## 1 Introduction  
The increasing population growth with urbanization in developing countries has led to increased production of plastic waste. Until the 1970s, rather small amounts of plastic waste were produced. Between the 1970s and 1990s, plastic waste generation tripled. In the single decade of the 2000s, plastic waste increased more than the respective amounts of the previous 40 years. Nowadays, more than 400 million tons/year of plastic waste are produced. With this trend, global production of plastic is forecasted to reach 1100 million tons by 2050 [1]. Until today, of 7 billion tons of plastic waste generated globally, only 10% has been recycled. Despite the last year's effort, about 75 to 199 million tons of plastic waste is found in the oceans. In this regard, plastic waste in ecosystems could almost triple, from 9 to 14 million tons per year in 2016 to 23-37 million per year, by 2040 [1]. Currently, it is widely accepted that the primary management approaches for plastic waste should be reuse and recycling. Indeed, reuse and recycling are related to various advantages such as low environmental impact, resource-saving, etc. However, there are factors of various origins (scientific, practical, cultural) that render practically impossible the recycling of 100% of the produced plastic waste, at least currently or in the immediate future. In general, plastic waste recycling is rather expensive [2]. More specifically, in various countries, including Greece, the social culture for recycling is only at its beginning, e.g., plastic, paper, metal, and glass waste are collected together. This lack of recycling culture creates practical problems for recycling. For example, additional cost, energy, and labor work are needed to separate the different types of recyclable waste. Though common domestic plastic waste contains mainly six thermoplastic polymers (low and high-density poly(ethylene), poly(propylene), poly(ethylene terephthalate), poly(vinyl chloride), and poly(styrene)), other polymers may be present. For proper recycling, further separation of each polymer is required. In addition, plastic waste from food packaging typically is not cleaned by the end-user prior to disposal, and thus a portion of plastic waste is \"contaminated\" by food residues. This contamination is translated to various problems of additional energy and cost or even may render impossible the cleaning of the plastic and thus its recycling. In addition, it is known that recycled polymers may suffer from deteriorated properties due to low purity and/or thermal oxidation during thermal processing. Recently, it was reported that various common polymers such as cellulose esters [3,4], poly(vinyl alcohol) [4,5], and poly(vinyl chloride) [5] do not exhibit any actual melting and on the contrary, their solid-liquid (or glass-to-rubber) transition is a peculiar effect of simultaneous softening and (minor) decomposition. This minor thermal degradation is independent of any thermal oxidation and may contribute to additional deterioration of the polymer's properties after recycling (additional thermal processing). Also, a considerable amount of plastics is already in oceans or (either legally or illegally) has been landfilled or disposed of on the ground's surface. Such plastic waste has little potential for recycling. Similarly, multilayered plastic waste and other composite plastic materials also have limited recycling potential [6] unless delamination is carried out [7]. Consequently, it is not surprising that various research efforts focus on management approaches to plastic waste other than recycling. Such methods include pyrolysis and gasification, which aim to degrade plastic in the absence of oxygen in a controllable manner in order to obtain high-added value gas, liquid, and solid products [8, 9, 10].