Sealing of joints or gaps formed by connecting metal and other solid, e.g., glass, plastic, and carbon fiber, components of assemblies is necessary in many industries. For example, aircraft manufacturing requires sealing joints and gaps in fuel tank and wing assemblies. Various polymeric sealants, including polysulfide rubbers, can be used to seal such joints and gaps.
The surface of polysulfide rubber requires a lengthy period of time, i.e., typically at least several hours, to harden. Before its surface hardens and forms a skin (known as skin formation or skinning), i.e., while the surface of polysulfide rubber sealant remains soft, it is prone to gather debris from other manufacturing processes. For example, metal debris such as small aluminum turnings with sharp edges and points that result from drilling often are become imbedded into the sealant. Removal of such debris is time-consuming, and often requires repair of the sealant.
Avoiding the problems resulting from slow skinning by delaying subsequent manufacturing steps is unacceptable. Thus, compositions have been used to increase the skinning rate. These compositions include compounds that are understood to react with free thiol groups at the ends of polysulfide chains to form cross-links between polymer chains, thus causing the polymer seal to harden at its surface. An example of a compound used in these compositions is 1-(diethylthiocarbamoyldisulfanyl)-N,N-diethyl-methanethioamide, commonly known as disulfiram, which can be represented by the following chemical structure:

An example of a disulfiram-containing composition also contains magnesium acetate and flammable solvents (about 75-90% by weight methanol, as well as ethanol, toluene, ethyl acetate, and methyl isobutyl ketone). These products, although they tend to increase the skinning rate, are still inadequate. Skinning is still unacceptably slow, and the flammable solvents are relatively slow to evaporate and also create safety issues.
Compositions containing hexavalent chromium (Cr6+) in flammable solvent(s) can also be used to increase the rate of skin formation. These compositions, of course, pose severe environmental and exposure problems.
Therefore, there is a need for compositions capable of causing rapid skin formation on polysulfide rubber sealants that are environmentally friendly and safe.