Plastics are light-weight, durable, and versatile, allowing their incorporation into a diverse range of applications. In recent years, the environmental, social and economic impact of plastics has been the topic of the political agenda, with a focus on sustainable production and the decoupling of adverse environmental effects from waste generation. The disposal of waste plastics has become a major worldwide environmental problem. The USA, Europe and Japan generate about 55 million tons of post consumer plastic waste [1]. These waste products were previously dumped in landfill sites, a non-sustainable and environmentally questionable option. The number of landfill sites and their capacity are moreover decreasing rapidly and in most countries the legislation on landfills is becoming increasingly stringent.

New sustainable processes have emerged, i.e. 1) the advanced mechanical recycling of post-consumer plastic waste as virgin or second grade plastic feedstock; and 2) thermal treatments to recycle the waste as virgin monomer, as synthetic fuel gas, as hydrocarbon feedstock, or as a heat source (incineration with energy recovery). These processes avoid land filling, where the non-biodegradable plastics remain a lasting environmental burden. Plastic solid waste (PSW) treatment can be divided in four methods, as illustrated in Figure 1, each individual method providing a unique set of advantages making it particularly suited and beneficial to a specific location, application or product requirement [2,3]. The purpose of recycling is to minimize the consumption of finite natural resources, and this is especially relevant in the case of plastics which account for the use of 4% - 8% of the global oil production [3]: re-using plastics is the required course of action, with the additional benefit of reducing emissions associated with plastic production [3]. The most appropriate recovery method is chosen considering the environmental, economic and social impact of a particular technique. Figure 1 illustrates the position of each recycling method within the production chain.

Thermo-chemical recycling refers to advanced technology processes which convert plastic materials into smaller molecules, usually liquids or gases, which are suitable for use as a feedstock for the production of new petrochemicals and plastics [1]. Products have moreover been proven to be useful as fuel. The technology behind its success is the de-polymerisation process that can result in a very profitable and sustainable industrial scheme, providing a high product yield and a minimum residual waste. Processes of pyrolysis, gasification, hydrogen-nation, and steam/catalytic cracking have been previously reported upon [1].

Recently, much attention has been paid to chemical recycling (mainly pyrolysis, gasification and catalytic degradation) as a method of producing various hydrocarbon fractions from PSW. By their nature, a number of polymers are advantageous for such treatment. Thermolysis is the treatment of PSW in the presence of heat at controlled temperatures and under a controlled environment. Thermolysis processes can be divided into pyrolysis (thermal cracking in an inert atmosphere), gasification (in the sub-stoichiometric presence of air usually leading to CO and CO₂ production) and hydrogenation (hydrocracking) [4].

Pyrolysis can be successfully applied to Polyethylene teraphthalate (PET), polystyrene (PS), polymethylmetacrylate (PMMA) and certain polyamides such as nylon, efficiently depolymerising them into constitutive monomers [5-7]. Polyolefins, and in particular polyethylene (PE), has been targeted as a potential pyrolysis feedstock for fuel (gasoline) production, or to produce waxes as feedstock for synthetic lubricants, albeit with a limited success.

The development of value added recycling technologies is highly desirable as it would increase the economic incentive to recycle polymers. Several methods for thermochemical recycling are presently in use, such as direct gasification, and degradation by liquefaction [8]. Various degradation methods for obtaining petrochemicals are presently under investigation, and conditions suitable for pyrolysis and gasification are being researched extensively [9].

Catalytic cracking and reforming facilitate the selective degradation of waste plastics. The use of solid catalysts such as silica-alumina, ZSM-5 or zeolites, effect tively converts polyolefins into liquid fuel, giving lighter fractions as compared to thermal cracking.

Gasification has recently been attracting increased attention as thermo-chemical recycling technique. Its main advantage is the possibility of treating heterogeneous and

contaminated polymers with limited use of pre-treatment, whilst the production of syngas creates different applications in synthesis reactions or energy utilisation. Gasification has been widely studied and applied for biomass and coal, with results reported and published in literature. The application for the treatment of plastic solid waste is less documented, although the number of publications increases exponentially.

Figure 2 shows different thermolysis schemes, main technologies and their main products obtained, as initially presented by Mastellone [10].