Water boosts reactive toughening of PET
Ferenc Ronkay, Béla Molnár, Edina Szabó, György Marosi, Katalin Bocz
Abstract
During melt-processing, the molecular weight of poly(ethylene terephthalate) PET, being prone to hydrolytic degradation, decreases proportionally with its initial moisture content. Therefore, to avoid undesired deterioration of the physical and mechanical properties, PET granules or recyclates are generally dried (the residual moisture must be generally lower than 500 ppm) just prior to melt-processing. In this research, in contrast to the general practice, PET granules with increasing moisture content in the range of 30-3520 ppm, as conditioned in a climate chamber, were melt blended with a constant amount (13%) of ethylene-butyl acrylate-glycidyl methacrylate (EBA-GMA) type reactive terpolymer by twin screw extrusion. It was found that moisture boosts the reactive toughening process, and consequently, the notched Izod impact strength increases by up to 600%. Notched impact strength higher than 50 kJ/m2 was reached only by using PET with optimal (1710 ppm) moisture level instead of fully dried granules, as starting material. It is also noteworthy that the tensile strength and stiffness of the blends showed little variation over the examined moisture content range. Scanning electron microscopic analyses show that the size of the dispersed phase decreases continuously with the increasing initial moisture content of PET, which is explained by the increasingly effective interfacial compatibilisation with the terpolymer. Compatibilisation reactions are accelerated by the low-molecular-weight PET chains of increased reactivity that are formed in situ in the presence of moisture during melt-processing. The observed phenomenon can be advantageously utilized for upgrading recycling technology.