Лако-красочные материалы — производство

A. Condensation Reactions

formaldehyde addition to melamine occurs more easily and completely than does addition to urea. The amino group in melamine accepts easily up to two molecules of formalde­hyde. Thus complete methylolation of melamine is possible, which is not the case with urea [1]. Up to six molecules of formaldehyde can be attached to one molecule of melamine. The methylolation step leads to a series of methylol compounds with two to six methylol groups. Because melamine is less soluble than urea in water, the hydrophilic stage proceeds more rapidly in MF resin formation. Therefore, hydrophobic intermediates of the MF condensation appear early in the reaction. Another important difference is that MF con­densation to give resins, and their curing, can occur not only under acid conditions, but also under neutral or even slightly alkaline conditions. The mechanism of the further reaction of methylol melamines to form hydrophobic intermediates is the same as for UF resins, with splitting off of water and formaldehyde. Methylene and ether bridges are formed and the molecular size of the resin increases rapidly. These intermediate con­densation products constitute the large bulk of the commercial MF resins. The final curing process transforms the intermediates to the desired MF insoluble and infusible resins through the reaction of amino and methylol groups which are still available for reaction.

A simplified schematic formula of cured MF resins has been given by Koehler [2] and Frey [3]. They emphasize the presence of many ether bridges besides unreacted methylol groups and methylene bridges. This is because in curing MF resins at temperatures up to 100°C, no substantial amounts of formaldehyde are liberated. Only small quantities are liberated during curing up to 150° C. However, UF resins curing under the same conditions liberate a great deal of formaldehyde.

At the condensation stage attention must be paid to the formation of hydrolysis products of the melamine before preparation starts. The hydrolysis products of melamine are obtained when the amino groups of melamine are gradually replaced by hydroxyl groups. Complete hydrolysis produces cyanuric acid (Formula 1).

Formula 1

Ammeline and ammelide can be regarded as partial amides of cyanuric acid. They are acid and have no use in resin production. They are very undesirable by-products of the manufacture of melamine because of their catalytic effect in the subsequent MF resin production, due to their acidic nature. If present, both must be removed from crude melamine by an alkali wash and/or crystallization of the crude melamine.