UREA FORMALDEHYDE RESIN
Reactions producing Amino resins:
The first stage is addition of Paraformaldehyde and Urea to produce mono and dimethylol urea. This reaction takes place under mildly alkaline or neutral conditions.
The second stage continues when the alkali conditions are changed into acidic conditions. A branched highly polar polymer is produced. This polymer is insoluble in solvents and incompatible with other resins, therefore a third stage is required in which an alcohol, commonly Butanol, reacts with the resin. This stage is known as etherification or specially butylation and in fact proceeds alongside the second stage and in direct competion with it.
The nature of the final etherified UF resin depends on the proportions of each of the starting raw materials, the amount and type of acid, and which particular alcohol is used in the etherification stage.
When the desired molecular weight and degree of etherification has been obtained (by measuring the water evolved), the reaction is stopped by neutralizing the acid and cooling the mixture. Vacuum distillation removes unreacted materials although the alcohol is often returned to be used as a solvent. Hydrocarbon solven Xylene is being used as a carrier.
The reactivity of an amino resin can, to a certain extent, be controlled by the ratio of the main functional groups present on the resin. The more formaldehyde used in the polymerization, the less ‘N-H’ function will be present. The more alcohol used the more ether groups that will be present. In addition, the choice of alcohol used is very important. The lower the molecular weight of the alcohol, the more reactive is the resin.
The N-H group is polar, as are the methylol groups. They produce hydrogen bonding which increases viscosity and decreases compatibility with other less polar resins. By increasing the formaldehyde content we reduced the amount of N-H function and hence, viscosity decreases and compatibility increases.