A non-formaldehyde bio-based wood adhesive and preparation method and application thereof

Abstract

This invention discloses a method for preparing a formaldehyde-free bio-based wood adhesive, specifically: dissolving gelatin in deionized water and heating until completely dissolved to obtain a gelatin solution; adding lipoic acid and stirring to obtain a gelatin-lipoic acid prepolymer solution; dissolving tannic acid in deionized water and ultrasonically stirring to obtain a tannic acid dispersion; adding sodium hydroxide solution dropwise to the gelatin-lipoic acid prepolymer solution, then adding the tannic acid dispersion and stirring to carry out a crosslinking reaction to obtain the formaldehyde-free bio-based wood adhesive. In this method, the bonding performance is enhanced through hydrogen bonding, electrostatic interaction, and dynamic disulfide bond exchange. The adhesive has a shear strength ≥2 MPa, meeting the bonding requirements of engineered wood products production. Furthermore, the core raw materials are all bio-based, and there is no formaldehyde release during the preparation and use process, meeting environmental standards.

Description

Technical Field

[0001] This invention belongs to the field of adhesive preparation technology, specifically relating to a method for preparing a formaldehyde-free bio-based wood adhesive, as well as the formaldehyde-free bio-based wood adhesive and its application. Background Technology

[0002] Most existing wood adhesives are aldehyde-based, including urea-formaldehyde resin, phenolic resin, and melamine-formaldehyde resin, which contain free formaldehyde and are harmful to the environment and human health. While existing bio-based adhesives (such as pure soy protein or gelatin glue) are environmentally friendly, they suffer from low bonding strength. Although chemical cross-linking agents (such as epichlorohydrin) can improve performance, they often introduce new toxicities. Therefore, developing a purely natural adhesive that achieves high-performance adhesion through intermolecular physical interactions is of great significance.

[0003] Gelatin is a natural bio-based polymer obtained from the hydrolysis of collagen, primarily extracted from connective tissues such as animal skin and bones. Its molecular chain contains abundant amino, carboxyl, hydroxyl, and amide groups, exhibiting excellent film-forming properties, adhesion, biocompatibility, and thermally reversible gelation characteristics. Due to its multiple functional properties, including gelling, foaming, and emulsifying, it is widely used in numerous industries such as food, materials, pharmaceuticals, and photography. Gelatin's molecular chain contains a large number of amide, carboxyl, and amino groups, resulting in good film-forming properties and cohesive strength. In wood adhesives, gelatin is often used as a macromolecular backbone; however, pure gelatin layers become extremely brittle after drying and have limited wetting properties on wood surfaces, leading to brittle delamination at the bonding interface.

[0004] Tannic acid is a polyphenolic compound widely found in plants and possesses extremely high chemical activity. The tannic acid molecule contains numerous phenolic hydroxyl groups, enabling it to physically cross-link with proteins (such as gelatin) through strong hydrogen bonds. While tannic acid can significantly increase the cross-linking density of gelatin systems, high concentrations often lead to a sharp increase in adhesive viscosity and uneven stress distribution in the cured adhesive layer, making it prone to microcracks and limiting its application in high-performance wood processing.

[0005] Lipoic acid is a sulfur-containing natural organic acid widely found in organisms as a coenzyme. Its unique molecular structure consists of a five-membered ring containing a disulfide bond (-SS-) and an aliphatic carboxyl side chain. Previous studies have largely focused on the antioxidant properties of lipoic acid in the biopharmaceutical field or on its use in chemical ring-opening polymerization via its disulfide bond. However, research on the use of lipoic acid as a functional modifier in physically cross-linked wood adhesive systems is relatively scarce.

[0006] Although existing gelatin / tannic acid physical crosslinking systems have achieved formaldehyde-free properties, they often face the following bottlenecks when applied to wood substrates: (1) Poor interfacial wettability: Lignin on the surface of wood has a certain hydrophobic tendency, making it difficult for the purely polar gelatin / tannic acid system to penetrate deep into the wood; (2) High brittleness of the adhesive layer: Although the strong hydrogen bond network improves strength, it also sacrifices toughness, causing the adhesive layer to break easily when the wood product is subjected to impact or natural deformation. Therefore, finding a natural regulator that can simultaneously improve the flexibility of the adhesive layer and enhance its affinity for the wood interface is the key to preparing high-performance all-bio-based wood adhesives. Summary of the Invention

[0007] The primary objective of this invention is to provide a method for preparing a formaldehyde-free bio-based wood adhesive, which is formaldehyde-free and has high bonding strength.

[0008] A second objective of the present invention is to provide the above-mentioned formaldehyde-free bio-based wood adhesive.

[0009] A third objective of this invention is to provide the application of the above-mentioned formaldehyde-free bio-based wood adhesive.

[0010] The first technical solution adopted in this invention is a method for preparing formaldehyde-free bio-based wood adhesive, which is implemented according to the following steps: Step 1: Dissolve gelatin in deionized water and heat it until completely dissolved to obtain a gelatin solution. Then add thioctic acid and stir to react, resulting in a gelatin-thioctic acid prepolymer solution. Step 2: Dissolve tannic acid in deionized water and stir ultrasonically until it is evenly dispersed to obtain a tannic acid dispersion. Step 3: Add sodium hydroxide solution dropwise to the gelatin-lipoic acid prepolymer solution obtained in Step 1, then add the tannic acid dispersion obtained in Step 2, stir to carry out cross-linking reaction, and obtain formaldehyde-free bio-based wood adhesive.

[0011] The invention is further characterized in that, In step 1, the mass percentage concentration of the gelatin solution is 10%-15%; the mass ratio of thioctic acid to gelatin is 1:10-30. The heating time of the gelatin is 5-20 minutes, and the heating temperature is 50-70℃; the stirring reaction time is 20-60 minutes, and the stirring speed is 150-400 rpm.

[0012] In step 2, the mass percentage concentration of the tannic acid dispersion is 0.01-2%. The ultrasonic stirring time is 2-10 min, and the ultrasonic frequency is 50-70 Hz.

[0013] In step 3, the cross-linking reaction time is 30-90 min, the cross-linking reaction temperature is 40-70℃, the stirring speed is 100 rpm-400 rpm, and the mass percentage concentration of the sodium hydroxide solution is 1%-25%.

[0014] The second technical solution adopted in this invention is a formaldehyde-free bio-based wood adhesive prepared by a method for preparing formaldehyde-free bio-based wood adhesive.

[0015] The third technical solution adopted in this invention is the application of formaldehyde-free bio-based wood adhesive in bonding wood.

[0016] The beneficial effects of this invention are: (1) Formaldehyde-free and environmentally friendly: The core raw materials of this invention are all bio-based materials. No formaldehyde is released during the preparation and use process, which meets environmental protection standards and is harmless to human health and the environment. (2) Excellent bonding performance: Hydrogen bonding, electrostatic interaction and dynamic exchange of disulfide bonds jointly enhance the bonding performance. The shear strength of the adhesive is ≥2 MPa, which can meet the bonding requirements of artificial board production.

[0017] (3) The preparation process is simple: no complex equipment is required, the reaction conditions are mild (50~60℃), the energy consumption is low, the raw materials are widely available and the cost is controllable, making it suitable for large-scale industrial production.

[0018] (4) Degradability: All raw materials are degradable and can be naturally degraded after disposal, without causing environmental pollution, which is in line with the concept of sustainable development. Attached Figure Description

[0019] Figure 1 Fourier transform infrared spectra of gelatin, tannic acid, thioctic acid, and formaldehyde-free bio-based wood adhesives; Figure 2 A comparison of the maximum shear strength of adhesives with different tannic acid concentrations; Figure 3 A comparison chart of the maximum shear strength of different wood adhesives. Detailed Implementation

[0020] The present invention will now be described in detail with reference to specific embodiments and accompanying drawings.

[0021] The method for preparing the formaldehyde-free bio-based wood adhesive of the present invention is specifically implemented according to the following steps: Step 1, Preparation of gelatin-lipoic acid prepolymer solution: First, dissolve gelatin in deionized water and heat it until completely dissolved to obtain a gelatin solution. Then, add lipoic acid and stir to react, resulting in a homogeneous gelatin-lipoic acid prepolymer solution. The mass percentage concentration of the gelatin solution is 10%-15%; The gelatin is heated for 5-20 minutes at a temperature of 50-70℃. The mass ratio of thioctic acid to gelatin is 1:10-30, the stirring reaction time is 20-60 minutes, and the stirring speed is 150-400 rpm. Step 2, prepare tannic acid dispersion; dissolve tannic acid in deionized water and stir ultrasonically until it is evenly dispersed to obtain tannic acid dispersion; The mass percentage concentration of the tannic acid dispersion is 0.01-2%; the ultrasonic stirring time is 2-10 min, and the ultrasonic frequency is 50-70 Hz. Step 3: Add sodium hydroxide solution dropwise to the gelatin-lipoic acid prepolymer solution obtained in Step 1, then add the tannic acid dispersion obtained in Step 2, stir to carry out cross-linking reaction, and obtain formaldehyde-free bio-based wood adhesive.

[0022] The sodium hydroxide solution has a mass percentage concentration of 1%-25%; the mass ratio of tannic acid to gelatin is 1:25-250. The cross-linking reaction time is 30-90 min, the cross-linking reaction temperature is 40-70℃, and the stirring speed is 100 rpm-400 rpm.

[0023] In the method of this invention, gelatin, tannic acid, and α-lipoic acid form a dual-network ester-free system dominated by a hydrogen-bonded supramolecular network and assisted by a dynamic SS network. Gelatin serves as the backbone, TA undertakes multi-point cross-linking, and LA provides flexible chains and sulfur sources. The whole relies on non-covalent interactions such as hydrogen bonds and electrostatic interactions to construct a physically cross-linked supramolecular structure. It belongs to a formaldehyde-free dynamic supramolecular adhesive system and is a formaldehyde-free, environmentally friendly, and high-strength bio-based wood adhesive.

[0024] Example 1 The method for preparing the formaldehyde-free bio-based wood adhesive of the present invention is specifically implemented according to the following steps: Step 1, Preparation of gelatin-lipoic acid prepolymer solution: Gelatin was dissolved in deionized water and heated to a solution state to obtain a gelatin solution with a mass percentage concentration of 15%. The gelatin was heated and dissolved for 10 minutes at a temperature of 60°C. Then, thioctic acid was added and stirred to obtain a homogeneous prepolymer solution with a mass ratio of thioctic acid to gelatin of 1:30. The stirring reaction time was 20 minutes and the stirring speed was 200 rpm. Example 2 The method for preparing the formaldehyde-free bio-based wood adhesive of the present invention is specifically implemented according to the following steps: Step 1, Preparation of gelatin-lipoic acid prepolymer solution: Gelatin was dissolved in deionized water and heated to a solution state to obtain a gelatin solution with a mass percentage concentration of 15%. The gelatin was heated and dissolved for 10 minutes at a temperature of 60°C. Then, lipoic acid was added and stirred to obtain a homogeneous prepolymer solution. The mass ratio of lipoic acid to gelatin was 1:30. The stirring reaction time was 20 minutes at a stirring speed of 200 rpm.

[0025] Step 2: Prepare a tannic acid dispersion. Dissolve tannic acid in deionized water and ultrasonically stir until uniformly dispersed to obtain a tannic acid dispersion. The mass percentage concentration of the tannic acid dispersion is 0.375 wt%. The ultrasonic stirring time is 5 min, and the ultrasonic frequency is 70 Hz.

[0026] Step 3: Add 0.01 ml of sodium hydroxide solution to the gelatin-lipoic acid prepolymer solution obtained in Step 1, add the tannic acid dispersion obtained in Step 2, stir to carry out cross-linking reaction, and obtain formaldehyde-free bio-based wood adhesive by static gelation.

[0027] The sodium hydroxide solution had a mass percentage concentration of 3.85%. The mass ratio of tannic acid to gelatin was 1:200, the cross-linking reaction time was 60 min, the cross-linking reaction temperature was 60℃, and the stirring speed was 200 rpm.

[0028] Example 3 The method for preparing the formaldehyde-free bio-based wood adhesive of the present invention is specifically implemented according to the following steps: Step 1, Preparation of gelatin-lipoic acid prepolymer solution: Gelatin was dissolved in deionized water and heated to a solution state to obtain a gelatin solution with a mass percentage concentration of 15%. The gelatin was heated and dissolved for 10 minutes at a temperature of 60°C. Then, lipoic acid was added and stirred to obtain a homogeneous prepolymer solution. The mass ratio of lipoic acid to gelatin was 1:30, and the reaction was stirred for 20 minutes at a stirring speed of 200 rpm.

[0029] Step 2: Prepare a tannic acid dispersion. Dissolve tannic acid in deionized water and ultrasonically stir until uniformly dispersed to obtain a tannic acid dispersion. The mass percentage concentration of the tannic acid dispersion is 0.75 wt%. The ultrasonic stirring time is 5 min, and the ultrasonic frequency is 70 Hz.

[0030] Step 3: Add 0.01 ml of sodium hydroxide solution to the gelatin-lipoic acid prepolymer solution obtained in Step 1, add the tannic acid dispersion obtained in Step 2, stir to carry out the cross-linking reaction, and obtain formaldehyde-free bio-based wood adhesive.

[0031] The sodium hydroxide solution had a mass percentage concentration of 3.85%. The mass ratio of tannic acid to gelatin was 1:100, the cross-linking reaction time was 60 min, the cross-linking reaction temperature was 60℃, and the stirring speed was 200 rpm.

[0032] Example 4 The method for preparing the formaldehyde-free bio-based wood adhesive of the present invention is specifically implemented according to the following steps: Step 1, Preparation of gelatin-lipoic acid prepolymer solution: Gelatin was dissolved in deionized water and heated to a solution state to obtain a gelatin solution with a mass percentage concentration of 15%. The gelatin was heated and dissolved for 10 minutes at a temperature of 60°C. Then, lipoic acid was added and stirred to obtain a homogeneous prepolymer solution. The mass ratio of lipoic acid to gelatin was 1:30, and the reaction was stirred for 20 minutes at a stirring speed of 200 rpm.

[0033] Step 2: Prepare a tannic acid dispersion. Dissolve tannic acid in deionized water and ultrasonically stir until uniformly dispersed to obtain a tannic acid dispersion. The mass percentage concentration of the tannic acid dispersion is 1.25 wt%. The ultrasonic stirring time is 5 min, and the ultrasonic frequency is 70 Hz.

[0034] Step 3: Add 0.01 ml of sodium hydroxide solution to the gelatin-lipoic acid prepolymer solution obtained in Step 1, add the tannic acid dispersion obtained in Step 2, stir to carry out the cross-linking reaction, and obtain formaldehyde-free bio-based wood adhesive.

[0035] The sodium hydroxide solution had a mass percentage concentration of 3.85%. The mass ratio of tannic acid to gelatin was 1:60, the cross-linking reaction time was 60 min, the cross-linking reaction temperature was 60℃, and the stirring speed was 200 rpm.

[0036] Example 5 The method for preparing the formaldehyde-free bio-based wood adhesive of the present invention is specifically implemented according to the following steps: Step 1, Preparation of gelatin-lipoic acid prepolymer solution: Gelatin was dissolved in deionized water and heated to a solution state to obtain a gelatin solution with a mass percentage concentration of 15%. The gelatin was heated and dissolved for 10 minutes at a temperature of 60°C. Then, lipoic acid was added and stirred to obtain a homogeneous prepolymer solution. The mass ratio of lipoic acid to gelatin was 1:30, and the reaction was stirred for 20 minutes at a stirring speed of 200 rpm.

[0037] Step 2: Prepare a tannic acid dispersion. Dissolve tannic acid in deionized water and ultrasonically stir until uniformly dispersed to obtain a tannic acid dispersion. The mass percentage concentration of the tannic acid dispersion is 1.5 wt%. The ultrasonic stirring time is 5 min, and the ultrasonic frequency is 70 Hz.

[0038] Step 3: Add 0.01 ml of sodium hydroxide solution to the gelatin-lipoic acid prepolymer solution obtained in Step 1, add the tannic acid dispersion obtained in Step 2, stir to carry out cross-linking reaction, and obtain formaldehyde-free bio-based wood adhesive by static gelation.

[0039] The sodium hydroxide solution had a mass percentage concentration of 3.85%. The mass ratio of tannic acid to gelatin was 1:50, the cross-linking reaction time was 60 min, the cross-linking reaction temperature was 60℃, and the stirring speed was 200 rpm.

[0040] Example 6 The method for preparing the formaldehyde-free bio-based wood adhesive of the present invention is specifically implemented according to the following steps: Step 1, Preparation of gelatin-lipoic acid prepolymer solution: First, dissolve gelatin in deionized water and heat it until completely dissolved to obtain a gelatin solution. Then, add lipoic acid and stir to react, resulting in a homogeneous gelatin-lipoic acid prepolymer solution. The mass percentage concentration of the gelatin solution was 10%; the gelatin was heated for 20 minutes at a temperature of 70°C. The mass ratio of thioctic acid to gelatin was 1:30, the stirring reaction time was 60 min, and the stirring speed was 400 rpm. Step 2, prepare tannic acid dispersion; dissolve tannic acid in deionized water and stir ultrasonically until it is evenly dispersed to obtain tannic acid dispersion; The mass percentage concentration of the tannic acid dispersion was 2%; the ultrasonic stirring time was 10 min, and the ultrasonic frequency was 50 Hz. Step 3: Add sodium hydroxide solution dropwise to the gelatin-lipoic acid prepolymer solution obtained in Step 1, then add the tannic acid dispersion obtained in Step 2, stir to carry out cross-linking reaction, and obtain formaldehyde-free bio-based wood adhesive.

[0041] The sodium hydroxide solution has a mass percentage concentration of 25%; the mass ratio of tannic acid to gelatin is 1:150. The cross-linking reaction time was 90 min, the cross-linking reaction temperature was 60℃, and the stirring speed was 300 rpm.

[0042] Figure 1 These are the Fourier transform infrared (FTIR) spectra of gelatin, tannic acid, lipoic acid, and formaldehyde-free bio-based wood adhesives. Gelatin exhibits a low-functional-group amorphous protein structure, while lipoic acid shows a peak density of 1692 cm⁻¹. -1The absorption peak at 3391 cm⁻¹ corresponds to the characteristic of a carboxyl carbonyl group (C=O). Tannic acid has an absorption peak at 3391 cm⁻¹. -1 The strong, broad peak at 3600-3000 cm⁻¹ corresponds to the stretching vibration of the polyphenolic hydroxyl group (-OH). In the spectrum of formaldehyde-free bio-based wood adhesives, the intensity of the phenolic hydroxyl peak of tannic acid is significantly reduced, while the carbonyl peak of lipoic acid shifts and merges. -1 The flattened peak shape and shift towards lower wavenumbers indicate the formation of a dense intermolecular hydrogen bond network within the system. During cold pressing, hydrogen bonds act as physical cross-linking points, giving the adhesive the ability to cure rapidly at room temperature and to bond tightly to the wood surface.

[0043] Figure 2 The figure shows the shear strength of formaldehyde-free bio-based wood adhesives at different tannic acid concentrations. As can be seen from the figure, the shear strength of the formaldehyde-free bio-based wood adhesive is the highest when the tannic acid concentration is 1.25%.

[0044] Figure 3 shows the comparison test results of shear strength of different wood adhesives. The test data shows that there are significant differences in the bonding mechanical properties of various types of commercially available adhesives. Formaldehyde-free bio-based wood adhesives can exhibit higher shear strength, stronger interfacial bonding ability, and better mechanical load-bearing performance, which can effectively improve the bonding reliability and overall stability of wood composite structures.

Claims

1. A method for preparing a formaldehyde-free bio-based wood adhesive, characterized in that, The specific steps are as follows: Step 1: Dissolve gelatin in deionized water and heat it until completely dissolved to obtain a gelatin solution. Then add thioctic acid and stir to react, resulting in a gelatin-thioctic acid prepolymer solution. Step 2: Dissolve tannic acid in deionized water and stir ultrasonically until it is evenly dispersed to obtain a tannic acid dispersion. Step 3: Add sodium hydroxide solution dropwise to the gelatin-lipoic acid prepolymer solution obtained in Step 1, then add the tannic acid dispersion obtained in Step 2, stir to carry out cross-linking reaction, and obtain formaldehyde-free bio-based wood adhesive.

2. The method for preparing the formaldehyde-free bio-based wood adhesive as described in claim 1, characterized in that, In step 1, the mass percentage concentration of the gelatin solution is 10%-15%; the mass ratio of thioctic acid to gelatin is 1:10-30.

3. The method for preparing the formaldehyde-free bio-based wood adhesive as described in claim 1, characterized in that, In step 1, the gelatin is heated for 5-20 minutes at a temperature of 50-70°C; the stirring reaction time is 20-60 minutes at a speed of 150-400 rpm.

4. The method for preparing the formaldehyde-free bio-based wood adhesive as described in claim 1, characterized in that, In step 2, the mass percentage concentration of the tannic acid dispersion is 0.01-2%.

5. The method for preparing the formaldehyde-free bio-based wood adhesive as described in claim 1, characterized in that, In step 2, the ultrasonic stirring time is 2-10 min and the ultrasonic frequency is 50-70 Hz.

6. The method for preparing the formaldehyde-free bio-based wood adhesive as described in claim 1, characterized in that, In step 3, the crosslinking reaction time is 30-90 min, the crosslinking reaction temperature is 40-70℃, the stirring speed is 100 rpm-400 rpm, and the mass percentage concentration of the sodium hydroxide solution is 1%-25%.

7. The formaldehyde-free bio-based wood adhesive prepared by the method for preparing the formaldehyde-free bio-based wood adhesive according to any one of claims 1-6.

8. The use of the formaldehyde-free bio-based wood adhesive as described in any one of claims 1-6 in bonding wood.

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