(447b) Plastic Waste Recycling: A Parametric Study of Dissolution Recycling Method

One of the current environmental issues is products service life made of polymeric materials and their subsequent processing. The materials service life very often exceeds the time for which we designed them. Therefore, plastics recycling is becoming a major scientific problem and economically and ecologically advantageous process needs to be developed, at its end a recycled polymer comparable to a freshly produced polymer will appear.

At present, two basic methods predominate: (i) mechanical (physical), and (ii) chemical recycling. Neither one is universal nor has significant advantages. The mechanical method is problematic in processing of wide range of materials with different densities, colours, additives, etc. The chemical recycling based on polymer pyrolysis is often accompanied by process difficulties. An approach in between these two methods is plastics recycling by dissolution.

This alternative is based on a polymer dissolution into a suitable selective solvent to create a polymer solution. The solution is further refined to get rid of added pigments and additives. This approach requires finding suitable solvents for both for original material dissolution and for the selective extraction of additives. The toxicity, impact on the environment and cost shall be considered.

Polyolefins, mainly semicrystalline polyethylene (PE) and polypropylene (PP) are the polymers with the highest annual production. Therefore, our work focuses on PE and PP dissolution in organic solvents. We have performed a parametric study of various parameters: temperature (up to 200 °C), pressure (up to 10 MPa) and solvent type. The solvents were chosen according to the affinity of the solvent to the studied material.

The experiments have been conducted in a 100 cm³ mixing vessel with a glass window for monitoring the visual changes in polymer dissolution. Subsequently, an extraction reagent was added to get rid of the potential additives in the dissolving material. Then the refined and dried polymer has been analysed by Differential Scanning Calorimetry (DSC) and Time-Domain Nuclear Magnetic Resonance (TD-NMR). We have studied the polymer crystallinity changes of the original (waste) and refined material. Promising preliminary results show good polymer characteristics recovery. The up-to-date challenge is the removal of the additives from the original material.

The understanding of interaction of the crystalline phase with the solvents will be based on our wide database of polyolefin thermodynamic data. Polymer crystallinity decreases with concentration of gaseous penetrants as measured by TD-NMR. And we observed by gel permeation chromatography (GPC) that short polymer chains are washed out in the presence of liquid penetrants that decrease polymer crystallinity. The obtained data broaden the understanding of diffusion and transport phenomena in semicrystalline polyolefins and allow the optimization of the dissolution process.