The production process of silane cross-linked polyethylene mainly includes three methods: two-step cross-linking process, one-step cross-linking process, and ethylene-silane copolymer cross-linking process.
Developed by Dow Corning in 1968. This method first prepares silane-grafted polyethylene pellets (component A) and catalyst masterbatch (component B), mixes and extrudes them into a cross-linkable polyethylene, and finally hydrolyzes the cross-linkable polyethylene to obtain cross link polyethylene.
The advantage of this method is that the graft reaction between silane and polyethylene is separated from the extrusion molding, which can avoid or reduce the negative impact of modifiers on the graft reaction of polyethylene, so the grafting rate and degree of crosslinking are high; since A and B materials are used, extrusion molding is only a physical extrusion process, so there is no special requirement for the extruder, and the extrusion molding line speed is high. Other modifiers can be conveniently added when A and B materials are mixed according to user needs to produce products with certain characteristics. Its disadvantage is that the production process is long, there are more production equipment, and it is prone to impurities.
Developed by Maillefer in 1974. This process directly adds polyethylene, resin, silane, peroxide, and crosslinking catalyst into the extruder to produce wires, cables, and pipes.
This process has a short production process, precise control, less impurities, and is conducive to industrial production. Moreover, the danger of water cross-linking is avoided because the grafted polymer does not need to be stored. The shortcomings are high technical requirements and large investment.
Developed by Mitsubishi Oil in Japan in 1986. In this method, ethylene and ethylene-silane co-polymerization occurs under high pressure in the traditional high-pressure polyethylene reactor.
The co-polymerization process is advanced and unique, and the silane cross link polyethylene produced has the following advantages:
The storage stability of prepared ethylene-silane Z-polymer is greatly improved, and the co-polymerization method has very few impurities, so it can improve the electrical properties of cross-linking materials, and also has corresponding improvements in heat resistance, chemical resistance, and mechanical properties. The molding and processing stability is improved, and fewer gases are generated during processing.
In recent years, some large foreign companies (such as BP in the UK, UCC in the United States, etc.) have successively launched ethylene-silane copolymer cable materials, while China is still blank in the production of cable materials using the ethylene-silane copolymerization method. Silane can be regularly distributed on the polymer chain, so a specific degree of cross-linking can be obtained with a small amount of silane.
As there is no need to use peroxide, volatile decomposition products will not be generated, which can avoid the formation of bubbles and voids. This is important for the application of cables and pipes. The co-polymerization method can reduce impurity contamination, ensure high cleanliness, and have good product performance, especially improved electrical performance. Copolymers prepared by the co-polymerization method have enhanced moisture resistance and greatly improved storage stability.