**1. Introduction**

Undoubtedly, plastics play a major role in our everyday life, since plastic parts are used in numerous applications, such as packaging (for instance, food containers), automotive industry, electric and electronic equipment (EEE), etc., due to their unique properties [1]. Some of their most important characteristics that necessitate their use in these applications are lightness, ease of processing, resistance to corrosion, transparency, and others. Nevertheless, their wide use in various applications in combination with the short life span of many plastic products leads to large amounts of end-of-life plastics. Taking all these into account, along with plastic nonbiodegradability, research has focused on exploring environmentally friendly approaches for their safe disposal [2]. Plastic handling involves collection, treatment, and afterward recycling. Unfortunately, finding environmentally friendly approaches for their disposal is no mean feat (**Figure 1**); due to the variation in types of plastics, which are often of unknown composition, the existence of polymer blends, or composites, multilayer structures with other materials apart from polymers, as well as the wide range

**Figure 1.** *Difficulties encountered during end-of-life plastic handling.*

of additives (such as UV and thermal stabilizers, antistatic agents, (brominated) flame retardants, colorants, plasticizers, etc.) they may contain [3, 4].

The disposal of post-consumer plastics occurs via landfilling, primary recycling, energy recovery, mechanical recycling, and chemical recycling [2]. Although landfilling is an undesirable, non-recycling method, since it results in serious environmental problems, such as soil and groundwater contamination, until now large amounts of end-of-life plastics still end up in landfilling [5, 6]. With a view to eliminating plastic landfilling, research has focused on recycling methods (**Figure 2**) that can be applied, which are primary recycling, recycling without quality losses, energy recoveryquaternary, mechanical or secondary recycling-downcycling into lower qualities and chemical or tertiary recycling-recovery of chemical constituents [7]:


**Figure 2.** *Recycling methods for post-consumer plastics.*

products, at nearly the same or lower performance level when compared with the original products [6]. Since mechanical recycling can be used only in case of homogeneous plastics, heterogeneous plastics require sorting and separation before their recycling. In mechanical recycling, the presence of brominated flame retardant (BFR) incorporated in plastics must be identified before its application, in order to avoid the possible formation of toxic substances, such as polybrominated dibenzo-p-dioxins/furans (PBDD/Fs) [9, 10]. Its main drawback is the fact that product's properties are deteriorated during every cycle [2]; and it should be underlined that each polymer can endure only a limited number of reprocessing cycles [11]. An additional challenge is the existence of mixed plastic waste (polymer blends), since different polymer types have different melting points and processing temperatures. In such cases, the processing temperature is usually set to the highest melting component. Nevertheless, this may result in overheating and possible degradation of the lower melting components and so, in reduced final properties [12].


In conclusion, during chemical recycling, plastics are converted into smaller molecules (mainly liquids and gases), which can be used for the production of new, valuable products; and that is why it is considered as an environmentally friendly and economically feasible technique. Furthermore, chemical recycling seems to be more advantageous than the other existing methods; taking into account, for instance, the fact that during chemical recycling, both heterogeneous and contaminated polymers can be treated, only with a limited pretreatment. Moreover, the energy consumption of the process is very low, if compared with that of mechanical recycling or energy recovery [6].

Chemical recycling comprises two processes: solvolysis and thermolysis. During solvolysis, polymers are dissolved in a solvent and treated with or without catalysts and initiators. *Solvolysis* can also be applied as a pretreatment before thermochemical processes (such as pyrolysis). During *thermolysis*, polymers are heated in an inert atmosphere (e.g., N2 atmosphere) in the absence of air or oxygen. It consists of various processes including (thermal and catalytic) pyrolysis, gasification, and hydrogenation (**Figure 3**) [13–14].

*Thermal pyrolysis* involves polymer cracking in an inert atmosphere (usually nitrogen atmosphere), at high temperatures, and in the absence of catalysts. During this, plastic waste is converted into liquids (pyrolysis oil), gases, and solid residues (chars) [6]. Various temperatures within the range of 300–900°C as well as different heating rates varying from 4 to 25°C/min and different retention times have been investigated in literature in order to find the optimal conditions [15]. When pyrolysis of brominated flame-retarded plastics occurs, the liquid fraction usually

**Figure 3.** *Chemical recycling routes.*

contains many brominated compounds that inhibit their reuse. In such cases, a pretreatment step before or during pyrolysis is of paramount importance, in order to obtain bromine-free products.

*Catalytic pyrolysis* involves polymer cracking in an inert atmosphere (usually nitrogen atmosphere) and in the presence of catalysts. It offers many advantages if compared with thermal pyrolysis, such as the fact that there are required lower temperatures and shorter reaction times; and so, in this case, less energy is consumed. Furthermore, the selectivity of the products is increased, since catalysts enhance the formation of high commercial value and quality products; and in the meantime, the formation of undesired products (e.g., brominated compounds) can be suppressed [15, 16]. As a consequence, various catalysts have been explored for pyrolysis of various types of plastics, including silica-alumina, zeolites (HZSM-5, etc.), mesoporous catalysts (MCM-41), metal-based catalysts, fluid catalytic cracking (FCC) catalysts, and minerals [9]. Among them, zeolites are the most widely investigated in case of nitrogen-containing polymers such as poly(acrylonitrile-butadiene-styrene) (ABS), since they promote the formation of aromatics [13]; but of course, their properties vary depending on the zeolite type.

*Gasification* includes partial oxidation or indirect combustion of polymers at high temperatures (up to 1600°C) and in the presence of oxygen. It results in the formation of two main products: CO and H2 (synthesis gas – syngas). Syngas can be used either in order to run a gas engine or it can be converted into hydrocarbon fuels via the Fischer-Tropsch process. More often than not, it is preferable to gain condensable liquids or petrochemicals as the main products; and that is the reason why pyrolysis is favored over gasification, since the latter requires multiple steps in order to obtain liquid products [13, 14].

*Hydrogenation* entails the conversion of large hydrocarbon molecules into lower-molecular-weight products. It takes place in hydrogen atmosphere, high pressure (approximately 100 atm), and at moderate temperatures between 150 and 400°C [14].

Generally, it should be underlined that pyrolysis can be considered as one of the best options for plastics recycling, since its advantages are aplenty. Specifically, pyrolysis enables material and energy recovery from polymer waste, as a very small amount of the energy content of waste is consumed for its conversion into valuable hydrocarbons. Furthermore, pyrolysis products are valuable, since they can be used as fuels or chemical feedstock. Last but not least, in case that flame retardants are present in plastic waste, via pyrolysis the formation of toxic substances may be restricted, due to the fact that it takes place in the absence of oxygen [17]. Of course, catalyst's presence, as mentioned previously, plays a vital role. Apart from catalysts, various other parameters, including temperature, heating rate, residence time, operating pressure, etc., can strongly affect the quality and distribution of pyrolysis products [6].
