Polyethylene silane cross-linking

Cross-linking polyethylene enhances its heat resistance (in terms of resistance to melt flow) since the network persists even about the crystalline melting point of the uncross-linked material. Cross-Knked polyethylene thus finds application in the cable industry as a dielectric and as a sheathing material. Three main approaches used for cross-linking polyethylene are (1) radiation cross-linking, (2) peroxide cross-linking, and (3) vinyl silane cross-linking. 

In the vinyl silane cross-linking process Sioplas process) developed by Dow, an easily hydrolysable trialkoxy vinyl silane, CH2=CHSi(OR)3, is grafted onto the polyethylene chain, the site activation having been achieved with the aid of a small amount of peroxide. The material is then extruded onto the wire. When exposed to hot water or low-pressure steam, the alkoxy groups hydrolyze and then condense to form a siloxane cross-link ... 

BP Chemicals have introduced a new technology for silane cross-linking in low-voltage cable insulations to produce more uniform cross-linking and greater shelf-life. The new silane-linked polyethylene compounds can be run on conventional extrusion equipment. The company has also produced a strippable double layer cable shield in cross-linkable polyethylene for medium voltage cables. 

Goto T. Material recycling of silane cross-linked polyethylene insulated cable using supercritical alcohol. Electrical Insulation News in Asia [trade journal— Institute of Electrical Engineers of Japan] October 2006. 

Atkinson J.R., and R.Z. Cicek. 1983. Silane cross-linked polyethylene for prosthetic applications. Part I. Certain physical and mechanical properties related to the nature of the material. Biomaterials 4 267-275. 

With silane cross-linking processes, the cross-linking reactions occur while the polymer is in the solid state. As such, the polyethylene material exists with both crystalline and amorphous regions being present. There will be differences in the degree of... 

Vulcanised (cross-linked) polyethylene is being used for cable application where service temperatures up to 90°C are encountered. Typical cross-linking agents for this purpose are peroxides such as dicumyl peroxide. The use of such agents is significantly cheaper than irradiation processes for the cross-linking of the polymer. An alternative process involves the use of vinyl silanes (see Section 10.9). 

Electron beam irradiation is one of the methods of cross-linking in fhis process. The other methods use peroxide, multifunctional azide, or an organofunctional silane. Polyethylene resins respond to electron beam irradiation well since the rate of cross-linking exceeds significanfly fhe chain scission. Polypropylene (PP) is prone to P-cleavage, which makes if difficult to cross-link by a free radical process. For fhaf reason, PP resins... 

Shah, G.B. Fuzail, M. Answar, J. Aspects of cross-linking of polyethylene with vinyl silane. J. Appl. Polym. Sci. 2004, 92, 3796-3803. 

Cross-linking of the outer casing is achieved by grafting a vinyl silane to polyethylene by means of a peroxide as part of the cable extrusion process. The chemistry is shown in Scheme 2.1. When the cable is subsequently soaked in water, the alkoxy silanes hydrolyse and crosslink within the matrix. It is an unresolved question whether polymers that have been modified by a free radical mechanism will survive for the expected lifetime of underground cables (ca. 75 years). 

POEs have been used in both plastic rmd rubber applications. These elastomeric polyethylenes have ability to be cross-linked via peroxide, irradiation, and moisture (if silane grafted). Potential applications include tubing, impact modifiers, low-voltage cable insulation, elastic films, foams, shoe soles, belts, automotive hoses, medical applications, gasket seals, and other electrical applications. 

---