**3. Update on MAP of plastic waste**

This chapter aims to report a comprehensive description of the new results obtained to provide an accessible description not only to specialized researchers but also to open this knowledge to a greater audience.

MAP of plastic wastes has emerged as a promising chemical method to displace waste plastic and generating fuels suitable to produce energy and chemical products. Several parameters such as the plastic origin and its composition, temperature, reaction time, MW power, MW absorber and eventually the catalyst present and its composition has been reported in the literature in different reviews during the last years [3, 7, 8]. The catalyst, if present, rapidly loses its activity due to the formation of char on its surface. Energy balance showed that 5 MJ of electric energy was required to process 1 kg of HDPE using a continuous MAP system (energy efficiency 89.6%). It is interesting to note that the requested energy could be generated from the gas products formed during pyrolysis (6.1 MJ), making the process self-sufficient from an energetic point of view.

Some of the main theoretical and experimental aspects of the microwave-materials interactions are reported alongside the issues related to a microwave pyrolytic process of materials [9]. Due to several parameters affecting the interactions between materials and microwaves a rigorous detection of the reaction temperature may be a hard task during MAP [10] both for its detection and uniformity of the distribution of temperature on the material during the overall pyrolytic run.

The MAP is furthermore a promising process available for other industrial applications allowing for energy recovery directly from plastic wastes. Moreover, it offers some interesting advantages, such as high energy efficiency, absence of waste production, lower reaction temperature if correlated to classical heating processes. In the following parts are reported the MAP of synthetic plastic produced through the polymerization of monomer obtained from mineral oils together with citations of MAP of biomasses (polymers from natural source) to produce bio-gas, bio-oil, and bio-char.

### **3.1 Disposal of a single type of plastic**

Disposal of a single type of plastic waste through pyrolysis is not the best choice when they are clean and not deteriorated, because they can be easily reused through mechanical recycling as reported in **Figure 1**. Mixed plastics may be hardly sent to a mechanical process because a large amount of expensive compatibilizers are required, due to the low miscibility of different plastics. Otherwise, if the plastics, even if of a single type, are contaminated or they have lost a large part of their performances, pyrolysis represents one the best choice. Waste or contaminated polyolefins were disposed through MAP using tyres or carbonaceous char as MW absorber [3, 11–15]. The presence of the absorber mixed with the char formed in the course of the pyrolysis process is not a problem because it may be easily recycled to the pyrolysis process or(depending on the absorber used) employed together with the char formed for several applications such as solid fuel, support for catalyst, filler for fiber-reinforced composites and so on. The study of MAP on single plastic is however important to understand the mechanism of the process, the nature and composition of the products, and the interaction among intermediates when pyrolysis of composites or mixed plastics is run.

### *3.1.1 Polyethylene (PE)*

PE is the most common plastic used today, it is produced as High-density Polyethylene (HDPE) through Ziegler-Natta catalysis or Low-Density Polyethylene (LDPE) from a radical polymerization. Other classes of intermediate products (between HDPE and LDPE) are present on the market. PE is a polymer, with production over 100.0 x 106 tons/y accounting for 34% of the total plastic market. It is a polymer, primarily used for packaging (plastic bags and films), geomembranes and containers including bottles, tanks, and so on). Waste or contaminated PE may be disposed of through MAP using tyres or carbonaceous char as MW absorber. HDPE was converted into waxy products (yield 80.2 -83.9 wt. %) while the yield of gas was 15.7-19.2 wt. % and that one of solid was 0.4—0.6 wt. % [9, 10, 14] when standard apparatus was employed. However HDPE was converted into a low viscosity liquid by using a very low MW power, but a long time was required to obtain a complete conversion. It is possible to use a pyrolytic apparatus containing a system able to fractionate the vapor formed, improving the residence time of the waxy products in the oven. In these conditions, a low viscosity and density liquid was obtained and the overall pyrolysis was improved. In all cases, the time of the process was strongly reduced concerning processes using classical thermal heating. The liquid fraction from HDPE contained linear alkanes and 1-alkenes with negligible formation of branched, cyclic, or aromatic hydrocarbons.

MAP of HDPE was realized also using a bed of activated carbon [11] obtaining a liquid containing hydrocarbons with a carbon chain length profile close to that one of a diesel. The chain of HDPE was broken, across all operating temperatures, and a lighter liquid product with a narrower range of chain lengths was formed if compared to the traditional pyrolysis using a bed of traditional coke. High-temperature

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

pyrolysis of plastic has been also reported by Jang et al. [16] without any correlation with other results previously reported in the literature.

The MAP of PE using a mixed catalyst containing cracking active components (γ-Al2O3, amorphous aluminum silicate, fly ash, zeolite molecular sieve, natural mineral clay, and solid acid) together with an MW absorber (C-containing compounds, Si-containing compounds, Fe-containing compounds, and polymers) and coking inhibiting component (alkali metal salt, P-containing compounds, B-containing compounds, magnesium sulfate, diethylpolysiloxane and p-tert-Butyl catechol) were reported [17]. The use of in-situ or ex-situ catalysts such as NiO and HY zeolite is reported to improve the quality of the products formed [18].
