Currently, there is a mismatch in the supply and demand of recycled post-consumer plastics. Within this project the influence of impurities on the quality of recycled PP and PE has been systematically studied.
Currently, there is a mismatch in the supply and demand of recycled post-consumer plastics. Potential clients search for highly pure recycled plastics for competitive prices to replace virgin plastics. The recycling facilities supply a small amount of highly pure recycled plastics and larger amounts of more impure recycled plastics. The highly pure recycled plastics are relatively easy to market, but aren’t and will not (soon) be food grade. The less pure recycled plastics have reduced properties and are more difficult to market. This project studied the quality of post-consumer recycled PE and PP (rPE and rPP) in relation to impurities that regularly occur in these recycled plastics to deepen our understanding of the quality of these recycled plastics and their applicability.
Analysis of current quality of recycled PP and PE
Several types of commercial rPE and rPP available in the market have been analysed. All received rPP samples (flakes or pellets) have a purity in the order of 90-95%. They contain more than 3% PE and several sub-percental polymeric and other materials impurities. The rPE samples have a similar purity, with PP as main impurity. The measured properties (MFI and colour) match with the specifications as reported in the corresponding datasheet.
Preparation of highly pure rPE and rPP
rPE and rPP washed milled goods with a high polymeric purity (high purity rPE and rPP) were manually prepared in the lab by rigorous sorting on the object level and subsequent mechanical recycling. Polymeric purity levels of more than 99.5% can be attained. The preparation of the high purity rPE and rPP samples is laborious and very difficult to achieve on an industrial scale. Furthermore, this material contains different grades of the target polymer and unknown amounts of additives coming from different types of post-consumer packages and packaging components.
Impact of impurities in PP and PE recycled plastics
Based on the impurities found in the commercial samples, different common polymeric impurities (PET, PS, PVC, flexibles) were added in different realistic amounts varying from 0.5 to 10% to the high purity samples. This results in relatively small variations of the measured properties.
Main findings for the effect of impurities on the properties of rPP:
- Clear reduction in the processability with presence of 0.5% PET or 0.5% metallised film (melt filters in extrusion process are clogged).
- Maximum flow (MFI) that can be achieved with rPP is limited to what we call medium flow region (~20 g/10 min at at 230°C). HDPE and LDPE even in small amounts decrease the MFI of the rPP materials. This is also true for most of the other added impurities but the effect is smaller. Samples where 50% of the bodies are replaced by bodies with in-mould labels (50IML) show a reversed behaviour, MFI increases compared to 100% rPP.
- All rPP samples have a green/grey colour, stemming from the different packages and inks used in the recycled packages. Colour gets darker when PET or in-mould labels are present.
- The thermomechanical behaviour, melting temperature and degree of crystallisation hardly change by adding impurities to rPP. This suggests that these parameters are mostly related to the different types of PP grades that are present in the matrix and not to added impurities.
- The presence of HDPE as impurity improves the impact strength.
Main findings for the effect of impurities on the properties of rPE:
- The melt flow clearly increases with 5 and 10% of rPP added to the recycled PE.
- Impact strength decreases when rPP is present in amounts higher than 5% in rPE.
- Impurities such as PS, PET and PVC strongly influence the colour of the recycled samples.
Comparison to virgin material
The largest differences however are observed when comparing the recycled materials to the virgin materials. Any comparison between a virgin polymer and a recycled plastic is hampered by the fact that virgin polymers are sold as tailor-made grades with specific properties to suit their use. Post-consumer recycled plastics are in their very nature a mixture of different tailor-made grades with consequently average properties. This situation is – besides legal and business barriers – an important reason for the current mismatch in supply and demand of recycled material.
Achieve higher level of circularity for PE and PP
To overcome this challenging situation and achieve a higher level of circularity for PE and PP, several options are possible. First of all, design-for-recycling of packaging in combination with better compliance to existing specifications of the sorted products will reduce the amount of impurities present in the washed milled goods and consequently in the pellets. Moreover, the introduction of quality specifications on intermediate products such as washed milled goods could also enhance the quality of the produced pellets. This study can be used as a first exploration. However, to create formal specifications, more in-depth research is needed.
A second route is to enhance the applicability of existing recycled plastics by improving processing and entering new application fields. In a follow-up project additional measurement methods would need to be added to widen the scope of applicability. As alternative, the recycled material can be made more suitable for a specific application by using additives, adjusting processing parameters and/or redesign the final product. This is called design-from-recycling and needs involvement of all stakeholders in the chain.
The most extensive pathway would be a new system based on advanced sorting technologies. Ideas are for example to sort on different grades or to use marking technology to enable food-safe applications for rPP and rPE.
Summarizing, there are multiple pathways to improve the circularity of PE and PP. Regardless which route(s) will be followed, active participation of all stakeholders is needed, including the EU and national governments to set out a policy in one or more of these directions.