JP2022133876A - Adhesive, wood board using the same, and method for producing adhesive - Google Patents

Abstract

[Problem] To provide an adhesive that inhibits the reaction between tannin and formaldehyde to stabilize the curing reaction and improve water resistance, and a method for producing the same. [Solution] By using tannin as the main agent, adding formaldehyde and glyoxal resin as crosslinking agents, and further using a curing accelerator in combination, it is possible to control the reaction between tannin and formaldehyde, stabilize the curing reaction, and at the same time improve water resistance. [Selected Figure] None

Description

TECHNICAL FIELD The present invention relates to an adhesive, a wooden board using the adhesive, and a method for producing the adhesive.

BACKGROUND ART Adhesives containing tannins have been researched and developed, for example, as adhesives for wooden boards and plywood.

For example, in Patent Document 1, a mixture of elongated wooden pieces and a tannin-based adhesive containing tannin derived from acacia is formed by aligning and laminating the elongated wooden pieces in one direction. A wood-based composite material obtained by heating and pressing a wood mat is disclosed.

JP 2009-179042 A

By the way, in a tannin-based adhesive containing formaldehyde as a cross-linking agent, the reactivity of tannin and formaldehyde is high, and the condensation reaction proceeds very quickly, so the curing reaction proceeds too quickly, and the adhesive force is effectively lost. difficult to demonstrate. If the condensation reaction proceeds rapidly, the viscosity of the adhesive increases and the applicability of the adhesive deteriorates. In addition, in order to slow down the condensation reaction, if formaldehyde is not sufficiently contained, many cross-linking points sufficient to develop high water resistance are not formed between tannin molecules, so it is an adhesive with low water resistance. turn into. Therefore, in order to slow down the condensation reaction, tannin-based adhesives that are reacted under alkaline conditions have been proposed. In this tannin-based adhesive, the use of hexamethylenetetramine makes it possible to slow down the condensation reaction and form sufficient cross-linking points. Since it is acidic, there is not only the problem that it is difficult to maintain the alkalinity, but there is also the problem that maintaining the alkalinity softens the wood itself and reduces the strength of the wood.

The present invention has been made in view of this point, and its object is to suppress the reaction between tannin and formaldehyde, stabilize the curing reaction, and improve water resistance.

In order to achieve the above object, an adhesive according to the present invention is characterized by containing tannin as a main ingredient and containing formaldehyde and glyoxal resin.

According to the above configuration, since glyoxal resin is contained in addition to tannin and formaldehyde, the probability of reaction between tannin and formaldehyde is low, and for example, the reaction between tannin and formaldehyde proceeds even at a relatively low temperature such as room temperature. The reaction is greatly slowed down and the curing reaction can be stabilized. Here, since the glyoxal resin has a relatively low viscosity, it is possible to reduce the viscosity of the adhesive by mixing it with an aqueous tannin solution having a relatively high viscosity, thereby improving the applicability of the adhesive. can be done. In addition, when the adhesive is cured by hot pressing, etc., tannin and formaldehyde react to form a cross-linked structure. A crosslinked structure is also formed between tannin and glyoxal resin by reaction with hydroxyl groups. Furthermore, the methylol groups of the glyoxal resin bind to the hydroxyl groups of the cellulose that constitutes wood fibers, wood chips and wood strands. As a result, the number of cross-linking points with tannin increases, and the glyoxal resin and cellulose are bound together, so that the water resistance can be greatly improved. Therefore, it is possible to suppress the reaction between tannin and formaldehyde, stabilize the curing reaction, and improve the water resistance.

Furthermore, the adhesive according to the present invention may contain a curing accelerator.

According to the above configuration, by adding a curing accelerator to the adhesive, it is possible to increase the reaction speed after the start of hot pressing, shortening the pressing time and reducing the amount of formaldehyde emitted accordingly. . Here, the curing accelerator is configured to accelerate curing by, for example, reacting with formaldehyde after hot pressing to generate a strong acid and adjust the pH of the adhesive to 1 to 3. Curing speed is not significantly improved.

3 to 6 parts by mass of formaldehyde and 10 to 21 parts by mass of glyoxal resin may be contained with respect to 100 parts by mass of tannin.

According to the above configuration, 3 to 6 parts by mass of formaldehyde and 10 to 21 parts by mass of glyoxal resin are contained with respect to 100 parts by mass of tannin. Water absorption thickness expansion rate 17% or less in the water absorption thickness expansion rate test specified in JIS A5905, and bending strength 15 MPa or more in the wet bending strength A test), and a predetermined formaldehyde emission grade (JIS A5905 specified It is possible to manufacture a wood fiber board that satisfies F☆☆☆☆) in the classification. Here, when formaldehyde is contained in an amount of less than 3 parts by mass with respect to 100 parts by mass of tannin, the curing reaction of the adhesive is remarkably slowed down, ensuring the strength of the wood fiber board manufactured using the adhesive. becomes difficult to do. Also, if formaldehyde exceeds 6 parts by mass with respect to 100 parts by mass of tannin, it becomes difficult to ensure formaldehyde emission grade of F☆☆☆☆ in the above classification. In addition, if the glyoxal resin is contained in an amount of less than 10 parts by mass with respect to 100 parts by mass of tannin, the number of cross-linking points will be insufficient, making it difficult to ensure the water resistance of the manufactured wood fiberboard. In addition, if the amount of glyoxal resin exceeds 21 parts by mass per 100 parts by mass of tannin, it becomes difficult to secure a formaldehyde emission grade of F☆☆☆☆ in the above classification.

A wood board according to the present invention is characterized by bonding at least one of wood fibers, wood chips, and wood strands using the adhesive described above.

According to the above configuration, in the adhesive, the reaction between tannin and formaldehyde can be suppressed, the curing reaction can be stabilized, and the water resistance can be improved, so the water resistance performance of the wooden board can be improved. .

Further, a method for producing an adhesive according to the present invention is a method for producing an adhesive containing tannin as a main ingredient and containing formaldehyde and glyoxal resin, wherein the tannin aqueous solution obtained by dissolving the tannin in water is and a cross-linking agent mixing step of adding and mixing the glyoxal resin to the aqueous tannin solution prepared in the aqueous solution preparation step and then adding and mixing the formaldehyde. .

According to the above production method, in the cross-linking agent mixing step, glyoxal resin is added to the tannin aqueous solution prepared in the aqueous solution preparation step and mixed, and then formaldehyde is added and mixed. For example, the reaction between tannin and formaldehyde, which proceeds even at a relatively low temperature such as room temperature, is greatly slowed down, and the curing reaction can be stabilized. Here, since the glyoxal resin has a relatively low viscosity, it is possible to reduce the viscosity of the adhesive by mixing it with an aqueous tannin solution having a relatively high viscosity, thereby improving the applicability of the adhesive. can be done. In addition, when the adhesive is cured by hot pressing, etc., tannin and formaldehyde react to form a cross-linked structure. A crosslinked structure is also formed between tannin and glyoxal resin by reaction with hydroxyl groups. Furthermore, the methylol groups of the glyoxal resin bind to the hydroxyl groups of the cellulose that constitutes wood fibers, wood chips and wood strands. As a result, the number of cross-linking points with tannin increases, and the glyoxal resin and cellulose are bound together, so that the water resistance can be greatly improved. Therefore, it is possible to suppress the reaction between tannin and formaldehyde, stabilize the curing reaction, and improve the water resistance.

After the cross-linking agent mixing step, a curing accelerator mixing step may be provided in which a curing accelerator is added to and mixed with the mixed liquid in which the glyoxal resin and the formaldehyde are mixed in the cross-linking agent mixing step.

According to the above production method, by adding a curing accelerator in the curing accelerator mixing step, it is possible to increase the reaction rate after the start of hot pressing, shortening the pressing time and reducing the amount of formaldehyde emitted accordingly. can be achieved. Here, the curing accelerator is configured to accelerate curing by, for example, reacting with formaldehyde after hot pressing to generate a strong acid and adjust the pH of the adhesive to 1 to 3. Curing speed is not significantly improved.

According to the present invention, since tannin is contained as a main ingredient and formaldehyde and glyoxal resin are contained, the reaction between tannin and formaldehyde can be suppressed, the curing reaction can be stabilized, and water resistance can be improved. .

1 is a schematic diagram showing a crosslinked structure of an adhesive according to a first embodiment of the present invention; FIG. 4 is a table showing the results of experiments (water resistance (thickness expansion rate)) performed on wood fiber boards that were made as prototypes using the adhesive according to the first embodiment of the present invention. 4 is a table showing the results of experiments (water resistance (flexural strength)) performed on wood fiber boards that were experimentally produced using the adhesive according to the first embodiment of the present invention. FIG. 4 is a table showing experimental results (formaldehyde emission amount) conducted on wood fiber boards manufactured by way of trial using the adhesive according to the first embodiment of the present invention. FIG.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail based on the drawings. In addition, the present invention is not limited to the following embodiments.

<<1st Embodiment>>
1 to 4 show a first embodiment of an adhesive, a wood fiber board using the adhesive, and a method for producing the adhesive according to the present invention. Here, FIG. 1 is a schematic diagram showing the crosslinked structure of the adhesive of this embodiment. 2, 3 and 4 show experimental results (water resistance (thickness expansion rate), water resistance (flexural strength) and formaldehyde It is a table showing the amount of radiation).

The adhesive of this embodiment contains tannin as a main ingredient, and also contains formaldehyde, glyoxal resin, and a curing accelerator. As a specific composition, the adhesive of the present embodiment contains about 3 to 6 parts by mass of formaldehyde, about 10 to 21 parts by mass of glyoxal resin, and 1 to 3 parts by mass of a curing accelerator per 100 parts by mass of tannin. contains the extent In this embodiment, the adhesive containing tannin, formaldehyde, glyoxal resin and curing accelerator is exemplified, but the curing accelerator in the adhesive may be omitted.

Tannins have a structure such as the following formula (1), for example, mimosa tannins extracted from the outer bark of acacia, queplacho tannins extracted from the heartwood of queplacho, radiata pine tannins, etc. have a molecular weight of 500 to 3000. degree of condensed tannins, and mixtures thereof. In addition, when an alkaline aqueous solution is used as a solvent for extracting tannin, although the yield of tannin is improved, the reactivity with formaldehyde is lost, resulting in a decrease in adhesive strength. Not suitable as Moreover, when hot water or hot water is used as a solvent for extracting tannins, it takes a long time to extract tannins, making it difficult to extract tannins efficiently. Therefore, it is preferable to extract tannin using a lower alcohol or a mixed solvent of a lower alcohol and an aqueous solvent. Here, in the formula (1), the portion indicated by the arrow is the highly reactive portion of the benzene ring (with which formaldehyde reacts, for example) (black circle portion in FIG. 1). Moreover, in Formula (1), R is hydrogen (H) or a hydroxyl group (OH). The tree species from which tannin is extracted is not particularly limited.

Examples of formaldehyde include formalin (aqueous formaldehyde solution), paraformaldehyde, hexamethylenetetramine, and mixtures thereof. Furthermore, urea resin containing formaldehyde, urea-melamine cocondensation resin, phenol resin, or the like may be used. That is, urea resin, urea-melamine cocondensation resin, phenolic resin, etc. contain excessive formaldehyde. A melamine co-condensation resin, a phenol resin, or the like may be added to form the adhesive of the present embodiment.

Glyoxal resin is obtained by reacting glyoxal with urea and formaldehyde to form a cyclic molecular structure, and has a basic structure represented by the following formula (2). Here, in formula (2), R is hydrogen (H) or a methylol group (CH ₂ OH).

As the curing accelerator, for example, an ammonium salt such as ammonium chloride or ammonium sulfate is preferable, but any strong acid or weak base salt can be used.

As shown in FIG. 1, the adhesive having the above structure has a crosslinked structure F formed by formaldehyde and a crosslinked structure G formed by glyoxal resin between a plurality of tannin T molecules or molecular linkages to form a three-dimensional network. By forming the structure, the wood fibers are bonded together to form a wood board such as a wood fiber board. Here, the crosslinked structure F is formed by addition reaction and dehydration condensation reaction between the benzene ring in tannin T and formaldehyde. Also, the crosslinked structure G is formed by a dehydration condensation reaction between the hydroxyl group in the tannin T and the methylol group in the glyoxal resin. Examples of wood fibers include wood chips, veneer chips, plywood chips, wood fibers such as bark, hemp, hemp, palm, palm, kenaf, sorghum, bamboo, straw, rice straw, and beet pulp. A plant fiber such as a fiber can be used, and a mixture thereof may be used. Further, in the present embodiment, the wood fiber board obtained by bonding the above-mentioned wood fibers is exemplified as the wood board. It may also be a wood board made by bonding a mixture of at least two of fibers, wood chips and wood strands.

Next, a method for producing the adhesive of the present embodiment will be described. The method for producing the adhesive of the present embodiment comprises the following steps of preparing an aqueous solution, mixing a cross-linking agent, and mixing a curing accelerator.

<Aqueous solution preparation step>
For example, 60 g of water at about 60 to 70 ° C. is placed in a container such as a beaker, and 60 g of dried tannin powder is added little by little while stirring to dissolve tannin in warm water to prepare a 50% by mass tannin aqueous solution. do. In the present embodiment, the manufacturing method of dissolving tannin powder in warm water to prepare a tannin aqueous solution was exemplified, but for example, extracting components obtained from the bark of Morishima acacia or acacia mangium, the heartwood of Keplaco, etc. are concentrated. Thus, a 50% mass % tannin aqueous solution may be prepared.

<Crosslinking agent mixing step>
A predetermined amount of glyoxal resin (10 to 21 parts by mass per 100 parts by mass of tannin in terms of solid content) is added to the 50% by mass aqueous tannin solution prepared in the aqueous solution preparation step, for example, 40% by mass solid content. After adding and mixing as an aqueous solution of, a predetermined amount of formaldehyde (3 to 6 parts by weight with respect to 100 parts by weight of tannin) is added and mixed, for example, as a 37% by weight aqueous solution (formalin).

<Curing accelerator mixing step>
In the cross-linking agent mixing step, a predetermined amount of ammonium chloride is added as a curing accelerator to the mixture of glyoxal resin and formaldehyde (1 to 3 parts by mass based on 100 parts by mass of tannin in terms of solid content). For example, it is added as a 25% by mass aqueous solution and mixed.

As described above, the adhesive of the present embodiment can be produced. Thereafter, the manufactured adhesive is sprayed on the disentangled wood fibers, molded into a mat, and then heated and pressurized to manufacture a wood fiber board.

Next, specific experiments conducted with the adhesive of this embodiment will be described.

First, mimosa tannin is used as tannin, formalin is used as formaldehyde, an aqueous solution having a molecular weight of several hundred as glyoxal resin and a resin content of 35 to 45% by mass is used, and a 25% by mass aqueous solution of ammonium chloride is used as a curing accelerator. , 3, 4, 5, 6 and 7 parts by weight of formaldehyde per 100 parts by weight of tannin, as shown in the tables of FIGS. Adhesives containing 10, 14, 21 and 29 parts by mass of glyoxal resin adhesives and, if necessary, 2 parts by mass (fixed amount) of ammonium chloride were prepared as samples.

Subsequently, 100 parts by mass of the fibrillated radiata pine wood fiber was sprayed with each of the above trial adhesives at a solid content ratio of 16 parts by mass, and the target density was 800 kg / m ³ , and the hot plate temperature of the hot press After heating and pressing at 180 ° C., pressing time 3 to 6 minutes, maximum pressure 4 MPa, target thickness 3 mm, both sides are sanded 0.15 mm each with 150 grit to make a wood fiber board with a thickness of 2.7 mm. I made a prototype.

Furthermore, the following water resistance performance evaluation and formaldehyde emission measurement were performed on the wood fiber boards that were made as prototypes.

～Water resistance performance evaluation～
Based on the water absorption thickness expansion coefficient test specified in JIS A5905, the thickness expansion coefficient after being immersed in water at 20 ° C. ± 1 ° C. for 24 hours was measured. Those with an expansion rate of 17% or less were evaluated as ◯, and those with a thickness expansion rate of more than 17% were evaluated as x.

Based on the wet bending strength A test specified in JIS A5905, immerse in hot water at 70 ° C. for 2 hours, then measure the bending strength after immersing in water at 20 ° C. for 1 hour, As shown in the table of FIG. 3, samples having a wet bending strength exceeding 15 MPa were evaluated as ◯, and samples having a wet bending strength of less than 15 MPa were evaluated as x.

～Formaldehyde emission measurement～
It was performed based on the desiccator method specified in JIS A1406, and as shown in the table of FIG. 3 mg/L or less, maximum 0.4 mg/L or less) was evaluated as ◯, and worse than that was evaluated as x. The test was performed with N=1, and the test piece was cured in an air-conditioned room at 20° C.±1° C. for about 2 days.

As the experimental results, as shown in the tables of FIGS. 2, 3 and 4, the adhesive contained 3 to 6 parts by mass of formaldehyde and 10 to 21 parts by mass of glyoxal resin per 100 parts by mass of tannin. If so, the wood fiber board using the adhesive has a water absorption thickness expansion rate of 17% or less in the water absorption thickness expansion rate test specified in JIS A5905, and bending strength in the wet bending strength A test. It was confirmed that water resistance of 15 MPa or more and formaldehyde emission grade of F☆☆☆☆ in the classification specified in JIS A5905 were satisfied.

As described above, according to the adhesive and its manufacturing method of the present embodiment, in the cross-linking agent mixing step, glyoxal resin is added to and mixed with the aqueous tannin solution prepared in the aqueous solution preparation step, and then formaldehyde is added. Since the tannins and formaldehyde are mixed together, the probability of reaction between tannin and formaldehyde is reduced, and the reaction between tannin and formaldehyde, which proceeds even at relatively low temperatures such as room temperature, is greatly slowed down, and the curing reaction can be stabilized. Here, since the glyoxal resin has a relatively low viscosity, it is possible to reduce the viscosity of the adhesive by mixing it with an aqueous tannin solution having a relatively high viscosity, thereby improving the applicability of the adhesive. can be done. In addition, when the adhesive is cured by hot pressing, etc., tannin and formaldehyde react to form a cross-linked structure. A crosslinked structure is also formed between tannin and glyoxal resin by reaction with hydroxyl groups. Furthermore, the methylol groups of the glyoxal resin bind to the hydroxyl groups of the cellulose that constitutes wood fibers, wood chips and wood strands. As a result, the number of cross-linking points with tannin increases, and the glyoxal resin and cellulose are bound together, so that the water resistance can be greatly improved. Therefore, it is possible to suppress the reaction between tannin and formaldehyde, stabilize the curing reaction, and improve the water resistance. Here, although some tannins have a high degree of polymerization and high viscosity at the time of extraction, the viscosity of the adhesive of the present embodiment is low, so the applicability of the adhesive can be improved. In addition, since the adhesive of this embodiment exhibits appropriate performance as an adhesive for wooden boards simply by mixing at room temperature, it is sufficient to store each raw material, and work such as self-condensation is unnecessary. As a result, the manufacturing process of wooden boards can be greatly simplified. Furthermore, since the adhesive of the present embodiment can react with tannin under acidic conditions, it can be mixed with existing adhesives such as urea resins and urea-melamine cocondensation resins, or mixed when manufacturing wooden boards. It can be used without or in combination.

Further, according to the adhesive and its manufacturing method of the present embodiment, the adhesive contains 3 to 6 parts by mass of formaldehyde and 10 to 21 parts by mass of glyoxal resin with respect to 100 parts by mass of tannin. using a predetermined water resistance (water absorption thickness expansion rate of 17% or less in the water absorption thickness expansion rate test specified in JIS A5905, and a bending strength of 15 MPa or more in the wet bending strength A test), In addition, it is possible to produce a wooden board that satisfies a predetermined formaldehyde emission grade (F☆☆☆☆ in the classification specified in JIS A5905).

In addition, according to the wood board using the adhesive of the present embodiment, the reaction between tannin and formaldehyde can be suppressed in the adhesive, the curing reaction can be stabilized, and the water resistance can be improved. The water resistance performance of the board can be improved. In addition, since hardening before molding into a wooden board can be suppressed, it is possible to improve the strength of the wooden board, stabilize performance, and reduce water absorption.

<<Other embodiments>>
In the above embodiment, formaldehyde was exemplified as one of the cross-linking agents, but in the present invention, instead of formaldehyde, for example, hexamethylenetetramine, isocyanate resins such as MDI (diphenylmethane diisocyanate), urea (urea) resins, It can also be applied to adhesives using compounds capable of reacting with tannin, such as urea/melamine cocondensation resins, amino resins, and epoxy resins.

In the above embodiments, glyoxal resin was exemplified as one of the cross-linking agents (reaction stabilizers). It can also be applied to an adhesive using a compound that has a thin molecular chain and reacts with the hydroxyl group of tannin to increase cross-linking points.

INDUSTRIAL APPLICABILITY As described above, the present invention is extremely useful because it can suppress the reaction between tannin and formaldehyde and stabilize the curing reaction.

Claims (6)

An adhesive containing tannin as a main ingredient and containing formaldehyde and glyoxal resin. The adhesive according to claim 1,
An adhesive characterized by containing a curing accelerator. In the adhesive according to claim 1 or 2,
An adhesive comprising 3 to 6 parts by mass of formaldehyde and 10 to 21 parts by mass of glyoxal resin per 100 parts by mass of tannin. A wood board comprising at least one of wood fibers, wood chips and wood strands bonded with the adhesive according to any one of claims 1 to 3. A method for producing an adhesive containing tannin as a main ingredient and containing formaldehyde and glyoxal resin,
An aqueous solution preparation step of preparing a tannin aqueous solution in which the tannin is dissolved in water;
a cross-linking agent mixing step of adding and mixing the glyoxal resin to the aqueous tannin solution prepared in the aqueous solution preparing step, and then adding and mixing the formaldehyde. In the method for producing an adhesive according to claim 5,
After the cross-linking agent mixing step, a curing accelerator mixing step of adding and mixing a curing accelerator to the mixed liquid in which the glyoxal resin and the formaldehyde are mixed in the cross-linking agent mixing step. manufacturing method.

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