Crosslinked polyethylene (XLPE) has been widely used in wire and cable industry for three reasons: good performance, low price and simple manufacturing process.


There are four popular cross-linking modes in cable industry at present:
1. Thermochemical crosslinking of peroxide (mostly used for insulation between high voltage and ultra-high voltage power cables of 10 kV and above);
2. Silane crosslinking in warm water (mostly used for insulation of medium and low voltage cables below 10kV);
3. Electron irradiation crosslinking (mostly used for insulation of medium and low voltage cables below 10kV);
4. In addition, there is a very small amount of ultraviolet cross-linking.


Among the four cross-linking methods, peroxide cross-linking is the most frequently used, followed by silane cross-linking, electron irradiation cross-linking is mostly used for special cables (such as high temperature resistant, halogen-free flame retardant cables), and ultraviolet cross-linking is only suitable for transparent and translucent materials. Among these cross-linking methods, the most prominent advantage of silane cross-linking is simple processing and low cost materials. Wire and cable factory users only need to buy materials, put them into conventional plastic extruders, extrude them in a general plastic cable production mode, and then put them into warm water or steam room (sauna bath) for about 6 hours to complete the cross-linking. The other crosslinking modes must go through special equipment (such as dry crosslinking line, electron accelerator, ultraviolet ray) to complete the crosslinking after the extrusion of the cable. This increases a lot of manufacturing costs.


The patent of silane crosslinked polyethylene expired as early as the early 1970s, which coincided with the reform and opening up of China and the great development of power industry and wire and cable. Therefore, this cross-linking method is rapidly popularized and promoted in the cable industry in China.


There are two kinds of silanes that can be crosslinked at present:
A-171: Vinyl trimethoxysilane
A-151: Vinyl triethoxysilane


Through decades of practice and comparison, because the grafting reaction of A-171 is faster than A-151, most countries in the world use A-171 as grafting monomer. The main catalyst for crosslinking is dibutyltin dilaurate (DBTD), and the initiator is diisopropylbenzene peroxide (DCP).


I. Varieties and Classification of Silane Crosslinking
The production sequence of silane crosslinked wires and cables is: first, silane monomer is grafted onto polyethylene macromolecule, and then shrinkage crosslinking under the action of catalyst. Therefore, silane crosslinking materials can be classified in three ways:


1. According to the mode of production:
Copolymerization: Ethylene monomers and silane monomers are polymerized in a certain proportion in the reactor; and from the molecular structure, silane can still be regarded as a group grafted on the main chain of polyethylene.
Two-step method: polyethylene and silane were Melt-grafted in a co-rotating twin screw to complete the first step; the second step was to cross-link with the catalyst after mixing extrusion and condensation.
One-step method: Polyethylene, silane and catalyst are mixed and absorbed into dry particles. The chemical reaction of grafting is completed during screw extrusion.


The advantages of copolymerization method are good and stable product quality, high content of grafted silane and low scorch rate, so that the crosslinking degree of the final product after catalytic reaction is higher.
Compared with the one-step method, the two-step method of silane grafting is completed in the granulator. Therefore, a certain grafting rate can be guaranteed, so that the final product quality is far more stable than one-step method.
The advantage of the one-step method is that the production cost is low, and the granulator only needs to mix silane and basic resin to absorb. Subsequent grafting and cross-linking are completed in the wire and cable factory as users. Therefore, the quality of the product is also related to the technical level and equipment level of the operator in the cable factory of the end user to a great extent.
The companies that produce copolymerization are all abroad, such as Nippon clone in Japan, Dow's and Nordic chemical industry in the United States, etc. The products produced by copolymerization are easy to process and of good quality, but because of the high price, they do not have a large domestic market.


2. According to the cross-linking mode:
Hot water or steam crosslinking;
Room temperature crosslinking.


3. According to whether flame retardant:
Ordinary (non-flame retardant);
Halogen-free flame retardant;
Halogen flame retardant.


Estimation of the Consumption of Manufacturers and Last Year
It is estimated that more than 200 domestic manufacturers can produce silane crosslinking materials. Especially in the past decade, because of the shaking tank production and the emergence of one-step silane crosslinking production lines, many large-scale wire and cable plants have begun to produce and sell their own products.


According to my statistics on the production of several major cable material factories in the country in 2016 at the beginning of this year, and the estimation of the consumption of cable material in the cable industry in the country are as follows:
The total consumption of cross-linked polyethylene is 45-500,000 tons.
Among them, silane polyethylene accounts for about half: 200-250,000 tons;
The consumption of silane is about 2500-3000 tons; imported silane accounts for a large proportion in the past few years and has been replaced by domestic materials in recent years.


III. Development Direction of Silane Crosslinking Materials in China
1. Take the road of copolymerization: Because the quality of cross-linking material produced by copolymerization is controllable and stable, it is very popular with users, but this method must be completed by the petrochemical enterprises which produce polyethylene, and the petrochemical enterprises are afraid that the output is too small and the users are too scattered to make up their minds. However, due to the excellent quality of copolymerization materials, it is necessary to take the road in the long run.
2. Increase the silane grafting rate of two-step method to further improve the crosslinkability.
3. The quality stability of room temperature crosslinking material needs to be improved.
4. Halogen-free flame retardant also needs to be improved.
5. It is necessary to further reduce the heat shrinkage performance.