The influence of processing on the mechanical properties and emissions of recycled polyolefin

Some recycled plastics would provide the mechanical, thermal, and optical properties to replace virgin materials in some applications, but they are not used because of their odor. The reason for the unpleasant odour of post-consumer recyclates is that they are containing volatile compounds from their former applications, which also may have undergone degradation in the recycling steps, like residues from food or drinks which were stored in the package. We found that a reduction of such emissions is possible in the melt processing step of the recycling in a previous work but saw a big influence of processing parameters as the material will also further degrade in this step [1]. Therefore, the aim of this work was to investigate the reduction of the emissions in a co-rotating twin screw extruder with vacuum degassing, with an emphasis on different temperature profiles and screw configurations. Previous tests have shown that the temperature is a major factor to reduce the emissions, but around 250°C degradation effects have a negative influence. Therefore, another test series was processed to validate this effect and to find the optimal temperature settings for the process. The untreated recyclate was processed with different screw configurations. The emissions of all the compounds were determined with gas chromatography according to VDA 277. Further, mechanical characterization with tensile and notched impact tests was carried out to investigate the effects of the different processing parameters on the materials behavior. In addition, also the Oxidation Induction Time was determined to check how much of the stabilizer is left in the material. The results were compared to find the best settings for the process. The first tests with different screws show that emissions can be reduced over 70% with the right screw configuration and process settings.