*6.2.2 PET recycle*

Globally, some 245 million tons of plastic are estimated to be in use annually [51]. The largest component is fibers with a quarter of this polymeric composition attributed to packaging [52]. The second largest component of the plastic mass is composed of PET bottles whose contribution expands annually. The oxygen content of PET identifies it to be a poor candidate for thermal recycling.

Secondary recycling based on mechanical technology enables the recycling of granulated plastic feedstock that has been processed from common waste plastic materials [15, 16, 53, 54]. The heated process of extrusion in a rotating screw configuration offers conditions that induce thermal softening of the plastic mass and plasticization. This process provides blending of the plastic melt, which permits the recycling of single, mixed, or blended plastic compositions. An example of the desired properties of vPET and rPET is displayed in (**Table 3**) [55]. The characteristics of rPET can vary widely with different sourced feedstocks. The components of a mechanical recycling process are shown in (**Figure 8**) targeting waste PET to provide the cleanest feedstock to the extrusion process. Thermal conditions of the extruder can lead to oxidative degradation of the plastic mass, and shear induced in the melt can result in undesirable polymer chain breaking, branching, and crosslinking. Polymer chain length degradation leads to reduced mechanical properties and lessens the features required for polymer processing into forms such as bottles, which require a higher molar weight polymeric mass.

The process conditions can be optimized to avoid some of these problematic issues. The chemical composition of the plastic mass in an extrusion process is important to the mechanical effects placed on the melt. The fluid mechanics of the extrusion is adjusted to allow the melt to begin to flow. The intrinsic viscosity of the melt rises with increased polymeric molecular weight. Heated extrusion processing is the primary technology employed in the mechanical recycling of plastics to form a granulated feedstock without the use of solvents.

**Figure 8.** *Sequence of PET recycling process components.*

### *Waste Material Recycling in the Circular Economy - Challenges and Developments*


**Table 3.**

*Virgin vs. recycled polyester characteristic properties (Reconstructed after Vardicherla) [27].*

Atmospheric oxygen can react with shear-induced radicals to form peroxy radicals, and this leads to radical-induced decomposition. Plastics with high oxygen permeability have been observed to exhibit enhanced thermo-oxidation rates within the melt. The chemical and physical forces accompanying extrusion leads to undesired changes in tensile strength, elongation, and many other properties for rPET. Inventive use of semi-closed or open-loop recycling involving the addition of vPET during the recycling process is employed to alleviate material properties decline experienced during extrusion.

#### *6.2.3 Contamination effects*

Contamination in the recycling feedstock occurs as a leftover from previous processing or desired modifications to the original feedstock but without proper treatment before recycling generally exerts significant negative influence on the quality and variability of the recycled plastic [30, 56, 57]. A list of contaminants (**Table 4**) found in PET feedstock have been identified for their significant contribution to quality decrease and performance variability increase of the regenerated polymer [57]. PET is hygroscopic and must be dried to remove water to reduce any chain length reduction through hydrolysis at the melt stage. Super cleaning techniques are used in some situations to ensure the optional flake quality for the later polymerization steps [58]. The label adhesives can release acetic acid, which will catalyze polymer chain reduction through hydrolysis, specifically during heatrelated segments of the extrusion process. In the United States, the majority of virgin and rPET infrastructure are in the southeast and midwest (**Table 5**).

EG degradation and recombination products have been identified as the source of discoloration and clarity losses in the polymer product. Carry-over degradation products from thermal and oxidative conditions lead to yellowing and reduction of *Are Reliable and Emerging Technologies Available for Plastic Recycling in a Circular… DOI: http://dx.doi.org/10.5772/intechopen.101350*


#### **Table 4.**

*Metal contamination found in rPET.*


#### **Table 5.**

*Applications of chemical recycling with PET feedstock.*

PET mechanical properties. Polyvinyl chloride (PVC) is used in bottle cap liners commonly connected with PET bottles and presents a major problem in the PET recycling process. Under thermal treatment conditions, PVC degrades to form hydrochloric acid, which in PET melt conditions leads to the reduction of the polymer chain length by hydrolysis reducing the value of the rPET [59, 60]. This problem is intensified, since PET and PVC have the same density and are difficult to separate.

Catalyst residues and processing additives such as trace metals (antimony, cobalt, and manganese) from the previous processing of consumer PET wastes promote transesterification and polycondensation reactions in the recycled PET as part of the heated extrusion process (**Table 6**). The recycled PET formed by the action of these trace metal has a chemically heterogeneous composition and may be affected by changes to the viscosity of the melt leading to batch–batch variability [60, 61]. Melt degradation can also be caused by the presence of pigments that are used to color plastics. Colored feedstock leads to the formation of an unattractive and lower commercial value gray color rPET product.

The mechanical recycling of complex and contaminated PET feedstock is difficult [62–64]. Mechanical recycled rPET is commonly characterized by intrinsic viscosities that are relatively low and heterogeneous. These properties have excluded rPET from direct incorporation in the production of bottle-grade PET, high-quality industrial fibers and films. Carpets, textile fibers for wearing apparel, and plastic containers designed for non-food applications have employed lower


#### **Table 6.**

*Contaminants to the PET recycling process and their effects.*

quality rPET. Contaminant removal has been accomplished through a thorough cleaning step, which removes the spectrum of contaminants encountered. There is a significant difference in the performance of the cleaning step depending on the relative level of contamination. Heavily contaminated PET feedstock is much more difficult to recycle to high value products. An array of chemicals employed as necessary adjuvants to plastic production may exert significant roles to the recycling process and may be emitted throughout the plastic's life cycle [64, 65].

#### *6.2.4 Market for PET recycling*

Plastic recycling has become a crucial practice for the recovery of the fossil fuel resources embedded in the waste along with society's desire to control plastic use to avoid the detrimental littering of much of the annual plastic mass [61, 65]. A recent survey of U.S plastics recyclability paints a rather negative picture for the currently available technology [66, 67]. The recycling of plastics is at a significant growth stage begging for significant expansion. For certain segments of this expansion, an infrastructure development is necessary requiring considerable investment to a market that counts margins in pennies per pound.

A recognition of the resource value of this plastic mass continues to underscore the waste's value to create products of significant economic value [68]. Evaluations of the importance of this reutilization of plastic waste have been estimated in billions of dollars and have a transformative effect on the U.S. chemical industry and has led to an unforeseen boost to the economy [69, 70].

PET offers remarkable opportunities for recycling. At the mechanical recycling level, recycled material can be reprocessed multiple times. Feedstock is available in large quantities as a high PET content material. Dedicated collection systems for the recycling of PET bottles or separation from more complex feedstock can provide the required composition needed for recycling. Waste collection logistics is assisting the plastic recycling effort through the introduction of collection rate mandates, disposal bans, and quotas for reuse and reprocessing [71, 72]. Collection rates and

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

plastic consumption in all markets continue to increase. Increased rPET content has been found to be highly desirable in the production of new bottles; this has been observed with the willingness of drink brands to pay slightly more for the rPET of the vPET. Furthermore, drink brands have agreed to increased rPET composition over the next few years.
