Surface Modifications and Applications of Aqueous Silica Sols

Doctoral thesis, 2010

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 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. About 1.5 GPTMS groups per nm2 silica surface were needed to affect significantly the properties of colloidal silica. Modification of conventional silica sols improves the stability toward gelling by electrolytes, allows the preparation of mixtures of various lattices with silica sols that are stable toward gelling, 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 was the aluminate modification of the silica particles. With 1.5 aluminosilicate sites per nm2 of core surface, well-dispersed composite particles having 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 wt-% of titania based on the weight of the core, were prepared. In a third type of modification colloidal silica was prepared with a high degree of 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. Highly structured silica sols but being stabilised with sodium can be used to make solid electrolytes in lead-acid batteries with improved properties compared with conventional electrolytes. Colloidal silica of different sizes can be used to improve the properties of concrete mixtures. 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 of the early strength while the large particle fraction will increase the ultimate mechanical properties.