Glass strengthening by organosilane water-based coatings
Abstract-We have studied an epoxysilane/aminosilane mixture coating deposited from an aqueous solution to strengthen flat glass. Indented glass coated with a mixed epoxysilane/aminosilane film of 100 nm thickness exhibited a significant increase in strength of 75%. The film, which had a 6 GPa Young’s modulus, adhered well and had a very high cohesive strength, due to formation of a highly crosslinked network by both the epoxy-amine reaction and silanol condensation. Closure stresses (stresses which tend to pull the fracture surfaces together) generated within the crack by the elastic response of the material filling the crack were probably the dominant strengthening mechanism in the present system. We have characterized the condensation of the silanes solution from which the film was deposited by NMR and IR spectroscopies, as well as by dynamic light scattering. The condensation in the mixture was noticeably enhanced and proceeded faster as compared to pure epoxysilane solution, and co-condensation also occurred. Oligomer growth proceeded slowly and resulted in destabilization of the solution: due to their hydrophobic character, the oligomers formed coalesced abruptly into 200 nm diameter particles. The progress of condensation in solution resulted in a wetting transition during the deposition of the silane film on glass by dip coating. More and more hydrophobic oligomers were produced as the reaction time increased. This resulted in adsorption of more hydrophobic aggregates onto the surface which eventually led to dewetting of the film. This, in turn, prevented sufficient penetration of the coating in the defects and strengthening effect disappeared.