(201e) Plastic Waste Pyrolysis Wax Production, Characterization, and Application

The complex challenge of recycling plastics can be enabled through innovations in chemical recycling, an example of which is thermochemical conversion processes such as pyrolysis. Fast pyrolysis is considered an effective method for transforming certain types of plastic waste into valuable products that can be upcycled into new plastics or high value chemicals. Polyethylene (PE), the most commonly used plastic globally with a consumption share of approximately 30%, can also be processed through such methods to yield useful products. However, one of the major challenges is ensuring the consistent and efficient production of high-quality, valuable products from transformed plastic waste. One promising high-quality product from pyrolysis is industrial wax, which can be used as a feedstock for many items including candles, crayons, adhesives, and cosmetics.

This study employed a novel liquid-fed waste plastic fast-pyrolysis pilot plant equipped with a vertical pyrolysis reactor, designed to effectively collect char (char chamber), to process low-density polyethylene (LDPE) from military Meals, Ready-to-Eat (MRE) bags and high-density polyethylene (HDPE) from MRE bags. The HDPE and LDPE bags are formulated with about 3% wt. nanoclay as a strengthening agent, which is the source of char from the process. The waste plastic feeds were combined with recycled pyrolysis wax in a 1:1 ratio (wt.) to form a liquid feed for pyrolysis. The fast pyrolysis reaction occurred at 460°C at vapor residence times less than 1 second, resulting in a wax yield rate of >70% and 100% conversion of the waste feed into gas, liquid, wax, and char products. The resulting pyrolysis wax underwent evaporation to remove light species and was hydrotreated using hydrogen gas and Pt/Al₂O₃ catalyst. The pyrolysis wax from each step and hydrotreated products were characterized via thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and gas chromatography-mass spectrometry (GC-MS) analysis. According to the TGA and GC-MS results, the hydrocarbon number ranges in pyrolysis wax became more similar to paraffin wax by evaporating 25% of the volatile species (<C20) from the crude wax sample. Interestingly, the alkene peaks in the GC-MS spectra decreased compared to the “shoulder” alkane peaks, disappearing as the molecular weight of the sample increased (>C20), with emergence of the alkane as confirmed by FTIR results. Additionally, the mild hydrotreatment successfully converted the alkene compounds to alkane within 2 hours, creating a suitable industrial wax alternative. These findings demonstrate the potential of pyrolysis wax as a promising alternative to paraffin wax in industrial wax applications and for candle-making purposes.