Interfacial Considerations in Polysulfide Sealant Bonding

Abstract

Abstract It is necessary to examine the sealant-coating interface and consider the polymer-primer interaction for a complete understanding of interfacial bonding. Why a polysulfide sealant sticks to a fresh or “imaged” polyurethane coating surface and not to an “aged” polyurethane surface has been determined. Also, we were able to artificially age polyurethane coated surfaces and correlate artificial aging with natural aging by surface tension and contact angle measurements. The polyurethane kinetic studies indicate that unreacted isocyanate (NCO) groups at the surface quickly deplete in a matter of few hours under 60°C/95 percent R.H. condition. Both heat and moisture reduce the NCO concentration in bulk but at a moderately slower rate. Room temperature cure of the polyurethane was found to be very slow. Disappearance of residual NCO groups during aging coupled with hardening of the polyurethane surface appears to be the primary reason for inadequate adhesion with a polysulfide sealant to an aged polyurethane coating. Titanate primer reacts with polysulfide resin to give chemically bonded product, whereas no reaction with “aged” and “unaged” polyurethane coated surfaces was detected. The strong solvency of the primer, however, softens and lowers the glass transition temperature (Tg) of the polyurethane coating. Insofar as the polysulfide adhesion improvement onto “aged” polyurethane surface is concerned, we found that an alkyl phosphatotitanate and mercaptofunctional silane are effective adhesion promoters, whereas oxirane-functional silane and a polymeric isocyanate are ineffective primers. The SEM study of sealant-coating interface revealed that water is one of the most potent desorbing agents. The SEM method is quite valuable and should therefore be further refined. The adhesion of polysulfide sealant to new polyurethane surface can be attributed to the reaction of residual NCO groups with the polysulfide to form polythiourethane that resist desorption by JRF and moisture. Since “aged” polyurethane surfaces contain too few residual NCO groups, the resultant adhesion of the polysulfide sealant is poor. The use of a titanate primer, however, produces adequate adhesion. The titanate interacts with the polysulfide but does not react with the polyurethane surface. The titanate and solvent combination, however, interfaces with the polyurethane surface by softening it and producing microroughness and thereby increasing mechanical interlocking. Another reason the primer functions is by removing or lifting a monomolecular (or thicker) water layer from the polyurethane surface and allowing the sealant to “wet” or get “closer” to polyurethane. Incorporation of epoxy modification in a polysulfide sealant improves adhesion to “aged” polyurethane surface because epoxy groups can react with urethane groups. Additionally, hydroxyl and nitrogen groups may produce increased attractive forces.