MXPA98006687A - Gypsum wood fiber product having improved water resistance - Google Patents

Gypsum wood fiber product having improved water resistance

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Publication number
MXPA98006687A
MXPA98006687A MXPA/A/1998/006687A MX9806687A MXPA98006687A MX PA98006687 A MXPA98006687 A MX PA98006687A MX 9806687 A MX9806687 A MX 9806687A MX PA98006687 A MXPA98006687 A MX PA98006687A
Authority
MX
Mexico
Prior art keywords
suspension
wax
gypsum
calcium sulfate
filter cake
Prior art date
Application number
MXPA/A/1998/006687A
Other languages
Spanish (es)
Inventor
Song Weixin
Original Assignee
United States Gypsum Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by United States Gypsum Company filed Critical United States Gypsum Company
Publication of MXPA98006687A publication Critical patent/MXPA98006687A/en

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Abstract

The present invention relates to an improved composite material;more particularly to a composite gypsum board product having improved water resistance which is especially useful for making building products. Specifically, the present invention relates to an improved gypsum/wood fiber building board having enhanced water resistance through the addition of a wax emulsion to the gypsum and wood fiber during the board manufacturing process.

Description

PLASTER AND WOOD FIBER PRODUCT. THAT HAS IMPROVED WATER RESISTANCE DESCRIPTION OF THE INVENTION The present invention relates to an improved composite material; more particularly with a gypsum panel product having improved water resistance, which is specifically useful for manufacturing construction products. Specifically, the present invention relates to an improved fiber board for construction made of gypsum / wood fiber, which has greater resistance to water through the addition of a wax emulsion to the gypsum and the wood fiber during the process of manufacture of wood fiberboard.
BACKGROUND AND PREVIOUS TECHNIQUE Certain properties of gypsum (calcium sulfate dihydrate) make it very popular for use in the manufacture of industrial and construction products; especially drywall. It is a full and generally cheap raw material which, through a process of dehydration and rehydration, can be emptied, molded or in other circumstances formed into useful forms. It is also incombustible and relatively dimensionally stable when exposed to moisture. However, because it is a REF-27960 brittle crystalline material, which has a relatively low tensile and flexural strength, its uses are typically limited to non-structural applications, which do not bear load and do not absorb impacts. Gypsum cardboard; also known as plasterboard or drywall, it consists of a rehydrated gypsum core sandwiched between a multiplicity of paper cover sheets, and is used extensively for interior wall and roof applications. Due to the fragility and low retention properties of nails and screws in its gypsum core, the conventional drywall, by itself can not withstand heavy loads or absorb significant impacts. Consequently, means for improving tensile strength, bending, nail and screw retention and impact resistance of plaster and gypsum construction products have been sought and still sought for a long time. Another readily available and available material, which is also widely used in construction products, is a lignocellulosic material particularly in the form of wood and paper fibers. For example, in addition to construction wood, board of small pieces of wood compressed at high pressure, cardboard of wood pulp and waste paper, thin cardboard, plywood and "hard" cardboard (wood pulp board or high-grade waste paper) density) are some of the forms of processed lignocellulosic material used in the construction industry. Such materials have better resistance to traction and bending than plaster. However, they are also generally more expensive, have little resistance to fire and are often susceptible to swelling or rolling when exposed to moisture. Therefore, means that can be provided to improve those limiting properties of use of construction products made of cellulosic material are also desirable. Previous attempts to combine the favorable properties of gypsum and cellulose fibers, particularly wood fibers, have had very limited success. Attempts to add cellulosic fibers, (or other fibers for that purpose), to the core of gypsum plaster and / or plasterboard have generally produced little or no improvement in strength due to the inability hitherto to achieve significant binding between the fibers and the plaster. U.S. Patent 4,328,178; 4,239,716; 4,392,896 and 4,645,548 disclose recent examples where wood fibers or other natural fibers were mixed in a stucco suspension (calcium sulfate hemihydrate) for the purpose of serving as reinforcements for a fiber board for rehydrated gypsum or the like. The American Patent 4, 734,163, teaches a process in which raw or lcined gypsum is finely milled and wet mixed with 5-10% paper pulp. The dough is partially dehydrated, formed into a cake and subsequently dehydrated by means of pressure rollers until the water / solids ratio is less than 0.4. The cake is cut into raw cartons, which after being t and cut, are stacked between two steel plates and placed in an autoclave. The temperature in the autoclave rises to approximately 140 ° C to convert the gypsum to the calcium alpha-hydrate. During the subsequent increasing cooling of the cartons of the container, the hemihydrate is rehydrated to dihydrate (gypsum) and gives integrity to the carton, the cartons are then dried and finished as necessary. U.S. Patent 5,320,677 to Baig discloses a composite product and a process for producing the product in which a dilute suspension of gypsum particles and wood fiber is heated under pressure to convert the gypsum to alpha calcium sulfate hemihydrate. Wood fibers have pores or voids on the surface and the alpha hemihydrate forms crystals inside, over and around the voids and pores of the wood fibers. The hot slurry is then dehydrated to form a filter cake, preferably using equipment similar to papermaking equipment, and before the slurry is too chilled to rehydrate the hemihydrate to gypsum, the filter cake is compressed to a cardboard of the desired configuration. The compressed filter cake is cooled and the hemihydrate is rehydrated to gypsum to form a dimensionally stable and useful fiberboard construction board. The cardboard is deburred and dried later. The process described in the '5,320,677 patent is distinguished from the first method in which the calcination of the gypsum takes place in the presence of wood fibers, whereas the gypsum is in the form of a diluted suspension, so that the suspension wets the wood fibers , taking the dissolved gypsum to the holes of the fibers, and the calcination forms acicular calcium sulphate alpha hemihydrate crystals in and around the voids. These prior art products, like ordinary gypsum board, gypsum slab, gypsum blocks, gypsum castings and the like have relatively little water resistance. When ordinary gypsum board, for example, is submerged in water, the board quickly absorbs a considerable amount of water, and loses a large part of its resistance. Actual tests have shown that when a 5.08 cm x 10.16 cm (2 inch x 4 inch) cylinder of gypsum board core material was immersed in water at approximately 21.11 ° C (70 ° F), the cylinder showed an absorption of 36% water after being submerged for 40 minutes. Many attempts have been made in the past to improve the water resistance to gypsum products. These attempts have included the incorporation of water-resistant materials, such as metal soaps, asphalts, waxes, resins, etc., into the suspension of calcium sulfate hemihydrate. They have also included attempts to coat the finished gypsum product with water resistant films or coating. A specific example of past attempts to make water-impermeable gypsum, integrating it by the addition of water-repellent substances is described in King and Camp Patent 2,198,776. This shows the incorporation of paraffin, wax, asphalt, etc., into the aqueous suspension by spraying the molten material into the suspension. U.S. Patent No. 2,432,963 describes the addition of a wax emulsion, such as paraffin wax and asphalt, in relative proportions of about 1 part to about 10 parts of asphalt by wax to the gypsum suspension. Since asphalt is a relatively poor solvent for paraffin waxes and similar waxes at ordinary temperatures, the solution formed at high temperatures tends, during cooling, to deposit microscopic wax crystals on the asphalt-wax surface, thus ensuring unusual water-repellent properties. U.S. Patent 2,526,537 describes the addition of potassium sulfate to such a combination of asphalt-wax. U.S. Patent 5,437,722 also discloses an emulsion based on paraffin wax for use with gypsum compositions. It is an object of the present invention to provide a gypsum board-fiber product of wood having the strength and dimensional stability of the product type described in U.S. Patent 5,320,677 and having improved water resistance. The present invention provides a method for manufacturing a gypsum board product having improved water resistance, which comprises: adding an aqueous wax emulsion to an aqueous suspension of a calcium sulfate material and guest particles, the emulsion is stable under the conditions in which the suspension is maintained; passing the suspension containing wax on a porous, flat forming surface to form a filter cake; removing a substantial portion of the water from the filter cake through the porous surface; compress the filter cake to form a cardboard and remove the additional water; and dry the cardboard to remove the remaining free water and make the cardboard core reach a temperature sufficient to melt the wax. The present invention further provides a method for making a gypsum board product having improved water resistance, which comprises: adding an aqueous wax emulsion to an aqueous suspension of a calcium sulfate material and guest particles, while such a suspension is at a temperature at which the calcium sulfate hemihydrate crystals are maintained, the emulsion is stable under the conditions in which the calcium hemihydrate crystals are maintained; passing the wax-containing suspension over a flat porous forming surface to form a filter cake before the temperature of the filter cake falls below the temperature at which the calcium hemihydrate is rapidly rehydrated to the dihydrate of calcium phosphate; remove a substantial portion of the water from the filter cake through the porous surface and cool the filter cake to a temperature at which rehydration begins, compress the filter cake to form a cardboard and remove the additional water, so that the calcium sulfate hemihydrate crystals around the hot particles are rehydrated in situ to calcium sulfate dihydrate crystals; and dry the cardboard to remove the remaining free water and make the core of the cardboard reach a temperature sufficient to melt the wax. A related object is to provide a process for producing such an improved gypsum board product, wherein a wax emulsion is added to a hot aqueous suspension of calcium sulfate hemihydrate with other substances having higher strength, such as a fiber. of wood, wherein the suspension containing hot wax is passed over a porous, flat forming surface, to form a shaped filter cake, which is processed to provide the gypsum board product. A more specific objective of the invention is to provide a paperless pressed fiber board which has uniformly good strength, including resistance to the start of nails and screws, in its entirety; that is dimensionally more stable; and that it is more resistant to water, that is to say, that it maintains its resistance even when exposed to water; that is very resistant; and that can be produced at practical costs. The main objectives are realized, according to the invention, by adding a wax emulsion to a hot suspension of calcium sulfate hemihydrate and a host particle of a stronger material, by passing the hot suspension over a flat forming surface, porous, to form a filter cake which dehydrates and compresses to form a cardboard before the hemihydrate is completely re-hydrated to gypsum. The main objectives are realized, preferably according to the invention, by adding a wax emulsion to a diluted suspension of ground gypsum which has been calcined under conditions which produce crystals of alpha acicular hemihydrate and around the gaps of a host particle. a stronger material, by passing the suspension to a flat, porous forming surface, to form a filter cake which dehydrates with minimal loss of the wax emulsion. The filter cake is compressed to form a paperboard before the hemihydrate is completely rehydrated to gypsum, after which the paperboard is dried under conditions that melt the wax inside the paperboard. It has been found that the addition of the wax emulsion not only improves the strength of the board to water, but the cardboard will retain its strength and in some cases, the addition of wax improves the strength of the product. The term "plaster", as used herein, means calcium sulfate in stable dihydrate state, ie, CaS? 4'2H20, and includes the mineral found in nature, the synthetically derived equivalents and the dihydrate material formed by the hydration of the hemihydrate ( stucco) or calcium sulfate anhydride. The term "calcium sulfate material", as used herein, means calcium sulfate in any of its forms, primarily calcium sulfate anhydride, calcium sulfate hemihydrate, calcium sulfate dihydrate and mixtures thereof. The meaning of the term "host particle" encompasses any microscopic particle, such as a fiber, a chip or a flake, from another substrate other than plaster. The particle which is generally insoluble in the liquid of the suspension, must also have accessible holes in it; either holes, cracks, fissures, voids, or other imperfections on the surface, which are penetrable by the rest of the suspension and within which calcium sulfate crystals can form. It is also desirable that such voids are present on an appreciable portion of the particle; it is evident that the more and more distributed the holes are, the greater and more geometrically stable will be the physical union between the plaster and the host particle. The substance of the host particle should desirably have properties not found in the gypsum, and, preferably, at least a higher tensile and flexural strength. A lignocellulosic fiber, particularly a wood fiber, is an example of a host particle especially well suited for the composite material and the process of the invention. Therefore, without intending to limit the material and / or particles that qualify as a "host particle", wood fibers are often used here later for convenience rather than the broader term. The meaning of the term "gypsum / wood fiber", which is sometimes abbreviated as "GWF", as used herein, covers a mixture of gypsum and host particles, for example, wood fibers, which are used to produce cartons wherein at least a portion of the gypsum is in the form of acicular calcium sulfate dihydrate crystals placed in and around the hollows of the host particles, wherein the dihydrates are formed in themselves by the hydration of the crystals of Acicular calcium sulfate hemihydrate in and around the voids of such particles. The GWF boards are preferably produced by the process of US Pat. No. 5,320,677. The term "wax emulsion", as used herein, means an aqueous emulsion of one or more waxes, which are emulsified through the use of one or more surfactants. The wax emulsion must comprise a wax or waxes adapted to provide water resistance to the finished product. The wax or waxes must be inert with respect to the gypsum and wood fibers that make up the product. The wood should be in the form of an emulsion which is stable under conditions of temperature and pressure under which the calcium sulfate alpha / wood fiber hemihydrate suspension emerges from the calcination process. More importantly, the wax emulsion should not only be stable in the presence of the different additives that are used to regulate the crystallization of the sem-ihydrate and the different accelerators or retarders that are used to adjust the procedure by which rehydration occurs to plaster, but that the wax emulsion should not interfere with the operation of those additives. More importantly, a high proportion of the wax must adhere to the gypsum / wood fiber particles during the procedure whereby the suspension is dehydrated to remove most of the water and a filter cake is formed, to prevent loss of wax with the water removed from the suspension. The melting point of the wax must be lower than the core temperature reached by the board during the final drying of the product. In the preferred embodiment, a cationic surfactant, such as a quaternary amine, was included in the wax emulsion at the time of adding the wax emulsion to the hot suspension. In the procedure, the uncalcined gypsum and the host particle were mixed together with sufficient liquid to form a diluted suspension, which was then heated under pressure to calcine the gypsum, converting it to an alpha hemihydrate of calcium sulfate. Although the micro-mechanics of the invention is not completely understood, it is believed that the menses of the diluted suspension wet the host particle, carrying the dissolved calcium sulfate to the voids therein. The hemihydrate eventually forms nuclei and forms crystals, predominantly acicular crystals, in si tu in and around the hollows of the host particle. Glass modifiers can be added to the suspension if desired. . The resulting compoon is a host particle physically entangled with calcium sulfate crystals. This entanglement not only creates a good bond between the calcium sulfate and the stronger host particle, but also prevents the migration of calcium sulfate away from the host particle when the hemihydrate is subsequently rehydrated to the dihydrate (gypsum). A plurality of such compo particles form a mass of material which can be compacted, compressed into cartons, cast, sculpted, molded, or otherwise formed, in the form desired for a final assembly. After the final assembly, the compo material can be cut, waxed, felted, perforated and in other circumstances machined. In addition, it exhibits the desirable fire resistance and dimensional stability of the gypsum plus certain improvements (particularly strength and strength) bestowed by the substance of the host particle. According to a preferred embodiment of the invention, the host particle is a paper fiber. The process for manufacturing a gypsum / wood fiber compo material, according to the invention, starts by mixing between about 0.5% to about 30%, and preferably between about 3% to 20%, by weight (based on the total solids), wood fibers with the respective complement of ground gypsum, but not calcined. The dry mixture is combined with sufficient liquid, preferably water, to form a dilute suspension having about 70% -95% by weight of water. The suspension is heated in a pressure vessel at a temperature sufficient to convert the gypsum to calcium sulfate hemihydrate. It is desirable to continuously agitate the suspension with stirring or gentle mixing to break up any clumps of fiber and keep all the particles in suspension. After the hemihydrate has precipitated from the solution and formed acicular hemihydrate crystals, the pressure in the product suspension is decreased when the suspension is discharged from the autoclave, and the wax emulsion is added. When still hot, the suspension is discharged through a head onto a continuous felt conveyor, such as the type used in papermaking operations, to form a filter cake and remove as much non-combined water as possible. Wax emulsion is added to the suspension, along with the selected process modifier or additives that improve properties, such as accelerators, retarders, weight reducing fillers, etc., - before the suspension is passed through of the head on the felt conveyor on which the filter cake is formed. Up to 90% of the water can be removed from the filter cake by the felt conveyor. As a result of the removal of water, the filter cake is cooled to a temperature at which rehydration can begin. However, it may still be necessary to provide additional external cooling to bring the temperature to a sufficiently low level to achieve rehydration within an acceptable time. Before extensive rehydration takes place, the filter cake is preferably wet compressed into a board of the desired thickness and / or den. If the cardboard is to be given a special surface texture or a laminated surface finish, this could preferably occur during or after this process step. During wet compression, which preferably takes place with a gradual cooling of the pressure to preserve the integrity of the product, two things occur. Additional water is removed, for example about 50-60% of the remaining water. As a consequence of the removal of additional water, the filter cake is further cooled to a temperature at which rapid rehydration occurs. The calcium sulfate hemihydrate is hydrated to gypsum, so that acicular calcium sulfate hemihydrate crystals are converted to gypsum crystals in and around the wood fibers. After some rehydration, the cartons can be cut and deburred, if desired, and then, after complete rehydration, be sent through an oven for drying. Preferably, the drying temperature should be kept low enough to avoid recalculation of any gypsum on the surface, but high enough to cause the core temperature of the board to exceed the melting point of the wax, at least briefly. To achieve maximum improvement in water resistance, it was coered essential to use a wax emulsion, which is stable in the GWF suspen at the temperature and in the existing chemical environment during the time in which the suspen becomes a Filtration cake and dehydrated. The stability of the wax emulsion is markedly improved by the use of a cationic emulsifier in the wax emulsion. It has been found that wax emulsions that are not sufficiently stable produce GWF boards with lower water resistance. Such emulsions also tend to separate from the filter cake and deposit on the equipment. The wax selected for the emulsion should have a sufficiently low melting point so that it melts and disperses perfectly through the GWF board when the board or panel is dried. A board or panel composed of gypsum / wood fiber, made according to the above procedure offers a GWF board having improved water resistance, as well as the synergistic compilation of the desirable characteristics offered by the prior art boards, for example , the cards made by the process of US Pat. No. 5,320,677. Because the paperboard of the present invention has improved water resistance, it offers better strength, including resistance to pick-up of nails and screws, on conventional gypsum board and the wood-based gypsum / paperboard boards of the prior art. In addition, this can occur in a range of dey and thickness. These and other features and advantages of the invention will be apparent to those skilled in the art, following the more detailed discussion of the invention, DETAILED DESCRIPTION OF THE PRESENT INVENTION The basic procedure begins by mixing uncalcined gypsum and guest particles (eg wood fiber or paper) with water to form a dilute aqueous suspension. The gypsum source can be crude byproduct ore or a desulfurization process of combustible gas or phosphoric acid. The gypsum should be of relatively high purity, that is, preferably at least about 92-96%, and finely ground, for example, from the 92-96% less than 100 mesh or smaller. Larger particles can prolong the conversion time. The plaster can be introduced either as a dry powder or via an aqueous suspension.
The host particle is preferably a cellulosic fiber which come from pulp, wood pulp, wood chips, and / or other plant fiber. It is preferred that the fiber be one whose surface is porous, hollow, scored and / or rough so that its physical geometry provides accessible gaps or gaps which accommodate the penetration of dissolved calcium sulfate. In any case the source, for example, wood pulp, also require previous cement products to remove lumps, separate the material of excessive dimensions and insufficient dimensions, and, in some cases, remove the materials that retard the resistance and / or contaminants that could adversely affect the calcination of the plaster; such as semicellulose, acetic acid, etc. The ground gypsum and the wood fiber are mixed with sufficient water to make a suspension containing about 5-30% by weight solids, although suspensions having approximately 5-20% by weight solids are preferred. The solids in the suspension should comprise from about 0.5% to 30% by weight of wood fibers and preferably from about 3% to 20% of wood fibers, the remainder being mainly gypsum.
CONVERSION TO SEMIHYDRATE The suspension is fed to a pressure vessel equipped with a continuous stirring or mixing device. Crystal modifiers, such as organic acids, be added to the suspension at this point, if desired, to stimulate or retard crystallization or decrease the calcination temperature. Steam is injected into the container to bring the interior temperature of the container to between about 212 ° F (100 ° C) and about 350 ° F, 177 ° C), and autogenous pressure; the lowest temperature is approximately the practical minimum at which the calcium sulfate dihydrate will be calcined to the hemihydrate state within a reasonable time; and the highest temperature is approximately the maximum temperature for calcining the hemihydrate without the undue risk of causing some of the calcium sulfate hemihydrate to be converted to anhydride. The temperature of the autoclave is preferably in the range of about 285 ° F (140 ° C) to 305 ° F (152 ° C). When the suspension is processed under these conditions for a sufficient period of time, for example of the order of 15 minutes, sufficient water will be removed from the calcium sulfate dihydrate molecule to convert it to the hemihydrate molecule. The solution, aided by continuous agitation to keep the particles in suspension, will get wet and will penetrate the open holes in the host fibers. When it reaches the saturation of the solution, the hemihydrate will block and start to form crystals in, on and around the gaps in the area of the walls of the host fibers. It is believed that during the operation of the autoclave, the dissolved calcium sulfate penetrates the voids of the wood fibers and subsequently precipitates as acicular hemihydrate crystals inside, over and around the voids and surfaces of the wood fibers. When the conversion is completed, the pressure in the autoclave is reduced, the desired additives are introduced, -including the wax emulsion, typically in the head, and the suspension is discharged onto a dehydration conveyor. Conventional additives can be added including accelerators, retardants, preservatives, flame retardants and agents to encourage resistance to suspension at this point in the process. It has been found that certain additives, such as the particle accelerator (to accelerate the hydration of the calcium sulfate hemihydrate to gypsum) can greatly affect the level of improvement in water resistance exhibited by the wax emulsion. As a result, potash over alum and other materials is preferred as an accelerator.
WAX EMULSION The present invention broadly contemplates the addition of sufficient wax, in the form of a stable emulsion, to the suspension to create a product having at least about 1% by weight of wax distributed through the product. The present invention contemplates the use of any wax or combination of waxes although paraffin waxes are favored to form the emulsion. The wax emulsion used in the present invention preferably comprises a combination of a paraffinic hydrocarbon, montane wax, polyvinyl alcohol and water; and may contain the additives conventionally employed in emulsions, including ulsifiers to assist in the formation of the emulsion and stabilizers to assist in the stabilization of the emulsion. A suitable nonionic wax emulsion of this type is available from Bakor, Inc., under the trademark Aqualite 71, which contains a wax composition that is reported to have a melting point of 167 ° F (75 ° C). The following commercial paraffin waxes can be used: Gypseal II by Conoco Aqualite 70 by Bakor DeWax PA-R-40 by Deforest Enterprises MICHEM 955 by Michelman Paraffin wax preferably has a melting point of 40 ° to 80 ° C. When melting is higher than 80 ° C, it becomes necessary to use a high drying temperature in the manufacture of conventional gypsum board and this results in poor water resistance in the gypsum board. If the melting point is below 40 ° C, the quality of the resulting gypsum board is lower. The mountain wax, also known as lignite wax, is a hard wax, found in nature, from dark to amber. It is insoluble in water but soluble in solvents such as carbon tetrachloride, benzene and chloroform. The mountain wax is used in an amount of about 1 to 200 parts, preferably 1 to 50 parts, by weight, per 100 parts of paraffinic hydrocarbon. Polyvinyl alcohol is usually prepared by the hydrolysis of polyvinyl acetate and is preferably a substantially complete hydrolyzed polyvinyl acetate. Suitably this should be at least 90% hydrolyzed polyvinyl acetate and preferably 97 to 100% hydrolyzed polyvinyl acetate. Suitably polyvinyl alcohol is soluble in water at elevated temperatures of about 60 ° C to about 95 ° C, but is insoluble in cold water. The polyvinyl alcohol is used in an amount of about 1 to 50, preferably 1 to 20 parts, by weight, per 100 parts of the paraffin wax. The polyvinyl alcohol provides adhesive characteristics as well as a greater resistance to water. The water that forms the aqueous vehicle of the emulsion normally used in one. amount from 35 to 80%, preferably from 50 to 60%, by weight, of the emulsion. Conventional emulsifiers that can be added to a wax to form the emulsion include nonionic surfactants such as alkylphenoxypoly (ethyleneoxy) ethanols, sorbitan fatty acid esters and polyoxyethylene sorbitan fatty acid esters, anionic surfactants such as saponified fatty acids, and cationic surfactants, discussed below, which are typically employed in an amount of 0.1 to 5%, by weight, of the emulsion. Conventional stabilizers that can be added to the wax emulsion include alkali metals or ammonium hydroxides, which are typically used in an amount of 0.1 to 1%, by weight, of the emulsion. In the present invention, a cationic emulsifier is preferably included in the emulsion. The cationic emulsifier may be the sole emulsifier or may be used in combination with other emulsifiers. A particularly preferred cationic emulsifier is the quaternary amine surfactant sold by ICI Surfactants under the trademark G-265. Other useful cationic emulsifiers include quaternary ammonium chlorides Tomah Q-17-2 (sold by Tomah Products, Inc.) and Ethoquad C / 25 (sold by AKZO Chemicals, Inc.). The chemical formula of Q-17-2 is as follows: CH2CH2OH R-0-CH2CH2CH2 - Ñ + - CH3 CI "V" \ CH2CH2OH The addition of the cationic surfactants, described above, as the sole emulsifier or in combination with other conventional emulsifiers in the wax emulsion, promote the stability of the wax emulsion under the high temperatures employed to produce the paperboard of the present invention. It has been found that under some conditions the wax emulsion, which does not include the cationic surfactant, can break and allow the wax particles to agglomerate, which results in the filter cake adhering to the forming equipment and decreasing the water resistance of the resulting cardboard. It is theorized that the addition of the cationic surfactant improves the retention of the wax in the filter cake and in the resulting paperboard because the positively charged cationic wax / surfactant binds to the negatively charged fiber surface. The wax emulsion can be prepared by heating the paraffinic hydrocarbon in the wax montana to a molten state and mixing them together. A hot aqueous solution of polyvinyl alcohol containing emulsifiers and stabilizers is passed through the hot paraffin and montane wax mixture through a colloid mill and the resulting emulsion is allowed to cool. Other types of equipment and methods may be used to prepare the emulsion. The wax emulsion is added to the aqueous gypsum / wood fiber suspension and mixed with the suspension in proportions to obtain from 0.5 to 20 parts, and preferably from about 1 to 3 parts, by weight of wax solids per 100 parts of plaster. Other ingredients such as foaming agents, dispersing agents and setting accelerators may be included in the suspension. It has been found that between 65 and 90% of the wax of the emulsion added to the suspension is retained on the gypsum / wood fiber product, the rest is lost in the dehydration step of the process. The wax solids content of the wax emulsion added to the suspension is not critical.
The wax emulsion is preferably added to the suspension after it has been released from the autoclave, preferably before the head, to provide sufficient time for the wax emulsion to thoroughly mix with the suspension prior to the formation of the filter cake and the dehydration step of the procedure. The temperature of the suspension at the time the emulsion is added is not critical, but it is essential that the wax emulsion be stable under the conditions of the suspension. In some embodiments, the temperature of the suspension may be high enough to maintain the crystals of calcium sulfate hemihydrate. In any case, the wax emulsion must be stable at the temperature of the suspension at the moment when the wax emulsion is mixed with the gypsum-wood fiber suspension and the wax emulsion must remain stable in the presence of the additives, such as accelerators, if present in the. suspension. The wax emulsion should remain stable through the steps of dehydration and carton formation, but it is important that a high proportion of the wax be retained in the filter cake through dehydration and carton formation.
DEHYDRATION The wax-containing, hot suspension is passed through the head which distributes the suspension on a flat, porous forming surface to produce a filter cake. The filter cake is dehydrated by evaporation, water when the suspension is released from the autoclave and water in the suspension passing through the porous forming surface, preferably aided by vacuum. Although dehydration causes cooling of the filter cake, additional external cooling must be applied during the dehydration step. Most water should be removed if possible while the temperature of the product is still relatively high and before the hemihydrate becomes substantially gypsum. Up to 90% of the water in the suspension is removed in the dehydration device, leaving a filter cake with approximately 35% by weight of water. In this step the filter cake preferably consists of wood fiber entangled with crystals of rehydrated calcium sulfate hemihydrate and can be made even in individual composite fibers or nodes, formed, emptied, or compacted at a high density.
Formation of the filter cake, dehydration of the filter cake is preferably carried out using the papermaking equipment of the type described in U.S. Patent 5,320,677, which forms part of this description.
COMPRESSION AND REHYDRATION The dehydrated filter cake is preferably compressed wet for a few minutes to further reduce the water content and to compact the filter cake to the desired shape, thickness and / or density before substantial rehydration of the hemihydrate occurs. Although the removal of the volume of water in the dehydration step will contribute significantly to lower the temperature of the filter cake, additional external cooling may be required to reach the desired rehydration temperature within a reasonable time. The temperature of the filter cake is preferably reduced to less than about 120 ° F (49 ° C), so that relatively rapid rehydration can take place. The rehydration crystallizes again the crystals of alpha semidhidrato in acicular gypsum crystals instead, physically entangled with the wood fibers.
Depending on the accelerators, retarders, crystal modifiers or other additives provided in the suspension, hydration can take only a few minutes to an hour or more. Due to the entanglement of acicular hemihydrate crystals with wood fibers, and the removal of most of the carrier liquid from the filter cake, the migration of calcium sulfate is also avoided, leaving a homogeneous composition. The rehydration affects the recrystallization of the semihydrate crystals to dihydrate crystals in situ, ie in and around the hollows of the wood fibers, thus preserving the homogeneity of the composition. The growth of the crystals also connects the calcium sulfate crystals on the adjacent fibers to form a total crystalline mass, with better resistance due to the reinforcement of the wood fibers. When hydration is complete, it is desirable to immediately dry the compound mass to remove the remaining free water. In other circumstances, the hygroscopic wood fibers would tend to retain, or absorb, the non-combined water which will evaporate at the end. If the calcium sulfate coating is set completely before the extra water is extracted, the fibers can contract and stay out of the gypsum when the non-combined water evaporates. Therefore, for optimal results it is preferable to remove as much free water in excess of the composite mass as possible before the temperature drops below the level at which hydration begins.
DRYING The compressed paperboard, which typically contains about 30% by weight of free water, is then rapidly dried at a relatively high temperature to reduce the free water content to about 0.5% or less in the final product. During the drying step it is important to raise the internal temperature of the final product enough, for a short period of time, to perfectly melt the wax. Obviously, the drying conditions that tend to calcinate the plaster should be avoided. It has been found desirable to carry out drying under conditions in which the product reaches a core temperature of at least 170 ° F (77 ° C), and preferably a core temperature of between about 170 ° C. F (77 ° C) and 200 ° F (93 ° C). The dry-set board can be cut and in other circumstances finished to the desired specification. When it finally sets, the single composite exhibits the desired properties given by both of its two components. The wood fibers increase the resistance, particularly the resistance to friction, of the gypsum matrix, while the gypsum acts as a coating and binder to protect the wood fiber., give fire resistance and decrease the expansion due to humidity. The following examples will serve to illustrate the preparation and testing of gypsum / wood fiber products with improved water resistance of the present invention, although it should be understood that those examples are set forth for illustrative purposes and that many other gypsum products can be made. Wood fiber that have better water resistance using appropriate variations.
EXAMPLE 1 A standard GWF paperboard product was produced as follows: A mixture of 92% by weight of uncalcined FGD gypsum (the by-product of the fuel gas desulfurization) and 8% by weight of corrugated paper fiber pulp were added to an autoclave, with agitation, with enough water to create a suspension having 15% by weight solids. The resulting suspension was heated under pressure at approximately 146.1 ° C (295 ° F) for 15 minutes, which allowed the gypsum to calcify to form alpha hemihydrate. The pressure in the suspension was released when the suspension was discharged from the autoclave. The resulting evaporation of the water cooled the suspension to approximately 82.22 at 100 ° C (180 to 212 ° F). Accelerators were added to the suspension which was then fed into the head of the formation line. The accelerators were 2% by weight of K2SO4 (Potassium) and 2% by weight of a calcium sulfate dihydrate coated with sugar (as described, for example, in US Pat. No. 3,813,312), based on the weight of the gypsum. The suspension was distributed on a porous transporter on which a filter cake was formed. The filter cake was passed through a vacuum dewatering device which removed approximately 80% of the water and the filter cake / suspension reached the temperature of approximately 48.88 ° C (120 ° F). The filter cake was compressed to a cardboard of approximately 0.762 cm (0.3 inches) in thickness as it was subjected to an additional vacuum treatment to remove more water and cool the cartridge to approximately 35 ° C (95 ° F), for better rehydration of the gypsum hemihydrate. After rehydration, the board was cut into panels and the panels were dried under conditions which caused the core of the cardboard to reach approximately 93.3 ° C (200 ° F) for a short period of time. The resulting panels were then tested, as reported below.
Example 2 A stabilized wax emulsion was prepared as follows. A solution was formed by mixing 10 parts by weight of a cationic surfactant (G-265) with 90 parts by weight of water with stirring. This solution was added to a wax emulsion (Bakor-Aqualite 71) at a weight ratio of 100 to 1030. This provides a stabilized wax emulsion which contains: Aqualite active components 37% Cationic surfactant 1% Water 62%.
EXAMPLE 3 A paperboard was prepared according to Example 1, which was compared with a cardboard made under the same conditions, but in which the wax emulsion of Example 2 was added to the suspension, before the head, in an amount sufficient to provide 3% by weight of wax solids per pound (454 grams) of gypsum. Samples of both cartons were tested to determine water absorption, density and strength. The results of the test, reported below, show not only that the absorption of water increased markedly but that the strength of the cardboard was improved by the addition of the wax emulsion.
TABLE 1 Wax Absorption Water Density Strength used for weight gain (pcf) (psi) 0% 39% 66.8 1123 3% 3.5% 66.2 1221 Measurements of water absorption, density and resistance were made on 14 specimens of each of five cartons, and the average of 70 measurements was reported in Table 1. Water Absorption, reported in Table 1, was determined by following the ASTM C-473 test method, which is based on the complete immersion of the specimens for 2 hours. The weight of each specimen before and after immersion was used to calculate the weight gain. The density was determined by dividing the measured weight by the measured volume, while the strength was determined as the modulus of rupture ("MOR") according to ASTM test method D1037.
EXAMPLE 4 A paperboard was prepared according to Example 1, except that a commercial wax emulsion (Bakor Aqualite 71) was added to the slurry rather than to the head in tffta sufficient quantity to provide 2 wt.% Of wax solids for each pound (454 grams) of plaster. The analysis of the resulting board showed that approximately 1.7% by weight of the wax, based on the weight of the plaster, it was retained in the finished, dry cardboard. The forms of the invention shown and described herein should be considered as illustrative only. It should be apparent to those skilled in the art that numerous modifications may be made thereto without departing from the spirit of the invention and scope of the appended claims.
It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (13)

1. A process for manufacturing a gypsum board product having water resistance, characterized in that it comprises: adding an aqueous wax emulsion to an aqueous suspension of calcium sulfate material and guest particles, the emulsion is stable under the conditions in which the suspension is maintained; passing the suspension containing wax on a porous, flat forming surface to form a filter cake; removing a substantial portion of the water from the filter cake through the porous surface; compress the filter cake to form a csrt and remove the additional water; and dry the cardboard to remove the remaining free water and make the core of the cardboard reach a temperature sufficient to melt the wax.
The process according to claim 1, characterized in that the amount of wax emulsion added to the suspension is sufficient to provide at least about 1% by weight of wax solids to the suspension, based on the weight of the calcium sulfate in her.
3. The process according to claim 2, characterized in that the amount of wax emulsion added to the suspension is sufficient to provide from about 1% to about 3% by weight of wax solids to the suspension, based on the weight of the calcium in it.
4. The process according to claim 1, characterized in that the wax emulsion comprises a cationic surfactant.
5. The process according to claim 1, characterized in that the wax emulsion comprises a quaternary amine cationic surfactant.
6. The process according to claim 1, characterized in that the wax emulsion comprises a paraffin wax.
The process according to claim 6, characterized in that the wax emulsion comprises a mixture of paraffin wax, mountain wax, and polyvinyl alcohol.
8. The process according to claim 1, characterized in that the suspension comprises ground calcium sulfate material and discrete lignocellulosic host particles, the cellulosic particles each having penetrable voids per suspension menstruation on a substantial portion of their bodies.
The process according to claim 1, characterized in that the host particles are wood fibers selected from the group consisting of chemically refined wood pulp, mechanically refined wood pulp, thermo-mechanically refined wood pulp and combinations of the above.
The process according to claim 8, characterized in that the solids in the suspension comprise from about 0.5 to about 30% by weight of the wood fibers.
11. The process according to claim 10, characterized in that the solids in the suspension comprise from about 3 to about 20% by weight of wood fibers.
12. A process for making a gypsum board product having better water resistance, characterized in that it comprises: adding an aqueous wax emulsion to an aqueous suspension of a calcium sulfate material and guest particles, while such a suspension is found a temperature at which the calcium sulfate hemihydrate crystals are maintained, the emulsion is stable under the conditions in which the calcium sulfate hemihydrate crystals are maintained; passing the suspension containing wax on a flat, porous forming surface to form a filter cake before the temperature of the filter cake falls below the temperature at which the calcium sulfate hemihydrate is rapidly rehydrated to calcium sulfate dihydrate; remove a substantial portion of the water from the filter cake through the porous surface and cool the filter cake to a temperature at which rehydration begins, compress the filter cake to form a cardboard and remove the additional water, so that the crystals of the calcium sulfate hemihydrate around the hot particles are rehydrated in their calcium crystals dihydrate crystals; and dry the cardboard to remove the remaining free water and make the core of the cardboard reach a temperature sufficient to melt the wax. The method according to claim 1, characterized in that it comprises: mixing ground gypsum and host particles together with sufficient water to form a suspension, the host particles each having hollows on their surface and / or inside their body penetrable by the percussion of the suspension containing suspended and / or dissolved gypsum and the suspension is sufficiently diluted to substantially wet the penetrable voids in the host particles and to encourage the formation of acrylesular calcium sulfate alpha hemihydrate crystals when heated under pressure; heating the suspension in a pressure vessel, with continuous agitation, at a temperature sufficient to calcify the gypsum to calcium sulfate alpha hemihydrate; maintaining the suspension at such temperature until at least some of the calcium sulfate hemihydrate has substantially crystallized in and around the voids in the host particles; add the aqueous wax emulsion to the suspension while the suspension is at a temperature at which the sulfate hemihydrate crystals. of calcium are maintained, the emulsion is stable under the conditions in which the calcium hemihydrate crystals are maintained; passing the suspension containing wax on the porous, flat forming surface, to form a filter cake before the temperature of the filter cake falls below the temperature at which the calcium sulfate hemihydrate crystals rehydrate rapidly to dihydrate crystals; cooling the filter cake to a temperature at which rehydration begins; compressing the filter cake to form a paperboard and to remove the additional water itself so that the calcium sulfate hemihydrate crystals in and around the holes in the host particles are rehydrated to form crystals of calcium sulfate dihydrate; and dry the cardboard. 'IMPROVED TO WATER -f _ __ ---- --- - SUMMARY OF THE INVENTION The present invention relates to a composite and improved material; more particularly with a product composed of cardboard or gypsum panel that has better resistance to water that is especially useful for manufacturing construction products. Specifically, the present invention relates to an improved gypsum / fiberboard construction board having improved water resistance through the addition of a wax emulsion to gypsum and wood fiber during the carton manufacturing process.
MXPA/A/1998/006687A 1996-12-20 1998-08-18 Gypsum wood fiber product having improved water resistance MXPA98006687A (en)

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