Effect of Melamine-Formaldehyde Structure on Cure Response of Thermoset Coatings

By Loren W. Hill, Shih-Bin Lee

Reactivity of etherified melamine-formaldehyde (MF) resins with polyol binders depends on the pattern of substitution on off-ring nitrogen atoms. These sites can be doubly substituted [–N(CH₂OCH₃)₂] or singly substituted [–N(H)(CH₂OCH₃)]. The (N–H) portion of singly substituted sites is referred to as an imino group. The main finding of this work is that acid catalyzed cure response is improved greatly when the imino content is reduced to very low levels.

The effect of degree of substitution on MF basicity and the site of protonation, ring, or side chain are key elements in the explanation of this cure response behavior. Properties were determined for films cured for 30 min at various temperatures with constant catalyst level and type (0.5 phr para-toluenesulfonic acid).

Resins of varying imino content were used. With the MF resin of lowest imino content, extensive cure of acrylic and polyester polyols was observed at cure temperatures as low as 180°F (82°C). Paint tests of hardness and solvent resistance were useful for determining cure response.

Dynamic mechanical analysis (DMA) on free films also indicated cure response differences. Storage modulus values from DMA were used to calculate crosslink densities of the thermoset films.