RU2789870C2 - Plasterboard of gypsum, containing high level of chloride salt and starch layer, as well as related method - Google Patents

Abstract

FIELD: building materials.

SUBSTANCE: group of inventions relates to plasterboard formed of synthetic gypsum and other gypsum sources having high concentrations of chloride salt, a method for the manufacture of plasterboard, and a wall system, in which plasterboard is used. The method for the manufacture of plasterboard includes preparation of an aqueous suspension containing a mixture of water and gypsum wall plaster. Moreover, gypsum wall plaster contains calcium sulfate semi-hydrate. At the same time, the aqueous suspension contains a mixture of at least 60% by wt. of the specified calcium sulfate semi-hydrate on a dry base, from about 500 wt.pt. to about 3000 wt.pt. of chloride-anions per 1,000,000 parts by weight (wt.pt.) of the specified calcium sulfate semi-hydrate and water. The mass ratio of water to calcium sulfate semi-hydrate is from 0.2:1 to 1.2:1. The aqueous suspension is distributed between a front paper covering sheet and a rear covering sheet. Moreover, each covering sheet has an inner surface and an outer surface. At the same time, the inner surface of at least one of the front paper covering sheet and the rear paper covering sheet is covered with a starch layer. Moreover, the gypsum suspension is in contact with the starch layer. After curing of calcium sulfate semi-hydrate with formation of a panel containing a gypsum core, the panel is dried and cut into gypsum plates. The wall system contains a frame, to which at least one specified plate of plasterboard is fixed. At the same time, the outer surface of the front paper covering sheet faces away from the frame.

EFFECT: increase in the efficiency of the manufacture of plasterboard, improvement of adhesion between a core of a plate and paper covering sheets.

10 cl, 7 dwg, 2 tbl

Description

FIELD OF TECHNOLOGY

[001] This invention relates to drywall formed from synthetic gypsum and other sources of gypsum having high concentrations of chloride salt. The present invention also provides a method of making gypsum board with a starch layer to improve the adhesion of the gypsum board core to the paper cover sheet compared to gypsum board without a starch layer, as well as a wall system where gypsum board is used.

BACKGROUND OF THE INVENTION

[002] In the construction of buildings, one of the most common building elements for construction and renovation is gypsum wallboard, often referred to as dry plaster, drywall, gypsum panels, gypsum paneling, and ceiling tiles. From a chemical point of view, gypsum is calcium sulfate dihydrate (CaSO ₄ • 2H ₂ O).

[003] Set gypsum (calcium sulfate dihydrate) is a well-known material that is used in such products. Set gypsum boards are often referred to as gypsum boards which comprise a board core (set gypsum core) sandwiched between two cover sheets, in particular between paper cover sheets. Such panels are usually used in the construction of plasterboard interior walls and ceilings of buildings. One or more denser areas, often referred to as "surface layers", may be applied in layers to any face of the board core, typically at the interface between the board core and the inner surface of the cover sheet or its coating. The denser areas may be adjacent to the less dense area of the gypsum layer that forms the gypsum layer of the drywall core.

[004] In the drywall manufacturing process, stucco (containing calcium sulfate hemihydrate), water, and other ingredients, if necessary, can be mixed, typically in a mixer, to form an aqueous gypsum slurry. Prior art terms: aqueous gypsum slurry, or aqueous slurry, or gypsum slurry are generally used for slurry both before and after the conversion of calcium sulfate hemihydrate to calcium sulfate dihydrate. The gypsum slurry is formed and discharged from the mixer onto a moving conveyor transporting a first cover sheet, optionally covered with a skim coat. If present, a top coat is applied up to the location where the gypsum slurry is discharged onto the first cover sheet. After applying the gypsum slurry to the first cover sheet, the gypsum slurry is overlaid with a second cover sheet, again optionally coated, to form a multi-layer assembly having the desired thickness. A forming plate, roller, and similar devices can assist in curing the desired thickness. The gypsum slurry is then left to set, forming set ( i.e., rehydrated) gypsum by reacting the calcined gypsum with water to form a matrix of crystalline hydrated gypsum ( i.e., calcium sulfate dihydrate, also known as set gypsum). Hydration of calcined gypsum is a necessary condition for the formation of a cohesive matrix of hardened gypsum crystals, which as a result gives strength to drywall. Heat may be applied to remove the remaining free ( ie, unreacted) water and obtain a dry product (eg using an oven). Thereafter, the set gypsum product is cut to form drywall boards having the desired length.

[005] Gypsum (calcium sulfate dihydrate and any impurities) suitable for use in wallboard can be obtained from both natural sources and synthetic sources, and then subjected to further processing.

[006] Natural gypsum can be used by calcining its calcium sulfate dihydrate to form a hemihydrate. Gypsum from natural sources is a mineral of natural origin and can be mined as hard rock. Gypsum of natural origin is a mineral commonly found in old salt lake deposits, in volcanic deposits and in clayey formations. When mined, raw gypsum is generally in the dihydrate form. Gypsum is also known as calcium sulfate dihydrate, calcium sulfate (terra alba) or natural gypsum (landplaster). This material is also obtained as a by-product of various industrial processes. For example, synthetic gypsum is a by-product of flue gas desulfurization processes in power plants. In gypsum, approximately two molecules of water are bound to each molecule of calcium sulfate.

[007] Plaster of Paris is also known as calcined gypsum, stucco, or calcium sulfate hemihydrate.

[008] In the event that calcium sulfate dihydrate from any source is heated sufficiently in a process called roasting or calcining, then the water of hydration is at least partially distilled off and calcium sulfate hemihydrate (CaSO ₄ •½H ₂ O) can be formed (as typically found in a material commonly referred to as "stucco" or calcium sulfate anhydrite (CaSO ₄ ), depending on temperature and duration of exposure. As used herein, the terms "stucco" and "calcined gypsum" refer to both the hemihydrate and anhydrite forms of calcium sulfate that they may contain. Calcination of gypsum to obtain a hemihydrate form occurs according to the following equation:

CaSO ₄ •2H ₂ O→CaSO ₄ •0.5H ₂ O + 1.5H ₂ O

[009] Calcined gypsum is capable of reacting with water to form calcium sulfate dihydrate, which is a solid product and is referred to herein as "set gypsum".

[0010] Gypsum can also be produced synthetically (referred to in the art as "singip") 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 above 150 °C, to form stucco ( i.e., calcined gypsum in the form of calcium sulfate hemihydrate and/or calcium sulfate anhydrite), which can be subsequently rehydrated to form hardened gypsum in the desired shape, such as slabs.

[0011] Synthetic gypsum obtained from power plants is generally suitable for use in gypsum panels intended for construction projects. Synthetic gypsum is a by-product of flue gas desulfurization processes from power plants (also known as desulfurization gypsum or desulfurized gypsum or DSG). Specifically, flue gas containing sulfur dioxide is scrubbed with lime or limestone, resulting in the formation of 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+½O ₂ →CaSO ₄ •2H ₂ O

The hemihydrate form can then be obtained by calcination in a similar manner to that used for natural gypsum.

[0012] However, many traditional coal-fired power plants are being closed in favor of cleaner energy sources. The shutdown of coal-fired power plants has led to a growing shortage of synthetic gypsum suitable for the production of gypsum panels. Lower quality synthetic gypsum is available from power plants and other sources, but this gypsum from alternative sources often contains fairly high concentrations of foreign salts, in particular magnesium or sodium salts, more specifically magnesium chloride and sodium chloride. Small amounts of potassium chloride and calcium chloride may also be present in synthetic gypsum from alternative sources. Foreign salts can be problematic due to their tendency to reduce adhesion between the board core and the cover sheets, in particular the back paper cover sheet.

[0013] It should be understood that this description of the prior art is provided by the inventors for the benefit of the reader, and should not be construed as a reference to the prior art or as an indication that any of these problems are of paramount importance in the art. Although the principles described may, in some respects and in some embodiments of the invention, overcome the problems inherent in other systems, it should be understood that the scope of protection of an innovation is determined by the appended claims, and not by the ability of the claimed invention to solve any particular problem noted in this document.

BRIEF DESCRIPTION OF THE INVENTION

[0014] The invention relates to gypsum boards having substantial adhesion between the board core and one or more cover paper sheets even when significant amounts of foreign salts, in particular chloride salts, and more particularly NaCl, KCl, MgCl ₂ and/or _CaCl2 . Methods are proposed for the production of gypsum boards from saline sources of gypsum, in particular from low quality synthetic gypsum. Improving the adhesion between the board core and the paper cover sheets can be realized by providing a layer of starch applied to the inner surface of at least one of the paper cover sheets, in particular the back paper cover sheet.

[0015] Thus, in one aspect, the invention provides gypsum boards comprising a board core containing set gypsum. Typically, it also includes one or more chloride salts, such as NaCl, KCl, MgCl ₂ or CaCl ₂ or any combination thereof.

[0016] In particular, the invention provides drywall containing:
the core layer of the slab containing the hardened gypsum,

a front paper cover sheet having an outer surface and an inner surface, the inner surface being in contact with the first face of the board core; And
a back paper cover sheet having an outer surface and an inner surface, the inner surface being in contact with a second face of the board core;

wherein the inner surface of at least one of the front paper cover sheet and the back paper cover sheet is coated with a layer of starch;

wherein the core layer is located between the first cover sheet and the second cover sheet,

wherein the core layer is formed as a result of solidification between the front paper cover sheet and the back cover sheet of an aqueous slurry containing a mixture of water and stucco, and the stucco contains calcium sulfate hemihydrate, while the aqueous slurry contains:

at least 60% wt. the specified calcium sulfate hemihydrate on a dry (anhydrous) basis,

from about 500 ppm up to about 3000 ppm chloride anions per 1,000,000 parts by weight of said calcium sulfate hemihydrate, and

water at a mass ratio of water to calcium sulfate hemihydrate from 0.2:1 to 1.2:1.

[0017] The inner surface of the front paper cover sheet having an outer surface and an inner surface is in contact with the first face of the board core. The inner surface of the back paper cover sheet having an outer surface and an inner surface is in contact with the second face of the board core. The inside surface of at least one of the front paper cover sheet and the back paper cover sheet is coated with a layer of starch. More specifically, the outer surface of the rear paper cover sheet may be attached to the wall after the drywall has been installed, and the outer surface of the front paper cover sheet may face outward from the wall after the drywall has been installed. One or both of the cover sheets may include a paper cover sheet, which may be made from the same or different paper materials.

[0018] Calcium sulfate hemihydrate is present in the precipitated aqueous suspension according to the invention in an amount of at least 60% wt. dry (anhydrous) materials of aqueous suspension. Preferably, calcium sulfate hemihydrate is at least 70% wt. dry (anhydrous) materials of an aqueous suspension, more preferably at least 80% wt. dry (anhydrous) materials of an aqueous suspension, even more preferably at least 90% wt. dry (anhydrous) materials of aqueous suspension. In typical wallboard compositions according to the invention, dry (anhydrous) materials in aqueous suspension contain at least 90% wt. or at least 95% wt. calcium sulfate hemihydrate. The use of calcium sulfate anhydrite is also contemplated, although it is preferably used in small amounts of less than 20% by weight. dry (anhydrous) materials of aqueous suspension.

[0019] Similarly, the calcium sulfate dihydrate present in the core layer of the gypsum board of the invention as a result of solidification of the aqueous suspension is at least 60% wt. core, preferably not less than 70% wt. and more preferably at least 80% wt. In typical compositions of wallboards, dry (anhydrous) materials in aqueous suspension contain at least 90% wt. or at least 95% wt. calcium sulfate dihydrate.

[0020] In one or more other aspects of the invention, the invention provides methods for making drywall having significant amounts of one or more foreign salts in the core of the board. In gypsum boards, substantial adhesion between the board core and one or more cover sheets, in particular the backing paper cover sheet, can occur even when one or more foreign salts are present. In this way, gypsum board with a starch layer is obtained to improve the adhesion of the gypsum board core to the cover sheet compared to gypsum board without a starch layer.

[0021] With respect to the method of the invention, a method for making drywall is provided, comprising:

preparation of an aqueous slurry containing a mixture of water and stucco, wherein the stucco contains calcium sulfate hemihydrate, the aqueous slurry containing a mixture of:

at least 60% wt. said calcium sulfate hemihydrate on a dry basis,

from about 500 ppm up to about 3000 ppm chloride anions per 1,000,000 parts by weight (parts by weight) of said calcium sulfate hemihydrate, and

water at a mass ratio of water to calcium sulfate hemihydrate from 0.2:1 to 1.2:1; and

distributing the aqueous suspension between the front paper cover sheet and the back paper cover sheet to form a panel, each cover sheet having an inner surface and an outer surface;
wherein the inner surface of at least one of the front paper cover sheet and the back paper cover sheet is coated with a starch layer, the gypsum slurry being in contact with the starch layer;
solidifying calcium sulfate hemihydrate to form a panel containing a gypsum core containing calcium sulfate dihydrate; And
drying the panel; and cutting the panel into gypsum boards having one or more predetermined dimensions.

[0022] From about 500 ppm up to about 3000 ppm chloride anions per 1,000,000 parts by weight of said calcium sulfate hemihydrate means that per 1,000,000 parts by weight of said calcium sulfate hemihydrate, the amount of chloride anions present is from about 500 ppm. up to about 3000 ppm

[0023] The concentration of chloride anions in the aqueous slurry used to make drywall products and carry out the methods of this invention can range from about 500 ppm to about 500 ppm. up to about 3000 ppm, typically from about 1000 ppm up to about 3000 ppm, more typically from about 2000 ppm. up to about 3000 ppm per 1,000,000 parts by weight of calcium sulfate hemihydrate.

[0024] The chloride anions in the aqueous suspension used for the articles and methods of the invention may be from any source. In general, one or more chloride salts are present in the gypsum source used to form the core of the board of the invention. The source of the gypsum may be a source of synthetic gypsum, in particular a low quality synthetic gypsum obtained from a flue gas stream from a power plant. Thus, in general, the source of chloride anions is one or more chloride salts in the stucco used to prepare the aqueous slurry. Chloride anions are formed from the chloride atoms of one or more chloride salts in stucco when the stucco is used in aqueous slurries. Typically, one or more chloride salts are any of NaCl, KCl, MgCl ₂ , CaCl ₂ or any combination thereof. However, chloride anions can also arise from impurities, such as one or more chloride salts contained in the water used to prepare the gypsum slurry.

[0025] As a rule, dry (anhydrous) components of aqueous gypsum slurry are used for products and methods according to the invention and, as a result, the board core contains less than 10% wt. portland cement or other hydraulic cement, more typically does not contain them or contains any combination of them. As a rule, the aqueous suspension contains dry (anhydrous) components, and, as a result, the core of the plate contains less than 10% wt. fly ash, more typically, it is absent. As a rule, the aqueous suspension contains dry (anhydrous) components and, as a result, the core of the plate contains less than 10% wt. calcium carbonate, more typically absent.

[0026] For the purposes of this description, a dry base means an anhydrous base.

[0027] All average molecular weights, percentages, and ratios used herein are by weight ( i.e., wt %) unless otherwise indicated.

[0028] In one or more other aspects of the invention, a wall system is provided comprising a frame to which at least one drywall board of the invention is attached, with the outer surface of the front paper cover sheet facing away from the frame. In this wall system, drywall can be on an interior wall or on the ceiling of a building. As a rule, the frame is made of wood or metal. Typically, at least one drywall board is attached to the frame by one or more screws, nails, adhesives, or other mechanical fasteners.

[0029] The advantages of the present invention may become apparent to those skilled in the art from a review of the following detailed description taken in conjunction with the examples and the appended claims. However, it should be noted that while the invention is capable of various forms, this description is intended to be illustrative and not intended to limit the invention.

BRIEF DESCRIPTION OF GRAPHICS

[0030] In FIG. 1 illustrates a cross-sectional view of a gypsum board according to the invention, in which the core of the board (gypsum core) is located between a front paper cover sheet and a back cover sheet, at least one of which is coated with a layer of starch.

[0031] In FIG. 2 illustrates a plan view (axially) of the drywall of FIG. 1, which better shows the arrangement of the plurality of perforations in the back cover sheet.

[0032] In FIG. 3 illustrates a perspective view of drywall of the invention attached to one side of a wall with a metal partition stud suitable for the wall system of the invention.

[0033] In FIG. 4 illustrates photographs showing the effect of a dilute starch solution on a 12" x 12" x 1/2" (30.48 cm x 30.48 cm x 1.27 cm) gypsum board containing 1200 ppm. Cl ^- from MgCl ₂ per million parts of calcium sulfate hemihydrate.

[0034] In FIG. 5 illustrates photographs showing the effect of a dilute starch solution on a 12" x 12" x 1/2" gypsum board containing 2100 ppm. Cl ^- from MgCl ₂ per million parts of calcium sulfate hemihydrate.

[0035] In FIG. 6 illustrates photographs showing the effect of dilute starch solution and perforation on a 12" x 12" x 1/2" gypsum board containing 2000 ppm. Cl ^- from NaCl per million parts of calcium sulfate hemihydrate.

[0036] In FIG. 7 illustrates photographs showing the effect of a dilute starch solution on a 12" x 12" x 1/2" gypsum board containing 30 ppm. Cl ^- from NaCl per million parts of calcium sulfate hemihydrate.

DETAILED DESCRIPTION OF THE INVENTION

[0037] The present invention proposes the possibility of using gypsum containing relatively high amounts of foreign salts in the core of a drywall board. Under normal circumstances, high salt concentrations in the board core can lead to insufficient adhesion between the board core and at least one of the front paper cover sheet and the back cover sheet, in particular the back cover sheet.

Surprisingly, [0038] a layer of starch located on the inner surface(s) of the covering sheet(s) can improve adhesion to a board core made from an aqueous slurry of stucco containing calcium sulfate hemihydrate and a large amount of foreign salts. Preferably, the starch layer is a continuous layer of starch distributed over the entire inner surface of the back cover sheet. Optionally, another layer of starch is also distributed over the entire inner surface of the front cover sheet. One or both of the cover sheets may be paper cover sheets, which may be made from the same or different paper materials. Optionally, additional starch and/or other additives may be present in the board core or in the aqueous gypsum slurry used to form the board core.

[0039] The core of the board may comprise a less dense region (layer) and further comprises one or more regions (layers) of high density in contact with the inner surface of the front paper cover sheet or back paper cover sheet or with a starch layer applied thereon.

[0040] In the event that at least one layer of starch is present in combination with a board core containing a large amount of foreign salts, then any of these boards can benefit from improved adhesion between the board core and the cover sheet(s) along compared to a stove, which is the same but devoid of the starch layer. Accordingly, the present invention provides gypsum boards containing significant amounts of foreign salts in the core of the board, and methods for manufacturing such gypsum boards using a source of gypsum containing significant amounts of foreign salts. Thus, to form drywall, a layer of starch applied to at least one of the cover sheets may allow the use of lower quality gypsum sources containing an excessive amount of extraneous salts. Otherwise, such sources of gypsum may not be suitable for forming drywall with sufficient adhesion between the board core and the cover sheets. The present invention effectively solves this problem.

[0041] Drywall

[0042] In FIG. 1 illustrates a wall panel 10 of the invention in which a gypsum core 12 (e.g., 0.5 inch (1.27 cm) thick) is sandwiched between a back paper cover sheet (back cover sheet) 14 and a facing paper cover sheet (front cover sheet). 16 , each of which may be a single or multi-layer paper. The inner surface of the back paper cover sheet 14 forms the engagement side 24 of the back paper cover sheet 14 which faces the gypsum core. The inner surface of the facing paper cover sheet 16 forms the engagement side 26 which faces the gypsum core 12 . The outer surface of the back paper cover sheet 14 faces the wall frame (see Fig. 3) of the room after the wall panel 10 is installed as an interior wall. The outer surface of the facing paper cover sheet 16 faces the interior of the room after the wall panel 10 is installed as an interior wall.

[0043] The starch layer 20 lies on the clutch side 24 of the back sheet 14 . An optional starch layer 20 lies on the bonding side 26 of the facing paper cover sheet 16 . An optional thin, dense layer of gypsum 22 lies between the gypsum core 12 and the facing paper cover sheet 16 to be in contact with the gypsum core 12 and the facing paper cover sheet 16' . If a thin dense layer 22 of gypsum is present, then the starch layer 20 is between the thin dense layer 22 of gypsum and the facing paper cover sheet 16' . Another optional thin, dense layer of gypsum (not shown) may lie between the gypsum core 12 and the back paper cover sheet 14' . In general, the gypsum core 12 and the thin dense layer 22 of gypsum have the same composition and are adjacent to each other. However, the core slurry (water slurry) used to form the gypsum core 12 was foamed, and the water slurry used to form the thin dense gypsum layer 22 was not foamed, so the thin dense gypsum slurry is denser than the core slurry.

[0044] Surprisingly, the starch layer 20 has been found to increase the effectiveness of the adhesion between the gypsum core 12 and the back sheet 14 . Improved bonding performance can be especially evident when the gypsum has a high salt concentration.

[0045] A layer of starch is applied to the back paper in an amount in the range of about 0.5-15 lb/MSF (2.4-73.2 g/m ² ), preferably about 0.5-5 lb/MSF (2.4 -24.4 g/m ² ), more preferably about 1-2 lb/MSF (4.9-9.8 g/m ² ) (MSF = 1000 square feet). The starch is used as a binder and can be native (raw) starch, pregelatinized starch, or a combination of native starch and pregelatinized starch.

[0046] In general, the relatively low density inner region (gypsum core 12 ) and the relatively high density region (thin dense gypsum layer 22 ) have the same composition and are adjacent to each other. However, the inner low density region may be formed from the gypsum slurry in the foamed state, while the high density region may be formed from the gypsum slurry that is not foamed, thus forming a denser layer. That is, the high density region may have lower porosity associated with it than the interior low density region. Typically, a thin, dense layer of gypsum 22 is applied to the inner surface of the selected cover sheet. If a high density region is also present on the selected cover sheet, then the starch layer 20 is between the cover sheet and the high density region (thin dense layer 22 of gypsum).

[0047] One or more chloride salts may be present in the core 12 of the board of this invention. If a high density region (thin dense layer 22 of gypsum) is included, then one or more chloride salts may also be present there. In general, the one or more chloride salts include any chloride salts from stucco or other ingredient used to prepare an aqueous slurry for making board.

[0048] The concentration of chloride anions in the aqueous slurry used to make the board core 12 and, if present, the high density region (thin dense layer 22 of gypsum) can range from about 500 ppm. up to about 3000 ppm, typically from about 1000 ppm up to about 3000 ppm, more typically from about 2000 ppm. up to about 3000 ppm per 1,000,000 parts by weight of calcium sulfate hemihydrate.

[0049] Manufacturing Methods

[0050] Various methods can be used to make the drywall of this invention from an aqueous slurry of gypsum containing calcium sulfate hemihydrate and a large amount of chloride salt. However, instead of a conventional gypsum source, a gypsum source containing one or more chloride salts may be used in the manufacturing process as described above.

[0051] The main material from which gypsum wallboard and other gypsum products are made is the hemihydrate form of calcium sulfate (CaSO ₄ •½H ₂ O), commonly referred to as "calcined gypsum" or "plaster gypsum", which is obtained by thermal transformation of the dihydrate form calcium sulfate (CaSO ₄ ).

[0052] The invention encompasses a method for making drywall, including:

preparation of an aqueous slurry containing a mixture of water and stucco, wherein the stucco contains calcium sulfate hemihydrate, the aqueous slurry containing a mixture of:

at least 60% wt. said calcium sulfate hemihydrate on a dry basis,

from about 500 ppm up to about 3000 ppm chloride anions per 1,000,000 parts by weight (parts by weight) of said calcium sulfate hemihydrate, and

water at a mass ratio of water to calcium sulfate hemihydrate from 0.2:1 to 1.2:1; and

distributing the aqueous slurry between the front paper cover sheet and the back paper cover sheet to form a panel, each paper cover sheet having an inner surface and an outer surface;
moreover, the inner surface of at least one of the front paper cover sheet and the back paper cover sheet is covered with a layer of starch, while the gypsum suspension is in contact with the layer of starch;
solidifying calcium sulfate hemihydrate to form a panel containing a gypsum core containing calcium sulfate dihydrate; And
drying the panel; and cutting the panel into gypsum boards having one or more predetermined dimensions.

[0053] Exemplary manufacturing techniques and equipment suitable for forming drywall in accordance with this invention can be found, for example, in US patent 7364676 and US patent application publication 2010/0247937, each of which is incorporated herein in its entirety by reference . Briefly, such processes may include unloading the cover sheet onto a moving conveyor. Since drywall is usually formed "face down", this cover sheet corresponds to the facing paper cover sheet 16 at the completion of the manufacturing process. The gypsum slurry can be prepared at a suitable ratio of water to calcium sulfate hemihydrate for application to the cover sheet.

[0054] For the production of gypsum board, stucco is mixed with water and additives to form an aqueous slurry that is continuously fed between continuous layers of paper on a board machine. One paper cover sheet is called the front paper sheet or facing sheet, and the other paper cover sheet is called the back paper sheet or back sheet. As the slab moves down the conveyor line to form the panel, the calcium sulfate recrystallizes or rehydrates back to its original stone state. As the gypsum hardens, the paper becomes chemically and mechanically bonded to the core layer of the board. The panel is then cut to length and transported through dryers to remove free moisture.

[0055] The dry and/or wet components of the gypsum slurry are fed into a mixer ( eg , a stick mixer) where they are mixed to form a gypsum slurry. The mixer has a main body and an outlet (for example, a receiving hopper with an outlet gate, known in the art, or an alternative device, for example, described in US patents 6494609 and 6874930, which are incorporated herein in their entirety by reference). In some process configurations, the outlet may include a slurry distributor with a single feed opening or with multiple feed openings, such as those described in US Patent Application Publications 2012/0168527 and 2012/0170403, which are incorporated herein in their entirety by reference. When using a slurry distributor with multiple feed openings, the outlet may include a suitable flow divider, such as described in US Patent Application Publication 2012/0170403. A blowing agent (generally soap) may be added to the outlet of the mixer (for example, to the closure, as described, for example, in US Pat. ). The slurry discharged from the outlet after adding all the ingredients, including the blowing agent, is the main gypsum slurry and is used to form the core of the board. This gypsum slurry is discharged onto a moveable front cover sheet.

[0056] As described above, one or both of the cover sheets in drywall may optionally be in boundary contact with a high density area or layer of the board core, also known as a skim coat. After curing, the top coat can adhere to the core of the board. In embodiments where foam is introduced into the outlet port, the secondary gypsum slurry stream may be removed from the mixer body to provide a slurry to form a cover layer. A top coat, if present, may be applied to the moving front paper cover sheet prior to applying the bulk of the gypsum slurry to form the core of the board, with the top coat generally occurring before the mixer.

[0057] As described above, a layer of starch is present on at least one cover sheet, in particular on the back cover sheet, to increase adhesion thereto. The starch layer may be pre-applied to the inner surface of at least one of the front paper cover sheet and the back cover sheet. Alternatively, the methods may further include applying a layer of starch to the inside surface of at least one of the front paper cover sheet and the back cover sheet. The starch may be applied to the inner surface(s) as a continuous layer of starch as defined above. More specifically, starch may be applied to the inner surface(s) of the front paper cover sheet and/or back paper cover sheet in an amount of from about 0.5-15 lb/MSF, preferably about 0.5-5 lb/MSF. , more preferably about 1 lb/MSF to about 2 lb/MSF. Suitable methods for applying a layer of starch to either the back paper cover sheet or the front paper cover sheet may include, for example, spraying, smearing, inkjet printing, screen printing, screen printing, roller application, dip coating or similar methods.

[0058] After mixing, foam is optionally added to the aqueous slurry to reduce the density of the article. Foam is formed by combining soap and water. The foam is then injected into the aqueous slurry after it exits the mixer via a hose or chute. The frothing ring is a device having a plurality of holes arranged in a ring perpendicular to the axis of the hose, so that when passing through the frothing ring, foam is forced under pressure into an aqueous suspension. Foam is generally added to the slurry charge to reduce the density of the core layer, but not to the skim coat slurry charge.

[0059] When the foam and slurry are brought together, the resulting slurry is moved and poured onto a conveyor covered with one piece of facing material. Another piece of facing material is placed over the slurry, forming a sandwich assembly with the slurry between the two facing materials. The multilayer assembly is fed onto a forming board whose height determines the thickness of the drywall board. The solid sandwich assembly is then cut with a cutting knife into appropriate lengths, typically eight to twelve feet (2.44 m to 3.66 m). During this treatment, the slurry is allowed to set (set) to form a board core containing a cohesive crystalline matrix of set gypsum.

[0060] The plates are then transferred to a drying oven. The oven temperature is typically 450°F (232°C) to 500°F (260°C) maximum. Preferably, the oven has three or more temperature zones. In the first zone, with which the wet plate is in contact, the temperature rises to the maximum temperature, and in the last two zones the temperature slowly decreases. The blower for the first zone is located at the outlet of the zone, blowing air in a countercurrent direction in relation to the direction of movement of the plate. In the second and third zones, the blowers are located at the entrance to the zone, directing the flow of hot air parallel to the movement of the plate. Less heat in the last zone prevents dry areas of the board from calcining, resulting in poor paper adhesion. Typical oven dwell time is about forty minutes, but time will vary depending on line performance, platen humidity, and other factors.

[0061] If a starch layer and a cover layer are applied to the cover sheet, then the starch layer is applied to the cover sheet(s) before the cover layer(s) is applied to it(s). Essentially, a layer of starch is placed between the top layer (high density area), if present, and the cover sheet. More specifically, the starch layer is in interface contact with the inner surface of the cover sheet and the top layer, if present, is in interface contact with the starch layer. If the coating layer is absent in this place, then the core of the plate is in boundary contact with the starch layer. After drying the gypsum board to form the set gypsum as described above, the skim layer(s) becomes one with the low density area in the board core, thus the high density area (layer) in the board core is in boundary contact. with a back paper cover sheet or a layer of starch applied thereon and/or with a front paper cover sheet or a layer of starch distributed thereon.

[0062] A gypsum core ( eg , gypsum core 12 in Fig. 1) made from a set gypsum core slurry typically has a thickness of 0.25 inches (0.635 cm) to 1.5 inches (3.81 cm) and a density of 15 to 55 pounds/cubic feet (0.2403 to 0.881 g/cm ³ ). When foamed, the gypsum core obtained from the hardened foamed gypsum slurry has a total void volume of 10 to 92% vol., in particular 25 to 90% vol. and more specifically, from 30 to 85% vol. Conversely, the resulting coating layer, if present, has a total void volume of less than 30% vol.

[0063] Gypsum and stucco (calcined gypsum)

[0064] The calcium sulfate hemihydrate component (generally contained in the starting material known as stucco or calcined gypsum) used to form the crystalline matrix of the gypsum panel core typically includes beta calcium sulfate hemihydrate, water soluble calcium sulfate anhydrite, alpha calcium sulfate hemihydrate, or mixtures of any or all of them, derived from natural or synthetic sources. In some aspects, the stucco may include non-gypsum minerals, such as small amounts of clays or other components that are associated with the gypsum source or added during the calcining, processing, and/or delivery of the stucco to the mixer. Stucco plaster may be fibrous or non-fibrous. As a rule, raw stucco contains at least 70% wt. calcium sulfate hemihydrate, preferably at least 80% wt. calcium sulfate hemihydrate, more preferably at least 85% wt. calcium sulfate hemihydrate and even more preferably at least 90% wt. calcium sulfate hemihydrate.

[0065] Chloride salts

[0066] The aqueous gypsum slurry for forming drywall of the invention comprises water and stucco, wherein the aqueous gypsum slurry also contains chloride anions. The chloride anions may be derived from one or more chloride salts from any source. Typically, one or more chloride salts are present in the gypsum source used to form the core of the board of the invention. Thus, all or at least most of the one or more chloride salts can be introduced into the gypsum slurry from the gypsum source used. The source of gypsum may be a source of synthetic gypsum, in particular low quality synthetic gypsum obtained from a power plant flue gas stream. Such a low quality gypsum source may not otherwise be suitable for forming wallboard without the use of at least one layer of starch, in accordance with the present invention. One or more chloride salts can also be formed from impurities, for example one or more chloride salts in the water used to prepare the aqueous gypsum slurry.

[0067] Chloride salts are any salts that contain chloride. Thus they include monovalent salts of a chloride anion and a monovalent cation such as sodium or potassium. Thus they include divalent salts of chloride anions and a divalent cation such as calcium or magnesium. Other chloride salts are also contemplated, for example trivalent salts of chloride anions and a trivalent cation.

[0068] The concentration of chloride anions of one or more chloride salts in the aqueous slurry used to make the core of the board and, if present, the high density region (thin dense layer of gypsum) of the invention can range from about 500 ppm. up to about 3000 ppm, typically from about 1000 ppm up to about 3000 ppm per 1,000,000 parts by weight of calcium sulfate hemihydrate, more typically from about 2000 ppm. up to about 3000 ppm per 1,000,000 parts by weight of calcium sulfate hemihydrate.

[0069] Starch

[0070] Various starches can be used for the starch layer of the invention and, if desired, also added to the aqueous gypsum slurry. However, the aqueous suspension may not contain starch.

[0071] The starch layer can be wet or dry (pre-applied) in contact with the gypsum core or, if present, with a dense layer of gypsum. The gypsum core does not harden by the time the starch layer comes into contact with it.

[0072] The starch layer does not contain gypsum, inorganic filler, organic filler, and latex or other polymer. The starch layer essentially consists of starch and water, but may optionally contain a small amount of STMP (sodium trimetaphosphate, sodium trimetaphosphate).

[0073] Any of the layers of starch (e.g., starch layer 20 ) may be a continuous layer of starch that is in contact with at least 90%, or preferably 100%, of the inside surface of the back paper cover sheet 14 or the front paper cover sheet. sheet 16 . A starch layer 20 that is "solid" refers to a layer having a substantial absence of breaks on the inside surface of the back paper cover sheet 14 or the front paper cover sheet 16 . ) back paper cover sheet 14 or front paper cover sheet 16 . Alternatively, when the starch layer 20 is continuous, there is no pattern on the layer. Accordingly, suitable methods for applying the starch layer 20 to the back paper cover sheet 14 or to the front paper cover sheet 16 may include, for example, spraying, spreading, inkjet printing, screen printing, screen printing, roller application, dip coating or the like. ways. The starch layer 20 may be pre-applied to the back paper cover sheet 14 or the front paper cover sheet 16 prior to coming into contact with the gypsum slurry. The starch in the starch layer 20 may be wet or dry when in contact with the gypsum slurry. As described below, the gypsum is in a wet state in contact with the starch layer 20 in order to form a more effective bond therewith. That is, the gypsum slurry is in contact with the starch layer 20 before the gypsum solidifies to form the core 12 of the board.

[0074] The starch in starch layer 20 may be pregelatinized (cooked) starch and/or raw starch. In this regard, starches are classified as carbohydrates and contain two types of polysaccharides: straight-chain amylose and branched amylopectin. Starch granules are semi-crystalline, as seen for example in polarized light, and they are insoluble at or near room temperature. Raw starches are characterized as being insoluble in cold water and having a semi-crystalline structure. Generally, raw starches are obtained by wet milling and are not modified by heating the wet starch as is the case with cooked starches. Pregelatinized or cooked starches are characterized as being cold water soluble and having a non-crystalline structure.

[0075] Crude starch can be obtained in the form of a suspension (dispersion) up to 50% wt. solid particles of starch in water. Cooked starch is a solution limited to a maximum of 10% wt. solid particles dissolved in water. The starch dispersion contains a maximum of 50% wt. total content of cooked starch and raw starch. A mixture of water with boiled starch and raw starch is useful because the boiled starch helps keep the raw starch in suspension. However, only raw starch can be used in the invention if the starch slurry is mixed very soon after mixing with water. Preferably, the starch suspension contains from 0.5 to 5% wt. boiled starch, more preferably 1-3% wt. boiled starch. Starch and water may optionally additionally include STMP in an amount in the range between 0-10% wt., preferably 3% wt. from starch solids.

[0076] Native (raw) starch is preferred. A starch gel can be obtained by dispersing starch particles in water. Without pre-cooking, starch can form a gel during the drying process.

[0077] Raw starch generally has a higher bulk density with less variability than that found in pregelatinized starch. This is advantageous because, for example, a stable density provides improved accuracy and stability of starch addition using a volumetric feeder. For example, the bulk density of starch in pounds per cubic foot (lb/cu.ft.) may be from about 35 lb/cu.ft. (0.5606 g/cm³) up to about 50 lb/cu.ft (0.8009 g/cm³), from about 35 lb/cu ft to about 45 lb/cu ft (0.7208 g/cm³), from about 37 lb/cu.ft (0.5927 g/cm³) up to about 50 lb/cu.ft, from about 37 lb/cu.ft up to about 45 lb/cu ft, from about 40 lb/cu ft (0.6407 g/cm³) up to about 50 lb/cu ft, from about 40 lb/cu ft up to about 47 lb/cu.ft (0.7529 g/cm³), from about 40 lb/cu.ft up to about 45 lb/cu ft, from about 41 lb/cu ft (0.6568 g/cm³) up to about 45 lb/cu.ft. etc.

[0078] Raw starch is typically in native granular form. The peak viscosity of raw starches may refer to the average molecular weight of the starch. The granulated raw form of starch may undergo at least some degree of gelatinization during the drywall manufacturing process ( e.g. during the kiln-drying process).

[0079] When raw starch is used, the raw starch may have an average molecular weight indicated by a peak viscosity of about 100 BU (Brabender units) to about 900 BU. The peak viscosity of raw starch can be determined according to the following method. Peak Brabender viscosity is measured using a Viscograph-E (C.W. Brabender) set to 75 rpm; 700 cmg. For viscosity measurement, starch is in suspension having a concentration of 15% solids in water. The starch slurry is heated from 25°C to 95°C at a rate of 3°C/min. Then it is kept at 95 °C for 10 minutes, after which it is cooled to 50 °C at a rate of -3 °C/min.

[0080] Typically, the peak viscosity of a raw starch having an average molecular weight may be from about 100 Brabender units to about 850 Brabender units, from about 120 Brabender units to about 875 Brabender units, from about 250 Brabender units to about 750 Brabender units , from about 400 Brabender units to about 700 Brabender units, from about 100 Brabender units to about 460 Brabender units, from about 100 Brabender units to about 300 Brabender units, from about 500 Brabender units to about 850 Brabender units, or from about 550 Brabender units up to about 850 Brabender units.

[0081] The properties of raw starches include low viscosity in cold water ( i.e. at 77 ºF (25 ºC)), in contrast to the properties of pregelatinized starches, including their instant high viscosity in cold water. The raw starches of the invention may have any suitable cold water viscosity. Typically, the viscosity in cold water is from about 1 centipoise (0.001 Pa s) to about 500 centipoise (0.5 Pa s), e.g. , from about 1 centipoise to about 400 centipoise (0.4 Pa s), from about 1 centipoise to about 300 centipoise (0.3 Pa s), from about 1 centipoise to about 200 centipoise (0.2 Pa s), or from about 1 centipoise to about 100 centipoise (0.1 Pa s). If necessary, raw starch may have a cold water viscosity of about 50 centipoise (0.05 Pa s), for example , about 40 centipoise (0.04 Pa s) or less, about 30 centipoise (0.03 Pa s) or less, about 20 centipoise (0.02 Pa s) or less, or about 10 centipoise (0.01 Pa s) or less in cold water (e.g., about 1 centipoise to about 50 centipoise, about 1 centipoise to about 40 centipoise, from about 1 centipoise to about 30 centipoise, about 1 centipoise to about 20 centipoise, about 5 centipoise to about 50 centipoise, about 5 centipoise to about 30 centipoise, about 5 centipoise to about 20 centipoise, etc.).

[0082] Cold water viscosity can be measured using a Brookfield viscometer using the test program below. Starch (20 g, dry) is added to water (180 g) placed in a Waring blender (Model 31BL92) over 15 seconds while mixing at low speed. The starch solution (200 g) is transferred to a beaker. Select agitator No. 2 and a speed of 60 rpm. The viscosity value measured after 20 seconds is used as the starch viscosity.

[0083] As used herein, the term "crude" also means that the starch has a degree of gelatinization of less than about 5% ( for example , less than about 3% or less than about 1%, including 0%) before it is applied to starch layer. Raw starch may be partially or fully gelatinized at elevated temperatures during the drywall manufacturing process, such as in an oven used in the drying step to remove excess water from the board core.

[0084] Cooked (pregelatinized) starches can be prepared by any suitable method, for example , in an extruder, as described in US patent application publication 2015/0010767, which is incorporated herein in its entirety by reference. Cooked starch can be prepared to have any desired properties ( eg viscosity, cold water solubility, cold water viscosity, etc.).

[0085] Cooked starches can be prepared by heating wet starch, and can also be prepared, for example , by extrusion methods, such as those described in U.S. Patent Application Publications 2015/0010767, 2014/0113128, and 2014/011312, each of which is incorporated herein. document in its entirety by reference. Cooked starches are called pregelatinized starches because the crystal structure of the starch granules melts and this leads to starch gelatinization, which is characterized by the disappearance of birefringence under a polarized light microscope. Gelatinization is a process in which starch is placed in water and heated (“boiled”), thus the crystal structure of the starch granules is transformed as the starch melts, after which the starch molecules can be dissolved in water, resulting in good dispersion. It has been found that when the starch granules are converted to a gelled form, the starch granules initially provide little viscosity in water because they are insoluble in water. As the temperature rises, the starch granules swell and the crystal structure transforms as the starch melts at the gelatinization temperature. Peak viscosity occurs when the starch granules reach their maximum swelling state. Further heating leads to the destruction of starch granules and the dissolution of starch molecules in water, while the viscosity decreases sharply. Upon cooling, the starch molecules can re-associate to form a three-dimensional (3-D) gel structure, with the viscosity increasing due to the gel structure.

[0086] Examples of starches that may be suitable for use in drywall boards and related methods described herein include, but are not limited to, one or more of native corn starches, native root starches, native tuber starches and/or chemically modified starches, specific generic examples of which include, for example , corn starch (regular, waxy and/or high amylose), type A wheat starch, type B wheat starch, pea starch, substituted/functionalized starches having substituted groups (eg, acetate, phosphate, hydroxyethyl, hydroxypropyl, alkylsulfonate, and the like) on the hydroxyl groups of starch, or any combination of the above starches.

[0087] Starch, whether raw or cooked, may have any suitable viscosity. The specific viscosity of the starch may contribute to the formation of the starch layer(s) of a certain thickness. Generally, one or more pregelatinized starches having an average viscosity of from about 20 centipoise (0.02 Pa s) to about 700 centipoise (0.7 Pa s) (measured by the VMA method) or raw starch with peak viscosity from about 100 Brabender units to about 900 Brabender units (measured by the method described in this document). Although, in accordance with the methods for measuring viscosity described herein, the viscosity index is determined by placing the starch under certain conditions, it should be understood that the starch does not need to be introduced into the gypsum board under these conditions.

[0088] To reduce the peak viscosities of raw or cooked starches, the starch molecules can be modified, for example to hydrolyze the glycosidic bonds between the glucose units to achieve the desired molecular weight. For example, such modifications may include acid modifications, enzymatic modifications, pyrolysis, and/or other methods. The most commonly used enzyme for the conversion of starch is α-amylase (alpha-amylase). The enzymatic hydrolysis reaction can be stopped by adjusting the pH or by heating. It should be understood that in order to obtain acid-modified starches, an aqueous suspension of unmodified starch may be treated with, for example , a small amount of acid, for example, in particular a strong acid, such as hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid or similar acids. The degree of depolymerization can be modified to the desired degree by adjusting the reaction time. For example, if proper fluidity is achieved, as determined, for example , by laboratory control during the process, then a weak alkali can be introduced to neutralize the acid and stop hydrolysis. As a result, acid-modified starches with varying degrees of fluidity (viscosity) can be obtained. In addition, some acid-modified starches can be used directly after neutralization without further purification, or they can be purified to remove salts. The need for purification of acid-modified starch may be determined by its end use. For example, starch modified with sulfuric acid and neutralized with calcium hydroxide may contain sulfate and calcium ions, which can be added to the stucco and aqueous slurry or brought into contact with the slurry since these ions are already present in the stucco. Thus, considerations for determining the desirability of purifying acid-modified starch include, for example, the identity of the acid and alkaline base and the acceptability of introducing the specific ions formed therefrom at a given location. For example, it may be desirable to avoid incorporating excessive amounts of alkali or alkaline earth metal halides with starch, and in such situations, purification may be beneficial.

[0089] Suitable pregelatinized starch may include, but is not limited to, pregelatinized corn flour, such as pregelatinized corn flour, available from Bunge, St. Louis, Missouri, having the following composition: moisture 7.5%, protein 8, 0%, oil 0.5%, crude fiber 0.5%, ash 0.3%; green strength 0.48 psi (3.309 kPa); and having a free bulk density of 35.0 lb/cu.ft (0.5606 g/cm ³ ). Other useful pregelatinized starches may include, but are not limited to, pregelatinized rice starch and pregelatinized wheat starch.

[0090] Pregelatinized starches may have any suitable viscosity. In the event that the pregelatinized starch is subjected to conditions corresponding to the VMA method, in which the amount of pregelatinized starch in water is 15% wt. based on the combined weight of the pregelatinized starch and water, the viscosity of the pregelatinized starch can be characterized as "medium" viscosity ( ie, a viscosity of about 20 centipoise to about 700 centipoise).

[0091] Typically, the viscosity of a suitable pregelatinized starch can be from about 20 centipoise to about 1000 centipoise (1 Pa s), for example , from about 20 centipoise to about 900 centipoise (0.9 Pa s), from about 20 centipoise to about 800 centipoise (0.8 Pa s), about 20 centipoise to about 700 centipoise, about 20 centipoise to about 500 centipoise, about 30 centipoise to about 200 centipoise, or about 100 centipoise to about 700 centipoise. Examples include, but are not limited to, pregelatinized corn starches, e.g. in its entirety by reference.

[0092] Typically, pregelatinized starch can be formulated to have the desired cold water solubility. Conventional pregelatinization techniques involve making the starch soluble in cold water and generally require the starch to be cooked in excess water. Extrusion allows the combination of heat and mechanical shear and is an energy efficient method that can be used to produce pregelatinized starch having a low moisture content and cold water solubility in a one step process. Cold water solubility is defined as any solubility in water at room temperature (about 25°C). Cold water soluble starches can have a cold water solubility greater than about 30% and can increase the strength of drywall. The solubility of pregelatinized starch in water is defined as the amount of starch that dissolves in water at room temperature divided by the total amount of starch.

[0093] For example, the cold water solubility of pregelatinized starch can be from about 30% to about 100%. In more specific examples, the cold water solubility of the pregelatinized starch can be from about 50% to about 100%.

[0094] Typically, pregelatinized starch may have a cold water (10% solids, 25°C) viscosity of about 10 BU to about 120 BU as measured by the Brabender method, e.g. , about 20 BU to about 110 BU, from about 30 BU to about 100 BU, or about 60 BU to about 70 BU. According to the Brabender viscosity measurement procedure referred to herein, viscosity is measured using a CW Brabender Viscograph instrument, such as the Viscograph-E, which uses reactive torque for dynamic measurement. It should be noted that, as defined herein, Brabender units (BU) are measured using a 16 fl oz (about 500 cc) sample cup with a 700 cmg cartridge at 75 rpm. One of skill in the art will also readily appreciate that Brabender units can be converted to other units of viscosity, such as centipoise ( e.g. cP = BU X 2.1 when the measuring cartridge is 700 cmg) or Krebs units as described document.

[0095] The starch layer(s) within the drywall boards of this invention may contain raw starch. Or the starch layer(s) may contain a mixture of raw starch and cooked starch. The ratio of raw starch to cooked starch can be selected, for example, to obtain the desired degree of fluidity for applying starch to the inner surface(s) of the back paper cover sheet and/or the front cover sheet, in particular the back cover sheet, more specifically, back paper cover sheet. The fluidity selected may depend on the technique by which the starch is applied to the inner surface of the cover(s) sheet(s). Flowability can be measured, for example , by the slump test method. Flow may be related to the viscosity of the starch or starch mixture, for example, from about 50 cP to about 2000 cP (2 Pa s), for example, from about 200 cP to about 1500 cP (1.5 Pa s), measured in accordance with the test using a Brookfield viscometer at 60 rpm with #5 spindle.

[0096] Any suitable ratio of raw starch to cooked starch may be included in the starch layer(s), such as 0:100 to 100:0 (as the combination is optional) or 1:100 to 100:1. The ratio of raw starch to cooked starch may be from about 90:10 to about 10:90, from about 70:30 to about 30:70, from about 60:40 to about 40:60, from about 90:10 to about 60: 40, about 90:10 to about 70:30, about 90:10 to about 80:20, about 80:20 to about 60:40, about 80:20 to about 70:30, or similar ratios. If desired, the ratio of raw starch to cooked starch may be about 25:75, about 30:70, about 35:65, about 50:50, about 65:35, about 70:30, about 75:25, and similar ratios.

[0097] Cooked starch and raw starch in combination may be dry or wet mixed for application to the inner surface(s) of the back paper cover sheet and/or the front cover sheet. Similarly, cooked or raw starch in pure form can be applied dry or wet to the inner surface(s) of the back paper cover sheet and/or the front cover sheet. Cooked starch and raw starch in combination can be applied to the inner surface(s) at the same time or one at a time. The cooked and raw starches may be mixed together in the starch layer, or the cooked and raw starches may be stacked on top of each other in any order.

[0098] The starch in the starch layer(s) is generally non-migratory starch. In this respect, migrating acid-modified starches differ from the starches present in the starch layer(s) and are known in the art to have shorter molecular chains. These shorter chain migrating starches can generally migrate in the gypsum slurry, but are not beneficial for increasing board strength. Migrating acid-modified starches typically have a molecular weight of less than 6000 daltons. Typically, the starches for inclusion in the starch layer(s) of the invention have a higher molecular weight than the migrating starches, for example at least about 15,000 Daltons or at least about 30,000 Daltons. The average molecular weight can be referred to as peak viscosity.

[0099] Any suitable amount of starch may be present in the starch layer(s). For example, the starch layer(s) may include an amount of starch in the range of, for example, about 0.1 lb/MSF (MSF = 1000 square feet) (0.5 g/m ² ) to about 35 lb/MSF ( 170.7 g/m ² ), for example, from about 1 lb/MSF to about 2 lb/MSF, or from about 2 lb/MSF to about 35 lb/MSF, or from about 1 lb/MSF to about 5 lb/ MSF, or from about 6 lb/MSF (29.3 g/m ² ) to about 20 lb/MSF (97.5 g/m ² ).

[00100] In specific examples, the starch layer(s) in the gypsum boards of the invention do not contain(s) gypsum, inorganic filler, organic filler, and latex or other synthetic polymers. In some cases, the starch layer(s) may consist essentially of starch and water, optionally with a small amount of sodium trimetaphosphate (STMP).

[00101] Organic functionalized starches can be used in the starch layer(s) within any of the drywall boards described herein. Organic functionalized starches can be formed through the interaction of any of the hydroxyl groups in the glucose monomer units of the starch, but they are easier to introduce into the primary alcohol group at C-6. Functional groups that may be introduced include, but are not limited to, acetate, phosphate, hydroxyalkyl, alkylsulfonate, and the like.

[00102] Sulfonated starches suitable for use in the gypsum boards of this invention can be synthesized by known sulfonation methods. During sulfonation, a fragment of sulfonic acid (-SO ₂ OH) or sulfonate salt (-SO ₃ ^- Na ⁺ ) is introduced into starch. In an exemplary sulfonation process, starch may be reacted with sodium vinyl sulfonic acid at 90° C. for 4 hours in aqueous NaOH to form an alkyl bonded sulfonate salt.

[00103] More generally, a sulfonic acid or sulfonate salt may be bonded to starch with an alkyl group or arylalkyl group, in particular C ₂ -C ₁₈ or C ₂ -C ₈ sulfonic acid or sulfonate salt, more specifically C ₂ - C ₅ alkylsulfonic acid or sulfonate salt. The addition of a sulfonic acid to a sulfonic acid or sulfonate salt via an alkyl or arylalkyl group can take place in the presence of a suitable base, such as 10% sodium hydroxide (NaOH). Suitable sulfonating agents may include, but are not limited to, sodium vinylsulfonate, styrenesulfonate, 2-chloroethanesulfonic acid, 3-chloropropanesulfonic acid, or 1,3-propanesultone. Although sulfonation can be carried out at a temperature of from about 50°C to about 100°C, it can be carried out at a temperature of from about 75°C to about 95°C, in particular at a temperature of from about 85°C to 95°C.

[00104] Additives

[00105] Other additives that may be present in the gypsum slurry used to form the board core may include, but are not limited to, reinforcing agents, foam (derived from a suitable blowing agent), dispersants, polyphosphates ( e.g. , sodium trimetaphosphate), starches, setting retarders, setting accelerators, recalcination inhibitors, binders, adhesives, secondary dispersants, leveling or non-levelling agents, thickeners, bactericides, fungicides, pH adjusters, buffers, colorants, reinforcing fillers, flame retardants, water repellents (e.g. siloxane), fillers and mixtures of the above.

[00106] Additives and other components of the gypsum slurry can be added to the mixer in various ways. For example, various combinations of components may be premixed before being fed into the mixer as one or more dry components and/or as one or more wet components. The individual components can likewise be introduced into the mixer in wet or dry form. When administered wet, the components may be incorporated into a carrier liquid, eg water, at any suitable concentration.

[00107] Fibers can optionally be used in the methods and compositions of this invention. The fibers may include mineral fibers (also known as mineral wool), glass fibers, carbon fibers and blends of such fibers, as well as other comparable fibers providing comparable benefits to wallboard. For example, glass fibers may be included in the gypsum core slurry and/or the overlay slurry, resulting in a core crystalline structure. In such aspects, the glass fiber may have an average length of about 0.5 inches (1.27 cm) to about 0.75 inches (1.91 cm) and a diameter of about 11 to about 17 microns. In other aspects, such glass fiber may have an average length of about 0.5 to about 0.675 (1.71 cm) inches and a diameter of about 13 to about 16 microns. In other aspects, E-glass fibers having a softening point greater than about 800°C or greater than at least about 900°C are used. Instead of or in combination with glass fiber, mineral wool or carbon fibers known to those skilled in the art may be used.

[00108] The fibers, if included, may be present in the gypsum core slurry and/or the cover layer slurry in amounts based on dry matter per 100 parts by weight. calcium sulfate hemihydrate, from about 0.5 to about 10 parts by weight; preferably, from about 1 to about 8 parts by weight; more preferably, from about 2 to about 7 parts by weight; and most preferably, from about 3 to about 6 wt.h. There may also be no fibers.

[00109] Optionally, one or more phosphate-containing compounds may also be included in the suspension, if desired. For example, these phosphate-containing components may include water-soluble components and may be in the form of an ion, salt or acid, namely, in the form of condensed phosphoric acids, each of which contains two or more units of phosphoric acid; salts or ions of condensed phosphates, each containing two or more phosphate units; and monobasic salts or monovalent ions of orthophosphates, as well as in the form of water-soluble acyclic polyphosphate salts. Illustrative examples are described in US Pat. Nos. 6,342,284; 6632550; 6815049; and 6822033, which are incorporated herein in their entirety by reference.

[00110] Phosphate-containing components can enhance green strength, resistance to permanent deformation ( eg , sag), dimensional stability, and the like. Trimetaphosphate compounds may be used, including, for example, sodium trimetaphosphate, potassium trimetaphosphate, lithium trimetaphosphate, and ammonium trimetaphosphate. Sodium trimetaphosphate (STMP) is widely used, although other phosphates may be suitable, including, 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 , acidic sodium pyrophosphate having the molecular formula Na ₂ H ₂ P ₂ O ₇ , ammonium polyphosphate having 1000-3000 phosphate repeating units and having the molecular formula (NH ₄ ) _n+2 P _n O _3n+1 where n=1000-3000, or polyphosphoric acid having two or more phosphoric acid repeating units and having the molecular formula H _{n+ 2} P _n O _{3 n+1} , where n is two or more.

[00111] Phosphates are typically added in dry form and/or liquid form in an aqueous solution along with dry ingredients added to the slurry mixer, along with liquid ingredients added to the mixer, or at other stages or procedures.

[00112] If phosphate is present, then it can be included in the composition of the gypsum suspension in dry form or in the form of an aqueous solution (For example, a phosphate solution with a concentration of from about 5% to about 20%, for example, about 10%). If phosphate is included, it may be present in any suitable amount (on a solids weight basis), for example, from about 0.01% to about 0.5% by weight of the stucco,For example, from about 0.03% to about 0.4%, from about 0.1% to about 0.3%, or from about 0.12% to about 0.4% by weight of the stucco. Phosphate may also be absent.

[00113] The gypsum slurry may optionally include at least one dispersant to improve flow. The dispersant(s) may be added to the gypsum slurry in dry form, optionally with other additives, and/or in liquid form, optionally with other liquid components. Examples of suitable dispersants include naphthalenesulfonates, eg polynaphthalenesulfonic acid and its salts (polynaphthalenesulfonates) and derivatives, which are condensation products of naphthalenesulfonic acids and formaldehyde, and polycarboxylate dispersants, eg polycarboxylic esters. Other examples of suitable dispersants include lignosulfonates or sulfonated lignin. Lignosulfonates are water-soluble anionic polyelectrolyte polymers that are by-products of wood pulp production using sulphite pulping.

[00114] Lower molecular weight dispersants may be desirable. Lower molecular weight naphthalenesulfonate dispersants may be preferred because they tend to require less water than higher viscosity, higher molecular weight dispersants. Thus, molecular weights from about 3,000 to about 10,000 ( eg , about 8,000 to about 10,000) may be desirable molecular weights for a dispersant. If desired, the molecular weight of polycarboxylate dispersants may be from about 20,000 to about 60,000 and may exhibit less set retardation than dispersants having a molecular weight greater than about 60,000.

[00115] Typical naphthalenesulfonates are a solution of naphthalenesulfonate in water with a solids content of naphthalenesulfonate ranging from about 35% wt. up to about 55% wt. However, if desired, the naphthalenesulfonates may be used in solid or powder form.

[00116] The dispersant, if present, may be included in the gypsum slurry in any suitable amounts (dry weight ratio), for example, from about 0.1% to about 5% by weight of the stucco, for example , from about 0.1 % to about 4%, from about 0.1% to about 3%, from about 0.2% to about 3%, from about 0.5% to about 3%, from about 0.5% to about 2.5 %, from about 0.5% to about 2%, from about 0.5% to about 1.5% and in similar amounts. Also, any one or more of the polynaphthalenesulfonates, polycarboxylic acid ethers, or lignosulfonates may be absent.

[00117] Accelerators and/or retarders can be added to the gypsum core slurry and/or the cover layer slurry to change the rate at which the calcium sulfate hemihydrate hydration reactions proceed. Suitable accelerators may include, for example, a wet gypsum set accelerator, a heat resistant set accelerator (HRA), or a climate set accelerator (CSA). "CSA" is an accelerator containing 95% calcium sulfate dihydrate ground with 5% sugar and heated to 250°F (121°C) to caramelize the sugar. CSA is available from USG Corporation and is manufactured under US Pat. Nos. 3,573,947 and 6,409,825, incorporated herein by reference. Another preferred set accelerator is potassium sulfate. Heat Resistant Set Accelerator (HRA), which is the preferred set accelerator, is calcium sulfate dihydrate freshly ground with sugar in a ratio of about 5 pounds (2.268 kg) to 25 pounds (11.340 kg) of sugar per 100 pounds (45.359 kg) of calcium sulfate dihydrate . This is further described in US Pat. No. 2,078,199, incorporated herein by reference. They are both preferred set accelerators. If an accelerator and/or retarder is present, then each of them can be included in the gypsum slurry in a dry matter amount of, for example , from about 0% to about 10% by weight of the stucco ( for example , from about 0, 1% to about 10%), eg about 0% to about 5% by weight of the stucco ( eg about 0.1% to about 5%). Suitable accelerators may include, for example, calcium sulfate dihydrate, carbohydrate-coated calcium sulfate, calcium sulfate dihydrate/organic phosphonate, and calcium sulfate dihydrate/organic phosphate. Accelerators and/or retarders may also be absent.

[00118] Foam (also known as foamed water), optionally, may be incorporated into the gypsum core slurry and/or the top coat slurry (preferably the gypsum core slurry) in amounts that provide the aforementioned reduced core density and panel weight. The blowing agent for producing foam is typically a soap or other suitable surfactant. Incorporation of foam into the gypsum core slurry in the proper amounts, formulations and process will produce the desired network and void distribution within the core of the finally dried wallboards. This void structure reduces the amount of gypsum and other core constituents, as well as core density and weight, while maintaining the desired structural and strength properties of the panel. If present, blowing agents may contain a major part by weight of the unstable component and a small part by weight of the stable component ( eg when combining the unstable component with a mixture of stable and unstable components). The weight ratio of the unstable component to the stable component is effective to form distributed air voids within the set gypsum core, as described in US Pat. Nos. 5,643,510; 6342284; and 6632550, which are incorporated herein in their entirety by reference. Approaches for adding foam to a gypsum core slurry are known in the art, and one example of such an approach is discussed in US Pat. No. 5,683,635, the disclosure of which is incorporated herein by reference. Voids from evaporating water, typically having a diameter of about 5 microns or less, also contribute to the overall distribution of voids along with the aforementioned air (foam) voids. The volume ratio of voids with a pore size greater than about 5 microns to voids with a pore size of about 5 microns or less is from about 0.5:1 to about 9:1, for example, from about 0.7:1 to about 9:1 , from about 1.8:1 to about 2.3:1 or similar ratios. The blowing agent is present in the gypsum slurry in an amount, such as less than about 0.5% by weight of the stucco, such as about 0.01% to about 0.5%, about 0.01% to about 0.2%. , about 0.02% to about 0.4%, about 0.02% to about 0.2%, about 0.01% to about 0.1%, or similar amounts. Also, foaming agents may be absent.

[00119] Components may also be included in the gypsum slurry to provide fire resistance and/or water resistance. Examples include, for example, siloxanes (water resistance); fibers; heat absorbing additives such as aluminum trihydrite (ATH, aluminum trihydrite), magnesium hydroxide or similar additives; and/or particles with a high coefficient of thermal expansion ( eg , expanding to about 300% or more of their original volume when heated for about one hour at 1560°F (849℃)). Further description of such additives can be found in US Pat. No. 8,323,785, which is incorporated herein by reference in its entirety. Vermiculite with a high coefficient of thermal expansion may be included, although other fire resistant materials may be included. If flame retardant or waterproof additives are present, they can be included in the composition, if desired, in any suitable amount depending on, for example , the degree of fire resistance and similar performance parameters. For example, if flame retardant or waterproof additives are included, they may individually be present in an amount of from about 0.5% to about 10% by weight of the stucco, for example, from about 1% to about 10%, from about 1% to about 8 %, from about 2% to about 10%, from about 2% to about 8% and in a similar amount. If the composition includes a siloxane, it is desirable that it be introduced in the form of an emulsion. The slurry can then be formed and dried under conditions that promote polymerization of the siloxane to form a highly crosslinked silicone resin. A catalyst may be added to the gypsum slurry to help polymerize the siloxane to form a highly crosslinked silicone resin. As the siloxane, a solvent-free methylhydrosiloxane liquid can be used. This product is a liquid siloxane containing no water or solvents. It is contemplated that, if desired, from about 0.3% to about 1.0% siloxane, based on the weight of the dry ingredients, may be used. For example, if desired, the gypsum slurry may contain from about 0.4% to about 0.8% siloxane based on the weight of dry stucco. Also, any one or more of these flame retardant and/or watertight components may be omitted. For example, siloxane may be absent.

[00120] Water

[00121] Water is added to the suspension in any amount that makes the suspension flowable. The amount of water used varies greatly depending on the application in which it is used, the strictly appropriate dispersant used, the properties of the calcium sulfate hemihydrate, and the additives used.

[00122] To better control the properties of both the slurry and the set gypsum, the water used to prepare the slurry should be as pure as practicable. It is well known that salts and organic compounds modify slurry set time and range widely from set accelerators to set inhibitors. Some impurities lead to disturbances in the structure, since a cohesive matrix of dihydrate crystals is formed, which reduces the strength of the set product. Thus, the strength and consistency of the product is improved by using water that is as free from contaminants as is practical.

[00123] Water may be present in the gypsum core slurry and/or the top layer slurry of this invention at a weight ratio of water to calcium sulfate hemihydrate of about 0.2:1 to about 1.2:1; preferably from about 0.3:1 to about 1.1:1; more preferably from about 0.6:1 to about 1:1; most preferably from 0.7:1 to 0.95:1; and typically about 0.85:1.

[00124] Rear paper cover sheet and front paper cover sheet

[00125] The back paper cover sheet and the front cover sheet may be made from any suitable paper material having any suitable basis weight.

[00126] The back and front cover sheets are made of paper. However, the paper materials for each cover sheet may be the same or different.

[00127] Various grades of paper can be used in gypsum panels, including Manila brand paper with a smooth film finish is often used as the facing paper cover sheet, and Newsline paper with a rougher finish is often used as the back paper cover sheet. As a rule, paper of both grades is multi-layered, with at least one decorative top layer and several layers of filler. However, if desired, at least one paper cover sheet or both paper cover sheets are made from a single layer of paper.

[00128] If desired, to improve strength ( e.g. , nail pull strength), especially for lower density drywall boards, one or both coversheets may be formed from paper having a basis weight of, e.g., at least about 45 pounds. /MSF (219.5 g/m ² ) ( for example , from about 45 lb/MSF to about 65 lb/MSF (317.0 g/m ² ), from about 45 lb/MSF to about 60 lb/MSF (292 .6 g/m ² ), about 45 lb/MSF to about 55 lb/MSF (268.3 g/m ² ), about 50 lb/MSF (243.9 g/m ² ) to about 65 lb/MSF MSF from about 50 lbs/MSF to about 60 lbs/MSF or similar basis weight). If desired, the front paper cover sheet may have a higher basis weight than the back cover sheet, which may provide improved nail pull resistance and handling. If desired, the back paper cover sheet may have a slightly lower basis weight ( for example , a basis weight of less than 45 lb/MSF, for example , from about 33 lb/MSF (161.0 g/m ² ) to 45 lb/MSF ( for example , from about 33 lb/MSF to about 40 lb/MSF (195.1 g/m ² )).

[00129] In addition, if desired, the back paper cover sheet may be a paper sheet that contains a plurality of perforations. Surprisingly, perforations in the back paper cover sheet can increase adhesion between the board core and the cover sheet. Without being limited by theory or mechanism, it is believed that the perforations provide a conduit for outward migration of chloride salts released from the board core, which would otherwise aggregate detrimentally at the interface between the board core and cover sheets. Thus, perforation can be proposed, in addition to the starch coatings described herein, as a mechanism to improve adhesion. For example, a layer of starch may be applied between the gypsum core and the front paper cover sheet to improve adhesion of the front paper cover sheet, and the back paper cover sheet may be perforated to improve adhesion of the back cover sheet. Also, for example, a layer of starch may be applied between the gypsum core and the front paper cover sheet and/or back paper cover sheet to improve adhesion of the respective sheet, and the back paper cover sheet may have perforations to improve adhesion of the back cover sheet. Exemplary equipment for perforating one or more cover sheets in drywall in a manner compatible with this invention is described in US Patent Application Publication 2018/0065336, which is incorporated herein by reference in its entirety.

[00130] The perforations in the back paper cover sheet may be of a shape, size, and perforation density suitable for maintaining drying and/or promoting adhesion in drywall.

[00131] Generally, perforations are substantially circular in shape. However, it should be appreciated that the perforations may have alternative geometries, such as oval, triangular, square, rectangular, or the like. Any combination of round and non-circular perforations may be present depending on the requirements for a particular application. Any combination of non-circular perforations may be present.

[00132] Each of the perforations in the cover sheet(s), particularly in the back cover sheet, may define a hole having a size that ranges from about 0.005 inches (0.013 cm) to about 0.1 inches (0.254 cm) in its largest transverse dimension, preferably from about 0.01 inch (0.025 cm) to about 0.1 inch, and more preferably from about 0.01 inch to about 0.02 inch (0.051 cm ). The term "largest transverse dimension" means the largest dimension measured for each perforation in the plane of the cover sheet(s). For round perforations, the largest transverse dimension is the diameter of the round hole.

[00133] Perforation coverage density refers to the number of perforations per unit area in the back cover sheet. The perforation density in the cover sheet(s) may be from about 5 to about 50 perforations/inch ² (1 inch ² = 6.45 cm ² ), preferably from about 12 to about 25 perforations/inch ² , for example , from about 10 to about 15 perforations/inch ² or, for example, from about 15 to about 20 perforations/inch ² .

[00134] If a perforated back paper cover sheet is used, in the method of making drywall, when the foam and slurry are brought together, the resulting slurry is moved and poured onto a conveyor covered with the first piece of facing material, which is the front paper cover sheet (face paper cover sheet) . Another piece of liner material, which is a perforated back paper cover sheet (back paper cover sheet), is placed over the slurry to form a slurry sandwich assembly between the two liner materials.

[00135] Typically, perforations occupy 0.1 to 10%, more typically 0.5 to 5%, of the surface of the back cover sheet.

[00136] In Fig. 2 illustrates a plan view (axial direction) of the wall panel 10' , in which an exemplary configuration of the perforations 21 in the back paper cover sheet 14' can be seen. As shown, the perforations 21 are structured in regular columns and rows. It should be appreciated that other patterned arrangements of the perforations 21 are also within the scope of the invention, and the depicted number, intervals, and coverage density of the perforations 21 should not be construed as limiting.

Systems

[00137] In FIG. 3 illustrates a perspective view of a typical building wall system 30 that may be used in the wall system of the present invention. On FIG. 2 illustrates a metal stud wall "frame" 32 that includes a plurality of metal studs 34, a top rail 36, a bottom rail 38. Drywall boards 10 (FIG. 1) may be attached in any known manner to one or both sides of the metal studs 34, to close the wall and form the inner surface or surfaces of the wall or ceiling. An exemplary "frame" wall with metal studs can be made in accordance with US Pat. . This metal frame system is merely illustrative, as wood or other metal frames may also be used.

[00138] The following examples provide additional explanation for the present invention, but, of course, should not be construed as limiting its scope.

EXAMPLES

[00139] Gypsum core (slurry) formulations were prepared with a high concentration of chloride in the form of NaCl or MgCl ₂ as indicated in Table 1. Samples designated as containing MgCl ₂ or NaCl plus starch refer to testing these samples with a layer of starch on the back paper cover sheet (see table 2 below). Thus, the board cores identified by Samples No. 1 and No. 2 are identical in composition to each other, as in Samples No. 3 and No. 4. All gypsum core formulations contained a small amount of boiled cornstarch.

Table 1 Compositions of gypsum cores

CORE 1200 ppm Cl ^- from MgCl ₂ 1200 ppm Cl ^- from MgCl ₂ + starch 2100 ppm Cl ^- from
NaCl 2100 ppm Cl ^- from NaCl + starch 2000 ppm Cl ^- from NaCl 30 ppm Cl from NaCl Sample identification No. 1 No. 2 No. 3 No. 4 No. 5 (no starch + perforation);
No. 6 (with starch + perforation) No. 7 (no starch);
No. 8 (with starch) Thickness ½ inch (1.27 cm) ½ inch (1.27 cm) ½ inch (1.27 cm) ½ inch (1.27 cm) ½ inch (1.27 cm) ½ inch (1.27 cm) Stucco (g) (calcium sulfate hemihydrate) 900 900 900 900 900 900 HRA (g) ¹ 3.6 3.6 3.6 3.6 3.6 3.6 Cooked (also known as pregelatinized) cornstarch (g) 2.7 2.7 2.7 2.7 2.7 2.7 Fiberglass (g) 2.5 2.5 2.5 2.5 2.5 2.5 NaCl (g) ---- ---- 3.46 3.46 3.30 0.049 MgCl ₂ (g) 1.45 1.45 ---- ---- ---- ---- 10% sodium trimetaphosphate solution (g) ² 5.4 5.4 5.4 5.4 5.4 5.4 1% retarder (g) ² 25 25 25 25 25 25 Excess water (g) 861 861 861 861 861 861

¹ HRA = heat resistant set accelerator

² grams of solution, % is equal to % wt.

[00140] 12" x 12" x 1/2" envelopes were made using Manila paper (46 lb/MSF (224.4 g/m ² )) as the front paper cover sheet and Newsline paper (40 lb/MSF) in as a back cover sheet. Before pouring the slurry formulations (Table 1) into the envelope, dilute starch solutions (Table 2) were brushed onto the base (inside) side of the back paper cover sheet used for Samples #2 and #4.

[00141] Table 2. Concentration and use of diluted starch

Sample identification No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 No. 8 Back paper solution starch free 4% raw starch + 4% cooked (also known as pregelatinized) cornstarch starch free 8% crude starch starch free 5% raw starch on perforated back paper starch free 5% raw starch Change in paper weight (g) 0 10 0 10 0 10 0 10 Dry Starch Weight (lbs/MSF (g/m ² )) 0 0.9 lb (408.2 g) raw starch + 0.9 lb cooked (also known as pregelatinized) corn starch 0 1.8 pounds (816.5 g) raw starch 0 1.1 lb (499 g) raw starch 0 1.1 lb (499 g) raw starch

[00142] Slurry formulations (Table 1) were prepared by soaking dry powders in solution for 10 seconds and mixing for 10 seconds in a Hobart mixer, followed by injection of the resulting foam for 4.5 seconds and mixing for another 2 seconds. The suspension thus obtained was poured into each of the envelopes. After the suspension had solidified, the envelope was sealed using paper tape. The sealed plate was dried at 450 ^° F (232.2 ^° C) for 20 minutes and then transferred to a 350 ^° F (176.7 ^° C) oven. After drying at 350 ^° F for 15 minutes, the board was further dried at 110 ^° F (43.3 ^° C) overnight.

[00143] The results of the starch treatment were evaluated by the X-notch method to test the adhesion of the paper to the core. Several X-shaped cuts were made on the back paper of the finished wallboard samples. The size of the X-shaped incision was approximately 2 inches x 2 inches (5.08 cm x 5.08 cm). Paper was peeled off from the center of the notch. The samples were evaluated qualitatively in comparison with each other by evaluating the amount of paper remaining on the gypsum core of the wallboard after peeling. With good adhesion to the gypsum core, a thin layer of paper remains on the core. Conversely, if there is poor adhesion to the core, then the paper peels off the core and leaves most of the core exposed.

[00144] In FIG. 4 illustrates photographs showing the effect of a dilute starch solution on a 12" x 12" x 1/2" slab containing 1200 ppm. Cl ^- from MgCl ₂ per million parts of calcium sulfate hemihydrate. Control Board No. 1 had no starch layer between the paper cover sheet and the gypsum core. Experimental Board No. 2 had a layer of starch between the paper cover sheet and the gypsum core.

[00145] In FIG. 5 are photographs showing the effect of a dilute starch solution on a 12" x 12" x 1/2" slab containing 2100 ppm. Cl ^- from MgCl ₂ per million parts of calcium sulfate hemihydrate. Control Board No. 3 had no starch layer between the paper cover sheet and the gypsum core. Experimental Board No. 4 had a layer of starch between the paper cover sheet and the gypsum core.

[00146] When comparing board samples #1 and #2 in FIG. 4 side by side, the experimental board #2 with the starch layer had less exposed gypsum at each "X" notch than the control board #1 without the starch layer. This indicates that the starch layer significantly improved the adhesion of the paper cover sheet to the gypsum core layer made from an aqueous gypsum slurry containing 1200 ppm. anions Cl ^- .

[00147] Similarly, when comparing samples #3 and #4 in FIG. 5 side by side, the experimental slab #4 with the starch layer had less exposed gypsum at each "X" notch than the control slab #2 without the starch layer. This indicates that the starch layer significantly improved the adhesion of the paper cover sheet to the gypsum core layer made from an aqueous gypsum slurry containing 2100 ppm. anions Cl ^- .

[00148] In FIG. 6 illustrates photographs showing the effect of a dilute starch solution from the same raw starch used in the above examples, and in addition, perforations on a 12" x 12" x 1/2" drywall board having a gypsum plaster core, containing 2000 ppm Cl ^- from NaCl per million parts of calcium sulfate hemihydrate. Figure 6 compares the control sample of the plate No. 5 with the experimental sample of the plate No. 6 according to this invention. Control Board No. 5 had a back paper cover sheet without perforations and a layer of starch between the back paper cover sheet and the gypsum core. Experimental Board No. 6 of this invention had a perforated paper cover sheet and a layer of starch between the back paper cover sheet and the gypsum core. For this perforated paper sheet, the distance between adjacent perforations was ¼ inch. The diameter of the perforations ranged from about 0.01 to 0.02 inches.

[00149] When comparing board samples #5 and #6 in FIG. 6, when the chloride anion was 2000 ppm, the experimental board sample No. 6 with the starch layer and perforation treatment had a smaller exposed gypsum area. Thus, the experimental sample of the plate No. 6 had better adhesion than the control sample of the plate No. 5.

[00150] In addition, a comparison of the experimental plate No. 6 in FIG. 6 with the experimental plate No. 2 in FIG. 5 with the same starch layer but without the perforations showed that the perforations further improved adhesion.

[00151] In FIG. 7 illustrates photographs showing the effect of using a dilute starch solution on a 12" x 12" x 1/2" drywall board having a gypsum core made from 30 ppm stucco gypsum. Cl ^- from NaCl per million parts of calcium sulfate hemihydrate. This stucco is a normal stucco with a low content of chloride salt. The diluted starch solution was the same as in the board samples in FIG. 6. In FIG. 7 illustrates Control Board #7 with a back paper cover sheet having no starch layer between the back paper cover sheet and the gypsum core. On FIG. 7 also illustrates board prototype No. 8 having a layer of starch between the back paper cover sheet and the gypsum core.

[00152] When comparing the control board No. 7 and the experimental board No. 8, when the content of chloride anion is 30 ppm, the open area of the control board No. 7 is similar to the area of the experimental board No. 8. This indicates that the starch layer did not significantly improve paper-core adhesion in these samples, which had low levels of chloride salts.

[00153] The foregoing are merely examples of the invention. One of ordinary skill in the art will appreciate that each of these examples may be used in various combinations with other aspects of the invention set forth herein.

[00154] ITEMS OF THE INVENTION

[00155] Various aspects of the present invention are described in the following paragraphs:

[00156] 1. Drywall, comprising:

the core layer of the slab containing the hardened gypsum,

a front paper cover sheet having an outer surface and an inner surface, the inner surface being in contact with the first face of the board core; And
a back paper cover sheet having an outer surface and an inner surface, the inner surface being in contact with a second face of the board core;

wherein the inner surface of at least one of the front paper cover sheet and the back paper cover sheet is coated with a layer of starch;

wherein the core layer is located between the first cover sheet and the second cover sheet,

wherein the core layer is formed as a result of solidification between the front cover paper sheet and the back cover sheet of an aqueous slurry containing a mixture of water and stucco, and the stucco contains calcium sulfate hemihydrate, while the aqueous slurry contains:

at least 60% wt. said calcium sulfate hemihydrate on a dry basis,

from about 500 ppm up to about 3000 ppm chloride anions per 1,000,000 parts by weight of said calcium sulfate hemihydrate, and

the specified water at a mass ratio of water to calcium sulfate hemihydrate from 0.2:1 to 1.2:1.

[00157] Claim 2. The drywall of claim 1, wherein the starch layer is a continuous layer of starch on the entire inner surface of at least one of the front paper cover sheet and the back cover sheet.

[00158] Claim 3. Drywall according to claim 1 or 2, wherein the starch layer contains raw starch.

[00159] Item 4. Drywall according to any one of paragraphs. 1-3, wherein the starch layer contains a mixture of raw starch and cooked starch.

[00160] Claim 5. Drywall according to any of the preceding claims, wherein the aqueous slurry contains from about 1000 ppm. up to about 3000 ppm chloride anions per 1,000,000 parts by weight of said calcium sulfate hemihydrate.

[00161] Claim 6. Drywall according to any one of the preceding claims, wherein a layer of starch is applied to the inside surface of the back cover sheet.

[00162] Claim 7. Drywall according to any one of the preceding claims, wherein a layer of starch is applied to the inside surface of the front paper cover sheet and the inside surface of the back cover sheet.

[00163] Claim 8. Drywall according to any one of the preceding claims, wherein the front paper cover sheet is the front paper cover sheet and the back paper cover sheet is the back paper cover sheet.

[00164] Claim 9. The drywall of any one of the preceding claims, wherein the board core has a low density region and at least one high density region, wherein at least one high density region is layered between the low density region and the front paper cover. sheet or back cover sheet.

[00165] Item 10. Drywall according to claim 9, wherein the low density area is located between the first high density area located in the form of a first layer in contact with the front paper cover sheet or a layer of starch applied thereto, and the second high density area, arranged as a second layer in contact with the backing paper cover sheet or the layer of starch deposited thereon.

[00166] Claim 11. The drywall of claim 9 or 10, wherein the low density region is formed from a foamed gypsum slurry and contains a plurality of voids.

[00167] Item 12. Drywall according to any one of the preceding claims, in which a layer of starch is applied to the inner surface in an amount of from about 0.5 lb/MSF to about 15 lb/MSF, from about 0.5 lb/MSF to about 5 lb /MSF, more preferably from about 1 lb/MSF to about 2 lb/MSF.

[00168] Claim 13. Drywall according to any of the preceding claims, wherein the set gypsum is formed from synthetic gypsum containing one or more chloride salts.

[00169] Item 14. A method of making drywall, comprising:

preparation of an aqueous slurry containing a mixture of water and stucco, wherein the stucco contains calcium sulfate hemihydrate, the aqueous slurry containing a mixture of:

at least 60% wt. said calcium sulfate hemihydrate on a dry basis,

from about 500 ppm up to about 3000 ppm chloride anions per 1,000,000 parts by weight (parts by weight) of said calcium sulfate hemihydrate, and

water at a mass ratio of water to calcium sulfate hemihydrate from 0.2:1 to 1.2:1; and

distributing the aqueous suspension between the front paper cover sheet and the back cover sheet, each cover sheet having an inner surface and an outer surface;
wherein the inner surface of at least one of the front paper cover sheet and the back paper cover sheet is coated with a starch layer, the gypsum slurry being in contact with the starch layer;
solidifying calcium sulfate hemihydrate to form a panel containing a gypsum core containing calcium sulfate dihydrate; And
drying the panel; and cutting the panel into gypsum boards having one or more predetermined dimensions.

[00170] Item 15. The method of claim 14, further comprising:
applying a layer of starch to the inner surface of at least one of the front paper cover sheet and the back paper cover sheet in the form of a continuous layer of starch covering the entire inner surface.

[00171] Item 16. The method of claim 14, further comprising:
applying a layer of starch to the inner surface of at least one of the front paper cover sheet and the back paper cover sheet in the form of a continuous layer of starch covering the entire inner surface, while the back paper cover sheet has perforations.

[00172] Item 17. The method according to any one of paragraphs. 14-16, wherein at least a portion of the gypsum slurry is in a foamed state when positioned between the front paper cover sheet and the back cover sheet.

[00173] Claim 18. The method of claim 17, wherein the first portion of the gypsum slurry is distributed in a non-foamed state as a first high density region in the form of a layer that is in contact with the front paper cover sheet or a layer of starch thereon, and the second portion The gypsum slurry is distributed in the foamed state as a low density area which is in contact with the first high density area.

[00174] Item 19. The method of claim 18, wherein the low density region is located between the first high density region and the second high density region in the form of a layer that is in contact with the back paper coversheet or the starch layer thereon.

[00175] Item 20. The method according to any one of paragraphs. 14-19 wherein the starch layer is applied in an amount of from about 0.5 lb/MSF to about 15 lb/MSF, preferably from about 0.5 lb/MSF to about 15 lb/MSF, more preferably from about 1 lb/MSF up to about 2 lbs/MSF.

[00176] Item 21. The method according to any one of paragraphs. 14-20, in which calcium sulfate hemihydrate contains synthetic gypsum containing one or more chloride salts.

[00177] Item 22. A wall system comprising a frame to which at least one drywall board according to any one of paragraphs is attached. 1-13 with the outer surface of the front paper cover sheet facing away from the frame.

[00178] Claim 23. The wall system of claim 22, wherein the drywall is on an interior wall or ceiling of a building.

[00179] Item 24. Wall system according to item 22 or 23, in which the frame is made of wood.

[00180] Item 25. The wall system of claim 22 or 23, wherein the frame is made of metal.

[00181] Item 26. Wall system according to any one of paragraphs. 22-25, wherein at least one drywall board is attached to the frame with one or more screws, nails, or adhesive.

[00182] This document describes the preferred embodiments of the present invention, including the best, according to the applicants, the way of implementing the invention. For those of ordinary skill in the art, the possibility of changing such preferred embodiments of the invention may become apparent upon reading the foregoing description. The inventors expect those skilled in the art to make use of such variations where appropriate, and the inventors expect the invention to be practiced in a manner other than specifically described herein. Accordingly, this invention contains all changes and equivalents to the subject matter of the invention, listed in the claims attached to this document, in accordance with the norms of the current legislation. Moreover, the invention covers any combination of the above described elements in all their possible variations, unless otherwise indicated in this document, or this clearly does not contradict the context.

[00183] All links mentioned herein are hereby incorporated by reference to the same extent as in the case of an individual and separate indication for each link to include its content by reference and set forth herein in its entirety.

[00184] The terms given in the singular and in the form of the term "at least one", as well as similar references in the context of the description of this invention (especially in the context of the following claims) are intended to cover both the singular and the plural, if this document does not indicate otherwise, or unless it is clearly contrary to the context. The use of the term "at least one" followed by a listing of one or more elements (e.g., "at least one of A and B") should be understood to mean one element selected from the listed elements (A or B) or any combination of two or more of the listed elements (A and B), unless otherwise indicated in this document, or unless it clearly contradicts the context. "Coupling" does not mean that the two layers are in direct contact. The terms "comprising," "having," "including," and "comprising" are intended to be non-limiting terms (i.e., meaning "including, but not limited to"), unless otherwise noted. The listing of ranges of values in this document is intended solely as a symbolic way of individually referring to each individual value belonging to a given range, unless otherwise noted in this document, with each individual value included in the description in the same way as individually specified in this document. All methods described herein may be performed in any acceptable order, unless otherwise specified in this document or clearly contradicts the context. The use of any and all examples or exemplary language (for example, "for example") given in this document is intended solely to better illustrate the invention and does not impose restrictions on the scope of the invention, unless otherwise stated. No wording in the description is intended to indicate any non-declared element as being essential to the practice of the invention.

[00185] Unless otherwise indicated, all percentages, ratios, and average molecular weights are by weight.

Claims (30)

1. Drywall containing: the core layer of the slab containing the hardened gypsum, a front paper cover sheet having an outer surface and an inner surface, the inner surface being in contact with the first face of the board core; And a back paper cover sheet having an outer surface and an inner surface, the inner surface being in contact with a second face of the board core; wherein the inner surface of at least one of the front paper cover sheet and the back paper cover sheet is coated with a layer of starch; wherein the core layer is located between the first cover sheet and the second cover sheet, wherein the core layer is formed as a result of solidification between the front cover paper sheet and the back cover sheet of an aqueous slurry containing a mixture of water and stucco, and the stucco contains calcium sulfate hemihydrate, while the aqueous slurry contains: at least 60% wt. said calcium sulfate hemihydrate on a dry basis, from about 500 ppm up to about 3000 ppm chloride anions per 1,000,000 parts by weight of said calcium sulfate hemihydrate, and the specified water at a mass ratio of water to calcium sulfate hemihydrate from 0.2:1 to 1.2:1. 2. Drywall according to claim 1, characterized in that the starch layer is a continuous layer of starch on the entire inner surface of at least one of the front paper cover sheet and the rear cover sheet, wherein the starch layer contains crude starch. 3. Drywall according to claim 1, characterized in that the starch layer is a continuous layer of starch on the entire inner surface of at least one of the front paper cover sheet and the back cover sheet, and the starch layer contains a mixture of raw starch and cooked starch. 4. Drywall according to claim. 1, characterized in that the aqueous suspension contains from about 1000 ppm. up to about 3000 ppm chloride anions per 1,000,000 parts by weight of said calcium sulfate hemihydrate. 5. Drywall according to claim 1, characterized in that a layer of starch is applied to the inner surface in an amount of from about 0.5 lb/MSF (2.4 g/m ² ) to about 15 lb/MSF (73.2 g/m ² ). 6. Method for the manufacture of drywall, including: preparation of an aqueous slurry containing a mixture of water and stucco, wherein the stucco contains calcium sulfate hemihydrate, the aqueous slurry containing a mixture of: at least 60% wt. said calcium sulfate hemihydrate on a dry basis, from about 500 ppm up to about 3000 ppm chloride anions per 1,000,000 parts by weight (parts by weight) of said calcium sulfate hemihydrate, and water at a mass ratio of water to calcium sulfate hemihydrate from 0.2:1 to 1.2:1; and distributing the aqueous suspension between the front paper cover sheet and the back cover sheet, each cover sheet having an inner surface and an outer surface; wherein the inner surface of at least one of the front paper cover sheet and the back paper cover sheet is coated with a starch layer, the gypsum slurry being in contact with the starch layer; solidifying calcium sulfate hemihydrate to form a panel containing a gypsum core containing calcium sulfate dihydrate; And drying the panel; and cutting the panel into gypsum boards having one or more predetermined dimensions. 7. The method of claim 6, further comprising: applying a layer of starch to the inner surface of at least one of the front paper cover sheet and the back paper cover sheet in the form of a continuous layer of starch covering the entire inner surface. 8. The method of claim 6, further comprising: applying a layer of starch to the inner surface of at least one of the front paper cover sheet and the back paper cover sheet in the form of a continuous layer of starch covering the entire inner surface, the rear paper cover sheet having perforations. 9. The method of claim 6 wherein the calcium sulfate hemihydrate comprises a synthetic gypsum containing one or more chloride salts. 10. A wall system comprising a frame to which at least one drywall board of claim 1 is attached, wherein the outer surface of the front paper cover sheet faces away from the frame.

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