Polyethylene terephthalate catalyzed by titanium (IV) butoxide : experimental design (DOE) and mechanism

(eng) Polyethylene terephthalate (PET) is one of the major polymers produced worldwide. However, the polycondensation of PET requires the use of a catalyst which is nowadays antimony-based. Even if there is still no restrictive legislation, antimony-based catalysts are suspected to be hazardous and have come under increasing scrutiny with regard to food packaging applications. PET producers are therefore searching for alternatives. A possibility is to use titanium alkoxides which are cheap, very efficient catalysts and said to be of low toxicity. Nevertheless, titanium alkoxides induce a strong yellowing of the polymer that is commercially unacceptable. Consequently, the major objective of the thesis is to find a synthesis route compatible with the industrial requirements in order to avoid the coloration arising from the use of titanium catalysts. As a polymerization process is influenced by large number of factors, it cannot be approached globally with classical methodologies. For this reason, the study of the PET synthesis proposed here will use a statistical methodology called “design of experiments” (DOE). The DOE method uses the multivariate analysis in order to model a phenomenon from a limited and well-defined number of experiments. The DOE study demonstrates that it is possible to synthesize PET suitable for soft-drinks bottles using titanium alkoxides. This result is obtained by combining low titanium catalyst concentration and moderate synthesis temperature with the addition of phosphoric acid during the polycondensation. The role of phosphoric acid is to limit the strong catalysis of the PET thermal degradation by the titanium which is the main cause of the yellowing. Finally, in the search for colored species, an unexpected biphenyl structure was identified in the PET degradation products. This structure is not directly responsible of the PET yellowness but provides important informations concerning the side-reactions arising from the thermal degradation.