Recycling of metallocene isotactic polypropylene: importance of antioxidants

Abstract

Recycling of plastics is absolutely essential in a circular economy, especially in the case of commodity polymers from fossil resources, like isotactic polypropylene (iPP). Therefore, evaluation of the factors that are decisive for an optimum performance of the recycled based materials becomes mandatory for the obtainment of new products with optimal properties. One of the most important aspects is the protection of the plastics materials not only against the external degradation agents, but also from the radicals generated during their previous service life. Accordingly, several materials have been prepared by extrusion based on virgin iPP with different amounts of the same polypropylene severely degraded, which has been used as model component to be recycled. Previous to the extrusion, a mixture of antioxidants was added to all the samples, and special attention has been paid to consumption of those additives during the extrusion. The results show an increasing reduction of antioxidants with rising content of the degraded material. But, importantly, a rather analogous mechanical response has been found for all the recycled materials in relation to the virgin iPP, pointing out a satisfactory dilution effect of the existing degradation points within the virgin polymeric chains, and indicating the very relevant action of the antioxidants used.

Improvement of Thermal Protection in Recycled Polyolefins through Hybrid Mesoporous Silica–Antioxidant Particles

“…Determination was performed following the same extraction protocol described in the Materials and Methods, and the ratio found between both species was 2.90 for the virgin PP and 2.27 for PPR. These results indicate, on one hand, that both pellets incorporated a rather equivalent content of Irgafos 168 and, on the other hand, that there was a massive consumption of Irgafos 168 during the extrusion of PPR, which is attributed to the existence of a high content of oxidative species, whose activity grows exponentially during processing at high temperatures under an air atmosphere [3]. In the rest of the systems, an additional content of the primary antioxidant Irganox 1076 was added following distinct procedures, and a completely different response was observed.…”

“…This degradation process can be magnified by the presence of free radicals within the polymer chains, developed by their exposure to damaging agents such as heat, humidity, and the sun, among others, during their previous shelf life. The absence of stabilizing agents during posterior processing can promote catalytic processes that will accelerate the propagation of these radicals [3,4].…”