Lignin-formaldehyde adhesive as well as preparation method and application thereof

Abstract

The invention discloses a lignin-formaldehyde adhesive as well as a preparation method and application thereof, and belongs to the technical field of efficient utilization of lignin. According to the invention, lignin is pyrolyzed into a micromolecular aromatic compound, and then a lignin liquid phase depolymerization product is mixed with a catalyst to prepare the self-crosslinking adhesive depending on a lignin degradation product. The lignin-formaldehyde adhesive provided by the invention has the advantages of low cost, high bonding strength and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of efficient utilization of lignin, and in particular relates to a lignin-formaldehyde adhesive and a preparation method and application thereof. Background Art

[0002] The information disclosed in this background technology section is only intended to enhance understanding of the overall background of the invention and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to those skilled in the art.

[0003] Phenolic resins are highly dependent on non-renewable fossil resources. Their raw material, phenol, is toxic, corrosive, and carcinogenic, leading to environmental risks and high pollution treatment costs during production and use. Lignin, a natural aromatic polymer, has structural units (such as coumarin and coniferyl alcohol) rich in phenolic hydroxyl and methoxy groups, similar to phenol in chemical properties, offering theoretical potential as a replacement for petroleum-based phenol. However, in practical applications, lignin suffers from structural defects such as a wide molecular weight distribution and a low density of active phenolic hydroxyl groups, resulting in insufficient crosslinking density and difficulty forming a uniform and stable network. Furthermore, existing modification methods (such as hydroxymethylation and demethylation) rely on strong acids and bases, high temperatures, and organic solvents, violating the principles of green manufacturing. Summary of the Invention

[0004] In order to address the deficiencies of the prior art, the present invention aims to provide a lignin-formaldehyde adhesive and a preparation method and application thereof, wherein the adhesive has low cost and good thermal stability.

[0005] Lignin's complex molecular structure, wide molecular weight range, and low reactivity lead to problems in existing lignin-based adhesives, such as poor performance and high energy consumption in the modification process. This invention achieves a self-crosslinking adhesive by thermochemically depolymerizing lignin and then grafting the depolymerization product with formaldehyde.

[0006] In order to achieve the above object, the technical solution of the present invention is: In a first aspect, the present invention provides a method for preparing a lignin-formaldehyde adhesive, comprising the following steps: The lignin pyrolysis liquid phase product and formaldehyde are mixed, a catalyst is added, and the mixture is heated and stirred to obtain the product.

[0007] In one or more embodiments, the raw material sources of lignin include, but are not limited to, broadleaf woody plants, herbaceous plants, and vascular plants; and the methods for obtaining lignin include, but are not limited to, acid, alkali, supercritical, aqueous two-phase, ionic liquid, organic solvent, enzyme, and other treatments. Lignin includes, but is not limited to, sulfate lignin, alkali lignin, hydrolyzed lignin, enzymatic lignin, and organic solvent lignin. The lignin is obtained from broadleaf woody plants, herbaceous plants, and vascular plants through acid, alkali, supercritical, aqueous two-phase, ionic liquid, organic solvent, and enzyme treatments, such as sulfate lignin, alkali lignin, hydrolyzed lignin, enzymatic lignin, and organic solvent lignin.

[0008] In one or more embodiments, the method for thermochemical depolymerization of lignin is: rapid pyrolysis at 550-700° C. for 2-10 min.

[0009] In one or more embodiments, the molar ratio of the lignin pyrolysis liquid phase product to formaldehyde is 1:0.2-1:0.6. A too high ratio of the lignin pyrolysis liquid phase product will result in a decrease in adhesive strength, and a too high ratio of formaldehyde will result in an increase in free formaldehyde content.

[0010] The present invention uses industrial lignin, which is condensed lignin. The liquid product of industrial lignin pyrolysis has a molecular weight of 120-1000 and is primarily composed of aromatic compounds, with a phenol content of less than 0.5%. Without depolymerization, industrial lignin cannot be cured and cross-linked to form an adhesive.

[0011] In one or more embodiments, the catalyst is a 30-40 wt% sodium hydroxide solution, and the addition amount is 1-5 wt% of the lignin pyrolysis liquid phase product, specifically 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, etc. Too low a catalyst addition amount will result in a decrease in bonding strength, while too high a catalyst addition amount will result in an excessively fast reaction rate, excessive condensation during the reaction process, and increased viscosity.

[0012] In one or more embodiments, the heating and stirring step involves stirring the mixed solution at 53-57°C for 30-60 minutes, then heating it to 80-85°C and maintaining it for 10-20 minutes. The two heating temperatures are used because lignin degradation products and formaldehyde react differently at different temperatures. Compared to a single heating step, the adhesive prepared by heating at two different temperatures in the present invention exhibits superior properties, such as bonding strength.

[0013] The stirring time during the first heating can be 30, 40, 50, or 60 minutes. A stirring time that is too long has no significant effect, while a stirring time that is too short can result in decreased bonding strength. During the second heating, the holding time can be 10, 15, or 20 minutes. A holding time that is too short can result in poor bonding strength, and excessive self-bonding can lead to decreased bonding strength and increased viscosity.

[0014] In a second aspect, the present invention provides a lignin-formaldehyde adhesive obtained by the above preparation method.

[0015] In a third aspect, the present invention provides a use of the above-mentioned lignin-formaldehyde adhesive in bonding board materials.

[0016] In a fourth aspect, the present invention provides a plywood comprising the above-mentioned lignin-formaldehyde adhesive.

[0017] The method for preparing the plywood comprises: applying the above-mentioned lignin-formaldehyde adhesive to wood veneers, combining the veneers with the adhesive and hot pressing them to obtain the plywood.

[0018] Preferably, the sizing amount is 50-200 g / m 2 Specific value is 50 g / m 2 , 100 g / m 2 , 150 g / m 2 , 200 g / m 2 Etc. If the glue amount is too low, the bonding strength will decrease, and if the glue amount is too high, it will cause glue overflow.

[0019] Preferably, the hot pressing pressure is 0.8-2.0 MPa, specifically 0.8 MPa, 0.9 MPa, 1.0 MPa, 1.1 MPa, 1.2 MPa, 1.3 MPa, 1.4 MPa, 1.5 MPa, 1.6 MPa, 1.7 MPa, 1.8 MPa, 1.9 MPa, 2.0 MPa, etc. If the hot pressing pressure is too low, the bonding strength will decrease, while if the hot pressing pressure is too high, the wood board will collapse.

[0020] Preferably, the hot pressing time is 6-30 min, specifically 6 min, 10 min, 12 min, 15 min, 20 min, 25 min, 30 min, etc. Hot pressing time that is too short or too long will lead to a decrease in bonding strength.

[0021] Preferably, the hot pressing temperature is 80-180° C., specifically 80° C., 90° C., 100° C., 110° C., 120° C., 130° C., 140° C., 150° C., 160° C., 170° C., 180° C., etc., preferably 140-160° C. Both too low and too high hot pressing temperatures will lead to a decrease in bonding strength.

[0022] One or more of the above technical solutions have the following advantages or beneficial effects: (1) The present invention first pyrolyzes lignin into small molecular aromatic compounds, overcoming the problems of low lignin reactivity, large molecular weight span, and large structural differences; then the lignin liquid phase depolymerization products are mixed with a catalyst to prepare an adhesive that relies on the self-crosslinking of the lignin degradation products.

[0023] (2) The lignin-formaldehyde adhesive provided by the present invention has the advantages of low cost and good thermal stability. Compared with existing commercial phenolic resin adhesives, the adhesive provided by the present invention has better bonding strength (greater than 1.8 MPa, further greater than 2.2 MPa, and further greater than 2.5 MPa). BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0025] Figure 1 A physical picture of the adhesive prepared in accordance with an embodiment of the present invention; Figure 2 This is a physical picture of the plywood prepared according to the embodiment of the present invention. DETAILED DESCRIPTION

[0026] In the present invention, unless otherwise specified, other test materials and instruments and equipment are conventional test materials in this field and can be purchased through commercial channels.

[0027] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0028] Example 1 Kraft lignin was rapidly pyrolyzed at 650°C for 5 min, and the volatile pyrolysis products were condensed to obtain lignin rapid pyrolysis liquid products. The lignin degradation liquid products and formaldehyde were evenly mixed in a molar ratio of 1:0.4. After adding 2wt% sodium hydroxide solution, the mixture was stirred at 55°C for 2 h, then heated to 80°C within 20 min and stirred for 15 min. The adhesive was obtained after cooling to room temperature.

[0029] According to GB / T 17657-2022 "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels", apply the adhesive evenly on the wood board, with a glue amount of 100 g / m on each side. 2After the veneers were assembled, they were placed on the hot plate of a hot press. The temperature was 140°C, the time was 12 minutes, and the pressure was 1.5 MPa. The measured bond strength was 2.55 MPa and the viscosity was 460 mPa.s.

[0030] Example 2 Kraft lignin was rapidly pyrolyzed at 650°C for 5 min, and the volatile pyrolysis products were condensed to obtain lignin rapid pyrolysis liquid products. The lignin degradation liquid products and formaldehyde were evenly mixed in a molar ratio of 1:0.4. After adding 2wt% sodium hydroxide solution, the mixture was stirred at 55°C for 2 h, then heated to 80°C within 20 min and stirred for 15 min. The adhesive was obtained after cooling to room temperature.

[0031] According to GB / T 17657-2022 "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels", apply the adhesive evenly on the wood board, with a glue amount of 100 g / m on each side. 2 After the veneers were assembled, they were placed on the hot plate of a hot press. The temperature was 160°C, the time was 12 minutes, and the pressure was 1.5 MPa. The measured bond strength was 2.28 MPa, and the viscosity was 460 MPa / s.

[0032] Example 3 Kraft lignin was rapidly pyrolyzed at 650°C for 5 min, and the volatile pyrolysis products were condensed to obtain lignin rapid pyrolysis liquid products. The lignin degradation liquid products and formaldehyde were evenly mixed in a molar ratio of 1:0.4. After adding 2wt% sodium hydroxide solution, the mixture was stirred at 55°C for 2 h, then heated to 80°C within 20 min and stirred for 10 min. The adhesive was obtained after cooling to room temperature.

[0033] According to GB / T 17657-2022 "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels", apply the adhesive evenly on the wood board, with a glue amount of 100 g / m on each side. 2 After the veneers were assembled, they were placed on the hot plate of a hot press. The temperature was 140°C, the time was 12 minutes, and the pressure was 1.5 MPa. The measured bond strength was 1.87 MPa and the viscosity was 420 MPa / s.

[0034] Comparative Example 1 According to GB / T 17657-2022 "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels", commercial phenolic resin adhesive (phenolic resin synthesized directly from phenol and formaldehyde) was evenly applied on the wood board, with an adhesive amount of 100 g / m on each side. 2The resulting wood panels were assembled and hot-pressed at 140°C for 12 minutes at a pressure of 1.5 MPa. After cooling to room temperature, plywood was obtained. The measured bond strength was 1.63 MPa.

[0035] Comparative Example 2 The difference from Example 1 is that the lignin is not subjected to depolymerization treatment. Specifically: Lignin and formaldehyde were directly mixed at a molar ratio of 1:0.4, and 2 wt% sodium hydroxide solution was added. The mixture was stirred at 55°C for 1 hour and then cooled to room temperature to produce an adhesive. The measured bonding strength was 0. Obviously, since industrial lignin is a condensed compound, it cannot form cured crosslinks without depolymerization.

[0036] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A method for preparing a lignin-formaldehyde adhesive, characterized in that: The following steps are involved: The lignin pyrolysis liquid phase product and formaldehyde are mixed, a catalyst is added, and the mixture is heated and stirred to obtain the product.

2. The preparation method according to claim 1, characterized in that The raw material sources of lignin include broadleaf woody plants, herbaceous plants, and vascular plants; Preferably, the method for obtaining lignin includes acid, alkali, supercritical, two-phase aqueous, ionic liquid, organic solvent, enzyme and other treatments; Preferably, the lignin includes sulfate lignin, alkali lignin, hydrolyzed lignin, enzymatic lignin, and organic solvent lignin; Preferably, the pyrolysis is carried out by a thermochemical depolymerization method, with rapid pyrolysis at 550-700°C for 2-10 minutes; Preferably, the molecular weight of the liquid phase product of lignin pyrolysis is 120-1000, and it is mainly aromatic compounds, with a phenol content of less than 0.5%.

3. The preparation method according to claim 1, characterized in that The molar ratio of the lignin pyrolysis liquid phase product to formaldehyde is 1:0.2-1:0.

6.

4. The preparation method according to claim 1, characterized in that The catalyst is a 30-40 wt% sodium hydroxide solution, and the added amount is 1-5 wt% of the lignin pyrolysis liquid phase product.

5. The preparation method according to claim 1, characterized in that Heating and stirring: the mixed solution is stirred at 53-57°C, and then heated to 80-85°C and kept warm; Preferably, the stirring time is greater than 30 min, preferably 30-60 min; Preferably, the holding time is 10-20 min.

6. A lignin-formaldehyde adhesive, characterized in that: The method is obtained by any one of claims 1 to 5.

7. Use of the lignin-formaldehyde adhesive obtained by the preparation method according to any one of claims 1 to 5 or the lignin-formaldehyde adhesive according to claim 6 in bonding boards.

8. A plywood, characterized in that: It comprises the lignin-formaldehyde adhesive according to claim 6.

9. The plywood according to claim 8, characterized in that The preparation method of the plywood comprises: applying lignin-formaldehyde adhesive to wood veneers, combining the veneers with adhesive and hot pressing to obtain the plywood.

10. The plywood according to claim 9, characterized in that The amount of glue applied is 50-200 g / m 2 ; Preferably, the hot pressing pressure is 0.8-2.0 MP; Preferably, the hot pressing time is 6-30 min; Preferably, the hot pressing temperature is 80-180°C.

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