Internal bond cross-linking agents

Rubber base adhesives can be used without cross-linking. When necessary, essentially all the cross-linking agents normally used in the vulcanization of natural rubber can be used to cross-link elastomers with internal double carbon-carbon bonds. A common system, which requires heat to work, is the combination of sulphur with accelerators (zinc stearate, mercaptobenzothiazole). The use of a sulphur-based cross-linking system with zinc dibutyldithiocarbamate and/or zinc mercaptobenzothiazole allows curing at room temperature. If the formulation is very active, a two-part adhesive is used (sulphur and accelerator are placed in two separate components of the adhesive and mixed just before application). 

Williams [11] demonstrated that the modification of the resins with the addition of cross-linking agents (e.g., trimethoxymethylmelamine) improves the values of internal bond (IB) strength by 28% for the uncatalyzed resin and 47% for the catalyzed formulation. 

Once the nonproductive stock is mixed in an internal mixer in the second-stage cross-linking agents, accelerators are added to the nonproductive stock, which can then be shaped into final form and cured. The curing process is a chemical cross-linking reaction that builds bonds between rubber molecules. 

Figure 4.11 EfTect of the cross-linking/plasticizing agent on the internal stress concentration (K is the concentration coefiicient) at the edges of the adhesive-bonded joints...

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