Volume 10 Issue 2
Improved Processability and the Processing-Structure-Properties Relationship of Ultra-High Molecular Weight Polyethylene via Supercritical Nitrogen and Carbon Dioxide in Injection Molding
Galip Yilmaz, Thomas Ellingham and Lih-Sheng Turng
1Polymer Engineering Center, Department of Mechanical Engineering, University of Wisconsin–Madison, Madison, WI 53706, USA
2Wisconsin Institute for Discovery, University of Wisconsin-Madison, Madison, WI 53715, USA
*Author to whom correspondence should be addressed.
Abstract
The processability of injection molding ultra-high molecular weight polyethylene (UHMWPE) was improved by introducing supercritical nitrogen (scN2) or supercritical carbon dioxide (scCO2) into the polymer melt, which decreased its viscosity and injection pressure while reducing the risk of degradation. When using the special full-shot option of microcellular injection molding (MIM), it was found that the required injection pressure decreased by up to 30% and 35% when scCO2 and scN2 were used, respectively. The mechanical properties in terms of tensile strength, Young’s modulus, and elongation-at-break of the supercritical fluid (SCF)-loaded samples were examined. The thermal and rheological properties of regular and SCF-loaded samples were analyzed using differential scanning calorimetry (DSC) and parallel-plate rheometry, respectively. The results showed that the temperature dependence of UHMWPE was very low, suggesting that increasing the processing temperature is not a viable method for reducing injection pressure or improving processability. Moreover, the use of scN2 and scCO2 with UHMWPE and MIM retained the high molecular weight, and thus the mechanical properties, of the polymer, while regular injection molding led to signs of degradation.
Keywords:ultra-high molecular weight polyethylene (UHMWPE); microcellular injection molding; supercritical fluid; supercritical N2; supercritical CO2