Non-combustible hardboard sheet

A process for producing a hardboard product having a fire rating of Class A according to ASTM Test E-84 comprising:

• (a) forming an aqueous slurry comprising from about 95% to about 98% water and from about 2% to about 5% solids, said solids consisting essentially of the following ingredients:
• (i) from about 60% to about 85% by weight mineral material, said mineral material consisting essentially of:
• (A) from about 20% to about 85% mineral fibers;
• (B) from 0% to about 50% perlite;
• (C) from 0% to about 25% of other mineral materials;
• (ii) from about 15% to about 40% of a binding system, said binding system consisting essentially of:
• (A) from about 5% to about 15% by weight cellulosic fibers;
• (B) from about 10% to about 35% of a heat and moisture re-activatable binding agent;
• (b) forming a relatively light product by depositing the said aqueous slurry on a forming screen;
• (c) drying the said formed product to a moisture content of no greater than about 1% by weight, said dried product having a density of from about 15 to about 23 pounds per cubic foot;
• (d) applying to each side of the dried product from about seven pounds per thousand square feet to about fifteen pounds per thousand square feet of water;
• (e) simultaneously heating at a temperature of from about 300.degree. F. to about 400.degree. F. and pressing with a minimum pressure of at least about 200 psi the formed product to reduce its thickness by a factor of at least 2.0 and to increase its density by a corresponding amount, the simultaneous heating and pressing being for a period of from about 30 seconds to about 5 minutes.

A process for producing a hardboard product having a fire rating of Class A according to ASTM Test E-84 from a relatively light dry product having the following ingredients:

• (i) from about 60% to about 85% by weight mineral material, said mineral material being composed of:
• (A) from about 20% to about 85% mineral fibers;
• (B) from 0% to about 50% perlite;
• (C) from 0% to about 25% of other mineral materials;
• (ii) from about 15% to about 40% of a binding system, said binding system being composed of:
• (A) from about 5% to about 15% by weight cellulosic fibers;
• (B) from about 10% to about 35% of a heat and moisture re-activatable binding agent;
• (iii) said relatively light product having a moisture content of no greater than about 3% by weight;
• said process comprising:
• (A) applying to the light, dry product from about seven pounds per thousand square feet to about fifteen pounds per thousand square feet of a solution comprising water;
• and, thereafter,
• (B) simultaneously heating and pressing the formed product to reduce its thickness by a factor of at least 1.5 and to increase its density by a corresponding amount.