can be used in the biodegradable composite material comprising the cellulose nanofibers and biodegradable resins according to the present invention include polylactic acid (PLA); microbial polysaccharides such as pullulan and curdlan; polycaprolactone (PCL); starch-based biodegradable polymers; all microbially derived biodegradable plastics, including polyhydroxyalkanoates (PHAs) such as polyhydroxybutyrate (PHB), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), and polyhydroxybutyrate valerate (PBAV); polyvinyl alcohol; polyamino acids; chitin and chitosan; all cellulose-based biodegradable plastics, including cellulose acetate, hydroxyethyl cellulose (HEC), and hydroxypropyl methylcellulose (HPMC); polyglycolic acid; polyethylene terephthalate succinate (PETS); polybutylene succinate (PBS), and polybutylene adipate-co-terephthalate. This includes any and all biodegradable resins such as (PBAT) and combinations thereof.

The CNFs that can be used in the biodegradable composite material containing cellulose nanofibers and biodegradable resins according to the present invention differ from those produced by conventional manufacturing methods (e.g., Patent Documents 4 and 5) in that they are obtained by subjecting wood of the size of wood chips or the like to a solvothermal treatment as is, without any prior chemical treatment.
More specifically, the CNF used in the present invention is produced by a manufacturing method including the steps of subjecting wood chips to solvothermal treatment, crushing the solvothermal treated wood chips, and subjecting the crushed wood chips to chemical treatment.
As used herein, solvothermal treatment refers to a treatment in which raw materials such as wood chips are immersed in a solvent and exposed to a subcritical to supercritical state under high temperature and pressure conditions for a certain period of time, and is sometimes referred to as hydrothermal treatment in particular when water is used as the solvent.

The wood chips used as the raw material in the above manufacturing method may be any raw material from which natural cellulose can be extracted, for example, wood chips of broadleaf or coniferous trees, as well as herbaceous plants. The wood chips in the present invention include pieces of wood, preferably having a size of 0.5 x 0.5 cm to 2.0 x 2.0 cm, more preferably 0.7 x 0.7 cm to 1.5 x 1.5 cm, and most preferably 0.8 x 0.8 cm to 1.2 x 1.2 cm.

In the solvothermal treatment step of the above-mentioned manufacturing method, wood chips or herbaceous plants as raw materials are immersed in a solvent and subjected to a subcritical to supercritical state under high temperature and high pressure conditions. Examples of the solvent used here include water, methanol, ethanol, propanol, pyrrolidone-based solvents such as N-methylpyrrolidone, acetate-based solvents such as butyl acetate, glycol ether-based solvents such as diethylene glycol monomethyl ether, ketone-based solvents such as methyl ethyl ketone, aromatic solvents such as toluene and xylene, and hydrocarbon solvents such as paraffin. The chips immersed in the solvent are treated for 60 to 180 minutes at 1 to 300 atmospheres and up to 400°C, preferably 2 to 250 atmospheres and 5 to 200°C, more preferably 25 to 100 atmospheres and 100 to 380°C, and most preferably 25 to 100 atmospheres and 150 to 250°C, which is within the range from subcritical to supercritical. These solvothermal treatments turn the chips into a soft, swollen, ground wood material. The manufacturing method of the present invention is characterized by subjecting the wood chips directly to the solvothermal treatment process. In conventional CNF manufacturing methods, wood chips are first chemically treated, but the above manufacturing method is characterized in that cellulose raw materials of a certain size, such as wood chips, are first subjected to solvothermal treatment without chemical treatment. When mixed with resin, CNF produced by this manufacturing method improves the physical properties of the composite material.

Then, the obtained ground wood is subjected to a crushing process to further reduce the pulp of the ground wood. This crushing process can be performed using a ball mill, disk mill, wet cutter mill, pressure homogenizer, etc. This crushing process breaks the wood into smaller ground wood particles of 0.05 to 0.5 mm.

Finally, the crushed wood fragments are chemically treated. Examples of chemical treatments include oxidation treatment, hydrolysis treatment, base treatment, or a combination of these. For the oxidation treatment, an oxidizing agent such as ozone, hypochlorous acid or its salts, persulfuric acid or its salts, or perorganic acid or its salts can be used. For this oxidation treatment, an oxidation catalyst such as an N-oxyl compound can be used in combination. For the hydrolysis treatment, hydrolases such as cellulase enzymes and hemicellulase enzymes can be used. For the base treatment, caustic soda, KOH, etc. can be used.

Lignin can also be added to wood chips before the solvothermal treatment. By adding lignin, the surface of the CNF produced is hydrophobized (hydrophobized CNF). A composite material produced by mixing hydrophobized CNF with resin has higher tensile strength than a composite material made of non-hydrophobized CNF without the addition of lignin, making this more preferable. The mixing ratio of ground wood and lignin is preferably ground wood/lignin (weight ratio) = 0.5 to 2, preferably 0.7 to 1.5, and more preferably 0.8 to 1.2.

Various additives can be added to the biodegradable composite material containing the cellulose nanofibers and biodegradable resin according to the present invention, as long as they do not impair the strength and biodegradability. For example, organic pigments and inorganic pigments can be added to the material for coloring purposes.

The biodegradable composite material according to the present invention can be used as a material to mold various resin molded products.
Examples of such resin molded products include cutlery products such as spoons, knives, and forks, containers, tableware, home appliance parts, automobile parts, electronic components, mobile phone parts, lenses, switches, optical discs, syringes, and other medical tools molded by injection molding; trays, containers, tableware, and the like molded by vacuum molding; bottles, containers, and the like molded by blow molding; films, sheets, bags, and the like molded by film molding or inflation molding; containers, tableware, home appliance parts, automobile parts, and the like molded by hot press molding; various foam molded products molded by foam molding, and all resin molded products that can be molded by other molding methods as well.

[Example 1]
1 kg of wood chips (1.0 x 1.0 cm) was mixed with 10 L of N-methylpyrrolidone and subjected to solvothermal treatment in an autoclave (200°C, 25 atm) for 2 hours. The resulting ground wood was heat-treated in a 10% aqueous solution of hypochlorous acid at 90°C for 1 hour to obtain the CNF of the present invention (solvothermal-treated/chemically-treated CNF: in the examples, the CNF of the present invention is simply referred to as "CNF").
30 g of the CNF produced above was blended with 100 g of polylactic acid (PLA; Natureworks) and mixed using a twin-screw extruder to prepare a biodegradable composite material containing approximately 23 wt% CNF.
The tensile properties (tensile strength, elongation at break, elastic modulus) and flexural properties (flexural strength) of PLA and the biodegradable composite material obtained by mixing it with CNF were measured using a precision universal testing machine Autograph AG-X and a small tabletop testing machine EZ-X (manufactured by Shimadzu Corporation), respectively, and the data was processed using "TRAPEZIUM LITE X" equipped with a macro function attached to the testing machine. The measurement results are shown in Table 1.

[Tensile property test]
The tensile strength, elastic modulus and breaking elongation of the biodegradable composite material were measured in accordance with JIS K 7161:1994.
Test speed: 20 mm/min Dumbbell test piece size: 1B size [Bending property test]
The bending strength of the biodegradable composite material was measured in accordance with JIS K 7171:1994.
Test speed: 2 mm/min. Test piece dimensions: 80×10×4 mm
Indenter tip radius R1: 5 mm
Support tip radius R2: 5 mm
Distance between supporting points: 60 mm

[Examples 2 to 11]
In the same manner as in Example 1, biodegradable composite materials containing approximately 23 to 30% by weight of CNF were obtained using polycaprolactone (PCL), polybutylene succinate (PBS), polyhydroxyalkanoate (PHA), polybutylene adipate-co-terephthalate (PBAT), starch-based resin, a mixed resin of starch-based resin and PLA (weight ratio 6:4), a mixed resin of starch-based resin and PBAT (weight ratio 6:4), cellulose acetate, hydroxyethyl cellulose (HEC), and hydroxypropyl methylcellulose (HPMC) instead of polylactic acid (PLA).
The tensile strength, tensile stroke, elastic modulus, flexural strength and flexural stroke of each biodegradable resin and the biodegradable composite material obtained by mixing each of them with CNF were measured. The measurement results are shown in Table 1.

It was confirmed that the tensile strength of both biodegradable resins was increased by blending CNF.

[Comparative Examples 1 to 3]
In the same manner as in Example 1, biodegradable composite materials containing approximately 2 wt. % of untreated CNF (the raw material for the CNF of the present invention) were obtained using polylactic acid (PLA), polybutylene succinate (PBS), and polybutylene adipate-co-terephthalate (PBAT), respectively.
The tensile strength of each biodegradable resin and the biodegradable composite material obtained by mixing each of them with untreated CNF was measured. The measurement results are shown in Table 2.

Untreated CNF had low dispersibility in biodegradable resins and could not be blended in amounts exceeding about 2% by weight.
Moreover, the incorporation of untreated CNF reduced the tensile strength.

[Example 12]
The effect of CNF blending on the biodegradable properties of polylactic acid (PLA) was investigated.
CNF was blended in amounts of 5 g, 11 g, or 30 g per 100 g of PLA to obtain biodegradable composite materials containing CNF at approximately 5 wt %, approximately 10 wt %, or approximately 23 wt %, respectively.
The results of the biodegradability properties of PLA and its biodegradable composites mixed with CNF are shown in Table 3.

[Biodegradability test]
The sample materials were freeze-pulverized to powder and then subjected to an enzymatic hydrolysis test using proteinase K.
2.8 mL of _{0.2 M} NaH2PO4 was mixed with 22.8 mL of _{0.2 M} Na2HPO4, and distilled water was added to make the total volume 50 mL to prepare a phosphate buffer with a pH of 7.7. Separately, a predetermined amount of proteinase K derived from Tritirachium album (white crystalline powder; Fujifilm Wako Pure Chemical Industries, Ltd.) was weighed and dissolved in the phosphate buffer to prepare an enzyme solution.
A specified amount of material was added to the enzyme solution, and the sample was prepared by thoroughly mixing it once with a vortex mixer, and then placed in a thermostatic shaker. After a specified time, the sample was removed, cooled on ice, filtered through a 0.2μ membrane filter, and the filtrate was diluted 5 times with distilled water. The water-soluble total organic carbon concentration (TOC) of the diluted filtrate was measured with a TOC measuring device (Shimadzu Corporation, TOC-VCSH).
The net TOC value of the material was determined by subtracting the TOC value derived from the enzyme and the TOC value derived from trace water-soluble components in the material from the TOC value of the filtrate. The TOC value derived from the enzyme was measured using the enzyme solution, and the TOC value derived from the material was measured using a suspension obtained by putting each material into phosphate buffer and thoroughly mixing it with a vortex.
The carbon content in the repeating unit of polylactic acid (-O-CH(CH3)-CO-; M=72) is C3=36, so the carbon content is exactly half of the weight of polylactic acid (36/72=50%). For example, if 10 mg of polylactic acid in 5 mL of phosphate buffer is 100% biodegraded (hydrolyzed) and becomes water-soluble, the carbon content in 5 mL of aqueous solution is 5 mg, so the TOC value is calculated as 5 mg/5 mL=1 mg/mL=1000 ppm. In this case, if the TOC value after the enzyme degradation test is 500 ppm, the biodegradation rate is 50%. The calculation was performed under the assumption that only the polylactic acid portion is hydrolyzed and water-soluble by the enzyme, but the cellulose nanofiber is not decomposed.

[Examples 13 to 17]
In the same manner as in Example 12, biodegradable composite materials were obtained that contained polycaprolactone (PCL), polybutylene succinate (PBS), polybutylene adipate-co-terephthalate (PBAT), polyhydroxybutyrate valerate (PBAV), a mixed resin of starch-based resin and PBAT, cellulose acetate, hydroxyethyl cellulose (HEC), and hydroxypropyl methylcellulose (HPMC), each of which contained about 5 to 25 wt % of CNF, instead of polylactic acid (PLA).
The results of the biodegradability properties of each biodegradable resin and the biodegradable composite materials obtained by mixing each of them with CNF are shown in Table 3.

As can be seen from Table 3, it was confirmed that adding 5% or more by weight of CNF not only increases the strength of the biodegradable resin, but also improves the decomposition rate.

[Comparative Example]
The results of the biodegradation properties of the biodegradable composite material containing approximately 2 wt % untreated CNF in polylactic acid (PLA) in the same manner as in Example 12 are shown in Table 3.
The results of the biodegradability properties of the biodegradable composites obtained by mixing PLA with untreated CNF are shown in Table 4.

Untreated CNF had poor dispersibility in biodegradable resins and could not be mixed in amounts exceeding about 2% by weight.
The inclusion of untreated CNF reduced biodegradability.

Untreated cellulose nanofibers have low dispersibility in resins and could not be mixed in amounts exceeding approximately 2% by weight. Furthermore, when the upper limit of 2% by weight is added to resin, both strength and biodegradability decrease. On the other hand, the cellulose nanofibers of the present invention have high dispersibility in biodegradable resins and can be mixed into resins according to the desired strength. Furthermore, surprisingly, the effect of improving the biodegradation rate of biodegradable resins has been demonstrated.

By blending the cellulose nanofibers of the present invention with a biodegradable resin, a biodegradable resin composition with improved strength and biodegradability can be obtained.

Claims (4)

A biodegradable composite material comprising a cellulose nanofiber and a biodegradable resin, the cellulose nanofiber content being 5 to 30% by weight in a total amount of 100% by weight of the cellulose nanofiber and the biodegradable resin, and the biodegradable resin is
selected from polylactic acid ; polycaprolactone; starch-based biodegradable polymers ; polyhydroxybutyrate valerates, polyhydroxyalkanoates ; cellulose acetate, hydroxyethyl cellulose, hydroxypropyl methylcellulose ; polybutylene succinate; polybutylene adipate-co-terephthalate copolymers, and combinations thereof;
A biodegradable composite material, in which the cellulose nanofibers are produced by a production method including, in this order, a step of subjecting wood pieces or wood chips to a solvothermal treatment, a step of crushing the solvothermal treated wood chips, and a step of subjecting the crushed wood chips to a chemical treatment to obtain cellulose nanofibers (CNF), wherein the solvothermal treatment is carried out under critical or subcritical conditions, and the chemical treatment is an oxidation treatment. The biodegradable composite material according to claim 1, wherein the wood chips or wood chips are softwood or hardwood chips or wood chips. The biodegradable composite material according to claim 1 or 2, further comprising a step of adding lignin prior to the solvothermal treatment step. A molded article formed using the biodegradable composite material according to any one of claims 1 to 3.