Additives for Polyurethane Systems

Water reacting with isocyanate produces carbon dioxide. In a coating that gas either forms bubbles in the film or pinholes as it escapes through a partially cured surface. Unlike entrained air, this foam is generated continuously during cure, so a defoamer that simply deaerates the wet paint is not addressing the mechanism.

The correct approach is eliminating the water source: dry solvents, dry fillers, moisture-controlled additives and proper container discipline. Defoamer selection then handles genuinely entrained air. High-build PU systems benefit from polyether siloxane copolymers that persist long enough to keep working as viscosity climbs during cure.

Viscosity in a curing PU system rises continuously, which makes the sag-versus-levelling window narrower than in a physically drying coating. Rheology additives must provide enough low-shear structure to prevent sagging in the first minutes while still allowing levelling before the viscosity becomes too high for flow.

For high-build applications this generally means associative thickeners with a fairly sharp shear-thinning profile rather than strong permanent structure. Test at the real film thickness on a vertical panel. Sag behaviour does not scale linearly from a thin drawdown.

PU systems are frequently specified precisely because the substrate is difficult, so adhesion promoters are often part of the package. Amino silanes are effective but react with isocyanate, so they belong on the polyol side and their consumption of isocyanate should be accounted for in mixing ratio.

Epoxy silanes are generally the safer choice where the additive must survive alongside isocyanate. On polyolefin substrates, chlorinated polyolefin promoters remain the practical answer, applied as a primer layer rather than blended into the topcoat.