Binder Not Dependent on pH

A binder developed at DuPont called Ludox SK [8,10] avoids the need for pH adjustments in order to maintain slurry stability. The product is based on aluminate modified colloidal silica. The aluminated product is deionized and because the alumina sites provide sufficient charge density the product is stable. Deionization to remove sodium hydroxide results in a pH which is acidic. As long as the pH remains above about 3, there is little danger of the alumina being leached from the product surface. 

This deionized colloidal silica is not a strong binder. The removal of the sodium hydroxide, which appears to act as a binder catalyst, and/or the presence of the alumina on the surface, weakens the overall bond strength developed. However, this acidic version of colloidal silica does form stable mixtures with several water soluble polymers, the most important of which is polyvinyl alcohol (PVA). PVA is itself a good film former and imparts this property of much improved film formation to colloidal silica even when present as only about 3-5% of the total solids. The mixture of PVA and acid stabilized colloidal 

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