Poly(ethylene terephthalate) foams: Correlation between the polymer properties and the foaming process

The foamability of two food-grade, high-molecular-weight poly(ethylene terephthalate)s (PETs) was investigated. Sorption tests were performed to determine the solubility and diffusivity of N(2) and CO(2) in molten polymers at 250 degrees C with a magnetic Suspension balance. Pressure-volume-temperature (pVT) data were also measured and used in the context of the Sanchez-Lacombe equation of state to predict the sorption isotherms. The thermal properties, in terms of the glass-transition, melting, and crystallization temperatures, were measured by differential scanning calorimetry analysis on the two high-molecular-weight PETs and, for comparison, on a bottle-grade PET. The rheological properties were measured to asses the improvement of the high-molecular-weight PET with respect to the bottle-grade one. Expansion tests were performed on the two high-molecular-weight grades and bottle-grade PETs with a batch foaming process with N(2), CO,, and an 80-20 wt %, N(2)-CO(2) mixture used as blowing agents. The whole processing window was explored in tern-is of temperature, pressure drop rate, and saturation pressure. The results of the foaming experiments were correlated to gas sorption and the thermal and rheological properties of the polymers in the molten state. The results proved the feasibility of foam processing these two high-molecular-weight grades, which gave, when compared to the bottle grade at specific foaming conditions, very low densities and fine morphologies