Cross-linking indirect promoters

Cross-linking of polymeric materials can also be promoted by the incorporation of additives often known as cross-linking enhancers [39] or cross-link promoters [33]. They are classified as (a) indirect cross-link promoters and (b) direct cross-link promoters. 

Bases like water or ammonia [33] have been found to enhance cross-linking in polymers like PP and ethylene-propylene elastomers. The mechanism of action of various indirect cross-link promoters is reported in the literature [77-81]. 

Active thrombin not only converts fibrinogen into fibrin, but also indirectly promotes its own synthesis by catalyzing the activation of factors V and Vlll. In addition, it catalyzes the activation of factor Xlll and thereby triggers the cross-linking of the fibrin. 

There are essentially two types of promoters of radiation cross-linking or prorads.32 One group, indirect cross-link promoters, do not enter directly into the cross-linking reaction, but merely enhance the formation of reactive species such as free radicals,... 

The water carrier is usually absorbed by at least one of the bonded surfaces though evaporation can also play some part. As water is removed, the dispersed plastic phase is forced into contact and congeals the resulting film binds the surfaces together. Cure rate depends on the speed of water extraction - ultimately, the absorbed water is lost by indirect evaporation. Direct evaporation of the emulsion to form a film is used in bonding of open weave materials but, even so, there is still a slower, subsequent release of absorbed water from the substrates. Normally, the cured adhesive film is completely thermoplastic but special acid catalysts, available for some formulations, will promote cross-links in the film to enhance environmental stability. 

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