Study of the Viscosity and Thermal Characteristics of Polyolefins/Solvent Mixtures: Applications for Plastic Pyrolysis

Abstract

Plastic pollution is one of the biggest environmental problems that the world is currently facing. Pyrolysis is a frontier technique aimed at converting plastic waste back into virgin-quality resin. However, the transfer of the waste plastic feed into the pyrolysis reactor must be optimized before the process can be upscaled to a continuous process. In this study, a new solvent that reduces the viscosity of molten plastic was introduced and characterized. The results revealed that the polymers are soluble in the ratio of up to 75 wt % plastic and 25 wt % solvent at 240 °C. The viscosity of pure low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polypropylene (PP) in the solvent was measured in different weight percentages of polymer in solvent (30–80 wt %) and at 160, 180, 200, 220, 240, and 260 °C. The viscosity decreased with the decreasing polymer-weight percentage and with increasing temperature. The viscosity of LDPE/solvent and PPs(isotactic)/solvent is much lower than for HDPE/solvent and PPp(polypropylene impact copolymer)/solvent. Differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) were applied to characterize the thermal behavior of LDPE, HDPE, and PP in the solvent in three different weight percentages (25, 50, and 75 wt %). The DSC results indicate that in the mixture of PPs/solvent and LDPE/solvent the melting point of PP and LDPE decreases as the amount of solvent increases. Overall, these results indicate that the selected solvent is an effective agent to prepare waste plastics for pyrolysis.

Evaluation of Pyrolysis Wax as a Solvent in Polyolefin Pyrolysis Processing

“…In a prior article, we demonstrated that paraffin wax is an effective solvent for dissolving waste plastics and reducing the mixture viscosity by temperature adjustment . In the results presented here, we compare paraffin wax and pyrolysis wax to each other.…”

“…In our previous study and pyrolysis pilot plant experiments, we demonstrated that the complete dissolution of polyolefin plastics in paraffin wax at a 1:1 (wt) ratio mixture is possible at temperatures over 200 °C, and their viscosities are suitable for flowing the mixture into a pyrolysis reactor. In addition, those prior experiments also demonstrated that both the plastics and the paraffin wax in the mixture were degraded into pyrolysis products (paraffins were converted to alkenes) .…”

“…In the results presented here, we compare paraffin wax and pyrolysis wax to each other. Figure A compares the TGA results of pyrolysis wax and paraffin wax (the physical and thermal properties of paraffin wax have previously been studied). While paraffin wax vaporizes at temperatures between 145 and 370 °C, pyrolysis wax vaporizes at temperatures somewhat lower and higher, between 45 and 475 °C, indicating that pyrolysis wax contains both lighter and heavier species than paraffin wax (see Figure for DTG and GC-MS results for both).…”

“…Copyrolysis of plastic with other materials, such as petroleum waste, coal, and oil waste, was studied to improve conversion and decrease the viscosity of the end product. , However, this method requires pyrolysis plants to be coupled with petroleum plants, and it is not environmentally friendly since fossil fuel is used as the solvent. As part of our previous study, paraffin wax was evaluated as a potential solvent for reducing the viscosity of various olefin plastic types for pyrolysis. To the best of our knowledge, no study has examined the effects of pyrolysis wax product, acting as a waste plastic solvent, on the viscosity of plastic (such as HDPE and PP)/solvent mixtures at high temperatures.…”

“…Due to the lack of data on the subject, this paper reports on the viscosity of HDPE/pyrolysis wax and PP/pyrolysis wax mixtures at different temperatures (220, 240, and 260 °C) and compared the results to experiments where paraffin wax was used to reduce plastic viscosity . It is important to note that pyrolysis wax is a byproduct of the pyrolysis process, thus making it economically valuable as a solvent via its internal recycling in the process, as opposed to paraffin wax, which is obtained via the crude oil refinement process and must be purchased as a process input.…”