Modified melamine formaldehyde resin is a polymer obtained by the reaction of melamine and formaldehyde, also known as melamine formaldehyde resin or melamine resin.
Physical properties: The cured melamine formaldehyde resin is colorless and transparent, stable in boiling water, and can even be used at 150°C. It has self-extinguishing properties, arc properties, and good mechanical properties.
Chemical properties: The raw materials are melamine and 37% formaldehyde aqueous solution. The molar ratio of formaldehyde to melamine is 2~3. Different numbers of N-hydroxymethyl substituents are generated in one step, and then further condensed into linear resins. The reaction conditions are different, and the molecular weight of the product is different. It can range from water-soluble to hardly soluble in water, or even insoluble and infusible solid. The pH value has a great influence on the reaction rate. The resin solution prepared above is not easy to store, and spray drying is commonly used in industry to produce a powdery solid. Melamine resin does not cure at room temperature. It is generally thermally cured at 130-150°C. Adding a small amount of acid to catalyze the curing rate can increase the curing rate.
Application in the leather industry: In the leather industry, melamine resin is a commonly used pre-tanning, re-tanning, and filling resin, among which trimethyl melamine resin is a widely used amino resin. Amino resin has a tanning mechanism like aldehyde tanning: the prepolymer penetrates the skin, and as the pH value drops, the prepolymer automatically condenses into molecules of a certain size in the skin. Through the condensation of the active -NHCH2OH in the molecule with the amino groups on the collagen peptide chain, a covalent cross-linking system is formed to achieve the purpose of tannins.
Due to the high reactivity of methylal, trimethyl melamine molecules are prone to polycondensation reactions, producing non-linear cross-linked molecules and losing tanning. Currently, the most widely used and obvious methods are to appropriately block the hydroxymethyl group to reduce its activity, prevent further condensation between molecules, or introduce hydrophilic groups into the molecular structure to increase water solubility.
At present, melamine resin tanning agents are mainly prepared at home and abroad by condensation of melamine, dicyandiamide alone or mixed with formaldehyde, and they are appropriately modified as needed.
Applications in paper industry:
Because melamine formaldehyde resin contains hydroxymethyl groups, it can form an etherified structure between fiber bundles. This cross-linking between different molecules produces water, making the paper moist. Therefore, it is mainly used as wet reinforcing agent and water agent in the paper industry.
Anionic modified melamine formaldehyde resin
It is mainly sulfonated melamine resin. In comparison, sulfamate-modified resin is better and widely used. This type of product can be cured under acidic, neutral, or alkaline conditions, and can be added to the slurry or used in coating. The problem is that when the modifier reacts with formaldehyde, precipitation and crystallization occur, and it is difficult to filter out.
Cationic modified melamine formaldehyde resin
Melamine resin modified with cationic reagents such as triethanolamine, dimethylamine, etc. has positive electricity, can be quickly absorbed by fibers, and can be cured at a higher pH value. It avoids the adverse effects of ordinary melamine-formaldehyde resin on the whiteness and durability of paper. At present, the main research in this field at home and abroad is to use small molecular alcohols, such as methanol, butanol, etc., as etherifying agents and a small amount of cationic modifiers under the catalysis of p-toluene sulfonic acid to block their active groups and improve product stability. .
The main methods to reduce the content of free formaldehyde are: using organic matter with oxidizing ability, such as formic acid and hydrogen peroxide, which can oxidize formaldehyde into CO2 and water. During the resin preparation process, formaldehyde neutralizers such as borax, propylene glycol, and sodium bisulfite are added, and vacuum distillation is used.
Applications in the coating industry
Melamine formaldehyde resin modified by etherification of low-alcohol substances (such as methanol, butanol, isobutane) is widely used in the coating industry and is mainly used as a cross-linking agent for water-based coatings. As a raw material for topcoats and primers, it can be formulated into amino resins with alkyd resins, polyester resins, acrylic resins, epoxy resins, etc., and is widely used in automobiles, furniture, home appliances and other industries.
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