In-line birefringence and dichroism methods for the monitoring of polymer film extrusion

Abstract

The main objective of this work is to use non-invasive optical techniques (birefringence and dichroism) for the in-line monitoring of film extrusion. Birefringence and dichroism experimental set-up are inspired on rheo-optical techniques used for investigating the interplay between flow-induced structures and flow properties [1,2]. Off-line optical measurements were first carried out on films obtained with a laboratory processing line for processing tape and filaments comprising extrusion and drawing stages, in order to elucidate the effect of processing conditions on film properties. Results from a full factorial design of experiments suggest that the temperature of the re-heating oven and the drawing ratio significantly affect the birefringence of films. Dichroism, crystallinity and tensile modulus are less affected by the above processing parameters. In-line birefringence and dichroism show same qualitative responses to changes in processing conditions. However, in-line birefringence values are roughly 1t] % larger than off-line data, whereas in-line and off-line dichroism data present a much better correlation. The mismatch identified for the difference in birefringence is intrinsically related to the chain relaxation upon film cooling and to the film aging occurring after collecting samples for off-line measurements.