Depthwise Viscosity Gradients in UV-Cured Epoxy Coatings

A magnetic microrheometer is used to characterize the development of viscosity at different depths in UV-cured epoxy coatings. Lateral magnetic particle velocities are tracked at different depths to quantify viscosity gradients. In general, viscosity build-up is faster near the coating surface, tending to produce a skin. The effects of process conditions on the viscosity gradient development, on the rate of viscosity increase, and on surface defects are studied. More severe gradients develop in thicker coatings and in those with higher photoinitiator concentration. Under some conditions, the skin layer wrinkles, indicating the development of local compressive stress. Curing at higher temperature, however, increases cure rates while reducing the viscosity gradients and wrinkling defects.