Surface Modifications and Applications of Aqueous Silica Sols

Abstract: Methods of modifying the surface of the particles of colloidal silica were developed and the effects of surface modification on the properties of colloidal silica were studied. Gamma-glycidoxypropyltrimethoxysilane (GPTMS) readily reacts with water in aqueous solutions to yield a hydrolysed silane that reacts readily with the silica surface. The epoxy functional groups were not affected during the hydrolysis but subsequent reaction with the silica surface opened the epoxy rings. The presence of silane groups on the particle surface was established by Si-NMR. Up to about 2.0 GPTMS groups could be attached per nm2 silica surface and about 1.5 GPTMS were needed to affect the properties of colloidal silica in a significant way. Modification of conventional silica sols by reacting them with GPTMS improves the stability toward gelling by electrolytes, prevents precipitation normally caused by freezing of silane-unmodified sols, allows the preparation of mixtures of various lattices with silica sols that are perfectly stable toward gelling, improves the water resistance of latex films, increases the hardness of latex coatings, and improves the properties of pigmented coatings. Methods were developed for making white composite pigments consisting of a silica core with a titania shell. A necessary step in the preparation was to modify the sol of core particles (Stöber sols) by reacting the cores with sodium aluminate to create strongly negatively charged aluminosilicate sites on the particle surface. With 1.5 aluminosilicate sites per nm2 of core surface, dispersions of well-dispersed composite particles having silica cores of uniform size in the range of 300 to 500 nm with a titania coating, the thickness of which corresponded to from 150 to 400 % by wt of titania based on the weight of the core, were prepared. In a third type of modification colloidal silica was prepared under conditions yielding sols of high structure, i.e. the silica particles having linear dimensions rather than being perfect spheres and being stabilised with amines. Such sols are very effective anionic components in dual retention aids in papermaking. Yet, similar type of modification of colloidal silica, with a high degree of structure, but being stabilised with sodium can be used to make solid electrolytes in lead-acid batteries. Mixing colloidal silica with sulphuric acid yields a solid electrolyte in the form of a gel with improved properties compared with conventional fluid electrolytes. Colloidal silica of different particle sizes can be used to improve the properties of concrete and concrete mixtures in different ways. Modifying colloidal silica to yield sols of wide particle size distributions provides a means to make concrete with improved properties. The small particle fraction of the sol will increase the workability time of concrete mixtures and the early strength. The large particle fraction will increase the ultimate mechanical properties of concrete.