Dynamics of uni and biaxial deformation and its effects on the thickness uniformity and surface roughness of poly (ether ether ketone) films

Abstract

AbstractThe uniformity of films and their surface roughness are of great importance in applications involving information recording (videotapes etc.). These properties of polymers processed with such film production techniques as tenter frame operations are not well understood. To understand the relationships between the process variables, such as stretching mode, rate of stretching, temperature of stretching, and material characteristics, melt cast amorphous PEEK films were subjected to systematic experiments involving variations of the process parameters. The results reveal that stress hardening in the stress strain curves plays an important role in defining the level of thickness uniformity and surface roughness. It the films are stretched in a way that they do not experience stress hardening, the resulting films were found to have significant variations in thickness, sometimes worse than those of precursor materials. In all cases, where films are subjected to substantial stress hardening, the above two parameters improved dramatically. This indicates that no matter what the stress response of the film, those films stretched to intermediate stretch ratios have poor thickness uniformity and surface quality. Once the stress hardening takes effect, the deterioration of the dimensional parameters was found to stop and reverse and ultimately reacha point in deformation space where the film quality was better than that of the original films. This is solely attributed to the stress hardening that allows the thinned sections of the films to sustain higher loads, thereby propagating the deformation to other underformed areas in the films, which in turn causes a “self‐leveling” effect. An important implication of this finding is that to obtain films of optimum thickness and surface quality, they need to be stretched beyond the (global) stress hardening point. Otherwise, less than desirable film quality may result.