Melamine resin
Amino resins (also: amino or amide resin ) are relatively low molecular weight thermosetting resins obtained by polycondensation of carbonyl compounds (often formaldehyde) and compounds having NH groups, such as urea ( urea resins, UF), melamine (melamine resins, MF) or dicyandiamide ( dicyandiamide, DD ) are obtained. The resins can be used (as adhesive resins for example ) or as a spray-dried powder as a molding composition as aqueous solutions. By the addition of hardeners, the low molecular weight resins are cross-linked and thus converted into insoluble, infusible thermoset. The term aminoplast is commonly used for both the precursor as well as yet to be cured by the final cured product.
Production
Generally aminoplasts obtained by polycondensation of formaldehyde with compounds containing two or more amino groups (such as urea, thiourea, melamine, cyanamide, diaminohexane ) was prepared. As an example, the synthesis of urea -formaldehyde resin:
This reaction takes place in a first step by the addition of urea to give N - hydroxymethyl groups, and then a chain growth on polycondensation with elimination of water. This reaction is usually carried out in basic solution, because OH - ions are required as a catalyst and form uncrosslinked pre-condensates. The precondensates are stable for several months at room temperature.
During curing under acidic conditions further polycondensation of linear chain molecules built up of the resin leads among themselves to shortcuts. The more formaldehyde was added, the more cross-connections there.
Use
The majority of the amino resins is used as an adhesive in the wood-based industry (eg in the chipboard as " Spanplattenleim " ) and as an insulating material. The largest share by far form the urea -formaldehyde resins ( urea resin, UF resins ), due to their widespread use in the wood -based panel industry. They are used in household items, body parts, and packaging materials. Another use is melamine resin in the manufacture of table tennis. Weatherproof table tennis tables are now being made for the most part with melamine resin surfaces.
The property to cure with gaseous formaldehyde, also allows the use as binders for foundry sand in sand casting.