**6.1 Plastic recycling strata**

The recycling process for plastics has been subdivided into four components: primary recycling, secondary recycling, tertiary recycling, and quaternary recycling [16–18]. Recovery of production scraps as a pure polymer stream or the extrusion of preconsumer polymer has been identified as the operational component of primary recycling. Secondary recycling involves sorting of waste polymer feedstock, waste polymer size reduction, cleaning the waste polymer flow, drying and extrusion to form desired recycled polymer products [19–24]. Multiple cycles of recycling at primary and secondary stages can be used to recover many polymers while maintaining expected performance characteristics.

Conversion of the polymer feedstock to its constituent monomers can be selected when the first two recycling stages do not offer the desired recycling flexibility, economic opportunity, or product quality [25]. This tertiary stage recycling can complement the earlier, more traditional recycling technology strata (**Figure 4**). A selective approach to converting the feedstock polymer to its monomeric components can retain significant feedstock value with the ability to convert the recycle stream to a spectrum of polymer applications. This depolymerized feedstock can be utilized in a spectrum of pathways leading to chemical production syntheses other than reforming recycled polyethylene terephthalate (rPET) as an upcycling endeavor. Chemical recycling utilizes reactions with the chemical structure of the polymer to convert the polymeric mass to monomers and related chemicals [15, 26–30]. The chemical recycling process permits the removal of material consigned to downcycling recycling technology such as colorants, process additives, and other contaminants. This removal provides a material that can be used as a feedstock for opportunities of upcycling plastic waste to access valueadded chemicals, thereby enhancing economic performance (**Figure 5**).

The nonselective technologies found useful at the last recycling stage are generally thermolytic such as pyrolysis and hydrocracking technologies that convert the polymer feedstock into monomer and pyrolytic oils [31, 32]. At a quaternary stage, incineration is employed to recover the energy content of the waste polymer feedstock [33–35]. The relegation of polymer feedstock to quaternary recycling can be attributed to feedstock complexity, processing economics, waste unsuitable to other forms of recycling, market forces, and waste polymer treatment demands and capabilities [24, 36, 37]. This stage of recycling recovers little value of the feedstock and may contribute to greenhouse gas emissions having other unintended environmental consequences. Selective chemical recycling of polymers has become more

**Figure 5.** *Tertiary plastic recycling strata.*

*Are Reliable and Emerging Technologies Available for Plastic Recycling in a Circular… DOI: http://dx.doi.org/10.5772/intechopen.101350*

attractive to engage significant process funding recently, since chemical depolymerization is designed to form the original monomers of the recycled polymer in successive recycling to approach continuous recyclability [38–40].
