**3. Chemical recycling methods**

Heaps of polymer wastes are generated due to the extensive use of polymers in many applications and generally life time of the product is exceeded by that of the polymers that the product is made of. The recycling strategies allow production of new polymeric materials from waste. There are existing and emerging approaches for recycling polymer waste which may contain thermoplastics or crosslinked polymers. The categories of recycling technologies that include primary, secondary (mechanical), tertiary (chemical), and quaternary approaches may be summarized as follows [17, 31, 44, 56–66]:

Primary recycling, also termed as closed-loop recycling, refers to reprocessing of industrial byproducts and pre-consumer scrap materials to give a product that will be used for the same purpose as the original one without loss of properties. Primary textile waste may be single or complex polymers that are usually easy to recycle.

Secondary recycling involves mechanical applications such as grinding, melting, and reforming, for processing post-consumer products into new ones with different physical/chemical properties. In contrast to primary recycling, extraction/dissolution and purification of materials is needed for secondary recycling since materials with unknown composition and purity are treated.

Tertiary recycling approach covers chemical processes such as pyrolysis and hydrolysis, in which chemical structure of waste is broken and converted to basic chemical constituents, monomers, or fuels. It is also known as feedstock recycling.

Quaternary recycling is waste-to-energy conversion process where the energy of fuel value of waste is recovered via incineration or pyrolysis.

Polymers, either orinating from natural or synthetic sources, can be recycled commercially by using one of the recycling processes aforementioned. **Figure 3** presents the polymer recycling approaches.

Other than the recycling approaches presented, biodegradable recycling is an emerging approach especially performed for natural based textile materials. Special microorganisms, enzymes, diverse bacteria and fungi are utilized for degradation of biological polymers (cellulose, chitin, wood, hemp) and organic compounds (PET, polylactic acid-PLA, 1,4butanediol, etc.) [59].

To recycle polymeric waste, chemical recycling technologies offer complementary solutions to mechanical recycling. Chemical recycling, in other words feedstock recycling, breaks down the synthetic fibers for repolymerization and yields the monomers of the polymers (or partially depolymerises to oligomers) by performing processes such as hydrolysis, pyrolysis, gasification, condensation, glycolysis, hydrocracking, dissolution etc [57, 59].

**Figure 3.**

*Polymer recycling approaches (adapted from [56]).*

#### **Figure 4.**

*Simplified process diagram of chemical recycling (adapted from [60, 67].*

For chemical recycling the synthetic fibers are chopped, pelletized, depolymerised by chemicals and repolymerised for fiber formation. An overview of the process is shown in **Figure 4**.

Before chemical recycling, the feedstock has to be carefully sorted and all nontextile elements and contaminants (buttons, zips, etc.) need to be removed. Missing or washed-out care labels in garments make manual sorting inaccurate and automated near-infrared identification techniques are required for successful sorting. After sorting, the feedstock is shredded into small fragments to facilitate the dissolving process. The recovery of synthetic fibers involves depolymerisation followed by the production of polymer pellets. A cellulosic pulp is obtained from the cotton. Solvents used are typically recovered to minimize waste and reduce processing costs [35].

Pyrolysis, sometimes referred to as thermolysis, is a thermo-chemical process that polymers are subjected to various temperatures and pressure levels in the presence of catalysts or reactive gases and, decomposed. Pyrolysis processes are economically viable for polyamide 6 polymers. For depolymerisation of polyester, frequently studied approaches are glycolysis, hydrolysis and metanolysis that use glycol, water and methanol, respectively [57].

### *An Evaluation of Recycled Polymeric Materials Usage in Denim with Lifecycle Assesment… DOI: http://dx.doi.org/10.5772/intechopen.99446*

Solvolysis is depolymerization of cross-linked polymers by using solvents to break covalent bonds in the tridimensional network. The process can be conducted in a wide range of temperature and pressure by employing different solvents. The need for high temperature, pressure and harsh chemicals limit the industrial adoption of solvolysis [17].

Chemical processes allow successful fiber-to-fiber recycling since undesired non polimeric constituents such as colorants, catalysts, surface treatments, backing materials, and other auxiliary chemicals used in textile production are removed. On the other hand, contamination rates as high as 20–30% by weight is economically feasible for chemical recycling. As a result of degradation or contamination of the physical quality during mechanical recycling, chemically recycled polymers offer better inherent quality properties. Another benefit for chemical recycling is that form of the polymer -bottle, jacket, industrial scrap, automobile component, etc.-to be recycled does not matter [58].

Despite its advantages, there are also limitations for large scale applications of chemical recycling processes due to the requirement of considerable amounts of energy inputs for the present methods and due to many uncertainties about their environmental impacts. Even though processes for chemical recycling are technically viable, the ecological and economical impacts need to be questioned. Technologies for chemically recycling of polyester and some other polymers have been existing for some time however, building and operating costs of chemical recycling facilities are higher than those of mechanical recycling facilities. Presence of additives and chemicals used during polymerization might also complicate the processing and affect the purity and quality of the monomers obtained after chemical recycling [35, 58, 59, 65].

Existing chemical processes for recycling polyester are expected to increase and new chemical recycling approaches for polyester in laboratory scale are expected to be developed until 2030. Chemical recycling of cotton and other cellulose based fibers are expected to be developed in full scale by 2030 [68].
