Method of manufacturing gypsum board with improved fire resistance

Abstract

The present disclosure is directed to a method of manufacturing gypsum board as well as the resulting gypsum board. The method comprises providing a gypsum slurry with a shrinkage-reducing additive comprising a metal salt of an acid, a silica, or a mixture thereof. As a result, the gypsum board includes gypsum and a shrinkage-reducing additive comprising a metal salt of an acid, a silica, or a mixture thereof. The gypsum board exhibits an area shrinkage of less than 10%.

Description

BACKGROUND OF THE INVENTION

Gypsum wallboards have been used as fire resistant building materials for many years. Generally, the fire resistance of the board is derived from the loss of crystalline water (“endothermic dehydration”) present in the gypsum molecular structure. However, when exposed to high temperatures, gypsum boards have a tendency to shrink. Such shrinkage can also result in cracks or voids to form within the core. In turn, this can affect the mechanical integrity and strength of the board and also create openings that may allow fire or hot air to penetrate through the board and/or a wall assembly containing such board. This can result in increased heat transfer and ultimately, failure of the board and/or the wall assembly.

As a result, a need exists for providing a gypsum board with improved resistance. In particular, a need exists for providing a gypsum board with reduced shrinkage and improved mechanical strength, for instance when exposed to high temperatures.

SUMMARY OF THE INVENTION

Aspects and advantages of the invention will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention.

In accordance with one embodiment of the present invention, a method for manufacturing a gypsum board is disclosed. The method comprises providing a gypsum slurry with a shrinkage-reducing additive comprising a metal salt of an acid, a silica, or a mixture thereof. The gypsum board exhibits an area shrinkage of less than 20%.

In accordance with another embodiment of the present invention, a gypsum board is disclosed. In particular the gypsum board includes gypsum and a shrinkage-reducing additive comprising a metal salt of an acid, a silica, or a mixture thereof. The gypsum board exhibits an area shrinkage of less than 20%.

These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:

FIG. 1 is a graphical representation of a Differential Scanning calorimetry (DSC) curve of temperature (° C.) and weight loss of a conventional gypsum sample;

FIG. 2 is a data plot of a typical E119 test measuring time and temperature of a conventional gypsum board;

FIG. 3 is a Thermal Mechanical Analyzer (TMA) plot of a length dimension, percent change in dimension and temperature of a conventional gypsum sample;

FIG. 4 is a comparison of Scanning Electron Microscope (SEM) images of gypsum crystal morphology when exposed to different temperatures;

FIG. 5 is a Thermal Mechanical Analyzer (TMA) plot of the percent change in dimension and temperature of a gypsum sample and a sample with 1% colloidal silica in accordance with one aspect of the present disclosure; and

FIG. 6 is a Scanning Electron Microscope (SEM) image of gypsum crystals containing colloidal silica.

DETAILED DESCRIPTION OF THE INVENTION

Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents.

Generally speaking, the present invention is directed to a method of manufacturing gypsum board as well as the resulting gypsum board. The method comprises providing a gypsum slurry with a shrinkage-reducing additive. The shrinkage-reducing additive may include a metal halide, a metal salt of an acid, a silica, or any combination thereof. Such shrinkage-reducing additives may also be present in the final gypsum board.

The present inventors have discovered that such shrinkage-reducing additives can be employed to be present at the gypsum crystal interfaces in order to minimize the shrinkage of the resulting board. Alternatively or simultaneously, the shrinkage-reducing additives can cause a defect in the gypsum crystal growth during the crystal growth stage at high temperatures. Nevertheless, without intending to be limited by theory, the present inventors have discovered a manner in which a gypsum board can have a lower shrinkage when exposed to high temperatures.

As determined herein, the shrinkage can be measured via thermal mechanical analyzer (TMA). The test is conducted using the E119 ramp rates with a sample size of 5 mm by 5 mm by 11 mm (11 mm thickness). Using TMA, a change in dimension, in particular thickness can be determined and as measured herein, the shrinkage and thickness is based on the values at 950° C. Without the shrinkage-reducing additive as disclosed herein, a gypsum board (i.e., one containing gypsum, the shrinkage-reducing additive, and other additives) may have an average shrinkage of greater than 20%. However, by employing a shrinkage-reducing additive as disclosed herein, the gypsum board may exhibit a shrinkage of 20% or less, such as 16% or less, such as 15% or less, such as 14% or less, such as 12% or less, such as 10% or less, such as 8% or less, such as 6% or less, such as 5% or less. The gypsum board may have an average shrinkage of greater than 0%, such as 0.5% or more, such as 1% or more, such as 2% or more, such as 3% or more, such as 5% or more, such as 10% or more. Such percentages are based on the initial thickness.

As another means for determining the effect of the shrinkage-reducing additive on gypsum, the shrinkage can be measured by determining an area shrinkage (i.e., of a face instead of a thickness). For such area shrinkage, it can be determined utilizing a cast gypsum bar (i.e., one containing only gypsum and the shrinkage-reducing additive) having dimensions of 1″×1″×11.25″ (face or back of 1″×11.25″ and a thickness of 1″), drying the bar at 45° C. until a constant mass is obtained, and conditioning the bar at 70° F. and 50% RH for 12 hours. After conditioning, the bars are placed into a muffle furnace and quickly ramped according to ramp rates as defined in ASTM E119 to a temperature of about 950° C. The percentage change in shrinkage is determined by comparing the area after heating with the initial 1″×11.25″ area.

In this regard, as illustrated in Table 2 and 3 of the examples, a conventional gypsum bar not containing a shrinkage-reducing additive as disclosed herein or any other additives may have an area shrinkage of 14-17%. Meanwhile, shrinkage-reducing additives may be employed to provide a reduction in the amount of shrinkage. For instance, such shrinkage-reducing additives may provide a gypsum board or bar that exhibits an area shrinkage of 20% or less, such as 17% or less, such as 15% or less, such as 14% or less, such as 13% or less, such as 12% or less, such as 10% or less, such as 8% or less, such as 6% or less, such as 4% or less, such as 3% or less. The area shrinkage may be 1% or more, such as 2% or more, such as 3% or more, such as 5% or more, such as 7% or more, such as 9% or more. Such percentages are based on the initial area.

The gypsum board may be produced according to any method generally known in the art. In this regard, the general method of making a gypsum board may not necessarily be limited by the present invention.

In general, the gypsum board includes a gypsum core that is produced from a gypsum slurry that is prepared as a mixture of dry and wet ingredients. The dry ingredients include primarily calcium sulfate hemihydrate and may include one or more additives that are known in the art, such as fiberglass, set accelerators, fillers (e.g., vermiculite), crystal modifiers (e.g., boric acid) and/or binders (e.g., starch).

The wet ingredients may include water and may include one or more additional components that are known in the art, such as paper pulp, potash, dispersants, set retarders, polymers, wax emulsion, silicone or siloxanes, surfactants, and thickening agents. Additional “gauging water” may be added to the gypsum slurry to achieve the desired flowability of the slurry. The dry ingredients, pulp paper solution and gauging water may comprise the basic chemical components of the gypsum core. In addition, it should be understood that the water to stucco (i.e., calcined gypsum) ratio is not necessarily limited. For instance, while it may be greater than 1 in one embodiment, in another embodiment, it may be 1 or less, such as 0.9 or less.

The dry ingredients are typically mixed in a high speed mixing apparatus. The premix of dry ingredients, the wet ingredients and any gauging water are combined in a mixer (e.g., a pin mixer) to form the gypsum slurry. The gypsum slurry is commonly output from the mixer to a canister and then discharged through an outlet chute or “boot”.

An aqueous foam may also be added to the gypsum slurry, to enhance the fluidity of the slurry and control the core density and weight of the gypsum board. The foam may comprise a mixture of foam water, a foaming agent (e.g., soap) and air, and can be pre-generated using various mechanical foam generation devices that are known in the art. The foam may be added to the gypsum slurry in the mixer or in the canister where additional mixing may occur.

Thereafter, a boot deposits and spreads the gypsum slurry onto a moving, continuous sheet of bottom (back) facing material. A moving, continuous sheet of top (face) facing material is placed on the gypsum slurry, to sandwich the slurry between the top and bottom facing materials and form the board. The board passes through a forming station which shapes the board into the desired thickness and width. Although the facing material is described as paper, other materials known in the art may be used as a facing material, such as fiberglass mat.

The board travels along a belt line for several minutes, during which time the stucco and water rapidly undergo a rehydration reaction and the board stiffens and sets into a solid form. In the rehydration reaction, the calcium sulfate hemihydrate is hydrated to form crystals of calcium sulfate dihydrate, according to the following equation:CaSO₄·0.5H₂O+1.5H₂O→CaSO₄·2H₂O+heat

The boards are then cut into a desired length and dried in a continuous kiln to evaporate excess (free) water, while the chemically bound water is retained in the newly formed gypsum crystals. In this regard, the gypsum board now contains an interlocking matrix of gypsum that is formed from stucco, water, and various other additives

In general, the fire resistance of gypsum board is derived from the loss of crystalline water present in the gypsum molecular structure. Gypsum contains two moles of crystalline water in its molecular structure, or approximately 21% water by weight, depending on purity. When exposed to high temperatures, gypsum goes through two principal dehydration reactions. In the first reaction, gypsum undergoes a dehydration reaction to form calcium sulfate hemihydrate.CaSO₄·2H₂O+Heat→CaSO₄·0.5H₂O+1.5H₂O  (Reaction 1) (Gypsum) (Hemihydrate (bassanite))

In the second reaction, the calcium sulfate hemihydrate is further dehydrated and converted to the soluble form of calcium sulfate anhydrite.CaSO₄·0.5H₂O+Heat→CaSO₄(AIII)+0.5H₂O  (Reaction 2) (Hemihydrate (bassanite)) (Soluble Anhydrite)

The dehydration reactions are endothermic reactions that absorb energy from the surroundings, usually in the form of heat. The endothermic reactions hold the gypsum board at a constant temperature until fully dehydrated, which slows the temperature rise of the gypsum during exposure to fire. The release of water in these dehydration reactions further delays the progress of fire.

Once the dehydration is completed, the temperature of the gypsum board increases and the calcium sulfate anhydrite transforms from the soluble form to the insoluble form (AII). This transformation is accompanied by a transition from a rhombohedral to an orthorhombic crystalline structure, which results in a denser mass and a dimensional contraction (shrinkage) of the gypsum board.

As discussed in Examples 1-4 below, FIGS. 1-3 provide information regarding the weight loss of a conventional gypsum sample as illustrated using DSC (FIG. 1), the time/temperature test in accordance with ASTM E119 (FIG. 2), and the change in dimension (i.e., thickness) as a function of temperature as determined according to TMA (FIG. 3). Meanwhile, FIG. 4 provides SEM images of gypsum crystal at various temperatures. These images show the transformation of gypsum crystals in a conventional gypsum boards a function of temperature. For instance, at 250° C., the gypsum crystals maintain their initial shape and size; however, the structure becomes more porous with increasing numbers of fissures and cracks typical of dehydration. At 450° C., the gypsum crystals maintain the needle-like shape and size, similar to the structure shown at 250° C.; however, the number of observed fissures and cracks is further increased. At 950° C., the gypsum crystals lose their needle-like morphology and transform into a smooth, dense surface. This morphology change is attributed to the sintering phenomenon.

The present inventors have discovered that by employing the shrinkage-reducing additives disclosed herein, such sintering phenomenon may be minimized. For instance, such shrinkage-reducing additives can be incorporated in order to minimize the extent to which the gypsum crystals are sintered. In addition, the inclusion of such shrinkage-reducing additive can allow for the formation of a defect in the crystal growth. As a result, the gypsum crystals are able to maintain their integrity for a longer period of time at higher temperatures without resulting in excessive shrinkage which in turn would result in failure of the board at such high temperatures and when tested in accordance with ASTM E119. Thus by minimizing the shrinkage, the gypsum board can have improved fire resistance as well as improved mechanical integrity and strength.

As a result, the specific surface area of the gypsum at a temperature of 950° C. may be about 40% or more, such as about 50% or more, such as about 70% or more, such as about 80% or more, such as about 90% more, such as about 100% or more, such as about 125% or more, such as about 150% or more, such as about 200% more the specific surface area of the gypsum at a temperature of 22° C. Without such shrinkage-reducing additive, the specific surface area of the gypsum at a temperature of 950° C. may be less than 40% of the specific surface area of the gypsum at a temperature of 22° C.

In order to provide the desired effect, the shrinkage-reducing additive may have a particular size. For instance, the shrinkage-reducing additive may have a particle size of 200 micrometers or less, such as 150 micrometers or less, such as 100 micrometers or less, such as 75 micrometers or less, such as 50 micrometers or less, such as 40 micrometers or less, such as 25 micrometers or less, such as 15 micrometers or less, such as 10 micrometers or less, such as 5 micrometers or less, such as 1 micrometer or less, such as about 900 nanometers or less, such as about 800 nanometers or less, such as about 600 nanometers or less, such as about 500 nanometers or less, such as about 300 nanometers or less, such as about 200 nanometers or less, such as about 100 nanometers or less, such as about 50 nanometers or less. The shrinkage-reducing additives may have a particle size of 5 nanometers or more, such as 10 nanometers or more, such as 20 nanometers or more, such as 30 nanometers or more, such as 40 nanometers or more, such as 50 nanometers or more, such as 100 nanometers or more, such as 250 nanometers or more, such as 500 nanometers or more, such as 750 nanometers or more, such as 1 micrometer or more, such as 5 micrometers or more, such as 10 micrometers or more, such as 20 micrometers or more, such as 50 micrometers or more, such as 100 micrometers or more. In one embodiment, such particle size may be an average particle size of the shrinkage-reducing additive.

In one embodiment, in order to provide a shrinkage-reducing additive at the gypsum crystal interface, the shrinkage-reducing additive may have a particular size. In this regard, the shrinkage-reducing additive may be one having a particle size of less than 1 micrometer, such as about 900 nanometers or less, such as about 800 nanometers or less, such as about 600 nanometers or less, such as about 500 nanometers or less, such as about 300 nanometers or less, such as about 200 nanometers or less, such as about 100 nanometers or less, such as about 50 nanometers or less. The shrinkage-reducing additives may have a particle size of 5 nanometers or more, such as 10 nanometers or more, such as about 20 nanometers or more, such as about 30 nanometers or more, such as about 40 nanometers or more, such as about 50 nanometers or more, such as about 100 nanometers or more, such as about 250 nanometers or more. In one embodiment, such particle size may be an average particle size of the shrinkage-reducing additive.

As indicated herein, a shrinkage-reducing additive is employed in the gypsum board. The shrinkage-reducing additive may include a metal salt of an acid, a silica, or any combination thereof. In one embodiment, the shrinkage-reducing additive may include a metal salt of an acid. In another embodiment, the shrinkage-reducing additive may include a silica.

In one embodiment, the shrinkage-reducing additive may include a metal salt of an acid, such as an organic acid. For instance, the salt may be a sulfate, a carbonate, a phosphate, or a mixture thereof. In one embodiment, the salt may be a sulfate. In another embodiment, the salt may be a carbonate. In another embodiment, the sale may be a phosphate.

In addition, the metal may be an alkaline earth metal or a transition metal. In one embodiment, the metal may be an alkaline earth metal. In another embodiment, the metal may be a transition metal. For instance, the metal may include, but is not limited to, strontium, barium, iron, copper, nickel, titanium, zirconium, manganese, cobalt, silver, aluminum, etc. In one embodiment, the metal may include barium, iron, or a mixture thereof. In one particular embodiment, the metal may include barium. In another particular embodiment, the metal may include iron.

In one embodiment, the shrinkage-reducing additive may be a metal sulfate. In particular, the shrinkage-reducing additive may include barium sulfate, iron sulfate, or a mixture thereof In one particular embodiment, the shrinkage-reducing additive may include barium sulfate. In another particular embodiment, the shrinkage-reducing additive may include iron sulfate. In one embodiment, the shrinkage-reducing additive does not include calcium sulfate.

In one embodiment, the shrinkage-reducing additive may be a metal carbonate. In particular, the shrinkage-reducing additive may include barium carbonate, iron carbonate, or a mixture thereof. In one particular embodiment, the shrinkage-reducing additive may include barium carbonate. In another particular embodiment, the shrinkage-reducing additive may include iron carbonate.

In one embodiment, the shrinkage-reducing additive may be a metal phosphate. In particular, the shrinkage-reducing additive may include barium phosphate, iron phosphate, or a mixture thereof. In one particular embodiment, the shrinkage-reducing additive may include barium phosphate. In another particular embodiment, the shrinkage-reducing additive may include iron phosphate.

In one embodiment, the shrinkage-reducing additive may be formed from a precursor shrinkage-reducing additive. For instance, the precursor shrinkage-reducing additive may be provided to the slurry and undergo a reaction to form the shrinkage-reducing additive. In one embodiment, the precursor shrinkage-reducing additive may include a metal halide. For instance, the metal of the metal halide may be any of the aforementioned metals referenced above. The halide may generally be any halide. The halide may be any halide, such as fluoride, chloride, bromide, iodide, or a mixture thereof. However, in one embodiment, the halide may be a chloride.

In one embodiment, when the shrinkage-reducing additive is a metal sulfate, such sulfate may be provided in the gypsum board via the precursor shrinkage-reducing additive. Such precursor shrinkage-reducing additive may be a metal halide wherein the metal of the metal halide is the same as the metal of the metal sulfate. In this regard, the metal halide may be provided to the slurry and via the existing sulfate (e.g., calcium sulfate) present within the slurry, a reaction may occur between the metal halide and the calcium sulfate to yield the metal sulfate shrinkage-reducing additive. For instance, a barium chloride precursor shrinkage-reducing additive may be employed to react with the calcium sulfate present in the slurry to yield barium sulfate. As another example, an iron chloride precursor shrinkage-reducing additive may be employed to react with the calcium sulfate present in the slurry to yield an iron sulfate.

The present inventors have discovered that such method allows for the synthesis of a precipitated metal sulfate. Such reaction allows for the synthesis of precipitated metal sulfate having a much smaller particle size than conventional and readily available metal sulfates, which have particle sizes in the micrometer-scale range. In addition, conversion of such conventional and readily available metal sulfates to such a nanometer-scale as required may not be practical. In this regard, the present inventors have discovered that providing a shrinkage-reducing additive via the aforementioned means can allow for even a further reduction in the amount of shrinkage.

In one embodiment, the shrinkage-reducing additive may be a silica. The silica may be a colloidal silica, a precipitated silica, or a mixture thereof. In one embodiment, the silica may be a colloidal silica. In another embodiment, the silica may be a precipitated silica. Without intending to be limited by theory, the silica may be chemically inert unlike the formation of the aforementioned metal salts of the acids which may be formed via an in situ reaction by the incorporation of a precursor shrinkage-reducing additive, such as a metal halide.

In one embodiment, the shrinkage-reducing additive may be colloidal silica. As generally known in the art, colloidal silica may be a suspension of fine, amorphous, generally spherical silica particles. Generally, colloidal silica may also be nonporous. Generally, the silica particles in the colloidal silica may be monodisperse with respect to particle size. For instance, the deviation in particle size may be 10% or less, such as 5% or less, such as 3% or less, such as 2% or less, such as 1% or less. In one embodiment, the colloidal silica may have a pH of 7 or more. The colloidal silica may have a solids content of 75% or less, such as 60% or less, such as 50% or less, such as 45% or less, such as 35% or less, such as 30% or less and 5% or more, such as 10% or more, such as 20% or more, such as 30% or more, such as 40% or more.

In one embodiment, the shrinkage-reducing additive may be precipitated silica. As generally known in the art, precipitated silica is produced by precipitation from a solution containing silicate salts. Generally, precipitated silica may be porous and may be present as an agglomerate. The precipitated silica may have a D50 of 500 nanometers or more, such as 1 micrometer or more, such as 2 micrometers or more, such as 3 micrometers or more, such as 5 micrometers or more, such as 10 micrometers or more, such as 15 micrometers or more, such as 25 micrometers or more to 100 micrometers or less, such as 75 micrometers or less, such as 50 micrometers or less, such as 40 micrometers or less, such as 25 micrometers or less, such as 20 micrometers or less, such as 15 micrometers or less, such as 10 micrometers or less, such as 5 micrometers or less, such as 4 micrometers or less, such as 1 micrometer or less. Such aforementioned D50, in one embodiment, may refer to the agglomerate size of the precipitated silica.

In addition to the above, the shrinkage-reducing additive may be one that does not burn at a temperature of 400° C. or more, such as about 500° C. or more, such as about 700° C. or more, such as about 900° C. or more.

The shrinkage-reducing additives may be present in an amount of about 20 wt. % or less, such as about 15 wt. % or less, such as about 10 wt. % or less, such as about 8 wt. % or less, such as about 5 wt. % or less, such as about 4 wt. % or less, such as about 3 wt. % or less, such as about 2 wt. % or less, such as about 1 wt. % or less, based on the weight of the stucco (i.e., calcined gypsum). The shrinkage-reducing additives may be present in an amount of about 0.05 wt. % or more, such as about 0.1 wt. % or more, such as about 0.25 wt. % or more, such as about 0.5 wt. % or more, such as about 0.75 wt. % or more, such as about 1 wt. % or more, such as about 2 wt. % or more, such as about 4 wt. % or more, such as about 5 wt. % or more, based on the weight of the stucco (i.e., calcined gypsum). In one embodiment, the shrinkage-reducing additive may be present in a final gypsum board in the aforementioned amounts wherein the amount is based on the amount of gypsum in the board.

It should be understood that such shrinkage-reducing additives may be used alone or in any combination. For instance, combinations may be effective at providing a synergistic effect to reduce the shrinkage.

In addition, it should be understood that the point of the process in which the shrinkage-reducing additives are incorporated is not necessarily limited. That is, the shrinkage-reducing additives may be incorporated into a gypsum slurry at various steps in the manufacturing process. In addition, where multiple shrinkage-reducing additives are employed, such shrinkage-reducing additives may be added to the gypsum slurry at the same or at different steps of the manufacturing process. However, in one preferred embodiment, the shrinkage-reducing additives are added to the gypsum slurry by mixing with the dry ingredients. For instance, such shrinkage-reducing additives may be provided in a mixer of the manufacturing process. Alternatively, the shrinkage-reducing additives may be added in the mill.

The gypsum board produced according to the method disclosed herein may have any combination of physical and/or mechanical properties as desired for the particular application. In this regard, the gypsum board disclosed herein is not necessarily limited by weight, density, or other mechanical properties (e.g., nail pull strength, flexural strength, core hardness, humidified bond load, etc.).

The board may have any weight as generally employed in the art. For instance, the board may have a weight of about 2500 lbs/MSF (lbs per one thousand square feet) or less, such as about 2400 lbs/MSF or less, such as about 2200 lbs/MSF or less, such as about 2000 lbs/MSF or less, such as about 1800 lbs/MSF or less, such as about 1500 lbs/MSF or less, such as about 1300 lbs/MSF or less. The board may have a weight of about 1000 lbs/MSF or more, such as about 1200 lbs/MSF or more, such as about 1400 lbs/MSF or more, such as about 1600 lbs/MSF or more, such as about 1800 lbs/MSF or more, such as about 2000 lbs/MSF or more. By about, the weight may vary by +/−5%, such as +/−3%, such as +/−2%, such as +/−1%.

The board may be any thickness as generally employed in the art. For instance, in one embodiment, the board may have a thickness of about ¼″. In another embodiment, the board may have a thickness of about ⅜″. In another embodiment, the board may have a thickness of about ½″. In a further embodiment, the board may have a thickness of about ⅝″. By about, the thickness may vary by +/−5%, such as +/−3%, such as +/−2%, such as +/−1%.

In one embodiment, the hardness (as determined in accordance with ASTM C473), may be about 9 lb_f or more, such as about 10 lb_f or more, such as about 11 lb_f or more, such as 12 lb_f or more, such as 13 lb_f or more. The hardness may be about 50 lb_f or less, such as about 40 lb_f or less, such as about 30 lb_f or less, such as about 25 lb_f or less, such as about 20 lb_f or less, such as about 18 lb_f or less, such as about 15 lb_f or less. The aforementioned hardness describes the core hardness, the end hardness, or both. In one embodiment, the aforementioned hardness describes the core hardness.

In one embodiment, the nail pull strength (as determined in accordance with C473), may be about 70 lb_f or more, such as about 75 lb_f or more, such as about 77 lb_f or more, such as about 80 lb_f or more, such as about 85 lb_f or more, such as about 90 lb_f or more, such as about 100 lb_f or more. The nail pull strength may be about 120 lb_f or less, such as about 110 lb_f or less, such as about 100 lb_f or less, such as about 90 lb_f or less, such as about 80 lb_f or less.

In one embodiment, the flexural strength in the perpendicular direction (as determined in accordance with ASTM C473) may be about 90 lb_f or more, such as about 100 lb_f or more, such as about 110 lb_f or more, such as about 130 lb_f or more, such as about 150 lb_f or more, such as about 160 lb_f or more, such as about 175 lb_f or more. The flexural strength in the perpendicular direction may be about 250 lb_f or less, such as about 225 lb_f or less, such as about 200 lb_f or less, such as about 180 lb_f or less, such as about 175 lb_f or less, such as about 150 lb_f or less, such as about 125 lb_f or less.

In one embodiment, the flexural strength in the parallel direction (as determined in accordance with ASTM C473) may be about 20 lb_f or more, such as about 25 lb_f or more, such as about 30 lb_f or more, such as about 40 lb_f or more, such as about 50 lb_f or more, such as about 60 lb_f or more. The flexural strength in the parallel direction may be about 80 lb_f or less, such as about 70 lb_f or less, such as about 60 lb_f or less, such as about 50 lb_f or less, such as about 40 lb_f or less.

In addition, the gypsum board may be manufactured so as to pass the necessary UL tests for fire-resistance. For instance, the gypsum board may pass ASTM E119, which is the standard test method for a fire test of a building construction and materials.

EXAMPLES

Example 1

Dehydration of a Conventional Gypsum Board

The dehydration of a conventional gypsum board was tested by thermogravimetric analysis and differential scanning calorimetry, as shown in FIG. 1. The energy required for complete dehydration of gypsum is about 625 kJ/kg, which is consumed in the separation and evaporation of the water molecules from the gypsum crystals. The dehydration curve shows that endothermic Reactions 1 and 2 take place between about 80° C. to 220° C., with respective peaks at about 160° C. and about 190° C. Once dehydration is complete, the temperature of the gypsum sample continues to rise. A third peak is shown between about 350° C. to 380° C., which corresponds to the exothermic transformation from soluble a anhydrite (AIII) to insoluble β anhydrite (AII), and the transition in the gypsum crystal structure.

Example 2

Fire Test of Conventional Gypsum Board

The industrial fire testing standard ASTM E119 provides strict requirements for testing wallboard in a controlled testing assembly to ensure that products meet certain fire resistance standards.

A conventional gypsum board was subjected to an ASTM E119 fire test. FIG. 2 shows the board temperature profiles during gypsum dehydration for both the exposed and unexposed sides of the gypsum board in the wall assembly, along with the furnace temperature ramp. The temperature of the gypsum board does not increase appreciably during the dehydration reaction, particularly on the unexposed board side. However, the temperature increase resumes when the dehydration reaction is finished, as shown by the temperature rise on the exposed side of the gypsum board.

Example 3

Thermomechanical Analysis of Conventional Gypsum Board

A conventional gypsum board was subjected to thermomechanical analysis (TMA) to determine the dimensional change with increasing temperature. The dehydration reaction and the transformation from soluble to insoluble anhydrite results in a series of shrinkages, as shown in FIG. 3. The first shrinkage starts at about 120° C. and ends at about 200° C., as a result of the gypsum crystal dehydration. The shrinkage is about −0.23% from its original thickness, depending upon gypsum purity and impurities. The second shrinkage starts at about 350° C. and is about 10 times greater than the first shrinkage (−2.3% vs. −0.23%). This shrinkage is the result of the change from soluble to insoluble anhydrite and the accompanying change from rhombohedral to orthorhombic crystal structure. The third shrinkage, and the largest one, starts at about 600° C. and shows shrinkage of about −23% at about 950° C., which is a result of the sintering process the insoluble anhydrite goes through at higher temperatures.

Example 4

A conventional gypsum board was analyzed by scanning electron microscopy (SEM) at temperatures of 22° C., 250° C., 450° C., and 950° C., as shown in FIG. 4. At 250° C., the gypsum crystals maintain their initial shape and size; however, the structure becomes more porous with increasing numbers of fissures and cracks typical of dehydration. At 450° C., the gypsum crystals maintain the needle-like shape and size, similar to the structure shown at 250° C.; however, the number of observed fissures and cracks is further increased. At 950° C., the gypsum crystals lose their needle-like morphology and transform into a smooth, dense surface. This morphology change is attributed to the sintering phenomenon. X-ray diffraction (XRD) analysis indicates an orthorhombic crystalline structure, which is typically seen in insoluble anhydrite.

This transformation is further evidenced by a comparison of the specific surface area (SSA) of the gypsum samples, as shown in Table 1 below. At 22° C., the sample has an SSA of 1.330 m²/g. As the sample is heated, fissures and cracks are introduced, which increase in number and size with increased temperature. The SSA of the gypsum sample increases to 4.819 m²/g at 250° C. and to 15.595 m²/g at 450° C. At 950° C., the sintering process increases the sample density and thus reduces the surface area to 0.505 m²/g.

TABLE 1
Specific Surface Area of Conventional
Gypsum Based on Temperature
                       Specific Surface Area
Sample                 (m2/g)
Cube Heated to 22° C.  1.330
Cube Heated to 250° C. 4.819
Cube Heated to 450° C. 15.595
Cube Heated to 950° C. 0.505

Example 5

The shrinkage performance of cast gypsum was determined. The gypsum was a cast gypsum bar having dimensions of 1″×1″×11.25″ (face or back of 1″×11.25″ and a thickness of 1″). The bar was dried at 45° C. until a constant mass was obtained. The bar was then conditioned at 70° F. and 50% RH for 12 hours. After conditioning, the bars were placed into a muffle furnace and quickly ramped according to ramp rates as defined in ASTM E119 to a temperature of about 950° C. All of the samples were dimensionally measured before and after heating where the initial 1″×11.25″ area was compared to the post heating area.

Table 2 shows the shrinkage-reducing additives, concentration of such shrinkage-reducing additives, and the percentage of area shrinkage that occurred for these shrinkage-reducing additives.

TABLE 2
Shrinkage performance of a gypsum bar with
various shrinkage-reducing additives
	Amount	Shrinkage
	(%, w/w based	area
Additive	on stucco)	(%)
None (Control)	n/a	14-16
Barium chloride	0.9	2.9
Barium carbonate	0.9	13.2
Barium sulfate	1.0	16.3
Aluminum phosphate	1.9	6.6
Iron chloride	1.0	6.3
Iron sulfate	1.0	14.7
Colloidal	12 nm, 40% solids	1.0	7.9
silica	12 nm, 25% solids	1.0	4.6
	22 nm, 34% solids	1.0	5.8
Precipitated	D50 of 18 μm	1.0	8.9
silica	D50 of 4 μm	1.0	8.2
	D50 of 2.5 μm	1.0	8.5

Example 6

The shrinkage performance of a gypsum board containing gypsum, a shrinkage-reducing additive, and at least one other additive was determined. The gypsum board was analyzed to determine the area shrinkage as described in Example 5 or via thermomechanical analysis to determine a dimensional (i.e., thickness) change with increasing temperature. The gypsum had a sample size of 5 mm by 5 mm by 11 mm. The sample was dried at 45° C. until a constant mass was obtained. The bar was then conditioned at 70° F. and 50% RH for 12 hours. The ramp rate was the E119 ramp rate.

Table 3 shows the shrinkage-reducing additives, concentration of such shrinkage-reducing additives, and the percentage of shrinkage that occurred for these shrinkage-reducing additives.

TABLE 3
Shrinkage performance of a gypsum board
with various shrinkage-reducing additives
		Amount	Area	Thickness
		(%, w/w based	Shrinkage	Shrinkage
	Additive	on stucco)	(%)	(%)
	None (Control 1)	n/a	16.34	—
	None (Control 2)	n/a	—	29
	Barium chloride	0.4	13.89	—
		1.0	11.68	—
		2.0	7.18	—
		5.0	13.20	—
	Colloidal silica	1.0	—	12

FIG. 5 includes a Thermal Mechanical Analyzer (TMA) plot of the percent change in dimension and temperature of a gypsum sample and a sample with 1% colloidal silica. As can be observed, a significant dimensional change for the gypsum control occurs before 950° C. Meanwhile, with the colloidal silica, such change is shifted to a higher temperature. Furthermore, FIG. 6 provides an SEM image of the gypsum crystals containing colloidal silica.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.

Claims

1. A method for manufacturing a gypsum board, the method comprising: providing a gypsum slurry comprising stucco and a shrinkage-reducing additive comprising a silica comprising colloidal silica, precipitated silica, or a mixutre thereof onto a bottom facing material, wherein the shrinkage-reducing additive is present in an amount of 0.05 wt. % or more to 2 wt. % or less based on the weight of the stucco, andproviding a top facing material on the gypsum slurry,wherein the gypsum board exhibits an area shrinkage of 10% or less, andwherein the gypsum board exhibits a nail pull strength of 70 lbf or more.

2. The method according to claim 1, wherein the silica comprises colloidal silica.

3. The method according to claim 1, wherein the silica comprises precipitated silica.

4. The method according to claim 1, wherein the shrinkage-reducing additive has a particle size of 5 nanometers or more to 100 micrometers or less.

5. The method according to claim 1, wherein the gypsum board exhibits an area shrinkage of 8% or less.

6. The method according to claim 1, wherein the shrinkage-reducing additive is present in an amount of 0.05 wt. % or more to 1 wt. % or less.

7. The method according to claim 1, wherein the shrinkage-reducing additive has an average particle size of 5 nanometers or more to 100 nanometers or less.

8. A gypsum board comprising: gypsum and a shrinkage-reducing additive comprising a silica wherein the shrinkage-reducing additive is present in an amount of 0.05 wt. % or more to 2 wt. % or less based on the weight of the gypsum, wherein the silica is from colloidal silica, precipitated silica, or a mixutre thereof,wherein the gypsum board exhibits an area shrinkage of 10% or less, andwherein the gypsum board exhibits a nail pull strength of 70 lbf or more.

9. The gypsum board according to claim 8, wherein the silica comprises colloidal silica.

10. The gypsum board according to claim 8, wherein the silica comprises precipitated silica.

11. The gypsum board according to claim 8, wherein the shrinkage-reducing additive has a particle size of 5 nanometers or more to 100 micrometers or less.

12. The gypsum board according to claim 8, wherein the gypsum board exhibits an area shrinkage of 8% or less.

13. The gypsum board according to claim 8, wherein shrinkage-reducing additive is present in an amount of 0.05 wt. % or more to 1 wt. % or less.

14. The gypsum board according to claim 8, wherein the shrinkage-reducing additive has an average particle size of 5 nanometers or more to 100 nanometers or less.