ABSTRACT: The crosslinking principle, classification, formulation, process and equipment of silane crosslinked polyethylene insulators for wires and cables are briefly described. Some characteristics of silane naturally crosslinked polyethylene insulators in application and use and factors affecting their crosslinking status are also introduced.
Key words: silane crosslinking; natural crosslinking; polyethylene; insulation; wire and cable
Silane crosslinked polyethylene cable material as insulation material for low-voltage power cables has been widely used in China's wire and cable industry. Compared with peroxide cross-linking and irradiation cross-linking, this material has the advantages of simple manufacturing equipment, convenient operation and low comprehensive cost. It has become the leading insulating material for low-voltage cross-linked cables.
1. Crosslinking Principle of Silane Crosslinked Cable Material
There are two main processes in the preparation of silane crosslinked polyethylene: grafting and crosslinking. In the process of grafting, free radicals were produced by the free initiator and pyrolysis free radicals, which lost H atom on tertiary carbon atom. The free radicals reacted with vinylsilane - CH = CH2 group to form a grafted polymer containing trioxysilyl group. During the crosslinking process, the grafted polymer was hydrolyzed to produce silanol under the action of water, and - OH condensed with the adjacent Si - O - H group to form Si - O - Si bond, which resulted in crosslinking between polymer macromolecules.
2. Silane Crosslinked Cable Material and Its Cable Production Mode
As you know, the production of silane cross-linked cable materials and their cables can be divided into two steps and one step. The difference between two-step method and one-step method is where the silane grafting process takes place. The grafting process is carried out in two-step method at the cable material manufacturer and in one-step method at the cable manufacturer. At present, the two-step silane crosslinked polyethylene insulation material with the largest market share in China consists of so-called A and B materials. A material is Polyethylene Grafted with silane and B material is catalyst master material. Its weight ratio is generally A:B = 95:5. A material and B material are made by cable material factory and sold in cable factory. After mixing A material and B material proportionally before using, cable factory can extrude them in ordinary extruder. Cable insulation cores are then crosslinked in warm water or steam.
There is another kind of two-step silane crosslinked polyethylene insulation material. The production mode of material A is different. It is to introduce vinyl silane into the synthesis of polyethylene to directly obtain polyethylene containing silane branched chains. This method is essentially the production technology of resin, which must be completed by large petrochemical enterprises. Material A of LINKLON silane material, which first entered China, belongs to this type. At present, DOW and BOREALIS silane materials are also of this type. There are no such products in domestic petrochemical enterprises.
There are also two types of one-step method. The traditional one-step method is to put all kinds of raw materials into a specially designed extruder to complete the grafting and extrusion of cable insulation cores by a special precision metering system according to the proportion of the ingredients. In this process, no granulation is needed, no participation of cable material factory is needed, and the cable factory alone completes the grafting and extrusion. The production equipment and formulation technology of this kind of one-step silane cross-linked cable are mostly imported from abroad, which is expensive.
Another kind of one-step silane cross-linked polyethylene insulating material is produced by cable material manufacturer. It is a special method to mix all raw materials according to the proportion of formula. It is sold after packaging. There is no material A and B. The cable factory can directly complete the grafting and extrusion of cable insulating core at the same time in one step in the extruder. The unique feature of this method is that the silane grafting process can be completed in the ordinary PVC extruder without the expensive special extruder, and the two-step method of mixing material A and material B before extrusion is omitted.


3. Formula Composition
The formulation of silane crosslinked polyethylene cable compound is generally composed of base resin, initiator, silane, antioxidant, inhibitor and catalyst.
(1) Low density polyethylene (LDPE) resins with melt index (MI) of 2 are generally used as base resins. Recently, linear low density polyethylene (LLDPE) has been used as or partly as base resins for this material with the development of synthetic resin technology and cost pressure. Different resins often have great influence on their grafting and crosslinking due to their different internal structure of macromolecule. Therefore, the formulation of different resins based on different substrates or the same type of resins produced in different factories will be revised accordingly.
(2) Diisopropylbenzene peroxide (DCP) is commonly used as initiator. The key problem is how to control the amount of initiator. Excessive amount of initiator leads to inadequate silane grafting; excessive amount of initiator leads to polyethylene cross-linking, which reduces its fluidity and makes the extruded insulating core surface rough and difficult to extrude. Because the amount of initiator is very small and sensitive, how to disperse uniformly is also very important. Generally, the initiator is added together with silane.
(3) Vinyl unsaturated silane is generally used for silane, including vinyl trimethoxysilane (A2171) and vinyl triethoxysilane (A2151). Because of the fast hydrolysis rate of A2171, the choice of A2171 is more. Similarly, silane also has a problem of adding quantity. At present, cable material manufacturers are trying to achieve its lower limit to reduce costs, because silane is imported and the price is more expensive.
(4) Antioxidants are added to ensure the stability of polyethylene processing and the anti-aging of cables. Antioxidants can inhibit the grafting reaction in the process of silane grafting. Therefore, in the process of grafting, the addition of antioxidants should be cautious, and the selection of the amount of antioxidants should consider the matching with the amount of DCP. In the two-step crosslinking process, most of the antioxidants can be added to the catalyst masterbatch, which can reduce the impact on the grafting process. In the one-step crosslinking process, antioxidants exist in the whole grafting process, so the selection of varieties and quantities is more important. The commonly used antioxidants are 1010, 168, 330, etc.
(5) Inhibitors are added to inhibit the side effects of some grafting and crosslinking processes. Adding an anti-crosslinking agent in the grafting process can effectively reduce the occurrence of C2C crosslinking and improve the processing fluidity. Additionally, adding an inhibitor which hydrolyzes silane before the grafting under the same conditions can reduce the hydrolysis of grafted polyethylene and improve the long-term stability of the grafted polyethylene.
(6) Catalysts are usually organotin derivatives (except natural cross-linking). The most common catalyst is dibutyltin dilaurate (DBDTL), which is usually added in the form of master batch. In the two-step method, the graft material (A) and the catalyst master material (B) are packaged separately, and the A and B are mixed together before being added to the extruder, so as to prevent the pre-crosslinking of A. In one-step silane crosslinked polyethylene insulation, the polyethylene in packaging has not been grafted, so there is no pre-crosslinking problem, so the catalyst does not need to be packed separately.
In addition, there are complex silanes on the market, which are composed of silane, initiator, antioxidant, some lubricants and anti-copper additives. One-step silane cross-linking method in cable factories generally uses this kind of compound silane. At present, most cable material factories do not use this kind of compound silane, because the price is too expensive.
Therefore, the formulation of silane crosslinked polyethylene insulating material is considered not very complicated and can be obtained from relevant data. However, the proper formulation of the product can only be finalized after some adjustment, which requires a full understanding of the role of the components in the formulation, the law of their influence on performance and their mutual influence.
At present, silane cross-linked cable materials (either two-step or one-step) are considered to be the only ones with chemical processes in extrusion. The extrusion granulation process of other cable materials, such as polyvinyl chloride (PVC) cable materials and polyethylene (PE) cable materials, is a physical blending process, even chemical cross-linking and irradiation cross-linked cable materials, regardless of the extrusion granulation process. There is no chemical process during extrusion of cables. Therefore, process control is more important in the production of silane cross-linked cable materials and cable insulation extrusion.