A starch-based non-isocyanate polyurethane wood adhesive and a preparation method thereof

Abstract

This application provides a starch-based non-isocyanate polyurethane wood adhesive and its preparation method, relating to the field of polyurethane adhesive technology. The invention modifies starch by enzymatic hydrolysis-oxidation with a silane coupling agent and a carbonate agent to generate a starch-based carbonate intermediate. This intermediate is then subjected to aminolysis with an aminolysis agent to obtain the starch-based non-isocyanate polyurethane wood adhesive. The starch-based non-isocyanate polyurethane wood adhesive obtained by this invention exhibits excellent water resistance and high shear strength. The addition of starch debranching enzyme improves the rheological properties and reactivity of starch. The addition of an oxidizing agent solution exposes more reaction sites, improving the compatibility between starch and the polymer. After carbonated starch, the reaction with diamine forms urethane bonds and constructs a three-dimensional network structure. The process is simple, the reaction is mild, and it is suitable for large-scale production.

Description

Technical Field

[0001] This invention relates to the field of polyurethane adhesives, and in particular to a starch-based non-isocyanate polyurethane wood adhesive and its preparation method. Background Technology

[0002] my country's timber industry is booming, with the output of engineered wood products increasing year by year, leading to a huge demand for adhesives. Globally, wood adhesives account for approximately 75% of total adhesive production, with formaldehyde-based adhesives still dominating. As the adhesive market continues to expand, the high price of global petroleum resources and environmental pollution are increasingly attracting attention. Synthetic resin adhesives, containing toxic substances such as formaldehyde and phenol, pose significant risks to the environment and human health during production, transportation, and use. Therefore, renewable and environmentally friendly biomass-based adhesives have become a research hotspot. Biomass resources, as natural polymer materials, are widely distributed and renewable, and many substances have been proven to be suitable as raw materials for wood adhesives.

[0003] Polyurethane adhesives, as an important class of non-formaldehyde adhesives, have shown excellent application potential in the wood industry. Traditional polyurethane adhesives are synthesized mainly from polyisocyanate monomers. Their molecules contain highly reactive isocyanate groups, which can form strong chemical bonds with active groups such as hydroxyl groups in wood, thus generating extremely strong adhesion. However, the isocyanate raw materials they rely on have two serious problems: toxicity and dependence on non-renewable resources. Currently, new methods to design isocyanate-free methods are being explored. Non-isocyanate polyurethane (NIPU) is a green technology in the polyurethane field. It abandons the toxic isocyanate raw materials that must be used in the synthesis of traditional polyurethanes. It achieves the green synthesis of polyurethanes through the addition reaction of cyclic carbonates and amino groups to form urethane bonds. The resulting non-isocyanate polyurethane materials have better chemical resistance and higher impermeability. However, because the main chain of non-isocyanate polyurethane contains a large number of hydroxyl groups, it has strong hydrophilicity. Especially when used as an adhesive in the plywood field, the wet bond strength of plywood is often unsatisfactory.

[0004] Chinese patent CN113603884B discloses a method for preparing non-isocyanate polyurethane, which requires first preparing an epoxide using bio-based polyphenols and epichlorohydrin under the action of a phase transfer catalyst, then preparing a five-membered cyclic carbonate using carbon dioxide, and finally adding a diamine for reaction. This invention involves a complex synthesis process, long reaction times in the steps involved, and requires sophisticated equipment. Chinese patent CN121718294A discloses a method for preparing a bio-based non-isocyanate polyurethane wood adhesive. This invention involves cyclocarbonating epoxidized soybean oil under high temperature and pressure and then performing a ring-opening addition reaction with furan-2,5-dicarboxyhydrazide, followed by further heating and polymerization with acrylamide, an initiator, and a crosslinking agent to form an adhesive containing bio-based non-isocyanate polyurethane and polyacrylamide. This invention also involves a complex synthesis process, the use of toxic reagents and hazardous chemicals, high cost of cyclic carbonate raw materials, and a low bio-content in the prepared non-isocyanate polyurethane.

[0005] Chinese patent CN121006170A discloses a method for preparing a starch-based adhesive and its application in wood processing. The method involves first preparing a starch-based adhesive using high-amylose starch and tannic acid as raw materials, then treating the wood surface with KH560 before applying the adhesive and pressing it into boards. The adhesive prepared after modifying the high-amylose starch improves the water resistance of the starch-based adhesive to some extent. However, because the kinetic energy of the starch chains significantly increases during aging treatment, it is difficult to maintain the original network structure, and its shear strength remains low. Furthermore, the need for prior wood surface treatment increases the workload and is detrimental to cost control during production.

[0006] Chinese patent CN116676058A discloses a preparation process for a high-strength, high-toughness starch adhesive. First, potassium permanganate is added to a starch solution and reacted, followed by the addition of glutaraldehyde to obtain modified starch. Then, under nitrogen protection, polytetrahydrofuran ether diol and isophorone diisocyanate are mixed and reacted with the addition of dibutyltin dilaurate. An acetone solution containing tetramethylolpropionic acid sulfoxide (POSS) is then mixed in to obtain a waterborne polyurethane. Finally, the starch adhesive is compounded with the waterborne polyurethane containing a three-dimensional cross-linked structure of POSS to obtain the adhesive. The preparation route for waterborne polyurethane follows the traditional polyurethane preparation method, using toxic isocyanates and high-cost, highly sensitive raw materials such as glutaraldehyde and POSS. The preparation process requires multiple solvent removal steps, making it cumbersome and lengthy, resulting in high preparation costs and relatively harsh synthesis conditions.

[0007] Chinese patent CN119144265A (publication date December 17, 2024) discloses a method for preparing a non-isocyanate polyurethane adhesive resistant to extreme environments. The method involves synthesizing a diglycidyl ether intermediate from a phenolic compound containing a rigid group, reacting it with carbon dioxide to obtain a cyclic carbonate monomer, and then reacting it with amine compounds containing flexible and rigid segments to prepare the non-isocyanate polyurethane adhesive. The reaction process involves high-pressure treatment and an organic solvent system.

[0008] Chinese patent CN114031767A (publication date February 11, 2022) discloses a method for preparing cyclic carbonate prepolymers and silane-modified non-isocyanate polyurethane resins. The method involves reacting a polymeric polyol with a diacid anhydride and adding glycerol carbonate to obtain a cyclic carbonate prepolymer. The modified non-isocyanate polyurethane is then obtained by introducing a diamine chain extender and an aminosilane coupling agent for chain extension. However, the preparation of the cyclic carbonate prepolymer requires a prolonged high-temperature reaction and nitrogen protection. Furthermore, the addition of silane modifiers for modification presents challenges in reaction control and operational difficulty.

[0009] As mentioned above, most industrial methods for synthesizing non-isocyanate polyurethanes currently use petrochemical raw materials. From a sustainable development perspective, green synthesis methods for non-isocyanate polyurethanes are becoming increasingly important. Therefore, it is urgent to utilize sustainable biomass resources to prepare non-isocyanate polyurethane adhesives that possess both resistance to damp heat aging and simple processing techniques. Summary of the Invention

[0010] To overcome the shortcomings of the prior art, this invention provides a starch-based non-isocyanate polyurethane wood adhesive with excellent comprehensive performance and its preparation method. The main reaction can be completed under normal pressure, the process is mild, and carbonate groups are directly constructed in situ on the starch backbone, reducing dependence on petrochemical products. The synthesis route is greener and simpler.

[0011] The technical solution of this invention is implemented as follows: A method for preparing a starch-based non-isocyanate polyurethane wood adhesive includes the following steps: S1: Starch-based carbonate intermediates are generated by modifying starch through enzymatic hydrolysis-oxidation with silane coupling agents and carbonate esterifying agents; S2: The starch-based carbonate intermediate is subjected to an aminolysis reaction with an aminolysis agent to obtain the starch-based non-isocyanate polyurethane wood adhesive. The carbonate esterifying agent is at least one selected from dimethyl carbonate, diethyl carbonate, diphenyl carbonate, ethylene carbonate, and propylene carbonate. The aminolysis reagent is at least one selected from ethylenediamine, 1,6-hexanediamine, 1,8-octanediamine, and 1,4-cyclohexanediamine.

[0012] This invention optimizes the starch structure through enzymatic hydrolysis and oxidative pretreatment. Hydroxyl groups are oxidized to carbonyl groups, and further oxidized to carboxyl groups. Simultaneously, some glycosidic bonds are broken, reducing molecular weight and activating functional groups. Then, a carbonate esterifying agent is used to introduce carbonate groups, generating cyclic and linear carbonated derivatives. Finally, these are cross-linked with diamines to form urethane bonds, resulting in a non-isocyanate polyurethane. This invention utilizes the reaction of hydroxyl groups in starch with a carbonate esterifying agent to generate a starch-based carbonate intermediate, which then undergoes an aminolysis reaction with diamine to form urethane bonds, thereby constructing a cross-linked network structure.

[0013] Enzymatic hydrolysis increases the number of reduced ends and free hydroxyl groups in the starch chains, enhancing reactivity and facilitating more uniform reactions in subsequent oxidation and carbonate steps, thereby increasing the crosslinking density and adhesive strength of the adhesive. Oxidation introduces polar groups, enhancing the compatibility between starch and polymers. Carboxyl and amine groups form ionic or hydrogen bonds. The carbonate agent undergoes transesterification with the hydroxyl groups of starch to generate starch methyl carbonate, which then crosslinks with diamine to construct a three-dimensional network structure.

[0014] In summary, this invention comprehensively utilizes the technical principles of enzymatic hydrolysis-oxidation treatment, carbonate esterification and ammonolysis reactions, as well as composite modification, to construct a highly efficient non-isocyanate polyurethane wood adhesive synthesis technology system.

[0015] Based on the above technical principles, in a specific embodiment of the present invention, the carbonate esterifying agent is dimethyl carbonate, and the aminolysis agent is 1,6-hexanediamine. The carbonate esterifying agent introduces carbonate groups, which then undergo an aminolysis reaction with the diamine to form urethane bonds. Similarly, the carbonate esterifying agent can be selected from diethyl carbonate, diphenyl carbonate, ethylene carbonate, propylene carbonate, or any combination thereof, all of which can undergo transesterification with the hydroxyl groups of starch to generate starch-based carbonate intermediates. Similarly, the aminolysis agent can be selected from ethylenediamine, 1,8-octanediamine, 1,4-cyclohexanediamine, or any combination thereof, all of which can undergo an aminolysis reaction with the starch-based carbonate intermediates to form urethane bonds, thereby constructing the three-dimensional network structure described in the present invention, and obtaining a starch-based non-isocyanate polyurethane wood adhesive with excellent water resistance and mechanical strength.

[0016] Preferably, the silane coupling agent is added in the form of a pre-hydrolyzed silane coupling agent solution, and the method for preparing the pre-hydrolyzed silane coupling agent solution includes the following: Add anhydrous ethanol as a diluent, and adjust the pH to 4-5 by adding water. Then slowly add the silane coupling agent to carry out the reaction. After the system is completely clear and transparent, a pre-hydrolyzed silane coupling agent solution is obtained. The mass ratio of the silane coupling agent, anhydrous ethanol and water is 1:5~15:1~5; The mass of the silane coupling agent is 0.5% to 5% of the dry starch mass; The silane coupling agent is at least one of KH550, KH560, and KH570.

[0017] Specifically, in the embodiments of the present invention, the silane coupling agent in step S1 is added in the form of a pre-hydrolyzed silane coupling agent solution, and the preparation method of the pre-hydrolyzed silane coupling agent solution is as follows: Step ①: Weigh a certain amount of anhydrous ethanol, add deionized water and mix and stir, then add acetic acid dropwise to adjust the pH to 4-5 to catalyze hydrolysis; the mass of anhydrous ethanol is 3 g and the mass of deionized water is 0.6 g. Step 2: KH560 was selected as the silane coupling agent. 0.3 g of the silane coupling agent was slowly added to the alcohol-water solution to carry out the hydrolysis reaction. Step 3: Perform magnetic stirring at 400 rpm for 20 minutes. The solution will become slightly turbid and slightly exothermic, indicating that hydrolysis has begun. Let it stand for later use.

[0018] Preferably, the method for preparing the enzymatically hydrolyzed-oxidized starch includes: enzymatically hydrolyzing starch with starch debranching enzyme, and oxidizing it with starch oxidant to obtain enzymatically hydrolyzed-oxidized starch; The starch debranching enzyme is at least one of pullulanase, isoamylase, and α-amylase; The starch oxidant is at least one of ammonium persulfate, sodium persulfate, and sodium hypochlorite.

[0019] More preferably, starch is added to water at a mass ratio of 1:1 to 4 to form an emulsion for enzymatic hydrolysis; The enzymatic hydrolysis was performed at a pH of 3-6, a temperature of 30-60 ℃, and a time of 0.5-2 h. Add 20-60 μL of starch debranching enzyme to every 20-50 g of starch; The amount of starch oxidant added is 3% to 10% of the dry starch; the starch oxidant is added in the form of a solution, and the mass concentration of the starch oxidant solution is 1% to 30%. The oxidation time is 20~60 min.

[0020] Specifically, in the embodiments of the present invention, in the preparation process of starch-based non-isocyanate polyurethane wood adhesive, the enzymatic hydrolysis-oxidation of starch in step S1 is prepared by the following process: S11: Mix starch with deionized water to form an emulsion, then add hydrochloric acid solution to adjust the pH to 4.5-5; the concentration of the hydrochloric acid solution is 0.5 mol / L. S12: Add starch debranching enzyme to the emulsion and heat in a constant temperature water bath to carry out enzymatic hydrolysis; S13: After the enzymatic hydrolysis reaction is completed, a starch oxidant solution is added for oxidation treatment to obtain enzymatically hydrolyzed-oxidized starch.

[0021] The starch oxidant is at least one of ammonium persulfate, sodium persulfate, and sodium hypochlorite. In a specific embodiment of the present invention, ammonium persulfate is selected as the starch oxidant. Sodium persulfate or sodium hypochlorite, or a combination thereof, can also oxidize starch and expose more reaction sites. The amount of starch oxidant used is 3% to 10% of the dry starch. Within this range, the wet bonding strength increases with the increase of the amount of starch oxidant. Specifically, in a specific embodiment of the present invention, the starch oxidant is added in the form of a solution, and the mass concentration of the starch oxidant solution is 1% to 30%. Specifically, in a specific embodiment of the present invention, the mass concentration of the starch oxidant is selected as 20%, the amount of starch oxidant used is about 8.67% of the dry starch, and the oxidation time is 40 min. In addition, the oxidation time can be 20 to 60 min.

[0022] The mass ratio of starch to deionized water is 1:1 to 4. In a specific embodiment of the present invention, the mass ratio of starch to deionized water is selected as 1:1. In addition, mass ratios of 1:2, 1:3 or 1:4 are also acceptable.

[0023] Preferably, by weight, the mixture comprises 20-50 parts starch, 0.5-10 parts silane coupling agent, 10-40 parts carbonate esterifying agent, and 30-60 parts aminolysis reaction agent.

[0024] Preferably, step S1 further includes a modification reaction between enzymatic hydrolysis-oxidation of starch and chemical modifiers and / or filler modifiers; The chemical modifier is at least one of sodium dodecyl sulfate, urea, and anhydrous potassium carbonate; The filler modifier is at least one of montmorillonite, nano-silica, and attapulgite; in a specific embodiment of the present invention, montmorillonite is selected as the filler modifier, and nano-silica and / or attapulgite can also achieve the same modification effect.

[0025] More preferably, the amount of chemical modifier added is 0.2 to 3.5 parts by weight, and the amount of filler modifier added is 0.2 to 3 parts; The mass concentration of the chemical modifier is 2%~12%; The mass concentration of the filler modifier is 2% to 10%.

[0026] Preferably, in step S1, the temperature of the modification reaction is 40~70 °C and the time is 0.5~1 h; In step S2, the temperature of the aminolysis reaction is 70~99 °C and the time is 0.5~2 h.

[0027] Preferably, an aminolysis agent is added dropwise to the starch-based carbonate intermediate over a period of 10-20 minutes.

[0028] This invention provides a starch-based non-isocyanate polyurethane wood adhesive prepared by the method described above.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The starch-based non-isocyanate polyurethane wood adhesive of the present invention has excellent water resistance and high shear strength. Adding starch debranching enzyme for enzymatic hydrolysis improves the rheological properties of starch and enhances its reactivity. Adding starch oxidant solution for oxidation exposes more reaction sites, improving the compatibility between starch and polymer. After carbonate esterification of starch, it reacts with diamine to form urethane bonds and constructs a three-dimensional network structure.

[0030] 2. The process of this invention is simple, the reaction conditions are mild, the enzymatic hydrolysis and oxidation are carried out at low temperature, and the carbonate esterification does not require high temperature and high pressure. Without the use of catalysts, non-isocyanate polyurethane adhesives can be prepared by adding different modifiers and then using a one-pot method. The synthesis route is simple and safe.

[0031] 3. The present invention uses fully bio-based raw materials and does not use isocyanates in the synthesis process. It is green, environmentally friendly, healthy, non-toxic and harmless. The non-isocyanate polyurethane adhesive products prepared have excellent water resistance and mechanical strength. Attached Figure Description

[0032] Figure 1 The tensile shear failure cross section of the pine wood chip bonding specimen prepared using the adhesive of Example 1 after being treated in a water bath at 63 ℃ for 3 h. Figure 2 The tensile shear failure cross section of a pine wood chip bonding specimen prepared using the adhesive of Example 2 after being treated in a 63 ℃ water bath for 3 h. Figure 3 The tensile shear failure section of a pine wood chip bonding specimen prepared using the adhesive of Example 3 after being treated in a 63 ℃ water bath for 3 h. Figure 4 The tensile shear failure cross section of a pine wood chip bonding specimen prepared using the adhesive of Example 4 after being treated in a 63 ℃ water bath for 3 h. Figure 5 The tensile shear failure section of the pine wood chip bonding specimen prepared using the adhesive of Comparative Example 1 after being treated in a water bath at 63 ℃ for 3 h. Figure 6 The tensile shear failure section of the pine wood chip bonding specimen prepared using the adhesive of Comparative Example 2 after being treated in a water bath at 63 ℃ for 3 h. Figure 7 The tensile shear failure section of the pine wood chip bonding specimen prepared using Comparative Example 3 adhesive after being treated in a 63 ℃ water bath for 3 h. Figure 8 The tensile shear failure section of the pine wood chip bonding specimen prepared using Comparative Example 4 adhesive after being treated in a 63 ℃ water bath for 3 h. Figure 9 The tensile shear failure section of the pine wood chip bonding specimen prepared using Comparative Example 5 adhesive after being treated in a 63 ℃ water bath for 3 h. Figure 10 The dry shear strength and wet shear strength of the pine wood chip bonded specimens prepared using the adhesives of Examples 1-4 and Comparative Examples 1-5 are as follows: Figure 10 In the figure, (a) represents the dry shear strength and wet shear strength of the pine wood chip bonded specimens prepared using the adhesives of Examples 1-4. Figure 10 (b) in the figure represents the dry shear strength and wet shear strength of the pine wood chip bonded specimens prepared using the adhesives of Comparative Examples 1-5. Detailed Implementation

[0033] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0034] Unless otherwise specified, the experimental methods used in the embodiments of this invention are all conventional methods.

[0035] Unless otherwise specified, all materials and reagents used in the embodiments of this invention are commercially available.

[0036] Example 1 A composite modified starch-based non-isocyanate polyurethane wood adhesive, comprising the following components by weight: Table 1. Components of Starch-Based Non-Isocyanate Polyurethane Wood Adhesive in Example 1

[0037] The main steps in preparing the pre-hydrolyzed silane coupling agent solution are as follows: Step ①: Weigh a certain amount of anhydrous ethanol, add deionized water and mix and stir, then add acetic acid dropwise to adjust the pH to 4-5 to catalyze hydrolysis; the mass of anhydrous ethanol is 3 g and the mass of deionized water is 0.6 g. Step 2: KH560 was selected as the silane coupling agent. 0.3 g of the silane coupling agent was slowly added to the alcohol-water solution to carry out the hydrolysis reaction. Step 3: Perform magnetic stirring at 400 rpm for 20 minutes. The solution will become slightly turbid and slightly exothermic, indicating that hydrolysis has begun. Let it stand for later use.

[0038] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 30 g starch with 30 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 40 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 21 g carbonate oxidant, stir at 500 rmp for 40 min to obtain a mixed liquid; (3) Add 3.9 g of pre-hydrolyzed silane coupling agent solution and a mixed modified solution of 0.5 g chemical modifier and 0.7 g filler modifier dispersed in 10 g dispersion medium to the mixture, and continue stirring at 50 ℃ for 40 min. (4) Heat the water bath to 90 °C, add 42 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0039] Example 2 The difference between this embodiment and Example 1 is that no filler modifier is added in the preparation method of enzymatic hydrolysis-oxidation starch. The components are as follows by weight: Table 2. Components of Starch-Based Non-Isocyanate Polyurethane Wood Adhesive in Example 2

[0040] The preparation method of the pre-hydrolyzed silane coupling agent solution is the same as in Example 1.

[0041] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 30 g starch with 30 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 40 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 21 g carbonate oxidant, stir at 500 rmp for 40 min to obtain a mixed liquid; (3) Add 3.9 g of pre-hydrolyzed silane coupling agent solution and 0.5 g of chemical modifier solution dispersed in 10 g of dispersion medium to the mixture, and continue stirring at 50 ℃ for 40 min. (4) Heat the water bath to 90 °C, add 42 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0042] Example 3 The difference between this embodiment and Example 1 lies in the chemical modifiers used in the preparation method of enzymatic hydrolysis-oxidation starch. Specifically, chemical modifier I is urea, chemical modifier II is anhydrous potassium carbonate, and no filler modifier is added. The components are as follows by weight: Table 3. Components of Starch-Based Non-Isocyanate Polyurethane Wood Adhesive in Example 3

[0043] The preparation method of the pre-hydrolyzed silane coupling agent solution is the same as in Example 1.

[0044] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 30 g starch with 30 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 40 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 21 g carbonate oxidant, stir at 500 rmp for 40 min to obtain a mixed liquid; (3) Add 3.9 g of pre-hydrolyzed silane coupling agent solution and a mixed modification solution of 0.5 g each of chemical modifier I and II dispersed in 10 g of dispersion medium to the mixture, and continue stirring at 50 ℃ for 40 min. (4) Heat the water bath to 90 °C, add 42 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0045] Example 4 The difference between this embodiment and Example 1 is that no filler modifier or chemical modifier is added in the preparation method of enzymatic hydrolysis-oxidation starch. The components are as follows by weight: Table 4. Components of Starch-Based Non-Isocyanate Polyurethane Wood Adhesive in Example 4

[0046] The preparation method of the pre-hydrolyzed silane coupling agent solution is the same as in Example 1.

[0047] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 30 g starch with 30 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 40 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 21 g carbonate oxidant, stir at 500 rmp for 40 min to obtain a mixed liquid; (3) Add 3.9 g of pre-hydrolyzed silane coupling agent solution to the mixture and continue stirring at 50 °C for 40 min; (4) Heat the water bath to 90 °C, add 42 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0048] Comparative Example 1 The difference between this comparative example and Example 1 is that the preparation method of enzymatic hydrolysis-oxidation starch does not include silane coupling agents, filler modifiers, and chemical modifiers. The components, by weight, are as follows: Table 5. Component list of starch-based non-isocyanate polyurethane wood adhesive for Comparative Example 1

[0049] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 30 g starch with 30 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 40 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 21 g carbonate oxidant, continue to keep warm at 500 rpm, and stir for 40 min; (3) Heat the water bath to 90 °C, add 42 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0050] Comparative Example 2 The difference between this comparative example and Example 1 is that the preparation method of enzymatic hydrolysis-oxidation starch does not include silane coupling agents and chemical modifiers, and metakaolin is used as the filler modifier. The components are as follows by weight: Table 6. Component list of starch-based non-isocyanate polyurethane wood adhesive in Comparative Example 2

[0051] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 30 g starch with 30 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 40 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 21 g carbonate oxidant, stir at 500 rmp for 40 min to obtain a mixed liquid; (3) Add the modified solution of 0.7 g filler modifier dispersed in 10 g dispersion medium to the mixture, and continue to stir at 50 ℃ for 40 min. (4) Heat the water bath to 90 °C, add 42 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0052] Comparative Example 3 The difference between this comparative example and Example 1 lies in the different chemical modifiers used in the preparation method of enzymatic hydrolysis-oxidation starch. Specifically, chemical modifier I is urea, chemical modifier II is polyvinyl alcohol, and no filler modifier is added. The components are as follows by weight: Table 7. Component list of starch-based non-isocyanate polyurethane wood adhesive in Comparative Example 3

[0053] The preparation method of the pre-hydrolyzed silane coupling agent solution is the same as in Example 1.

[0054] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 30 g starch with 30 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 40 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 21 g carbonate oxidant, stir at 500 rmp for 40 min to obtain a mixed liquid; (3) Add 3.9 g of pre-hydrolyzed silane coupling agent solution and 0.5 g of chemical modifier mixed solution dispersed in 10 g dispersion medium to the mixture, and continue stirring at 50 ℃ for 40 min. (4) Heat the water bath to 90 °C, add 42 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0055] Comparative Example 4 The difference between this comparative example and Example 1 is that, in the preparation method of enzymatic hydrolysis-oxidation starch, no silane coupling agent and filler modifier were added, and sodium lignosulfonate was used as the chemical modifier. The components are as follows by weight: Table 8. Component list of starch-based non-isocyanate polyurethane wood adhesive (Comparative Example 4)

[0056] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 30 g starch with 30 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 40 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 21 g carbonate oxidant, stir at 500 rmp for 40 min to obtain a mixed liquid; (3) Add the modified solution of 0.5 g chemical modifier dispersed in 10 g dispersion medium to the mixture, and continue to stir at 50 ℃ for 40 min; (4) Heat the water bath to 90 °C, add 42 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0057] Comparative Example 5 This comparative example demonstrates the preparation of a pure starch adhesive. The main steps are as follows: Mix 30 g of starch with 40 g of deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, heat to 50 ℃, and stir at a constant temperature for 1 h to obtain a uniform pregelatinized solution for later use; heat to 90 ℃, stir at a constant temperature for 1 h, and after complete gelatinization, cool to room temperature to obtain pure starch adhesive.

[0058] Application Example 1 The performance of the starch-based non-isocyanate polyurethane adhesives prepared in Examples 1-4 and Comparative Examples 1-5 was evaluated.

[0059] I. Performance Testing Methods (1) Viscosity: The viscosity of the adhesive was measured using a digital viscometer in accordance with GB / T 14074-2017 "Test Methods for Adhesives and Resins for Wood Industry"; the measurement was performed in parallel for 3 times (unit mPa·s), and the average value was taken as the result.

[0060] (2) Solid content: Fold tin foil into an open rectangular container, weigh the container, and dry it in a forced-air drying oven for 30 min. Place it on a desiccator for 10 min and weigh it using an analytical balance. Repeat this operation until the weight difference between the two measurements is less than 0.005 g, and record it as solid content. m 1. Add 5 g of adhesive to the container, weigh the container again, and record the weight. m 2; The container is dried in a forced-air drying oven at 100 ℃. Record the result when the difference in mass between the initial and final drying cycles is less than 0.1% of the final mass. m 3. Repeat the experiment three times, take the average value, and calculate the solid content of the adhesive according to the following formula: .

[0061] (3) pH value: The pH value of the adhesive was determined by using a rotary digital pH meter in accordance with the requirements of the national standard GB / T 14074-2017 "Test Methods for Adhesives and Resins for Wood Industry". The average value was taken after repeating the test three times.

[0062] (4) Gluing performance: Masson pine wood chips (80 mm × 20 mm × 5 mm) were selected, and after gluing and assembly, they were hot-pressed to form wood chip bonding test specimens. The process parameters were: hot-pressing temperature 200 ℃, glue application amount 180 g / m 2The hot-pressing pressure was 1.0-1.1 MPa, and the hot-pressing time was 30 min. The specimens were processed according to the national standard GB / T 33333-2016. One group was treated with equilibrium moisture content (called dry state), and the other group was soaked in water at 63 ℃ for 3 h (called wet state). The tensile shear strength of the treated glued samples was tested using a universal testing machine to obtain the glued strength and wood breakage rate of the specimens.

[0063] II. Results Analysis Figures 1-9 The tensile shear failure sections of pine wood chip bonding specimens prepared with the adhesives of the examples and comparative examples after being treated in a water bath at 63 ℃ for 3 h are shown.

[0064] Figure 1 The morphology of the glued area after the specimen was damaged is shown. Tearing of wood fibers and wood residue are visible in some parts of the cross-section, indicating a moderate degree of wood damage.

[0065] Figure 2 The morphology of the glued area after specimen failure is shown. The cross-section exhibits numerous irregularly distributed areas of wood peeling, indicating a relatively high proportion of wood damage. Figure 1 There has been an increase.

[0066] Figure 3 The morphology of the glued area after the specimen was damaged is shown. The cross-section shows a large area of ​​wood fibers torn along the wood grain, indicating significant wood damage.

[0067] Figure 4 The morphology of the glued area after the specimen was damaged is shown. Only local wood peeling and fiber residue were found on the cross-section, while the rest of the area mainly showed damage to the glued interface.

[0068] Figure 5 The morphology of the glued area after the specimen was damaged is shown. Only a small amount of wood fiber or sawdust remains on the cross-section, indicating that the degree of damage to the wood inside is relatively low.

[0069] Figure 6 The morphology of the glued area after the specimen was damaged is shown. The cross-section is basically intact, with only a very small amount of wood tearing visible. The damage mainly occurred in the glue layer or the glue interface.

[0070] Figure 7 The morphology of the glued area after the specimen was damaged is shown, and there are small areas of wood damage and wood fiber residue in the cross section.

[0071] Figure 8 The morphology of the glued area after the specimen was damaged is shown. A small amount of wood peeling can be seen on the cross-section. Overall, the damage is mainly due to the glue layer or the glue interface.

[0072] Figure 9The morphology of the glued area after the specimen was damaged is shown. No obvious wood fiber tearing or wood residue was observed on the cross-section. The specimen was basically separated along the glue layer or the glue interface.

[0073] Overall, Figures 1 to 9 In this context, the higher the wood breakage rate, the larger the area of ​​torn wood fibers and remaining wood on the cross-section; among them... Figure 3 The damage to the timber was most obvious. Figure 9 No damage to the timber was found.

[0074] The test results of the examples and comparative examples are shown in Table 9 below.

[0075] Table 9. Overall Performance of Starch-Based Non-Isocyanate Polyurethane Adhesives

[0076] Depend on Figure 10 As shown in Table 9, the unmodified pure starch adhesive (Comparative Example 5) has excessively high viscosity, making application difficult. Its wet bonding strength is 0, and its water resistance is extremely poor. The addition of dimethyl carbonate and 1,6-hexanediamine forms a cross-linked network (Comparative Example 1), which improves the performance, but its strength remains low and cannot meet application requirements.

[0077] The wet strength was improved after the addition of filler modifier and chemical modifier. The adhesives prepared by composite modification (Examples 1 and 3) had a maximum wet bonding strength of 1.88±0.39 MPa, indicating that the addition of chemical modifier enabled the reactive sites on the starch skeleton to react more fully, and the resulting crosslinking system was more stable and firm, with a significant improvement in water resistance.

[0078] Compared to Comparative Example 5, the addition of various modifiers can effectively reduce the viscosity of the system, making it easier to apply adhesive when bonding wood, and making it more conducive for the adhesive to penetrate into the cavities and pores of wood cells. After hot pressing and curing, it forms glue nails that enhance its shear strength.

[0079] In Comparative Example 2, the addition of metakaolin as a filler resulted in higher viscosity and increased colloid thickness. Furthermore, the formation of an inorganic particulate dispersion phase increased the risk of salting out of the adhesive. The addition of silane coupling agent (Example 4) improved the bonding interface and enhanced water resistance. The wet bonding strength was 1.18 ± 0.19 MPa. This is because the silanol formed after hydrolysis can connect the hydrophilic surfaces with hydroxyl groups, thereby reducing interfacial debonding.

[0080] Compared with other comparative examples, the starch-based non-isocyanate polyurethane adhesive prepared in Example 3 has the highest bonding strength and dry wood breakage rate, as well as suitable viscosity and pH value. In particular, the wet bonding strength (1.88 MPa) is 54.10% higher than that of other reported starch-based non-isocyanate polyurethane adhesives (1.22 MPa). The product has excellent overall performance and can be promoted and applied in the plywood industry.

[0081] Example 5 A composite modified starch-based non-isocyanate polyurethane wood adhesive, comprising the following components by weight: Table 10. Components of Starch-Based Non-Isocyanate Polyurethane Wood Adhesive in Example 5

[0082] The preparation method of the pre-hydrolyzed silane coupling agent solution is the same as in Example 1.

[0083] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 50 g starch with 50 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 60 μL of pullulanase with 2000 U / g, heat to 50 ℃, stir at constant temperature for 1 h, and obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 32 g carbonate oxidant, stir at 500 rmp for 40 min to obtain a mixed liquid; (3) Add 8 g of pre-hydrolyzed silane coupling agent solution and 0.8 g of chemical modifier solution dispersed in 10 g of dispersion medium to the mixture, and continue stirring at 50 ℃ for 40 min. (4) Heat the water bath to 90 °C, add 55 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0084] Example 6 A composite modified starch-based non-isocyanate polyurethane wood adhesive, comprising the following components by weight: Table 11 Components of Starch-Based Non-Isocyanate Polyurethane Wood Adhesive in Example 6

[0085] The preparation method of the pre-hydrolyzed silane coupling agent solution is the same as in Example 1.

[0086] The main steps of the preparation method of the above-mentioned starch-based non-isocyanate polyurethane wood adhesive are as follows: (1) Mix 20 g starch with 20 g deionized water, adjust the pH to 4.5-5 with 0.5 mol / L dilute hydrochloric acid, add 20 μL of pullulanase with 2000 U / g and heat to 50 ℃, stir at constant temperature for 1 h to obtain a uniform pregelatinized liquid for later use. (2) Place the pregelatinized liquid in a 50 ℃ constant temperature water bath, add 13 g starch oxidant solution with a mass concentration of 20% and 11 g carbonate oxidant, stir at 500 rpm for 40 min to obtain a mixed liquid; (3) Add 1 g of pre-hydrolyzed silane coupling agent solution and 0.8 g of chemical modifier solution dispersed in 10 g of dispersion medium to the mixture, and continue stirring at 50 ℃ for 40 min. (4) Heat the water bath to 90 °C, add 32 g of aminolysis reagent dropwise in a constant pressure funnel for 10 min, keep warm and stir for 60 min, cool to room temperature, and discharge to obtain starch-based non-isocyanate polyurethane wood adhesive.

[0087] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive, characterized in that, Includes the following steps: S1: Starch-based carbonate intermediates are generated by modifying starch through enzymatic hydrolysis-oxidation with silane coupling agents and carbonate esterifying agents; S2: The starch-based carbonate intermediate is subjected to an aminolysis reaction with an aminolysis agent to obtain the starch-based non-isocyanate polyurethane wood adhesive. The carbonate esterifying agent is at least one selected from dimethyl carbonate, diethyl carbonate, diphenyl carbonate, ethylene carbonate, and propylene carbonate. The aminolysis reagent is at least one selected from ethylenediamine, 1,6-hexanediamine, 1,8-octanediamine, and 1,4-cyclohexanediamine.

2. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive according to claim 1, characterized in that, The silane coupling agent is added in the form of a pre-hydrolyzed silane coupling agent solution, and the preparation method of the pre-hydrolyzed silane coupling agent solution includes the following: Anhydrous ethanol was added as a diluent, and water was added to adjust the pH to 4-5. Then, silane coupling agent was added to carry out the reaction, and a pre-hydrolyzed silane coupling agent solution was obtained. The mass ratio of the silane coupling agent, anhydrous ethanol and water is 1:5~15:1~5; The mass of the silane coupling agent is 0.5% to 5% of the dry starch mass; The silane coupling agent is at least one of KH550, KH560, and KH570.

3. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive according to claim 1, characterized in that, The method for preparing enzymatically hydrolyzed-oxidized starch includes: hydrolyzing starch with starch debranching enzyme, adding starch oxidant for oxidation, and obtaining enzymatically hydrolyzed-oxidized starch; The starch debranching enzyme is at least one of pullulanase, isoamylase, and α-amylase; The starch oxidant is at least one of ammonium persulfate, sodium persulfate, and sodium hypochlorite.

4. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive according to claim 3, characterized in that, Starch is added to water at a mass ratio of 1:1 to 4 to form an emulsion for enzymatic hydrolysis. The enzymatic hydrolysis was performed at a pH of 3-6, a temperature of 30-60 ℃, and a time of 0.5-2 h. Add 20-60 μL of starch debranching enzyme to every 20-50 g of starch; The amount of starch oxidant added is 3% to 10% of the dry starch; the starch oxidant is added in the form of a solution, and the mass concentration of the starch oxidant solution is 1% to 30%. The oxidation time is 20~60 min.

5. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive according to claim 1, characterized in that, By weight, the ingredients are: 20-50 parts starch, 0.5-10 parts silane coupling agent, 10-40 parts carbonate esterifying agent, and 30-60 parts aminolysis reaction agent.

6. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive according to claim 1, characterized in that, Step S1 further includes a modification reaction between enzymatic hydrolysis-oxidation of starch and chemical modifiers and / or filler modifiers; The chemical modifier is at least one of sodium dodecyl sulfate, urea, and anhydrous potassium carbonate; The filler modifier is at least one of montmorillonite, nano-silica, and attapulgite.

7. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive according to claim 6, characterized in that, By weight, the amount of chemical modifier added is 0.2 to 3.5 parts, and the amount of filler modifier added is 0.2 to 3 parts; The mass concentration of the chemical modifier is 2%~12%; The mass concentration of the filler modifier is 2% to 10%.

8. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive according to claim 1, characterized in that, In step S1, the temperature of the modification reaction is 40~70 ℃ and the time is 0.5~1 h; In step S2, the temperature of the aminolysis reaction is 70~99 °C and the time is 0.5~2 h.

9. A method for preparing a starch-based non-isocyanate polyurethane wood adhesive according to claim 1, characterized in that, An aminolysis agent is added dropwise to the starch-based carbonate intermediate over a period of 10-20 minutes.

10. A starch-based non-isocyanate polyurethane wood adhesive prepared by a method according to any one of claims 1 to 9.

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