Moisture-resistant gypsum boards and slurries for making same

1. A gypsum board comprising:

a cover sheet; and

a gypsum layer disposed on said cover sheet with said gypsum layer comprising a reaction product of;

an isocyanate having an isocyanate functionality wherein said isocyanate is liquid at 25° C. and comprises a diphenylmethane diisocyanate;

water; and

stucco.

2. The gypsum board of claim 1 wherein there is an excess of said water relative to said stucco and wherein said liquid isocyanate reacts with said excess of said water to form a polymer matrix in said gypsum layer for increasing moisture resistance of said gypsum board.

3. The gypsum board of claim 1 wherein said liquid isocyanate has a dynamic viscosity of from about 1 to about 2500 cps at 25° C.

4. The gypsum board of claim 1 wherein said liquid isocyanate is selected from the group of 2,4′-diphenylmethane diisocyanate, a modified isocyanate different from said 2,4′-diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanate, and combinations thereof.

5. The gypsum board of claim 4 wherein said liquid isocyanate further comprises 4,4′-diphenylmethane diisocyanate.

6. The gypsum board of claim 5 wherein said 2,4′-diphenylmethane diisocyanate is present in an amount of from about 10 to about 55 parts by weight based on 100 parts by weight of said liquid isocyanate, and said 4,4′-diphenylmethane diisocyanate is present in an amount of from about 45 to about 90 parts by weight based on 100 parts by weight of said liquid isocyanate.

7. The gypsum board of claim 5 wherein said modified isocyanate is present in an amount of from about 15 to about 35 parts by weight based on 100 parts by weights of said liquid isocyanate, and said 4,4′-diphenylmethane diisocyanate is present in an amount of from about 65 to about 85 parts by weight based on 100 parts by weight of said liquid isocyanate.

8. The gypsum board of claim 5 wherein said polymethylene polyphenyl polyisocyanate is present in an amount of from about 30 to about 75 parts by weight based on 100 parts by weight of said liquid isocyanate, and said 4,4′-diphenylmethane diisocyanate is present in an amount of from about 20 to about 70 parts by weight based on 100 parts by weight of said liquid isocyanate.

9. The gypsum board of claim 1 wherein said liquid isocyanate is reacted in an amount of from about 0.1 to about 10 parts by weight based on 100 parts by weight of said stucco.

10. The gypsum board of claim 1 wherein said liquid isocyanate is reacted with an active hydrogen containing species component in an amount of from about 1 to about 10 parts by weight based on 100 parts by weight of said liquid isocyanate to form a prepolymer.

11. The gypsum board of claim 1 wherein said gypsum layer is substantially free of a supplemental foaming agent yet comprises a plurality of voids in said gypsum layer.

12. The gypsum board of claim 1 wherein said gypsum layer absorbs no more than 20% of water in 2 hours as defined according to ASTM C473.

13. The gypsum board of claim 12 wherein said gypsum layer is substantially free of conventional moisture-resistant additives yet is moisture-resistant as defined by ASTM C473.

14. The gypsum board of claim 1 wherein said gypsum layer is substantially free of cellulose fiber.

15. A gypsum board comprising a cover sheet and a gypsum layer disposed on the cover sheet, said gypsum layer comprising a reaction product of:

an isocyanate having an isocyanate functionality;

water; and

stucco;

in the presence of an emulsifying agent for increasing miscibility of said isocyanate with said water;

wherein said emulsifying agent is selected from the group of sulfates, sulfonates, and combinations thereof.

16. The gypsum board of claim 15 wherein there is an excess of said water relative to said stucco and wherein said isocyanate reacts with said excess of said water to form a polymer matrix in said gypsum layer for increasing moisture resistance of the gypsum board.

17. The gypsum board of claim 15 wherein said isocyanate is selected from the group of an isocyanate which is liquid at 25° C. and has a dynamic viscosity of from about 1 to about 2500 cps at 25° C., an isocyanate which is solid at 25° C., and a combination thereof.

18. The gypsum board of claim 15 wherein said isocyanate is selected from the group of 2,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethane diisocyanate, a modified isocyanate different from said 2,4′-diphenylmethane diisocyanate and said 4,4′-diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanate, and combinations thereof.

19. The gypsum board of claim 15 wherein said isocyanate is reacted in an amount of from about 0.1 to about 7 parts by weight based on 100 parts by weight of said stucco.

20. The gypsum board of claim 15 wherein said emulsifying agent comprises ammonium ether sulfate.

21. The gypsum board of claim 15 wherein said emulsifying agent is present in an amount of from 0.1 to about 2 pounds (lbs) per 1000 square feet (msf) of said gypsum layer.

22. A method of manufacturing gypsum board, said method comprising the steps of:

combining water, stucco, and an isocyanate having an isocyanate functionality wherein the isocyanate is liquid at 25° C. and comprises diphenylmethane diisocyanate to form a slurry, wherein the slurry comprises a reaction product of; the isocyanate having the isocyanate functionality, the water, and the stucco; and

applying the slurry to a cover sheet to form a gypsum layer on the cover sheet.

23. The method of claim 22 wherein there is an excess of the water relative to the stucco and wherein the liquid isocyanate reacts with the excess of the water to form a polymer matrix in the gypsum layer for increasing moisture resistance of the gypsum board.

24. The method of claim 22 wherein the step of combining is further defined as:

reacting the liquid isocyanate and the water to form a composition; and

reacting the composition and the stucco to form the slurry.