Synthesis and Structure of Environmentally Friendly Hybrid Clay/Organosilane Nanocomposite Coatings

Abstract

Environmentally friendly polysiloxane and clay/polysiloxane composite coatings were synthesized on aerospace-grade aluminum alloy AA2024-T3 substrates from mildly acidified aqueous solution. The polysiloxane coatings were synthesized by acid-catalyzed hydrolysis and condensation of organosilane coupling agents such as glycidoxypropyltrimethoxysilane (GPTMS) and tetramethoxysilane (TMOS) followed by solution drop-casting onto the substrate to form self assembled nanoparticles, coating. The epoxy polysiloxane coating formed by condensation reaction of hydrolyzed TMOS and GPTMS was subsequently cured with aminosilane coupling agent to form cross-linked polysiloxane coating. Clay/polysiloxane coating was formed by dispersing about 0.1–0.3wt% of clay in the hydrolyzed TMOS/GPTMS solution followed by solution casting and the resulting clay/polysiloxane composite coating was subsequently cured with aminosilane coupling agent. The structure and composition of organosilane coupling agents and hybrid polysiloxane coatings were determined by reflection–absorption infrared spectroscopy (RAIR) and X-ray diffraction spectrometry (XRD). The hydrolysis, condensation and curing reactions of TMOS and the organosilane coupling agents were studied by analyzing thin films cast on aluminum alloy substrate after a predetermined reaction time by using RAIR. The XRD results show that the resulting polysiloxanes are semi-crystalline polymers. Wide angle XRD analysis indicated that clay dispersed in clay/polysiloxane composite coating is either highly intercalated or partially exfoliated. This inference was drawn from the disappearance of d001 diffraction peak for clay from the XRD spectrum of clay/polysiloxane coatings cured at 100 °C for 2.5 h.