CHARACTERIZATION AND RHEOLOGICAL INVESTIGATION OF POLYMER MICROGELS USED IN AUTOMOTIVE COATINGS

The mechanism by which polymer microgels impart shear-thinning behavio r and a yield stress to the Newtonian, low molecular weight resin of h igh-solids solvent-borne automotive coatings is not well understood. T oward identifying this mechanism, we measured the viscosity of several microgel dispersions as a function of particle volume fraction as wel l as the size and polydispersity of the particles. Variations in the r elative ''softness'' of the particles, due to differences in the steri c layer thickness or crosslink density of the particles, gave rise to differences in the viscometry behavior. When dispersed in acrylic poly mer solutions, the microgels induced a yield stress that displayed a p ower-law dependence on particle concentration. Dynamic oscillatory mea surement were used to investigate the nature of the structure formed b y the microgels. The mechanism of structure formation can be attribute d to depletion flocculation when the polymer does not absorb to the mi crogels and to bridging flocculation when the polymer does adsorb.