Effect of boron oxide on surface hydroxyl coverage of aluminoborosilicate glass fibres: a ¹⁹F solid state NMR study

Solid state ¹⁹F MAS NMR studies of (3,3,3-trifluoropropyl)dimethylchlorosilane (TFS) probe molecules covalently bound to low surface area glass fibres are used for the quantitative analysis of hydroxyl sites on the surfaces of model E-glass fibres. The intensity of the ¹⁹F MAS NMR signal from the fluorinated TFS probe molecule reports on the total concentration of accessible hydroxyl units, while chemical shift and spin lattice relaxation rates are interpreted in terms of the surface morphology. Studies of a commercial aluminoborosilicate microfibre and four model E-glass fibres are presented. Accessible hydroxyl coverages in the range of 0.50-1.44 OH/nm² are measured and it is found that the presence of 1.8-5.5 mol% B₂O₃ reduces the surface hydroxyl concentration by a factor of two. The effect of the corresponding coordination of the boron species on the hydroxyl coverage and microporosity of the surface is discussed.