High-strength urea-formaldehyde glue and preparation method thereof

Abstract

The invention relates to a high-strength urea-formaldehyde glue and a preparation method thereof, and belongs to the technical field of resin adhesives, the high-strength urea-formaldehyde glue comprises formaldehyde with the mass concentration of 46-52%, urea, melamine, organic montmorillonite powder and modified cellulose nanocrystals, the molar ratio of formaldehyde to urea is 0.8-1.05, the adding amount of melamine is 2-10% of the mass of urea, the adding amount of organic montmorillonite powder is 1-5% of the mass of modified cellulose nanocrystals, and the adding amount of organic montmorillonite powder is 1-5% of the mass of modified cellulose nanocrystals. The use amount of the organic montmorillonite powder is 1-2% of the mass of the urea, and the use amount of the modified cellulose nanocrystals is 2-3% of the mass of the urea. On the basis of adopting a high-concentration formaldehyde material, the organic montmorillonite powder and the modified cellulose nanocrystals are added, so that the performance of the urea-formaldehyde resin is further improved, and the strength and the water resistance of the urea-formaldehyde resin are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of resin adhesives, and particularly relates to a high-strength urea-formaldehyde adhesive and a preparation method thereof. Background Art

[0002] Urea-formaldehyde resin adhesive is a thermosetting resin formed by the polycondensation of urea and formaldehyde under the catalysis of acid or alkali. It has the characteristics of low cost, high bonding strength, easy coloring, and easy curing. Due to a series of the above advantages of urea-formaldehyde resin adhesive, it is widely used in the wood processing industry such as plywood, particleboard, and medium-density fiberboard, and also in industrial fields such as interior decoration, paper and fabric sizing, etc. However, urea-formaldehyde resin adhesive also has some disadvantages, such as poor water resistance and weather resistance, and free formaldehyde will be released during the curing process, which has a certain impact on the environment and human health.

[0003] Currently, in the improvement of urea-formaldehyde resin adhesive, it is mainly achieved by modifying urea-formaldehyde resin, optimizing the preparation process, adding additives, etc. Commonly used additives mainly include nano-silica, nano-titanium dioxide, nano-manganese dioxide, nano-aluminum oxide, etc. Although the mechanical properties and water resistance of urea-formaldehyde resin adhesive can be improved, the current urea-formaldehyde resin still needs to be improved in terms of performance.

[0004] Our company is committed to the development and research of urea-formaldehyde resin, and has developed a method for producing urea-formaldehyde resin adhesive using high-concentration formaldehyde, which reduces the water content ratio in formaldehyde. The produced urea-formaldehyde resin adhesive has a high solid content and does not require dehydration, solving the problem of generating formaldehyde wastewater and achieving certain results. On this basis, further research and development still need to be carried out in order to further improve the performance of urea-formaldehyde resin adhesive. Summary of the Invention

[0005] The present invention aims to provide a high-strength urea-formaldehyde adhesive and a preparation method thereof. On the basis of using high-concentration formaldehyde as raw material, by adding organic montmorillonite powder and modified cellulose nanocrystals, the performance of urea-formaldehyde resin is further improved.

[0006] The technical solution adopted by the present invention to achieve its purpose:

[0007] A high-strength urea-formaldehyde adhesive, comprising formaldehyde with a mass concentration of 46%-52%, urea, melamine, organic montmorillonite powder, and modified cellulose nanocrystals, wherein the molar ratio of formaldehyde to urea is 0.8-1.05, the addition amount of melamine is 2%-10% of the mass of urea, the dosage of organic montmorillonite powder is 1%-2% of the mass of urea, and the dosage of modified cellulose nanocrystals is 2%-3% of the mass of urea.

[0008] Further, the organic montmorillonite powder is montmorillonite modified by cetyltrimethylammonium bromide or montmorillonite modified by octadecyltrimethylammonium bromide.

[0009] Furthermore, the organic montmorillonite powder is surface-treated with a silane coupling agent:

[0010] A. Immerse the organic montmorillonite in the silane coupling agent solution and disperse it ultrasonically for 15 - 20 min;

[0011] B. Heat to 30 - 40 °C and keep warm for 5 - 6 h. After treatment, centrifuge, filter, and dry to obtain the surface-modified organic montmorillonite powder.

[0012] Furthermore, the modified cellulose nanocrystals are epoxy-based silane coupling agent-modified cellulose nanocrystals.

[0013] Furthermore, the epoxy-based silane coupling agent-modified cellulose nanocrystals are obtained by immersing the cellulose nanocrystals in the epoxy-based silane coupling agent solution, dispersing them ultrasonically for 15 - 20 min, adjusting the pH to 4 - 4.5, stirring for 8 - 10 h, and then centrifuging, filtering, and drying after treatment.

[0014] Furthermore, the mass ratio of the organic montmorillonite powder to the modified cellulose nanocrystals is 1.5 - 2.

[0015] A preparation method of a high-strength urea-formaldehyde adhesive comprises the following steps:

[0016] S1. Put a formaldehyde solution with a concentration of 46% - 52% into the reaction kettle, adjust the pH value to 7.5 - 8.0, heat to 50 °C - 55 °C, and then add melamine and the first batch of urea, start the stirrer, and stir evenly;

[0017] S2. Continue heating, raise the temperature to 88 °C - 92 °C, stop heating, and react for 30 min - 40 min under this condition;

[0018] S3. Lower the temperature to 83 °C - 87 °C, adjust the pH value to 4.5 - 5.0, start heating, raise the temperature to 88 °C - 92 °C, and carry out polycondensation reaction until the required viscosity;

[0019] S4. Adjust the pH value to 7.5 - 8.0, lower the temperature to 75 °C - 80 °C, add the second batch of urea, and react for 20 min - 30 min;

[0020] S5. Adjust the pH value to 7.0 - 7.5, add the third batch of urea, and keep warm at 55 °C - 60 °C for 10 min - 20 min;

[0021] S6. Add organophilic montmorillonite powder and epoxy group silane coupling agent modified cellulose nanocrystals into the liquid material in step S5, mix evenly, heat to 60°C - 70°C, stir and keep warm for 30 min - 40 min, then cool to 30°C - 35°C, adjust the pH value to 7.5 - 8.0, and discharge to obtain high-strength urea-formaldehyde resin adhesive.

[0022] Further, the addition amount of the first batch of urea is added according to the molar ratio of formaldehyde to urea of 1.9 - 2.0, and the addition amount of the second batch of urea is added according to the molar ratio of formaldehyde to the sum of the first and second batches of urea of 1.2 - 1.4.

[0023] Further, during the whole preparation process, when heating, the heating rate is 1.5°C / min for temperature increase.

[0024] The beneficial effects of the present invention are as follows:

[0025] 1. Using formaldehyde with a concentration of 46 - 52% reduces the moisture content ratio in formaldehyde. The produced urea-formaldehyde resin adhesive has a high solid content and does not require dehydration, solving the problem of generating formaldehyde wastewater.

[0026] 2. Organophilic montmorillonite powder as a filler can improve the mechanical properties, heat resistance, and flame retardancy of urea-formaldehyde resin. Although the organophilic montmorillonite powder has good dispersibility, it still needs to be improved. Through the modification of silane coupling agent, the compatibility between organophilic montmorillonite powder and polymer is further improved, thereby improving the mechanical properties of urea-formaldehyde resin. At the same time, due to the modification of silane coupling agent, the dispersibility of organophilic montmorillonite powder in urea-formaldehyde resin is better.

[0027] 3. Cellulose nanocrystals have high strength, high modulus, and high aspect ratio, which can enhance the mechanical properties of urea-formaldehyde resin and improve its dimensional stability. They can also improve the thermal stability and flame retardancy of the composite material. The addition of epoxy group silane coupling agent modified cellulose nanocrystals can enhance the mechanical properties of urea-formaldehyde resin adhesive, while reducing the formaldehyde release and improving the water resistance and thermal stability of urea-formaldehyde resin adhesive.

[0028] 4. The combined use of organophilic montmorillonite powder and epoxy group silane coupling agent modified cellulose nanocrystals can produce a synergistic effect and further improve the enhancement effect. Specific embodiments

[0029] The present invention will be described in detail below with reference to specific embodiments. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention. Those parts not described in detail in the present invention are all prior arts.

[0030] Example 1

[0031] The organophilic montmorillonite powder is surface-treated with a silane coupling agent as follows (the organophilic montmorillonite powder is montmorillonite powder modified with cetyltrimethylammonium bromide):

[0032] A. Immerse the organophilic montmorillonite powder into the silane coupling agent solution and perform ultrasonic dispersion for 20 min;

[0033] B. Heat to 30 °C, perform heat-preserving grinding treatment for 5.5 h, and after treatment, perform centrifugation, filtration, and drying to obtain the surface-modified organophilic montmorillonite powder.

[0034] The cellulose nanocrystals modified with epoxy-based silane coupling agent are prepared by immersing the cellulose nanocrystals into an epoxy-based silane coupling agent solution (such as 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane ethanol solution, 5 wt%), performing ultrasonic dispersion for 20 min; adjusting the pH to 4, and performing stirring treatment for 8 h. After treatment, perform centrifugation, filtration, and constant-temperature drying at 60 °C to obtain the cellulose nanocrystals modified with epoxy-based silane coupling agent. Among them, the cellulose nanocrystals are prepared by acid hydrolysis of microcrystalline cellulose.

[0035] Example 2

[0036] A high-strength urea-formaldehyde adhesive includes 200 g of formaldehyde with a mass concentration of 46%, 184 g of urea, 5.5 g of melamine, 2.7 g of the surface-modified organophilic montmorillonite powder prepared in Example 1, and 4.1 g of cellulose nanocrystals modified with epoxy-based silane coupling agent.

[0037] A preparation method of a high-strength urea-formaldehyde adhesive includes the following steps:

[0038] S1. Put 200 g of a 46% formaldehyde solution into the reaction kettle, adjust the pH value to 7.8 with 30% NaOH solution, heat to 50 °C at a heating rate of 1.5 °C / min, then add 5.5 g of melamine and 92 g of the first batch of urea, and start the stirrer to stir evenly;

[0039] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 90 °C, stop heating, and react for 35 min under this condition;

[0040] S3. Cool down to 85 °C, adjust the pH value to 4.5 with 20% NH 4 Cl solution, start heating, raise the temperature to 90 °C, and perform polycondensation reaction until the required viscosity of 19 s (hot measurement, coating 4 cup);

[0041] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add 39.5 g of the second batch of urea, and react for 20 min;

[0042] S5. Maintain the pH value at 7.5, add 52.5 g of the third batch of urea, and keep it warm at 55 °C for 20 min;

[0043] S6. Add 2.7 g of the surface-modified organic montmorillonite powder prepared in Example 1 and 4.1 g of the epoxy group silane coupling agent-modified cellulose nanocrystals to the material liquid in Step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain the high-strength urea-formaldehyde resin adhesive.

[0044] Example 3

[0045] A high-strength urea-formaldehyde resin adhesive, comprising 200 g of formaldehyde with a mass concentration of 50%, 200 g of urea, 6 g of melamine, 2 g of the surface-modified organic montmorillonite powder prepared in Example 1, and 4 g of the epoxy group silane coupling agent-modified cellulose nanocrystals.

[0046] A preparation method of a high-strength urea-formaldehyde resin adhesive, comprising the following steps:

[0047] S1. Put 200 g of a 50% formaldehyde solution into the reaction kettle, adjust the pH value to 8.0 with 30% NaOH solution, heat to 55 °C at a heating rate of 1.5 °C / min, then add 6 g of melamine and 105 g of the first batch of urea, start the stirrer, and stir evenly;

[0048] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 92 °C, stop heating, and react for 300 min under this condition;

[0049] S3. Cool down to 87 °C, adjust the pH value to 4.8 with 20% NH 4 Cl solution, start heating, raise the temperature to 92 °C, and carry out polycondensation reaction until the required viscosity is 19 s (hot test, coating 4 cup);

[0050] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add 49 g of the second batch of urea, and react for 20 min;

[0051] S5. Maintain the pH value at 7.5, add 46 g of the third batch of urea, and keep it warm at 60 °C for 10 min;

[0052] S6. Add 2 g of the surface-modified organic montmorillonite powder prepared in Example 1 and 4 g of the epoxy group silane coupling agent-modified cellulose nanocrystals to the material liquid in Step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain the high-strength urea-formaldehyde resin adhesive.

[0053] Example 4

[0054] A high-strength urea-formaldehyde glue, comprising 200 g of formaldehyde with a mass concentration of 46%, 184 g of urea, 5.5 g of melamine, 2.7 g of surface-modified organic montmorillonite powder prepared in Example 1, and 3.8 g of epoxy-silane coupling agent-modified cellulose nanocrystals.

[0055] A preparation method of a high-strength urea-formaldehyde glue, comprising the following steps:

[0056] S1. Put 200 g of a 46% formaldehyde solution into a reaction kettle, adjust the pH value to 7.8 with 30% NaOH solution, heat it to 50 °C at a heating rate of 1.5 °C / min, then add 5.5 g of melamine and 92 g of the first batch of urea, start the stirrer, and stir evenly;

[0057] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 90 °C, stop heating, and react for 35 min under this condition;

[0058] S3. Cool down to 85 °C, adjust the pH value to 4.5 with 20% NH 4 Cl solution, start heating, raise the temperature to 90 °C, and carry out polycondensation reaction until the required viscosity is 19 seconds (hot measurement, coating cup No. 4);

[0059] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add 39.5 g of the second batch of urea, and react for 20 min;

[0060] S5. Maintain the pH value at 7.5, add 52.5 g of the third batch of urea, and keep it warm at 55 °C for 20 min;

[0061] S6. Add 2.7 g of the surface-modified organic montmorillonite powder prepared in Example 1 and 3.8 g of epoxy-silane coupling agent-modified cellulose nanocrystals to the liquid material in step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain the high-strength urea-formaldehyde glue.

[0062] Example 5

[0063] A high-strength urea-formaldehyde glue, comprising 200 g of formaldehyde with a mass concentration of 50%, 200 g of urea, 6 g of melamine, 2 g of surface-modified organic montmorillonite powder prepared in Example 1, and 5 g of epoxy-silane coupling agent-modified cellulose nanocrystals.

[0064] A preparation method of a high-strength urea-formaldehyde glue, comprising the following steps:

[0065] S1. Charge 200 g of formaldehyde solution with a concentration of 50% into the reaction kettle, adjust the pH value to 8.0 with 30% NaOH solution, heat it to 55 °C at a heating rate of 1.5 °C / min, then add 6 g of melamine and 105 g of the first batch of urea, start the stirrer, and stir evenly;

[0066] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 92 °C, stop heating, and react for 30 min under this condition;

[0067] S3. Cool down to 87 °C, adjust the pH value to 4.8 with 20% NH 4 Cl solution, start heating, raise the temperature to 92 °C, and carry out polycondensation reaction until the required viscosity of 19 seconds (hot test, coating cup No. 4);

[0068] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add 49 g of the second batch of urea, and react for 20 min;

[0069] S5. Maintain the pH value at 7.5, add 46 g of the third batch of urea, and keep it at 60 °C for 10 min;

[0070] S6. Add 2 g of the surface-modified organophilic montmorillonite powder prepared in Example 1 and 5 g of epoxy group silane coupling agent-modified cellulose nanocrystals to the liquid material in Step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain high-strength urea-formaldehyde resin adhesive.

[0071] Example 6

[0072] A high-strength urea-formaldehyde resin adhesive, comprising 200 g of formaldehyde with a mass concentration of 52%, 208 g of urea, 6 g of melamine, 4.1 g of the surface-modified organophilic montmorillonite powder prepared in Example 1, and 6.15 g of epoxy group silane coupling agent-modified cellulose nanocrystals.

[0073] A preparation method of a high-strength urea-formaldehyde resin adhesive, comprising the following steps:

[0074] S1. Charge 200 g of formaldehyde solution with a concentration of 57% into the reaction kettle, adjust the pH value to 8.0 with 30% NaOH solution, heat it to 50 °C at a heating rate of 1.5 °C / min, then add 6 g of melamine and 106 g of the first batch of urea, start the stirrer, and stir evenly;

[0075] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 90 °C, stop heating, and react for 40 min under this condition;

[0076] S3. Cool down to 87 °C, use 20% NH4 Adjust the pH value of the Cl solution to 4.8, start heating, raise the temperature to 90 °C, and carry out polycondensation reaction until the required viscosity of 19 seconds (thermal measurement, cup 4);

[0077] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add the second batch of 54 g of urea, and react for 20 min;

[0078] S5. Maintain the pH value at 7.5, add the third batch of 48 g of urea, and keep warm at 55 °C for 20 min;

[0079] S6. Add 4.1 g of surface-modified organic montmorillonite powder prepared in Example 1 and 6.15 g of epoxy group silane coupling agent-modified cellulose nanocrystals to the liquid material in step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain high-strength urea-formaldehyde glue.

[0080] Comparative Example 1

[0081] A high-strength urea-formaldehyde glue, comprising 200 g of formaldehyde with a mass concentration of 46%, 184 g of urea, 5.5 g of melamine, and 2.7 g of surface-modified organic montmorillonite powder prepared in Example 1.

[0082] A preparation method of a high-strength urea-formaldehyde glue, comprising the following steps:

[0083] S1. Put 200 g of 46% formaldehyde solution into the reaction kettle, adjust the pH value to 7.8 with 30% NaOH solution, heat to 50 °C at a heating rate of 1.5 °C / min, then add 5.5 g of melamine and the first batch of 92 g of urea, start the stirrer, and stir evenly;

[0084] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 90 °C, stop heating, and react for 35 min under this condition;

[0085] S3. Cool down to 85 °C, adjust the pH value to 4.5 with 20% NH 4 Cl solution, start heating, raise the temperature to 90 °C, and carry out polycondensation reaction until the required viscosity of 19 seconds (thermal measurement, cup 4);

[0086] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add the second batch of 39.5 g of urea, and react for 20 min;

[0087] S5. Maintain the pH value at 7.5, add the third batch of 52.5 g of urea, and keep warm at 55 °C for 20 min;

[0088] S6. Add 2.7 g of the surface-modified organic montmorillonite powder prepared in Example 1 to the liquid material in Step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain a high-strength urea-formaldehyde resin adhesive.

[0089] Comparative Example 2

[0090] A high-strength urea-formaldehyde resin adhesive, comprising 200 g of formaldehyde with a mass concentration of 46%, 184 g of urea, 5.5 g of melamine, and 4.1 g of epoxy-silane-coupling-agent-modified cellulose nanocrystals.

[0091] A preparation method of a high-strength urea-formaldehyde resin adhesive, comprising the following steps:

[0092] S1. Charge 200 g of a 46% formaldehyde solution into a reaction kettle, adjust the pH value to 7.8 with 30% NaOH solution, heat to 50 °C at a heating rate of 1.5 °C / min, then add 5.5 g of melamine and the first batch of 92 g of urea, start the stirrer, and stir evenly;

[0093] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 90 °C, stop heating, and react under these conditions for 35 min;

[0094] S3. Cool down to 85 °C, adjust the pH value to 4.5 with 20% NH 4 Cl solution, start heating, raise the temperature to 90 °C, and carry out polycondensation reaction until the required viscosity of 19 s (hot test, coating cup No. 4);

[0095] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add the second batch of 39.5 g of urea, and react for 20 min;

[0096] S5. Maintain the pH value at 7.5, add the third batch of 52.5 g of urea, and keep warm at 55 °C for 20 min;

[0097] S6. Add 4.1 g of epoxy-silane-coupling-agent-modified cellulose nanocrystals to the liquid material in Step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain a high-strength urea-formaldehyde resin adhesive.

[0098] Comparative Example 3

[0099] A high-strength urea-formaldehyde resin adhesive, comprising 200 g of formaldehyde with a mass concentration of 46%, 184 g of urea, 5.5 g of melamine, and 6.8 g of the surface-modified organic montmorillonite powder prepared in Example 1.

[0100] A preparation method of a high-strength urea-formaldehyde glue, comprising the following steps:

[0101] S1. Put 200 g of a formaldehyde solution with a concentration of 46% into a reaction kettle, adjust the pH value to 7.8 with 30% NaOH solution, heat it to 50 °C at a heating rate of 1.5 °C / min, then add 5.5 g of melamine and 92 g of the first batch of urea, start the stirrer, and stir evenly;

[0102] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 90 °C, stop heating, and react for 35 min under this condition;

[0103] S3. Cool down to 85 °C, adjust the pH value to 4.5 with 20% NH 4 Cl solution, start heating, raise the temperature to 90 °C, and carry out polycondensation reaction until the required viscosity is 19 seconds (hot test, coating 4 cup);

[0104] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add 39.5 g of the second batch of urea, and react for 20 min;

[0105] S5. Maintain the pH value at 7.5, add 52.5 g of the third batch of urea, and keep it warm at 55 °C for 20 min;

[0106] S6. Add 6.8 g of the surface-modified organic montmorillonite powder prepared in Example 1 to the liquid material in Step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain the high-strength urea-formaldehyde glue.

[0107] Example 4

[0108] A high-strength urea-formaldehyde glue, comprising 200 g of formaldehyde with a mass concentration of 46%, 184 g of urea, 5.5 g of melamine, and 6.8 g of epoxy group silane coupling agent-modified cellulose nanocrystals.

[0109] A preparation method of a high-strength urea-formaldehyde glue, comprising the following steps:

[0110] S1. Put 200 g of a formaldehyde solution with a concentration of 46% into a reaction kettle, adjust the pH value to 7.8 with 30% NaOH solution, heat it to 50 °C at a heating rate of 1.5 °C / min, then add 5.5 g of melamine and 92 g of the first batch of urea, start the stirrer, and stir evenly;

[0111] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 90 °C, stop heating, and react for 35 min under this condition;

[0112] S3. Cool down to 85 °C, adjust the pH value to 4.5 with 20% NH 4 Cl solution, start heating, raise the temperature to 90 °C, and carry out polycondensation reaction until the required viscosity of 19 seconds (thermal measurement, cup 4);

[0113] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add the second batch of urea 39.5 g, and react for 20 min;

[0114] S5. Maintain the pH value at 7.5, add the third batch of urea 52.5 g, and keep warm at 55 °C for 20 min;

[0115] S6. Add 6.8 g of epoxy silane coupling agent modified cellulose nanocrystals to the material liquid in step S5, mix evenly, heat to 65 °C, stir and keep warm for 30 min, then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain high-strength urea-formaldehyde glue.

[0116] Comparative Example 5

[0117] A urea-formaldehyde glue, comprising 200 g of formaldehyde with a mass concentration of 46%, 184 g of urea, and 5.5 g of melamine.

[0118] A preparation method of urea-formaldehyde glue, comprising the following steps:

[0119] S1. Put 200 g of 46% formaldehyde solution into the reaction kettle, adjust the pH value to 7.8 with 30% NaOH solution, heat to 50 °C at a heating rate of 1.5 °C / min, then add 5.5 g of melamine and the first batch of urea 92 g, start the stirrer, and stir evenly;

[0120] S2. Continue to heat at a heating rate of 1.5 °C / min, raise the temperature to 90 °C, stop heating, and react for 35 min under this condition;

[0121] S3. Cool down to 85 °C, adjust the pH value to 4.5 with 20% NH 4 Cl solution, start heating, raise the temperature to 90 °C, and carry out polycondensation reaction until the required viscosity of 19 seconds (thermal measurement, cup 4);

[0122] S4. Adjust the pH value to 7.5 with 30% NaOH solution, cool down to 80 °C, add the second batch of urea 39.5 g, and react for 20 min;

[0123] S5. Maintain the pH value at 7.5, add the third batch of urea 52.5 g, keep warm at 55 °C for 20 min; then cool to 35 °C, adjust the pH value to 8.0 with 30% NaOH solution, and discharge to obtain urea-formaldehyde glue.

[0124] Performance detection:

[0125] The high-strength urea-formaldehyde glue obtained in the above examples was detected according to "Urea-formaldehyde, phenol-formaldehyde, and melamine-formaldehyde resins for wood industry adhesives" (GB / T 14732-2017). The results are shown in Table 1.

[0126] Table 1

[0127]

[0128]

[0129] After analysis, adding organically modified montmorillonite powder with surface modification and cellulose nanocrystals modified with epoxy-based silane coupling agent has no impact on the probation period and little impact on the curing time. However, it has obvious improvement on the free formaldehyde and the bonding strength after soaking, indicating that under the technical process of the present invention, adding organically modified montmorillonite powder with surface modification and cellulose nanocrystals modified with epoxy-based silane coupling agent can improve the strength of urea-formaldehyde resin and enhance the water resistance.

[0130] The formaldehyde release amount was detected according to the method of determining formaldehyde release amount - desiccator method in "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels" (GB / T 17657-2013). The results are shown in Table 2.

[0131] Table 2

[0132] Index Formaldehyde release amount (mg / L) Example 2 0.02 Example 3 0.02 Example 4 0.03 Example 5 0.03 Example 6 0.02 Comparative Example 5 0.04

[0133] Result analysis shows that adding organically modified montmorillonite powder with surface modification and cellulose nanocrystals modified with epoxy-based silane coupling agent will not have a negative impact on the formaldehyde release amount.

Claims

1. A high-strength urea-formaldehyde adhesive, characterized in that: The method comprises a formaldehyde solution having a mass concentration of 46% to 52%, urea, melamine, organic montmorillonite powder and modified cellulose nanocrystals; wherein the molar ratio of formaldehyde to urea is 0.8 to 1.05, the amount of melamine added is 2 to 10% of the quality of the urea, the amount of organic montmorillonite powder used is 1 to 2% of the quality of the urea, and the amount of modified cellulose nanocrystals used is 2 to 3% of the quality of the urea.

2. A high-strength urea-formaldehyde adhesive according to claim 1, characterized in that: The organic montmorillonite powder is montmorillonite modified by hexadecyltrimethylammonium bromide or montmorillonite modified by octadecyltrimethylammonium bromide.

3. The high-strength urea-formaldehyde adhesive according to claim 1, characterized in that: The organic montmorillonite powder is surface treated with a silane coupling agent: A. Immerse the organic montmorillonite in the silane coupling agent solution and disperse it by ultrasonic for 15-20 minutes; B. Heat to 30-40°C, keep warm for 5-6 hours, centrifuge, filter and dry to obtain surface-modified organic montmorillonite powder.

4. The high-strength urea-formaldehyde adhesive according to claim 1, characterized in that: The modified cellulose nanocrystals are cellulose nanocrystals modified by epoxy silane coupling agents.

5. The high-strength urea-formaldehyde adhesive according to claim 1, characterized in that: The epoxy silane coupling agent modified cellulose nanocrystals are prepared by immersing the cellulose nanocrystals in an epoxy silane coupling agent solution, dispersing them by ultrasonic wave for 15-20 minutes, adjusting the pH to 4-4.5, stirring for 8-10 hours, and centrifuging, filtering and drying to obtain the epoxy silane coupling agent modified cellulose nanocrystals.

6. The high-strength urea-formaldehyde adhesive according to claim 1, characterized in that: The mass ratio of the organic montmorillonite powder to the modified cellulose nanocrystals is 1.5-2.

7. A method for preparing high-strength urea-formaldehyde adhesive, characterized in that: The following steps are involved: S1. Add a 46% -52% formaldehyde solution to the reactor, adjust the pH to 7.5-8.0, heat to 50 ℃ -55 ℃, then add melamine, the first batch of urea, start the stirrer and stir evenly; S2. Continue heating to 88°C-92°C, stop heating, and react for 30min-40min under this condition; S3. Cool down to 83-87°C, adjust the pH value to 4.5-5.0, start heating, raise the temperature to 88-92°C, and perform polycondensation reaction to the required viscosity; S4. Adjust the pH value to 7.5-8.0, cool to 75°C-80°C, add the second batch of urea, and react for 20min-30min; S5. Adjust the pH value to 7.0-7.5, add the third batch of urea, and keep warm at 55°C-60°C for 10min-20min; S6. Add organic montmorillonite powder and epoxy silane coupling agent modified cellulose nanocrystals to the liquid of step S5, mix evenly, heat to 60°C-70°C, stir and keep warm for 30min-40min, then cool to 30°C-35°C, adjust the pH value to 7.5-8.0, discharge and obtain high-strength urea-formaldehyde glue.

8. The method for preparing a high-strength urea-formaldehyde adhesive according to claim 7, characterized in that: The first batch of urea is added at a molar ratio of formaldehyde to urea of ​​1.9-2.0, and the second batch of urea is added at a molar ratio of formaldehyde to the sum of the first batch of urea and the second batch of urea of ​​1.2-1.

4.

9. The method for preparing a high-strength urea-formaldehyde adhesive according to claim 7, characterized in that: During the entire preparation process, the temperature was increased at a rate of 1.5°C / min.

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