Thermal stresses and orientations in acrylic sheet

Two copolymers of poly(methyl methacrylate) with 3% and 10% ethyl acrylate, respectively, were extruded and calendered, with the same line speed and roll temperatures. The residual stresses and anisotropies, induced by the complex processing deformations and temperature variations, were characterized by monitoring the dimensional changes and sample curvatures resulting from subsequent heat treatments. In both copolymers, two distinct recovery regimes were observed: heat treatments just below Tg caused relaxation of residual thermal stresses and at temperatures above Tg led to recovery of chain extended orientations. It was found that the similar processing conditions induced similar residual thermal stresses in the two copolymers, but the higher viscosity of the 3% EA sheet led to greater residual orientations. Across the width of the sheets, little variations were seen in the thermal stresses, but orientations were lower at the edges. Some curvature changes observed following heat treatment well below Tg, were interpreted as resulting from aging effects.