PET degradation

Acids such as sulfuric or nitric acids or bases such as sodium hydroxide may catalyze the hydrolysis of PET. It has been demonstrated that the rate of alkaline PET hydrolysis increases in the presence of quaternary ammonium compounds.26 27 Niu et al.26 reported an increase in the rate of alkaline PET degradation in the presence of dodecylbenzyldimethylammonium chloride at 80°C. Polk et al.27 reported increases in the rate of sodium hydroxide depolymerization of PET in the presence of trioctylmethylammonium chloride, trioctyl-methylammonium bromide, and hexadecyltrimethylammonium bromide at 80° C. 

The vapour pressures of the main volatile compounds involved in esterification and polycondensation are summarized in Figure 2.25. Besides EG and water, these are the etherification products DEG and dioxane, together with acetaldehyde as the main volatile product of thermal PET degradation. Acetaldehyde, water and dioxane all possess a high vapour pressure and diffuse rapidly, and so will evaporate quickly under reaction conditions. EG and DEG have lower vapour pressures but will still evaporate from the reaction mixture easily. 

A particularly relevant thermo-oxidative study on PET degradation and PBT reported the degradation products observed for ethylene dibenzoate [39], The products observed paralleled those of the photolysis and photo-oxidation reports discussed above with benzoic acid, vinyl benzoate, 2-hydroxyethylene dibenzoate, 2-carboxymethoxy benzoate and the coupling product, 1,4-butylene dibenzoate, being reported. The 2-hydroxyethylene dibenzoate and 2-carboxymethoxy... 

PET degrades thermally during processing at 280-300 °C. The chain scission is catalyzed by transesterification catalysts (zinc, manganese or cobalt acetates, alkyl titan-ates). PET is stabilized by tributyl phosphate (46) or triphenyl phosphate (47), considered as heat stabilizers and metal deactivators (Karayannidis et al., 1998). 

In contrast to polymerisates, polycondensates can not be depolymerized under inert conditions. Decomposition usually leads to the destruction of the chemical structure and the monomers. The thermal decomposition of PET starts at about 300°C in an inert atmosphere [25]. Between 320 and 380°C the main products are acetaldehyde, terephthalic acid, and carbon oxides under liquefaction conditions. The amounts of benzene, benzoic acid, acetophenone, C1-C4 hydrocarbons, and carbon oxides increase with the temperature. This led to the conclusion that a P-CH hydrogen transfer takes place as shown in Eigure 25.8 [26]. Today the P-CH-hydrogen transfer is replaced as a main reaction in PET degradation by several analytic methods to be described in the following sections. The most important are thermogravimetry (TG) and differential scanning calorimetry (DSC) coupled with mass spectroscopy and infrared spectroscopy. 

Controlled degradation of PP (BASF, Exxon) was done using a single-screw extruder with an air and polymer feed. The Eastman Kodak PET degradation process involves the coextrusion of PET and ethylene glycol. 

Depending on the depolymerization agent, polyester chemolysis methods have been classified as follows glycolysis, methanolysis, hydrolysis, ammono-lysis, aminolysis and combined processes. Figure 2.1 summarizes the different alternatives for PET chemolysis, as well as the type of products derived from each one. All these PET degradation alternatives are reviewed in the following sections. 

An alternative glycolytic method of PET degradation is based on treatment at... 

Hydrolytic treatments can serve not only as a PET degradation method, but may simultaneously enable the separation of hydrolysable and non-hydro-lysable polymers present in the plastic waste stream. Thus, Saleh and Wellman64 have proposed the separation of PET and polyolefin mixtures by treatment with water from about 200 °C up to the critical temperature of water under autogenous pressure. The resulting liquid phase contains the hydrolysis products, TPA and ethylene glycol, whereas the solid phase is formed by the non-reacted polyolefins. 

While PLA was invented about 150 years ago, its moisture sensitivity and relatively high cost meant it did not have any packaging applications. Most uses of PLA were for medical applications where cost is not a harrier, and its biocompatibility is extremely important. When PET degrades, it produces lactic acid, which is also produced naturally in the human body (associated with muscle fatigue, for example). The advantages of something that can be implanted in the human body and then will slowly degrade and disappear over time are obvious. 

FIGURE 6.2 PET degradation by glycolysis, hydrolysis, and methanolysis. (After De Winter, W. 1992. Die Makromol. Chem., Macromol. Symp., 57, 253.)... 

As was noted in Chapter 4, polyesters such as polyethylene terephthalate (PET) degrade mainly in a thin surface layer under the influence of light and oxygen. This suggests that photostabilisation in the bulk of an article will not be optimally used at best, portions of the additive distributed in the interior will act as a reservoir to replace stabiliser lost in the outer regions during any of the chemical or physical processes under way. 

FIGURE 2.2 PET degradation by glycolysis, hydrolysis, and methanolysis. (After De Winter, W. 1992. 

Enzymatically released aromatic PET degradation products can be monitored by UV detection at 240-255 nm, following separation by reversed-phase high performance liquid chromatography (HPLC). Soluble hydrolysis products of PET films, fibers, and cyclic PET trimers (CTR) that have been identified include TPA, mono(2-hydroxyethyl) terephthalate (MHET), Z A(2-hydroxyethyl terephthalate) (BHET), 1,2-ethylene-mono-terephthalate-OTono(2-hydroxyethyl terephthalate) (EMT), and 1,2-ethylene-/ A-terephthalate (EBT) [8, 33, 38, 40, 90, 102] (Fig. 1). 

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