Cross-linking processes

Kanegafuchi Chemical of Japan has iatroduced a chemical cross-linking process for produciag PE foams by the bead technique similar to EPS. 

Radiation cure of methyl vinyl sihcone mbber results ia a random cross-linking process with no differentiation between methyl and vinyl groups... 

It was found that Zs levels off at dose > 20 krad. The cross-linking process in dioxane occurs at times longer than several milliseconds. Similar results were obtained for PHS in dioxane, however the radiolytic yield of scission, G(S), measured from the initial slope of the plot of Zs vs. D, is different for the two polymers G(S) is about 12 for PBS and about 4 for PHS, in contrast to irradiation of the pure polymers, where for both at 30° G(S) 1080. 

The basis of model calculations for copolymerization, branching and cross-linking processes is the stochastic theory of Flory and Stockmayer (1-3). This classical method was generalized by Gordon and coworkers with the more powerful method of probability generating functions with cascade substitution for describing branching processes (4-6). With this method it is possible to treat much more complicated reactions and systems (7-9). 

The commonly observed, and technically troublesome, formation of gel in the polymerization of dienes testifies to the occurrence of cross-linking processes during their polymerization. According to Eq. (13), gel should begin to form when p reaches the critical value pc = l/y , where is the weight average degree of polymerization of the primary molecules. Thus... 

Another type of network imperfection, resulting from cross-linking of two units not distantly related structurally, is indicated in Fig. 94. Cross-linkages such as B are wasted (except insofar as the loop may be involved in entanglements not otherwise operative). The proportion of these short path cross-linkages should be small ordinarily but could become very large if the cross-linking process were carried out in a dilute solution of the polymer. 

Whereas k = 1.3 is derived from the above-presented NSE data, k = 2.75 is expected for a four-functional PDMS network of Ms = 5500 g/mol on the basis of Eq. (67). Similar discrepancies were observed for a PDMS network under uniaxial deformation [88]. Elowever, in reality this discrepancy may be smaller, since Eq. (67) provides the upper limit for k, calculated under the assumption that the network is not swollen during the cross-linking process due to unreacted, extractable material. Regardless of this uncertainty, the NSE data indicate that the experimentally observed fluctuation range of the cross-links is underestimated by the junction constraint and overestimated by the phantom network model [89],... 

A3. The end-to-end distance of the free strands is practically unchanged by the cross-linking process, i.e., the extent of chemical modification of the polymer chains produced by high energy radiation should be small. 

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