JP7792108B1 - Carboxylic acid cross-linked chitin-based natural polymer composition and its manufacturing method - Google Patents

Abstract

The present invention provides a coating composition that is applicable as a hard coating agent and is composed solely of biodegradable materials.
[Solution] The carboxylic acid cross-linked chitin-based polymer composition produces a thermosetting coating with a pencil hardness of 4H or higher, and the weight ratio of citric acid to chitosan (citric acid/chitosan) is 0.5 to 3. Chitin nanofibers can also be contained in a weight ratio (chitin nanofiber/chitosan) of 0.5 or less. A method for producing a carboxylic acid cross-linked chitin-based polymer composition includes a film-forming solution preparation step of mixing chitosan, citric acid, and an aqueous acetic acid solution to prepare a film-forming solution containing 1.0 to 20.0 wt% chitosan and having a weight ratio (citric acid/chitosan) of 0.5 to 3, and a cured film formation step of applying the film-forming solution to form a coating film and then thermally curing the coating film at a heating temperature of 100°C to 200°C to form a cured coating film.
[Selected Figure] Figure 2

Description

Application of Article 30, Paragraph 2 of the Patent Act. Presented at Chitose International Horam 2024, held at Chitose Institute of Science and Technology from September 3rd to 4th, 2024.

The present invention relates to a coating composition composed of a highly biodegradable chitin-based natural polymer and a method for producing the same.

Non-biodegradable petroleum-derived synthetic resins, plastics, films, paints, adhesives, etc. pose environmental problems such as soil and marine pollution when disposed of. For this reason, there is a demand for the development of biodegradable materials that are sustainable resources with low environmental impact to replace petroleum-derived materials. Chitin, which is a by-product of food processing such as crab and shrimp, and its derivative chitosan, are attracting attention as biologically derived biodegradable polymer materials second only to cellulose, and are expected to have a significant effect in reducing environmental impact.
Petroleum-derived synthetic polymer materials are used not only for plastic moldings, ordinary films, and coating materials, but also for hard coating agents intended to protect coated objects by providing durability or scratch resistance. If these hard coating agents are made biodegradable, they can have a significant effect in reducing environmental impact.

Patent Document 1 discloses a citric acid-crosslinked gel-like elastic material containing elastin, collagen, chitosan, and a water-soluble solvent such as water or aqueous ethanol. Patent Document 2 discloses a method for preparing a polymer hydrogel by crosslinking an aqueous solution containing a hydrophilic polymer consisting of carboxymethyl cellulose or hydroxyethyl cellulose and a polycarboxylic acid selected from _C4 to _C12 dicarboxylic acids, tricarboxylic acids, or tetracarboxylic acids, using the polycarboxylic acid as a crosslinking agent. However, the compositions disclosed in Patent Documents 1 and 2 are both polymer hydrogels and do not satisfy the film hardness (pencil hardness of 4H or more) required for coating compositions, and there is no disclosure regarding film hardness.
Patent Document 3 discloses a composition with a pencil hardness of 4H or higher, which is made by crosslinking cellulose nanofibers with a resin binder that contains hydroxyl groups, carboxyl groups, amino groups, thiol groups, and double bonds and is soluble or dispersible in water or organic solvents, with an initiator or photoinitiator that contains an isocyanate group, a carbodiimide group, or an acryloyl group. However, this composition contains a resin binder that is a petroleum-derived plastic material, and is not made solely from biodegradable plastics.

Non-Patent Document 1 discloses a microcrystalline cellulose-incorporated polyvinyl alcohol adhesive using 0.3 wt% citric acid and 0.5 wt% citric acid as crosslinking agents as a latent wood adhesive, with pencil hardness of 4H and 3H, respectively. However, the composition is intended to reduce the use of petrochemical-derived plastic materials, and contains polyvinyl alcohol, a petroleum-derived plastic material.
Non-Patent Document 2 discloses the structure and properties of a composite film made of food-use polyvinyl alcohol (PVA) and citric acid-crosslinked chitosan (CS), and discloses that a CS/PVA ratio of 1/1 has excellent mechanical properties. However, there is no disclosure about the film hardness (pencil hardness) of the composite film. Furthermore, the composition contains polyvinyl alcohol, a petroleum-derived plastic material.

Japanese Patent Application Laid-Open No. 2022-119739 Special Publication No. 2010-535911 JP 2015-160870 A

Polymer Bulletin (2023)80:8013-8030 Carbohydrate Polymers 312 (2023)120842

The objective is to provide a coating composition made solely from biodegradable materials that can be used as a hard coating agent and has a significant effect in reducing environmental impact.

The present inventors have conducted extensive research in light of the above-mentioned problems, and as a result have been able to propose a coating composition in which chitin-based natural polymers, i.e., chitin, chitosan, and chitin nanofibers, which are biodegradable materials that have a low environmental impact and are unlikely to cause resource depletion, are crosslinked with a naturally occurring carboxylic acid crosslinking agent, thereby solving the above-mentioned problems.
Specifically, this can be achieved in the following manner.

(Aspect 1) A carboxylic acid-crosslinked chitin-based polymer composition in which the weight composition ratio of citric acid to chitosan and chitin nanofibers (citric acid/chitosan and chitin nanofibers) is 0.5 or more and 3 or less, and the pencil hardness of a thermoset coating containing chitin nanofibers at a weight composition ratio to chitosan (chitin nanofibers/chitosan) of 0.5 or less is 4H or more.

(Aspect 2) A method for producing a carboxylic acid-crosslinked chitin-based polymer composition, which produces a thermosetting coating having a pencil hardness of 4H or greater, comprising: a film-forming solution preparation step of mixing chitosan, citric acid, and an aqueous solution of acetic acid to prepare a film-forming solution having a chitosan content of 1.0 to 20.0 wt % and a weight composition ratio of citric acid to chitosan (citric acid/chitosan) of 0.5 to 3; and a cured film formation step of applying the film-forming solution to form a coating, and then thermally curing the coating at a heating temperature of 100°C to 200°C to form a thermosetting coating.

(Aspect 3) A method for producing a carboxylic acid-crosslinked chitin-based polymer composition, in which the thermosetting coating contains chitin nanofibers in a weight ratio (chitin nanofibers/chitosan) of 0.5 or less and has a pencil hardness of 4H or more, comprises: a film-forming solution preparation step of mixing chitosan, chitin nanofibers, citric acid, and an aqueous solution of acetic acid to prepare a film-forming solution having a chitosan and chitin nanofiber content of 1.0 to 20.0 wt % and a weight composition ratio of citric acid to chitosan (citric acid/chitosan) of 0.5 or more and 3 or less; and a cured coating formation step of applying the film-forming solution to form a coating, and thermally curing the coating at a heating temperature of 100°C to 200°C to form a thermosetting coating.

We can provide a coating composition made entirely of biodegradable materials, such as chitin, chitosan, and chitin nanofiber, which are chitin-based natural polymers that are biodegradable materials with a low environmental impact and little risk of resource depletion, crosslinked with a naturally derived carboxylic acid crosslinker to produce a coating with a pencil hardness of 4H or higher. Therefore, using a naturally derived biodegradable plastic material as a hard coating agent can have a significant effect in reducing the environmental impact.

This is the chemical structure of chitin and chitosan. 1 is a schematic diagram showing a method for producing a carboxylic acid-crosslinked chitin-based polymer composition of the present invention. FIG. 1 is a graph showing the coating hardness of the carboxylic acid cross-linked chitin-based polymer composition of the present invention by a pencil hardness test. 1 is an SEM photograph showing a fracture surface of a carboxylic acid-crosslinked chitin-based polymer composition of the present invention. 1 is a graph showing the surface contact angle and film thickness of a carboxylic acid-crosslinked chitin-based polymer composition of the present invention. 1 is an SEM photograph showing the water resistance of a carboxylic acid cross-linked chitin-based polymer composition of the present invention. 1 is an SEM photograph showing the water resistance of a carboxylic acid cross-linked chitin-based polymer composition of the present invention. 1 is a graph showing stress-strain data of a carboxylic acid cross-linked chitin-based polymer composition of the present invention.

Modes for implementing the present invention are described below. However, the present invention is not limited to the described embodiments.

The carboxylic acid cross-linked chitin-based natural polymer composition of the present invention is a carboxylic acid cross-linked chitin-based natural polymer composition in which the pencil hardness of the thermosetting coating is 4H or more and which is composed only of biodegradable materials consisting of a chitin-based natural polymer and a carboxylic acid cross-linking agent.
Hereinafter, chitin, chitosan, chitin nanofibers, and a carboxylic acid crosslinking agent for crosslinking chitin-based natural polymers, which are chitin-based natural polymers constituting the carboxylic acid crosslinked chitin-based natural polymer composition of the present invention, will be described.

1. Constituents of the Carboxylic Acid Cross-Linked Chitin-Based Polymer Composition The carboxylic acid cross-linked chitin-based polymer composition of the present invention is composed of a chitin-based natural polymer (chitin, chitosan) and a carboxylic acid cross-linking agent that cross-links the chitin-based natural polymer to form a three-dimensional cross-linked structure, and chitin nanofibers can also be added.

(1.1) Chitin and Chitosan Chitin is an aminopolysaccharide with a fibrous structure consisting of long chains (hundreds to thousands) of N-acetyl-D-glucosamine units. It is a natural polymer widely distributed as a component of the exoskeletons of crustaceans such as shrimp and crabs, the exoskeleton of insects, shellfish, and mushrooms. Biologically derived chitin may be in any form, such as fibrous or granular. It is extracted and processed from the shells and exoskeletons of crustaceans, insects, or krill.
As shown in Figure 1, chitosan is a polysaccharide containing amino groups, which is obtained by deacetylating chitin through alkali treatment. The higher the degree of deacetylation, i.e., when the degree of acetylation is about 50% or more, the more soluble it becomes in an aqueous acid solution such as dilute acetic acid.
Chitosan has highly reactive functional groups, such as amino and hydroxyl groups, which give it excellent properties, such as the ability to adsorb odorous components and formaldehyde, and antibacterial properties. It is also highly biodegradable. Furthermore, the hydrogen ionization of the amino groups at pH levels below 6 increases its solubility, and this cationized chitosan is biodegradable, antibacterial, antifungal, non-toxic, and biocompatible.

Chitosan can be produced by deacetylating chitin under heating conditions with a strong alkali such as sodium hydroxide.
Chitin deacetylation is carried out by heating and stirring crab shells or chitin powder after acid decalcification in a 30-60 wt% aqueous solution of sodium hydroxide (80-120°C, 0.5-5 hours). The average degree of deacetylation of chitosan can be controlled by adjusting the sodium hydroxide concentration, reaction temperature, and reaction time.
The average degree of deacetylation of the chitosan of the present invention is 50% or more, preferably 70% to 100%, in view of the solubility in an aqueous acid solution such as dilute acetic acid and the progress of the crosslinking reaction with a naturally occurring carboxylic acid crosslinker.
The average deacetylation degree of chitosan can be measured by elemental analysis, conductometric titration, FT-IR, etc. The following formula is an example of a formula for calculating the average deacetylation degree.
Average deacetylation degree (%) = [(number of amino groups contained in chitin nanofibers) / (number of amino groups contained in chitin nanofibers + number of acetamide groups)] x 100
The chitosan of the present invention also includes chitosan derivatives, specifically compounds in which some of the amino groups of chitosan or some of the hydroxyl groups in the same molecule have been chemically modified by acylation, etherification, esterification, or other chemical reactions.

(1.2) Chitin Nanofibers Chitin nanofibers obtained by mechanically defibrating chitin can be used.
Chitin nanofibers can be obtained by mechanically defibrating chitin. For this process, deproteinized and decalcified chitin nanofibers are treated with a weak acid (pH 3-4) followed by mechanical defibration using a millstone grinder, high-pressure homogenizer, freeze-pulverizer, or other device. For example, a wet grinding method (see JP 2005-270891 A) has been disclosed, in which a dispersion of polysaccharides such as chitin is sprayed from a pair of nozzles at a high pressure of 70-250 MPa, causing the jets to collide with each other and pulverize the mixture. Another high-pressure injection method (see JP 2011-056456 A) has been disclosed, in which a biomass dispersion fluid is sprayed at a high pressure of 100-240 MPa against a hard impact body and pulverized. It has also been disclosed that the defibration process can be performed in the presence of microbubbles generated by a swirling liquid flow microbubble generator (see JP 2017-94218 A). In a swirling flow microbubble generator, the shear force and microbubbles generated by the swirling flow inside the gas-liquid generation tank act synergistically at the same time. Therefore, by using microbubbles generated by a swirling flow microbubble generator in the fiber defibration process, it is possible to efficiently obtain thin, long, and uniform bio-nanofibers with excellent super-specific surface area, nano-size, and molecular alignment properties, with low energy and low cost.
The width (or diameter) of chitin nanofibers is about 2 nm to about 200 nm, preferably about 2 nm to about 100 nm, more preferably about 2 nm to about 50 nm, for example, about 5 nm to about 20 nm, about 2 nm to about 20 nm, etc. Chitin nanofibers with such a width (or diameter) have excellent dispersibility in aqueous media.
The aspect ratio of chitin nanofibers (fiber length/fiber width) is usually about 10 or more (about 10 to about 100,000), preferably about 100 or more, for example, 200 to about 10,000.

The chitin nanofibers of the present invention can be surface-chitosanized chitin nanofibers.

(1.3) Carboxylic Acid Crosslinking Agents As crosslinking agents for chitin-based natural polymers, dialdehydes such as glyoxal and genipin can be used, but it is preferable to use green crosslinking agents such as naturally occurring polycarboxylic acids.
Examples of the carboxylic acid crosslinking agent include polycarboxylic acids such as citric acid, tartaric acid, malic acid, oxalic acid, malonic acid, succinic acid, glutaric acid, adipic acid, fumaric acid, maleic acid, citraconic acid, itaconic acid, lactic acid, and phthalic acid, as well as salts thereof. Citric acid is preferred from the standpoints of safety and cost.
The content of the carboxylic acid crosslinking agent can be appropriately determined relative to the chitin-based natural polymer so that the thermoset coating of the carboxylic acid crosslinked chitin-based polymer composition formed into a thermally cured coating maintains a pencil hardness of 4H or more. For example, the weight composition ratio (citric acid/chitosan) of the carboxylic acid crosslinking agent (citric acid) to the chitin-based natural polymer (chitosan) is preferably 0.5 or more and 3.0 or less.

2. Method for Producing Carboxylic Acid Cross-Linked Chitin-Based Polymer Composition The production of the carboxylic acid cross-linked chitin-based polymer composition of the present invention comprises a film-forming solution preparation step of mixing chitosan, or chitosan and chitin nanofibers, which is a natural chitin-based polymer, with citric acid, which is a carboxylic acid cross-linking agent, in an aqueous acetic acid solution to prepare a film-forming solution, and a cured film formation step of applying the film-forming solution to form a coating film, and then thermally curing the coating film at a heating temperature of 100°C to 200°C to form a thermoset coating film.
The film-forming solution preparation step and the cured film formation step will be described below in that order.

(2-1) Film-forming solution preparation step In the film-forming solution preparation step for producing the carboxylic acid cross-linked chitin-based polymer composition of the present invention, a film-forming solution is prepared by mixing chitosan, which is a chitin-based natural polymer, or chitosan and chitin nanofibers, with a carboxylic acid cross-linking agent (citric acid) that cross-links the chitin-based natural polymer to form a three-dimensional cross-linked structure, in an aqueous acetic acid solution.

The film-forming solution of the present invention contains a 1.0 to 20.0 wt % aqueous solution of acetic acid as a solvent.
The reason why an aqueous acetic acid solution was used as the solvent is that in order to dissolve chitin-based natural polymers (chitosan), the pH must be 6 or less, preferably 2.5 to 4.5, and acetic acid (pK _a 4.76) is suitable from the viewpoints of safety and cost.

The amounts of chitosan and citric acid blended in the film-forming solution (acetic acid aqueous solution) are such that the chitosan content is 1.0 to 20.0 wt % and the weight composition ratio of citric acid to chitosan (citric acid/chitosan) is 0.5 to 3.0.
If the chitosan content is less than 1.0 wt %, the continuous film forming property is insufficient, and if it exceeds 20.0 wt %, the coating film forming property is insufficient.
If the weight ratio of citric acid to chitosan (citric acid/chitosan) is less than 0.5, the surface hardness (pencil hardness) of the cured film will be less than 4H, and if it exceeds 3.0, the film-forming properties will be insufficient.

Chitin nanofibers can be added to the film-forming solution. When chitin nanofibers are added to the film-forming solution, the weight composition ratio of chitin nanofibers to chitosan (chitin nanofibers/chitosan) is preferably 0.5 or less.
This is because when the weight composition ratio of chitin nanofibers to chitosan (chitin nanofibers/chitosan) exceeds 0.5, the coating film uniformity is insufficient.

(2-2) Cured Film Forming Step The cured film forming step for producing the carboxylic acid cross-linked chitin-based polymer composition of the present invention involves applying a film-forming solution to form a coating film, and then thermally curing the coating film at a heating temperature of 100°C to 200°C to form a cured film.
The heat treatment of the coating film is intended to promote and complete the reaction of the carboxylic acid crosslinking agent (citric acid).
The heat treatment method can be any heating method that is normally used for coating films, and may be either a contact heating method or a non-contact heating method. Examples include a method in which the coating film is heated by contact with the cylinder surface (heating roll, heating platen) of a cylinder dryer heated to a predetermined temperature, a non-contact heating method in which the coating film is treated by exposing it to a high-temperature atmosphere in a hot air circulation dryer or the like for a predetermined period of time, and a treatment method using a far-infrared dryer or an electromagnetic induction heating furnace.
The temperature of the heat treatment varies depending on the heat treatment method, but can be, for example, 100 to 220°C, preferably 120 to 200°C, and more preferably 140 to 180°C. The heat treatment time varies depending on the heat treatment method, and in the case of non-contact heating using a hot air circulation dryer or the like, it is, for example, 30 to 90 minutes, more preferably 40 to 60 minutes. In the case of contact heating using a heated roll and/or heated platen or the like using a cylinder dryer or the like, it is, for example, 0.5 seconds to 30 minutes, more preferably 1 second to 3 minutes. In this case, the temperature of the portion of the coating film that comes into contact with the heated roll and/or heated platen or the like can be, for example, 130 to 250°C, and preferably 180 to 220°C.

The carboxylic acid cross-linked chitin-based polymer composition of the present invention will be specifically described below by showing embodiments, but it should not be construed that the present invention is limited to these embodiments.
Table 1 lists the components of the carboxylic acid cross-linked chitin-based polymer composition of the present invention and examples of evaluation of physical properties.

Example 1
1. Preparation of film-forming solutions Chitosan solutions were prepared by dissolving 1.0 g of chitosan (CS), 2.5 g of acetic acid (AA), and varying amounts of citric acid (CA) (0.0 g, 1.0 g, 1.5 g, 2.0 g, 3.0 g) in water, and then diluted with half the amount of water to prepare film-forming solutions (CA_1.1-CA_1.5).
0.1 g of chitin nanofibers (NÅNO) was added to the film-forming solutions (CA_1.1-CA_1.5) to prepare film-forming solutions (CA_2.1-CA_2.5).
The chitosan (CS), chitin nanofiber (NÅNO), acetic acid (AA), and citric acid (CA) used were as follows.
Chitosan: Low molecular weight chitosan (deacetylation degree 75-85%, MW 50-190 kDa; Batch #MKBB9037, manufactured by Sigma-Aldrich)
・Chitin nanofiber 1-3% aqueous solution (NÅNO Batch #2302016, manufactured by Omura Paint Co., Ltd.)
・Acetic acid 99.7% acetic acid manufactured by Junsei Chemical Co., Ltd. ・Citric acid 98% citric acid manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

2. Formation of cured coatings The coating solutions (CA_1.1-CA_1.5, CA_2.1-CA_2.5) were applied to microscope slides (L 76 mm × W 26 mm × H 0.8-1.0 mm, manufactured by Matsunami Glass Industry Co., Ltd.) using a hand coater (RK Print coat Instruments, size 8), and the slides were heated at curing temperatures (160°C, 180°C, 200°C) for 1 hour to form cured coatings (CA_1.1-CA_1.5, CA_2.1-CA_2.5).

3. Physical properties of cured films The cured films (CA_1.1-CA_1.5, CA_2.1-CA_2.5) were evaluated for surface hardness (pencil hardness), wettability (surface contact angle), film thickness, and water resistance.

(3.1) Surface hardness (pencil hardness)
The surface hardness of the cured coating was evaluated using the Wolff-Wilborn method (ASTM D3363). A pencil (Hi-uni, manufactured by Mitsubishi Pencil Co., Ltd.) with a pencil hardness of 10B-10H was attached to a pencil hardness tester (Anhui East Electronic Technology HT-6510P) at a 45° angle, and the coating scratch hardness was measured under a load of 500 g. The measurement was performed five times, and the presence or absence of scratches was observed under a microscope.
Figure 3 shows the pencil hardness test results (right) of the cured films (CA_1.1-CA_1.5, CA_2.1-CA_2.5) and the scratch marks (left) from the pencil hardness test.
The pencil hardness of the cured films (CA_1.1-CA_1.5, CA_2.1-CA_2.5) was comparable to that of the cured films containing citric acid (CA_1.2-CA_1.5, CA_2.2-CA_2.5) (5H-8H), while the cured films without citric acid (CA_1.1, CA_2.1) had a pencil hardness of 6B-2H (Table 1 and Figure 3).
As shown in the SEM photographs of fracture surfaces in Figure 4, (a) the cured coating (CA_1.1) that does not contain citric acid (CA) is soft and the cross section is not clear, while (b) the cured coating (CA_2.3) that does contain citric acid (CA) has a clear cross section.
There is no clear difference in pencil hardness between the cured film with 0.1 g of chitin nanofiber (NÅNO) added (CA_2.2-CA_2.5) and the cured film without 0.1 g of chitin nanofiber (NÅNO) added (CA_1.2-CA_1.5).

(3.2) Wettability (surface contact angle)
The wettability of the coating was measured using a contact angle meter (DM 500, manufactured by Kyowa Interface Science Co., Ltd.). 3 μL was dropped onto a horizontal surface of the coating, and the static contact angle was measured at room temperature. Measurements were taken five times at different locations, and the contact angle was calculated using measurement software (FAMAS).
Figure 5(a) is a graph showing the measured surface contact angles (static contact angles) of cured films (CA_1.1-CA_1.5, CA_2.1-CA_2.5). The contact angle of the cured films containing citric acid (CA) (CA_1.2-CA_1.5, CA_2.2-CA_2.5) was 57.4±2.0°. All static contact angles were below 90°, indicating hydrophilicity. The contact angle of the cured films not containing citric acid (CA) (CA_1.1, CA_2.1) was 26.8±4.5°. The smaller contact angle of the cured films not containing citric acid (CA) (CA_1.1, CA_2.1) is believed to be due to the amount of amino groups that do not contribute to crosslinking.

(3.3) Coating Thickness The thickness of the cured coating was measured using a coating thickness step meter (optical profiler Bruker Dektak XT-S).
Figure 5(b) is a graph showing the measured thickness of the cured coatings (CA_1.1-CA_1.5, CA_2.1-CA_2.5). The coating thickness is approximately 1-5 μm, and tends to increase with the amount of citric acid (CA) added. The increase in coating thickness with the amount of citric acid (CA) added is thought to be due to an increase in the viscosity of the coating solution.

(3.4) Water Resistance A weighed film was placed in a conical centrifuge tube (50 mL, manufactured by Thermo Scientific) containing 25 mL of deionized water, and the film was left at room temperature for 24 hours before collapse (damage) and swelling were evaluated by SEM observation. The results were as follows:
<Destruction and swelling of the coating>
No breakdown of the coating was observed in either the cured coatings heat-cured at 200°C (CA_1.1-CA_1.5, CA_2.1-CA_2.5) or the cured coating heat-cured at 180°C (CA_2.1-CA_2.5).
Slight disintegration was observed on the edge of the cured coatings (CA_1.1-CA_1.5) that were heat-cured at 180°C. In addition, the cured coatings that did not contain citric acid (CA) (CA_1.1, CA_2.1) showed slight yellowing of the water.
The cured coatings (CA_1.1-CA_1.5, CA_2.1-CA_2.5) that were heat-cured at 160°C were observed to disintegrate and peel off from the coated glass plates.
In warm water, the cured films (CA_1.1-CA_1.5, CA_2.1-CA_2.5) all showed an increase in weight due to the swelling action of chitosan (CS).
<SEM Observation>
Figure 6 shows SEM images of a cured coating (CA_1.3) before and after a water resistance test. The left side shows the results after the test (after 24 hours at room temperature), and the right side shows the results before the test, at (a) 200°C, (b) 180°C, and (c) 160°C. Figure 7 shows SEM images of a cured coating (CA_2.3) before and after a water resistance test. The left side shows the results after the test (after 24 hours at room temperature), and the right side shows the results before the test, at (a) 200°C, (b) 180°C, and (c) 160°C.
The cured coating (CA_1.3) without chitin nanofibers (NÅNO) added showed a homogeneous surface, whereas the cured coating (CA_2.3) with chitin nanofibers (NÅNO) added showed a heterogeneous surface.
No surface damage was observed for the cured coatings (CA_1.3) and (CA_2.3) heat-cured at 200°C and 180°C, respectively. However, slight surface damage was observed for the cured coatings (CA_1.3) and (CA_2.3) heat-cured at 160°C.

4. Preparation of thick film samples: 2.6 g of chitosan (CS) was added to a citric acid solution prepared by dissolving 0.4 g of citric acid (CA) in 10.0 g of water to prepare a mixed paste. This paste was wrapped in aluminum foil and pressurized for 1 hour at temperatures (160°C, 180°C, 200°C) using a hydraulic mug (Masada Seisakusho Co., Ltd. MH-15P). After 15 minutes of pressurization at 10 MPa, the mixture was left for 45 minutes to prepare a thick film sample (CS_neat).
A mixed paste was prepared by mixing 2.0 g of chitosan (CS) and 0.6 g of chitin and adding this to a citric acid solution prepared by dissolving 0.4 g of citric acid (CA) in 10.0 g of water. This paste was wrapped in aluminum foil and pressurized for 1 hour at temperatures (160°C, 180°C, 200°C) using a hydraulic mug (Masada Seisakusho Co., Ltd. MH-15P). After 15 minutes of pressure at 10 MPa, the mixture was left for 45 minutes to prepare a thick film sample (CS_chitin).
The chitosan (CS), chitin, and citric acid (CA) used were as follows:
Chitosan: Low molecular weight chitosan (deacetylation degree 75-85%, MW 50-190 kDa; Batch #MKBB9037, manufactured by Sigma-Aldrich)
Chitin, deacetylation degree 1.6%, Batch #0926-25, manufactured by Koyo Chemical Co., Ltd. Citric acid, 98% citric acid, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

(4-1) Evaluation of Mechanical Properties The mechanical properties (Stress-Strain Analysis) of the thick film samples (CS_chitin, CS_neat) were evaluated using a dumbbell tensile tester (AND MCT-2150) at a tensile speed of 10 mm/min.
The ultimate tensile strength was calculated by the formula TS=UF/A.
Here, the ultimate tensile strength (TS), the cross-sectional area of the break region (A), and the ultimate force (UF) are
The results of the mechanical property evaluation are shown in Table 2 and FIG. 8 (Stress-Strain curves).
<Test Results>
The thick film samples (CS_chitin, CS_neat) pressed at temperatures (180°C, 200°C) for 1 hour fractured directly at the end of the gradient without deformation.
The thick film samples (CS_chitin, CS_neat) pressed at a temperature (160°C) for 1 hour deformed before breaking.

Example 2
1. Preparation of film-forming solutions Chitosan solutions were prepared by dissolving 3.0 g of chitosan (CS), 4.5 g of acetic acid (AA), and varying amounts of citric acid (CA) (0.0 g, 0.5 g, 1.0 g, 1.5 g, 2.0 g) in water, and film-forming solutions (CA_3.1 to CA_3.5) were prepared.
The chitosan (CS), acetic acid (AA), and citric acid (CA) used were as follows:
Chitosan: Koyo Chitosan FM-80 (deacetylation degree 84.4%, viscosity 22 mPa·S, manufactured by Koyo Chemical Co., Ltd.)
・Acetic acid 99.7% acetic acid manufactured by Fujifilm Wako Pure Chemical Industries, Ltd. ・Citric acid crystalline manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

2. Formation of cured films The film-forming solutions (CA_3.1 to CA_3.4) were applied to microscope slides (L 76 mm × W 26 mm × H 1.0 ± 0.05 mm, manufactured by Muto Chemical Co., Ltd.) using a paint brush, and each was heated at a curing temperature (160°C, 180°C, or 200°C) for 1 hour to form cured films (CA_3.1 to CA_3.4).

3. Physical properties of cured films The surface hardness, water resistance, and solvent resistance of the cured films (CA_3.1 to CA_3.4) were evaluated.

(3.1) Pencil Hardness The surface hardness of the cured film was measured using a pencil hardness tester (manufactured by TP Giken Co., Ltd.) in accordance with the pencil hardness measurement method of JIS K5600-5-4. A pencil (Hi-uni, manufactured by Mitsubishi Pencil Co., Ltd.) with a pencil hardness of 10B-10H was attached at an angle of 45° and the scratch hardness of the film was measured under a load of 750 g±10 g. The measurement was carried out five times, and the presence or absence of scratches was observed under a microscope.

(3.2) Water Resistance Deionized water was dropped onto the cured film and immediately rubbed with a finger to check the degree of adhesion and stickiness (tack) on the surface of the coating film, and the water resistance was evaluated according to the following criteria.
× The coating film melts and becomes sticky. △ The film peels off. 〇 There is no peeling or tackiness.

(3.3) Evaluation of Solvent Resistance The solvents used were lacquer thinner, methyl ethyl ketone (MEK), Neoethanol P-7 (composition: ethanol 85%, isopropyl alcohol 5%, normal propyl alcohol 10%), and paint thinner. A cloth (clean rag) soaked in each solvent was passed back and forth over the cured coating 10 times with a load of 500 g, and the condition of the coating was observed and evaluated according to the following criteria.
x: No adhesion of the cured product to the cloth was found, but the cloth was discolored.
△: Dissolved cured product adhered to the cloth.
Good: No hardened material adheres to the cloth.

<Summary>
(1) Chitosan and chitin nanofiber (NÅNO)-added chitosan were crosslinked with a naturally occurring carboxylic acid crosslinker (citric acid) and cured at a heating temperature of 160°C to 200°C. The thermoset coatings exhibited a pencil hardness of 4H or more and water resistance.
(2) The addition of chitin nanofiber (NÅNO) improved the water resistance of the thermoset coating.

The carboxylic acid-crosslinked chitin-based polymer composition of the present invention can be used as a hard coating agent.

Claims (3)

A carboxylic acid-crosslinked chitin-based polymer composition in which the weight ratio of citric acid to chitosan and chitin nanofibers (citric acid/chitosan and chitin nanofibers) is 0.5 or more and 3 or less, and the weight ratio of chitin nanofibers to chitosan (chitin nanofibers/chitosan) is 0.5 or less, and the pencil hardness of a thermoset coating film formed therefrom is 4H or more. A method for producing a carboxylic acid cross-linked chitin-based polymer composition, which produces a thermosetting film with a pencil hardness of 4H or greater, comprising: a film-forming solution preparation step in which chitosan, citric acid, and an aqueous acetic acid solution are mixed to prepare a film-forming solution having a chitosan content of 1.0 to 20.0 wt% and a weight composition ratio of citric acid to chitosan (citric acid/chitosan) of 0.5 to 3; and a cured film formation step in which the film-forming solution is applied to form a coating, and the coating is thermally cured at a heating temperature of 100°C to 200°C to form a thermosetting film. A method for producing a carboxylic acid-crosslinked chitin-based polymer composition, comprising: a film-forming solution preparation step in which chitosan, chitin nanofibers, citric acid, and an aqueous solution of acetic acid are mixed to prepare a film-forming solution containing 1.0 to 20.0 wt% chitosan and chitin nanofibers, and in which the weight ratio of citric acid to chitosan (citric acid/chitosan) is 0.5 to 3; and a cured film formation step in which the film-forming solution is applied to form a coating, and the coating is thermally cured at a heating temperature of 100°C to 200°C to form a thermoset coating. The thermoset coating contains chitin nanofibers in a weight ratio of chitin nanofibers to chitosan (chitin nanofibers/chitosan) of 0.5 or less, and the pencil hardness of the thermoset coating is 4H or greater.