Mechanical Properties of Acrylic-Melamine Resin Cured Films for High Solids Coatings

Abstract

A series of different molecular weight, hydroxyl contained acrylic oligomers having same monomer compositions were cross-linked with monomeric melamine resin. The study of the cured films included the effect of oligomer molecular weight on physical properties as function of baking temperature and UV-condensation cycle weathering exposure time. Dynamic visco-elastic measurement and tensile testing were mainly used in this study.
Physical properties of high solids acrylic coating films were greatly influenced by oligomer molecular weight as follows ;
(i) The influence of baking temperature on physical properties of cured films was increased with decreasing oligomer molecular weight.
(ii) Knoop hardness number, glass transition temperature (T_g) and tensile strength were decreased, and tangent δ, molecular weight between cross-links (M_c) and elongation were increased with decreasing oligomer molecular weight. In this study, it was found that the molecular weight effect on physical properties depended on the free terminal group for the oligomer M_n>1000 ; for the oligomer M_n>1000, the existence of non-functional or mono-functional oligomers promoted the deterioration of physical properties.
(iii) T_g of the film depends on the oligomer's T_g, and it can be explained in the formula T_g=T_g∞-K/M_n.
(iv) Concerning highly cross-linked polymers as in high solids coating films, the formula log₁₀G=7. 0+293 ρ/M_c is more accurate than the rubber elasticity theory formula for the calculation of molecular weight between cross-links.
(v) The change of physical properties of lower molecular weight oligomer system during the exposure in the accelerated weathering test was greater than that of higher molecular weight oligomer system.