Evaluation of the reprocessability of polypropylene by the implementation of Ultrasonic Micro Injection Molding

Abstract

Polypropylene (PP) is one of the most consumed commodity thermoplastics worldwide, thereby, it is critical to propose new alternatives for the recycling of its post-industrial and post-consumer waste streams. This research evaluated methodically the use of the novel Ultrasonic Micro Injection Molding (UMIM) technology, to identify the changes in morphological, mechanical, thermal and rheological properties, after the reprocessing of pure regrind material for several consecutive cycles. Proper process parameters were obtained by a Design of Experiments, achieving a reduction of micro defects, in addition to thermal stability and an enhancement of the mechanical properties of recycled PP (increase of 36% Young’s modulus, 20% yield stress, 13% ultimate stress, 26% strain, 48% toughness). The tests showed that PP was able to withstand up to five reprocessing cycles until presenting the first signs of mechanical performance downgrading. A better understanding of the mechanochemical effects and degradation is provided by Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Fourier Transform Infrared (FTIR) spectra and dynamic rheology. The results of this work set UMIM in a more mature stage for its incorporation to the industry, while contributing to the circular economy practice.