Synthesis and Characterization of Nitrile Containing Polysiloxanes and Their Corresponding Networks as Aircraft Sealant Materials

Polysiloxane networks have excellent oxidative and good UV environmental stability, flexibility at low temperatures, and thermal stability at higher temperatures. This wide service temperature range makes these materials a candidate class of materials for high performance adhesives and sealants, and in particular for applications on high speed aircraft. Polar polysiloxane networks were prepared with cyanopropyl substituents to lower any propensity for the materials to swell in hydrocarbon fuels and to improve adhesion to metal substrates. 1,3,5,7-tetramethyl,1,3,5,7-tetrahydrocyclotetrasiloxane (D4H) was hydrosilated with allyl cyanide to yield the corresponding 3-cyanopropylmethylcyclotetrasiloxane monomer (D4CN). Controlled molecular weight oligomers with vinyl termination were prepared in equilibrium reactions using a basic catalyst. These oligomers were then crosslinked with various hydride functional crosslinking reagents to yield model networks for mechanical and adhesion studies. The network properties of nonpolar polydimethylsiloxane (PDMS), polar poly[methyl(3,3,3-trifluoropropyl)siloxane] (PMTFPS), and the novel polar poly(3-cyanopropylmethyl-siloxane) (PCPMS) were investigated as a function of sidechain chemical structures. Effects of increasing crosslink density were investigated for the PDMS networks by adding a difunctional siloxane dimer with vinyl groups. Moduli and tensile strengths increased while percent elongation decreased as the crosslink density was increased. All networks were thermally stable above 300 °C in both air and N2 (when heated at a rate of 10 °C/min.) and exhibited Tgs lower than -55 °C. The polar networks swelled to a much lesser extent (at least one order of magnitude) than the nonpolar networks in hydrocarbons and Jet fuel. Cohesive failure was observed for the polar networks via metal to metal (Al foil substrate to Al and Ti adherends) 180° peel test. The PCPMS elastomers had average load values twice those of the PDMS networks independent of crosslink density.