Method of making water-resistant gypsum-based article
A moisture resistant gypsum-based product, e.g., a gypsum board, is made by adding a small amount of a siloxane to the aqueous slurry used to make the gypsum-based product along with a small amount of a dead burned magnesium oxide catalyst to enhance the curing of the siloxane. In the preferred embodiment, the siloxane is formed into an aqueous emulsion in situ with no chemical emulsifier.
1. A method for making a water-resistant gypsum-based board comprising:
(A) mixing a siloxane emulsion with the gauging water used to prepare said gypsum-based board;
(B) mixing a small amount of dead-burned magnesium oxide with calcined gypsum, wherein the amount of magnesium oxide is from 0.1 to about 0.5 wt % of the gypsum;
(C) mixing said siloxane emulsion/gauging water mixture with said calcined gypsum/magnesium oxide mixture to form an aqueous slurry;
(D) allowing said dead-burned magnesium oxide to catalyze cross-linking of said siloxane; and
(E) shaping said slurry and allowing said shaped slurry to set to form a set gypsum-based, water-resistant board.
2. A method according to claim 1 wherein said magnesium oxide produces a temperature rise of no more than 1 degree Fahrenheit in the Temperature Rise System.
3. A method according to claim 1 wherein said magnesium oxide has a surface area of at least 0.3 square meters per gram as measured by BET.
4. A method according to claim 1 wherein said magnesium oxide has a loss on ignition of less than 0.1% by weight.
5. A method according to claim 1 wherein the amount of magnesium oxide is from about 0.2 to about 0.4 wt % of the gypsum.
6. A method according to claim 1 wherein said siloxane is poly(methyl hydrogen siloxane).
7. A method according to claim 6 wherein said poly (methyl hydrogen siloxane) comprises from about 0.3 to about 1.0 wt. % of the dry ingredients of the set of gypsum board.
8. A method according to claim 6 wherein the quantity of said poly (methyl hydrogen siloxane) is about 0.4 to about 0.8 wt. % of the dry ingredients of the set gypsum board.
9. The method of claim 1 further comprising selecting the siloxane in the siloxane emulsion from the group consisting of a linear, hydrogen-modified siloxane and a cyclic, hydrogen-modified siloxane.
10. A method for making a water-resistant gypsum-based board comprising:
(A) mixing a quantity of a siloxane fluid with a portion of the gauging water used to prepare said gypsum-based board in a high intensity mixer to form a siloxane/water emulsion in situ;
(B) mixing a small amount of dead-burned magnesium oxide with calcined gypsum;
(C) mixing said siloxane/water emulsion with the balance of the gauging water;
(D) mixing said gypsum/magnesium oxide mixture with the siloxane/water mixture of step (C) to form an aqueous slurry;
(E) allowing said dead-burned magnesium oxide to catalyze cross-linking of said siloxane; and
(F) shaping said slurry and allowing said shaped slurry to set to form a set gypsum-based, water resistant board.
11. A method according to claim 10 wherein said siloxane/water emulsion is formed in the absence of an emulsifier.
12. A method according to claim 10 wherein about 25 parts by weight of gauging water are mixed with each part by weight of siloxane in step (A).
13. A method according to claim 10 wherein the amount of magnesium oxide is from about 0.1 to about 0.5 wt % of the gypsum.
14. A method for incorporating siloxane while forming a water-resistant, gypsum-based board, comprising:
(A) mixing a quantity of siloxane fluid with a portion of the gauging water used to prepare said gypsum-based board in a high intensity mixer to form a siloxane emulsion;
(B) mixing said siloxane emulsion with balance of the gauging water, calcined gypsum and from about 0.1 to about 0.5 wt % of the gypsum of dead-burned magnesium oxide to form an aqueous slurry;
(C) allowing said dead-burned magnesium oxide to catalyze cross-linking of said siloxane; and
(D) shaping and allowing said slurry to set to form a set gypsum-based, water-resistant board, said board absorbing less than about 10% of its own weight in water when immersed at 70° F. for two hours in accordance with ASTM Standard 1396 within 24 hours.