Double crosslinking type starch-based wood adhesive and preparation method thereof

Abstract

The present application relates to the technical field of adhesive, in particular to a double crosslinking type starch-based wood adhesive and a preparation method thereof, aiming at solving the technical problems of poor water resistance, long curing time and complex process. In the present application, starch is pre-dispersed and nano-silica modified by octadecyl is added to form a physical crosslinking network of hydrophobic barrier through ultrasonic dispersion; then, alkali lignin, epoxy soybean oil, citric acid, bio-based furan ring-containing di-epoxy monomer and glycerol are sequentially added to form double chemical crosslinking through one-pot heating and stirring; then, copper-histidine organic metal complex is added as a catalyst, and a defoaming agent is added after cooling to obtain the starch-based wood adhesive. The present application significantly improves the water resistance and mechanical strength of the adhesive, and the preparation process is simple and environmentally friendly.

Description

Technical Field

[0001] This invention relates to the field of adhesive technology, specifically to a double cross-linked starch-based wood adhesive and its preparation method. Background Technology

[0002] Commonly used adhesives in the wood industry include synthetic resin adhesives such as urea-formaldehyde resin, phenolic resin, and melamine-formaldehyde resin. While these adhesives possess high strength and durability, they pose a problem of releasing free formaldehyde. Urea-formaldehyde and phenolic adhesives release harmful formaldehyde during production and use, polluting the environment. To reduce formaldehyde hazards, formaldehyde-free adhesives such as water-based polyurethane have gained attention. However, water-based isocyanate adhesives, for example, have high viscosity, short pot life, and high cost, limiting their application. Meanwhile, petroleum-based adhesives rely on non-renewable resources, and their prices fluctuate with crude oil prices. Starch, as a green and renewable resource, is inexpensive and abundant. Due to its molecular structure being similar to wood cellulose, it exhibits good adhesion to wood and paper. Starch-based adhesives have advantages such as high strength, non-toxicity, non-corrosiveness, and safety, and have long been widely used in paper products and other fields. However, natural starch adhesives have extremely poor water resistance; the adhesive layer easily softens or loses its adhesive strength after exposure to moisture or water, limiting their application in wood-based panel bonding. To improve the water resistance of starch adhesives, existing technologies employ various modification methods, including chemical crosslinking (reaction with aldehydes, anhydrides, isocyanates, etc.), graft copolymerization, the addition of hydrophobic additives, and two-component adhesive systems. However, existing modification methods often have shortcomings: either introducing aldehydes is not environmentally friendly, or requires two or more pretreatment steps, complicating the process, or necessitates prolonged hot pressing for full curing. Furthermore, some modifications can make the adhesive brittle or reduce its shelf life, all of which limit the widespread application of starch-based adhesives. Therefore, there is an urgent need for a new technology for starch-based wood adhesives that combines environmental friendliness, high water resistance, short curing time, and simple processing. Summary of the Invention

[0003] The purpose of this invention is to provide a double-crosslinked starch-based wood adhesive and its preparation method, overcoming the shortcomings of existing starch-based adhesives such as poor water resistance, long curing time, and complex processes, thereby improving the water resistance and mechanical strength of the adhesive while maintaining simple processing and environmentally friendly characteristics. This is achieved by introducing a physical crosslinking enhancer and a double chemical crosslinking network into the starch matrix. Its core components include: starch, octadecyl-modified silica nanoparticles, alkali lignin, epoxidized soybean oil, citric acid, bio-based furan-containing diepoxy monomer, glycerol, and a copper-histidine organometallic complex catalyst. The synergistic effect of these components achieves high-performance adhesion and enables rapid curing of the adhesive at relatively low temperatures.

[0004] The specific technical solution is as follows:

[0005] A method for preparing a double crosslinked starch-based wood adhesive includes the following steps:

[0006] The component content is as follows: 100 parts by weight of starch as the reference, 3-8 parts of octadecyl modified silica nanoparticles, 15-25 parts of alkali lignin, 8-12 parts of epoxidized soybean oil, 8-12 parts of citric acid, 10-18 parts of bio-based furan ring-containing diepoxy monomer, 5-10 parts of glycerol, 0.5 parts of organosilicon defoamer, and 0.8-1.5 parts of catalyst.

[0007] S1: Add starch to deionized water, stir and swell, then add octadecyl-modified silica nanoparticles, and sonicate to uniformly disperse and adsorb the nano-silica onto the surface of the starch particles to prepare a starch suspension.

[0008] Furthermore, the starch used is either corn starch or tapioca starch.

[0009] Furthermore, the method for preparing octadecyl-modified silica nanoparticles is as follows: silica nanoparticles with a particle size of 30 nm and octadecyltrimethoxysilane are added to toluene and refluxed at 110 °C for 6 hours to prepare octadecyl-modified silica nanoparticles.

[0010] Furthermore, the ultrasonic frequency was set to 300W and the duration to 30 minutes.

[0011] S2: Add alkali lignin, epoxidized soybean oil, citric acid, bio-based furan ring-containing diepoxy monomer and glycerol sequentially to the starch suspension prepared in step S1, heat to 80°C and stir to prepare a chemically cross-linked mixture.

[0012] Furthermore, the preparation method of the bio-based furan ring-containing diepoxy monomer is as follows: 5-hydroxymethylfurfural is dissolved in anhydrous ethanol, and ammonia and Raney nickel catalyst are added. The reaction is carried out for 8 hours under a hydrogen atmosphere and at a temperature of 80°C to obtain 5,5'-difuran. Then, epichlorohydrin and tetrabutylammonium bromide are added for catalytic reaction to prepare the bio-based furan ring-containing diepoxy monomer.

[0013] Furthermore, the stirring speed was 200 rpm, and the stirring time was 2 hours.

[0014] S3: Add copper-histidine organometallic complex catalyst to the chemical crosslinking mixture prepared in step S2, maintain the temperature at 85°C, set the stirring speed to 300 rpm for 30 minutes, then stop heating, cool to 50°C, add organosilicon defoamer, stir at 200 rpm for 10 minutes, and finally cool to room temperature to prepare a double crosslinked starch-based wood adhesive.

[0015] Furthermore, the catalyst was prepared by mixing CuCl2•2H2O with L-histidine in a 1:2 molar ratio.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1. This invention, by adding octadecyl-modified silica nanoparticles, can form a hydrophobic barrier around starch particles and construct a physical cross-linking network through hydrogen bonds and van der Waals forces between particles, thereby enhancing the structural stability of starch colloids and improving the water resistance of adhesives.

[0018] 2. The preparation method of the present invention adopts a one-pot method, which eliminates the need for pre-modifying starch in steps or storing the two components separately, thus simplifying the process.

[0019] 3. By introducing lignin grafting and furan ring structures, this invention imparts higher strength and rigidity to the adhesive layer, thereby improving the water resistance and bonding strength of the double crosslinked starch-based wood adhesive.

[0020] 4. This invention utilizes a copper-histidine complex to catalyze the ring-opening of epoxy resin, enabling the adhesive to cure and bond in a short time through hot pressing, thus saving energy. Attached Figure Description

[0021] Figure 1 This is a flowchart illustrating the preparation process of the double crosslinked starch-based wood adhesive of the present invention.

[0022] Figure 2 The image shows a comparison of the shear strength of the adhesives prepared in Examples 1-3 and Comparative Examples 1-3 when used in plywood.

[0023] Figure 3 The graph shows a comparison of the curing time of the adhesives prepared in Examples 1-3 and Comparative Examples 1-3 for plywood. Detailed Implementation

[0024] The following embodiments further explain and illustrate the technical solution of the present invention. It is particularly noted that each specific embodiment is a concretization and explanation of the technical solution and should not be considered as a limitation on the scope of protection of the present invention. Those skilled in the art still have the right to modify the technical solutions of these embodiments and make equivalent substitutions for some or all of the technical features, and these modifications or substitutions do not change the essence of the corresponding technical solutions, nor do they cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions described in the present invention. (See attached...) Figure 1 The diagram shows the preparation process of a double cross-linked starch-based wood adhesive. The detailed preparation steps are as follows:

[0025] 1. Component content

[0026] The ingredients are: 100 parts starch (as a reference), 3-8 parts octadecyl modified silica nanoparticles, 15-25 parts alkali lignin, 8-12 parts epoxidized soybean oil, 8-12 parts citric acid, 10-18 parts bio-based furan ring-containing diepoxy monomer, 5-10 parts glycerol, 0.5 parts organosilicon defoamer, and 0.8-1.5 parts catalyst.

[0027] 2. Preparation of starch suspension

[0028] Starch was added to deionized water and stirred until it swelled. Octadecyl-modified silica nanoparticles were then added. An ultrasonic disperser was set to a frequency of 300W for 30 minutes to uniformly disperse and adsorb the nano-silica onto the surface of the starch particles, thus obtaining a starch suspension.

[0029] Long-chain alkyl-modified silica is hydrophobic and can form a hydrophobic barrier around starch particles. It can also build a physical cross-linking network through hydrogen bonds and van der Waals forces between particles, thereby enhancing the structural stability of starch colloids. This physical cross-linking helps to improve the rigidity and dimensional stability of the colloid layer and reduce water penetration into the colloid layer.

[0030] Choose either corn starch or tapioca starch.

[0031] The method for preparing octadecyl-modified silica nanoparticles is as follows: 1 part of silica nanoparticles with a mass particle size of 30 nm and 1.5 parts of octadecyltrimethoxysilane are added to 20 parts of toluene by volume, and the mixture is refluxed at 110 °C for 6 hours to prepare octadecyl-modified silica nanoparticles.

[0032] 3. One-pot chemical modification and double crosslinking

[0033] Alkali lignin, epoxidized soybean oil, citric acid, bio-based furan ring-containing diepoxy monomer and glycerol were added sequentially to the starch suspension prepared above. The mixture was heated to 80°C and stirred at 200 rpm for 2 hours to prepare a chemically cross-linked mixture.

[0034] Epoxidized soybean oil grafted with lignin contains a hydrophobic long chain at one end and polar groups such as hydroxyl groups at the other. This allows it to bind to the hydrophobic silica surface through hydrophobic interactions and hydrogen bonds. Simultaneously, its polar groups interact with starch molecules, acting as a "molecular bridge" to connect the hydrophobic and hydrophilic phases, improving the compatibility and interfacial bonding strength of the system components. Citric acid undergoes esterification with the hydroxyl groups on starch molecules under elevated temperatures. One molecule of citric acid has a trifunctional group and can form ester bonds with different starch chains, introducing the first layer of chemical cross-linking network. This process reduces the number of free hydroxyl groups in starch molecules, lowers the hydrophilicity of the adhesive layer, and improves water resistance. The bio-based furan-ring-containing dieoxy monomer is derived from furfural, a renewable resource. The molecule contains a furan ring and two epoxy groups. A small amount of ring-opening begins at higher system temperatures, but the reaction is mainly complete in subsequent catalytic steps. Glycerol can insert between starch molecular chains to reduce hydrogen bonding, increase system fluidity, and prevent excessive brittleness caused by cross-linking reactions, thereby adjusting the toughness and workability of the adhesive.

[0035] The preparation method of bio-based furan ring-containing diepoxy monomer is as follows: 1 part by mass of 5-hydroxymethylfurfural is dissolved in 16 times the volume of anhydrous ethanol, and 4 times the volume of ammonia and 40 wt% Raney nickel catalyst are added. The reaction is carried out for 8 hours under a hydrogen atmosphere at 1 MPa and a temperature of 80℃ to obtain 5,5'-difuran. Then, 3.5 times the molar amount of epichlorohydrin and 15 wt% tetrabutylammonium bromide are added for catalytic reaction to prepare the bio-based furan ring-containing diepoxy monomer.

[0036] 4. Catalytic crosslinking and post-treatment

[0037] Add copper-histidine organometallic complex catalyst to the chemically crosslinked mixture prepared above, maintain the temperature at 85°C, stir at 300 rpm for 30 minutes to make the colloid thicker but still flowable, then stop heating, cool to 50°C, add organosilicon defoamer, stir at 200 rpm for 10 minutes, and finally cool to room temperature to prepare a double crosslinked starch-based wood adhesive.

[0038] This catalyst is a complex formed by divalent copper ions and the amino acid histidine, which can efficiently activate epoxy groups. After the catalyst is added, the diepoxy monomer in step S2 rapidly undergoes a ring-opening addition reaction with the hydroxyl groups on starch molecules and the phenolic hydroxyl groups on lignin molecules, forming a three-dimensional cross-linked network, thereby introducing a second layer of chemical cross-linking. Due to the high rigidity of the furan ring structure, it plays a role in increasing the stiffness of the network, giving the cured adhesive layer high mechanical strength and heat resistance. The introduction of the copper-histidine catalyst greatly reduces the curing time, achieving rapid curing, which is significantly more efficient and energy-saving compared to the long curing time required without a catalyst.

[0039] The catalyst was prepared by mixing CuCl2•2H2O with L-histidine in a 1:2 molar ratio.

[0040] Example 1

[0041] Table 1 Raw Material Information Table

[0042]

[0043] S1: Take 100g of corn starch and add it to 500ml of deionized water. After stirring and swelling, add 5g of octadecyl modified silica nanoparticles. Set the ultrasonic disperser frequency to 300W and the time to 30 minutes to uniformly disperse and adsorb the nano silica onto the surface of the starch particles to obtain a starch suspension.

[0044] The method for preparing octadecyl-modified silica nanoparticles is as follows: 10g of silica nanoparticles with a particle size of 30nm and 15g of octadecyltrimethoxysilane are refluxed in 200ml of toluene at 110℃ for 6 hours to prepare octadecyl-modified silica nanoparticles.

[0045] S2: Add 20g of alkali lignin, 10g of epoxidized soybean oil, 10g of citric acid, 15g of bio-based furan ring-bisepoxy monomer and 8g of glycerol to the starch suspension prepared above in sequence, heat to 80℃, and stir at 200rpm for 2 hours to prepare a chemically cross-linked mixture.

[0046] The preparation method of the bio-based furan ring-containing diepoxy monomer is as follows: 12.6g of 5-hydroxymethylfurfural is dissolved in 200ml of anhydrous ethanol, and 50ml of ammonia and 5g of Raney nickel catalyst are added. The reaction is carried out for 8 hours under a hydrogen atmosphere of 1MPa and a temperature of 80℃ to obtain 5,5'-difuran. Then, 37g of epichlorohydrin and 2g of tetrabutylammonium bromide are added for catalytic reaction to prepare the furan ring-containing bio-based diepoxy monomer.

[0047] S3: Add 1g of copper-histidine organometallic complex catalyst to the chemically crosslinked mixture prepared in step S2, maintain the temperature at 85℃, stir at 300rpm for 30 minutes to make the colloid thicker but still flowable, then stop heating, cool to 50℃, add 0.5g of organosilicon defoamer, stir at 200rpm for 10 minutes, and finally cool to room temperature to prepare a double crosslinked starch-based wood adhesive.

[0048] The catalyst was prepared by mixing CuCl2•2H2O with L-histidine in a 1:2 molar ratio.

[0049] Example 2

[0050] The preparation method is the same as in Example 1, except that:

[0051] In step S1, cassava starch is used; the content of octadecyl-modified silica nanoparticles is 3g.

[0052] In step S2, 15g of alkali lignin, 8g of epoxidized soybean oil, 8g of citric acid, 10g of bio-based furan ring-bisepoxy monomer and 5g of glycerol are added sequentially to the starch suspension prepared in step S1.

[0053] In step S3, 0.8 g of copper-histidine organometallic complex catalyst is added.

[0054] Example 3

[0055] The preparation method is the same as in Example 1, except that:

[0056] In step S1, the content of octadecyl-modified silica nanoparticles is 8g;

[0057] In step S2, 25g of alkali lignin, 12g of epoxidized soybean oil, 12g of citric acid, 18g of bio-based furan ring-containing biepoxide monomer and 10g of glycerol are added sequentially to the starch suspension prepared in step S1.

[0058] In step S3, 1.5g of copper-histidine organometallic complex catalyst is added.

[0059] Comparative Example 1

[0060] The preparation method of Example 1 was followed, but without the addition of octadecyl-modified silica nanoparticles. All other steps were the same.

[0061] Comparative Example 2

[0062] The preparation method of Example 1 is followed, but without the addition of a bio-based furan ring-containing diepoxy monomer and without a second chemical crosslinking. All other steps are the same.

[0063] Comparative Example 3

[0064] The preparation method of Example 1 was followed, but without the addition of a copper-histidine catalyst. All other steps were the same.

[0065] Double-crosslinked starch-based wood adhesives were prepared based on Examples 1-3 and Comparative Examples 1-3, and their comprehensive performance was compared. The adhesives prepared according to this invention were used in veneer bonding tests: poplar veneers were coated with adhesive (adhesive application amount on one side 280 g / m²). 2 The plywood samples were prepared by stacking three layers of plywood, hot-pressing and curing at 120℃ and 1.0MPa. The bonding strength and water resistance were then tested according to standard GB / T 9846-2015 "Ordinary Plywood". The specific test results are shown in Table 2. Figure 2 , Figure 3 As shown:

[0066] Table 2. Comparison of overall performance between Examples 1-3 and Comparative Examples 1-3

[0067]

[0068] As can be seen from the above comparison results, in Comparative Example 1, the dry shear strength is slightly lower than that in Example 1. After boiling in water, the shear strength is significantly reduced, indicating that the lack of hydrophobic nano-silica leads to a significant reduction in the water penetration resistance and crack resistance of the adhesive layer. This proves that the physical crosslinking enhancement and hydrophobic barrier effect of nano-silica are very important for improving water resistance. In Comparative Example 2, the dry shear strength is low, and after boiling in water, the shear strength is significantly reduced, indicating that the lack of a dense network formed by epoxy ring-opening crosslinking (secondary chemical crosslinking) results in poor water resistance. In Comparative Example 3, no catalyst was added, which means that the adhesive needs to be cured at a higher temperature or for a longer time. Therefore, hot pressing at 120°C and 1.0 MPa requires a longer time.

Claims

1. A method for preparing a double cross-linked starch-based wood adhesive, characterized in that, The composition and preparation process of the adhesive are as follows: starch is pre-dispersed and then nano-silica with octadecyl surface modification is added and ultrasonically dispersed to form a physical cross-linking network with a hydrophobic barrier; then alkali lignin, epoxidized soybean oil, citric acid, bio-based furan ring-containing diepoxy monomer and glycerol are added in a one-pot process and heated and stirred to form a chemical cross-linking; then copper-histidine organometallic complex is added as a catalyst to efficiently activate epoxy groups.

2. The preparation method of the double crosslinked starch-based wood adhesive as described in claim 1, characterized in that, The components are as follows: 100 parts by weight of starch (as a reference), 3-8 parts by weight of octadecyl-modified silica nanoparticles, 15-25 parts by weight of alkali lignin, 8-12 parts by weight of epoxidized soybean oil, 8-12 parts by weight of citric acid, 10-18 parts by weight of bio-based furan ring-containing diepoxy monomer, 5-10 parts by weight of glycerol, 0.5 parts by weight of organosilicon defoamer, and 0.8-1.5 parts by weight of catalyst.

3. The preparation method of the double cross-linked starch-based wood adhesive as described in claim 1, characterized in that, The specific steps include the following: S1: Add starch to deionized water, stir and swell, then add octadecyl-modified silica nanoparticles, and sonicate to uniformly disperse and adsorb the nano-silica onto the surface of the starch particles to prepare a starch suspension. S2: Add alkali lignin, epoxidized soybean oil, citric acid, bio-based furan ring-containing diepoxy monomer and glycerol sequentially to the starch suspension prepared in step S1, heat to 80°C and stir to prepare a chemically cross-linked mixture. S3: Add copper-histidine organometallic complex catalyst to the chemical crosslinking mixture prepared in step S2, maintain the temperature at 85°C, set the stirring speed to 300 rpm for 30 minutes, then stop heating, cool to 50°C, add organosilicon defoamer, stir at 200 rpm for 10 minutes, and finally cool to room temperature to prepare a double crosslinked starch-based wood adhesive.

4. The preparation method of the double crosslinked starch-based wood adhesive as described in claim 3, characterized in that, The starch mentioned in step S1 can be corn starch or tapioca starch.

5. The preparation method of the double crosslinked starch-based wood adhesive as described in claim 3, characterized in that, The octadecyl-modified silica nanoparticles described in step S1 are prepared by refluxing silica nanoparticles and octadecyltrimethoxysilane in toluene at 110°C for 6 hours.

6. The preparation method of the double crosslinked starch-based wood adhesive as described in claim 3, characterized in that, The ultrasound in step S1 is set to a frequency of 300W and a duration of 30 minutes.

7. The method for preparing the double cross-linked starch-based wood adhesive as described in claim 3, characterized in that, The bio-based furan ring-containing diepoxy monomer described in step S2 is prepared by dissolving 5-hydroxymethylfurfural in anhydrous ethanol, adding ammonia and Raney nickel catalyst, and reacting for 8 hours under a hydrogen atmosphere and at a temperature of 80°C to obtain 5,5'-difuran. Then, epichlorohydrin and tetrabutylammonium bromide are added for catalytic reaction to prepare the bio-based furan ring-containing diepoxy monomer.

8. The preparation method of the double crosslinked starch-based wood adhesive as described in claim 3, characterized in that, The stirring described in step S2 is set to a stirring speed of 200 rpm and a stirring time of 2 hours.

9. The method for preparing the double cross-linked starch-based wood adhesive as described in claim 3, characterized in that, The catalyst described in step S3 is prepared by mixing CuCl2•2H2O and L-histidine in a 1:2 molar ratio.

10. The double-crosslinked starch-based wood adhesive prepared by the preparation method according to any one of claims 1-9, characterized in that, The aforementioned double cross-linked starch-based wood adhesive has a dry shear strength of over 1.63 MPa, a shear strength of over 1.60 MPa after boiling in water for 24 hours, and a hot-press curing time of less than 4 minutes.

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