Effect of processing condition and antioxidants on visual properties of multilayer post-consumer recycled high density polyethylene films.

Mechanical recycling is a promising approach to reduce the environmental impact of plastic packaging waste. However, the presence of defects in recycled materials results in final products with relatively poor visual and/or mechanical properties. In this work, the origin of the visual defects in post-consumer recycled HDPE (PCR HDPE), as well as the effects of processing method, processing condition and the addition of antioxidants on the visual defects were studied in multilayer flexible polyethylene films. The nature of the defects in the film samples were investigated by combining optical microscopy, energy dispersive X-ray (EDX), hot stage microscopy, solvent extraction, and differential scanning calorimetry (DSC) techniques. It was found that the defects in PCR film samples can be mainly categorized as fiber defects and point gels. Hot stage microscopy results show that point gels can be subcategorized in two groups: (a) non-melting, non-deformable gels, and (b) melting, deformable defects. In addition, it was found that deformation of molten, deformable defects increased at higher temperatures specifically above 200 °C. Further characterizations showed that the observed deformable defects are highly entangled high molecular weight HDPE. The effect of processing temperature, processing with a twin-screw extruder and the addition of antioxidant on the visual defects in film samples were also discussed in detail. It was shown that increasing processing temperature and using twin-screw extruders were two approaches that could reduce considerably the number of defects. The addition of antioxidants was also shown to improve the film quality especially at lower processing temperatures.