Crosslinking technology is an important technique for improving the performance of polyethylene (PE). PE modified by crosslinking can significantly improve its mechanical properties, environmental stress cracking resistance, chemical and drug corrosion resistance, creep resistance, electrical properties, and importantly, it can increase the temperature resistance level from 70°C to over 100°C, thereby greatly expanding the application range of PE. Currently, cross-linked polyethylene (CLPE) has been widely used in pipe, film, cable material, and foam product industries.
During radiation crosslinking, the free radicals of the polymer are generated by the irradiation of high-energy rays such as Y-rays, electron beams, and neutron beams. In laboratory experiments, Y-rays are generally produced by a radiation source. In industry, a large electronic accelerator is commonly used to generate electron beams to cause crosslinking of the polymer. Radiation crosslinking mainly uses the free radicals generated by breaking the C-C and C-H bonds in PE as a result of the irradiation to induce crosslinking.
PE's radiation sensitivity problem is a research focus of the radiation crosslinking method to produce cross-linked polyethylene. The general approach to solving this problem is to add sensitizers and sensibilizers to PE, or to change the radiation atmosphere. Common sensitizers include tetra-acrylate of pentaerythritol and trimethylolpropane triacrylate; common sensibilizers include silicon tetrachloride, carbon tetrachloride, sodium fluoride, and carbon black. The use of acetylene atmosphere is one of the common methods for the radiation sensitization of PE fibers.
The advantages of utilizing radiation crosslinking method for cross link polyethylene production are that crosslinking and extrusion are separated, product quality is easy to control, production efficiency is high, and waste rate is low. In addition, crosslinking does not require additional free radical initiators, maintaining the cleanliness of the material and improving its electrical properties. It is particularly suitable for small cross-section, thin-wall insulated cables that are difficult to produce by chemical crosslinking. However, radiation crosslinking also has some disadvantages, such as the need to increase the acceleration voltage of electron beams for thick materials; for circular objects such as wires and cables, rotation or multiple beams are needed to achieve uniform radiation; it requires high initial investment costs, and operation and maintenance technology is complex, with strict safety protection requirements during operation.
The peroxide crosslinking method, also known as chemical crosslinking method, induces crosslinking of PE by causing a series of free-radical reactions through the high-temperature decomposition of peroxides. The difference from radiation crosslinking method is that:
The crosslinking process must have a crosslinking agent, that is, peroxide exists;
The crosslinking reaction must be carried out at a certain temperature.
Using peroxide to crosslink PE can produce high-quality crosslinked products, but during the processing of the products, the extrusion temperature must be kept very low, otherwise early crosslinking may result in carbonization, which affects the quality of the product and even damages the equipment. The extrusion speed of crosslinkable PE is strictly limited by this temperature limit. Moreover, when extruding products, continuous heating is required in high-temperature and high-pressure and dozens of meters (even hundreds of meters) of special pipelines, which requires large equipment space, high energy consumption, and low production efficiency. Therefore, this technique is limited in application to small and medium-sized production enterprises.
The use of crosslinking agents and co-crosslinking agents together can significantly improve the crosslinking effect. Co-crosslinking agents can improve the degree of crosslinking, reduce the probability of degradation, and appropriately reduce the amount of crosslinking agents used. Co-crosslinking agents contain monomers or polymers with a sulfur, oxime, or C-C- structure in the molecule, and common varieties include oxime and methyl methacrylate. The main development direction of peroxide crosslinking for PE in recent years is to graft polar monomers onto PE chains. Polar monomers include maleic anhydride, acrylic acid, acrylamide, and acrylic ester, etc. Crosslinked PE with grafted branches has improved compatibility with metals, fillers, or other polymers.