Foams based on poly(ethylene terephthalate) nanocomposites with enhanced thermal stability

Novel poly(ethylene terephthalate) nanocomposites have been developed, incorporating commercial bottle grade polyester and modified organoclay, which exhibit enhanced thermal and dynamic-mechanical properties and improved foaming behavior. A benefit arising from the use of an organoclay is that this class of material is generally able to withstand the high processing temperatures of engineering polymers, such as poly(ethylene terephthalate), and not undergo decomposition. This offers a great advantage over the use of organic modifiers, whose volatile by-products are known to promote thermal degradation of the polymer matrix under such conditions. In this work, a novel organoclay was obtained via intercalation of bis-(hydroxyethyl terephthalate) in sodium montmorillonite layers and then used to prepare poly(ethylene terephthalate)-clay nanocomposites via melt compounding by using a twin-screw extruder. Thermogravimetric analysis indicated that the onset decomposition temperatures of poly(ethylene terephthalate) nanocomposites incorporating this organoclay were higher than those of controls based on commercial organoclays. However, the inclusion of this new nanofiller also led to a drastic decrease of melt viscosity, which in turn suppressed foaming. In order to compensate for the effects of the viscosity reduction, pyromellitic dianhydride was used in the reactive extrusion process of poly(ethylene terephthalate) nanocomposites. Rheological analysis showed a strong improvement in viscosity of these poly(ethylene terephthalate)/pyromellitic dianhydride/organoclay nanocomposites, to levels of the same order of magnitude as reference commercial foaming grade poly(ethylene terephthalate), or higher. Samples were foamed using carbon dioxide as physical blowing agent in a batch foaming process. Comparison of sample morphology, performed by scanning electron microscopic analysis, indicated very good results in some samples in terms of cellular morphology and foam density.