16.6 ELASTOMERS

There are over 30 broad groups of chemical types of elastic polymers. These are arranged by ASTM D 1418 into categories of materials having similar chemical chain structures. There are several problems with joining elastomer materials.

One problem is the significant variation that can exist within a given chemical type. This is due to differences in average molecular weight, molecular weight (MW) distribution, polymerization processes, variation of structural arrangement, copolymer or terpolymer ratios, etc. There also can be significant differences in the compounding recipe for a given product. This is due to differences and latitude of choice in fillers and reinforcing agents, liquid plasticizers, compounding agents, and the like. Thus, almost an infinite number of compounds are possible for a given generic elastomer such as neoprene or nitrile, and each variation can significantly affect the adhesion properties of the material.

Elastomer materials specifications usually do not focus on the adhesive properties, but mainly address the chemical and physical properties of the rubber. Thus, the supplier has wide latitude within the specification to make changes in the compound formulation that could be disastrous to the adhesive bond. One solution is to qualify every new lot of elastomer material for adhesion as well as the more standard properties.

Besides curing systems, fillers, and plasticizers, an elastomeric compound may contain protective chemicals such as antioxidants, antiozone agents, waxes, and fungicides. Some of these are purposefully designed to "bloom" or to come to the surface of the elastomer either during processing or...