JP7714539B2 - Gypsum board containing highly absorbent paper and related methods - Google Patents

Description

In building construction, one of the more common architectural elements for construction and remodeling is gypsum wallboard, often known as drywall, gypsum board, gypsum panels, gypsum paneling, and ceiling tiles. In chemical terms, gypsum is calcium sulfate _dihydrate ( _CaSO4.2H2O ).

Set gypsum (calcium sulfate dihydrate) is a well-known material used in such products. Panels containing set gypsum are often called gypsum boards, which comprise a board core (set gypsum core) sandwiched between two cover sheets, typically paper cover sheets. Such panels are commonly used in the construction of drywall for the interior walls and ceilings of buildings. One or more denser regions, often referred to as "skim coats," may be included as layers on either side of the board core, usually at the interface between the board core and the inner surface of the cover sheets or coating thereon. The denser regions may be adjacent to less dense regions of the gypsum layer, providing the gypsum core layer of the gypsum board.

During the manufacture of gypsum board, stucco (containing calcium sulfate hemihydrate), water, and optionally other ingredients may be mixed, typically in a mixer, to form an aqueous gypsum slurry. The terms aqueous gypsum slurry or aqueous slurry or gypsum slurry are typically used to refer to the slurry both before and after the calcium sulfate hemihydrate is converted to calcium sulfate dihydrate. The gypsum slurry is formed and discharged from the mixer onto a moving conveyor carrying a first cover sheet, optionally having a skim coat. If present, the skim coat is applied upstream from where the gypsum slurry is discharged onto the first cover sheet. After the gypsum slurry is applied to the first cover sheet, a second cover sheet, also optionally having a skim coat, is applied to the gypsum slurry to form a sandwich assembly with the desired thickness. Forming plates, rollers, etc. may help set the desired thickness. The gypsum slurry is then hardened by forming set (i.e., rehydrated) gypsum through a reaction between the calcined gypsum and water to form a matrix of crystalline hydrated gypsum (i.e., calcium sulfate dihydrate, also known as set gypsum). The desired hydration of the calcined gypsum promotes the formation of an interlocking matrix of set gypsum crystals, thereby imparting strength to the gypsum board. Heat may be applied (e.g., in a kiln) to drive off any remaining free (i.e., unreacted) water and obtain a dry product. The set gypsum product is then cut to form gypsum boards having desired lengths.

Gypsum (calcium sulfate dihydrate and impurities) suitable for use in wallboard may be obtained from both natural and synthetic sources and then further processed.

Natural gypsum may be used by calcining its calcium sulfate dihydrate to produce the hemihydrate form. Gypsum from natural sources is a naturally occurring mineral that can be mined in rock form. Naturally occurring gypsum is a mineral commonly found in old salt lake formations, volcanic deposits, and clay layers. When mined, raw gypsum is generally found in the dihydrate form. Gypsum is also known as calcium sulfate dihydrate, terra alba, or land plaster. This material is also produced as a by-product of various industrial processes. For example, synthetic gypsum is a by-product of the flue gas desulfurization process from power plants. In gypsum, there are approximately two molecules of water associated with each molecule of calcium sulfate.

Plaster of Paris is also known as plaster of Paris, stucco, calcium sulfate hemihydrate, calcium sulfate hemihydrate, or calcium sulfate hemihydrate.

When calcium sulfate dihydrate from either source is heated sufficiently in a process called calcination or calcination, the water of hydration is at least partially driven off and, depending on the temperature and exposure time, can form calcium sulfate hemihydrate ( _CaSO4.1 / _2H2O ) (commonly provided in materials called "stucco") or calcium sulfate anhydrite ( _CaSO4 ). As used herein, the terms "stucco" and "calcined gypsum" refer to both the hemihydrate and anhydrite forms of calcium sulfate that may be present therein. Calcination of gypsum to produce the hemihydrate form proceeds according to the following equation:
CaSO ₄・2H ₂ O→CaSO ₄・0.5H ₂ O+1.5H ₂ O

Calcined gypsum can react with water to form calcium sulfate dihydrate, which is a rigid product and is referred to herein as "set gypsum."

Gypsum may also be obtained synthetically (referred to in the art as "syngyp"), for example, as a by-product of industrial processes such as flue gas desulfurization from power plants. Natural or synthetic gypsum can be calcined at high temperatures, typically greater than 150°C, to form stucco (i.e., calcined gypsum in the form of calcium sulfate hemihydrate and/or calcium sulfate anhydrite), which may subsequently be rehydrated to form set gypsum into a desired shape, such as a board.

Synthetic gypsum obtained from power plants is typically suitable for use in gypsum panels for construction projects. Synthetic gypsum is a by-product of the flue gas desulfurization process from power plants (also known as desulfurized gypsum, desulfogypsum, or DSG). Specifically, flue gas containing sulfur dioxide is wet washed with lime or limestone, which produces calcium sulfite in the following reaction:
CaCO ₃ +SO ₂ →CaSO ₃ +CO ₂
Calcium sulfite is then converted to calcium sulfate in the following reaction:
CaSO ₃ +2H ₂ O+1/2O ₂ →CaSO _{4.2H 2} O
The hemihydrate form may then be produced by calcination in a manner similar to that used for natural gypsum.

However, many traditional coal-fired power plants are being closed in favor of more environmentally friendly energy sources. The closure of coal-fired power plants has led to an increasing shortage of synthetic gypsum suitable for the manufacture of gypsum panels. While lower-quality synthetic gypsum is available from power plants and other alternative sources, gypsum from these alternative sources often contains fairly high concentrations of extraneous salts, particularly magnesium or sodium salts, especially magnesium chloride and sodium chloride. Small amounts of potassium chloride and calcium chloride may also be present in synthetic gypsum from alternative sources. Extraneous salts can be problematic because they tend to reduce the adhesion between the board core and the cover sheets, especially the backing paper cover sheets.

It will be recognized that this background description has been provided by the inventors to assist the reader and is not intended as a recitation of prior art or as an indication that any of the problems presented have been recognized in the art per se. While the principles described may, in some respects and embodiments, alleviate problems inherent in other systems, it will be understood that the scope of the protected innovation is defined by the scope of the appended claims, and not by the ability of the claimed invention to solve any particular problem described herein.

The present invention relates to gypsum boards and methods for producing gypsum boards that exhibit good adhesion between the set gypsum layer and the board's cover sheet. The invention is particularly applicable to boards formed from stucco containing a high content of salt impurities. Stucco is typically formed by calcining gypsum from natural or synthetic sources. In nature, it is a common, abundant mineral that can be mined from the earth. Synthetic forms of gypsum can be obtained as a by-product from flue gas desulfurization (FGD) processes associated with coal-fired power plants that burn high-sulfur coal. At power plants, sulfur dioxide emissions are removed by wet scrubbing processes. When limestone slurry is injected, synthetic gypsum precipitates after fly ash removal. For example, some forms of synthetic gypsum contain a high content of salt impurities, which then remain in the stucco formed as calcined gypsum. Salt impurities in synthetic gypsum can result, for example, from high-salt coal. These salt impurities have been found to adversely affect adhesion between the cover sheet (e.g., formed from paper) and the set gypsum layer (e.g., board core) within the board.

The present invention provides a product and method of manufacture in which at least one set gypsum layer is sandwiched between two cover sheets, at least one of which is highly absorbent paper. In this manner, the present invention allows for improved adhesion even when the set gypsum layer is formed from stucco derived from low-quality synthetic gypsum that contains undesirable extraneous salts, including, for example, chloride salts such as NaCl, KCl, _MgCl2 , and/or _CaCl2, which have been found to interfere with adhesion between the paper and the core.

Thus, in one aspect, the present invention provides a gypsum board including a set gypsum core disposed between a face cover sheet and a back cover sheet, the set gypsum core being formed from a slurry including water, stucco, and a high salt impurity content (e.g., chloride salts). At least one of the cover sheets is a highly absorbent paper. As used herein, highly absorbent paper refers to paper that absorbs more water compared to conventional paper grades. Optionally, a face skim coat layer can be disposed between a first side of the set gypsum core and the face cover sheet, and a back skim coat layer can be disposed between a second side of the set gypsum core and the back cover sheet. In some embodiments, at least the back cover sheet is a highly absorbent paper.

In another aspect, the present invention provides a method for manufacturing gypsum board. The method includes mixing at least water and stucco having a high salt impurity content to form a first slurry. The first slurry is applied to form a hardened gypsum board core in adhesive relationship to a face cover sheet. The board core has a first side and a second side. The first side faces the face cover sheet. A back cover sheet is applied to the second side of the board core in adhesive relationship to form a board precursor. At least one of the cover sheets is a highly absorbent paper (e.g., in some embodiments, at least the back cover sheet). The board precursor is dried to form a board. Optionally, a face skim coat layer and a back skim coat layer can be provided on either side of the board core in any suitable manner. For example, in some embodiments, a second slurry including at least stucco and water is applied to the face before the first slurry is applied to the face. The second slurry forms a face skim coat disposed between the face cover sheet and the board core. Similarly, if desired, a third slurry containing at least stucco and water can be applied to the backing paper before the back cover sheet is applied onto the board core. The third slurry forms a back skim coat disposed between the back cover sheet and the board core. The second and third slurries can be the same or different and, in preferred embodiments, are generally denser than the first slurry.

1 is a photograph of three boards after the adhesion test discussed in Example 1.

1 is a photograph of three boards after the adhesion test discussed in Example 1.

The present invention is based, at least in part, on a board comprising at least one set gypsum layer sandwiched between two cover sheets, at least one of which takes the form of highly absorbent paper to improve adhesion between the set gypsum layer and the cover sheet. The present invention is particularly useful for gypsum boards in which the set gypsum layer is formed from a stucco slurry containing a significant amount of extraneous salt. For example, in some embodiments, the salt is a chloride salt, such as sodium chloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl ₂ ), and/or calcium chloride (CaCl ₂ ).

Such salts can be found, for example, in stucco slurries where the stucco is derived from low-quality synthetic gypsum. In this regard, board manufacturing facilities typically source gypsum, which is then calcined to form stucco. The stucco is then reacted with water to form a gypsum (i.e., calcium sulfate dihydrate) layer of the desired dimensions. It has been found that if the low-quality synthetic gypsum contains significant amounts of salt impurities, such salts remain in the stucco after calcination and are therefore present in the stucco slurry. In some embodiments, the stucco slurry contains high-quality salts, e.g., at least about 150 ppm chloride anion per 1,000,000 parts by weight of the calcium sulfate hemihydrate, e.g., from about 150 ppm to about 2,000 ppm chloride anion per 1,000,000 parts by weight of the calcium sulfate hemihydrate. The presence of significant amounts of salt impurities has been found to interfere with adhesion between the board core and the paper cover sheets.

Highly absorbent paper absorbs more moisture than conventional paper. The use of high-salt stucco can cause salt to migrate to the surface of the paper cover sheet, between layers of the paper cover sheet, and to the interface between the paper cover sheet and the board core, potentially resulting in poor adhesion and delamination. While not wishing to be bound by any particular theory, it is believed that the use of highly absorbent paper increases the amount of salt impurities that migrate to the outside of the cover paper, and correspondingly reduces the amount of salt that migrates to the interface between the cover sheet and the core. Furthermore, during the kiln drying process, as moisture evaporates from the surface of the board, highly absorbent paper tends to absorb and evaporate moisture more quickly than conventional paper, resulting in more salt migrating to the outside of the paper.

The highly absorbent paper can have any suitable weight and thickness. Generally, the weight of a paper is determined by its basis weight, which refers to its weight per unit area. This can be expressed as pounds per thousand square feet (1000 square feet) of paper. In some embodiments, the highly absorbent paper has a basis weight of about 30 lb/MSF to about 70 lb/MSF, e.g., about 42 lb/MSF to about 60 lb/MSF, e.g., about 45 lb/MSF to about 55 lb/MSF. In some embodiments, the highly absorbent paper has a caliper of about 7 mils to about 20 mils, e.g., about 10 mils to about 15 mils (e.g., about 12 mils). The surface water absorption over 60 seconds, expressed in g/ ^m², is measured using the Cobb test. This test determines how many grams of water are absorbed by a 100 ^cm² paper surface using the Cobb test. The Cobb test procedure is described in TAPPI T 441.

The highly absorbent paper can have any suitable water absorption capacity greater than that of conventional paper. The adhesive side Cobb value refers to the inside of the paper that is in contact with the gypsum slurry, and the liner side Cobb value refers to the outside of the paper that is not in contact with the slurry, and these Cobb values are measured in accordance with the TAPPI standard as described in the TAPPI T 441 test procedure. For example, in some embodiments, the highly absorbent paper has an absorbency of at least about 1.8 g/100 cm ² , e.g., from about 1.8 g/100 cm ² to about 3 g/100 cm ² , e.g., from about 1.8 g/100 cm ² to about 2.9 g/100 cm ² , from about 1.8 g/100 cm ² to about 2.7 g/100 cm ² , from about 1.8 g/100 cm ² to about 2.5 g/100 cm ² , from about 2 g/100 cm ² to about 3 g/100 cm ² , from about 2 g/100 cm ² to about 2.9 g/100 cm ² , from about 2 g/100 cm ² to about 2.7 g/100 cm ² , from about 2 g/100 cm ² to about 2.5 g/100 cm ² , or about 2.1 g/100 cm ² to about 3 g/100 cm ² , about 2.1 g/100 cm ^{2 to} about 2.9 g/100 cm ² , about 2.1 g/100 cm 2 to about 2.7 g/100 cm ² , about 2.1 g/100 cm ² to about 2.5 g/100 cm ² , about 2.2 g/100 cm ² to about 3 g/100 cm ² , about ² g/100 cm ² to about 2.9 g/100 cm 2 , about 2.2 g/100 cm ² to about 2.7 g/100 cm ² , or about 2.2 g/100 cm ² to about 2.5 g/100 cm ^{2 , about 2.4 g/100 cm 2 to about 3 g/100 cm 2 , about} 2.4 g/100 cm ² to about 2.9 g/100 cm ² , such as about 2.4 g/100 cm ² to about 2.7 g/100 cm ² .

In some embodiments, the highly absorbent paper has a liner-side Cobb value of at least about 0.5 g/100 cm ² , e.g., from about 0.5 g/100 cm ² to about 1.5 g/100 cm ² , e.g., from about 0.5 g/100 cm ^{2 to} about 1.2 g/100 cm ² , from about 0.7 g/100 cm ² to about 1.5 g/100 cm ² , from about 0.7 g/100 cm ² to about 1.2 g/100 cm ² , from about 0.9 g/100 cm ² to about 1.5 g/100 cm ² , from about 0.9 g/100 cm 2 to about 1.2 g/100 cm ² , etc. Examples of highly absorbent paper include USG's Newsline Hi-Abs and News SHW HiAbs. Newsline Hi Abs has a basis weight of about 45 lb/MSF to about 48 lb/MSF. Newsline SHW Hi Abs is an ultra-absorbent paper with a basis weight of about 53 to about 63.5 lb/MSF.

In embodiments where the board has only one highly absorbent cover sheet (e.g., a back cover sheet), the other cover sheet (e.g., a front cover sheet) can have any suitable basis weight and thickness. For example, in some embodiments, the other cover sheet can have a basis weight of from about 10 lb/msf to about 60 lb/msf, e.g., from about 10 lb/msf to about 55 lb/msf, from about 10 lb/msf to about 50 lb/msf, from about 10 lb/msf to about 40 lb/msf, from about 20 lb/msf to about 60 lb/msf, from about 20 lb/msf to about 5 The sheet can have a basis weight of 5 lb/msf, about 20 lb/msf to about 50 lb/msf, about 20 lb/msf to about 40 lb/msf, about 30 lb/msf to about 60 lb/msf, about 30 lb/msf to about 55 lb/msf, about 30 lb/msf to about 50 lb/msf, about 30 lb/msf to about 40 lb/msf, etc. In some embodiments, the other cover sheet has a weight of about 15 lb/MSF to about 35 lb/msf, e.g., about 20 lb/MSF to about 33 lb/msf, about 20 lb/MSF to about 31 lb/msf, about 20 lb/MSF to about 29 lb/msf, about 20 lb/MSF to about 27 lb/msf, about 15 lb/MSF to about 31 lb/msf, etc. Paper of such weight can have a nominal thickness of about 0.005 inches to about 0.015 inches thick, e.g., 0.007 to about 0.03 inches (e.g., about 0.01 inches). In some embodiments, the other cover sheet can be in the form of paper having a thickness of about 0.008 inches to about 0.013 inches.

Optionally, the board can include a top skim coat and/or a back skim coat. In preferred embodiments, the skim coat layers are generally denser and much thinner than the set gypsum core. In some embodiments, the layers forming the set gypsum core cumulatively contribute the greatest amount to the thickness of the gypsum layers and the board as a whole. In some embodiments, the set gypsum core comprises a substantial thickness (e.g., at least about 90%, at least about 92%, at least about 95%, or at least about 97%) of the total thickness of all gypsum layers. In some embodiments, the top and/or back skim coat layers have a dry thickness of from about 0.125 inch (1/8 inch) to about 0.016 inch (1/64 inch). In preferred embodiments, the at least one skim coat layer has a thickness of about 0.08 inches to about 0.02 inches, e.g., about 0.08 inches to about 0.03 inches, about 0.07 inches to about 0.02 inches, about 0.07 inches to about 0.03 inches, about 0.06 inches to about 0.02 inches, about 0.06 inches to about 0.03 inches, about 0.05 inches to about 0.02 inches, about 0.05 inches to about 0.03 inches, about 0.04 inches to about 0.02 inches, or about 0.04 inches to about 0.03 inches.

In some embodiments, one or both skim coats can be prepared from a slurry containing skim coat starch, as described in U.S. Patent Application No. 62/930,965, filed November 5, 2019, which is incorporated herein by reference. Surprisingly and unexpectedly, it has been found that including skim coat starch in a very thin back skim coat further strengthens the bond between the core and back cover sheet. While not wishing to be bound by any particular theory, the presence of skim coat starch in the thin skim coat is effective in strengthening the bond between the cover sheet and the gypsum core, as it is believed that the starch acts as an adhesive to firmly bond the paper fibers to the gypsum crystals in the core.

In a wall assembly, the board can be attached to a substrate, usually a stud in a framing structure. In a wall assembly, the back surface of the board (i.e., the outer surface of the back cover sheet) faces inward toward the studs, while the front surface of the board (i.e., the outer surface of the front cover sheet) is visible when the board is installed and hanging in use.

The stucco slurry used to create the set gypsum layer of the board contains stucco, for example, in the form of calcium sulfate alpha hemihydrate, calcium sulfate beta hemihydrate, and/or calcium sulfate anhydrous. In addition to stucco and water, the board core is preferably formed from an agent that contributes to its low density, preferably a foaming agent, although in some embodiments, low-density fillers (e.g., perlite, low-density aggregate, etc.) can be used. Various foaming agent regimes are well known in the art. The foaming agent can be included to create an air bubble distribution within the crystalline matrix of the set gypsum. In some embodiments, the foaming agent comprises a major weight portion of an unstable component and a minor weight portion of a stable component (e.g., when unstable and stable/unstable blends are combined). The weight ratio of the unstable component to the stable component is effective to create an air bubble distribution within the set gypsum core. See, for example, U.S. Patent Nos. 5,643,510, 6,342,284, and 6,632,550. In some embodiments, the foaming agent comprises an alkyl sulfate surfactant.

Many commercially known foaming agents are available and may be used in accordance with embodiments of the present disclosure, such as the HYONIC line of soap products (e.g., 25AS) from GEO Specialty Chemicals (Ambler, Pennsylvania). Other commercially available soaps include Polystep B25 from Stepan Company (Northfield, Illinois). The foaming agents described herein may be used alone or in combination with other foaming agents. Foam can be pre-generated and then added to the stucco slurry. Pre-generation can occur by inserting air into the aqueous foaming agent. Methods and apparatus for generating foam are well known. See, for example, U.S. Patent Nos. 4,518,652, 2,080,009, and 2,017,022.

In some embodiments, the blowing agent comprises, consists of, or consists essentially of at least one alkyl sulfate, at least one alkyl ether sulfate, or any combination thereof, but is essentially free of olefins (e.g., olefin sulfates) and/or alkynes. Essentially free of olefins or alkynes means that the blowing agent contains either (i) 0 wt. % or no olefins and/or alkynes based on the weight of the stucco, or (ii) an ineffective amount, or (iii) an insignificant amount of olefins and/or alkynes. An example of an ineffective amount would be an amount below the threshold amount for achieving the intended purpose of using the olefin and/or alkyne blowing agent, as would be understood by one of ordinary skill in the art. An insignificant amount may be, for example, less than about 0.001 wt. %, e.g., less than about 0.0005 wt. %, less than about 0.001 wt. %, less than about 0.00001 wt. %, etc., based on the weight of the stucco.

Some types of unstable soaps, according to embodiments of the present disclosure, are alkyl sulfate surfactants having different chain lengths and different cations. Suitable chain lengths can be, for example, _C8 to _C12 , such as _C8 to _C10 or _C10 to _C12 . Suitable cations include, for example, sodium, ammonium, magnesium, or potassium. Examples of unstable soaps include, for example, sodium dodecyl sulfate, magnesium dodecyl sulfate, sodium decyl sulfate, ammonium dodecyl sulfate, potassium dodecyl sulfate, potassium decyl sulfate, sodium octyl sulfate, magnesium decyl sulfate, ammonium decyl sulfate, blends thereof, and any combination thereof.

Some types of stable soaps, according to embodiments of the present disclosure, are alkoxylated (e.g., ethoxylated) alkyl sulfate surfactants having different (generally longer) chain lengths and different cations. Suitable chain lengths can be, for example, _C10 to _C14 , e.g., _C12 to _C14 , or _C10 to _C12 . Suitable cations include, for example, sodium, ammonium, magnesium, or potassium. Examples of stable soaps include, for example, sodium laureth sulfate, potassium laureth sulfate, magnesium laureth sulfate, ammonium laureth sulfate, blends thereof, and any combination thereof. In some embodiments, any combination of stable and unstable soaps from this list can be used.

Examples of foaming agent combinations and their addition in preparing foamed gypsum products are disclosed in U.S. Patent No. 5,643,510, incorporated herein by reference. For example, a first foaming agent that forms stable foam and a second foaming agent that forms unstable foam can be combined. In some embodiments, the first foaming agent is a soap, such as an alkoxylated alkyl sulfate soap having an alkyl chain length of 8 to 12 carbon atoms and an alkoxy (e.g., ethoxy) group chain length of 1 to 4 units. The second foaming agent is optionally a non-alkoxylated (e.g., non-ethoxylated) alkyl sulfate soap having an alkyl chain length of 6 to 20 carbon atoms, e.g., 6 to 18 or 6 to 16 carbon atoms. Adjusting the individual amounts of these two soaps is believed to allow control of the board foam structure until approximately 100% stable soap or approximately 100% unstable soap is achieved, according to some embodiments.

In some embodiments, as described in U.S. Patent Publication Nos. US 2017/0096369 A1, US 2017/0096366 A1, and US 2017/0152177 A1, a fatty alcohol can optionally be included with the blowing agent, for example, in a premix for preparing the foam. This can result in improved foam stability, thereby allowing for better control of the size and distribution of the foam (air) voids. The fatty alcohol can be any suitable aliphatic fatty alcohol. As defined throughout this specification, "aliphatic" refers to alkyl, alkenyl, or alkynyl, and can be substituted or unsubstituted, branched or unbranched, and saturated or unsaturated, and will be understood to be represented in connection with some embodiments by the carbon chains described herein, e.g., _Cx - _Cy (x and y are integers). Thus, the term aliphatic also refers to chains with heteroatom substitution that maintain the hydrophobicity of the group. The fatty alcohol may be a single compound or a combination of two or more compounds. In some embodiments, any fatty alcohol is a _C6 - _C20 fatty alcohol (e.g., C6 _{- C18} , _C6 -C116, C6- _C14 , _C6 - _C12 , _C6 - _C10 , _C6 - _C8 , C8- _C16 , C8- _C14 , _{C8 - C12} , C8- _C10 , _C10 - _C16 , _C10 - _C14 , _{C10 - C12} , _{C12 -C16 , C12} - _C14 , or _C14 - _C16 aliphatic fatty alcohol, etc.). Examples include octanol, nonanol, _decanol , _undecanol , dodecanol, or any combination thereof.

In some embodiments, the optional foam stabilizer comprises a fatty alcohol and is essentially free of a fatty acid alkyloamide or a carboxylic acid tauride. In some embodiments, the optional foam stabilizer is essentially free of glycol, although glycol may be included in some embodiments, for example, to allow for a higher surfactant content. Essentially free of any of the above components means that the foam stabilizer contains either (i) 0% by weight of any of these components, or (ii) an ineffective amount, or (iii) an insignificant amount of any of these components. An example of an ineffective amount would be an amount below the threshold amount for achieving the intended purpose of using any of these components, as would be understood by one of ordinary skill in the art. An insignificant amount, as would be understood by one of ordinary skill in the art, may be, for example, less than about 0.0001% by weight, e.g., less than about 0.00005% by weight, less than about 0.00001% by weight, less than about 0.000001% by weight, etc., based on the weight of the stucco.

It has been found that suitable void distribution and wall thickness (independently) can be effective in enhancing strength, particularly in lower density boards (e.g., less than about 35 pcf). See, e.g., US 2007/0048490 and US 2008/0090068. Generally, evaporation water voids, having voids of about 5 μm or less in diameter, also contribute to the total void distribution along with the aforementioned air (bubble) voids. In some embodiments, the volume ratio of voids having a pore size greater than about 5 microns to voids having a pore size of about 5 microns or less is from about 0.5:1 to about 9:1, e.g., from about 0.7:1 to about 9:1, from about 0.8:1 to about 9:1, from about 1.4:1 to about 9:1, from about 1.8:1 to about 9:1, from about 2.3:1 to about 9:1, from about 0.7:1 to about 6:1, from about 1.4:1 to about 6:1, from about 1.8:1 to about 6:1, from about 0.7:1 to about 4:1, from about 1.4:1 to about 4:1, from about 1.8:1 to about 4:1, from about 0.5:1 to about 2.3:1, from about 0.7:1 to about 2.3:1, from about 0.8:1 to about 2.3:1, from about 1.4:1 to about 2.3:1, from about 1.8:1 to about 2.3:1, etc.

As used herein, void size is calculated from the maximum diameter of individual voids within the core. The maximum diameter is the same as the ferret diameter. The maximum diameter of each defined void can be obtained from an image of the sample. The image can be taken using any suitable technique, such as a scanning electron microscope (SEM), which provides a two-dimensional image. The pore size of a large number of voids can be measured in an SEM image, so that the randomness of the void cross-sections (pores) can provide an average diameter. This calculation can be improved by taking void measurements in multiple images randomly positioned throughout the core of the sample. In addition, constructing a three-dimensional model of the core based on several two-dimensional SEM images can also improve the void size calculation. Another technique is X-ray CT scanning analysis (XMT), which provides three-dimensional images. Another technique is optical microscopy, where optical contrast can be used to assist, for example, in determining the depth of the void. Voids can be measured either manually or using image analysis software, such as ImageJ, developed by the NIH. Those skilled in the art will understand that manual determination of void size and distribution from an image can be determined by visual observation of the dimensions of each void. Samples can be obtained by sectioning the gypsum board.

The foaming agent can be included in the stucco slurry in any suitable amount, depending, for example, on the desired density. A foaming agent solution is prepared, for example, at about 0.5% (w/w). An appropriate amount of air is mixed with the foaming agent solution and added to the slurry. Depending on the amount of air required, the concentration of the foaming agent solution can vary from about 0.1% to about 1% (w/w). Because the skim coat layer has a higher density, the slurry for forming the skim coat layer can be made with fewer (or no) bubbles.

When included, the fatty alcohol may be present in the stucco slurry in any suitable amount. In some embodiments, the fatty alcohol is present in the core slurry in an amount from about 0.0001% to about 0.03% by weight of the stucco, e.g., from about 0.0001% to about 0.025% by weight of the stucco, from about 0.0001% to about 0.02% by weight of the stucco, or from about 0.0001% to about 0.01% by weight of the stucco. Because the slurry for the skim coat layer contains little or no foam, fatty alcohol may not be necessary in the skim coat layer or may be included in a lesser amount, for example, from about 0.0001% to about 0.004% by weight of the stucco, e.g., from about 0.00001% to about 0.003% by weight of the stucco, from about 0.00001% to about 0.0015% by weight of the stucco, or from about 0.00001% to about 0.001% by weight of the stucco.

Other ingredients known in the art may also be included in the board core slurry, including, for example, accelerators, retarders, and the like. Accelerators can be in various forms (wet gypsum accelerators, heat-resistant accelerators, and weather-stabilizing accelerators). See, for example, U.S. Pat. Nos. 3,573,947 and 6,409,825. In some embodiments in which accelerators and/or retarders are included, the accelerators and/or retarders may each be present in the stucco slurry in an amount on a solids basis of about 0% to about 10% by weight of the stucco (e.g., about 0.1% to about 10%), for example, about 0% to about 5% by weight of stucco (e.g., about 0.1% to about 5%).

Additionally, in some embodiments, the gypsum layer can further comprise at least one dispersant to enhance fluidity. The dispersant may be included in the stucco slurry in dry form with other dry ingredients and/or in liquid form with other liquid ingredients. Examples of dispersants include naphthalene sulfonates, such as polynaphthalene sulfonic acid and its salts (polynaphthalene sulfonates) and derivatives, which are condensation products of naphthalene sulfonic acid and formaldehyde, and polycarboxylate dispersants, such as polycarboxylic acid ethers, e.g., PCE211, PCE111, 1641, 1641F, or PCE 2641 type dispersants, e.g., MELFLUX 2641F, MELFLUX 2651F, MELFLUX 1641F, MELFLUX 2500L dispersants (BASF), and Coatex, Inc. COATEX Ethacryl M available from Reed Lignin; and/or lignosulfonates or sulfonated lignins. Lignosulfonates are water-soluble anionic polyelectrolyte polymers that are by-products of wood pulp production using sulfite pulping. An example of a lignin useful for practicing the principles of embodiments of the present disclosure is Malaspel C-21 available from Reed Lignin.

Low molecular weight dispersants are generally preferred. For naphthalene sulfonate dispersants, in some embodiments, they are selected to have a molecular weight of about 3,000 to about 10,000 (e.g., about 8,000 to about 10,000). In some embodiments, higher water demand naphthalene sulfonates can be used, for example, with molecular weights greater than 10,000. As another example, for PCE211-type dispersants, in some embodiments, the molecular weight can be about 20,000 to about 60,000, which results in less retardation than dispersants with molecular weights greater than 60,000.

One example of a naphthalene sulfonate is DILOFLO, available from GEO Specialty Chemicals. DILOFLO is a 45% naphthalene sulfonate solution in water, although other aqueous solutions ranging, for example, from about 35% to about 55% by weight solids, are also readily available. The naphthalene sulfonate may be used in dry solid or powder form, such as LOMAR D, available from GEO Specialty Chemicals. Another example of a naphthalene sulfonate is DAXAD, available from GEO Specialty Chemicals (Ambler, Pennsylvania).

When included, dispersants can be present in any suitable amount. In some embodiments, for example, the dispersant can be present in the stucco slurry in an amount, for example, from about 0% to about 0.5%, e.g., from about 0.01% to about 0.7%, e.g., from about 0.01% to about 0.4%, from about 0.1% to about 0.2%, etc., based on the weight of the stucco.

In some embodiments, the gypsum layer can optionally be further formed from at least one phosphate-containing compound to enhance green strength, dimensional stability, and/or sag resistance. For example, in some embodiments, useful phosphate-containing components include water-soluble components, which may be in the form of ions, salts, or acids, i.e., condensed phosphoric acids, each of which contains two or more phosphoric acid units, condensed phosphate salts or ions, each of which contains two or more phosphate units, monobasic salts or monovalent ions of orthophosphate, and water-soluble acyclic polyphosphate salts. See, for example, U.S. Patent Nos. 6,342,284, 6,632,550, 6,815,049, and 6,822,033.

When added in some embodiments, phosphate compositions can enhance green strength, resistance to permanent deformation (e.g., sagging), dimensional stability, etc. Green strength refers to the strength of the board while it is still wet during manufacturing. Due to the rigors of the manufacturing process, lack of sufficient green strength can cause the board precursor to be damaged on the manufacturing line.

For example, trimetaphosphate compounds including sodium trimetaphosphate, potassium trimetaphosphate, lithium trimetaphosphate, and ammonium trimetaphosphate can be used. Although sodium trimetaphosphate (STMP) is preferred, other phosphates may be suitable, for example, sodium tetrametaphosphate, sodium hexametaphosphate having from about 6 to about 27 repeating phosphate units and having the molecular formula Na _n+2 P _n O _3n+1 (where n=6-27), tetrapotassium pyrophosphate having the molecular formula K ₄ P ₂ O ₇ , trisodium dipotassium tripolyphosphate having the molecular formula Na ₃ K ₂ P ₃ O ₁₀ , sodium tripolyphosphate having the molecular formula Na ₅ P ₃ O ₁₀ , tetrasodium pyrophosphate having the molecular formula Na ₄ P ₂ O ₇ , aluminum trimetaphosphate having the molecular formula Al(PO ₃ ) ₃ , sodium acid pyrophosphate having the molecular formula Na ₂ H ₂ P ₂ O ₇ , sodium acid pyrophosphate having 1,000-3,000 repeating phosphate units and having the molecular formula (NH ₄ ) _n+2 P _n O _3n+1 (where n=1,000-3,000), or polyphosphates with two or more repeating phosphate units and the molecular formula H _n+2 P _n O _3n+1 (where n is 2 or greater).

When included, polyphosphate can be present in any suitable amount. To illustrate, in some embodiments, polyphosphate can be present in the slurry in an amount of, for example, about 0.1% to about 1%, e.g., about 0.2% to about 0.4% by weight of the stucco, or about 0% to about 0.5%, e.g., about 0% to about 0.2% by weight of the stucco. Thus, the dispersant and polyphosphate, optionally, can be present in any suitable amount in the stucco slurry.

The board core can have any suitable density useful for contributing to a desired overall composite board density, e.g., a core density of from about 16 pcf (about 260 kg/m ³ ) to about 40 pcf, e.g., from about 18 pcf to about 40 pcf, 18 pcf to about 38 pcf, 18 pcf to about 36 pcf, 18 pcf to about 32 pcf, 20 pcf to about 40 pcf, 20 pcf to about 36 pcf, 20 pcf to about 32 pcf, 22 pcf to about 40 pcf, 22 pcf to about 36 pcf, 22 pcf to about 32 pcf, 26 pcf to about 40 pcf, 26 pcf to about 36 pcf, or 26 pcf to about 32 pcf. In some embodiments, the board core has an even lower density, such as about 30 pcf or less, about 29 pcf or less (about 460 kg/m ³ ), about 28 pcf or less, about 27 pcf or less (about 430 kg/m ³ ), about 26 pcf or less, etc. For example, in some embodiments, the core density is from about 12 pcf (about 190 kg/m ³ ) to about 30 pcf, from about 14 pcf (about 220 kg/m ³ ) to about 30 pcf, 16 pcf to about 30 pcf, 16 pcf to about 28 pcf, 16 pcf to about 26 pcf, 16 pcf to about 22 pcf (about 350 kg/m ³ ), 18 pcf to about 30 pcf, 18 pcf to about 28 pcf, 18 pcf to about 26 pcf, 18 pcf to about 24 pcf, 20 pcf to about 30 pcf, 20 pcf to about 28 pcf, 20 pcf to about 26 pcf, 20 pcf to about 24 pcf, 22 pcf to about 28 pcf, etc.

In some embodiments, composite boards made according to the present disclosure meet the testing protocol according to ASTM Standard C473-10. For example, in some embodiments, when the board is cast at a thickness of ½ inch, the dried board has a nail pull resistance of at least about 67 lb _f (pounds force), e.g., at least about 68 lb _f , at least about 70 lb f, at least about 72 lb f, at least about 74 lb _f , at least about 75 lb _f , at least about 76 lb _f , at least about 77 lb _{f , etc.} , as determined according to ASTM C473-10 (Method B). In various embodiments, the nail pull resistance is from about 67 lb _f to about 100 lb _f , from about 67 lb _f to about 95 lb _{f, from about 67 lb f to} about 90 lb _f , from about 67 lb _f to about 85 lb _f , from about 67 lb _f to about 80 lb _f , from about 67 lb _f to about 75 lb _f , from about 68 lb _f to about 100 lb _f , from about 68 lb _f to about 95 lb _f , from about 68 lb _f to about 90 lb _f , from about 68 lb _f to about 85 lb _f , from about 68 lb _f to about 80 lb _f , from about 70 lb _f to about 100 lb _f , from about 70 lb _f to about 95 lb _f , about 70 lb _f ~90lb _f , approximately 70lb _f ~approximately 85lb _f , approximately 70lb _f ~approximately 80lb _f , approximately 72lb _f ~approximately 100lb _f , approximately 72lb _f ~approximately 95lb _f , approximately 72lb _f ~approximately 90lb _f , approximately 72lb _f ~approximately 85lb _f , approximately 72 lb _f to approximately 80 lb _f , approximately 72 lb _f to approximately 77 lb _f , approximately 72 lb _{f to} approximately 75 lb f , approximately 75 lb _f to approximately 100 lb _f , approximately 75 lb _f to approximately 95 lb _f , approximately 75 lb _f to approximately 90 lb _f , approximately 75 lb _f to approximately 85 lb _f , approx. 75lb _f ~ approx. 80lb _f , about 75 lb _f to about 77 lb _f , about 77 _{lb f} to about 100 lb _f , about 77 lb _f to about 95 lb _{f , about 77 lb f to} about 90 lb _f , about 77 lb _f to about 85 lb f , or about 77 lb _f to about 80 lb _f .

In some embodiments, the composite gypsum board can have an average core hardness of at least about 11 lb _f , e.g., at least about 12 lb _f , at least about 13 lb _f , at least about 14 lb _f , at least about 15 lb f, at least about 16 lb _f , at least about 17 lb _f , at least about 18 lb _f , at least about 19 lb _f , at least about 20 lb _f , at least about 21 lb _f , or at least about 22 lb _f , as determined according to ASTM C473-10 _, Method B. In some embodiments, the board has a strength of about 11 lb _f to about 25 lb _f , e.g., about 11 lb _f to about 22 lb _f , about 11 lb _f to about 21 lb _f , about 11 lb _f to about 20 lb _f , about 11 lb _f to about 19 lb _f , about 11 lb _f to about 18 lb _f , about 11 lb _f to about 17 lb _f , about 11 lb _f to about 16 lb _f , about 11 lb _f to about 15 lb _f , about 11 lb _f to about 14 lb _f , about 11 lb _f to about 13 lb _f , about 11 lb _f to about 12 lb _f , about 12 lb _f to about 25 lb _f , about 12 lb _f to about 22 lb _f , about 12lb _f to about 21lb _f , about 12lb _f to about 20lb _f , about 12lb _f to about 19lb _f , about 12lb _f to about 18lb _f , about 12lb _f to about 17lb _f , about 12lb _f to about 16lb _f , about 12lb _f to about 15lb _f , about 12lb _f to about 14lb _f , about 12lb _f to about 13lb _f , about 13lb _f to about 25lb _f , about 13lb _f to about 22lb _f , about 13lb _f to about 21lb _f , about 13lb _f to about 20lb _f , about 13lb _f ~Approx. 19lb _f , approx. 13lb _f to about 18lb _f , about 13lb _f to about 17lb _f , about 13lb _f to about 16lb _f , about 13lb _f to about 15lb _f , about 13lb _f to about 14lb _f , about 14lb _f to about 25lb _f , about 14lb _f to about 22lb _f , about 14lb _f to about 21lb _f , about 14lb _f to about 20lb _f , about 14lb _f to about 19lb _f , about 14lb _f to about 18lb _f , about 14lb _f to about 17lb _f , about 14lb _f to about 16lb _f , about 14lb _f to about 15lb _f , approx. 15lb _f ~ approx. 25lb _f , about 15lb _f to about 22lb _f , about 15lb _f to about 21lb _f , about 15lb _f to about 20lb _f , about 15lb _f to about 19lb _f , about 15lb _f to about 18lb _f , about 15lb _f to about 17lb _f , about 15lb _f - about 16lb _f , about 16lb _f - about 25lb _f , about 16lb _f - about 22lb _f , about 16lb _f - about 21lb _f , about 16lb _f - about 20lb _f , about 16lb _f - about 19lb _f , about 16lb _f - about 18lb _f , about 16lb _f ~ approx. 17lb _f , approximately 17 lb _f to approximately 25 lb _f , approximately 17 lb _f to approximately 22 lb _f , approximately 17 lb _{f to} approximately 21 lb f , approximately 17 lb _f to approximately 20 lb _f , approximately 17 lb _f to approximately 19 lb _f , approximately 17 lb _f to approximately 18 lb _f , approximately 18 lb _f to approximately 25 lb _f , about 18lb _f to about 22lb _f , about 18lb _f to about 21lb _f , about 18lb _f to about 20lb _f , about 18lb _f to about 19lb _f , about 19lb _f to about 25lb _f , about 19lb _f to about 22lb _f , about 19lb _f ～Approx. 21lb _f , approx. 19lb The core hardness may be from about 20 lb _f to about 20 lb _f , from about 21 lb _f to about 25 lb _f , from about 21 lb _f to about 22 lb _f , or from about 22 lb _f to about 25 lb _f .

With respect to flexural strength, in some embodiments, when cast into a ½ inch thick board, the dried board has a flexural strength in the machine direction of at least about 36 lb _f (e.g., at least about 38 lb _f , at least about 40 lb _f , etc.) and/or at least about 107 lb _f (e.g., at least about 110 lb _f , at least about 112 lb _f , etc.) in the cross direction, as determined according to ASTM Standard C473-10. In various embodiments, the board has a strength of about 36 lb _f to about 60 lb _f , e.g., about 36 lb _f to about 55 lb _f , about 36 lb _f to about 50 lb _f , about 36 lb _f to about 45 lb _f , about 36 lb _f to about 40 lb _f , about 36 lb _f to about 38 lb _f , about 38 lb _f to about 60 lb _f , about 38 lb _f to about 55 lb _f , about 38 lb _f to about 50 lb _f , about 38 lb _f to about 45 lb _f , about 38 lb _f to about 40 lb _f , about 40 lb _f to about 60 lb _f , about 40 lb _f to about 55 lb _f , about 40 lb _f to about 50 lb _f , or a machine direction flexural strength of about 40 lb _f to about 45 lb _f . In various embodiments, the board has a strength of about 107 lb _f to about 130 lb _f , e.g., about 107 lb _f to about 125 lb _f , about 107 lb _f to about 120 lb _f , about 107 lb _f to about 115 lb _f , about 107 lb _f to about 112 lb _f , about 107 lb _f to about 110 lb _f , about 110 lb _f to about 130 lb _f , about 110 lb _f to about 125 lb _f , about 110 lb _f to about 120 lb _f , about 110 lb _f to about 115 lb _f , about 110 lb _f to about 112 lb _f , about 112 lb _f to about 130 lb _f , about 112 lb _f The laminate may have a cross-machine bending strength of from about 112 lb _f to about 125 lb _f , from about 112 lb _f to about 120 lb _f , or from about 112 lb f to about 115 lb _f .

Advantageously, in various embodiments at various board densities described herein, the dry composite gypsum board has a compressibility of at least about 170 psi (1,170 kPa), e.g., from about 170 psi to about 1,000 psi (6,900 kPa), from 170 psi to about 900 psi (6,200 kPa), from about 170 psi to about 800 psi (5,500 kPa), from about 170 psi to about 700 psi (4,800 kPa), from about 170 psi to about 60 0 psi (4,100 kPa), about 170 psi to about 500 psi (3,450 kPa), about 170 psi to about 450 psi (3,100 kPa), about 170 psi to about 400 psi (2,760 kPa), about 170 psi to about 350 psi (2,410 kPa), about 170 psi to about 300 psi (2,070 kPa), or about 170 psi to about 250 psi (1,720 kPa). In some embodiments, the board has a compressive strength of at least about 450 psi (3,100 kPa), at least about 500 psi (3,450 kPa), at least about 550 psi (3,800 kPa), at least about 600 psi (4,100 kPa), at least about 650 psi (4,500 kPa), at least about 700 psi (4,800 kPa), at least about 750 psi (5,200 kPa), at least about 800 psi (5,500 kPa), at least about 850 psi (5,850 kPa), at least about 900 psi (6,200 kPa), at least about 950 psi (6,550 kPa), or at least about 1,000 psi (6,900 kPa). Furthermore, in some embodiments, the compressive strength may be limited by any two of the foregoing points. For example, the compressive strength may be between about 450 psi and about 1,000 psi (e.g., between about 500 psi and about 900 psi, between about 600 psi and about 800 psi, etc.). As used herein, compressive strength is measured using a material testing system commercially available as an ATS machine model 1610 from Applied Test Systems (Butler, Pennsylvania). The load is applied continuously without impact at a rate of 1 inch/minute.

Gypsum boards according to embodiments of the present invention can be made on a typical gypsum wallboard manufacturing line. For example, board manufacturing techniques are described, for example, in U.S. Pat. No. 7,364,676, U.S. Patent Application Publication No. 2010/0247937, and U.S. Patent Application No. 16/581,070. Briefly, the process typically involves discharging a cover sheet onto a moving conveyor. Because gypsum boards are typically formed "face down," this cover sheet is, in such embodiments, the "face" cover sheet. Face and/or back skim coats, as known in the art, can be included as needed.

In some embodiments, one or both skim coat layers have an average dry core hardness at least about 1.5 times greater than the average dry core hardness of the board core, where the average core hardness is measured in accordance with ASTM C-473-07, e.g., at least about 2 times greater, 2.5 times greater, 3 times greater, 3.5 times greater, 4 times greater, 4.5 times greater, etc., each of these ranges can have any mathematically suitable upper limit, e.g., 8, 7, 6, 5, 4, 3, or 2.

The slurries for forming the board core and for forming the skim coat layer can be formed in any suitable manner. For example, a single mixer can be used to develop both slurry streams. The mixer can be, for example, in the form of a "pin mixer" or "pinless mixer," as needed, in which the raw materials are agitated. Alternatively, two or more separate mixers can be used. The mixers can be in series or disconnected. Examples of mixers are described in European Patent No. 1 637 302 B1, European Patent No. 2 929 996 B1, European Patent Application No. 3 342 571 A1, and U.S. Patent Application No. 2017/0008192 A1. If necessary for efficiency, the mixer used for the skim coat layer can have a smaller mixing volume capacity in some embodiments, since the amount of slurry that needs to be applied to the skim coat layer is less than the amount of slurry applied to form the board core. The "main" mixer (i.e., for forming the board core slurry) includes a body and a discharge conduit (e.g., a gate-canister-boot arrangement known in the art or a modified outlet design (MOD) arrangement as described in U.S. Pat. Nos. 6,494,609 and 6,874,930). The blowing agent can be added into the mixer's discharge conduit (e.g., into the gate as described in U.S. Pat. Nos. 5,683,635 and 6,494,609).

It will be appreciated that in some embodiments, the discharge conduit may include a slurry distributor having either a single feed inlet or multiple feed inlets, such as those described in U.S. Patent Application Publication No. 2012/0168527 A1 (Application No. 13/341,016) and U.S. Patent Application Publication No. 2012/0170403 A1 (Application No. 13/341,209). In those embodiments, by using a slurry distributor having multiple feed inlets, the discharge conduit may include a suitable flow divider, such as those described in U.S. Patent Application Publication No. 2012/0170403 A1.

As understood in the art, the board is typically formed simultaneously and continuously in a sandwich construction. The face cover sheet moves as a continuous ribbon on a moving conveyor. After exiting the mixer, a face skim coat layer slurry is applied to the (e.g., moving) face cover sheet. Also, as known in the art, hard edges, if desired, can be formed from the same slurry stream that forms the skim coat layers (e.g., face and/or back skim coat layers) for convenience, for example.

The board core slurry is then applied over the skim coat layer and covered with a second cover sheet (typically a "back" cover sheet) to form a wet assembly in the form of a sandwich structure that is the final product board precursor. The back (bottom) cover sheet can support a back skim coat layer, which can optionally include skim coat starch as described herein, to enhance adhesion between the backing paper and the board core. The back skim coat layer can be formed from the same or a different gypsum slurry as the top skim coat layer. In some embodiments, a skim coat layer is applied to the back side of the board, i.e., in adhesive relationship to the bottom (back) cover sheet, but no skim coat layer is applied between the core and the top cover sheet.

In some embodiments, the cover (face-down at the wet end of the board machine) can be manufactured to be slightly wider than the width of the final board product, since the edge of the paper can be folded over the edge of the board to meet the backing paper (face-up at the wet end of the board machine) to form a board envelope. For example, for a nominally 48-inch wide board, the cover can have a width of about 50 inches or more (e.g., about 50 to about 52 inches, e.g., about 50.375 inches). Correspondingly, in some embodiments, the backing paper can be manufactured to be narrower than the width of the board. Thus, for a nominally 48-inch wide board, the backing paper can have a width of less than about 48 inches (e.g., about 46.5 inches to about 47.5 inches, e.g., about 47.125 inches).

The resulting wet assembly is then conveyed to a forming station where the product is sized (e.g., via a forming plate) to the desired thickness and one or more knife sections where it is cut to the desired length. The wet assembly is allowed to harden to form an interlocking crystalline matrix of set gypsum, and excess water is removed using a drying process (e.g., by transporting the assembly through a kiln).

It is also common to use vibration in the manufacture of gypsum board to remove large voids or air pockets from the deposited slurry. Each of the above steps, as well as the processes and equipment for carrying out such steps, are known in the art.

The present invention is further described by the following exemplary sections. However, the present invention is not limited to the following sections.

(1) A gypsum board or a method for producing a gypsum board described in this specification.

(2) A gypsum board including a hardened gypsum core disposed between a front cover sheet and a back cover sheet, the hardened gypsum core being formed from a slurry containing water, stucco, and a high salt impurity content, and at least one of the cover sheets being highly absorbent paper.

(3) The gypsum board of (2), further comprising a surface skim coat disposed between the surface cover sheet and the set gypsum core.

(4) The gypsum board of (2) or (3), further comprising a back skim coat disposed between the back cover sheet and the set gypsum core.

(5) A gypsum board according to any one of (2) to (4), wherein the back cover sheet is highly absorbent paper.

(6) A gypsum board according to any one of (3) to (5), wherein the front and/or back skim coat has a dry thickness of about 0.125 inches (1/8 inch) to about 0.016 inches (1/64 inch), for example, about 0.08 inches (1/12 inch) to about 0.03 inches (1/32 inch).

(7) The gypsum board described in any one of (2) to (6), wherein the high salt impurities include at least about 150 ppm chloride anions per 1,000,000 parts by weight of the stucco.

(8) A gypsum board according to any one of (2) to (7), wherein the salt impurities include chloride salts such as sodium chloride, potassium chloride, magnesium chloride, or calcium chloride.

(9) The gypsum board according to any one of (2) to (8), wherein the highly absorbent paper has a bond-side Cobb value of at least about 2.1 g/100 ^cm2 according to a Cobb test.

(10) The gypsum board according to any one of (2) to (9), wherein the highly absorbent paper has a bond-side Cobb value of about 2.1 g/100 cm ² to about 3.1 g/100 cm ² according to a Cobb test.

(11) A gypsum board according to any one of (2) to (10), wherein the highly absorbent paper has a basis weight of about 35 lb/MSF to about 65 lb/MSF, for example, about 38 lb/MSF to about 60 lb/MSF, for example, about 42 lb/MSF to about 55 lb/MSF.

(12) A gypsum board according to any one of (2) to (11), wherein the highly absorbent paper has a caliper of about 10 mils to about 15 mils.

(13) A method for manufacturing gypsum board, comprising: (a) mixing at least water and stucco having a high salt impurity content to form a first slurry; (b) applying the first slurry to a face cover sheet to form a board core in adhesive relationship thereto, the board core having a first side and a second side, the first side facing the face cover sheet; (c) applying a back cover sheet to the second side of the board core in adhesive relationship thereto to form a board precursor, at least one of the cover sheets being a highly absorbent paper; and (d) drying the board precursor to form a board.

(14) The method of (13), further comprising applying a second slurry containing at least stucco and water to the face sheet to form a face skim coat disposed between the face cover sheet and the board core.

(15) The method of (13) or (14), further comprising applying a third slurry containing at least stucco and water to the backing paper to form a back skim coat disposed between the back cover sheet and the board core, wherein the second and third slurries are the same or different.

(16) The method according to any one of (13) to (15), wherein the back cover sheet is highly absorbent paper.

(17) The method according to any one of (13) to (16), wherein the front and/or back skim coat has a dry thickness of from about 0.125 inches (1/8 inch) to about 0.016 inches (1/64 inch), for example, from about 0.08 inches (1/12 inch) to about 0.03 inches (1/32 inch).

(18) The method of any of (13) to (17), wherein the high salt impurities include at least about 150 ppm chloride anions per 1,000,000 parts by weight of the stucco.

(19) The method according to any one of (13) to (18), wherein the salt impurities include chloride salts such as sodium chloride, potassium chloride, magnesium chloride, or calcium chloride.

(20) The method according to any one of (13) to (19), wherein the highly absorbent paper has a bond-side Cobb value of at least about 2.1 g/100 ^cm2 according to the Cobb test.

(21) The method according to any one of (13) to (20), wherein the highly absorbent paper has an adhesive side Cobb value of about 2.1 g/100 cm ² to about 3.1 g/100 cm ² according to the Cobb test.

(22) The method of any of (13) to (21), wherein the highly absorbent paper has a basis weight of about 40 lb/MSF to about 65 lb/MSF, for example, about 42 lb/MSF to about 60 lb/MSF, for example, about 45 lb/MSF to about 55 lb/MSF.

(23) The method of any of (13) to (22), wherein the highly absorbent paper has a caliper of about 10 mils to about 15 mils.

Please note that the preceding paragraphs are illustrative and non-limiting. Other exemplary embodiments will be apparent from the entire description of this specification. Those skilled in the art will also understand that each of these embodiments may be used in various combinations with the other embodiments provided herein.

The following examples further illustrate the invention but, of course, should not be construed as in any way limiting its scope.
Example 1

This example demonstrates the effectiveness of using highly absorbent paper as a cover sheet for wallboard containing a set gypsum layer prepared from stucco with a high content of salt impurities. The highly absorbent paper strengthens the bond between the paper and the set gypsum core when the set gypsum layer is formed from a slurry containing a high concentration of salt.

Specifically, a wallboard manufacturing production line prepared three 1/2-inch thick boards (1A-1C). These boards contained a set gypsum layer prepared according to the formulation in Table 1 sandwiched between front and back cover sheets. Board 1A was a control board in which the set gypsum core was prepared without high-salt impurities. The set gypsum core of each of boards 1B and 1C was prepared with 600 ppm chloride ion introduced by adding a mixture of sodium chloride and magnesium chloride to the formulation set forth in Table 1. The amounts of the ingredients in Table 1 are listed in lb/MSF.

In Table 1, HRA refers to heat accelerator. Pregelatinized starch is pregelatinized cornstarch with a cold water viscosity of 90 centipoise. Uncooked starch is uncooked acid-modified cornstarch with a hot water viscosity of 180 BU. The dispersant is naphthalene sulfonic acid. The retarder is pentasodium diethylenetriamine pentaacetate. STMP refers to sodium trimetaphosphate. Foam was prepared using the HYONIC line of soap products (e.g., 25AS) from GEO Specialty Chemicals (Ambler, Pennsylvania) and Polystep B25 from Stepan Company (Northfield, Illinois).

The front cover sheet (manila) of each of Boards 1A-1C was a paper having a basis weight of 50 lb and of a conventional (regular) composition without high-absorbency properties. The back cover sheet (Newsline) of Comparative Boards 1A and 1B was a paper having a basis weight of 47 lb and of a conventional composition, and the back cover sheet of Board 1C was a highly absorbent paper having a basis weight of 47 lb. Comparative Board 1A had a weight of 1307 lb/MSF, and Boards 1B and 1C had a weight of 1305 lb/MSF.

Tests were conducted to determine the effect on the adhesion between each cover sheet and the set gypsum layer of each board. After the boards came off the production line, they were cut into 5.5" x 5.875" samples. A 1/8" deep linear score was made on the surface of each board, 1.0" from and parallel to one of the 5.875" edges, and each board was conditioned overnight in a 75°F/50% relative humidity ("RH") chamber. The conditioned samples were then placed in a 90°F/90% RH chamber. Humidified adhesion tests were performed on both the front and back of the boards that had been in the 90°F/90% RH chamber for 3 hours, 16 hours, and 1 week, respectively. The humidified boards were tested according to the following Humidified Board Test.

The moistened board was snapped along the scores without tearing or stressing the paper on the back of the board, and the larger (4.5" x 5.875") section of the board sample was rotated and pressed face-up downward, attempting to force the backing paper on the back of the board to peel away from the larger section. The force was increased until the two sections of the board were completely separated. A higher moistened bond load indicates better adhesion between the paper and the core. The back of the larger section was then examined to determine the percentage of the backing surface that had completely separated from the core (referred to as the "failure rate").

As shown in Table 2, the top side of the boards in all three conditions exhibited similar humidified bond loads and 0% failure of the bond between the paper and core. Figure 1 shows photographs of all three conditions after the bond test. As shown in Figure 1, good adhesion between the paper and core was observed in each condition.

However, the wet adhesion results from the backside of the boards were different, as shown in Table 3. Figure 2 shows photographs of all three conditions after the adhesion test. As shown in Figure 2, conditioned board 1B showed poor adhesion between the paper and core, while conditioned boards 1A and 1C showed good adhesion between the paper and core. Control board 1A, with no added salt, showed a high wet adhesion load and 0% failure rate between the regular backing paper and gypsum core. On the other hand, when 600 ppm chloride was added to the gypsum slurry, board 1B showed a much lower wet adhesion load and a failure rate of over 60% between the regular Newsline backing paper and gypsum core.

Unlike Board 1B, Board 1C, formed from the same stucco slurry formulation as Board 1B, exhibited a much higher wet bond load and 0% failure rate for the paper-to-core bond when highly absorbent paper was used as the backing. This indicates that highly absorbent paper strengthens the paper-to-core bond, especially when the wallboard contains a high concentration of salt.

All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

In the context of describing the present invention (particularly in the context of the claims below), the use of the terms "a," "an," "the," and "at least one," and similar referents, are to be construed to encompass both the singular and the plural, unless otherwise stated herein or the context clearly contradicts. The use of the term "at least one" followed by a list of one or more items (e.g., "at least one of A and B") is to be construed to mean one item selected from the listed items (A or B), or any combination of two or more of the listed items (A and B), unless otherwise stated herein or the context clearly contradicts. The terms "comprising," "having," "including," and "containing" are to be construed as open-ended terms (i.e., meaning "including but not limited to"), unless otherwise indicated. The recitation of ranges of values herein is 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 herein as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or clearly contradicted by context. The use of any and all examples or exemplary language (e.g., "etc.") provided herein is intended merely to facilitate a better understanding of the invention and does not impose a limitation on the scope of the invention unless otherwise recited in the claims. No language herein should be construed as indicating any non-claimed element as essential to the practice of the invention.

Preferred embodiments of the present invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of these preferred embodiments may become apparent to those skilled in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as necessary, and the inventors intend the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims (17)

A gypsum board comprising a set gypsum core disposed between a face cover sheet and a back cover sheet, the set gypsum core being formed from a slurry comprising water, stucco, and high-salt impurities , the high-salt impurities comprising at least 150 ppm of chloride ions (Cl-) per stucco, ^the back cover sheet comprising paper , the paper having a glue-side Cobb value of at least 2.1 g/100 ^cm2 according to a Cobb test, the paper having a basis weight of 35 lb/MSF to 65 lb/MSF, and a thickness of 0.005 inch to 0.015 inch, the paper enabling superior adhesion between the set gypsum core and the back cover sheet of the ^gypsum board compared to a reference board made with a back cover sheet comprising paper having a glue-side Cobb value of less than 2.1 g/100 cm2 . The gypsum board of claim 1, further comprising a front skim coat disposed between the front cover sheet and the set gypsum core, and/or a back skim coat disposed between the back cover sheet and the set gypsum core. 3. The gypsum board of claim 2 , wherein the face and/or back skim coat has a dry thickness of 0.125 inch (1/8 inch) to 0.016 inch (1/64 inch ) . 10. The gypsum board of claim 1 , wherein the high salt impurities include sodium chloride, potassium chloride, magnesium chloride, calcium chloride, or any combination thereof. 2. The gypsum board of claim 1, wherein the paper has a glue-side Cobb value of 2.1 g/100 ^cm2 to 3.1 g/100 ^cm2 according to the Cobb test. 10. The gypsum board of claim 1, wherein the paper has a basis weight of 40 lb/MSF to 65 lb/MSF. 10. The gypsum board of claim 1, wherein the paper has a caliper of 10 mils to 15 mils. A method for manufacturing gypsum board, comprising:
(a) mixing at least water and stucco containing high salt impurities to form a first slurry, the high salt impurities comprising at least 150 ppm chloride ions (Cl ⁻ ) per stucco;
(b) applying the first slurry to form a board core in adhesive relationship to a face cover sheet, the board core having a first side and a second side, the first side facing the face cover sheet;
(c) applying a back cover sheet in adhesive relationship to the second surface of the board core to form a board precursor, the back cover sheet comprising paper , the paper having a glue-side Cobb value of at least 2.1 g/100 ^cm2 according to the Cobb Test , the paper having a basis weight of 35 lb/MSF to 65 lb/MSF, the paper having a caliper of 0.005 inch to 0.015 inch, the paper enabling superior adhesion between the board core and the back cover sheet of the gypsum board compared to a reference board made with a back cover sheet comprising paper having a glue-side Cobb value of less than 2.1 g/100 ^cm2 ;
(d) drying the board precursor to form a board;
A method comprising: 10. The method of claim 8 , further comprising applying a second slurry comprising at least stucco and water to the facing to form a face skim coat disposed between the face cover sheet and the board core. 10. The method of claim 9, further comprising applying a third slurry comprising at least stucco and water to the backing paper to form a back skim coat disposed between the back cover sheet and the board core, the second and third slurries being the same or different. 9. The method of claim 8, wherein the gypsum board further comprises a face skim coat disposed between the face cover sheet and the board core, and/or a back skim coat disposed between the back cover sheet and the board core, the face and/or back skim coat having a dry thickness of 0.125 inches ( 1/8 inch) to 0.016 inches (1/64 inch ) . 9. The method of claim 8 , wherein the high salt impurities include sodium chloride, potassium chloride, magnesium chloride, or calcium chloride . 9. The method of claim 8 , wherein the paper has a glue side Cobb value of 2.1 g/100 ^cm2 to 3.1 g/100 ^cm2 according to the Cobb test. The method of claim 8 wherein the paper has a basis weight of from 40 lb/MSF to 65 lb/ MSF . The method of claim 8 , wherein the paper has a caliper of 10 mils to 15 mils. The paper has a hardness of at least 0.5 g/100 cm according to the Cobb test. ² The gypsum board of claim 1 having a liner-side Cobb value of The paper has a hardness of at least 0.5 g/100 cm according to the Cobb test. ² 9. The method of claim 8, wherein the liner side cob value is