Ureidosilanes on E-glass fibres: Deposition and surface characteristics

The behaviour of methoxy- and ethoxy-ureidosilanes deposited onto E-glass fibres from both organic and aqueous solutions is reviewed. γ-Ureidopropyltri(m)ethoxysilanes are used as coupling agents for paints to metal surfaces, but they have not found much use as coupling agents for E-glass fibres. On metal surfaces, the silane is deposited with the ureido-functional group outermost, available for reaction with a paint/polymer matrix. On E-glass surfaces, however, unless specific conditions are employed, ureidosilanes tend to deposit with a highly cross-linked siloxane surface. The silanol groups condense not only with silanols of adjacent silanes and the E-glass surface, but also with aluminium hydroxides leached from the E-glass surface. This forms a highly cross-linked aluminosiloxane outer layer. The aluminosiloxane forms at all pHs between 4 and 10, and also when a range of organic deposition solvents is used. 29Si-NMR results are presented which illustrate the high level of condensation; silane to silicate binding via the amino group was not detected. A small proportion of Si-O-Si (silane-glass) bonds was inferred, indicating that a small number of sparsely distributed Si-O-Si links bound the silane to the surface. Electrokinetic analysis of the surfaces confirmed patchy deposition with a siloxane outer surface. Using AFM and ab initio calculations, it was estimated that the patchwise deposition of the silane was in the form of bilayers (or multiples of bilayers). The bilayer structure consisted of siloxane outer layers, with the inner ureido layers forming hydrogen bonds. A graphical fit using the mole and volume fractions of silane and organic solvents confirmed the various types of siloxane surfaces as deposited from different organic solvents. Chlorinated solvents tended to produce a ureido outer surface, which was condensed, but solvent-swollen; deposition from alcohol and acetone resulted in a siloxane surface. Toluene and THF both produced minimally condensed ureido surfaces, which were easily removed by washing with methylene chloride.