**1. Introduction**

During last decades, the great population increase worldwide together with the need of people to adopt improved conditions of living led to a dramatical increase of the consumption of polymers (mainly plastics). The world's annual consumption of plastic materials has increased from around 5 million tones in the 1950s to nearly 100 million tones today. Since the duration of life of plastic wastes is very small (roughly 40% have duration of life smaller than one month), there is a vast waste stream that reaches each year to the final recipients creating a serious environmental problem. The presently most common practice of handling such waste streams is to incinerate them with energy recovery or to use them for land-filling. Disposing of the waste to landfill is becoming undesirable due to legislation pressures (waste to landfill must be reduced by 35% over the period from 1995 to 2020), rising costs and the poor biodegradability of commonly used polymers (Achilias et al., 2009). Therefore, recycling seems to be the best solution.

The recycling of waste polymers can be carried out in many ways. Four main approaches have been proposed presented in Scheme 1 (Karayannidis and Achilias, 2007; Scheirs, 1998):


<sup>\*</sup> Lefteris Andriotis, Ioannis A. Koutsidis, Dimitra A. Louka, Nikolaos P. Nianias, Panoraia Siafaka, Ioannis Tsagkalias and Georgia Tsintzou

*Laboratory of Organic Chemical Technology, Department of Chemistry, Aristotle University of Thessaloniki, Thessaloniki, Greece* 

Recent Advances in the Chemical Recycling

poly(methyl methacrylate) (PMMA) and nylon.

Scheme 2. Chemical recycling techniques of PET.

**2. Chemical recycling of poly(ethylene terephthalate) PET** 

and Karayannidis, 2004).

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 5

which could be useful as feedstock for a variety of downstream industrial processes or as transportation fuel. In this sense, among the techniques proposed for recycling of waste polymers the most challenging method is chemical or feedstock recycling and various technologies have been successfully demonstrated and continue to be developed (Achilias

The aim of this chapter is to provide a critical review of the methods proposed and/or applied during mainly **the last decade**, on the chemical recycling of polymers. In this way, the state-of-the-art of the chemical recycling methods of several polymers will be presented. Polymers that will be studied include the widely used plastics, based on poly(ethylene terephthalate) (PET), polypropylene (PP), polystyrene (PS), low density polyethylene (LDPE), high density polyethylene (HDPE), poly(vinyl chloride) (PVC), polycarbonate (PC),

PET is a polyester with functional ester groups that can be cleaved by some reagents, such as water (hydrolysis), alcohols (alcoholysis), acids (acidolysis), glycols (glycolysis), and amines (aminolysis). Thus, chemical recycling processes for PET are divided as follows: (i) Hydrolysis, (ii) Glycolysis, (iii) Methanolysis and (iv) other processes (Scheme 2). According to the reagent used different products are obtained (Karayannidis and Achilias, 2007; Karayannidis et al. 2006; Karayannidis et al., 2005; Karayannidis et al. 2002; Kosmidis et al., 2001). The different process options for chemical recycling of PET waste may be categorized as follows: (i) regeneration of base monomers (methanolysis for dimethyl terephthalate (DMT) and hydrolysis for producing pure Terephthalic acid (TPA) and ethylene glycol (EG)); (ii) conversion into oligomers (glycolysis or solvolysis); (iii) use of glycolyzed waste for value-added products; (iv) conversion into speciality chemicals by aminolysis or ammonolysis; (v) conversion into speciality intermediates for use in plastics and coatings.

recycled resin is reduced due to chain scission reactions caused by the presence of water and trace acidic impurities. Strategies for maintaining the polymer average molecular weight during reprocessing include intensive drying, reprocessing with degassing vacuum, the use of chain extender compounds, etc.


Apart from the aforementioned methods, direct reuse of a plastic material (i.e. PET) could be considered as a "zero order" recycling technique (Nikles and Farahat, 2005). In a lot of countries it is a common practice PET-bottles to refilled and reused. However, this should be done with a great care since plastic bottles are more likely than glass to absorb contaminants that could be released back into food when the bottle is refilled. Moreover, refill of a PET-bottle with a high-alcoholic-degree drink may lead to degradation of the macromolecular chains with unexpected results.

Scheme 1. Polymer Recycling Techniques.

The objective of a plastic management policy, in accordance with the principles of sustainable development (development that meets the needs of present generation without compromising the ability of future generations to meet their needs), should be not only the reuse of polymeric materials but also the production of raw materials (monomers), from which they could be reproduced, or other secondary valuable products,

3. *Chemical or Feedstock recycling (tertiary recycling)* has been defined as the process leading in total depolymerization of PET to the monomers, or partial depolymerization to oligomers and other chemical substances. The monomers could subsequently re-

4. *Energy recovery (Quaternary recycling)* refers to the recovery of plastic's energy content. Incineration aiming at the recovery of energy is currently the most effective way to reduce the volume of organic materials. Although polymers are actually high-yielding energy sources, this method has been widely accused as ecologically unacceptable owing to the health risk from air born toxic substances e.g. dioxins (in the case of

Apart from the aforementioned methods, direct reuse of a plastic material (i.e. PET) could be considered as a "zero order" recycling technique (Nikles and Farahat, 2005). In a lot of countries it is a common practice PET-bottles to refilled and reused. However, this should be done with a great care since plastic bottles are more likely than glass to absorb contaminants that could be released back into food when the bottle is refilled. Moreover, refill of a PET-bottle with a high-alcoholic-degree drink may lead to degradation of the

The objective of a plastic management policy, in accordance with the principles of sustainable development (development that meets the needs of present generation without compromising the ability of future generations to meet their needs), should be not only the reuse of polymeric materials but also the production of raw materials (monomers), from which they could be reproduced, or other secondary valuable products,

vacuum, the use of chain extender compounds, etc.

polymerized to regenerate the original polymer.

chlorine containing polymers).

macromolecular chains with unexpected results.

Scheme 1. Polymer Recycling Techniques.

recycled resin is reduced due to chain scission reactions caused by the presence of water and trace acidic impurities. Strategies for maintaining the polymer average molecular weight during reprocessing include intensive drying, reprocessing with degassing which could be useful as feedstock for a variety of downstream industrial processes or as transportation fuel. In this sense, among the techniques proposed for recycling of waste polymers the most challenging method is chemical or feedstock recycling and various technologies have been successfully demonstrated and continue to be developed (Achilias and Karayannidis, 2004).

The aim of this chapter is to provide a critical review of the methods proposed and/or applied during mainly **the last decade**, on the chemical recycling of polymers. In this way, the state-of-the-art of the chemical recycling methods of several polymers will be presented. Polymers that will be studied include the widely used plastics, based on poly(ethylene terephthalate) (PET), polypropylene (PP), polystyrene (PS), low density polyethylene (LDPE), high density polyethylene (HDPE), poly(vinyl chloride) (PVC), polycarbonate (PC), poly(methyl methacrylate) (PMMA) and nylon.
