Structure of waterborne organic composite coatings

The mechanical properties and structure of composite films made of high-T-g polystyrene (PS) nodules dispersed in a low-T-g poly(butyl acrylate) (PBuA ) matrix were studied by means of dynamic mechanical spectrometry and small -angle neutron scattering. At temperatures between the T-g of PS and PBuA, the mechanical properties depend drastically on the spatial distribution of the glassy PS domains. For films cast from mixtures of PS and PBuA latexes , an aggregation of PS particles into dense clusters dispersed in the PBuA matrix was observed. As a consequence, a large density of contacts between PS particles in the dry films ensured a strong mechanical reinforcement abo ve a percolation threshold of 30% PS volume fraction. Extensive coalescence of PS particles occurred at their contacts upon annealing the films above the T-g of PS, leading to further mechanical reinforcement. For films cast from core-shell particles, the aggregation phenomenon was prevented, depend ing on the coverage of the PS core by the PBuA shell. An efficient core enc apsulation in the core-shell morphology led to poor contact between PS core s, the elastic moduli were then close to that of the PBuA matrix, and the P S coalescence upon annealing was prevented.