JP7354780B2 - Organic-inorganic composite hydrogel precursor composition, organic-inorganic composite hydrogel, and method for producing the same - Google Patents

Description

The present invention relates to an organic-inorganic composite hydrogel precursor composition, an organic-inorganic composite hydrogel, and a method for producing the same.

A gel has properties intermediate between a liquid and a solid, and is a stable state in which a substance such as an organic polymer forms a three-dimensional network in a solvent such as water. In particular, hydrogels whose solvent is water are called hydrogels, and have been developed for use as functional materials in medical, food, and sports-related fields. In particular, in order to have uniform transparency, strong mechanical properties, water absorption, biocompatibility, etc., efforts have been made to combine them with various materials and create crosslinked structures.

For example, inventions related to organic-inorganic composite hydrogels in which water is included in a three-dimensional network formed by a composite of a water-soluble organic polymer and a water-swellable clay mineral have been described (for example, patent documents (See 1). According to the organic-inorganic composite hydrogel described in Patent Document 1, it is described that it has a light transmittance of 95% or more, a water absorbency of 10 times or more relative to dry weight, and can be stretched 10 times or more.

However, when producing these hydrogels, it was thought that their synthesis was only possible in the absence of molecular oxygen, due to the radical polymerization of organic monomers. As a result, it has been difficult to apply it to industrial applications, such as use at civil engineering construction sites or construction construction sites. Furthermore, when containing water-swellable clay minerals in water, it is necessary that the water-swellable clay minerals be dispersed as uniformly as possible in water, but the dispersion speed is slow, and in addition, Since moderate agitation is required to prevent the formation of , it has been difficult to perform this work at civil engineering and construction sites. Furthermore, after water-swellable clay minerals are dispersed in water, the viscosity of the dispersion increases over time, and it may form a so-called card house structure and gel itself, and can remain in a water-dispersed state for a long period of time. It was not advisable to preserve it.

To solve the above problem, a method for producing an organic-inorganic composite hydrogel has been proposed, which comprises a step of mixing an aqueous solution containing a water-soluble organic monomer and a phosphonic acid-modified hectorite, a polymerization initiator, and a polymerization accelerator (for example, , see Patent Document 1). Although this manufacturing method is suitable for on-site construction, when longer-term storage stability is required for the material, the stability of the aqueous solution containing the water-soluble organic monomer and phosphonic acid-modified hectorite is insufficient. There was a problem.

International Publication No. WO2019/009025

The problem to be solved by the present invention is to provide an organic-inorganic composite hydrogel precursor composition that has long-term storage stability.

The present inventors have discovered that an organic-inorganic composite hydrogel precursor composition containing a water-soluble organic monomer, a water-swellable clay mineral, an oxyradical compound, and water can solve the above problems, and have completed the present invention. did.

That is, the present invention provides an organic-inorganic composite hydrogel precursor composition containing a water-soluble organic monomer (A), a water-swellable clay mineral (B), an oxyradical compound (C), and water. It is something to do.

The organic-inorganic composite hydrogel precursor composition of the present invention has excellent long-term storage stability, excellent workability, and is not subject to restrictions such as manufacturing location, so it can be applied to various industrial uses such as civil engineering construction sites. can.

The organic-inorganic composite hydrogel precursor composition of the present invention contains a water-soluble organic monomer (A), a water-swellable clay mineral (B), an oxyradical compound (C), and water.

Examples of the water-soluble organic monomer (A) include monomers having a (meth)acrylamide group, monomers having a (meth)acryloyloxy group, and acrylic monomers having a hydroxyl group.

Examples of the monomer having the (meth)acrylamide group include acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N-methylacrylamide, N-ethylacrylamide, N-isopropylacrylamide, and N-cyclopropylacrylamide. , N,N-dimethylaminopropylacrylamide, N,N-diethylaminopropylacrylamide, acryloylmorpholine, methacrylamide, N,N-dimethylmethacrylamide, N,N-diethylmethacrylamide, N-methylmethacrylamide, N-ethyl Examples include methacrylamide, N-isopropylmethacrylamide, N-cyclopropylmethacrylamide, N,N-dimethylaminopropylmethacrylamide, and N,N-diethylaminopropylmethacrylamide.

Examples of the monomer having the (meth)acryloyloxy group include methoxyethyl acrylate, ethoxyethyl acrylate, methoxyethyl methacrylate, ethoxyethyl methacrylate, methoxymethyl acrylate, and ethoxymethyl acrylate.

Examples of the acrylic monomer having a hydroxyl group include hydroxyethyl acrylate, hydroxyethyl methacrylate, and the like.

Among these, from the viewpoint of solubility and physical properties of the resulting organic-inorganic hydrogel, it is preferable to use monomers having a (meth)acrylamide group, such as acrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, N- It is more preferable to use isopropylacrylamide and acryloylmorpholine, even more preferable to use N,N-dimethylacrylamide and acryloylmorpholine, and N,N-dimethylacrylamide is particularly preferable from the viewpoint of facilitating polymerization.

In addition, the above-mentioned water-soluble organic monomer (A) may be used alone or in combination of two or more types.

The content of the water-soluble organic monomer (A) in the organic-inorganic composite hydrogel precursor composition of the present invention is preferably 1 to 50% by mass, more preferably 5 to 30% by mass. It is preferable that the content of the water-soluble organic monomer (A) is 1% by mass or more because a hydrogel with excellent mechanical properties can be obtained. On the other hand, it is preferable that the content of the water-soluble organic monomer is 50% by mass or less because the liquid can be easily prepared.

The water-swellable clay mineral (B) forms a three-dimensional network structure with the polymer of the water-soluble organic monomer, and becomes a constituent element of the organic-inorganic hydrogel.

The water-swellable clay mineral (B) is not particularly limited, but includes water-swellable smectite, water-swellable mica, and the like.

Examples of the water-swellable smectite include water-swellable hectorite, water-swellable montmorillonite, and water-swellable saponite.

Examples of the water-swellable mica include water-swellable synthetic mica.

Among these, from the viewpoint of stability of the dispersion, it is preferable to use water-swellable hectorite and water-swellable montmorillonite, and it is more preferable to use water-swellable hectorite.

The water-swellable clay mineral (B) may be naturally derived, synthesized, or surface-modified. Examples of surface-modified water-swellable clay minerals include phosphonic acid-modified hectorite, fluorine-modified hectorite, etc.; Preferably, a light is used.

As the phosphonic acid-modified hectorite, for example, pyrophosphoric acid-modified hectorite, etidronic acid-modified hectorite, alendronic acid-modified hectorite, methylenediphosphonic acid-modified hectorite, phytic acid-modified hectorite, etc. can be used. These phosphonic acid-modified hectorites may be used alone or in combination of two or more.

In addition, the above-mentioned water-swellable clay mineral (B) may be used alone or in combination of two or more types.

The content of the water-swellable clay mineral (B) in the organic-inorganic composite hydrogel precursor composition of the present invention is preferably 1% by mass or more, since the mechanical properties of the resulting hydrogel are further improved. More preferably, it is at least % by mass. Further, the content of the water-swellable clay mineral (B) in the organic-inorganic composite hydrogel precursor composition of the present invention should be 20% by mass or less, since it can further suppress an increase in the viscosity of the composition. is preferable, and more preferably 10% by mass or less.

The oxyradical compound (C) acts as a polymerization inhibitor, and the composition of the present invention exhibits long-term storage stability.

Examples of the oxy radical compound (C) include 2,2,6,6-tetramethyl-4-piperidine-1-oxyl, [2,6-di-tert-butyl-4-[(3,5- Examples include di-tert-butyl-4-oxo-2,5-cyclohexadien-1-ylidene)methyl]phenyloxy] radical. These oxy radical compounds (C) may be used alone or in combination of two or more. Note that other polymerization inhibitors can also be used within a range that does not impair the effects of the present invention.

The content of the oxyradical compound (C) is 20 to 20% of the water-soluble organic monomer (A), considering the balance between the polymerizability of the water-soluble organic monomer (A) and long-term storage stability of the composition A range of 200 ppm is preferred, and a range of 20 to 100 ppm is more preferred.

Furthermore, the organic-inorganic composite hydrogel precursor composition of the present invention may contain an organic solvent other than water, has small mass change even under open air conditions, and has excellent mechanical properties such as substrate adhesion and breaking strength. Since a hydrogel that can be stably retained is obtained, the volatility is 0.1 g or less per 1 cm ² · 1 hour in an open system at 60 ° C and 1 atm (0.1 g / cm ² · hr · 60 ° C · 1 atm or less) The amount is preferably 0.05 g or less, more preferably 0.01 g or less, and even more preferably 0.01 g or less. Specifically, since solvents that are easily miscible with water are preferable, glycerin (0.001 g or less/cm ² · hr · 60°C · 1 atm), diglycerin (0.001 g or less / cm ² ·hr · 60 °C · 1 atm) ), ethylene glycol (0.01g or less/ ^cm2・hr・60℃・1atm), propylene glycol (0.001g or less/ ^cm2・hr・60℃・1atm), polyethylene glycol (0.001g or less/ ^cm2・hr・60° C.・1 atm) and other polyhydric alcohols are preferred, and glycerin and diglycerin are more preferred. These organic solvents may be used alone or in combination of two or more. Further, it is desirable that these organic solvents are uniformly contained in the organic-inorganic composite hydrogel of the present invention.

The mass ratio of water to the organic solvent (water/organic solvent) in the organic-inorganic composite hydrogel precursor composition of the present invention has a small mass change even under open air conditions, and has a high Since an organic-inorganic hydrogel having excellent various physical properties can be obtained, it is important that the ratio is 60/40 to 20/80, and preferably 50/50 to 30/70.

The organic-inorganic composite hydrogel precursor composition of the present invention preferably contains cellulose nanofibers because the storage stability is further improved and the resulting hydrogel has better breaking strength and adhesion between hydrogels.

The cellulose nanofibers are obtained by fibrillating and/or refining various types of cellulose.

As the cellulose, pulp, cotton, paper, regenerated cellulose fibers such as rayon, cupro, polynosic acetate, etc., cellulose produced by bacteria, cellulose derived from animals such as sea squirt, etc. can be used. Further, the surface of these celluloses may be chemically modified if necessary.

Defibration and/or micronization of the cellulose nanofibers can be performed, for example, by adding cellulose to water or a defibrating resin such as polyester resin, and mechanically applying shearing force.

As a means for applying shearing force, it is possible to use an extruder such as a bead mill, an ultrasonic homogenizer, a single-screw extruder, or a twin-screw extruder, or a known kneading machine such as a Banbury mixer, a grinder, a pressure kneader, or a two-roll mill. can. Note that the above means may be used alone or in combination of two or more.

The cellulose nanofibers are modified cellulose nanofibers obtained by defibrating and/or making cellulose fine to produce cellulose nanofibers, then adding a modifying compound and reacting with the cellulose nanofibers. There may be.

Modifying compounds include chemically bonding functional groups such as alkyl groups, acyl groups, acylamino groups, cyano groups, alkoxy groups, aryl groups, amino groups, aryloxy groups, silyl groups, and carboxyl groups to cellulose nanofibers. Examples include compounds that are modified by

Moreover, even if the compound is not chemically bonded, the cellulose nanofibers may be modified in such a manner that the compound to be modified is physically adsorbed onto the cellulose nanofibers. Examples of the physically adsorbable compound include surfactants, and any of anionic, cationic, and nonionic compounds may be used.

Although the fiber diameter and fiber aspect ratio of the cellulose nanofibers are not particularly limited, the fiber diameter is preferably 1000 nm or less, more preferably 100 nm or less.

Commercially available cellulose nanofibers include "Selish" manufactured by Daicel FineChem Co., Ltd., "LeoCrysta" manufactured by Daiichi Kogyo Seiyaku Co., Ltd., "BiNFi-s" manufactured by Sugino Machine Co., Ltd., and "BiNFi-s" manufactured by Nippon Paper Industries Co., Ltd. "cellenpia" and the like.

The content of the cellulose nanofibers in the organic-inorganic composite hydrogel precursor composition of the present invention is preferably 0.1 to 50% by mass, more preferably 0.5 to 30% by mass, More preferably, it is 1 to 20% by mass. It is preferable that the content of cellulose nanofibers is 0.1% by mass or more, since it is possible to synthesize a hydrogel that has better storage stability, and has excellent breaking strength and adhesive strength between hydrogels. On the other hand, it is preferable that the content of cellulose nanofibers is 50% by mass or less because the precursor composition can be easily prepared.

In the organic-inorganic composite hydrogel precursor composition of the present invention, for example, cellulose nanofibers are added to a mixed solution of a water-soluble organic monomer (A), a water-swellable clay mineral (B), an organic solvent, and water as necessary. It can be easily prepared by adding, further mixing and stirring the oxyradical compound (C).

Since the organic-inorganic composite hydrogel manufacturing method of the present invention can easily obtain an organic-inorganic composite hydrogel having a three-dimensional network structure, the organic-inorganic composite hydrogel precursor composition, a polymerization initiator, and a polymerization accelerator are combined. A preferred method is to polymerize the water-soluble organic monomer (A) in a mixed solution (X) containing the following. The obtained water-soluble organic monomer polymer forms a three-dimensional network structure together with the water-swellable clay mineral, and becomes a constituent element of the organic-inorganic composite hydrogel.

The polymerization initiator preferably has a solubility in water at 20° C. of 50 g/100 ml or more, since the polymerization of the water-soluble organic monomer (A) can proceed sufficiently even in an air atmosphere.

Examples of the polymerization initiator include water-soluble peroxides and water-soluble azo compounds having a solubility in water of 50 g/100 ml or more at 20°C.

Examples of the water-soluble peroxide include ammonium persulfate, sodium persulfate, and t-butyl hydroperoxide.

Examples of the water-soluble azo compound include 2,2'-azobis(2-methylpropionamidine) dihydrochloride, 4,4'-azobis(4-cyanovaleric acid), and the like.

Among these, from the viewpoint of interaction with the water-swellable clay mineral (B), it is preferable to use water-soluble peroxides, and it is more preferable to use ammonium persulfate and sodium persulfate.

In addition, the said polymerization initiator may be used individually, or may be used in combination of 2 or more types.

The molar ratio of the polymerization initiator to the water-soluble organic monomer (A) in the mixed solution (X) is such that the polymerization of the water-soluble organic monomer (A) can sufficiently proceed even in an air atmosphere. Therefore, the range is preferably from 0.01 to 0.1, and more preferably from 0.01 to 0.05.

Examples of the polymerization accelerator include tertiary amine compounds, thiosulfates, and ascorbic acids.

Examples of the tertiary amine compound include N,N,N',N'-tetramethylethylenediamine and 3-dimethylaminopropionitrile.

Examples of the thiosulfate include sodium thiosulfate and ammonium thiosulfate.

Examples of the ascorbic acids include L-ascorbic acid and sodium L-ascorbate.

Among these, from the viewpoint of affinity and interaction with water-swellable clay minerals, it is preferable to use tertiary amine compounds, and it is more preferable to use N,N,N',N'-tetramethylethylenediamine.

In addition, the said polymerization accelerator may be used individually, or may be used in combination of 2 or more types.

The content of the polymerization accelerator in the liquid mixture (X) is preferably 0.01 to 1% by mass, more preferably 0.05 to 0.5% by mass. A content of 0.01% by mass or more is preferable because it can efficiently promote the synthesis of organic monomers for the resulting hydrogel. On the other hand, when the content is 1% by mass or less, the dispersion can be used without agglomerating before polymerization, and handling properties are improved, which is preferable.

The step of mixing the organic-inorganic composite hydrogel precursor composition, the polymerization initiator, and the polymerization accelerator to prepare a mixed solution (X) includes adding the polymerization initiator to the organic-inorganic composite hydrogel precursor composition. The agent and the polymerization accelerator may be mixed as they are, or an aqueous solution of the polymerization initiator or an aqueous solution of the polymerization accelerator may be mixed.

The mixed liquid (X) contains the organic-inorganic composite hydrogel precursor composition, the polymerization initiator, and the polymerization accelerator, but may further contain an organic crosslinking agent, a preservative, a thickener, etc., if necessary. May contain.

The method for producing an organic-inorganic composite hydrogel of the present invention includes mixing the organic-inorganic composite hydrogel precursor composition, the polymerization initiator, and the polymerization accelerator; This method polymerizes the organic monomer (A), but it has excellent workability because it does not require post-processes such as heating or ultraviolet irradiation.

The polymerization temperature is preferably 10 to 80°C, more preferably 20 to 80°C. It is preferable that the polymerization temperature is 10° C. or higher because the radical reaction can proceed in a chain manner. On the other hand, it is preferable that the polymerization temperature is 80° C. or lower because the water contained in the dispersion can be polymerized without boiling.

The polymerization time varies depending on the type of the polymerization initiator and the polymerization accelerator, but it is carried out for a period of several tens of seconds to 24 hours. In particular, in the case of radical polymerization using heating or redox, the time is preferably 1 to 24 hours, more preferably 5 to 24 hours. It is preferable that the polymerization time is 1 hour or more because the polymer of the water-swellable clay mineral (B) and the water-soluble organic monomer (A) can form a three-dimensional network. On the other hand, since the polymerization reaction is almost completed within 24 hours, the polymerization time is preferably 24 hours or less.

In the method for producing an organic-inorganic composite hydrogel of the present invention, since the organic-inorganic composite hydrogel precursor composition has excellent long-term storage stability, the organic-inorganic composite hydrogel precursor composition is prepared and then transported to the site of use. be able to. Moreover, since the organic-inorganic composite hydrogel can be easily produced even in an air atmosphere, it can be suitably used in on-site construction applications such as civil engineering construction sites and construction sites.

The present invention will be explained in more detail below with reference to specific examples.

(Example 1: Production and evaluation of organic-inorganic composite hydrogel precursor composition (1))
In a flat-bottom glass container, 40 mL of pure water, 50 mL of purified glycerin, 4.8 g/1.44 g of phosphonic acid-modified synthetic hectorite ("Laponite RDS"/"Laponite S-482" manufactured by BIC Chemie Japan Co., Ltd.), and 20 g of dimethylacrylamide. , N,N'-methylenebisacrylamide (20 mg) was added thereto, and an aqueous solution was prepared by stirring. The appearance of this aqueous solution was a uniform pale white color, and no particular aggregates were observed. Next, 10 g of cellulose nanofibers ("Selish KY100G" manufactured by Daicel FineChem Co., Ltd.; hereinafter abbreviated as "CNF (1)") were added little by little to this aqueous solution while stirring, and finally 2, 2, 6, Organic-inorganic composite hydrogel precursor composition (1 ) was adjusted.

(Example 2: Production and evaluation of organic-inorganic composite hydrogel precursor composition (2))
In a flat-bottom glass container, 40 mL of pure water, 50 mL of purified glycerin, 4.8 g/1.44 g of phosphonic acid-modified synthetic hectorite ("Laponite RDS"/"Laponite S-482" manufactured by BIC Chemie Japan Co., Ltd.), and 20 g of dimethylacrylamide. , N,N'-methylenebisacrylamide (20 mg) was added thereto, and an aqueous solution was prepared by stirring. The appearance of this aqueous solution was a uniform pale white color, and no particular aggregates were observed. Next, 10 g of CNF (1) was added little by little to this aqueous solution while stirring, and finally [2,6-di-tert-butyl-4-[(3,5-di-tert-butyl-4-oxo- 0.00055 g (5 ppm) of 2,5-cyclohexadien-1-ylidene)methyl phenyloxy] radical (Galvinoxyl) (hereinafter abbreviated as "oxyradical compound (C-2)") was mixed and dissolved to form an organic and inorganic compound. A composite hydrogel precursor composition (2) was prepared.

(Comparative Example 1: Production and evaluation of organic-inorganic composite hydrogel precursor composition (R1))
In a flat-bottom glass container, 40 mL of pure water, 50 mL of purified glycerin, 4.8 g/1.44 g of phosphonic acid-modified synthetic hectorite ("Laponite RDS"/"Laponite S-482" manufactured by BIC Chemie Japan Co., Ltd.), and 20 g of dimethylacrylamide. , N,N'-methylenebisacrylamide (20 mg) was added thereto, and an aqueous solution was prepared by stirring. The appearance of this aqueous solution was a uniform pale white color, and no particular aggregates were observed. Next, 10 g of CNF (1) was added little by little to this aqueous solution with stirring to prepare an organic-inorganic composite hydrogel precursor composition (R1).

(Comparative Example 2: Production and evaluation of organic-inorganic composite hydrogel precursor composition (R2))
In a flat-bottom glass container, 40 mL of pure water, 50 mL of purified glycerin, 4.8 g/1.44 g of phosphonic acid-modified synthetic hectorite ("Laponite RDS"/"Laponite S-482" manufactured by BIC Chemie Japan Co., Ltd.), and 20 g of dimethylacrylamide. , N,N'-methylenebisacrylamide (20 mg) was added thereto, and an aqueous solution was prepared by stirring. The appearance of this aqueous solution was a uniform pale white color, and no particular aggregates were observed. Next, 10 g of CNF (1) was added little by little to this aqueous solution while stirring, and finally 0.0055 g (50 ppm) of 1,4-hydroxybenzene was mixed and dissolved to prepare an organic-inorganic composite hydrogel precursor composition (R2). did.

[Evaluation of long-term storage stability]
The glass bottle containing the organic-inorganic composite hydrogel precursor composition obtained above was sealed and stored in a thermostat at 80° C., and the number of days until gelation was evaluated.

Table 1 shows the evaluation results obtained above.

It was confirmed that the organic-inorganic composite hydrogel precursor compositions of the present invention of Examples 1 and 2 have excellent long-term storage stability.

On the other hand, Comparative Example 1 was an example in which no polymerization inhibitor was used, but it was confirmed that the storage stability was insufficient.

Comparative Example 2 is an example in which no oxyradical compound was used as a polymerization inhibitor, but it was confirmed that the storage stability was insufficient.

(Example 3: Production and evaluation of organic-inorganic composite hydrogel (1))
10 g of glycerin and 80 μL of tetramethylethylenediamine (hereinafter abbreviated as “TEMED”) were placed in a flat bottom glass container and stirred to prepare a uniform TEMED aqueous solution.
In a 200 mL glass beaker, put 110 g of the organic-inorganic composite hydrogel precursor composition (1) obtained in Example 1 above, and add 0.5 g of sodium persulfate (hereinafter abbreviated as "NPS") therein. Stir until dissolved. Further, the TEMED aqueous solution prepared above was added, and stirring was continued until uniformly mixed to prepare a mixed solution (X-1).
The mixture (X-1) obtained above was immediately cast into a 2 mm gap between two glass plates with a thickness of 5 mm placed together via a 2 mm spacer, and left undisturbed at room temperature for 24 hours. An organic-inorganic composite hydrogel (1) with a thickness of 2 mm was produced. When the state of the cast liquid was checked after 24 hours, a uniform, colorless and transparent organic-inorganic composite hydrogel sheet was obtained.

[Evaluation of breaking strength and elongation rate of organic-inorganic composite hydrogel]
Using the organic-inorganic composite hydrogel sheet produced above, a tensile test was conducted according to the test method of JIS K 6251:2010 to evaluate the breaking strength. The sample for the tensile test was in the shape of a JIS No. 3 dumbbell. The conditions for the tensile test were as follows: Using a tensile testing machine (Autograph AG-10KNX, manufactured by Shimadzu Corporation), the sample was stretched at 23° C. and at a tensile speed of 500 mm/min until it broke.
The breaking strength was 0.55 MPa, and the elongation between the gauge lines was 1100%.

(Example 4: Production and evaluation of organic-inorganic composite hydrogel (2))
An organic-inorganic composite hydrogel sheet was obtained in the same manner as in Example 3, except that the organic-inorganic composite hydrogel precursor composition (2) obtained in Example 2 was used.
The breaking strength was 0.55 MPa, and the elongation between the gauge lines was 1100%.

Claims (5)

An organic-inorganic composite hydrogel precursor composition containing a water-soluble organic monomer (A), a water-swellable clay mineral (B), an oxyradical compound (C), and water. The organic-inorganic composite hydrogel precursor composition according to claim 1, wherein the water-swellable clay mineral (B) is phosphonic acid-modified hectorite. Claim 1 containing a low-volatile solvent having a volatility of 0.1 g or less per 1 cm ² 1 hour (0.1 g/cm ² hr 60°C 1 atm or less) in an open system at 60 ℃ 1 atm. or the organic-inorganic composite hydrogel precursor composition according to 2. An organic-inorganic composite hydrogel, which is a reaction product of the organic-inorganic composite hydrogel precursor composition according to any one of claims 1 to 3. A method for producing an organic-inorganic composite hydrogel, comprising a step of mixing the organic-inorganic composite hydrogel precursor composition according to any one of claims 1 to 3, a polymerization initiator, and a polymerization accelerator.