Chemical structure, hydrolytic crosslinking

The rate of reversion, or hydrolytic instability, depends on the chemical structure of the base polymer, its degree of crosslinking, and the permeability of the adhesive or sealant. Certain chemical linkages such as ester, urethane, amide, and urea can be hydrolyzed. The rate of attack is fastest for ester-based linkages. Ester linkages are present in certain types of polyurethanes and anhydride cured epoxies. Generally, amine cured epoxies offer better hydrolytic stability than anhydride cured types. 

The chemical origins of this field can be traced back over 150 years to the reports by Liebig, Wohler, and Rose [12,13] that phosphorus pentachloride and ammonia react to yield a white, crystalline, "organic"-type compound since shown to be hexachlorocyclotriphosphazene (5). Later, in 1897, Stokes [14] described the conversion of this compound to an insoluble, hydrolytically sensitive elastomer subsequently known as "inorganic rubber". The intractibility of inorganic rubber is now known to be due to its crosslinked structure, and its hydrolytic sensitivity to the presence of phosphorus-chlorine bonds. 

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