Light weight gypsum board

Abstract

This invention provides low density gypsum products. This invention also provides a method of making the low density gypsum products.

Claims

1. A light weight gypsum board comprising: a set gypsum core disposed between two cover sheets; the set gypsum core comprising a gypsum crystal matrix having a pore size distribution comprising (i) water voids having a pore size less than about 5 microns in diameter, (ii) air voids having a pore size of at least about 5 microns and less than about 50 microns in diameter, (iii) air voids having a pore size from about 50 microns to about 100 microns in diameter, and (iv) air voids having a pore size greater than about 100 microns in diameter, the air voids having a pore size greater than about 100 microns in diameter comprising at least about 20% of the total void volume of the set gypsum core, the voids measured using scanning electron photomicrograph imaging; the air void pore size having greatest frequency is a diameter of about 100 microns or less; the gypsum crystal matrix disposed such that the set gypsum core has an average core hardness of at least about 11 pounds as determined in accordance with ASTM C-473; and the board having a density of 33 pcf or less.

2. The light weight gypsum board of claim 1, wherein at least about 50% of the total void volume are air voids having a pore size greater than about 50 microns in diameter.

3. The light weight gypsum board of claim 1, wherein the board density is from about 24 pcf to 33 pcf or less.

4. The light weight gypsum board of claim 1, the set gypsum core formed from a slurry comprising water, foam, stucco, naphthalenesulfonate dispersant, and pregelatinized starch.

5. The light weight gypsum board of claim 3, the set gypsum core formed from a slurry comprising water, foam, stucco, and a pregelatinized starch, the pregelatinized starch in an amount from about 0.5% to about 10% by weight based on the weight of the stucco.

6. The light weight gypsum board of claim 3, the set gypsum core formed from a slurry comprising water, foam, stucco, and trimetaphosphate compound chosen from the group consisting of sodium trimetaphosphate, potassium trimetaphosphate, lithium trimetaphosphate, and ammonium trimetaphosphate, the trimetaphosphate compound being in an amount from about 0.12% to about 0.4% by weight based on the weight of the stucco.

7. The light weight gypsum board of claim 5, wherein the slurry includes naphthalenesulfonate dispersant in an amount from about 0.1% to about 3.0% by weight based on the weight of the stucco.

8. A light weight gypsum board comprising a set gypsum core disposed between two cover sheets, wherein the set gypsum core comprises (a) a gypsum crystal matrix with a distribution of air voids and water voids disposed therein, the air voids with a pore size of about 5 microns or greater and water voids with a pore size less than about 5 microns, in a volume ratio of air voids to water voids from about 1.8 to 1 to about 9 to 1, (b) a distribution of air voids having pore sizes of at least about 5 microns to less than about 100 microns, (c) a distribution of air voids further having pore sizes greater than about 100 microns in diameter such that the air voids having a pore size greater than about 100 microns in diameter comprise at least about 20% of the total void volume of the set gypsum core, the voids measured using scanning electron photomicrograph imaging, the air void pore size having greatest frequency is a diameter of about 100 microns or less, and (d) the board has a density from about 24 pcf to 33 pcf, and a ratio of density (pcf) to average core hardness (lb) of less than about 3.2, wherein the core hardness is determined in accordance with ASTM C473.

9. The light weight gypsum board of claim 8, wherein the volume ratio of air voids to water voids is from about 2.3 to 1 to about 9 to 1.

10. The light weight gypsum board of claim 8, wherein the board, when at a thickness of about ½ inch, has a dry weight from about 1000 lb/MSF to about 1300 lb/MSF.

11. The light weight gypsum board of claim 8, wherein at least about 50% of the total void volume comprises air voids having a pore size greater than about 50 microns in diameter.

12. The light weight gypsum board of claim 8, the set gypsum core formed from a slurry comprising water, foam, stucco, naphthalenesulfonate dispersant, and a pregelatinized starch, wherein the naphthalenesulfonate dispersant is in an amount from about 0.1% to about 3.0% by weight based on the weight of the stucco, and the pregelatinized starch is in an amount from about 0.5% to about 10% by weight based on the weight of the stucco.

13. The light weight gypsum board of claim 12, wherein the slurry further comprises trimetaphosphate compound chosen from the group consisting of sodium trimetaphosphate, potassium trimetaphosphate, lithium trimetaphosphate, and ammonium trimetaphosphate, the trimetaphosphate compound being in an amount from about 0.12% to about 0.4% by weight based on the weight of the stucco.

14. The light weight gypsum board of claim 1, wherein the air voids having a pore size of about 5 microns or greater and water voids having a pore size of less than about 5 microns are in a volume ratio from about 1.4:1 to about 2.3:1.

15. The light weight gypsum board of claim 1, wherein the air voids having a pore size of about 5 microns or greater and water voids having a pore size of less than about 5 microns are in a volume ratio from about 2.3:1 to about 9:1.

16. The light weight gypsum board of claim 1, wherein air voids having a pore size from about 50 microns to about 100 microns in diameter comprise at least about 30% of the total void volume of the set gypsum core.

17. The light weight gypsum board of claim 6, wherein the trimetaphosphate compound is sodium trimetaphosphate, and the slurry further comprises pregelatinized starch in an amount from about 0.5% to about 10% by weight based on the weight of the stucco and naphthalenesulfonate dispersant in an amount from about 0.1% to about 3.0% by weight based on the weight of the stucco.

18. The light weight gypsum board of claim 1, wherein the board density is from about 24 pcf to about 32 pcf or less.

19. The light weight gypsum board of claim 1, wherein the board density is from about 24 pcf to about 31 pcf or less.

20. The light weight gypsum board of claim 8, wherein the board density is from about 24 pcf to about 32 pcf or less.

21. The light weight gypsum board of claim 8, wherein the board density is from about 24 pcf to about 31 pcf or less.

22. The light weight gypsum board of claim 1, wherein the average air void pore size is less than about 100 microns in diameter.

23. The light weight gypsum board of claim 8, wherein the average air void pore size is less than about 100 microns in diameter.

24. A light weight gypsum board comprising: a set gypsum core disposed between two cover sheets; the set gypsum core comprising a gypsum crystal matrix having a pore size distribution comprising (i) water voids having a pore size less than about 5 microns in diameter, (ii) air voids having a pore size of at least about 5 microns and less than about 50 microns in diameter, (iii) air voids having a pore size from about 50 microns to about 100 microns in diameter, and (iv) air voids having a pore size greater than about 100 microns in diameter, the air voids having a pore size greater than about 100 microns in diameter comprising at least about 20% of the total void volume of the set gypsum core, the voids measured using three-dimensional imaging acquired by X-ray CT-scanning analysis (XMT); the air void pore size having greatest frequency is a diameter of about 100 microns or less; the gypsum crystal matrix disposed such that the set gypsum core has an average core hardness of at least about 11 pounds as determined in accordance with ASTM C-473; and the board having a density of about 33 pcf or less.

25. The light weight gypsum board of claim 24, wherein the board density is from about 24 pcf to about 33 pcf or less.

26. The light weight gypsum board of claim 24, wherein the board density is from about 24 pcf to about 32 pcf or less.

27. The light weight gypsum board of claim 24, wherein the board density is from about 24 pcf to about 31 pcf or less.

28. The light weight gypsum board of claim 24, the set gypsum core formed from a slurry comprising water, foam, stucco, naphthalenesulfonate dispersant, and a pregelatinized starch.

29. The light weight gypsum board of claim 28, wherein the pregelatinized starch is in an amount from about 0.5% to about 10% by weight based on the weight of the stucco.

30. The light weight gypsum board of claim 28, wherein the slurry further comprises trimetaphosphate compound chosen from the group consisting of sodium trimetaphosphate, potassium trimetaphosphate, lithium trimetaphosphate, and ammonium trimetaphosphate, the trimetaphosphate compound being in an amount from about 0.12% to about 0.4% by weight based on the weight of the stucco.

31. The light weight gypsum board of claim 28, wherein the naphthalenesulfonate dispersant is in an amount from about 0.1% to about 3.0% by weight based on the weight of the stucco.

32. The light weight gypsum board of claim 30, wherein the trimetaphosphate compound is sodium trimetaphosphate, the pregelatinized starch is in an amount from about 0.5% to about 10% by weight based on the weight of the stucco, and the naphthalenesulfonate dispersant is in an amount from about 0.1% to about 3.0% by weight based on the weight of the stucco.

33. A light weight gypsum board comprising a set gypsum core disposed between two cover sheets, wherein the set gypsum core comprises (a) a gypsum crystal matrix with a distribution of air voids and water voids disposed therein, the air voids with a pore size of about 5 microns or greater and water voids with a pore size less than about 5 microns, in a volume ratio of air voids to water voids from about 1.8 to 1 to about 9 to 1, (b) a distribution of air voids having pore sizes of at least about 5 microns to less than about 100 microns, (c) a distribution of air voids further having pore sizes greater than about 100 microns in diameter such that the air voids having a pore size greater than about 100 microns in diameter comprise at least about 20% of the total void volume of the set gypsum core, the voids measured using three-dimensional imaging acquired by X-ray CT-scanning analysis (XMT), the air void pore size having greatest frequency is a diameter of about 100 microns or less, and (d) the board has a density from about 24 pcf to 33 pcf, and a ratio of density (pcf) to average core hardness (lb) of less than about 3.2, wherein the core hardness is determined in accordance with ASTM C473.

34. The light weight gypsum board of claim 33, wherein the volume ratio of air voids to water voids is from about 2.3 to 1 to about 9 to 1.

35. The light weight gypsum board of claim 33, wherein the board, when at a thickness of about inch, has a dry weight from about 1000 lb/MSF to about 1300 lb/MSF.

36. The light weight gypsum board of claim 33, wherein at least about 50% of the total void volume comprises air voids having a pore size greater than about 50 microns in diameter.

37. The light weight gypsum board of claim 33, the set gypsum core formed from a slurry comprising water, foam, stucco, naphthalenesulfonate dispersant, and a pregelatinized starch, wherein the naphthalenesulfonate dispersant is in an amount from about 0.1% to about 3.0% by weight based on the weight of the stucco, and the pregelatinized starch is in an amount from about 0.5% to about 10% by weight based on the weight of the stucco.

38. The light weight gypsum board of claim 37, wherein the slurry further comprises trimetaphosphate compound chosen from the group consisting of sodium trimetaphosphate, potassium trimetaphosphate, lithium trimetaphosphate, and ammonium trimetaphosphate, the trimetaphosphate compound being in an amount from about 0.12% to about 0.4% by weight based on the weight of the stucco.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a scanning electron photomicrograph of a cast gypsum cube sample (11:08) at 15× magnification illustrating one embodiment of the present invention.

(2) FIG. 2 is a scanning electron photomicrograph of a cast gypsum cube sample (11:30) at 15× magnification illustrating one embodiment of the present invention.

(3) FIG. 3 is a scanning electron photomicrograph of a cast gypsum cube sample (11:50) at 15× magnification illustrating one embodiment of the present invention.

(4) FIG. 4 is a scanning electron photomicrograph of a cast gypsum cube sample (11:08) at 50× magnification illustrating one embodiment of the present invention.

(5) FIG. 5 is a scanning electron photomicrograph of a cast gypsum cube sample (11:30) at 50× magnification illustrating one embodiment of the present invention.

(6) FIG. 6 is a scanning electron photomicrograph of a cast gypsum cube sample (11:50) at 50× magnification illustrating one embodiment of the present invention.

(7) FIG. 7 is a scanning electron photomicrograph of a cast gypsum cube sample (11:50) at 500× magnification illustrating one embodiment of the present invention.

(8) FIG. 8 is a scanning electron photomicrograph of a cast gypsum cube sample (11:50) at 2,500× magnification illustrating one embodiment of the present invention.

(9) FIGS. 9-10 are scanning electron photomicrographs of a cast gypsum cube sample (11:50) at 10,000× magnification illustrating one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(10) It has unexpectedly been found that gypsum wallboard made using a gypsum-containing slurry including stucco, pregelatinized starch, and a naphthalenesulfonate dispersant, and an appropriate amount of soap foam, provides not only very low board core densities of about 10 to 30 pcf (and thus low board weight), but also low dusting upon normal board handling and upon working, such as, for example, cutting, sawing, routing, score/snapping, nailing or screwing down, or drilling, when the total void volume of the set gypsum core is from about 80% to about 92%. This wallboard is consequently easier to cut than other known products. The introduction of the soap foam produces small air (bubble) voids, which on average can be less than about 100 microns in diameter, but are generally greater than about 10 microns in diameter, and preferably greater than about 20 microns in diameter. The invention requires that these small air bubbles, along with evaporative water voids (generally about 5 microns in diameter, or less, normally less than about 2 microns in diameter), are generally evenly distributed throughout the set gypsum core in the finished wallboard products. For example, the set gypsum core can have a total void volume from about 80% to about 92% wherein at least 60% of the total void volume comprises air voids having an average diameter greater than about 10 microns and at least 10% of the total void volume comprises water voids having an average diameter less than about 5 microns. It is believed that the low density board core prepared in this manner with a total void volume of the set gypsum core from about 80% to about 92% as air and water voids (total core void volume) captures a substantial amount of the small dust and other debris in the voids exposed on cutting, sawing, routing, snapping, nailing or screwing down, or drilling the boards so that dust generation is significantly reduced and does not become air-borne.

(11) The rehydration of calcium sulfate hemihydrate (stucco) and consequent hardening requires a specific, theoretical amount of water (1½ moles water/mole of stucco) to form calcium sulfate dihydrate crystals. However, the commercial process generally calls for excess water. This excess process water produces evaporative water voids in the gypsum crystal matrix which are generally substantially irregular in shape, and also are linked to other water voids, forming irregular channels in a generally continuous network between set gypsum crystals. In contrast, air (bubble) voids are introduced into the gypsum slurry using soap foam. The air voids are generally spherical/round in shape, and also are generally separated from other air voids and thus generally discontinuous. The water voids can be distributed within the walls of the air voids (see, for example, FIGS. 8-10).

(12) The effectiveness of dust capture depends upon the composition of the set gypsum core. It has been found that the naphthalenesulfonate dispersants, if the usage level is high enough, can cross-link to the pregelatinized starch to bind the gypsum crystals together after drying, thus increasing dry strength of the gypsum composite. Further, it has now unexpectedly been found that the combination of the pregelatinized starch and the naphthalenesulfonate dispersant (organic phase) provides a glue-like effect in binding the set gypsum crystals together, and when this formulation is combined with a particular void volume and void distribution, larger sized fragments are generated on score/snapping of the finished wallboard. Larger gypsum fragments generally produce less air-borne dust. In contrast, if a conventional wallboard formulation is used, smaller fragments are generated and thus more dust. For example, conventional wallboards can generate dust fragments on saw cutting having an average diameter of about 20-30 microns, and a minimum diameter of about 1 micron. In contrast, the gypsum wallboards of the present invention generate dust fragments on saw cutting having an average diameter of about 30-50 microns, and a minimum diameter of about 2 microns; score/snapping can produce even larger fragments.

(13) In softer wallboards, dust can be captured in both the water voids and air voids (e.g. capture of small gypsum needles as single crystal dust). Harder wallboards favor dust capture in the air voids, since larger chunks or fragments of the set gypsum core are generated on working of these boards. In this case the dust fragments are too large for the water voids, but are trapped in the air voids. It is possible, according to one embodiment of the present invention, to achieve increased dust capture by introducing a preferred void/pore size distribution within the set gypsum core. It is preferred to have a distribution of small and large void sizes, as a distribution of air and water voids. In one embodiment, a preferred air void distribution can be prepared using soap foam. See Examples 6 and 7 below.

(14) The ratio of air voids (greater than about 10 microns) to water voids (less than about 5 microns) within the set gypsum core can range from about 1.8:1 to about 9:1. A preferred ratio of air voids (greater than about 10 microns) to water voids (less than about 5 microns) within the set gypsum core can range from about 2:1 to about 3:1. In one embodiment, the void/pore size distribution within the set gypsum core should range from about 10-30% of voids less about 5 microns and from about 70-90% of voids greater than about 10 microns, as a percentage of total voids measured. Stated in another way, the ratio of air voids (greater than 10 microns) to water voids (less than 5 microns) within the set gypsum core ranges from about 2.3:1 to about 9:1. In a preferred embodiment, the void/pore size distribution within the set gypsum core should range from about 30-35% of voids less about 5 microns and from about 65-70% of voids greater than about 10 microns, as a percentage of total voids measured. Stated in another way, the ratio of air voids (greater than 10 microns) to water voids (less than 5 microns) within the set gypsum core ranges from about 1.8:1 to about 2.3:1.

(15) It is preferred that the average air (bubble) void size be less than about 100 microns in diameter. In a preferred embodiment, the void/pore size distribution within the set gypsum core is: greater than about 100 microns (20%), from about 50 microns to about 100 microns (30%), and less than about 50 microns (50%). That is, a preferred median void/pore size is about 50 microns.

(16) Soap foam is preferred to introduce and to control the air (bubble) void sizes and distribution in the set gypsum core, and to control the density of the set gypsum core. A preferred range of soap is from about 0.2 lb/MSF to about 0.6 lb/MSF; a more preferred level of soap is about 0.45 lb/MSF.

(17) Soap foam must be added in an amount effective to produce the desired densities, and in a controlled manner. In order to control the process, an operator must monitor the head of the board forming line, and keep the envelope filled. If the envelope is not kept filled, wallboards with hollow edges result, since the slurry cannot fill the necessary volume. The envelope volume is kept filled by increasing the soap usage to prevent rupture of air bubbles during manufacturing of the board (for better retaining the air bubbles), or by increasing the air blast rate. Thus, generally, the envelope volume is controlled and adjusted either by increasing or decreasing the soap usage, or by increasing or decreasing the air blast rate. The art of controlling the head includes adjustments to the “dynamic slurry” on the table by adding soap foam to increase slurry volume, or by decreasing soap foam usage to decrease slurry volume.

(18) According to one embodiment of the present invention, there are provided finished gypsum-containing products made from gypsum-containing slurries containing stucco, pregelatinized starch, and a naphthalenesulfonate dispersant. The naphthalenesulfonate dispersant is present in an amount of about 0.1%-3.0% by weight based on the weight of dry stucco. The pregelatinized starch is present in an amount of at least about 0.5% by weight up to about 10% by weight based on the weight of dry stucco in the formulation. Other ingredients that may be used in the slurry include binders, waterproofing, agents paper fiber, glass fiber, clay, biocide, and accelerators. The present invention requires the addition of a soap foam to the newly formulated gypsum-containing slurries to reduce the density of the finished gypsum-containing product, for example, gypsum wallboard, and to control dusting by introduction of a total void volume of from about 75% to about 95%, and preferably from about 80% to about 92%, in the form of small air (bubble) voids and water voids in the set gypsum core. Preferably, the average pore size distribution will be from about 1 micron (water voids) to about 40-50 microns (air voids).

(19) Optionally, the combination of from about 0.5% by weight up to about 10% by weight pregelatinized starch, from about 0.1% by weight up to about 3.0% by weight naphthalenesulfonate dispersant, and a minimum of at least about 0.12% by weight up to about 0.4% by weight of trimetaphosphate salt (all based on the weight of dry stucco used in the gypsum slurry) unexpectedly and significantly increases the fluidity of the gypsum slurry. This substantially reduces the amount of water required to produce a gypsum slurry with sufficient flowability to be used in making gypsum-containing products such as gypsum wallboard. The level of trimetaphosphate salt, which is at least about twice that of standard formulations (as sodium trimetaphosphate), is believed to boost the dispersant activity of the naphthalenesulfonate dispersant.

(20) A naphthalenesulfonate dispersant must be used in gypsum-containing slurries prepared in accordance with the present invention. The naphthalenesulfonate dispersants used in the present invention include polynaphthalenesulfonic acid and its salts (polynaphthalenesulfonates) and derivatives, which are condensation products of naphthalenesulfonic acids and formaldehyde. Particularly desirable polynaphthalenesulfonates include sodium and calcium naphthalenesulfonate. The average molecular weight of the naphthalenesulfonates can range from about 3,000 to 27,000, although it is preferred that the molecular weight be about 8,000 to 22,000, and more preferred that the molecular weight be about 12,000 to 17,000. As a commercial product, a higher molecular weight dispersant has higher viscosity, and lower solids content, than a lower molecular weight dispersant. Useful naphthalenesulfonates include DILOFLO, available from GEO Specialty Chemicals, Cleveland, Ohio; DAXAD, available from Hampshire Chemical Corp., Lexington, Mass.; and LOMAR D, available from GEO Specialty Chemicals, Lafayette, Ind. The naphthalenesulfonates are preferably used as aqueous solutions in the range 35-55% by weight solids content, for example. It is most preferred to use the naphthalenesulfonates in the form of an aqueous solution, for example, in the range of about 40-45% by weight solids content. Alternatively, where appropriate, the naphthalenesulfonates can be used in dry solid or powder form, such as LOMAR D, for example.

(21) The polynaphthalenesulfonates useful in the present invention have the general structure (I):

(22) ##STR00001##
wherein n is >2, and wherein M is sodium, potassium, calcium, and the like.

(23) The naphthalenesulfonate dispersant, preferably as an about 45% by weight solution in water, may be used in a range of from about 0.5% to about 3.0% by weight based on the weight of dry stucco used in the gypsum composite formulation. A more preferred range of naphthalenesulfonate dispersant is from about 0.5% to about 2.0% by weight based on the weight of dry stucco, and a most preferred range from about 0.7% to about 2.0% by weight based on the weight of dry stucco. In contrast, known gypsum wallboard contains this dispersant at levels of about 0.4% by weight, or less, based on the weight of dry stucco.

(24) Stated in an another way, the naphthalenesulfonate dispersant, on a dry weight basis, may be used in a range from about 0.1% to about 1.5% by weight based of the weight of dry stucco used in the gypsum composite formulation. A more preferred range of naphthalenesulfonate dispersant, on a dry solids basis, is from about 0.25% to about 0.7% by weight based on the weight of dry stucco, and a most preferred range (on a dry solids basis) from about 0.3% to about 0.7% by weight based on the weight of dry stucco.

(25) The gypsum-containing slurry can optionally contain a trimetaphosphate salt, for example, sodium trimetaphosphate. Any suitable water-soluble metaphosphate or polyphosphate can be used in accordance with the present invention. It is preferred that a trimetaphosphate salt be used, including double salts, that is trimetaphosphate salts having two cations. Particularly useful trimetaphosphate salts include sodium trimetaphosphate, potassium trimetaphosphate, calcium trimetaphosphate, sodium calcium trimetaphosphate, lithium trimetaphosphate, ammonium trimetaphosphate, and the like, or combinations thereof. A preferred trimetaphosphate salt is sodium trimetaphosphate. It is preferred to use the trimetaphosphate salt as an aqueous solution, for example, in the range of about 10-15% by weight solids content. Other cyclic or acyclic polyphosphates can also be used, as described in U.S. Pat. No. 6,409,825 to Yu et al., herein incorporated by reference.

(26) Sodium trimetaphosphate is a known additive in gypsum-containing compositions, although it is generally used in a range of from about 0.05% to about 0.08% by weight based on the weight of dry stucco used in the gypsum slurry. In the embodiments of the present invention, sodium trimetaphosphate (or other water-soluble metaphosphate or polyphosphate) can be present in the range of from about 0.12% to about 0.4% by weight based on the weight of dry stucco used in the gypsum composite formulation. A preferred range of sodium trimetaphosphate (or other water-soluble metaphosphate or polyphosphate) is from about 0.12% to about 0.3% by weight based on the weight of dry stucco used in the gypsum composite formulation.

(27) There are two forms of stucco, alpha and beta. These two types of stucco are produced by different means of calcination. In the present inventions either the beta or the alpha form of stucco may be used.

(28) Starches, including pregelatinized starch in particular, must be used in gypsum-containing slurries prepared in accordance with the present invention. A preferred pregelatinized starch is pregelatinized corn starch, for example pregelatinized corn flour available from Bunge Milling, St. Louis, Mo., having the following typical analysis: moisture 7.5%, protein 8.0%, oil 0.5%, crude fiber 0.5%, ash 0.3%; having a green strength of 0.48 psi; and having a loose bulk density of 35.0 lb/ft.sup.3. Pregelatinized corn starch should be used in an amount of at least about 0.5% by weight up to about 10% by weight, based on the weight of dry stucco used in the gypsum-containing slurry.

(29) The present inventors have further discovered that an unexpected increase in dry strength (particularly in wallboard) can be obtained by using at least about 0.5% by weight up to about 10% by weight pregelatinized starch (preferably pregelatinized corn starch) in the presence of about 0.1% by weight to 3.0% by weight naphthalenesulfonate dispersant (starch and naphthalenesulfonate levels based on the weight of dry stucco present in the formulation). This unexpected result can be obtained whether or not water-soluble trimetaphosphate or polyphosphate is present.

(30) In addition, it has unexpectedly been found that pregelatinized starch can be used at levels of at least about 10 lb/MSF, or more, in the dried gypsum wallboard made in accordance with the present invention, yet high strength and low weight can be achieved. Levels as high as 35-45 lb/MSF pregelatinized starch in the gypsum wallboard have been shown to be effective. As an example, Formulation B, as shown in Tables 1 and 2 below, includes 45 lb/MSF, yet produced a board weight of 1042 lb/MSF having excellent strength. In this example (Formulation B), a naphthalenesulfonate dispersant as a 45% by weight solution in water, was used at a level of 1.28% by weight.

(31) A further unexpected result may be achieved with the present invention when the naphthalenesulfonate dispersant trimetaphosphate salt combination is combined with pregelatinized corn starch, and optionally, paper fiber or glass fiber. Gypsum wallboard made from formulations containing these three ingredients have increased strength and reduced weight, and are more economically desirable due to the reduced water requirements in their manufacture. Useful levels of paper fiber can range up to about 2% by weight based on the weight of dry stucco. Useful levels of glass fiber can range up to about 2% by weight based on the weight of dry stucco.

(32) Accelerators can be used in the gypsum-containing compositions of the present invention, as described in U.S. Pat. No. 6,409,825 to Yu et al., herein incorporated by reference. One desirable heat resistant accelerator (HRA) can be made from the dry grinding of landplaster (calcium sulfate dihydrate). Small amounts of additives (normally about 5% by weight) such as sugar, dextrose, boric acid, and starch can be used to make this HRA. Sugar, or dextrose, is currently preferred. Another useful accelerator is “climate stabilized accelerator” or “climate stable accelerator,” (CSA) as described in U.S. Pat. No. 3,573,947, herein incorporated by reference.

(33) Water/stucco (w/s) ratio is an important parameter, since excess water must eventually be driven off by heating. In the embodiments of the present invention, a preferred w/s ratio is from about 0.7 to about 1.3.

(34) Other gypsum slurry additives can include accelerators, binders, waterproofing agents, paper or glass fibers, clay, biocide, and other known constituents.

(35) Cover sheets may be made of paper as in conventional gypsum wallboard, although other useful cover sheet materials known in the art (e.g. fibrous glass mats) may be used. Paper cover sheets provide strength characteristics in the gypsum wallboard. Useful cover sheet paper includes Manila 7-ply and News-Line 5-ply, available from United States Gypsum Corporation, Chicago, Ill.; and Grey-Back 3-ply and Manila Ivory 3-ply, available from Caraustar, Newport, Ind. The paper cover sheets comprise top cover sheets, or face paper, and bottom cover sheets, or back paper. A preferred back cover sheet paper is 5-ply News-Line. A preferred face cover sheet paper is Manila 7-ply.

(36) Fibrous mats may also be used as one or both of the cover sheets. One useful fibrous mat is a glass fiber mat in which filaments of glass fiber are bonded together by an adhesive. Preferably the fibrous mats will be nonwoven glass fiber mats in which filaments of glass fiber are bonded together by an adhesive. Most preferably, the nonwoven glass fiber mats will have a heavy resin coating. For example, Duraglass nonwoven glass fiber mats, available from Johns-Manville, having a weight of about 1.2-2.0 lb/100 ft.sup.2, with about 40-50% of the mat weight coming from the resin coating, could be used. Other useful fibrous mats include, but are not limited to, woven glass mats and non-cellulosic fabrics.

(37) The following examples further illustrate the invention. They should not be construed as in any way limiting the scope of the invention.

EXAMPLE 1

Sample Gypsum Slurry Formulations

(38) Gypsum slurry formulations are shown in Table 1 below. All values in Table 1 are expressed as weight percent based on the weight of dry stucco. Values in parentheses are dry weight in pounds (lb/MSF).

(39) TABLE-US-00001 TABLE 1 Component Formulation A Formulation B Stucco (lb/MSF) (732) (704) sodium 0.20 (1.50) 0.30 (2.14) trimetaphosphate Dispersant 0.18 (1.35) 0.58 .sup.1 (4.05) (naphthalenesulfonate) Pregelatinized starch 2.7 (20) 6.4 (45) (dry powder) Board starch 0.41 (3.0) 0 Heat resistant  (15)  (15) accelerator (HRA) Glass fiber 0.27 (2.0) 0.28 (2.0) Paper fiber 0 0.99 (7.0) Soap* 0.03 (0.192) 0.03 (0.192) Total Water (lb.) 805  852  Water/Stucco ratio   1.10   1.21 *Used to pregenerate foam. .sup.1 1.28% by weight as a 45% aqueous solution.

EXAMPLE 2

Preparation of Wallboards

(40) Sample gypsum wallboards were prepared in accordance with U.S. Pat. No. 6,342,284 to Yu et al. and U.S. Pat. No. 6,632,550 to Yu et al., herein incorporated by reference. This includes the separate generation of foam and introduction of the foam into the slurry of all of the other ingredients as described in Example 5 of these patents.

(41) Test results for gypsum wallboards made using the Formulations A and B of Example 1, and a normal control board are shown in Table 2 below. As in this example and other examples below, nail pull resistance, core hardness, and flexural strength tests were performed according to ASTM C-473. Additionally, it is noted that typical gypsum wallboard is approximately ½ inch thick and has a weight of between about 1600 to 1800 pounds per 1,000 square feet of material, or lb/MSF. (“MSF” is a standard abbreviation in the art for a thousand square feet; it is an area measurement for boxes, corrugated media and wallboard.)

(42) TABLE-US-00002 TABLE 2 Control Formulation A Formulation B Lab test result Board Board Board Board weight (lb/MSF) 1587 1066 1042 Nail pull resistance (lb) 81.7 50.2 72.8 Core hardness (lb) 16.3 5.2 11.6 Humidified bond load (lb) 17.3 20.3 15.1 Humidified bond 0.6 5 11.1 failure (%) Flexural strength, 47 47.2 52.6 face-up (MD) (lb) Flexural strength, 51.5 66.7 78.8 face-down (MD) (lb) Flexural strength, 150 135.9 173.1 face-up (XMD) (lb) Flexural strength, 144.4 125.5 165.4 face-down (XMD) (lb) MD: machine direction XMD: across machine direction

(43) As illustrated in Table 2, gypsum wallboards prepared using the Formulation A and B slurries have significant reductions in weight compared to the control board. With reference again to Table 1, the comparisons of the Formulation A board to the Formulation B board are most striking. The water/stucco (w/s) ratios are similar in Formulation A and Formulation B. A significantly higher level of naphthalenesulfonate dispersant is also used in Formulation B. Also, in Formulation B substantially more pregelatinized starch was used, about 6% by weight, a greater than 100% increase over Formulation A accompanied by marked strength increases. Even so, the water demand to produce the required flowability remained low in the Formulation B slurry, the difference being about 10% in comparison to Formulation A. The low water demand in both Formulations is attributed to the synergistic effect of the combination of naphthalenesulfonate dispersant and sodium trimetaphosphate in the gypsum slurry, which increases the fluidity of the gypsum slurry, even in the presence of a substantially higher level of pregelatinized starch.

(44) As illustrated in Table 2, the wallboard prepared using the Formulation B slurry has substantially increased strength compared with the wallboard prepared using the Formulation A slurry. By incorporating increased amounts of pregelatinized starch in combination with increased amounts of naphthalenesulfonate dispersant and sodium trimetaphosphate, nail pull resistance in the Formulation B board improved by 45% over the Formulation A board. Substantial increases in flexural strength were also observed in the Formulation B board as compared to the Formulation A board.

EXAMPLE 3

½ Inch Gypsum Wallboard Weight Reduction Trials

(45) Further gypsum wallboard examples (Boards C, D and E), including slurry formulations and test results are shown in Table 3 below. The slurry formulations of Table 3 include the major components of the slurries. Values in parentheses are expressed as weight percent based on the weight of dry stucco.

(46) TABLE-US-00003 TABLE 3 Control Formulation Formulation Formulation Board C Board D Board E Board Trial formulation component/parameter Dry stucco (lb/MSF) 1300 1281 1196 1070 Accelerator (lb/MSF) 9.2 9.2 9.2 9.2 DILOFLO .sup.1 (lb/MSF) 4.1 (0.32%) 8.1 (0.63%) 8.1 (0.68%) 8.1 (0.76%) Regular starch (lb/MSF) 5.6 (0.43%) 0 0 0 Pregelatinized corn 0  10 (0.78%)  10 (0.84%)  10 (0.93%) starch (lb/MSF) Sodium trimetaphosphate 0.7 (0.05%) 1.6 (0.12%) 1.6 (0.13%) 1.6 (0.15%) (lb/MSF) Total water/stucco 0.82 0.82 0.82 0.84 ratio (w/s) Trial formulation test results Dry board weight 1611 1570 1451 1320 (lb/MSF) Nail pull resistance (lb) .sup. 77.3.sup.† 85.5 77.2 65.2 .sup.†ASTM standard: 77 lb .sup.1 DILOFLO is a 45% Naphthalensulfonate solution in water

(47) As illustrated in Table 3, Boards C, D, and E were made from a slurry having substantially increased amounts of starch, DILOFLO dispersant, and sodium trimetaphosphate in comparison with the control board (about a two-fold increase on a percentage basis for the starch and dispersant, and a two- to three-fold increase for the trimetaphosphate), while maintaining the w/s ratio constant. Nevertheless, board weight was significantly reduced and strength as measured by nail pull resistance was not dramatically affected. Therefore, in this example of an embodiment of the invention, the new formulation (such as, for example, Board D) can provide increased starch formulated in a usable, flowable slurry, while maintaining the same w/s ratio and adequate strength.

EXAMPLE 4

Wet Gypsum Cube Strength Test

(48) The wet cube strength tests were carried out by using Southard CKS board stucco, available from United States Gypsum Corp., Chicago, Ill. and tap water in the laboratory to determine their wet compressive strength. The following lab test procedure was used.

(49) Stucco (1000 g), CSA (2 g), and tap water (1200 cc) at about 70° F. were used for each wet gypsum cube cast. Pregelatinized corn starch (20 g, 2.0% based on stucco wt.) and CSA (2 g, 0.2% based on stucco wt.) were thoroughly dry mixed first in a plastic bag with the stucco prior to mixing with a tap water solution containing both naphthalenesulfonate dispersant and sodium trimetaphosphate. The dispersant used was DILOFLO dispersant (1.0-2.0%, as indicated in Table 4). Varying amounts of sodium trimetaphosphate were used also as indicated in Table 4.

(50) The dry ingredients and aqueous solution were initially combined in a laboratory Warning blender, the mixture produced allowed to soak for 10 sec, and then the mixture was mixed at low speed for 10 sec in order to make the slurry. The slurries thus formed were cast into three 2″×2″×2″ cube molds. The cast cubes were then removed from the molds, weighed, and sealed inside plastic bags to prevent moisture loss before the compressive strength test was performed. The compressive strength of the wet cubes was measured using an ATS machine and recorded as an average in pounds per square inch (psi). The results obtained were as follows:

(51) TABLE-US-00004 TABLE 4 Sodium trimetaphos- Wet cube Wet cube Test phate, grams DILOFLO .sup.1 weight compressive Sample (wt % based on (wt % based on (2″ × 2″ × 2″), strength, No. dry stucco) dry stucco) g psi 1 0 1.5 183.57 321 2 0.5 (0.05)  1.5 183.11 357 3 1 (0.1) 1.5 183.19 360 4 2 (0.2) 1.5 183.51 361 5 4 (0.4) 1.5 183.65 381 6 10 (1.0)  1.5 183.47 369 7 0 1.0 184.02 345 8 0.5 (0.05)  1.0 183.66 349 9 1 (0.1) 1.0 183.93 356 10 2 (0.2) 1.0 182.67 366 11 4 (0.4) 1.0 183.53 365 12 10 (1.0)  1.0 183.48 341 13 0 2.0 183.33 345 14 0.5 (0.05)  2.0 184.06 356 15 1 (0.1) 2.0 184.3  363 16 2 (0.2) 2.0 184.02 363 17 4 (0.4) 2.0 183.5  368 18 10 (1.0)  2.0 182.68 339 .sup.1 DILOFLO is a 45% Naphthalensulfonate solution in water

(52) As illustrated in Table 4, Samples 4-5, 10-11, and 17, having levels of sodium trimetaphosphate in the about 0.12-0.4% range of the present invention generally provided superior wet cube compressive strength as compared to samples with sodium trimetaphosphate outside this range.

EXAMPLE 5

½ Inch Light Weight Gypsum Wallboard Plant Production Trials

(53) Further trials were performed (Trial Boards 1 and 2), including slurry formulations and test results are shown in Table 5 below. The slurry formulations of Table 5 include the major components of the slurries. Values in parentheses are expressed as weight based on the weight of dry stucco.

(54) TABLE-US-00005 TABLE 5 Plant Plant Control Formulation Control Formulation Board 1 Trial Board 1 Board 2 Trial Board 2 Trial formulation component/parameter Dry stucco (lb/MSF) 1308 1160 1212 1120 DILOFLO .sup.1 (lb/MSF) 5.98 (0.457%) 7.98 (0.688%) 7.18 (0.592%) 8.99 (0.803%) Regular starch (lb/MSF) 5.0 (0.38%) 0 4.6 (0.38%) 0 Pregelatinized corn 2.0 (0.15%)  10 (0.86%) 2.5 (0.21%) 9.0 (0.80%) starch (lb/ASF) Sodium trimetaphosphate 0.7 (0.05%) 2.0 (0.17%) 0.6 (0.05%) 1.6 (0.14%) (lb/MSF) Total water/stucco 0.79 0.77 0.86 0.84 ratio (w/s) Trial formulation test results Dry board weight 1619 1456 1553 1443 (lb/MSF) Nail pull resistance (lb) .sup. 81.5.sup.† 82.4 80.7 80.4 Flexural strength, 41.7 43.7 44.8 46.9 average (MD) (lb) Flexural strength, 134.1 135.5 146 137.2 average (XMD) (lb) Humidified bond .sup.2 load, 19.2 17.7 20.9 19.1 average (lb) Humidified bond .sup.2, 3 1.6 0.1 0.5 0 failure (%) .sup.†ASTM standard: 77 lb MD: machine direction XMD: across machine direction .sup.1 DILOFLO is a 45% Naphthalensulfonate solution in water .sup.2 90° F./90% Relative Humidity .sup.3 It is well understood that under these test conditions, percentage failure rates <50% are acceptable.

(55) As illustrated in Table 5, Trial Boards 1 and 2 were made from a slurry having substantially increased amounts of starch, DILOFLO dispersant, and sodium trimetaphosphate, while slightly decreasing the w/s ratio, in comparison with the control boards. Nevertheless, strength as measured by nail pull resistance and flexural testing was maintained or improved, and board weight was significantly reduced. Therefore, in this example of an embodiment of the invention, the new formulation (such as, for example, Trial Boards 1 and 2) can provide increased trimetaphosphate and starch formulated in a usable, flowable slurry, while maintaining substantially the same w/s ratio and adequate strength.

EXAMPLE 6

½ Inch Ultra-Light Weight Gypsum Wallboard Plant Production Trials

(56) Further trials were performed (Trial Boards 3 and 4) using Formulation B (Example 1) as in Example 2, except that the pregelatinized corn starch was prepared with water at 10% concentration (wet starch preparation) and a blend of HYONIC 25 AS and PFM 33 soaps (available from GEO Specialty Chemicals, Lafayette, Ind.) was used. For example, Trial Board 3 was prepared with a blend of HYONIC 25 AS and PFM 33 ranging from 65-70% by weight of 25 AS, and the balance PFM 33. For example, Trial Board 4 was prepared with a 70/30 wt./wt. blend of HYONIC 25 AS/HYONIC PFM 33. The trial results are shown in Table 6 below.

(57) TABLE-US-00006 TABLE 6 Trial Board 3 Trial Board 4 (Formulation B plus (Formulation B plus HYONIC soap blend HYONIC soap blend Lab test result 65/35) (n = 12) 70/30) (n = 34)* Board weight 1106 1013 (lb/MSF) Nail pull 85.5 80.3 resistance.sup.a (lb) Core hardness.sup.b >15 12.4 (lb) Flexural strength, 55.6 .sup. 60.3 .sup.1 average.sup.c (MD) (lb) Flexural strength, 140.1  .sup. 142.3 .sup.1 average.sup.d (XMD) (lb) *Except as marked. .sup.1 n = 4 MD: machine direction XMD: across machine direction .sup.aASTM standard: 77 lb .sup.bASTM standard: 11 lb .sup.cASTM standard: 36 lb .sup.dASTM standard: 107 lb

(58) As illustrated in Table 6, strength characteristics as measured by nail pull and core hardness were above the ASTM standard. Flexural strength was also measured to be above the ASTM standard. Again, in this example of an embodiment of the invention, the new formulation (such as, for example, Trial Boards 3 and 4) can provide increased trimetaphosphate and starch formulated in a usable, flowable slurry, while maintaining adequate strength.

EXAMPLE 7

Percentage Void Volume Calculation in ½ Inch Thick Gypsum Wallboard Core as a Function of Board Weight and Saw Cutting Results

(59) Further trials were performed in order to determine void volumes and densities (Trial Boards No. 5 to 13) using Formulation B (Example 1) as in Example 2, except that the pregelatinized corn starch was prepared with water at 10% concentration (wet starch preparation), 0.5% glass fiber was used, and naphthalenesulfonate (DILOFLO) was used at a level of 1.2% by weight as a 45% aqueous solution. Soap foam was made using a soap foam generator and introduced into the gypsum slurry in an amount effective to provide the desired densities. In the present example, soap was used at a level from 0.25 lb/MSF to 0.45 lb/MSF. That is, the soap foam usage was increased or decreased as appropriate. In each sample, the wallboard thickness was ½ inch, and the core volume was assumed to be uniform at 39.1 ft.sup.3/MSF. Void volumes were measured across 4 ft wide wallboard samples from which the front and back paper was removed. The front and back papers can have a thickness in the range 11-18 mil (each side). Void volumes/pore sizes and pore size distribution were determined by scanning electron microscopy (see Example 8 below) and X-ray CT-scanning technology (XMT).

(60) TABLE-US-00007 TABLE 7 Foam Pore Evap. Pore Total Core Trial Board Foam Void Size Evaporative Size Void Board Core Board Weight Volume.sup.1 Distribution Void Volume.sup.2 Distribution Volume.sup.3 Density No. (lb/MSF) (ft.sup.3/MSF) (%).sup.† (ft.sup.3/MSF) (%).sup.† (%) (pcf).sup.4 5 1600-1700 15 54 12.7 46 70.8 39-41 (Control) 6 1400 19.6 66 10.3 34 76.5 34 7 1300 21.1 69  9.4 31 78.0 31 8 1200 20.9 68 10.0 32 79.0 28 9 1100 21.1 67 10.4 33 80.6 26 10 1000 20.9 65 11.1 35 81.8 23 11 900 23.4 71  9.5 29 84.1 21 12 800 25.5 76  8.1 24 85.9 18 13 500 31.5 88  4.5 12 92.1 10 .sup.1>10 micron air (bubble) voids .sup.2<5 micron water voids .sup.3Based on uniform core vol. = 39.1 ft.sup.3/MSF; i.e., Total core void volume = foam void vol. + evaporative void vol./39.1 × 100 .sup.4Based on uniform core vol. = 39.1 ft.sup.3/MSF; i.e., Board core density (pcf) = Board weight (lb/MSF) - weight of paper cover sheets (lb/MSF)/39.1 ft.sup.3/MSF = Board weight (lb/MSF) − 90 lb/MSF/39.1 ft.sup.3/MSF .sup.†Percent of total voids measured

(61) As illustrated in Table 7, trial board samples having total core void volumes ranging from 79.0% to 92.1% were made, which correspond to board core densities ranging from 28 pcf down to 10 pcf, respectively. As an example, saw cutting of Trial board 10, having a total core void volume of 81.8% and a board core density of 23 pcf, generated about 30% less dust than control board. As an additional example, if wallboards with a conventional formulation having less binder (as starch with or without dispersant) were made that had significantly less that about 75-80% total core void volume, significantly greater dust generation would be expected on cutting, sawing, routing, snapping, nailing or screwing down, or drilling. For example, conventional wallboards can generate dust fragments on saw cutting having an average diameter of about 20-30 microns, and a minimum diameter of about 1 micron. In contrast, the gypsum wallboards of the present invention will generate dust fragments on saw cutting having an average diameter of about 30-50 microns, and a minimum diameter of about 2 microns; score/snapping will produce even larger fragments.

(62) It has been shown that the combination of several key components used to make the gypsum-containing slurry, namely: stucco, naphthalenesulfonate dispersant, pregelatinized corn starch, sodium trimetaphosphate, and glass and/or paper fibers, in combination with a sufficient and effective amount of soap foam, can have a synergistic effect in producing a useful low density gypsum wallboard that also dramatically reduces gypsum dust formation during knife cutting, saw cutting, score/snapping, drilling, and normal board handling.

EXAMPLE 8

Determination of Air Bubble Void Sizes and Water Void Sizes in Trial Board No. 10, and Gypsum Crystal Morphology

(63) Cast gypsum cubes (2 inch×2 inch×2 inch) from the plant trial to prepare Trial Board No. 10 were analyzed by scanning electron microscopy (SEM). Air bubble voids and evaporative water voids were observed and measured, as well as gypsum crystal size and shape.

(64) Three sample cubes were made and labeled 11:08, 11:30, and 11:50, respectively. FIGS. 1 to 3 illustrate the air bubble void sizes and distribution for each sample at 15× magnification. FIGS. 4 to 6 illustrate the air bubble void sizes and distribution for each sample at 50× magnification.

(65) At higher magnifications, water voids were observed, for example in the generally substantially larger air bubble void walls, as shown in FIGS. 7 to 10 for sample cube 11:50, up to 10,000× magnification. Almost all of the gypsum crystals were needles; few platelets were observed. The density and packing of the needles varied on the surfaces of the air bubble voids. Gypsum needles were also observed in the water voids in the air bubble void walls.

(66) The SEM results demonstrate that in the gypsum-containing products made according to the present invention, the air and water voids are generally evenly distributed throughout the set gypsum core. The observed void sizes and void distributions also demonstrate that sufficient free space is formed as air and water voids (total core void volume) such that a substantial amount of the gypsum dust produced will be captured in the surrounding voids exposed upon normal board handling and during the cutting, sawing, routing, snapping, nailing or screwing down, or drilling and does not become air-borne.

EXAMPLE 9

Dust Capture in Low Dust Gypsum Wallboard

(67) If a wallboard were prepared according to the teachings of the present invention as in Example 7, it is expected that the gypsum dust produced on working the wallboard would comprise at least 50% by weight gypsum fragments larger than about 10 microns in diameter. At least about 30% or more of the total dust generated by working the wallboard by cutting, sawing, routing, score/snapping, nailing or screwing down, and drilling, would be captured.

(68) The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

(69) Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention.