Moisture scavengers

Certain coating systems such as polyurethane or coatings contai­ning metallic pigments such as aluminum or zinc are very sensitive to moisture. Moisture can get into the coatings by a variety of routes, such as by being dissolved in solvents or absorbed onto the surface of pigments and extenders. To avoid the potential problems due to moisture, generally, properly dried and moisture-free raw materials must be used. Despite this, it is always advisable to use moisture scavengers to remove any residual traces of water or to take care of any possible ingress of moisture during and after manufacturing of paint.

In polyurethane systems, isocyanate curatives are reactive to moisture. The reaction between isocyanate and water produces car­bon dioxide and an amine, which will cause gassing in the can or an increase in viscosity, which ultimately leads to gelation. To overcome this problem, aluminum silicate-based molecular sieves and finely divided silica gel are used to react with water. Some other options such as monomeric isocyanates, oxazolidines and triethyl orthoformate have also been used for this purpose. In coa­ting systems containing finely divided metallic zinc and aluminum pigments, these metals react with water to produce hydrogen gas. Moisture scavengers such as molecular sieves and silica gels are used to prevent the problem.

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