Hydrated alkali metal silicates are known fire-proofing materials and are extensively employed in building construction. Under the high temperatures existing during a fire, the water of hydration is driven off causing the composition to puff, expanding by up to 25 to 40 times its original volume. Thus, when combined with fire-stop materials these materials intumesce to provide a layer of insulation against fire and smoke, said layer being full of bubbles and holes from the expansion of the water of hydration. The foaming pressure helps to seal apertures and passages in building structures making these fireproofing materials useful in fire-stops, see U.S. Pat. No. 4,364,210.
Alkali metal silicates can also be incorporated into proofing materials such as asphalt shingles in order to convert these shingles into a fire retardant Class A or B form. Alkali metal silicate particles may be placed in an asphalt layer in between the top layer of asphalt and roofing granules and the substrate of organic felt or fiberglass mat. In the event of a fire on a roof, the intumescent silicate particles expand to form a thermal barrier which retards ignition of the roofing deck.
The formation of alkali metal silicate gels by the addition of such materials as sodium aluminate or boric acid solutions to alkali metal silicates at elevated temperatures is known (see U.S. Pat. No. 4,297,252, Example 3, and U.S. Pat. No. 3,498,807, column 3, lines 42-56).
It is also known that the production of a solid, hydrated alkali metal silicate in which the moisture content is controlled can require carefully controlled drying conditions (see U.S. Pat. No. 3,895,995). Typically, alkali metal silicate suspensions or gels described in the literature are dried or cured at temperatures ranging from 90.degree. to 136.degree. C. and below the boiling point of the suspension or solution.
One difficulty with silicate based materials is their degradation on exposure to water or high relative humidity for extended periods of time. Water is known to leach away the alkali metal oxide from the silicate particles, reducing their ability to intumesce. Various solutions to this problem have been proposed in the past, including a coating which covers alkali metal silicate particles (see U.S. Pat. No. 4,218,502).
it is the object of this invention to provide an alkali metal silicate material which retains its intumescence after long exposure to water in order to ensure that its effectiveness as a fire protection agent will last a long time under outdoor weathering conditions.