Waste polymeric materials valorization through microwave assisted pyrolysis

Microwave assisted pyrolysis (MAP) was exploited to convert several classes of waste polymers into a solid, a liquid, and a gas product with promising application as energy, power, and chemical sources. The polymeric material tested were: tires, high density poly(ethylene) (HDPE), poly(propylene) (PP), poly(styrene) (PS), multilayer packaging beverage (WMP), wood pellets (WP), poly(lactic acid) (PLA), and corn-derived plastic bags (CDP). Pyrolysis experiments were carried out in a batch laboratory scale reactor, using a microwave (MW) oven operating at 2.45 GHz, an energy output up to 6KW, and for some pyrolysis a fractionating system, directly connected to the pyrolysis oven. The presence of a fractionating system minimized the influence of different MW powers enhancing the residence time and usually improving the quality of the liquid product. MAP of tire might be run without the addition of a MW absorber. The liquid obtained showed a reduced density (from 0.92 to 0.88 g/cm3), and viscosity (from 3.92 to 1.25 cPs) when working with the fractionating system. Furthermore aromatic and olefinic compounds were formed in high amount. Different tire affected the MAP results: Tires containing a large amount of aromatics (styrene copolymers) were pyrolyzed faster than tire containing large amount of natural rubber. X-ray diffraction of solid from MAP of tire showed two different crystalline forms of ZnS, spharelite or wurtzite were present due to the different amount of waste tires employed. Their presence of these compounds suggested that tires were heated to a temperature higher than the usually accounted. MAP of polyolefins, HDPE, PP, and PS was performed using different MW absorbers (tire, carbon, or Fe) and MW power, obtaining a high quality liquid fraction with tailoring properties. From HDPE a mixture of linear alkanes, the corresponding 1-alkenes, and a very low amount of aromatics was obtained. On the contrary liquids from MAP of PP contained a mixture of methyl branched alkanes and alkenes, some aromatics and dienes. From MAP of PS a clear and low viscosity liquids were always collected together with low amount of gas (3.0 wt %) and solid (0.9 wt %). Using a MW power of 3 KW the styrene in the liquid was increased up to 65.98 %. MAP of WMP might by run with or without a MW absorber, and five products were always collected: char, gas, unscratched Al, and two liquid fractions. The organic phase liquid contained large amount of hydrocarbons, useful as fuel or the source of chemicals. The other phase contained large amount of water and oxygenated organic compounds, such as acetic acid, 2-hydroxypropan-2-one and levoglucosan. MAP of WP might be run without a MW absorber but the pyrolysis could not be completed. WP were converted into a char which preserved their former shape and a two phase liquid. These latter phases were characterized using an innovative chromatographic methodology. MAP of PLA was performed using different MW absorbers (tire, carbon, or Fe) and MW power, obtaining a liquid rich in lactide (the cyclic dimer of lactic acid) and other oxygenated hydrocarbons. The collected lactide was enantiopure and it was collected, separate, and purified directly from the liquid. Up to 9 wt% of initial PLA was recovered as L-lactide. MAP CDP was performed using different MW absorbers (carbon or Fe) and MW power, obtaining a three phase liquid and large amount of gas accordingly to the pyrolysis conditions. The liquid after spinning separate in three phases, upper, medium, and lower. Medium fractions were mainly composed of water (70 wt%), acids, alcohols, and anydrosugars and their pyrolysis products. Upper and lower fractions showed close properties and composition except that the lower fraction was a wax. Indeed the lower fraction was composed of oligomers and the upper phase was composed of the pyrolyzed form of these oligomers. The upper fractions contained large amount of aromatic acid and phthalates and their derivatives.