NCO Concentration Research Articles

Interfacial Considerations in Polysulfide Sealant Bonding

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.

Isocyanate Reactions with Difunctional Polyisobutylenes

Abstract The reactions of isocyanates with carboxy terminated polyisobutylenes, CTPIB, and with hydroxy terminated polyisobutylenes, HTPIB, have been studied in detail. In the case of HTPIB specific emphasis has been given to an hydroxy-ester functionality prepared by the base catalyzed reaction of CTPIB with propylene oxide. Isocyanate reactions with polymeric carboxyl groups were studied to observe if conditions could be established to remove quickly the undesirable carbon dioxide by-product. A potential advantage of this reaction would be the formation of a more stable amide link compared with that of a urethane linkage. In capping reactions with CTPIB and diisocyanates (where NCO group concentrations are in excess), the course of the reaction essentially follows second order kinetics with respect to carboxyl utilization. Bulk reactions, run under vacuum, facilitated the removal of CO2 and markedly increased the rate of reaction. Even so, the reaction required relatively high concentrations of tertiary amine catalysts suggesting a dual role for the base. Aromatic diisocyanates with chlorine substitution were several fold more reactive with CTPIB than was toluene diisocyanate, and gave indications of a better selectivity. Sulfonyl isocyanates possess still greater reactivity. The selectivity of the isocyanate reaction with polymeric COOH is poor when using common diisocyanates such as TDI. The predominant extension of prepolymers is far less probably than in the case of hydroxyl based systems. However, tough, dense, and flexible networks can be formed from initial products of 2000 number average molecular weight. The reactivity of the secondary hydroxyl ester terminal functionality of polyisobutylene, 2° HTPIB, with diisocyanates was comparable to that of commercial polyether or polyester diols which are largely primary hydroxyl. This comparable activity is explained by the fact that in bulk reactions the hydrocarbon backbone of 2° HTPIB provides a reaction medium with a lower dielectric constant and thus a more advantageous environment. In capping reactions followed by IR monitoring of OH consumption, reaction rates also followed second order kinetics with respect to OH consumption when the NCO concentration was in excess. In contrast to isocyanate-polymeric COOH systems, the reaction with HTPIB required no catalysts for extensive consumption of OH groups at moderate temperatures. The HTPIB-toluene diisocyanate reaction was far more selective, and this resulted in a greater potential for extension with the prepolymer. The physical properties of extended and crosslinked networks reflected this selectivity. For a given molecular weight level, networks with HTPIB-diisocyanate prepolymers were more extensible and had higher strengths than did CTPIB based counter parts. Fractionation of original starting materials into narrower molecular weight ranges with slightly improved degrees of functionality improved tensile strengths and extensibilities of subsequent HTPIB based networks. Interesting blocked polymer networks were formed with HTPIB and polyether diols (for example polytetramethyleneglycol). These two liquids which were immiscible, in the molecular weight range of Mn−2000, formed transparent elastic networks of high strength after mutual capping with TDI and subsequent extension and crosslinking by a combination of aromatic diamines and low molecular weight aliphatic diols.