Simultaneous Promotion of the Mechanical Flexibility and Dielectric Strength of Impact Polypropylene Copolymers Containing Multifold H-Shape Long-Chain-Branching Structures for Recyclable Power Cable Insulation Application

Abstract

In the synthesis of impact polypropylene copolymers (IPCs) containing multifold H-shape long-chain-branching (LCB) structures by synchronizing IPC production with ω-alkenylmethyldichlorosilane copolymerization-hydrolysis (ACH) chemistry, some small amounts of ethylene are introduced into the first-stage propylene polymerization to tune the chain structure of the polypropylene (PP) matrix, aiming to promote the innovated heterophasic copolymers in mechanical flexibility with collateral damage on electrical properties minimized. This has resulted in the incorporation of a few ethylene units into the otherwise net PP matrix that significantly reduces the average sequence length of PP chains, leading to a profound increase in mechanical flexibility. To one's surprise, such a mechanical flexibility enhancement has not been, as previously hypothesized, compromised by loss of dielectric strength. As a matter of fact, simultaneous enhancements in both mechanical flexibility and electrical breakdown strength have been achieved with the long-chain-branched IPCs (LCB-IPCs) incorporated with ethylene in the PP matrix. The LCB-IPCs with ethylene-incorporated PP matrices are also characterized by the many structural and property privileges ascribable to their multifold H-shape LCB structures, including fine spherulite morphology, fine and stable rubber phase dispersion morphology, strong interfacial adhesion, and the resulting high elongation ratio, high electrical breakdown strengths under both alternating current (AC) and direct current (DC) electrical voltages, and excellent elongational flow properties characterized by strong strain-hardening effect, all benefiting potential power cable insulation applications.

Hierarchical Structural Evolution, Electrical and Mechanical Performance of Polypropylene Containing Intrinsic Elastomers under Stretching and Annealing for Cable Insulation Applications

“…in the homopolypropylene porous powders. , As a result, a PP-based multiphase system is achieved in the reactors, including homopolypropylene as the continuous phase and PP-based intrinsic elastomer as the dispersion phase. − Particularly, owing to the continuous in situ copolymerization, the dispersed phase size of in-reactor alloy (in the scale of less than a micron dimension) is much smaller and more uniform than that of traditional blends, thus improving comprehensive performance effectively. , Furthermore, in-reactor alloy technology makes it possible to produce a multiphase polymer with sequential polymerization . The possibility of PP in-reactor alloy used as cable insulation was also explored by Dong et al via introducing H-shaped long-chain-branching (LCB) structures. − It is well-established that the microscopic structure (e.g., phase morphology and crystalline structure) of the PP-based multiphase system is highly dependent on the external fields during processing (e.g., temperature, shear, or elongational flows). − The spherical “islands” in the typical “islands-in-the-sea” phase structure via traditional melting compounding can be easily deformed into in situ ellipsoids or micro/nanofibrils with the aid of a strong elongational flow field. Also, flow fields are demonstrated to be efficient to induce conformational ordering of polymer molecular chains, accelerating the formation of heterogeneous nucleation sites and enhancing the crystallization kinetics .…”