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 r oll temperatures. The residual stresses and anisotropies, induced by the co mplex processing deformations and temperature variations, were characterize d by monitoring the dimensional changes and sample curvatures resulting fro m subsequent heat treatments. In both copolymers, two distinct recovery reg imes were observed: heat treatments just below Tg caused relaxation of resi dual 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 h igher viscosity of the 3% EA sheet led to greater residual orientations. Ac ross the width of the sheets, little variations were seen in the thermal st resses, but orientations were lower at the edges. Some curvature changes ob served following heat treatment well below Tg, were interpreted as resultin g from aging effects.