To obtain the strongest possible bonds to rubber with an epoxy adhesive, the surface of the rubber should be cyclicized using the following treatment: Immerse the rubber in concentrated sulfuric acid (Sp. Gr., 1.84) for (natural) 5-10 minutes, or (synthetic) 10-15 minutes. Alternatively, a paste of concentrated sulfuric acid and barytes can be used. This is made by adding barytes to the acid to a consistency that will not run. After washing thoroughly with water and drying, the brittle surface of the rubber should be broken by flexing so that a finely cracked surface is produced. It may be necessary to wash with dilute caustic solution (about 0.2%) to insure neutralization of residual acid which, if not removed, will consume some of the curing agent, and weaker the strength of the bond. The rubber should be thoroughly dried before the epoxy adhesive is applied.
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