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; Lih-Sheng Turng

doi:10.3390/polym10010036

Improved Processability and the Processing-Structure-Properties Relationship of Ultra-High Molecular Weight Polyethylene via Supercritical Nitrogen and Carbon Dioxide in Injection Molding

Polymers (Basel). 2017 Dec 30;10(1):36. doi: 10.3390/polym10010036.

Authors

Galip Yilmaz^{1

2}, Thomas Ellingham^{3

4}, Lih-Sheng Turng^{5

6}

Affiliations

¹ Polymer Engineering Center, Department of Mechanical Engineering, University of Wisconsin⁻Madison, Madison, WI 53706, USA. gyilmaz@wisc.edu.
² Wisconsin Institute for Discovery, University of Wisconsin-Madison, Madison, WI 53715, USA. gyilmaz@wisc.edu.
³ Polymer Engineering Center, Department of Mechanical Engineering, University of Wisconsin⁻Madison, Madison, WI 53706, USA. ellingham@wisc.edu.
⁴ Wisconsin Institute for Discovery, University of Wisconsin-Madison, Madison, WI 53715, USA. ellingham@wisc.edu.
⁵ Polymer Engineering Center, Department of Mechanical Engineering, University of Wisconsin⁻Madison, Madison, WI 53706, USA. turng@engr.wisc.edu.
⁶ Wisconsin Institute for Discovery, University of Wisconsin-Madison, Madison, WI 53715, USA. turng@engr.wisc.edu.

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: microcellular injection molding; supercritical CO2; supercritical N2; supercritical fluid; ultra-high molecular weight polyethylene (UHMWPE).