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Silica gels are related products to precipitated silicas, and while many of their applications are outside of polymers (such as use as desiccants), they do find some applications as particulate polymer additives, principally in paints and coatings. Patterson (1994) describes the production and uses.
The principal production method for silica gels is similar to that for precipitates and starts with the production and acid neutralization of sodium silicate. The difference is that the primary particles are built up into a massive threedimensional structure, instead of into small aggregates. This structure is highly porous and entraps all of the aqueous phase. The gel is then washed to remove by-product sodium salts, dried, and milled to produce small, highly porous particles. Two different drying processes are used, resulting in different classes of product. Conventional drying leads to significant shrinkage and loss of structure due to capillary forces and results in products known as xerogels. This structure collapse can be mitigated by various alternative drying methods, which reduce the capillary forces. One way is to replace the water by a liquid such as alcohol or acetone, which has a lower surface tension, and then to carry out critical point drying. Such products are known as aerogels.
All silica gels are high-specific-surface-area, high-oil-absorption products, with their properties mainly determined by the following factors: pH and method of gel formation Washing procedures Drying process
Any particle size reduction employed (e.g., milling)
Milled xerogels are the most commonly met types in polymer applications. Their principal uses are as flatting and matting agents in coatings and as anti-blocks in polymer film and in battery separators. In many of these, they compete with precipitated silicas.
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