Effect of Quenching Treatment on Morphology and Properties of Eco-friendly Polypropylene Copolymer for Cable Insulation

Polypropylene (PP) based cable insulating material has raised great attention due to the recyclability and superior dielectric properties. In this paper, isotactic PP (iPP) as a common commercial PP and ethylene-propylene copolymer (EPC) with natural cooling(-2°C/min) and quenching process (- 90°C/min) were prepared to investigate the role of quenching on morphology and properties of iPP and EPC. It is found that the elongation at break of quenched iPP and EPC significantly increased by 105% and 121%, respectively. In addition, the breakdown strength of iPP decrease 6% while that of EPC increase 13% after quenching treatment. It is shown by XRD results that β-crystal is introduced in both quenched iPP and EPC specimens. The parallel lamellas of β-crystal will slide under the external force, which can result in the increased elongation at break. Moreover, the crystallinity of quenched iPP decreased by 14%, which can cause the expansion of amorphous regions and free volume. The energy accumulation of carriers under applied electric field will be significantly enhanced, which can result in the decreased breakdown strength. As for quenched EPC, non-crystallizable ethylene-propylene copolymer segments tends to be restricted in limited dimensions to form dispersed amorphous microspheres. Deep traps which are introduced by rubber microspheres are considered to be an important reason for the increased breakdown strength.