Because cross-linked polyethylene is a large molecular size, it is unmelted and insoluble, and its heat resistance is significantly improved. With low cross-linking degree, the Vicat softening point does not change much; with high cross-linking degree, the Vicat softening point can be increased by 30℃ to 40℃. The long-term use temperature of cross-linked polyethylene is 95℃-100℃. The polyethylene with a molecular weight of 200,000 has only 12-13 silane monomers connected to each molecule chain, so the distance between cross-linking points of silane-cross-linked polyethylene is large. Due to the good flexibility of the -Si-O- and -Si-C- bonds, the low-temperature resistance of cross-linked polyethylene is good and can be used at -50℃ or even -70℃.
Furthermore, because the molecular structure of silane-cross-linked polyethylene is different from that formed by peroxide cross-linking between the intermolecular -C-C- bonds, all three silane oxygens can undergo hydrolysis condensation reactions, which can form a stereoscopic network cross-linking. Therefore, its thermomechanical properties are generally better than peroxide-cross-linked polyethylene with -C-C- planar structure. Even if the cross-linking degree of silane cross link polyethylene is 15%-20% lower than that of peroxide cross-linking, their heat distortion temperatures are still equivalent.
Cross-linked polyethylene has better chemical resistance than ordinary polyethylene. On the one hand, the strength is increased after cross-linking. On the other hand, after the silane long chain is grafted onto the polyethylene chain, the long chain continuously enters the tightly stacked crystal lattice, thus increasing the number of connecting molecules between crystals. In addition, the structural long side chain entanglement strengthens the amorphous region between crystal chips, which enhances the resistance to environmental stress, especially in the higher temperature range below 100℃.
Cross link polyethylene has less crystalline region, uniform density, and introduces non-polar -Si-O- and -Si-C- bonds, which makes its electrical insulation basically unchanged. Therefore, it is still a good insulating material, and its insulation is still high at higher temperatures and harsh chemical media. It is commonly used as a cable shielding layer.
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