Kinetics and Mechanistic Investigation of Hydrotalcite‐Catalyzed Melt Synthesis of Poly(ethylene terephthalate)

Abstract

Summary: Hydrotalcite‐catalyzed polycondensation of BHET was studied by thermogravimetry to elucidate the kinetics. The reaction was found to follow second‐order kinetics with respect to hydroxyl end groups. The overall activation energy of the polycondensation was found to decrease with increasing catalyst concentration before it leveled out to the value of 93 kJ · mol−1 at high catalyst concentration. This is a result of the uncatalyzed reaction that takes place parallel to the catalyzed one. The activation energy of the uncatalyzed path was found to be 156 kJ · mol−1. IR spectroscopy and XRD showed that the monomer intercalates between the layers of hydrotalcite at the beginning of the reaction enabling the complexation of oxygen‐containing functional groups with the hydroxide groups of the catalyst. Based on these findings a polycondensation mechanism is proposed. One end group of the monomer is activated in the form of alkoxide that counterbalances the positive charge of the hydrotalcite layer. This alkoxide group attacks an ester carbonyl group fixed close to it, generating a new ester bond and a glycoxide species. The role of hydrotalcite is to activate the reactants, rendering the attacking hydroxyl group more nucleophilic and the ester carbonyl group more electrophilic, and at the same time fixing the reactant together in a favorable geometry.Intercalation of BHET in the gallery of HT layers in a twisted alkoxide form.magnified imageIntercalation of BHET in the gallery of HT layers in a twisted alkoxide form.

Novel catalysts based on titanium dioxide/silicon dioxide for poly(ethylene terephthalate)

“…Poly(ethylene terephthalate) (PET) is one of the most important thermoplastics because of its balance of excellent mechanical properties, good thermal properties, and low cost 1–4. The global demand for PET was 37 million tons in 2004, and the output of PET will grow 8–10% annually in the next 10 years 5, 6…”

“…For the first step, a catalyst is not necessary because pure terephthalatic acid and ethylene glycol (EG) can undergo direct esterification under a certain pressure to form BHET while water is distilled out of the reaction system. This process is widely applied in industry 5, 6. However, the polycondensation of BHET must be catalyzed if a technically useful polymer is to be obtained because the reaction without a catalyst is too slow and many side reactions will occur when the melt is heated at high temperatures for a long time.…”

“Nothing Can Hide Itself from Thy Heat”: Understanding Polymers via Unconventional Applications of Thermal Analysis

“…The results have revealed that the new catalyst, hydrotalcite, is more active than the standard antimony‐based compounds. Further insights into the kinetics of hydrotalcite‐catalyzed synthesis of poly(ethylene terephthalate) measured by TGA as the rate of ethylene glycol outgassing has been obtained by El‐Toufaili et al …”

Hydrotalcites Catalyze the Acidolysis Polymerization of Phenolic Acid to Create Highly Heat‐Resistant Bioplastics

“…The polymer chains seem too large to intercalate into the gaps of the HT's. However, the literature on PET preparation has reported that aromatic compounds such as terephthalic acid intercalated effi ciently into HT layers, where polymer chains ends can react with them [ 11 ] . In the present case, coumarate compounds and a number of acetylated chain ends for the hyperbranched poly(4HCAco -DHCA)s might react around the surface of the layer portions, which can effectively catalyze their acidolysis.…”