Method for catalyzing melamino-formaldehyde resin methylation reaction by solid acid

Abstract

The invention discloses a method for catalyzing melamino-formaldehyde resin methylation reaction by solid acid. According to the invention, epoxy silane coupling agent grafted modified nanocellulose is introduced into sulfonated styrene-divinyl benzene cross-linked copolymer microspheres for in-situ crystal cross-linking, so that the problems that the traditional sulfonated polystyrene resin is poor in mechanical strength and solution solubility resistance, low in cycle index, easy to swell and fall off and the like are effectively solved; the prepared solid acid resin has the characteristics of high reaction activity, high proton transfer rate, high mechanical strength, high cyclicity and easiness in solid-liquid separation, and by utilizing the solid acid resin to catalyze the etherification reaction of the melamino-formaldehyde resin, the reaction temperature and the production energy consumption are reduced, the reaction time is shortened, the product quality and the production efficiency are improved, and the production cost is reduced. The problems of environmental protection and energy consumption caused by the generation of a large amount of VOCs in the etherification reaction of the traditional liquid catalyst for synthesizing the high-etherified melamine formaldehyde resin and the by-product inorganic salt in the subsequent process are solved.

Description

Technical Field

[0001] This invention belongs to the field of solid acid catalysis technology and relates to a method for the methyl etherification reaction of melamine-formaldehyde resin catalyzed by solid acid. Background Technology

[0002] Amino resins, as important polymer materials, are widely used in coatings, adhesives, and processing materials. Their unique chemical and physical properties give them a significant position in the industrial field. Common amino resins include melamine-based resins, urea resins, and aniline resins. Among them, hexamethylolamine (HMA) resin has attracted much attention due to its excellent heat and chemical resistance, as well as its superior coverage and adhesion. In recent years, research on amino resins has received increasing attention, especially in green chemistry and low-VOC coatings. Many studies have attempted to reduce the environmental impact of the production process by optimizing the synthesis process and catalyst selection. The synthesis of hexamethylolamine resin mainly involves hydroxylation and etherification reactions. By controlling the reaction conditions, different degrees of crosslinking and polymerization can be achieved, improving the overall performance of the material. Traditional hexamethylolamine resin synthesis processes often use liquid acid catalysts. This process is generally simple, but the neutralization and separation of the catalyst are complex, leading to increasingly prominent waste problems. Therefore, it is of great significance to seek a new catalyst to replace the traditional liquid catalyst to reduce VOC emissions and improve the overall performance of the resin.

[0003] Solid acid catalysts are catalysts that exhibit acidic properties in chemical reactions and exist in solid form. These catalysts are typically composed of acidic active centers that can "donate" protons or "accept" paired electrons, thereby "combining" with reactants in the reaction system, altering the reaction rate and selectivity, and promoting the chemical reaction to occur. Solid acid catalysts have high reactivity and are recyclable; after deactivation, they can be reactivated to regain catalytic activity.

[0004] Currently, the most common solid acidic resins on the market include sulfonated polystyrene resins, perfluorosulfonic acid resins, and inorganic carrier-supported resins. Among them, sulfonated polystyrene resins are widely used due to their low price, high acid strength, and strong catalytic activity. Sulfonated polystyrene resins prepared by traditional methods exhibit strong swelling properties, which easily leads to a decrease in mechanical strength, and the sulfonic acid groups are prone to loss. Therefore, preparing a sulfonated polystyrene resin with high mechanical strength and low sulfonic acid group loss rate is of great significance for this experiment. Summary of the Invention

[0005] The main objective of this invention is to provide a solid acidic resin to improve the mechanical strength of traditional sulfonated styrene resin and reduce the loss of sulfonic acid groups. This resin is used as a catalyst to catalyze the etherification reaction of melamine-formaldehyde resin, thereby solving the environmental and energy consumption problems of generating a large amount of VOCs and subsequent inorganic salts as byproducts in the etherification reaction of traditional liquid catalysts for synthesizing highly etherified melamine-formaldehyde resin.

[0006] The solid acid resin is prepared by: sulfonating styrene-divinylbenzene cross-linked copolymer microspheres using a gas sulfonation method; adding the sulfonated microspheres to an organic solvent and stirring at 50-85℃ for 2-5 hours to allow the microspheres to fully swell; then adjusting the temperature to 40-70℃ and adding modified nanocellulose crystal aerogel powder grafted with an epoxy silane coupling agent and a cross-linking accelerator, wherein the mass ratio of the sulfonated microspheres to the aerogel powder is 1-1.5:1, and the amount of cross-linking accelerator added is 0.1-1% of the total mass of the system; raising the system temperature to 80-100℃ for in-situ cross-linking for 6-12 hours; after the reaction, washing with an alcohol solvent and performing solvent gradient replacement, and finally freeze-drying to obtain the solid acid resin.

[0007] The organic solvent is dimethyl sulfoxide and / or tetrahydrofuran.

[0008] The crosslinking accelerator is citric acid and / or p-toluenesulfonic acid.

[0009] The alcohol solvent is anhydrous ethanol and tert-butanol.

[0010] The preparation method of the styrene-divinylbenzene crosslinked copolymer microspheres is as follows: A 3-10% (w / w) polyvinyl alcohol aqueous solution is heated to 50-80°C as a dispersion medium; a mixture of styrene and divinylbenzene at a mass ratio of 7-20:1 is prepared, and then 0.5-2% (w / w) of benzoyl peroxide (based on the mass of styrene and divinylbenzene) is added and shaken until completely dissolved; the styrene-divinylbenzene mixture is dropwise added to the polyvinyl alcohol aqueous solution at a volume ratio of 1:3-5, and the mixture is stirred at 70-90°C for 3-5 hours; after the microspheres have solidified, the reaction continues for 1-2 hours before stirring is stopped; finally, the microspheres are washed with hot water and dried.

[0011] The gas sulfonation method is as follows: styrene-divinylbenzene cross-linked copolymer microspheres are placed in an SO3 atmosphere and heated to carry out a sulfonation reaction. After the reaction is complete, the microspheres are immersed in an aqueous glycerol solution with a concentration of not less than 10% and treated at 60-80℃ for 2-4 hours. The microspheres are then removed and dried.

[0012] Nitrogen or an inert gas is added to the SO3 atmosphere.

[0013] The preparation method of the modified nanocellulose crystal aerogel grafted with epoxy silane coupling agent is as follows: a cellulose crystal suspension with a mass fraction of 2-8% is prepared and ultrasonically dispersed, and then added dropwise to a 2.0-5.0 wt% CaCl solution to obtain spherical cellulose crystal hydrogel. The volume ratio of the suspension to the CaCl solution is 1:2-4. After filtration, the hydrogel is immersed in an epoxy silane coupling agent solution with a mass content of 3-10%, heated at 60-80℃ for 1-2 hours, and the product is washed with anhydrous ethanol and then freeze-dried to obtain the modified nanocellulose crystal aerogel powder grafted with epoxy silane coupling agent.

[0014] The solid acid resin prepared above was used to catalyze the methyl etherification reaction of melamine-formaldehyde resin.

[0015] The specific operation of the solid acid resin-catalyzed methyl etherification reaction is as follows: Hexamethylol melamine, methanol, and solid acid resin are mixed, with a molar ratio of hexamethylol melamine to methanol of 1:6-15, and a mass ratio of solid acid resin to the sum of hexamethylol melamine and methanol of 1:20-30; the mixture is stirred at 60-80℃ for 2-5 hours, then filtered and separated. The solid acid resin is washed, dried, and recycled for reuse. The pH of the product solution is adjusted to 8-9 using an alkaline solution. Finally, after heating to remove water and alcohol, etherified melamine-formaldehyde resin is obtained.

[0016] The preparation method of the hexamethylol melamine is as follows: the pH value of the formaldehyde aqueous solution is adjusted to 8-9 with alkaline solution at 60-75℃, and melamine is added in batches. After reacting for 1-2 hours, the mixture is filtered, washed, and dried. The molar ratio of formaldehyde to melamine used is 1:8-10.

[0017] A method for preparing a highly etherified melamine-formaldehyde resin is as follows: the etherified melamine-formaldehyde resin obtained above, methanol and solid acid resin are mixed, stirred and reacted at 60-80℃ for 2-5 hours, and then filtered and separated. The solid acid resin is washed, dried and recycled for reuse. The pH of the product solution is adjusted to 8-9 with alkaline solution. Finally, the highly etherified melamine-formaldehyde resin is obtained by heating to remove water and alcohol.

[0018] The molar ratio of the etherified melamine-formaldehyde resin to methanol is 1:4-10, and the mass ratio of the solid acid resin and the etherified melamine-formaldehyde resin to methanol is 1:20-30.

[0019] Nanocellulose crystals (NCC) can significantly improve the mechanical strength and stability of materials in humid environments, but the following problems exist when used in composite sulfonated polystyrene resins: a severe mismatch between hydrophilicity and hydrophobicity; NCC has a large number of hydroxyl groups on its surface, exhibiting strong hydrophilicity, while sulfonic acid is hydrophobic, and forced introduction can lead to separation, causing NCC to spontaneously aggregate; weak interfacial bonding; NCC and sulfonated polystyrene resin spheres are bonded by hydrogen bonds, which have low bond energy and are easily detached under external forces; and a conflict between functionality and mechanical properties; high NCC content can easily block the proton transport channels of sulfonated polystyrene resin, leading to a decrease in functionality. To address the issue that high NCC content can block proton transport channels, this invention involves heat-treating pure NCC to form ester bonds between the surface hydroxyl groups and sulfonic acid groups, creating a cross-linked network. This network is then freeze-induced to form an aerogel structure, whose porous structure effectively transports protons. Furthermore, to address the weak interfacial bonding and hydrophobicity issues, an epoxy silane coupling agent is used to construct a Si-OC covalent bond structure to modify the NCC, resolving the problems of weak interfacial bonding and significant hydrophobicity differences between NCC and sulfonated polystyrene resin.

[0020] This invention introduces an epoxy silane coupling agent-grafted nanocellulose into copolymer microspheres for in-situ crystal crosslinking, effectively solving the problems of poor mechanical strength and solution resistance, low cycle count, and easy swelling and shedding of traditional sulfonated polystyrene resin. The resulting solid acid resin has the characteristics of high reactivity and proton transport rate, high mechanical strength, high cycleability, and easy solid-liquid separation, which is beneficial to reducing reaction temperature and production energy consumption, shortening reaction time, and improving product quality and production efficiency. Attached Figure Description

[0021] Figure 1 Infrared spectrum of hexahydroxymethyl melamine prepared in Example 1.

[0022] Figure 2 Infrared spectrum of the solid acid resin catalytic etherification reaction product prepared in Example 1.

[0023] Figure 3 Comparison of solid acid catalysis experimental results between Example 1 and Comparative Example 1. Detailed Implementation Example 1

[0024] A. Preparation of styrene-divinylbenzene crosslinked copolymer microspheres: Weigh 16g of polyvinyl alcohol and dissolve it in 384ml of deionized water, then heat to 60℃ for later use; prepare a mixture of 83.7g of styrene and 7.3g of divinylbenzene, add 0.91g of benzoyl peroxide, and stir until completely dissolved; add the monomer mixture solution dropwise to the polyvinyl alcohol solution at 300rpm, and after the addition is complete, heat the system to 80℃ and react for 4h; after the polymer microspheres are formed, continue stirring for 1.5h to observe the curing effect; after stopping heating and stirring, allow to cool naturally to room temperature, filter, wash, and dry to obtain white microspheres.

[0025] B. Gas sulfonation: The dried copolymer microspheres were loaded into a jacketed glass reactor. SO3 diluted to 4% with dry N2 was introduced into the reactor at a flow rate of 30 ml / min and reacted at 30°C for 1.5 h. The temperature was then raised to 45°C and the reaction was continued for 2 h. After the reaction was completed, N2 was continuously introduced to purge the microspheres. The sulfonated microspheres were then immersed in a 10% glycerol aqueous solution and treated at 80°C for 2 h. The microspheres were then removed and dried.

[0026] C. Modified nanocellulose crystal aerogel grafted with epoxy silane coupling agent: 100 ml of cellulose crystal suspension with a mass fraction of 5% was prepared and ultrasonically dispersed, and then added dropwise to 200 g of CaCl solution with a mass concentration of 2.5% and matured for 2 h to obtain spherical cellulose crystal hydrogel microspheres; after filtration, the hydrogel was immersed in 100 g of epoxy silane coupling agent solution with a mass concentration of 6% and heated at 70 °C for 2 h. After the reaction was completed, the product was washed with anhydrous ethanol and then frozen to obtain modified nanocellulose crystal aerogel powder grafted with epoxy silane coupling agent.

[0027] D. Solid acid resin: 12g of sulfonated microspheres were added to 500ml of dimethyl sulfoxide and stirred at 80℃ for 3h to allow the microspheres to fully swell. After the system was cooled to 50℃, 10g of modified nanocellulose crystal aerogel powder grafted with epoxy silane coupling agent and 1.4g of p-toluenesulfonic acid were added. The system temperature was raised to 80℃ for in-situ crosslinking for 12h. After the reaction was completed, the solvent and catalyst were removed by washing three times with ethanol. Gradient replacement was performed using 50%, 75%, and 100% concentrations of tert-butanol. Finally, the solid acid resin was obtained by freeze drying.

[0028] E. Preparation of hexamethylolmelamine: Weigh 250.0 g of a 37% formaldehyde aqueous solution into a three-necked flask and incubate in a water bath at 65°C for 30 min. Adjust the pH of the formaldehyde solution to 8-9 using 1.0 mol / L NaOH solution. Weigh 40.63 g of melamine and add it to the pH-adjusted formaldehyde solution in four batches. After the melamine is completely dissolved, react at 65°C for 1.5 h. After the reaction is complete, filter using a vacuum water pump. Dry the resulting solid product to remove formaldehyde and water, obtaining hexamethylolmelamine.

[0029] F. Primary etherification reaction: Weigh 61.2 g of the prepared hexamethylol melamine into a three-necked flask, add 76.90 g of methanol and 4.6 g of the solid acid resin from step D as a catalyst, and react at 60 °C with a stirring speed of 300 r / min for 3 h. After the reaction, filter and separate; wash and dry the solid acid resin for reuse; adjust the pH of the product solution to 8-9 with 1.0 mol / L NaOH solution, and heat in an 80 °C oven to remove water and alcohol to obtain etherified melamine-formaldehyde resin. Example 2

[0030] A. Preparation of hexamethylolmelamine: Weigh 250.0 g of 37% formaldehyde solution into a three-necked flask and incubate in a water bath at 65°C for 30 min. Adjust the pH of the formaldehyde solution to 8-9 using 1.0 mol / L NaOH solution. Weigh 40.63 g of melamine and add it to the pH-adjusted formaldehyde solution in four batches. After the melamine is completely dissolved, raise the temperature to 70°C and react for 1.5 h. After the reaction is complete, filter using a vacuum water pump. Dry the resulting solid product to remove formaldehyde and water, obtaining hexamethylolmelamine.

[0031] B. Primary etherification reaction: Weigh 61.2 g of the prepared hexamethylolmelamine into a three-necked flask, add 76.90 g of methanol and 4.6 g of the solid acid resin prepared in Example 1 as a catalyst, and react at 65°C with a stirring speed of 300 r / min for 3 h. After the reaction, filter and separate, wash and dry the solid acid resin for reuse; adjust the pH of the product solution to 8-9 using 1.0 mol / L NaOH solution, and place the reaction product solution in an oven to remove water and alcohol. Example 3

[0032] A. Preparation of hexamethylolmelamine: Weigh 250.0 g of 37% formaldehyde solution into a three-necked flask and incubate in a water bath at 65°C for 30 min. Adjust the pH of the formaldehyde solution to 8-9 using 1.0 mol / L NaOH solution. Weigh 40.63 g of melamine and add it to the pH-adjusted formaldehyde solution in four batches. After the melamine is completely dissolved, raise the temperature to 70°C and react for 1.5 h. After the reaction is complete, filter using a vacuum water pump. Dry the resulting solid product to remove formaldehyde and water, obtaining hexamethylolmelamine.

[0033] B. Primary etherification reaction: Weigh 61.2 g of the prepared hexamethylolmelamine into a three-necked flask, add 76.90 g of methanol and 4.6 g of the solid acid resin prepared in Example 1 as a catalyst, and react at 70°C with a stirring speed of 300 r / min for 3 h. After the reaction, filter and separate, wash and dry the solid acid resin for reuse; adjust the pH of the product solution to 8-9 using 1.0 mol / L NaOH solution, and place the reaction product solution in an oven to remove water and alcohol. Example 4

[0034] A. Preparation of hexamethylolmelamine: Weigh 250.0 g of 37% formaldehyde solution into a three-necked flask and incubate in a water bath at 65°C for 30 min. Adjust the pH of the formaldehyde solution to 8-9 using 1.0 mol / L NaOH solution. Weigh 40.63 g of melamine and add it to the pH-adjusted formaldehyde solution in four batches. After the melamine is completely dissolved, raise the temperature to 70°C and react for 1.5 h. After the reaction is complete, filter using a vacuum water pump. Dry the resulting solid product to remove formaldehyde and water, obtaining hexamethylolmelamine.

[0035] B. Primary etherification reaction: Weigh 61.2 g of the prepared hexamethylolmelamine into a three-necked flask, add 76.90 g of methanol solution and 4.6 g of the solid acid resin prepared in Example 1 as a catalyst, and react at 65°C with a stirring speed of 300 r / min for 3 h. After the reaction, filter and separate, wash and dry the solid acid resin for reuse; adjust the pH of the product solution to 8-9 using 1.0 mol / L NaOH solution, and place the reaction product solution in an oven to remove water and alcohol.

[0036] C. Secondary etherification reaction: Weigh 72.4 g of the product obtained from the primary etherification reaction in step B into a three-necked flask, add 25.6 g of methanol and 3.2 g of the solid acid resin prepared in Example 1 as a catalyst, and react at 65°C with a stirring speed of 300 r / min for 3 h. After the reaction, filter and separate, wash and dry the solid acid resin for reuse; adjust the pH of the product solution to 8-9 using 1.0 mol / L NaOH solution, and place the reaction product solution in an oven to remove water and alcohol to obtain highly etherified melamine-formaldehyde resin.

[0037] Table 1. Free formaldehyde content and hydroxylation amount of the products from the primary etherification in Examples 1-3 and the product from the secondary etherification in Example 4. Example Free formaldehyde content / % Hydroxymethyl amount Tolerance 1 0.069 3.7 2.8 2 0.084 4.2 3.2 3 0.049 4.2 3.0 4 0.052 5.6 5 or more

Claims

1. A method for preparing a solid acid resin, characterized in that, The specific operation of the preparation method is as follows: styrene-divinylbenzene crosslinked copolymer microspheres are sulfonated using a gas sulfonation method; the sulfonated microspheres are added to an organic solvent and stirred at 50-85℃ for 2-5 hours to allow the microspheres to fully swell; then the temperature is adjusted to 40-70℃ and modified nanocellulose crystal aerogel powder grafted with epoxy silane coupling agent and a crosslinking accelerator are added, wherein the mass ratio of sulfonated microspheres to aerogel powder is 1-1.5:1, and the amount of crosslinking accelerator added is 0.1-1% of the total mass of the system; the system temperature is raised to 80-100℃ for in-situ crosslinking for 6-12 hours; after the reaction is completed, the mixture is washed with an alcohol solvent and a solvent gradient replacement is performed; finally, the mixture is freeze-dried to obtain a solid acid resin.

2. The preparation method according to claim 1, characterized in that, The organic solvent is dimethyl sulfoxide and / or tetrahydrofuran; the crosslinking accelerator is citric acid and / or p-toluenesulfonic acid; and the alcohol solvent is anhydrous ethanol and / or tert-butanol.

3. The preparation method according to claim 1, characterized in that, The preparation method of the styrene-divinylbenzene crosslinked copolymer microspheres is as follows: A 3-10% (w / w) polyvinyl alcohol aqueous solution is heated to 50-80°C as a dispersion medium; a mixture of styrene and divinylbenzene at a mass ratio of 7-20:1 is prepared, and then 0.5-2% (w / w) of benzoyl peroxide (based on the mass of styrene and divinylbenzene) is added and shaken until completely dissolved; the styrene-divinylbenzene mixture is dropwise added to the polyvinyl alcohol aqueous solution at a volume ratio of 1:3-5, and the mixture is stirred at 70-90°C for 3-5 hours; after the microspheres have solidified, the reaction continues for 1-2 hours before stirring is stopped; finally, the microspheres are washed with hot water and dried.

4. The preparation method according to claim 1, characterized in that, The gas sulfonation method is as follows: styrene-divinylbenzene cross-linked copolymer microspheres are placed in an SO3 atmosphere and heated to carry out a sulfonation reaction. After the reaction is complete, the microspheres are immersed in an aqueous glycerol solution with a concentration of not less than 10% and treated at 60-80℃ for 2-4 hours. The microspheres are then removed and dried.

5. The preparation method according to claim 4, characterized in that, Nitrogen or an inert gas is added to the SO3 atmosphere.

6. The preparation method according to claim 1, characterized in that, The preparation method of the modified nanocellulose crystal aerogel grafted with epoxy silane coupling agent is as follows: a cellulose crystal suspension with a mass fraction of 2-8% is prepared and ultrasonically dispersed, and then added dropwise to a 2.0-5.0 wt% CaCl solution to obtain spherical cellulose crystal hydrogel. The volume ratio of the suspension to the CaCl solution is 1:2-4. After filtration, the hydrogel is immersed in an epoxy silane coupling agent solution with a mass content of 3-10%, heated at 60-80℃ for 1-2 hours, and the product is washed with anhydrous ethanol and then freeze-dried to obtain the modified nanocellulose crystal aerogel powder grafted with epoxy silane coupling agent.

7. The application of the solid acid resin prepared by the method according to any one of claims 1-6 in the catalytic methyl etherification reaction of melamine-formaldehyde resin.

8. The application according to claim 7, characterized in that, The specific operation of the solid acid resin-catalyzed methyl etherification reaction is as follows: Hexamethylol melamine, methanol, and solid acid resin are mixed, with a molar ratio of hexamethylol melamine to methanol of 1:6-15, and a mass ratio of solid acid resin to the sum of hexamethylol melamine and methanol of 1:20-30; the mixture is stirred at 60-80℃ for 2-5 hours, then filtered and separated. The solid acid resin is washed, dried, and recycled for reuse. The pH of the product solution is adjusted to 8-9 using an alkaline solution. Finally, after heating to remove water and alcohol, etherified melamine-formaldehyde resin is obtained.

9. The application according to claim 8, characterized in that, The preparation method of the hexamethylol melamine is as follows: the pH value of the formaldehyde aqueous solution is adjusted to 8-9 with alkaline solution at 60-75℃, and melamine is added in batches. After reacting for 1-2 hours, the mixture is filtered, washed, and dried. The molar ratio of formaldehyde to melamine used is 1:8-10.

10. A method for preparing highly etherified melamine-formaldehyde resin, characterized in that, The specific operation of the preparation method is as follows: the etherified melamine-formaldehyde resin obtained in claim 8, methanol and solid acid resin are mixed, the molar ratio of etherified melamine-formaldehyde resin to methanol is 1:4-10, and the mass ratio of solid acid resin and the sum of the mass of etherified melamine-formaldehyde resin and methanol is 1:20-30; after stirring and reacting at 60-80℃ for 2-5 hours, the mixture is filtered and separated, and the solid acid resin is washed, dried and recycled for reuse; the pH of the product solution is adjusted to 8-9 using alkaline solution; finally, after heating to remove water and alcohol, highly etherified melamine-formaldehyde resin is obtained.

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