Primary and secondary methods are mostly mechanical, and they generally produce lower quality materials than the virgin plastic. The most attractive method is chemical depolymerization, since it produces virgin materials, but today is only applied to limited plastic like PET and polystyrene. This review, excerpted from The Catalyst Group Resources’ study “Plastics Recycling and the Circular Economy: Catalytic and Compatibilization Solutions,” will highlight some emerging technologies for chemical recycling of polyolefins as well as compatibilizers that are reducing some of the drawbacks of primary or secondary recycling. It is designed to assist the plastics and polyolefin industries in the identification of new pipeline technologies and strategic commercial directions which will help speed the resolutions to the challenge of plastic waste in an economical way.

Chemical recycling

Pyrolysis is the most common approach to chemical recycling of polymers, with companies like GreenMantra, Plastic Energy, SABIC, and Neste proposing solutions, but a techno-economic analysis has revealed that it is not cost-effective.¹ Catalytic pyrolysis is being researched as it occurs under lower temperature conditions and offers potential flexibility in the product slate.  Heterogeneous catalysts like metal-doped acidic zeolites are the most promising.  Hydrocarbon products from catalytic pyrolysis show broad molecular weight distributions and may include alkanes and aromatics, which have use as chemical feedstocks, waxes or can be fed to a naphtha cracker to produce olefin monomers.

Another promising technology is the use of CuCO3 as a catalyst for the pyrolysis and cracking of HDPE at moderate temperatures reported by Singh et al., leading to the production of liquid hydrocarbons (Figure 1).² Research into catalytic pyrolysis continues at many universities, sometimes using techniques familiar to industry such as hydrocracking.  For example, the Institute for Cooperative Upcycling of Polymers has reported lab-scale hydrocracking of a commercial grade PE bag to liquid n-alkanes with a narrow molecular weight distribution suitable for use as a lubricant.³ Unfortunately, this process is too slow to be commercialized.

Figure 1.   Mechanisms of Waste HD-PE Plastic Using Pyrolysis-Catalytic Cracking
Over CuCO3

Another unique and interesting approach is the process developed by the Fraunhofer Institute in collaboration with CreaCycle GmbH. Here, waste is dissolved in a proprietary solvent blend depending on the polymer. Undissolved impurities (additives, inks, etc.) are removed and the polymer is recovered by precipitation and drying, allowing for the recovery of a near-pure polymer that can be reused. This process has been piloted by Unilever in Indonesia at a rate of 3 tons per day using roughly 15% of the energy of primary polymer production.⁴

Compatibilization routes

In contrast to catalytic or thermal technologies for recovery of monomers or polymers, compatibilizers aim to ease recycling by avoiding the need to separate different types of polymers before primary or secondary recycling.  In recycle streams, polyolefin resins are typically not fully separated and are thus often commingled.  In addition, multilayer plastics may consist of otherwise incompatible materials, or PET bottles may still have some PP bottle cap material present. The presence of multiple polymers often leads to recycled plastics with variable or poor mechanical properties.  Compatibilizers aim to reduce the cost of recycling by allowing for a blended plastic with a desired balance of properties. The most common compatibilizer technologies for polyolefins are either reactive or non-reactive types.

An interesting reactive compatibilizer technology has been recently introduced by Kenrich Petrochemicals.  The compatibilizer is a phosphate-titanium catalyst combined with aluminosilicate and is suitable for PE/PP and PE/PP/PET blends commonly found in recycle streams.  Added into an extruder at low rates, the compatibilizer catalyzes repolymerization while also providing non-reactive compatibilizer functionality.  This technology is capable of increasing the melt index of the blend, even through multiple heat cycles in an extruder.⁵  This technology is capable of being extended to other blend types such as PET/polycarbonate and PE/polyamide.

In summary, having technologies that allow for waste to be brought back to its original raw materials or allow for primary and secondary recycling while overcoming the effects of contaminants, are a major part of the move to a circular economy for plastics. Chemical recycling technologies are in the early stages, with many technologies proposed for different polymers. Compatibilizers could be an interesting bridge technology and may present a cost-effective solution when considering eliminating the need for plastic sorting.  Research is continuing in both of these areas and bears further watching.