Hydrolytic Degradation of PET

Thominette and Verdu studied the hydrolysis of PET (with equal to 27,000 g/mol) in boiling water. The evolution of molar mass is followed by size exclusion chromatography. It appears that 

Under acidic or basic conditions, the rate of hydrolysis is enhanced. Under acidic conditions, the hydrolysis involved protonation of the in-chain oxygen atom of the ester function followed by reaction with water to produce hydroxyl and carboxyl end groups compounds. Under basic conditions, the hydroxide anion attacks the carbonyl oxygen atom to produce same compounds. The hydrolysis reaction can be followed by measuring the increase in the concentration of carboxyl ends with time by using classical end group analysis (14,15). 

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In both cases hydrolytic degradation of PET was performed without any enzymic catalysis. Up to now there are no reliable indications available in the literature that microbes and enzymes can attack aromatic polyesters such as PET, PBT or poly(ethylene naphthalate) [2,41,42]. 

The hydrolytic degradation of a series of homo- and copolyesters analogous to PET and PEI based on the mentioned L-arabinitol, xylitol, o-mannitol, and galactitol were relatively fast at temperatures 10°C above their respective Tg [53]. 

Intramolecular photoinduced electron transfer (PET) may lead to the generation of acid (i.e., protons) from an anthracene-based sulphonium salt. An amphiphilic anthracene-based photoacid generator (An-PAG) has been synthesized and was reported to induce acid-catalyzed hydrolytic degradation of certain lipids upon UV exposure (Shum et al., 2001). At present, the application of this technique is limited by the short wavelength required to trigger the response. 

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. 

Many articles have been published on the hydrolytic degradation of poly(ethylene terephthalate) (PET) under acidic conditions [1-20] and basic conditions [21-30], but far fewer on similar degradation studies on poly(butylene terephthalate) (PBT) [31, 32], poly(trimethylene terephthalate) (PTT) [33] and poly(ethylene naphthalate) (PEN) [34, 35],... 

Studies on the waste from PET bottles hydrolytically depolymerised in a high-pressure autoclave equipped with a stirrer concluded that the degree of degradation of PET increases as the particle size of PET decreases, reaching a maximum of 24.61 % at a size of 1 mm x 1 mm [a.396]. The rate constants of hydrolysis of PET were found to be greater at higher particle size, in contrast to lower particle size, if catalyst (lead acetate) is used. Further,... 

PTT polymer pellets must be dried to a moisture level of <30 ppm, preferably in a close-loop hot air dryer, to avoid hydrolytic degradation during melt processing. Drying is carried out with 130 °C hot air with a dew point of < -40 °C for at least 4 h. Because of the faster crystallization rate, PTT pellets are already semicrystalline after pelletizing, and do not require pre-crystallization prior to drying as with PET. The dried polymer is extruded at 250-270 °C into bulk continuous filaments (BCFs), partially oriented yam (POY), spin-draw yam (SDY) and staple fiber. 

Too little has been published about the flow properties of PET as a criterion for processing. The results of melt flow index (MFI) testing conditions do not correlate with the processing behavior in the case of PET. This may be caused by the discrepancy between the shear rates in testing and processing. MFI is defined as the amount of polymer melt (in g) extruded within 10 min through an orifice of specified diameter at a standard load and temperature. In the case of PET, this method was not very popular until recently due to the sensitivity of this material to hydrolytic degradation. 

In addition to inherently faster crystallization kinetics in a water-heated mold, PBT offers several other benefits compared to PET. First of all, the lower processing temperatures required for PBT make it less susceptible to hydrolytic degradation and thus drying is not as critical as in the case of PET. Thermoplastic composites made from PBT also tend to have a higher % elongation to break (Table 15.2) [14, 15]. Although this attribute does not necessarily show... 

The relationships shown in Fig. 14.11 were experimentally determined for direct extrusion of non-dried PET with two vacuum zones at 5 mbar absolute pressure the 20% torque increase in the ZSK Me PLUS may be used to halve hydrolytic degradation at the same throughput or to increase throughput by 75% by additional speed increases while maintaining the same quality. The same process was successfully transferred to non-dried PLA. 

PET undergoes acid-catalyzed hydrolytic degradation during melt processing. The humidity must be kept below 0.005 %, which may be achieved by the addition of bis(2,6-di-tot butylphenylcarbodiimide) (50) (Karayannidis, 1998). 

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. 

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