AN INVESTIGATION OF THE SPATIAL AND OPTICAL BEHAVIOR OF BIAXIALLY DRAWN PET FILM

The paper investigates the evolution of the spatial and optical proper ties of particle-doped poly(ethylene terephthalate) PET film, as a fun ction of the degree of biaxial extension. Mean surface roughness and p eak particle concentration were the main spatial parameters studied. A sharp increase in the average asperity height has been recorded for e arly forward draw, followed by an abrupt decline that continues almost linearly through the remaining sideways draw. This behavior has been explained in terms of the unequal stress distribution and subsequent y ielding in the polymer film surrounding the particles. The peak concen tration drops initially due to stretching of the film surface, but rem ains constant through the transverse draw. The wide angle haze was fou nd to be an exponential function of forward extension, similar to the stress/strain curve for PET. However, the surface haze followed the sa me trend as surface roughness, suggesting a probable correlation betwe en the two. A consideration of the transversal particle migration sugg ests that the screw extrusion part of the process may be instrumental in the augmentation of the base surface roughness of the film leaving the slit die.