Effect of crystallinity on unimodal and bimodal cell structures,in a continuous production of HDPE microcellular foamed sheets using supercritical CO2 (ScCO2)

Crystallinity is a controlling parameter in the development of microcellular foam final structure. In this research, using a well-controlled continuous microcellular foamed sheet production system, the effect of crystallinity on the final structure of the microcellular foam is studied. To produce microcellular foamed sheets, different levels of the supercritical carbon dioxide (ScCO₂) is dissolved in the polymer at high pressure and the foamed material is stabilized using a four roll apparatus at different roll temperatures. Crystallization occur by delay, however this lag time can be controlled by controlling temperature gradient. The higher decrease in the roll temperature results in higher temperature gradient which increases the crystallization rate during the cell growth, causing bimodal cell structures which indicates secondary nucleation. Moreover, as the amount of gas increases, the cell formation time increases, hence, at even higher temperatures we observed the effect of secondary nucleation, resulting in the occurrence of bimodal cell structures.