X-ray radioscopy in-situ studies in thermoplastic polymer foams

[Display omitted] ► Bubble nucleation, growth & coalescence have been studied in viscous PE & PP systems. ► Results indicate that bubble nucleation and growth velocity are anticorrelated. ► Coalescence presents long relaxation times over 10s. ► Coalescence highly influences on the surrounding structure due to the high viscosity. The current work addresses results on two types of in-situ studies: bubble nucleation/growth and bubble coalescence for two thermoplastic materials (polypropylene and polyethylene) foamed by a chemical blowing agent (azodicarbonamide) under varying temperature conditions. The foaming process of these high viscosity materials has been in-situ monitored for the first time using a high resolution X-ray radiography set up. The studies were carried out in specially designed X-ray transparent furnaces reaching temperatures over the melting point of the polymer and the decomposition temperature of the blowing agent (which generates the expanding gas). Sequences of radiographies were typically acquired at 0.5Hz. Image processing allowed evaluating several foam features such as cell nucleation rate and coalescence detection/quantification. Results on pore nucleation and growth indicate a high influence of heating rate for this variable and reveal an apparent competition between pore growth velocity and the number of nucleated pores. Results on cell coalescence highlight special features of this highly viscous systems in comparison to low viscosity ones (aqueous and metallic foams). Ruptures in these materials take much longer times (up to 10s) and generate a large interaction with the surrounding cells making more difficult the quantification of the process.