Other Crosslinking Agents

There exist a considerable number of compounds containing labile chlorine which bring about sulfurless vulcanisation at levels of approximately 3 phr. Basic chemicals such as lead oxides and amines are needed. It may be assumed that diene rubbers are crosslinked by such systems through the formation of C-C links this would mean, initially, hydrogen chloride is split off and later neutralised by the base. Examples of chemicals that act in this maimer are  

4-dichlorobenzotrichloride (a.38) hexachloro-p-xylene (414) chloranil (416) bis-chloromethylxylene (410) benzene disulfochloride (407) 

Apart from the list above some N-bearing molecules are described in the literature (a.37). 

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Novolac network degradation mechanisms vary from tiiose of resole networks due to differences in crosslinking metiiods. Nitrogen-containing linkages must also be considered when HMTA (or other crosslinking agent) was used to cure novolac networks. For example, tribenzylamines, formed in HMTA-cured novolac networks, decompose to cresols and azomethines (Fig. 7.50). 

This is attributable to the protective effect of the PS in the blends. Further work on this interesting topic, using other crosslinking agents, mixtures of other plastics that need recycling, and electron irradiation, is warranted. 

Vulcanization or cross-linking chains by reacting the double bond in natural rubber with sulfur and other crosslinking agents is used to limit stress induced flow in natural rubber. Various means were tried to crosslink the synthetic elastomer ethylene propylene rubber (EPR). The technique commonly used is based on experience with natural rubber. A small amount of a diene [a monomer with two double bonds] is incorporated into the EPR chain to furnish sites for vulcanization reactions. 

Others crosslinking agent include poly(styrene sulfonic acid-co-maleic acid) (PVA/PSSA-MA) and poly(acrylic acid-co-maleic acid) (PVA/PAA-MA). Blending of PVA with sulfonated proton-conducting polymers, like sPEEK, Nafion, PSSA, is another approach to achieve the high conductivity requirements... 

Caprolactam, a white solid that melts at 69°C, can be obtained either in a fused or flaked form. It is soluble in water, ligroin, and chlorinated hydrocarbons. Caprolactam s main use is to produce nylon 6. Other minor uses are as a crosslinking agent for polyurethanes, in the plasticizer industry, and in the synthesis of lysine. 

Serious deviations of the polymer network structure from the ideal one can have several causes. One of them is the crosslinking agent involvement in intramolecular cycle formation. The contribution of this reaction grows with the system dilution as well as when the crosslinker units in the chain are close one to the other, i.e. its fraction in the copolymer increases. All this is in good agreement with the observed trend. 

All known methods of initiation are used for inducing three-dimensional polymerization of AAm, AAc, and their copolymerization [37, 38]. Some special initiating systems have been also elaborated [39]. There is a wide choice of crosslinking agents various A,A -bis-acrylamides (methylene, ethylene, isopro-pylidene) as well as dimethacrylates of oligoethylene glycols and some others. 

The synthesis of polysulfide elastomers involves the use of a small amount of trichloroalkane in addition to dichloroalkane and sodium sulfide in order to form a branched polymer. The prepolymer is treated with a mixture of sodium hydrosulfide and sodium sulfite followed by acidification to convert all end-groups to thiol groups. Further polymerization and crosslinking is achieved by oxidative coupling of the thiol end-groups by treatment with lead dioxide, p-quinone dioxime, or other oxidizing agent... 

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