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How Sodium Silicate Sealants Take Their Set
Unlike resin impregnation sealants which are made up of 100% solids, sodium silicate impregnation sealants are made up of 40-60% solids with the balance being water. For sodium silicate impregnation sealants to be effective when sealing against other compounds made up of water, the sodium silicate must be cured properly.
Silicates normally hardened by loss of water thus sodium silicate impregnates will dry and cure in air. It is important, however, to provide an avenue for the moisture to escape to ensure as complete a cure as possible. Air drying castings impregnated with sodium silicate is permissible, but is not the process of choice because the drying time and resultant cure of the sodium silicate is unpredictable and hard to duplicate.
Oven Drying or Baking
The better choice for curing sodium silicate is oven drying or baking because the time and temperature can be controlled and duplicated. Also, an oven provides for a more complete drying of the sealant. The more water removed from the silicate, the higher will be its resistance to water. The amount of water remaining in the silicate bond is governed by the temperature to which it has been exposed (assuming exposure was long enough to establish equilibrium). The temperature should be raised gradually as once the surface of the film has skinned over, escape of residual moisture is blocked. Puffing and blistering (intumescence) may occur as the retained moisture builds pressure and is finally expelled. The temperature is preferably held first around 200F (below the boiling point) until all moisture that can be evaporated at that temperature has escaped. Thereafter it may be raised according to the degree of dehydration required. Infra-red heating is especially recommended. In many cases a period of air drying prior to the baking cycle is effective. Completely dehydrated silicate is extremely heat resistant, softening at about 1200F. The last traces of moisture are lost only when this temperature is approached, but baking between 500F and 700F reducing the solubility of silicate sufficiently for many commercial moisture resistant requirements.
If you would like further information regarding sodium silicate or resin sealants, please contact IMPCO, Inc. at (401) 521-2490.