Nanostructured polyurethane ceramer coatings for aircraft

Abstract

A new low VOC (volatile organic compounds) polyurethane/oligosiloxane coating was formulated using 1,6-hexamethylene diisocyanate (HDI) isocyanurate, alkoxysilane-functionalized HDI isocyanurate, tetraethyl orthosilicate (TEOS) oligomers, and three cycloaliphatic polyesters. One series of polyesters was synthesized using 1,4-cyclohexanedimethanol (CHDM) with 1,4-cyclohexanedicarboxylic acid (1,4-CHDA) and 1,3-cyclohexanedicarboxylic acid (1,3-CHDA). A second series of polyesters was synthesized using 2-butyl-2-ethyl-1,3-propanediol (BEPD) with 1,4-CHDA or 1,4-CHDA, and 1,3-CHDA, respectively. The polyurethane provided the general mechanical properties and the polysiloxane functioned as an adhesion promoter and corrosion inhibitor. The continuous organic polyurethane phase was coupled with the inorganic polysiloxane phase via the alkoxysilane-functionalized HDI isocyanurate. The general coatings, tensile, and viscoelastic properties were evaluated for the ceramer coatings as functions of the polyester and concentration of TEOS oligomers. Damage tolerance was evaluated via fracture toughness and development of the film deformation zone prior to crack propagation. The coatings’ properties were dominated by the organic phase. The film morphology was investigated using the combination of SEM (scanning electron microscope) and X-ray analysis. Phase separation was observed for ceramer coatings resulting in SiO₂ particles. The size of the preceramic silicon-oxo-particles was controlled by the T_g of the organic phase and the concentration of TEOS oligomers.