**6.2 Chemical recycling example—Polyethylene terephthalate (PET)**

PET is the most common thermoplastic polymer of the polyester family of organic polymers [41, 42]. It is a naturally transparent and semi-crystalline plastic, which is used for packaging, for manufacturing stretch-blown bottles, sheet, and thermoforms, and for producing fibers for textile products [43]. The important characteristics of PET include resistance to water, high strength to weight ratio, and wide availability as an economic and recyclable plastic [44].

As condensation polymers, polyesters are constructed from the reaction of multifunctional carboxylic acids and polyols in which water or alcohol is eliminated to form a growing polymeric chain. The condensation polymer construction of PET provides a unique chemistry leading to its formation through the elimination of water or alcohol depending on the composition of monomers. PET as a condensation polymer has unique chemistry controlling its assembly or disassembly. It is important to understand the synthesis of PET to enable the depolymerization option to recycle PET optimally.

Exhibiting significant mechanical properties, PET's rigid polymer chains and high melting point contribute resistance to fatigue and plastic deformation as desirable features. Resistance to solvents, chemicals, and hydrolysis at normal application temperatures identify PET as a highly desirable component to a host of applications. PET's manufacture is affected with the unattractive disadvantage of a relatively slow rate of crystallization leading to increased processing time of a slow cooling cycle necessitating nucleating agents. As a crystallizable polymer, PET exhibits properties of different desirable states of physical order ranging from amorphous (transparent) to crystallized that are transparent and opaque [45].

#### *6.2.1 PET synthetic technology*

PET synthesis is configured in two steps [46]. An esterification of terephthalic acid (TPA) or dimethyl terephthalate (DMT) reaction with ethylene glycol (EG) can be used to form a prepolymer mixture product composed of bis(2-hydroxyethyl) terephthalate (BHET) and short-chain oligomers (**Figure 6**).

**Figure 6.** *Polyethylene terephthalate (PET) synthesis.*

The condensation reaction requires the removal of water or methyl alcohol to force the reaction to completion. The growing polymer formed by equilibrium esterification is susceptible to depolymerization by hydrolysis or methanolysis, so the removal of any solvolytic agent is important to build molar mass.

A second step is transesterification in melt phase leading to the polycondensation of short chains to form the molar weight range and intrinsic viscosity required for an intended application [47]. The prepolymer formation step can be conducted with DMT or TPA as one monomer with EG or another polyol as a monomeric partner. Even at the prepolymer step, the oligomeric mixture is difficult to purify and this emphasizes the need for high purity starting materials. DMT can be purified by distillation or crystallization from a melt. Heating TPA beyond its boiling point leads to decomposition, and purified TPA can be produced by recrystallization at process production scale. Currently, TPA is the monomer of choice for more than 70% of the PET production across the globe [48]. Where DMT is employed, purification is done by distillation to remove higher boiling constituents and light molecular weight esters. High-purity DMT is achieved by recrystallization. The quality and color of high-purity DMT are required to produce high molecular weight PET.

Industrially, PET is produced by the reaction of EG with the DMT or TPA in the presence of a catalyst (**Figure 7**) [48]. High conversions to condensed polymer in the first stage are established from a stoichiometric balance of reacting groups. Excess EG is employed in industrial processes, which is removed and recycled as part of the process. Early processes synthesized PET at industrial scale through the two-step polymerization reaction between DMT and a 30–50% excess of EG in the presence of a catalyst involving transesterification and polycondensation [49]. In the first step, transesterification of DMT with EG leads to the initial formation of BHET and a small number of assorted oligomers. This reaction is conducted at atmospheric pressure at a temperature range from 150 to 210°C under an inert atmosphere to minimize oxidative side reactions. Methanol is removed to force the reaction to completion with the formation of BHET and hydroxyethyl–terminated oligomers [49].

Polycondensation is the chemistry of the second step, which involves increasing the temperature to 270–280° C, exceeding the melting temperature of PET, and initiating the application of high vacuum (10–50 Pa). During this stage, EG is formed and removed as a by-product. The product PET has a degree of

**Figure 7.** *rPET synthesis from PET waste using depolymerization.*

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

polymerization of approximately 100. The equilibrium reaction of polycondensation requires the removal of excess EG through high vacuum and intensive mixing of the molten PET in the reactor. At the polycondensation stage, the intrinsic viscosity of the reaction mixture increases as the molar weight of the polymer increases. Removal of volatiles from the molten phase becomes a rate limiting process. Thermal degradation of the polymer occurs as heating more than 300° C is applied due to PET's limited thermal stability.

Judicious selection of polymerization catalyst and temperature control can be used to reduce the reaction time of the two steps. Significant investigation of the tertiary strata options for PET recycling has been conducted (**Figure 8**) leading to commercial opportunities for the recycle industry. Ammonia can be used to recycle PET to different materials using similar chemistry [50].
