Millboard has been used commercially for many years. It serves as thermal insulation in gaskets, linings for fire-safe cabinets, and in the glass making industry as a float roll covering material.
In the past, asbestos fibers of either the chrysotile or amphibole type have been incorporated into the starting mixture for such boards.
U.S. Pat. No. 1,594,417 (Kobbe) shows the use of a short-fibre asbestos combined with a cement binding agent and congealed sulfur to produce a high-resilient, tough, high-strength millboard.
U.S. Pat. No. 1,678,345 (Mattison) incorporates asbestos fibers into a mixture of hydraulic and calcium and aluminum carbonate to produce a millboard product.
U.S. Pat. No. 3,334,010 (Moore) utilizes both chrysotile asbestos and crocidolite (amphibole) asbestos in a millboard product.
These fibers strengthen the resulting product and provide heat resistance in high-temperature applications.
Asbestos has been shown to be a health hazard, however, and it is therefore desireable to find a suitable material for millboard use which does not contain asbestos fibers.
Asbestos-free millboard products have been developed to meet this need. One example is U.S. Pat. No. 4,308,070 (Cavicchio) which discloses a combination of a cellulosic fiber, barytes (barium sulphate), cement, and an inorganic filler such as talc, diatomaceous earth, silicates, and carbonates.
Millboards composed of washed ceramic fiber and incorporating various fillers have also been used as roll coverings for float line rolls in the manufacture of glass. These materials contain approximately twenty percent of unfiberized material, or shot, of a size less than 100 mesh (0.0059 inches). This unfiberized material causes microscopic defects in the glass sheet as it passes over the float line rolls. Another disadvantage is that these millboard products become dusty once the binder is removed.