Hydrolysis of PET

Chemical Properties. The hydrolysis of PET is acid- or base-catalyzed and is highly temperature dependent and relatively rapid at polymer melt temperatures. Treatment for several weeks in 70°C water results in no significant fiber strength loss. However, at 100°C, approximately 20% of the PET tenacity is lost in one week and about 60% is lost in three weeks (47). In general, the hydrolysis and chemical resistance of copolyester materials is less than that for PET and depends on both the type and amount of comonomer. 

Hydrolysis, although a simple method in theory, yields terephthalic acid (TPA), which must be purified by several recrystallizations. The TPA must be specially pretreated to blend with ethylene glycol to form premixes and slurries of the right viscosities to be handled and conveyed in modern direct polyesterification plants. Hie product of the alkaline hydrolysis of PET includes TPA salts, which must be neutralized with a mineral acid in order to collect the TPA. That results in the formation of large amounts of inorganic salts for which commercial markets must be found in order to make the process economically feasible. There is also the possibility that the TPA will be contaminated with alkali metal ions. Hydrolysis of PET is also slow compared to methanolysis and glycolysis.1... 

Methanolysis products are separated and purified by distillation. BHET, the monomer obtained by PET glycolysis, is normally purified by melt filtration under pressure. One of the problems encountered in neutral hydrolysis of PET is that the terephthalic acid isolated contains most of the impurities initially present in the PET waste. Hence very elaborate purification processes are required to obtain terephthalic acid of commercial purity. 

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. 

Neutral hydrolysis of PET is usually earned out under pressure (1-4 MPa) at temperatures of 200-300°C.12 High-purity TPA and EG may be obtained by the hydrolytic depolymerization of PET in an autoclave with excess water. PET hydrolysis occurs faster in the molten state than as a solid therefore, it is... 

Hydrolysis of PET with Sodium Hydroxide in Ethylene Glycol... 

Aluminium trihydrate (ATH) decomposes, absorbing energy from the flame and evolving water vapour which blankets and smothers the flame. The resulting water vapour at 230 °C can cause massive hydrolysis of PET. 

Brominated flame retardants can induce degradation namely, acid-catalyzed hydrolysis of PET. 

Fig. 3 Top row. Hydrolysis of PET with 7% crystallinity with a Humicola insolens cutinase for (a) 0, (b) 12 and (c) 48 h resulting in weight losses of 0, 18% and 54%, respectively. Bottom row. Hydrolysis of PET with (a) 34.8% crystallinity (blank), (b) a lipolytic enzyme (no weight loss) and (c) 1 M NaOH (modified from [14, 18])...

Almansa E, Heumann S, Eberl A et al (2008) Enzymatic surface hydrolysis of PET enhances bonding in PVC coating. Biocatal Biotrans 26 365-370... 

Liebminger S, Eberl A, Sousa E et al (2007) Hydrolysis of PET and bis-(benzoyloxyethyl) terephthalate with a new polyesterase from Penicillium citrinum. Biocatal Biotrans 25 171-177... 

Figure 6.7 Reaction pathway proposed for the reaction of terephthalic acid produced by the hydrolysis of PET. (Reproduced with permission from Elsevier)...

In the series described thus far, it was found that the degradation of waste plastics proceeds efficiently by both thermal pyrolysis and hydrolysis in a steam atmosphere. A wax and carbonaceous residue produced by the hydrolysis of PET are decomposed by reaction with steam over an FeOOH catalyst, the activity of which remains stable in a steam atmosphere. However, the liquid product from generated from the process mentioned above contains a large amount of heavy oil, as shown in Figure 6.10. Both catalysts and chemical processes are required for efficiently upgrading the quality of the heavy oil. 

Base-catalyzed hydrolysis of PET can be used to obtain TPA and oxalic acid [19], PET is completely hydrolyzed to TPA and EG at 250°C in concentrated aqueous NaOH solution. Then EG is converted to oxalic acid and CO2 by the introduction of oxygen. Since the sodium terephthalate formed is stable to oxidation, the yield of TPA reaches 100%. EG is converted to oxalic acid by base-catalyzed oxidation with a maximum yield of 61% ... 

Figure 25.18 Fluidized-bed reactor for hydrolysis of PET [44]. (Reprinted from Polymer Degradation and Stability, Vol. 85, G. Grause etal., pp. 571, 2004, with permission from Elseveir)...

Masuda et al. [43] have shown that hydrolysis of PET in a fixed-bed reactor at 450°C in an atmosphere of 70% steam and 30% nitrogen led to a yield of 87% TPA and less than 1% carbonaceous residue. 

Scheme 1.73. Acid hydrolysis of PET producing chain scission and generation of acid and alcohol end groups. The carboxylic acid catalyses further hydrolysis.

Almansa, E., Heumann, S., Eberl, A., Fischer-Colbrie, G., Martinkova, L., Marek, J., Cavaco-Paulo, A., and Guebitz, G.M. (2008) Enzymatic surface hydrolysis of PET enhances bonding in PVC coating. Biocatal. Biotransformation, 26 (5), 365-370. 

The alkaline hydrolysis of PET involves treating the polyester with an aqueous solution of sodium hydroxide (4-20 wt%) under pressure at temperatures between 200 and 250 °C for periods of several hours.53,54 Under these conditions the sodium salt of TPA is formed and by acidification TPA is recovered from the solution as a precipitate. It has been observed that the rate of the PET alkaline hydrolysis increases in the presence of quaternary ammonium compounds. Thus, Niu et al.55 have reported on the alkaline degradation of PET fibres with addition of dodecylbenzyldimethylammonium chloride (DBDMAC) into the reaction mixture. A sharp increase in the PET hydrolytic degradation at 80 °C was observed with DBDMAC concentrations in the range 0-1.0 g/1, especially for the least crystalline fibres. The authors concluded that the rate enhancement by quaternary ammonium compounds occurs preferentially on the amorphous regions of the PET fibres. 

The influence of proton concentration on hydrolysis rate is evident in the hydrolysis of PET in hydrochloric acid [1]. The rate does not respond to increases in acid concentration <3 M, but increases rapidly as concentration is increased above this value. The explanation for this appears to lie with the low dielectric constant of PET, which means that the acid in the polymer is considerably less ionised than in aqueous solution. Similar observations bave been made in experiments carried out in sulfuric acid [16] and nitric acid [13, 17]... 

Acid-catalysed hydrolysis of PET is facilitated by carboxyl end groups in the polymer, but this is not the case with alkaline-catalysed hydrolysis. The reaction is therefore largely confined to the outer layers of a substrate under alkaline attack, although the irreversibility of the reaction and likely removal of products from the reaction site means that prolonged exposure can result in the gradual eating away of the polymer. This phenomenon has been used to good effect by the fibre industry, where alkaline treatments have been used to produce aesthetically useful effects on fabric surfaces [22, 23]. 

Hydrolysis Hydrolysis of PET with water, usually with acid or base catalyst, results in direct production of TA and EG. The process can be slow and produces lower yields than with other methods, and has been commercially less applied than approaches (1) and (2). 

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