Crosslinking principle of silane crosslinked cable material

There are two main processes to make silane crosslinked polyethylene: grafting and crosslinking. In the process of grafting, under the action of free initiator and pyrolyzed free radical, the polymer loses the H atom on the tertiary carbon atom and generates free radical. The free radical reacts with - CH = CH2 of vinyl silane to form a graft polymer containing trioxysilane group.
In the process of crosslinking, the grafted polymer first hydrolyzes under the action of water to form silanol, and then - Oh condenses with adjacent si-o-h groups to form Si-O-Si bond, which makes the polymer macromolecules crosslink.

Silane crosslinked cable material and its production mode
As you know, the production of silane crosslinked cable materials and cables can be divided into two steps and one step. The difference between two-step method and one-step method lies in where the silane grafting process is carried out. The two-step method is carried out at the cable manufacturer's place, and the one-step method is carried out at the cable manufacturer's place.
At present, the two-step silane crosslinked polyethylene insulating material with the largest market share in China is composed of the so-called a material and B material. A material is the polyethylene that has been grafted with silane, B material is the catalyst master material, and its weight ratio is generally a: B = 95:5. A material and B material are made by the cable material factory and sold in the cable factory. Before use, the cable factory mixes a material and B material in proportion, and then in the ordinary extruder The cable insulation core can be extruded, and then the insulation layer can be crosslinked in warm water or steam.
There is also a kind of two-step silane crosslinked polyethylene insulation material. The production mode of material a is different. When synthesizing polyethylene, vinyl silane is introduced to directly obtain polyethylene containing silane branch chain. This method is essentially the production technology of resin, which must be completed by large petrochemical enterprises.
Material a of linklon silanes, which first came into China, belongs to this type. At present, Dow and Borealis silanes are also of this type, which are not available in domestic petrochemical enterprises.
There are two types of one-step process. The traditional one-step process is to put all kinds of raw materials into a specially designed special extruder by a special precision metering system according to the proportion in the formula to complete the grafting and extrusion of cable insulation core in one step. In this process, there is no need to granulate, no need to participate in the cable material factory, and the cable factory alone completes the process. Most of the production equipment and formulation technology of the one-step silane crosslinked cable are imported from abroad, and the price is expensive.
Another kind of one-step silane crosslinked polyethylene insulation material is produced by the cable material manufacturer, which mixes all raw materials together by a special method according to the proportion in the formula and sells them after packaging. There is no difference between material a and material B. the cable factory can directly complete the grafting and extrusion of cable insulation core in one step in the extruder. The unique feature of this method is that it can complete the silane grafting process in ordinary PVC extruder without expensive special extruder, and the work of mixing material a and B before extrusion in two-step method is omitted.
Formula composition
The formulation of silane crosslinked polyethylene cable material is generally composed of base resin, initiator, silane, antioxidant, polymerization inhibitor, catalyst, etc.
(1) the base resin is generally low-density polyethylene (LDPE) resin with melt index (MI) of 2, but recently with the development of synthetic resin technology and cost pressure, linear low-density polyethylene (LLDPE) is also used as or part of the base resin of the material.
Different resins often have great influence on their grafting and crosslinking due to the difference of macromolecular internal structure, so the formula of different base resin or the same type of resin from different manufacturers will be modified accordingly.
(2) diisopropylperoxybenzene (DCP) is commonly used as initiator, the key is to grasp the quantity, too little leads to insufficient silane grafting; too much leads to polyethylene cross-linking, so as to reduce its fluidity, rough surface of extruded insulated core, and difficult extrusion. Because the amount of initiator is very small and sensitive, how to disperse uniformly is also very important. Generally, it is added together by dissolving it into silane.
(3) generally, vinyl unsaturated silane is used as silane, including vinyl trimethoxysilane (a2171) and vinyl triethoxysilane (a2151). Because of the fast hydrolysis rate of a2171, there are many a2171. In the same way, silane also has a problem of adding quantity. At present, every cable material manufacturer tries to achieve its lower limit to reduce the cost, because silane is imported and the price is expensive.
(4) antioxidants are added to ensure the stability of polyethylene and the aging resistance of the cable. The antioxidants inhibit the graft reaction in silane grafting process. Therefore, the addition of antioxidants should be carefully considered in the process of grafting, and the matching of DCP dosage should be considered in the selection of additives.
In the two-step crosslinking process, most of the antioxidants can be added to the catalyst masterbatch, which can reduce the influence on the grafting process. In the one-step crosslinking process, antioxidant exists in the whole grafting process, so the selection of its variety and quantity is more important. The commonly used antioxidants are 1010, 168, 330, etc.
(5) the addition of polymerization inhibitor is to inhibit the side reactions of some grafting and cross-linking processes. Adding an anti cross-linking agent in the grafting process can effectively reduce the occurrence of C2C cross-linking, so as to improve the processing fluidity. In addition, adding an polymerization inhibitor which will be hydrolyzed prior to the silane on the grafting under the same conditions can reduce the hydrolysis of the grafted polyethylene and improve the long-term stability of the grafted material 。
(6) the catalyst is usually organotin derivative (except for natural crosslinking), and the most common one is laurel