*3.1.2 Polypropylene (PP)*

PP is a thermoplastic polymer obtained through stereospecific polymerization with properties close to those of PE, but slightly hard, and resistant to a higher temperature. It is white, non-polar, and used in a wide variety of applications. It is the second large produced plastic, after PE. PP has been pyrolyzed using tyres or carbonaceous char as MW absorber [14] obtaining a low density (0.746-0.760 g/ mL) and viscosity (0.63-0.95 cP) liquid (yield 56.5-74.7 wt. %), a char (yield 9.1- 12.0 wt. %) and a gaseous fraction (yield 13.3 - 34.4 wt. %) containing hydrogen and aliphatic hydrocarbons. Liquid from MAP of PP was formed as a mixture of methyl-branched alkane and alkenes, and sometimes aromatics. The composition was a function of pyrolysis conditions.

Similar results were later reported by Suriapparo et al. [19], using different MW absorbers such as graphite, aluminum, silicon carbide, activated carbon, lignin, and fly ash. The optimal conditions were an MW power of 450 W and a ratio of PP: graphite = 100: 1, but only an oil yield of 48 wt. % was reached. The oil had a heating value of 44.45 MJ/Kg and contained 50 wt. % of C3-C6 gaseous hydrocarbons. The reaction was repeated with 50 g of PP under the same conditions, obtaining an 83 % of energy recovery in the oil. Alkenes and cycloalkanes were the major compounds in the oil, but their relative yields varied significantly with different absorbers. The catalytic activity of the absorber was hypothesized even if the results may be explained with the different efficiency in the conversion of MW into heat and the transfer of the heat to the polymer. The MAP was also carried out in the same conditions using commercial PE and polyisoprene.

### *3.1.3 Polystyrene (PS)*

PS is an amorphous, colorless, and transparent thermoplastic polymer synthesized from an aromatic alkene (styrene) through radical polymerization. PS can be solid or foamed, the PS used for general purpose is clear, hard, and brittle. It have a low-density and a high barrier to heat and noise when expanded (EPS) but a low barrier to oxygen and water vapor and a relatively low melting point. PS is one of the most widely used plastics, after PE and PP, it can be naturally transparent, but can be colored. It is employed as protective packaging, containers, lids, bottles, trays, tumblers, and so on. Reverse polymerization of waste PS was realized obtaining styrene and other aromatics through MAP, using tyres or carbonaceous char as MW absorber [20–25]: Tyres may be employed if the final use of the liquid is as a diesel fuel, carbon if the liquid formed (styrene) must be directly used to produce new PS. Working at atmospheric pressure a clear and low viscosity liquid containing styrene as the major product was always collected together with a low amount of char and gas. Using a MW power of 3 kW and 100 g of PS together with 47.3 g of carbon, it gave a liquid (yield 86.5 wt. %) containing a higher amount of single-ring aromatic compounds, as evaluated by Gas Chromatographic-Mass Spectrometric analysis (GC–MS) (aromatics C6–C10 93.9%, among which styrene is 66.0%) a char (yield 9.8 wt. %) and a gas (yield 3.7 wt. %). Improvements in residence time, by using low MW power or a fractionating system directly inserted over the oven and before the collecting system, allowed to obtain a liquid with low viscosity and density even if the char yield was increased to 10.0%. If the process was realized at reduced pressure (21.3 KPa) the liquid product was formed with a yield of 84.3 wt. % containing the monomeric styrene in a concentration of 71.9 % (determined by GS-MS).

Waste PS was also pyrolyzed in the presence of aluminum as MW absorber, at temperatures as high as the melting point of aluminum, obtaining styrene and other substituted aromatic compounds [22]. The rate of MAP and yield of the products was found to depend on the size, shape, and form of the aluminum. The reaction was faster using the coil, slower for strips, and negligible for the cylindrical form. The products of the pyrolysis were found to contain 88 wt. % of liquid (substituted benzene together with polycyclic aromatics and condensed ring aromatics), 9–10 wt. % of gas, and a low amount of char.

A pyrolysis process was also run in a batch reactor for MAP using activated carbon [21]. The quality of the oil from pyrolysis of PS was assessed for the possible applicability of the liquid in fuel production. The best conditions were a MW power of 450 W and a polymer/activated carbon ratio of 10:1, resulting in an oil yield of 93.0 wt. %. The liquid contained alkenes 8.4 wt. %, α-methyl styrene 1.0 wt. %, condensed ring aromatics 23.2 wt. %, and benzene derivatives 26.8 wt. %. The C9-C12 aromatics were 93.0 wt. % while it was not reported the amount of styrene (C8H8) in the liquid.

### *3.1.4 Poly(vynylchloride) (PVC)*

PVC is another largely used polymer obtained through radical polymerization of vinyl chloride. It is employed in two basic forms: rigid and flexible. The main uses are in construction for pipes, doors, windows, non-food packaging, food-covering sheets, and cards. It can be made softer and more flexible by the addition of plasticizers. In this form, it is used in plumbing, electrical cable insulation, imitation leather, flooring, signage, phonograph records, inflatable products, and many applications where it replaces rubber. Together with cotton or linen, it has been used in the production of canvas. According to its thermodynamic stability, the pyrolysis of PVC was obtained with the initial loss of HCl followed by cracking the hydrocarbon chain, so the gas contains HCl and low molecular weight hydrocarbons (ethene and propene) while the main compounds present in the liquid fraction were aromatic hydrocarbons such as benzene, toluene, dialkylbenzenes and so on, formed through Diels Alder reactions involving polyunsaturated compounds formed as intermediates in the course of the MAP process.

### *3.1.5 Polyesters*

The most widespread synthetic polyester fiber is obtained by polycondensation of ethylene glycol and terephthalic acid (Polyethylentherephtalate: PET). It is also mainly used to produce bottles devoted to contain sparkling water and carbonated soft drinks due to the low permeability to CO2. PET may be also disposed of through pyrolysis with the formation of a gaseous fraction containing carbon dioxide (22.7%) and carbon monoxide (13.3%) due to decarboxylation and decarbonylation of the ester groups [26] while the liquid product contains aromatic

### *Mixed or Contaminated Waste Plastic Recycling through Microwave - Assisted Pyrolysis DOI: http://dx.doi.org/10.5772/intechopen.100179*

hydrocarbons (benzene and toluene) and oxygenated compounds (benzoic acid, acetaldehyde, and benzaldehyde). However, the yield of liquid product was lower and PET may be more efficiently recycled through microwave-assisted reverse polymerization (hydrolysis) in the presence of methanol obtaining a high yield of ethene glycol and dimethyl terephthalate the monomer employed for the synthesis of PET.

The microwave-assisted reverse polymerization (hydrolysis) of polycarbonate (PC) in the presence of NaOH using tetrahydrofuran/water as solvent has been also reported [27] obtaining bisphenol-A, the main monomer employed for the synthesis of PC.

## *3.1.6 Polylactic acid (PLA)*

PLA is a thermoplastic polyester formally obtained by condensation of lactic acid. It can be prepared by ring-opening polymerization of lactide, the cyclic dimer of the lactic acid. PLA has become a popular material due to its production from a renewable resource (lactic acid is industrially produced by fermentation of glucose, fructose, or galactose using lactic acid bacteria). Its widespread application has been hindered by numerous physical and processing shortcomings. PLA is the most widely used plastic filament material in 3D printing.

A reverse polymerization of PLA may be realized through MAP using different MW absorbers (tyre, carbon, Fe), apparatus set-up, and MW power [28]. A liquid rich in 3,6-dimethyl-1,4-dioxane-2,5-dione (lactide, one of the starting monomer for the production of PLA) and other oxygenated organic compounds were formed. The pure form of l-lactide was separated from the collected liquid by crystallization, while the meso-form of lactide remained in the mother liquors and was identified through GC–MS analysis. Up to 27.7% of lactide was obtained and might be recovered. Furthermore, simple acids such as acetic and propionic acids (up to 17.1 %), carbonyl compounds, and fragments of PLA backbone randomly cleaved were present in the liquid collected.

When the pyrolysis of PLA was run in the presence of tyres (used as MW absorber) the products showed the presence of cross-reactions between the intermediates formed from PLA and tyres affecting both yield and characteristics of the liquid. Aromatic compounds were formed from tyres pyrolysis and they improved the solvent properties of the liquid, preventying the crystallization of lactide.
