Nondestructive evaluation of sintering and degradation for rotational molded polyethylene

Developments in new sensor technologies and data processing are helping to increase the number of applications of nondestructive characterization methods. In this study, two major physiochemical phenomena affecting product quality of rotationally molded polyethylene parts, namely sintering and degradation, were evaluated using both traditional characterization techniques and a newer alternative ultrasonic-based method. Oven temperature and heating cycle time were controlled to produce six different process conditions for rotomolding. Increasing peak internal air temperature (PIAT) inside the mold produced a reduction in surface voids (pitting) and increased the impact strength for produced parts, which can be related to greater densification during sintering. Contrary to these characterizations denoting improved part quality, degradation was detected for PIAT above 220 °C by an increase in surface carbonyl groups by Fourier-transform infrared spectroscopy (FT-IR) and an increase in zero-shear viscosity, both relatable to thermo-oxidative free radical reactions. The newly proposed monitoring technique applying propagating ultrasonic guided waves showed that its data-rich spectral features based on harmonic frequencies were positively correlated to the same sintering and degradation properties observed above. Coupled with multivariate statistical analysis, the nondestructive ultrasonic technique shows great promise for combining multiple analyses in a single sensor technology, making it well suited to the implementation of advanced manufacturing methodologies in polymer processing practices.