Influence of washing media and temperature on the properties of recycled post-consumer flexible polyethylene packaging waste

Other conference contribution, 2023

Plastic packaging products, typically flexible film packaging, has been the largest application area for polymeric materials for many decades, constituting a major source of waste in society. Plastics packaging is also the fastest growing application area but considered to be the most challenging for material recycling. Complexities and challenges in recycling requires better understanding on influence of processing parameters, e.g., compounding and washing, on properties of the recycled materials made of post-consumer flexible plastic packaging waste.

In our previous studies the focus was more on understanding first the properties of untreated post-consumer flexible polyethylene packaging waste and secondly the influence of selected compounding parameters. Two different temperature profiles and screw configurations were used in compounding to assess the influence. The results showed that the studied compounding parameters had no great influence on the melting temperature, crystallinity, stiffness and strength of the samples. However, thermo-oxidative degradation apparently occurred after washing especially with industrial-scale washing at 70-80°C and deteriorated further when compounded at a high temperature profile. The elongation at break of injection moulded samples made of unwashed waste increased with high temperature compounding but for the washed waste the opposite was observed. Overall results indicated the importance of washing step and its influence on final properties.

The work continued with a focus on the influence of washing media and the temperature on properties of recycled post-consumer flexible polyethylene packaging waste. Laboratory-scale washing was done with three different washing medias: water, water + NaOH and water + detergent. Washings were done both at 25 and 40°C, referred to as hot and cold washing, respectively. The compounding and injection moulding parameters were kept the same for all samples and as in the previous work. The properties of intermediates and the moulded samples were analysed with high temperature gel permeation chromatography, capillary rheometry and both the thermal and mechanical properties were assessed.

The results showed that neither the washing media nor the washing temperature studied in this work had a significant influence on the thermal properties of the samples. The oxidation induction temperatures were on a similar level for all samples, being between 225-232°C, implying that the samples still contained a significant amount of active stabilizers. Similarly, the viscosity curves were in a similar range which was supported with the melt-mass flow rate measurements. However, the pressure losses at capillary flow were differed indicating different elasticity of the melts. Rheotens type of measurements clearly showed differences in the tenacity of melts. Washing with NaOH resulted in the lowest strain at break for the melts whereas the washing with only water and with detergent showed similar levels of strain at break of melts. The melt strengths were in a similar range for all the samples. Concerning the mechanical properties of injection moulded samples, the elongation at break was the most influenced property, the samples washed with NaOH had the lowest values whereas the other washings resulted in mouldings with similar elongation at break. The Young’s modulus and the tensile strength values were less changed by the washings. In general the washing temperature had no major impact.