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| Preparation of Long-Chain Branched Polypropylene Insulation Material and Its High-Temperature DC Electrical Properties |
| Yang Jiaming1, Wang Kai1, Zhao Hong1, Yang Xu1, Zhao Xindong1, Zhan Yunpeng2, Hou Shuai2 |
1. State Key Laboratory of High-Efficiency Special Cable Technology Harbin University of Science and Technology Harbin 150080 China;
2. CSG Electric Power Research Institute Guangzhou 510663 China |
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Abstract Thermoplastic polypropylene (PP) insulated power cables are promising alternatives to cross-linked polyethylene (XLPE) due to their eco-friendly remoldability and recyclability. However, homopolypropylene’s high crystallinity-induced hardness and low zero-shear viscosity (causing extrusion eccentricity) limit its direct application. Thus, impact copolymer polypropylene (IPC), containing ethylene-propylene rubber segments, was selected as the base material, with its chain structure and local state traps regulated via maleic anhydride (MAH) graft modification and anhydride-amino imidization reaction to synergistically improve rheological and DC electrical properties.
Firstly, MAH-grafted IPC (IPCgM) was prepared via melt grafting with an optimized formulation (100 phr PP, 0.2 phr dicumyl peroxide, 1 phr MAH, 0.5 phr styrene). Then, 3,3′-diaminopropylamine (DA) was added at 0.2/0.5/1.0 phr and melt-blended at 200℃ for 10 minutes to form viscosity-modified IPCv. Fourier transform infrared (FTIR) spectroscopy confirmed successful imidization, with MAH conversion rates of 22.1%, 68.0%, and 82.4% for IPCv0.2, IPCv0.5, and IPCv1.0. Gel content tests (0±0.002% for all samples) verified no crosslinking, preserving PP’s thermoplasticity.
Melt flow rate (MFR) and rotational rheological tests (0.01~100 rad/s) showed IPCv’s viscosity first increased then decreased with DA content. Fitted via the Carreau model, IPCv0.5 exhibited the highest zero-shear viscosity (3 554 178.40 Pa·s), significantly higher than IPC (3 676.54 Pa·s) and IPCgM (236 015.16 Pa·s). It also had the lowest characteristic angular frequency (ωc=5.01 rad/s) and longest relaxation time (τc=0.199 s), enhancing chain entanglement to suppress eccentricity. Mechanical testing results demonstrated that IPCv0.5 exhibited the maximum tensile strength (32.16 MPa), as well as the minimum elastic modulus (250 MPa) and yield strength (11.51 MPa). Dynamic mechanical analysis (DMA) in tensile mode revealed a reduced storage modulus (486.67 MPa at 25℃) and disappearance of the EPR phase’s -30℃ tanδ peak, indicating improved compatibility and flexibility.
Differential scanning calorimetry (DSC) tests showed modified IPC had higher crystallization temperature (improving processing efficiency) but unchanged melting temperature. IPCv’s crystallinity decreased with DA content, with IPCv0.5 at 29.93% (vs. 35.27% for IPC) and IPCv1.0 slightly recovering to 30.38% due to imidization competition. Reduced crystallinity enhanced flexibility by weakening chain constraints.
Furthermore, 80℃ DC conductivity and space charge tests showed IPCv0.5 maintained PP-comparable conductivity. Its space charge distribution was significantly improved, with a maximum electric field strength of ~-44.8 kV/mm and an 18% reduction in electric field distortion rate compared to IPC.
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Received: 20 October 2025
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2026, Vol. 41 
