Tannin-based adhesives, plywood, preparation methods and applications

Abstract

The present invention belongs to the technical fields of wood adhesives and wood processing, and relates to tannin-based adhesives, plywood thereof, preparation methods and applications. Aiming at the technical problems in the prior art that the raw materials for preparing tannin-based adhesives contain fossil resource raw materials (such as urea, phenol), and pH regulators need to be used in the preparation process, resulting in a relatively high formaldehyde emission and a complex preparation process, the tannin-based adhesive provided in this application contains tannin degradation products, organic solvents, aldehyde-based compounds or non-aldehyde-based compounds; the molar ratio of formaldehyde to tannin degradation products is 0.5 to 1.5; the mass of aldehyde-based compounds or non-aldehyde-based compounds is 5 to 15% of the mass of tannin degradation products; the maximum formaldehyde binding capacity of tannin degradation products is 0.9 to 1.3 g / g. The raw materials do not contain fossil resources, reducing the formaldehyde emission and having a simple preparation process. The present invention also provides plywood, which meets the strength index requirements of both Class I plywood and Class II plywood, and the formaldehyde emission reaches the F☆☆☆☆ level.

Description

Technical Field

[0001] The present invention belongs to the technical fields of wood adhesives and wood processing. Specifically, it relates to tannin-based adhesives, plywood thereof, preparation methods, and applications. Background Art

[0002] It is of practical significance to synthesize biomass-based adhesives using renewable biomass resource raw materials to replace fossil resource raw materials for wood-based panel production. Currently, raw condensed tannins cannot completely replace fossil resource raw materials such as phenol for the synthesis of wood adhesives. The reasons are their relatively high molecular weight, large steric hindrance, and a large number of reaction sites that are far apart. When the molecular weight of tannin itself is large (500 - 3000 Da) and the degree of condensation with formaldehyde is not high, the tannin adhesive can no longer wet and diffuse, and then loses its fluidity, resulting in the termination of the reaction and low bonding strength. Because of the large molecular weight, restricted by the molecular size and shape, the remaining active positions in the molecule are too far apart to further participate in the reaction, leading to a brittle adhesive layer and low bonding strength. Also, due to the large molecular weight and many reaction sites, the reaction rate of tannin with formaldehyde is too fast (larch tannin is more than 10 times that of phenol), resulting in a short storage period and active period of the tannin adhesive. In addition, due to the low degree of reaction with formaldehyde and the presence of a large number of residual hydrophilic groups, the water resistance of the tannin adhesive is poor.

[0003] To solve the problems existing in the use of the above-mentioned tannin adhesives, urea (Peng Jinda, Deng Shuangqi, Liao Dongsen, et al., Study on the blending properties of UF resin and myrica rubra tannin adhesive [J], Journal of Southwest Forestry University (Natural Science), 2022, 42(6): 140 - 150) or phenol (Dong Jiying, Wu Zhigang, Lei Hong, et al., Bonding properties of tannin-phenol-formaldehyde resin [J], Journal of Forest and Environment, 2018, 38(1): 118 - 122) is often added during the synthesis of adhesives, forming a copolymerization system with tannin or aldehyde compounds, or forming a blend system with phenol / formaldehyde resin (such as Chinese Invention Application No. 92107343.7, titled Preparation method of Chinese black wattle tannin glue with active phenolic resin as crosslinking agent, application date: March 31, 1992) or phenol-urea-formaldehyde resin (such as Chinese Invention Application No. 921073445, titled Chinese black wattle tannin glue with active phenol-urea-formaldehyde resin as crosslinking agent, application date: March 31, 1992). In these co-condensation resin adhesives and blend resin adhesives, the amount of tannin used is less, and the advantages of tannin as a renewable biomass resource cannot be fully utilized. In fact, tannin cannot completely replace fossil resource raw materials for the preparation or synthesis of wood adhesives.

[0004] Degrading condensed tannins is one of the means to solve the defects such as low bonding strength of tannin-based adhesives. The Chinese patent application number is 2020104128512, and the invention name is a degrading condensed tannin modified phenolic resin wood adhesive and its preparation method. The application date is May 15, 2020. The disclosed solution realizes the degradation activation of condensed tannins under acidic conditions. The degrading condensed tannins are stirred and reacted with phenol, formaldehyde solution, alkaline solution, polyethyleneimine and distilled water to prepare a degrading condensed tannin modified phenolic resin wood adhesive. The Chinese patent application number is 2020113007964, and the invention name is a modifier for urea-formaldehyde resin adhesive and its application, a modified urea-formaldehyde resin adhesive and its application. The application date is November 19, 2020, which discloses the use of tannin-degrading bacteria to degrade tannins and form a modifier with aqueous formaldehyde solution and soluble aluminum salt for the modification of urea-formaldehyde resin. The Chinese patent application number is 2020112713382, and the invention name is a two-component adhesive and its application in artificial boards. The application date is November 13, 2020, which discloses the use of sodium bisulfite, sodium sulfite and sodium metabisulfite to degrade tannins. The degraded tannins react with polycarbonyl small molecules to prepare component 1, and then form a two-component adhesive with component 2 formaldehyde-based resin. The Chinese patent application number is 2017112152598, and the invention name is a preparation method of a grape skin tannin modified phenolic resin adhesive. The application date is November 18, 2017, which discloses a method of combining sulfite extraction and biodegradation to prepare a grape skin tannin modified phenolic resin adhesive with low molecular weight and low degree of polymerization of condensed tannins. Although the above solutions use degraded tannins, the usage method is similar to that of undegraded tannins, which is used for the modification of urea-formaldehyde resin or phenolic resin. The above applications do not explain the degradation degree of condensed tannins, nor reflect the reaction characteristics of tannin degradation products.

[0005] The Chinese patent application number is 2019107345000, and the invention name is an ultrasonic-assisted photocatalytic selective degradation of condensed tannins and its degradation degree control method. The application date is November 8, 2019; and the Chinese patent application number is 201910734125X, and the invention name is an ultrasonic-assisted hydroxyl radical selective degradation of condensed tannins and its degradation degree control method. The application date is November 12, 2019, which proposes a controllable degradation method for condensed tannins. The principle is that there is a corresponding relationship between the molecular weight of condensed tannins and the formaldehyde binding amount, and the degradation degree can be controlled according to the change of the formaldehyde binding amount. However, the above two patents do not involve the specific application of degradation products. How to apply tannin degradation products to adhesives to solve the problems that pH regulators need to be used during the preparation of adhesives, the preparation process is complex, the formaldehyde release amount in the finished product is high, and the strength does not meet the standard is an urgent problem to be solved. Summary of the Invention

[0006] 1. Technical problems to be solved by the invention

[0007] In view of the technical problems in the prior art that the raw materials for preparing tannin-based adhesives contain fossil resource raw materials (such as urea and phenol), and pH regulators need to be used in the preparation process, resulting in a relatively high formaldehyde release amount and a complex preparation process, the tannin-based adhesive provided by this application does not contain fossil resources in the raw materials, reduces the formaldehyde release amount, and has a simple preparation process. This application also provides plywood, which meets the strength index requirements of both Class I plywood and Class II plywood, and the formaldehyde release amount reaches F ☆☆☆☆ level.

[0008] 2. Technical solutions

[0009] To achieve the above object, the technical solutions provided are as follows:

[0010] The tannin-based adhesive of the present invention comprises the following components: tannin degradation product, organic solvent, aldehyde compound or non-aldehyde compound;

[0011] When the aldehyde compound is formaldehyde, the molar ratio of the formaldehyde to the tannin degradation product is 0.5 - 1.5;

[0012] When the aldehyde compound is non-formaldehyde, the mass of the aldehyde compound or non-aldehyde compound is 5 - 15% of the mass of the tannin degradation product;

[0013] The tannin degradation product is prepared by photocatalytic degradation, hydroxyl radical degradation or superoxide anion radical degradation of raw condensed tannins; the maximum formaldehyde binding capacity of the tannin degradation product is 0.9 - 1.3 g / g.

[0014] Further, the aldehyde compound is formaldehyde, glyoxal or furfural; the non-aldehyde compound is hexamethylenetetramine or furfuryl alcohol.

[0015] Further, the theoretical solid content of the tannin-based adhesive is 40 - 60 wt%.

[0016] Preferably, the theoretical solid content of the tannin-based adhesive is 45%.

[0017] Further, the organic solvent is methanol with a content of 60 - 80 wt%.

[0018] Preferably, the concentration of the methanol aqueous solution is 70%.

[0019] Further, the raw condensed tannins are one or more of extracts from black wattle bark, acacia bark, larch bark, shortleaf pine bark, spruce bark, hemlock bark, red root bark, phyllanthus emblica bark, myrica rubra bark, albizia kalkora bark, casuarina equisetifolia bark, albizia chinensis bark, willow bark, red hardwood wood, ephedra root, rumex acetosa root, grape pomace or dioscorea cirrhosa tuber.

[0020] Preparation method of tannin-based adhesive, comprising the following steps:

[0021] Comprising the step of dissolving tannin degradation product in an organic solvent to obtain an aqueous solution of degradation product and organic matter;

[0022] Comprising the step of mixing the aqueous solution of degradation product and organic matter with an aldehyde compound or a non-aldehyde compound to obtain the tannin-based adhesive; the mixing temperature ≤ 15°C;

[0023] When the aldehyde compound is formaldehyde, the molar ratio of formaldehyde to tannin degradation product in the tannin degradation product is 0.5 - 1.5; when the aldehyde compound is non-formaldehyde, the mass of the aldehyde compound or non-aldehyde compound is 5 - 15% of the mass of the tannin degradation product;

[0024] The tannin degradation product is prepared by photocatalytic degradation, hydroxyl radical degradation or superoxide anion radical degradation of raw condensed tannin.

[0025] Preferably, the dosage of other aldehyde or non-aldehyde compounds is 5% or 10% of the tannin degradation product (dry weight).

[0026] Preferably, the mixing of the aqueous solution of degradation product and organic matter with the aldehyde compound or non-aldehyde compound is carried out in an ice-water bath.

[0027] Application of tannin-based adhesive, applying the tannin-based adhesive to the production of plywood.

[0028] Plywood, prepared using the tannin-based adhesive, the bonding strength of the plywood simultaneously meets the strength index requirements of Class I plywood and Class II plywood, and the formaldehyde release amount of the plywood reaches F ☆☆☆☆ Level.

[0029] Furthermore, when using the tannin-based adhesive for plywood pressing, both sides of the veneer are coated with glue, and the glue coating amount is 200 - 240 g / m 2 .

[0030] Preparation method of plywood, the unit hot pressing pressure is 1.0 - 1.5 MPa, the hot pressing temperature is 120 - 180°C, and the hot pressing time is 40 - 60 s / mm.

[0031] 3. Beneficial effects

[0032] Adopting the technical solution provided by the present invention, compared with the existing well-known technologies, it has the following beneficial effects:

[0033] (1) The tannin-based adhesive of the present invention comprises a tannin degradation product, an organic solvent, an aldehyde group compound or a non-aldehyde group compound; the molar ratio of formaldehyde to the tannin degradation product in the tannin degradation product is 0.5 to 1.5; the mass of the non-formaldehyde aldehyde group compound or the non-aldehyde group compound is 5 to 15% of the mass of the tannin degradation product; the tannin degradation product is prepared by photocatalytic degradation, hydroxyl radical degradation or superoxide anion radical degradation of the original condensed tannin; the maximum formaldehyde binding amount of the tannin degradation product is 0.9 to 1.3 g / g, and it has the advantages that the raw materials do not contain fossil resources (such as phenol, urea and other reinforcing agents), the formaldehyde release amount is reduced, and it can be used after being uniformly mixed at low temperature, etc.

[0034] (2) The plywood of the present invention is prepared by using the above-mentioned tannin-based adhesive, and simultaneously meets the strength index requirements of Class I plywood and Class II plywood, and the formaldehyde release amount reaches F ☆☆☆☆ level.

[0035] (3) The preparation method of the tannin-based adhesive of the present invention does not use a pH value regulator during the preparation process, the preparation process is simple, and the environmental pollution is small. Description of the Drawings

[0036] Figure 1 It is a comparison chart of the bonding strength when the resin adhesive prepared in Example 1 is detected according to the bonding strength detection method of Class I plywood, where A is the BT-0.5F resin adhesive, B is the BT-1.0F resin adhesive, and C is the BT-1.5F resin adhesive.

[0037] Figure 2 It is a comparison chart of the bonding strength when the resin adhesive prepared in Example 1 is detected according to the bonding strength detection method of Class II plywood, where A is the BT-0.5F resin adhesive, B is the BT-1.0F resin adhesive, and C is the BT-1.5F resin adhesive.

[0038] Figure 3 It is a comparison chart of the formaldehyde release amount of the resin adhesive prepared in Example 1, where A is the BT-0.5F resin adhesive, B is the BT-1.0F resin adhesive, and C is the BT-1.5F resin adhesive.

[0039] Figure 4 It is a comparison chart of the bonding strength when the resin adhesive prepared in Example 2 is detected according to the bonding strength detection method of Class I plywood, where A is the LT-5%H resin adhesive, B is the LT-10%H resin adhesive, and C is the LT-15%H resin adhesive.

[0040] Figure 5It is a comparison chart of the bonding strength when the resin adhesive prepared in Example 2 is detected according to the bonding strength detection method for Class II plywood. Among them, A is the LT-5%H resin adhesive, B is the LT-10%H resin adhesive, and C is the LT-15%H resin adhesive.

[0041] Figure 6 It is a comparison chart of the formaldehyde release amount of the resin adhesive prepared in Example 2. Among them, A is the LT-5%H resin adhesive, B is the LT-10%H resin adhesive, and C is the LT-15%H resin adhesive.

[0042] Figure 7 It is a comparison chart of the bonding strength when the resin adhesive prepared in Example 3 is detected according to the bonding strength detection method for Class I plywood. Among them, A is the AT-5%G resin adhesive, A is the AT-10%G resin adhesive, and C is the AT-15%G resin adhesive.

[0043] Figure 8 It is a comparison chart of the bonding strength when the resin adhesive prepared in Example 3 is detected according to the bonding strength detection method for Class II plywood. Among them, A is the AT-5%G resin adhesive, A is the AT-10%G resin adhesive, and C is the AT-15%G resin adhesive.

[0044] Figure 9 It is a comparison chart of the formaldehyde release amount of the resin adhesive prepared in Example 3. Among them, A is the AT-5%G resin adhesive, A is the AT-10%G resin adhesive, and C is the AT-15%G resin adhesive. Detailed implementation manners

[0045] The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments; based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0046] Example 1

[0047] The tannin-based adhesive, preparation method and plywood of this embodiment are as follows:

[0048] The reactants were raw myrica tannin (undegraded tannin, for the comparative example), with an average degree of polymerization of 7.19 (mDP = 7.19) and a formaldehyde binding capacity of 0.31 g / g, named BT0; the myrica tannin degradation product after 4 hours of photocatalytic degradation (with the maximum formaldehyde binding capacity, for the example), at this time the average degree of polymerization of the degradation product was 3.90 (mDP = 3.90) and the formaldehyde binding capacity was 1.28 g / g, named BT4; the myrica tannin degradation product after 9 hours of photocatalytic degradation (myrica tannin was over-degraded, for the comparative example), at this time the average degree of polymerization of the degradation product was 1.77 (mDP = 1.77) and the formaldehyde binding capacity was 0.45 g / g, named BT9. The other reactant was a 37% aqueous formaldehyde solution. The solvent used was a 70% methanol aqueous solution. The theoretical solid content of the resin adhesive was 45%.

[0049] Dissolve the tannin raw materials of BT0, BT4, and BT9 in the methanol aqueous solution respectively, place the mixture in an ice-water bath and stir magnetically for 10 minutes, then add the aqueous formaldehyde solution according to the molar ratios of formaldehyde / tannin of 0.5, 1.0, and 1.5 respectively, and continue stirring for 10 minutes to obtain the formaldehyde-based tannin adhesives. The dosage of the methanol aqueous solution was calculated based on the molar ratio and the theoretical solid content. The obtained adhesives were named: BT—0.5F, BT—1.0F, and BT—1.5F respectively. The specific formula is shown in Table 1.

[0050] Table 1 Formulation of the resin adhesive in Example 1

[0051]

[0052] Prepare five-layer plywood. The textures of the five-layer veneers are crisscrossed. The veneers are poplar veneers with a moisture content of 10 - 12%. Double-sided sizing is used, and the sizing amount is 200 g / m 2 ². Hot pressing conditions: The hot pressing temperatures are 100 °C, 120 °C, 140 °C, 160 °C, and 180 °C respectively. The pre-pressing unit pressure is 1.0 MPa, the pre-pressing time is 20 min, the hot pressing unit pressure is 1.0 MPa, and the hot pressing time is 50 s / mm. When detected according to the I-type plywood bonding strength detection method, the trend graph of the bonding strength changing with the increase of the hot pressing temperature is shown in Figure 1 When detected according to the II-type plywood bonding strength detection method, the trend graph of the bonding strength changing with the increase of the hot pressing temperature is shown in Figure 2 The trend graph of the formaldehyde release amount changing with the hot pressing temperature is shown in Figure 3 .

[0053] From Figure 1As can be seen from 2 and 3, using BT4 as the raw material, with a molar ratio of formaldehyde to BT4 of 1.0 and a hot pressing temperature of 140 °C, it meets the strength standard of Class I plywood, with a maximum value of 1.12 MPa; using BT4 as the raw material, with a molar ratio of formaldehyde to BT4 of 1.0 and a hot pressing temperature of 160 °C, it meets the strength standard of Class II plywood, with a maximum value of 1.72 MPa; on the premise of meeting the strength requirements of Class I and Class II plywood, the BT4 tannin-formaldehyde resin adhesive has an extremely low formaldehyde release amount of 0.12 mg / L, which can reach F ☆☆☆☆ level.

[0054] Example 2

[0055] The tannin-based adhesive, preparation method and plywood of this example are as follows:

[0056] The reactants are respectively raw larch tannin (undegraded tannin, for the comparative example), with an average degree of polymerization of 8.12 (mDP = 8.12) and a formaldehyde binding amount of 0.38 g / g, named LT0; the larch tannin degradation product degraded by hydroxyl radicals for 5 hours (with the largest formaldehyde binding amount, for the example), at this time the average degree of polymerization of the degradation product is 2.51 (mDP = 2.51) and the formaldehyde binding amount is 1.02 g / g, named LT5; the larch tannin degradation product degraded by hydroxyl radicals for 10 hours (the larch tannin is over-degraded, for the comparative example), at this time the average degree of polymerization of the degradation product is 1.65 (mDP = 1.65) and the formaldehyde binding amount is 0.41 g / g, named LT10. The other reactant is hexamethylenetetramine. The solvent used is 70% methanol aqueous solution. The theoretical solid content of the resin adhesive is 45%.

[0057] Dissolve LT0, LT5 and LT10 tannin raw materials in methanol aqueous solution respectively, place the mixture in an ice-water bath and stir magnetically for 10 minutes, then add hexamethylenetetramine according to the ratios of hexamethylenetetramine to tannin of 5%, 10% and 15% respectively, and continue to stir for 10 minutes to obtain the tannin-hexamethylenetetramine resin adhesive. The dosage of methanol aqueous solution is calculated according to the feeding ratio and the theoretical solid content. The obtained adhesives are named LT-5%H, LT-10%H and LT-15%H respectively, and the specific formula is shown in Table 2.

[0058] Table 2 Formulas of the resin adhesives in Example 2

[0059]

[0060]

[0061] Prepare five-layer plywood. The textures of the five veneers crisscross. The veneers are poplar veneers with a moisture content of 12%. Double-sided sizing is adopted, and the sizing amount is 200 g / m 2。Hot pressing conditions: The hot pressing temperatures are 100°C, 120°C, 140°C, 160°C, and 180°C respectively. The pre-pressing unit pressure is 1.0 MPa, the pre-pressing time is 20 min, the hot pressing unit pressure is 1.0 MPa, and the hot pressing time is 50 s / mm. When tested according to the gluing strength detection method for Class I plywood, the trend graph of the gluing strength changing with the increase of the hot pressing temperature is shown in Figure 4 。When tested according to the gluing strength detection method for Class II plywood, the trend graph of the gluing strength changing with the increase of the hot pressing temperature is shown in Figure 5 。The trend graph of the formaldehyde release amount changing with the hot pressing temperature is shown in Figure 6 。

[0062] From Figure 4 5 and 6, it can be seen that using LT5 as the raw material, with the addition amount of hexamethylenetetramine being 5%, and the hot pressing temperature being 140°C, it meets the strength standard of Class I plywood, and the maximum value is 0.90 MPa; using LT5 as the raw material, with the addition amount of hexamethylenetetramine being 5%, and the hot pressing temperature being 140°C, it meets the strength standard of Class II plywood, and the maximum value is 0.95 MPa; on the premise of meeting the strength requirements of Class I and Class II plywood, the LT5 tannin - hexamethylenetetramine resin adhesive has an extremely low formaldehyde release amount of 0.15 mg / L, which can reach F ☆☆☆☆ level.

[0063] Example 3

[0064] The tannin-based adhesive, preparation method and plywood of this example are as follows:

[0065] The reactants are respectively raw mimosa tannin (undegraded tannin, for the comparative example), with an average degree of polymerization of 9.75 (mDP = 9.75) and a formaldehyde binding amount of 0.35 g / g, named AT0; the mimosa tannin degradation product degraded by superoxide anion radical for 10 hours (with the maximum formaldehyde binding amount, for the example), at this time the average degree of polymerization of the degradation product is 2.37 (mDP = 2.37) and the formaldehyde binding amount is 1.11 g / g, named AT10; the mimosa tannin degradation product degraded by superoxide anion radical for 24 hours (the mimosa tannin is over-degraded, for the comparative example), at this time the average degree of polymerization of the degradation product is 1.33 (mDP = 1.33) and the formaldehyde binding amount is 0.47 g / g, named AT24. Another reactant is glyoxal. The solvent used is 70% methanol aqueous solution. The theoretical solid content of the resin adhesive is 45%.

[0066] Dissolve AT0, AT10, and AT24 tannin raw materials in an aqueous methanol solution respectively. Place the mixture in an ice-water bath and stir magnetically for 10 minutes. Then add glyoxal according to the ratios of 5%, 10%, and 15% of glyoxal to tannin respectively, and continue stirring for 10 minutes to obtain tannin-glyoxal resin adhesives. The dosage of the aqueous methanol solution is calculated based on the feeding ratio and the theoretical solid content. The obtained adhesives are named AT-5%G, AT-10%G, and AT-15%G respectively. The specific formula is shown in Table 3.

[0067] Table 3 Formulation of the resin adhesive in Example 3

[0068]

[0069]

[0070] Prepare a five-layer plywood. The textures of the five veneers are crisscrossed. The veneers are poplar veneers with a moisture content of 12%. Apply glue on both sides, and the glue application amount is 200 g / m 2 . Hot pressing conditions: The hot pressing temperatures are 100 °C, 120 °C, 140 °C, 160 °C, and 180 °C respectively. The pre-pressing unit pressure is 1.0 MPa, the pre-pressing time is 20 min, the hot pressing unit pressure is 1.0 MPa, and the hot pressing time is 50 s / mm. When detected according to the detection method of the bonding strength of Class I plywood, the trend graph of the bonding strength changing with the increase of the hot pressing temperature is shown in Figure 7 . When detected according to the detection method of the bonding strength of Class II plywood, the trend graph of the bonding strength changing with the increase of the hot pressing temperature is shown in Figure 8 . The trend graph of the formaldehyde release amount changing with the hot pressing temperature is shown in Figure 9 .

[0071] From Figure 7 , 8, and 9, it can be seen that using AT10 as the raw material, with a glyoxal addition amount of 10% and a hot pressing temperature of 160 °C, it meets the strength standard of Class I plywood, and the maximum value is 0.81 MPa; using AT10 as the raw material, with a glyoxal addition amount of 10% and a hot pressing temperature of 160 °C, it meets the strength standard of Class II plywood, and the maximum value is 1.04 MPa; on the premise of meeting the strength requirements of Class I and Class II plywood, the AT10 tannin-glyoxal resin adhesive has an extremely low formaldehyde release amount of 0.14 mg / L, which can reach F ☆☆☆☆ level.

Claims

1. Preparation method of tannin-based adhesive, characterized in that: The raw materials of the tannin-based adhesive are composed of the following components: tannin degradation products, organic solvents, aldehyde compounds or non-aldehyde compounds; including the following steps: including the step of dissolving tannin degradation products in an organic solvent to obtain an aqueous solution of degradation product and organic matter; including the step of mixing the aqueous solution of degradation product and organic matter with an aldehyde compound or a non-aldehyde compound to obtain the tannin-based adhesive; the mixing temperature ≤ 15 °C; when the aldehyde compound is formaldehyde, the molar ratio of formaldehyde to tannin degradation product is 0.5 - 1.5; when the aldehyde compound is non-formaldehyde, the mass of the aldehyde compound or non-aldehyde compound is 5 - 15% of the mass of the tannin degradation product; the tannin degradation product is prepared by photocatalytic degradation, hydroxyl radical degradation or superoxide anion radical degradation of raw condensed tannins; the maximum formaldehyde binding capacity of the tannin degradation product is 0.9 - 1.3 g / g; the aldehyde compound is formaldehyde, glyoxal or furfural; the non-aldehyde compound is hexamethylenetetramine or furfuryl alcohol.

2. Tannin-based adhesive, characterized in that:

3. The tannin-based adhesive according to claim 2, wherein: Prepared by the method according to claim 1.

4. The tannin-based adhesive according to claim 3, wherein: The theoretical solid content of the tannin-based adhesive is 40 - 60 wt%.

5. The tannin-based adhesive according to claim 4, characterized in that: The organic solvent is methanol with a content of 60 - 80 wt%.

6. Application of tannin-based adhesives, characterized in that: The raw condensed tannins are one or more of the extracts of black wattle bark, acacia bark, larch bark, shortleaf pine bark, spruce bark, hemlock bark, redroot bark, phyllanthus emblica bark, myrica rubra bark, albizia kalkora bark, casuarina equisetifolia bark, albizia chinensis bark, willow bark, red hardwood wood, ephedra root, dock root, grape pomace or Dioscorea cirrhosa tuber.

7. Plywood, characterized in that: Prepared from the tannin-based adhesive according to any one of claims 2-5, the plywood has a bonding strength that meets the strength index requirements of both Class I plywood and Class II plywood, and the formaldehyde emission of the plywood reaches F ☆☆☆☆ level.

8. The plywood according to claim 7, characterized in that: Use the tannin-based adhesive for plywood pressing, apply glue to both sides of the veneer, and the glue application amount is 200~240 g / m 2 .

9. The preparation method of the plywood according to any one of claims 7 or 8, characterized in that: Apply the tannin-based adhesive according to any one of claims 2 - 5 to the production of plywood. The unit hot pressing pressure is 1.0 - 1.5 MPa, the hot pressing temperature is 120 - 180 °C, and the hot pressing time is 40 - 60 s / mm.

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