Study of birefringence character of injection‐ and compression‐molded polycarbonate and its interpretation

Abstract

AbstractBirefringence development in molded polycarbonate is of great importance in the manufacture of optical discs. It is important in characterizing birefringence to realize that we are dealing not with a scalar but with the anisotropy of the refractive index tensor–and with the orientation and shape of the refractive index ellipsoid. This usually varies heterogeneously; i.e., point‐to‐point through the thickness of molded parts. We present here an experimental study of the shapes and orientations of refractive index ellipsoids in injection‐ and compression‐molded polycarbonate parts. We consider the variations in ellipsoid character as a function of molding conditions such as injection rate and quench. In injection‐molded parts, the refractive index ellipsoid is coaxial with the flow direction in the center of the parts, but varies with position in the direction of the mold walls. In some cases, the extinction angle reaches a maximum at the mold wall and in other cases, there is an intermediate maximum. In the central “flow‐thickness” direction plane, the transverse axis of the refractive index ellipsoid is perpendicular to this plane. The birefringence (in principal axis coordinates) in the flow‐thickness direction plane exhibits an intermediate maximum whose position and magnitude vary with molding conditions. In some cases, multiple maxima exist. In compression‐molded parts, a principal axis is normal to the mold wall. The results are interpreted in terms of the Rheo‐Optical Law using flow‐induced orientation and residual stress mechanisms. If the contribution of the residual stress to the birefringence can be subtracted, we may compute biaxial orientation factors. We do this for a case that has low residual quench stresses.