The effect of processing parameters on glass fiber birefringence development and relaxation

Birefringence of oxide glass fibers drawn from glass melts through an orifi ce has been detected previously [H. Stockhorst, R. Bruckner, J. Non-Cryst. Solids 49 (1982) 471; H. Stockhorst, R. Bruckner, J. Non-Cryst. Solids, 86 (1985) 105.] This birefringence is a measure of anisotropy in glass structu re that can influence fiber performance properties. Birefringence is produc ed during fiber drawing as the fiber is rapidly stretched in the viscoelast ic glass transition range. The birefringence is 'frozen' into the glass dur ing rapid cooling. With a simple drawing apparatus using Borosilicate glass (Corning code 7740) preforms, we produce glass fibers for a range of proce ss conditions and measure their as-drawn birefringence. The development of birefringence in glass fibers is found to depend on the amount of deformati on, the deformation rate, and temperature. Results for various process para meters show that increasing draw ratio, increasing elongation rate, and dec reasing draw temperature increase birefringence. Post-process annealing is used to examine the time and temperature dependent glass fiber birefringenc e relaxation. Birefringence is found to completely relax in the temperature range close to the glass transition range as expected, but it is also note d that birefringence shows substantial (although incomplete) relaxation in a temperature range well below the glass transition. This low temperature r elaxation indicates that the relaxation process may be due to a distributio n of relaxation times. A modified stretched exponential and a pair of Jeffr ey elements in parallel are used to describe this distribution and capture birefringence relaxation in a wide range of temperatures below the glass tr ansition range. (C) 1999 Elsevier Science B.V. All rights reserved.