Crosslinking silane-grafted

The attachment of trialkoxysilane functionality to polyolefins (HDPE, LDPE, PP) though grafting of vinylsilanes e.g. 40, 41) or silane functional acrylates e.g. 42) has been widely studied. The principal application of these materials is the preparation of moisture curable crosslinked polyolefins that arc widely used in the cable industry. Silane treatment has also been used for surface modification of polyolefins " and silane grafted polyolefins might also serve as precursors to graft copolymers. 

Fig. 18. Crosslinking step of silane-grafted HFFR compound.

Various applications are targeted by crosslinked PP. Crosslinking to a gel content of 55% proved to be beneficial for cable insulation. Both crosslinking via grafting of silanes initiated by peroxide decomposition and radiation in the presence of polyfunctional monomers result in a substantial increase of the resistance to electrical breakage in high-voltage tests. 

Polyethylene can be crosslinked by decomposition of organic peroxides, hydrolysis of vinyl-silane grafted polyethylene or by high-energy (P or y) irradiation. Design a suitable experiment to determine the crosslink density and present the relevant equations. 

Crosslinking is a critical step for a successful foaming process. Methods to improve crossUnking efficiency have been analyzed empirically. One possibility is to add crossUnking promoters [28,37], such as triallyl-1,3,5-benzenetricarboxylate or triallyl cyanurate. Another method is by using POs that are silane grafted to enhance physical properties and processabiUty [38]. 

Narkis M, Tzur A, Vaxman A and Fritz H G (1985) Some properties of silane-grafted moisture-crosslinked polyethylene, Polym Eng Sci 25 857-862. 

Silicone nanospheres with different particle diameters, crosslinking density, and chemical functionalization are accessible by aqueous hydrolysis-condensation sequences of silane and siloxane precursors [1-3] and subsequent isolation. Grafting of functionalized particles with organopolymers [1] or surface modification [2] results in nanosized silicone domains which are readily dispersible in monomeric and polymeric systems. A variety of these versatile, tailor-made products will soon be launched by Wacker on a commercial scale. 

In this case the final crosslinking reaction is not an addition polymerization reaction but the hydrolysis and condensation reaction of alkoxy silanes incorporated through a separate step of addition graft copolymerization. 

Grafting reaction Reaction of a monomer and a polymer to form a grafted polymer or copolymer Grafting vinyl silanes onto polyolefins for crosslinked polyolefins acrylic acids onto polyolefins for hydrophilicity maleic anhydride onto polyolefins for compatabilization... 

The function /is constant for a given grafting and whatever the substrate Me value. Values of Wq are also in agreement with values quoted in the literature. Indeed, usual work of adhesion values He between 40 and 70 mj m for organic-organic contacts, such as between two silanated siheas [10], and between 40 and 145 mJ m for a contact between a raw material such as silica and a silanated silica. Finally we have reported in Fig. 3.7 the evolution of the dissipative function / versus Me, the mass between crosslinks. A linear relationship is obtained. Therefore a modified expression for G, the separation energy, is proposed (Eq. (8), where/(v, T) represents a viscoelastic dissipation function which depends only on the temperature T and separation rate v). 

The choice of peroxide used is determined by the temperature of its decomposition. Peroxide should be effectively dispersed in the polymer melt brfore a substantial homolysis of 0—0 bonds can occur. For such a purpose, dicumyl peroxide which may be dissolved in vinyl trimethoxy silane (b.p. 120 °C) is suitable. The required degree of crosslinking was attained if 2 % w. of silane with 5-10% w. of peroxide were added to polyethylene. At the silylation, grafting should not commence before both compounds (peroxide, ane) are well dispersed in the polymer melt [141]. NonhcMno-geneous dispersion of additives reduces efficiency of grafting and of subsequent crosslinking. 

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). 

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