Vinyl silane cross-linking

A silane—the method is called cross-linking via silane or moisture-based vinyl silane cross-linking. The resulting PE is also called PEX-C in the European standards. 

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 ... 

Gross-Linking. A variety of PE resins, after their synthesis, can be modified by cross-linking with peroxides, hydrolysis of silane-grafted polymers, ionic bonding of chain carboxyl groups (ionomers), chlorination, graft copolymerization, hydrolysis of vinyl acetate copolymers, and other reactions. 

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). 

Silicones can be prepared in such a way that they contain only one percent or less of vinyl groups and silicon hydrides, which undergo a catalytic hydrosilylation reaction to give the desired cross-linking (Figure 18.3). Vinyl silanes are made by Si-H addition to acetylene, and thus two hydrosilylations are involved. 

In the Mukaiyama aldol additions of trimethyl-(l-phenyl-propenyloxy)-silane to give benzaldehyde and cinnamaldehyde catalyzed by 7 mol% supported scandium catalyst, a 1 1 mixture of diastereomers was obtained. Again, the dendritic catalyst could be recycled easily without any loss in performance. The scandium cross-linked dendritic material appeared to be an efficient catalyst for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene. The Diels-Alder adduct was formed in dichloromethane at 0°C in 79% yield with an endo/exo ratio of 85 15. The material was also used as a Friedel-Crafts acylation catalyst (contain-ing7mol% scandium) for the formation of / -methoxyacetophenone (in a 73% yield) from anisole, acetic acid anhydride, and lithium perchlorate at 50°C in nitromethane. 

Cross-linked beads made by vinylic polymerisations usually have some residual polymerisable functionalities [8], which allows the secondary imprinting polymerisation to covalently attach to the bead structure giving very stable particles. Where glass or silica is used a pretreatment with vinyl [9] or methacryloxy [2] silane derivatives ensures stable covalent attachment, while at the same time blocking surface silanol groups which might otherwise interfere with imprinting. 

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

The reduction of dichlorosilanes or chloroalkyl chlorosilanes stimulates the formation of linear chains, whereas the same reaction using trichlorosilanes or similar compounds may support cross-linked products. Cross-linking can also be obtained by the use of vinyl silanes [33-35] or Si-H-modified compounds. A silylation reaction occurs at higher temperatures, also leading to branched products. 

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). 

Organofunctional polysiloxanes with silanol groups can be cross-linked at low temperatures with tetrabutyl titanium, methyl triacetoxy-silane, and similar compounds and are thus used as cold vulcanization materials. In contrast, hot vulcanizable silicones possess one vinyl group in about 500-1000 monomeric units and can be cross-linked with peroxides. All silicon rubbers are filled with highly dispersed silica since the unfilled rubber is practically a useless elastomer. 

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