(154t) Novel Chemi-Mechanical Recycling Process for Blending of Polyethylene and Polypropylene

Valorizing mixtures of plastics is limited by the cost of separation and improved technologies that produce high-value products form plastic mixtures are desperately needed. Hydrothermal and supercritical water conditions are well known for their use in deconstructing plastics into fuels and other useful products. For example, hydrothermal depolymerization in the absence of oxygen can reduce char formation relative to that obtained under pyrolysis conditions, thereby maximizing yields of more valuable oil products. Addition of oxygen to hydrothermal water results in supercritical water oxidation, a technology that depolymerizes plastics, converts them into CO₂, and releases energy. Accordingly, hydrothermal and supercritical water is often considered exclusively as a deconstruction solvent, with corresponding production of fuels or energy with much less value than the parent polymers. That stated, at temperatures greater than polymer melting and less than rapid depolymerization, hydrothermal water possesses properties that may be appropriate for polymer swelling and moderate chemical rearrangement, including branch and cross link formation. Swelling, branch formation, and cross-linking under hydrothermal conditions therefore has potential as a new method of polymer recycling that incorporates both mechanical and chemical principles and that results in a product with comparable value or even greater value than the plastic feed.

We hypothesize that hydrothermal water can be used for blending immiscible commercial polymers at reaction conditions above polymer melting temperatures but less than depolymerization. The proposed process can be termed “chemi-mechanical” recycling as it aims to create a high-quality blend of ordinarily immiscible polymers, such as polyethylene (PE) and polypropylene (PP). Chemi-mechanical recycling employs a high-temperature reactor and water to blend otherwise immiscible polymers; rapid cooling can preserve the polymers in this blended state, resulting in a polymer-polymer composite with superior properties to those obtained from thermal mixing alone.

Experimental results with PE-PP mixtures indicate that hydrothermal treatment can be categorized into three regimes: 1) a melting regime, under which neither depolymerization nor blending occur, 2) a depolymerization regime, under which waxes or oils can be produced, and 3) an intermediate regime under which compatibilization occurs. In this regime, controlled reactions and solvent swelling promote micro-scale blending resulting in a composite that may have superior properties to those possible from traditional recycling of PE and PP. In this intermediate regime, we observe minimal molecular weight degradation and improved molecular mixing On the other hand, temperatures at the supercritical point of water correspond to the depolymerization regime, producing a brittle material due to partial depolymerization. Temperatures greater than the critical point produce waxes. Results from differential scanning calorimetry (DSC), seen below, show that at elevated temperatures there is a significant loss in both melting temperature (T_m) and percent crystallinity (X_c). However, treatment at temperatures below the supercritical point (SC), results in PE-PP blends which exhibit minimal degradation, as indicated by measured melt temperatures. In addition to melting temperatures, gel-permeation chromatography (GPC) results show that treatment above the critical temperature results in significant loss in molecular weight. At temperatures much less than the critical point, the molecular weight is unchanged after treatment. However, when treated at temperatures near the critical point, only a modest reduction of molecular weight is observed.

As the main issue with these blends is incompatible mixing, SEM was conducted to analyze the phases and degree of mixing. Figure 2a shows SEM images of melt blended PE and PP in the absence of hydrothermal conditions. As seen, the two polymers are not well mixed in this sample and suffer from interfacial adhesion failure. The morphology of the sample treated at a temperature above the critical point can be seen in Figure 2b, which shows intimate mixing of PE and PP. The significant loss in molecular weight observed for this sample means these reaction conditions not ideal for a reusable product. Finally Figure 2c shows well-distributed PP beads in a PE matrix obtained after processing at temperatures less than the critical point, indicating intimate mixing. When combined with Figure 1, we therefore observe both intimate mixing and retention of molecular weight in PE-PP samples treated hydrothermally at temperature below the critical point.

In summary, treatment of a PP-PE mixture in water near its critical point produces an intimate blend which retains its original molecular weight with negligible loss and with retention of its original melting characteristics. The results presented here establish a new, previously undiscovered regime in hydrothermal water that combines chemical and mechanical recycling as a new chemi-mechanical approach that transforms mixed plastic streams into a compatibilized product.