Лако-красочные материалы — производство

Waterbased binders are increasingly used in low VOC coatings, and like all other binders, demand for waterborne alkyds is continuously growing. Waterborne alkyd resins can be broadly categorized into two groups: water-reducible alkyds and alkyd emulsions.

2.3.8.1 Water-reducible alkyds

Water-reducible alkyds, in general, are prepared by formulating alkyds with an acid value in the range of 40 to 60 mg KOH/g. The free carboxylic acid groups are then partially or fully neutralized with a fugitive base such as ammonia or a volatile organic amine to provide enough polarity to make them water-reducible. Such sys­tems typically require 5 to 20 % water-miscible co-solvents such as glycol ethers or alcohols. In initial attempts, they were prepared by halting the conventional polymerization of alkyd at a higher acid value; this often resulted in lower MW. The improved two-step pro­cess can produce higher MW polymers, which involves synthesis of a hydroxyl-rich alkyd by the conventional technique followed by reaction with phthalic anhydride or, preferably, trimellitic anhydride under milder conditions than direct esterification to give an acid func­tional polymer. Short to medium oil length alkyds are more common for this type of mixture. One of the important concerns with water — reducible alkyds is their poor hydrolytic stability on storage. A novel core shell type of polymer design has also been adopted to reduce their tendency for hydrolysis, in which acrylic modified alkyds are designed with carboxylic groups on the acrylic chain. Once a wet film is applied, water, solvent, and neutralizing amine evaporate and the film cross-links by autoxidation. Since there are a fairly large number of residual carboxylic acid groups left in the cross-linked binder, the water resistance, and particularly the alkali resistance, of the films is reduced.