Kinetics of poly(ethylene terephthalate) glycolysis by diethylene glycol. I. Evolution of liquid and solid phases
Introduction
PET is widely used in packaging (soft-drink bottles, films). Thus, it is an important source of waste but different recycling methods exist. Glycolysis is one of the most attractive methods. Glycolysis by ethylene glycol (EG) leads to the bis-2-hydroxyethyl terephthalate (BHET) monomer (which can be used for PET synthesis). PET solvolysis by other diols or polyols can also lead to specific products like polyurethane foams, unsaturated polyesters, plasticizers, alkyd resins, etc. [1], [2].
In the literature, glycolysis products obtained at equilibrium have been characterized and showed that they do not result from a simple reaction but from multiple alcoholysis reactions [3], [4]. On the other hand, very few studies relate to reactions occurring in the first stages, where a liquid phase coexists with a more or less crystallized solid phase. Thus, one knows few things on crystallinity evolution, glycol diffusion and glycolysis mechanisms. We have reported kinetic data on solvolysis of PET by several glycols [5], which suggest that the ability of the glycol to solvate the polyesters plays a significant role. From visual inspection of quenched, partially glycolysed PET, Campanelli et al. [6] deduced that the reaction occurs in a single homogeneous phase above 245 °C, whereas below 245 °C solid PET is dispersed into a liquid phase. Yoshioka et al. [7] have assigned the rate increase of PET hydrolysis by sulfuric acid solution at 150 °C after 3 h of reaction, to the appearance of cracks on the PET surface.
A more precise description was given by Kurokawa et al. [8] who have studied the methanolysis of PET, catalysed (by aluminium triisopropoxide) or not. At 200 °C, without catalyst, a very large majority of the PET chips (92%) were found unchanged, though cracks could be observed. With catalyst, at 180–190 °C, a modest yield in monomers (dimethyl terephthalate + EG) was obtained, while the PET which remained solid presented the same type of cracks and its surface became rough. When the temperature was raised from 190 to 200 °C, the yield of the catalysed reaction increased abruptly while the PET, which remained solid, was found in the form of powder. They proposed a three-step mechanism for the reaction: (1) cleavage at a tie molecule connecting PET crystals (chain length shortened to 1/3); (2) depolymerization, at random positions, to oligomers; and (3) depolymerization of the oligomers to monomers, which was only possible with catalyst.
We previously described kinetic features of PET glycolysis by diols or triols [5] and of model polyesters [9]. Here, we describe more detailed kinetic features but limited to the glycolysis of PET by DEG. The object of this work is a better understanding of the reactivity in these systems constituted initially of a liquid (glycol) and a solid phase (PET), which evolve to a single homogeneous liquid phase. In this part, we report the results obtained with an experimental device allowing isothermal kinetic monitoring of both the liquid and the solid phases. We will describe in part II, to be published, the effect of some experimental conditions (catalyst, temperature, initial PET morphology) [10].
Section snippets
Materials
Poly(ethylene terephthalate) (PET) bottle preforms (manufactured from ICI resin “B95A Laser”) were cut into 20 × 20 mm fragments (thickness: 4 mm). Diethylene glycol (DEG) was purchased from the market and used without purification.
Analysis
The determination of the free glycols ratio (DEG remaining and EG formed) by gas chromatography (GC) and size exclusion chromatography (SEC) analyses were performed as previously described [5].
The molecular distribution of the polyesters, which remained solid, was
Kinetic study
We described earlier how to follow the glycolysis kinetics of PET [5] and model polyesters [9], in conventional stirred reactor. In these studies, the system evolution was followed through analyses of the liquid phase by:
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UV spectrometry (dissolution of aromatic polyesters).
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Gas chromatography (decrease of free DEG mass fraction, from both solubilisation of polyesters and incorporation of DEG moieties in polyesters chains; increase of free ethylene glycol – EG – mass fraction).
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SEC (molecular
Conclusions
This experiment with a rotatable basket allowed us to better understand the PET glycolysis, at least at 220 °C and without catalyst. The study of the changes of both the solid and the liquid phases was possible. The solid phase changes according to two essential characteristics:
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Decreases until stabilization of the average molecular weight. More exactly, the shape of molecular distribution does not change but shifts towards the lower molar masses. One does not notice any formation of very low
Acknowledgements
This work was supported financially by ADEME (Agence de l'Environnement et de la Maîtrise de l'Energie).
The authors would like to thank Prof. A. Jonquières (ENSIC, Nancy), Prof. A. Fradet (UPMC, Paris), G. Denis (Nestle Waters) and D. Menkès for their contribution to this work. They also thank N. Ruscassier and M. Jérôme for the SEM characterization, and J. Dubois for his advises in SEC.
References (19)
- et al.
Comparative reactivity of glycols in PET glycolysis
Polym Degrad Stab
(2006) - et al.
Polym Degrad Stab
(2003) - et al.
Reactivity of polyesters in glycolysis reactions; unexpected effect of the chemical structure of the polyester glycolic unit
Polym Degrad Stab
(2006) - et al.
Polymer
(2000) - et al.
Vib Spectrosc
(2004) - et al.
Polym Degrad Stab
(2000) - et al.
Ind Eng Chem Res
(1997) - et al.
J Appl Polym Sci
(1999)
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