Research on torsional laminated extrusion for improving the permeability and mechanical properties of HDPE/PA6 composite for type IV storage tank liners

Abstract

Polymers are advantageous in increasing hydrogen storage density of tanks due to their lightweight. Moreover, a uniform aggregate structure can enhance their advantages. However, achieving such as structure solely through screw extrusion can be challenging in liner moldings. This study reported a micro‐nano torsional laminated extrusion equipment that can be used before the die to effectively improve the composite orientation and reduce intercrystalline defects through divides, laminated thinning, and biaxial stretching of the melt with a strong drag flow field. The resulting HDPE/PA6 achieved a reduced helium permeation coefficient by up to 20.1% with a more pronounced effect on the diffusion than that on the dissolution. The mechanical property tests demonstrated an improvement in strength by up to 24.5% and modulus by up to 49.1% with the expanded range of toughness fracture. However, PA6 and HDPE exhibited limitations in terms of the moldable size and storage ability owing to their melt strength and gas barrier properties, respectively. Based on the analysis of the melt strength thresholds of HDPE/PA6, the PA6 content in the composites should be <70 wt% for blow molding and <50 wt% for welding. This study provides a novel approach for forming large‐diameter and thin‐walled liner billets through extrusion and welding.