WO1998051981A1

WO1998051981A1 - Glass fiber reinforced ceramic molding compositions

Info

Publication number: WO1998051981A1
Application number: PCT/US1998/009649
Authority: WO
Inventors: Martin C. Flautt; Yadi Delaviz; Gary Gao
Original assignee: Owens Corning
Priority date: 1997-05-15
Filing date: 1998-05-12
Publication date: 1998-11-19
Also published as: AU7481298A

Abstract

A glass fiber containing ceramic molding composition, and method of molding ceramic articles therewith, that provides enhanced strength, drying and shrinkage characteristics to the uncured molded body.

Description

GLASS FIBER REINFORCED CERAMIC MOLDING COMPOSITIONS

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY

The present invention relates to the manufacture of noncementitious ceramic articles. More particularly, the present invention relates to the inclusion of glass fibers in the moldable ceramic-forming composition used in the manufacture of such articles to improve the strength and fracture toughness of the green body before and during firing, and the resultant ceramic body after firing. The process is useful in a wide range of applications relating to the production of traditional ceramic articles such as whiteware, including wash basins and toilets ("sanitary ware"), tiles, dinner ware, and electrical porcelain, and finer texture ceramics used in electrical, biological/biochemical, magnetic, optical, nuclear and high temperature/high strength applications. Additionally, the invention relates to a process for coating glass fibers with metals or metal oxides to impart properties desirable for specific applications, including the reinforcement of ceramics, and to an apparatus for dispersing the glass fibers into the ceramic molding compositions.

BACKGROUND OF THE INVENTION

Noncementitious ceramics are well known and widely used as the material of choice for a variety of traditional ceramic products such as, porcelain (for figurines, vases, etc.), white ware (dinner ware, sanitary ware, floor tile, etc.), and structural products (brick, tile, terra cotta, etc.) as well as more advanced fine ceramics used in applications requiring specific electrical, magnetic or optical characteristics as well as high thermal resistance and mechanical strength. Ceramics exhibit many useful properties including good mechanical strength, chemical durability, and hardness.

As a general matter, there are a number of ways in which such ceramic products can be manufactured. Typically, these involve molding, casting or extruding (hereinafter referred to collectively as "molding") a ceramic-forming slurry or paste containing from about 15 to 75 percent water to form a ceramic "green body" which is then dried and fired to form the ceramic article. Depending on the complexity of the shape of the desired object, the objects are often either molded directly from the ceramic-forming molding composition, or carved or machined from a molded green body. For example, one of the more common molding techniques for directly forming the desired article, slip casting, relies upon using a porous mold and a ceramic slip which may consist of a wide variety of powdered ball clay, kaolin, nonplastics and other silica and silicon compounds, as well as various metal oxides, mixed with water in such a way that the mixture attains a suitable low viscosity for pouring or pumping into the mold. The porous mold is typically formed of gypsum which absorbs some of the water from the slip causing solid matter in the slip to form a cake on the mold's surface that exhibits the proper molded shape. The volume of liquid slip remaining in the mold after it has "set up" in the foregoing manner is drained from the mold and the mold is then removed to provide the green body.

Alternatively, ceramic articles can be formed from higher viscosity molding compositions containing greater amounts of organic plasticizers and less water. Such articles are often formed by standard compression or injection molding or extrusion techniques, or by machining a molded ceramic green body.

Regardless of the method of forming the ceramic into the shape of the desired article, the resulting green body is then dried to remove excess water and fired at an elevated temperature in an oven to cure the ceramic. The drying and firing processes can be complex, and if not performed properly, can cause the piece to crack or break.

In spite of the fact that these methods of producing ceramic articles are well developed and have been commercially successful for a long time, numerous drawbacks still exist with these production processes. First, it is often difficult to obtain a uniform moisture content within the green body. If the moisture content varies throughout the green body, the body can experience differential shrinkage during firing, which results in residual stresses which can cause cracks or defects. Furthermore, before firing, the green bodies are not very strong or tough and are, therefore, susceptible to deformation or breakage. Accordingly, although current processes for manufacturing ceramic ware are successful and produce acceptable products, there remain continuing needs not only to reduce production time but also to improve product quality, productivity, production yield and to reduce the necessary wall thickness of the ceramic body. To a great extent, these needs are satisfied through the practice of the present invention. SUMMARY OF THE INVENTION

The present invention provides a means for manufacturing noncementitious ceramic articles with reduced drying time requirements and a reduced incidence of deformation or breakage of the ceramic green body. Accordingly, the present invention provides a more efficient process for the manufacture of ceramic articles through increased productivity, less scrap and reduced wall thickness requirements.

According to the invention, the above goals are obtained by the incorporation of chopped glass fibers into the ceramic molding or ceramic glaze composition prior to molding. Dispersion of the glass fibers throughout the resulting green body imparts strength and toughness to the green body and facilitates drying of the green body. However, the addition of appropriately sized glass fibers through the ceramic molding composition in amounts sufficient to provide these beneficial attributes can alter the rheology of the molding composition and affect its moldability. Accordingly, the invention further encompasses the inclusion of additional water, gelation agents or deflocculating agents that counteract the rheoiogical effect of fiber addition and maintain the molding composition's rheoiogical properties at a level suitable for molding. By combining glass fibers and gelation or deflocculating agents, the rheology of the molding composition can be controlled to provide optimum molding characteristics.

Additionally, the present invention provides a method and apparatus for dispersing the chopped glass fibers into the ceramic molding composition, and maintaining the fibers in such a dispersed state until molding of the composition occurs. In particular, the invention provides a high shear mixer that can be mounted within existing storage tanks to disperse the fibers throughout the ceramic molding composition contained within the tank. Alternatively, the invention includes a detached high shear mixer connected to the storage tank via suitable plumbing to allow the ceramic molding composition to be withdrawn from the tank, pass through the high shear mixer to have glass fibers dispersed throughout, and returned to the storage tank. Additionally, for high solids ceramic processes, the glass fibers can first be dispersed in water or other liquid media and subsequently added to the ceramic molding composition. Furthermore, the present invention provides a process for coating glass fibers with metals or metal oxides to impart specific physical properties desirable for specific applications. In particular, the invention includes a process for coating glass fibers with certain metal oxides that increase the high temperature stability of the fibers and which may prevent fluxing of the fibers at the temperatures encountered during firing of the ceramic bodies. Consequently, such coated fibers may retain their identity in the cured ceramic products and provide improved fracture toughness to the ceramic articles. Such increases in fracture toughness may allow the wall thickness of the ceramic article to be reduced and provide a savings in raw material costs.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross-sectional view of a ceramic molding composition storage tank containing a high shear mixer for dispersing glass fibers throughout the molding composition contained in the tank.

Figure 2 is a graph showing increases in green body flexural strength or modulus of rupture (MOR) as a function of fiber length at glass fiber loadings of 0.5 and 1.0 percent by weight.

Figure 3 is a graph showing increases in green body fracture toughness as a function of fiber length at a glass fiber loading of 0.5 percent by weight.

Figure 4 is a graph showing green body fracture toughness or work of fracture (WOF) as a function of fiber loading for glass fibers of various lengths.

Figure 5 is a graph showing the reduction in linear shrinkage that occurs during drying/firing of ceramic compositions containing 0.5 percent by weight glass fibers.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

In the process of the present invention, the flexural strength and fracture toughness of the green body is increased by the incorporation of glass fiber reinforcement. During casting processes, fibers are concentrated near the surface because of the diffusion of water towards the outer part surface and into the porous mold. In injection, extrusion, and compression molding, fibers tend to align parallel to the surface due to the flow front and shear forces at the wall of the mold. Fibers may also be concentrated at and parallel to the surface by adding them directly to the glaze coating. In each of these processes, the enriched concentration of fibers near the surface of the part provides added benefit in the surface toughness. This reduces the sensitivity of the part to surface defects incurred during molding or handling and preferentially improves toughness in the surface region where tensile stresses are highest.

Additionally, the inclusion of the glass fibers within the ceramic molding composition facilitates drying of the green body as the glass fiber network tends to diffuse moisture to the surface which both increases the drying rate, and helps eliminate moisture differentials within the green body to provide a more uniform water content. As a result, less differential shrinkage occurs during drying/firing of the green body which reduces the occurrence of stress related cracks in the article during the drying/firing process. Moreover, the fiber network tends to reduce the amount of linear shrinkage of the green body that occurs during drying/firing.

Accordingly, inclusion of the glass fibers within the ceramic molding composition decreases the drying time of the green body prior to firing, reduces linear shrinkage of the green body during drying/firing, helps eliminate moisture differentials that can result in crack formation, and provides the green body with increased strength and fracture toughness, all of which lead to a more efficient molding process with a reduced percentage of defective or broken parts resulting in scrap.

Moreover, the inclusion of glass fibers also has the potential to reduce stresses in the fired microstructure. In some ceramic compositions, this is accomplished by enrichment in the composition by the glass fibers which flux into the ceramic body during firing. Localized compositional modifications may also modify the coefficient of thermal expansion which can blunt or deflect the propagation of cracks through the part, thereby improving toughness in the fired body. Through enrichment of microstructure or the presence of distinct fibers, the incorporation of fibers can toughen regions with stress risers such as sharp corners or double to single wall dropoffs. In the process of the invention, chopped glass fiber segments are dispersed throughout the ceramic molding composition prior to its introduction into the mold. The glass fiber segments can be made by any method known in the art that will provide chopped glass fibers in a form capable of being dispersed as individual filaments throughout the ceramic-forming composition. Preferably, the fiber segments are formed by "wet-chopping" a fiber strand (chopping the fiber strand after the application of an aqueous sizing composition) and maintaining the fiber segments in a wet state until they are added to the ceramic composition. Preparing the fibers in this manner reduces the filament to filament adherence that results when the sizing composition is allowed to dry, and enhances the dispersability of the fibers. Preferably, the fiber segments added to the ceramic composition have a moisture content of from about 0.5 to 30 percent by weight to aid dispersability of the fibers. More preferably, the moisture content of the fiber segments is from about 5 to 25 percent, with glass fibers having a moisture content of from about 8 to 12 percent being generally preferred.

The ceramic molding composition can be of any formulation known to the art to be useful in molding noncementitious ceramic articles. Exemplary formulations may comprise, on a weight percent basis, from about 20 to 35% ball clay, 20-35% kaolin and 30-60% nonplastic materials. In the practice of the invention, the ceramic molding composition may be formulated in various ways. For example, a dry mixture of glass fibers and ceramic-forming powders may be prepared that can be dry pressed or that can be hydrated to form a moldable composition. To this end, the glass fibers may be dry blended with the other ingredients of the ceramic-forming composition, or the fibers may be added to a ceramic slurry and spray dried to form the dry composition. Preferably, however, the glass fibers are added directly to a ceramic slurry to form the moldable ceramic composition. Such slurries may contain from about 5 to 75 percent water by weight, depending on the technique intended to be used to shape the composition. Any conventional equipment that will disperse the glass fiber substantially homogeneously throughout the ceramic molding composition may be used to accomplish this. Preferably, the glass fibers are dispersed throughout the molding composition by a high shear mixing apparatus. In commercial molding facilities that formulate the molding composition in large batches and maintain the molding composition in large storage tanks equipped with a low speed high torque mixer to keep the colloidal ceramic-forming particles in suspension, dispersion of the glass fibers throughout the composition can be accomplished by cycling the molding composition in the storage tank through a high shear mixer with the glass fibers, and returning the glass fiber containing ceramic-forming composition to the tank. The high shear mixing also cleans cations from the particle surface according to the Hofmeister series: H>Ca>Mg>Na>K. Accordingly, the divalent calcium and magnesium cations will be cleaned from the particle surface first which helps to reduce coagulation during the fiber dispersion. After the initial dispersion of the glass fibers throughout the ceramic-forming composition, the low speed high torque tank mixer will maintain the glass fibers in a dispersed state.

Although the high shear mixer can be located remotely from the storage tank containing the molding composition, and the molding composition pumped through suitable piping from the tank to the mixer and back to the tank, a particularly preferred high shear mixing apparatus for dispersing the glass fibers is one designed to be mounted within the storage tank as depicted in Figure 1. As shown in Figure 1 , inclusion of high shear mixer 10 within storage tank 20 provides the tank with a two- stage mixing system comprising a high speed high shear mixer 10 to initially disperse the chopped fiber in the ceramic-forming composition, and a low speed high torque mixer 30 to maintain the fibers and colloidal ceramic-forming particles in a dispersed state.

High speed mixer 10 is formed of a cylindrical chamber 11 with open ends 12 and 13. Preferably chamber 11 is mounted near the side of tank 20 at a height such that open end 12 is above the level of the molding composition in the tank 40 and open end 13 is below the level of the molding composition. Preferably, from about 50 to 80 percent of the chamber is submerged within the ceramic molding composition contained in the tank.

During operation, glass fiber is introduced into the mixing chamber through open end 12, and the ceramic molding composition enters the chamber through open end 13. Inside the chamber, a high shear mixing blade 14 disperses the glass fiber throughout the ceramic molding composition and imparts a flow to the mixer contents that forces a portion of the mixed composition back out through open end 13. Optionally, mixing chamber 11 may have additional openings located adjacent the mixing blade to improve egress of the mixed composition from the chamber. Mixing blade 14 is rotationally driven by motor 15 and drive means 16 at high speeds, preferably from about 350 to 500 revolutions per minute, to shear the glass fibers into the molding composition.

Inside tank 20, low speed high torque mixer 30 maintains the glass fiber ceramic particle dispersion. Mixer 30 can be of any design capable of performing this function. Preferably mixer 30 has blades or paddles 31 rotationally driven by motor 32 and drive means 33 at low speeds, preferably from about 14 to 20 revolutions per minute. Further, mixer 30 preferably has multiple blades or paddles to induce flow vertically as well as circularly within the tank, and is located within tank 20 so as to create an upward flow of the molding composition at open end 13 of mixing chamber 11 to ensure a good flow of molding composition through the high shear mixer.

The glass fiber segments used in the molding process of the invention may be of any size that will disperse substantially homogeneously throughout the ceramic molding composition and not deleteriously effect the moldability of the composition. Preferably, the chopped glass fibers are from about 0.5 to 10 mm in length and from about 5 to 20 microns in diameter. More preferably, the fibers are from about 1 to 6 mm in length and 9 to 14 microns in diameter.

In the method of the invention, the glass fibers may be incorporated into the molding composition in any amount that will impart the desired physical properties to the resulting green body or final ceramic article, but not interfere with the molding process. For example, inclusion of the glass fibers in the molding composition typically raises the viscosity of the composition. This can be problematic if not taken into account, especially in the molding of articles having complex shapes. If the viscosity of the molding composition becomes too high, incomplete filling of the mold may occur or other defects in the formed article may result. Moreover, as the viscosity of the composition rises, obtaining a substantially homogeneous dispersion of the fibers becomes more difficult. On the other hand, if the viscosity becomes too low, or plasticity or thixotropy is lost, then the casting or compaction rate can become problematic. As a result, the amount of fiber added to the composition must be balanced with the viscosity necessary to obtain good fiber dispersion and molding characteristics for the desired article. Generally, the addition of glass fibers in an amount of from about 0.1 to 2.0 % of the dry weight of the molding composition provides compositions exhibiting enhanced green body strength and toughness, reduced shrinkage and improved drying characteristics. Preferably, the amount of glass fiber added to the molding composition is from about 0.5 to 1.0 % of the molding composition's dry weight. As illustrated in Figures 2 through 5, the inclusion of glass fibers in such amounts can significantly increase the flexural strength and fracture toughness of the green body, as well as decrease the differential and linear shrinkage of the green body upon drying.

However, it has been found that in some applications, molding compositions containing such fiber loadings are too viscous to obtain good dispersion of the fibers or mold well. These problems have been overcome by the addition of water and/or certain compounds that act as deflocculation agents when added to the molding composition in conjunction with the glass fibers to at least partially offset the rise in viscosity due to the glass fiber addition.

The rheology of the ceramic molding composition may be largely controlled by the size distribution of the colloidal particles and the plasticity of the dispersion from cationic exchange. The surface of the ceramic particles is primarily electronegative; thus by altering the ionic concentration of the aqueous suspension by adding compounds that are monovalent and compatible with the molding composition and the molding process, deflocculation of the ceramic particles occurs. This deflocculation lowers the viscosity of the molding composition. Consequently, the increase in viscosity that accompanies the addition of glass fibers with surface cations is at least partially offset by the deflocculation of the ceramic particles.

Useful deflocculating agents include sodium silicate, sodium polyacrylate and sodium carbonate, or other materials appropriate for the specific composition and colloidal distribution and cationic exchange relative to the Hofmeister series. As one skilled in this art will recognize, the particular deflocculating agent to be added to the ceramic molding composition, and the amount used, depends upon a number of factors, such as the size and type of the ceramic-forming particles in the molding composition, the amount of fiber added, the type of molding being performed, and the complexity of the part being molded. The defiocculants may be added to the ceramic composition before, after or together with the glass fibers, however, adding the defiocculants before the glass fibers is generally preferred.

If the thixotropy or plasticity of the system is reduced by the fiber addition beyond that considered useful for good molding, then gelation or thixotropic agents may be added. Useful gelation or thixotropic agents include calcium sulfate, calcium carbonte, magnesium sulfate, calcium chloride and hydrochloric acid.

The composition of the glass fibers that may be included in the ceramic molding compositions to provide one or more of the above-described benefits vary widely. Suitable fibers include those made from compositions comprising, on a weight percent basis, from about 50 to 85% silica (Si0₂), 5 to 25% alumina (Al₂0₃), 0 to 12% magnesia (MgO), 0 to 25% calcium oxide (CaO), 0 to 10% boron oxide (B₂0₃), 0 to 15% sodium oxide (Na₂0) and potassium oxide (K₂0), and trace amounts of other oxides. However, to maximize the strength, drying and shrinkage benefits resulting from the inclusion of glass fibers, the glass fibers used in the process of the invention preferably withstand temperatures up to approximately 1200°C, the temperature at which essentially all shrinkage has occurred during the drying/firing process, without melting and fusing into the ceramic body. Preferred glass fibers include glass fibers comprising from about 65 to 85% by weight silica (Si0₂), 15 to 25% by weight alumina (Al₂0₃), and 0 to 12% by weight magnesia (MgO). Preferably, the fibers comprise from about 65 to 72% silica, 18 to 25% alumina, and 4 to 12% magnesia. Such fibers maintain their integrity during firing temperatures up to about 1000°C to 1500°C, thereby minimizing warpage and surface defects. Exemplary glass compositions include:

Softening Point Fiberizing Liquidus

85% Si0₂/15% AI₂0₃ 1250°C 1930°C

65% SiO₂/25% AI₂O₃/10% MgO 1050°C 1590°C

However, as the firing temperature increases above about 1000°C, even such high softening temperature glass fibers may be fluxed by the alkali components of the molding composition and fuse into the fired ceramic body. While at such point during the firing process, the green body/ceramic article is typically sufficiently cured that the glass fibers are no longer needed to impart strength and toughness or reduce shrinkage so as to prevent deformation during firing, it is believed that retaining the integrity of the glass fibers in the resultant ceramic articles may be beneficial to impart increased toughness. To this end, it has been discovered that coating the fibers with suitable metal oxides or phosphates may prevent fluxing of the glass fibers at temperatures up to about 1250° to 1500°C, the maximum temperature range typically encountered in the firing of most ceramics.

Suitable metal oxides include zirconia, titania, tin oxide, vanadia, silicon carboxynitride and aluminosilicates. The preferred metal oxides include zirconia, titania and tin oxide, with the most preferred being tin oxide. Suitable phosphate containing compounds, or metal phosphates, include pyrophosphate and polyphosphate compounds which include metals of groups I, II and III of the periodic table. These coatings can be applied to the fibers by liquid or vapor deposition, or by incorporating the metal oxide or phosphate, or precursor thereof into the sizing composition coated on the fibers during the fiber-forming operation, or the precursor can be applied separately to sized fibers in-line during the fiber-forming operation or off-line after fiber formation by any technique known in the art.

Preferably, however, the metal oxide or metal phosphate coatings are generated on the surface of sized fibers in an off-line deposition process that facilitates the uniform coating of filaments throughout a fibrous tow. In contrast to inline processing which limits the deposition time for the tow, restricts the process to atmospheric pressure, and prohibits the use of more toxic chemicals, coating the fibers off-line: (1) affords more time for the deposition to be completed (a period of several minutes rather than milliseconds); (2) can be performed in a variety of atmospheres, including vacuum; and (3) minimizes or eliminates human exposure to any toxic chemicals. In this process, the fibers are coated with the metal oxide or its precursor and heated to a suitable temperature in an oven or reactor. Depending on the coating desired, the deposition reactant can be in the form of solid powders, a liquid, or even a gas at room temperature. In the case where the reactant is a solid powder or liquid, the reactant may be added to the fiber strand as it enters the reactor or prior thereto, or the reactant may be applied to the fibers previously as a component of the sizing or a separate coating. Regardless of when the reactant is added to the fibers or strand, the reactant laden sized fiber strands are deposited into the reactor and the reactor is heated to the appropriate temperature for reaction. This temperature is typically between 200-500°C. In the case where the reactant is a liquid or gas, the reactant is preferably dosed into the reactor after the reactor has been evacuated and heated to the appropriate reaction temperature.

By using this process, a wide variety of materials, including metal oxides, can be coated onto sized glass fibers. For example, the following compositions have been deposited onto glass fiber strands by such process:

Coating Composition Reactant

Magnesium oxide Magnesium Acetate

Titanium oxide Titanium Isopropoxide

Titanium (acetylacetonate)-(i-propanolate) Silicon oxycarbide Polyurea Silazane

Tin oxide Monobutyltin trichloride/H20

Copper/copper oxide Copper Acetylacetonate

Copper Acetate in water (solution was added directly to strand) Molybdenum Molybdenum carbonyl

Aluminum Aluminum Acetylacetonate

Nickel Nickel Acetate

Chromium Chromium Acetylacetonate

Zirconium Zirconium Acetylacetonate

Calcium pyrophosphate Dicalcium Phosphate

However, the foregoing examples are merely illustrative, the process can be used to coat glass fibers with any compound that will react or decompose to form a metal, metal oxide, or metal phosphate on the glass fiber surface. Accordingly, depending on the coating composition, a variety of properties may be imparted to the glass strands. For example, such coatings may affect or improve electrical conductivity, thermal conductivity, refractive index, heat capacity, color, coupling to polymer matrix, abrasion resistance, handlability, modulus or strength of the glass fibers. In particular, glass filaments coated with copper have exhibited significantly better abrasion resistance than uncoated filaments.

Moreover, applying such coatings to multifilament strands by this process advantageously permeates the strand and imparts a substantially uniform coating of the material to each filament. Moreover, such coatings do not aggressively bond the filaments of the strand together. To the contrary, strands coated by this process can typically be filamentized easily, often times simply by bending or flexing the strand. As such, the coating method is particularly useful for coating multifilament strands intended to be chopped and separated into discrete filaments and dispersed throughout a matrix to provide reinforcement.

Preferably, the fibers are coated with tin oxide according to the following procedure. First, the fibers are coated with an organotin precursor to tin oxide. Suitable organotin compounds include tetra-n-butyltin, tetra-methyltin, di-n-butyltin diacetate and di-n-butyltin dichloride. A preferred organotin compound is typically monobutyl trichlorotin (MBTC), which is applied neat or via an aqueous solution. The precursor may be applied using an applicator as the fibers are formed, or by running the strand through a bath containing the solution. The precursor-coated strands are then collected into a collection can, or wound onto a stainless steel core. The can or wound package is then placed into an oven and heated to a temperature of from about 300°C to 500°C, preferably about 400°C, for approximately 10 to 60 minutes, preferably about 30 minutes, in air. During this heat treatment, the MBTC reacts with oxygen and water to produce a tin oxide coating on the surface of the filaments. The process is believed to be, in part, a chemical vapor deposition (CVD), because the precursor may become volatile and subsequently react at surface sites on the glass fibers within the strands. The tin oxide coated fiber strands can then be pulled directly from the can or wound package and chopped to the desired length for use in the fabrication of ceramic articles.

Alternatively, the glass fibers may be coated by feeding chopped, uncoated glass fiber strands into a fluidized bed reactor which is heated to a temperature of from about 300° to 500°C, preferably about 400°C, and applying a mixture of MBTC and water into the fluidized bed. The ensuing CVD reaction should yield the formation of a tin oxide coating on the surface of the chopped glass filaments.

By coating the glass fibers used in the invention with tin oxide, it appears possible to reduce the loss in fiber tensile strength which occurs upon exposure of the fibers to the high temperatures encountered during firing. When uncoated glass fibers are incorporated into a ceramic component (such as sanitary ware), the fibers typically flux and dissolve into the ceramic matrix during firing at temperatures of from about 1150 to 1200°C. However, visible evidence demonstrates that a coating of tin oxide on S glass ZenTron™ fibers (glass fibers composed of about 70% Si0₂, 20% Al₂0₃ and 10% MgO, obtained from Owens-Corning) minimizes crystallization of the fiber when exposed to temperatures of 1200°C for 150 minutes. Uncoated ZenTron™ fibers demonstrate extreme crystallization when heated to 1200°C for 150 minutes; Whereas, the tin oxide coated ZenTron™ fibers show little crystallization or other morphological changes after heating to 1200°C for 150 minutes. Accordingly, it is believed that tin oxide coatings on glass fibers may mitigate fluxing of the glass fiber during sintering of the ceramic. By retaining the fibrous morphology of the coated glass, the ceramic body may demonstrate enhanced fracture toughness. However, the desirability of such coatings are limited by their compatibility with the end use of the ceramic article. For example, such coatings may be undesirable for ceramic articles used as piezo electrics. Moreover, in some ceramic systems, the mullite enrichment that results from fluxing of the fibers during firing may be desirable.

In addition to the optional coating of, for example, tin oxide, the glass fibers used in the process of the invention are typically coated with a sizing composition to protect the glass fibers from being weakened as a result of abrasion during processing, as well as to facilitate the wetting of the coated fibers by the ceramic molding composition to enhance both the dispersability of the fibers throughout the composition and the coupling between the dried/cured ceramic material and the glass fibers. Preferably, the zeta potential of the sizing composition is similar to the isoelectric point and pH of the ceramic-forming slurry to aid in dispersing the fibers throughout the slurry. Exemplary sizing compositions for glass fibers to be used as reinforcement for ceramic materials are set forth in Table I, wherein the amounts indicated are the respective weight percentages of the various components of the sizing composition.

Table

¹Steaπc ethanolamide available from Alpha Owens Corning

²Hydroxy Ethyl Cellulose available from Union Carbide ³ Epoxy-cresol-novolac film former available from Shell

⁴5-chloro-2-methyl-4-ιsothιazolιn-3-one cupric nitrate magnesium nitrate, magnesium chloride solution available from Drew Chemicals

⁵Polyvιnyl alcohol water dispersion available from Air Products

⁶80% solids hexadecyldimethylamine oxide solution available from Armak Chemicals ⁷Petroleum distillate available from Henkel

⁸Catιonιc polyacrylamide film former available from Nalco

⁹Polydιmethyl siloxane solution available from G E Si cone

¹⁰Sιlylated polyazamide available from OSI of Witco/Union Carbide

¹¹Gamma-Amιnopropyltπethoxy silane available from OSI of Witco/Union Carbide ¹²bιs-3-trιmethoxysιlylpropyl amine available from OSI of Witco/Union Carbide

¹³Phenylamιnopropyltrιmethoxy silane available from OSI of Witco/Union Carbide

¹⁴Amιnoalkyl tnmethoxy silane formulation available from Dow Corning

¹⁵Alkylphenol-hydroxypolyoxyethylene surfactant available from Union Carbide

These compositions may be applied to the glass fibers during the fiber forming operation, or subsequent thereto, by any method known in the art

In the process of the invention, a suitable quantity of appropriately size-coated glass fibers of suitable length and diameter are sheared into a ceramic molding composition in such a manner that the glass fibers are dispersed substantially homogeneously throughout the composition If the viscosity of the molding composition increases to an unacceptable level as a result of the fiber addition, suitable quantities of an appropriate deflocculating agent may be added to reduce the viscosity to an acceptable level. Thereafter, the molding composition is supplied to a mold and formed into a green body of a desired shape. The green body is then dried and fired at elevated temperatures to fuse the ceramic particles and transform them into a ceramic material. The inclusion of glass fibers in the ceramic green bodies have been found to reduce drying times by as much as 50 to 90%, increase the impact or fracture toughness of the green body by as much as 3600%, increase the flexural strength of the green body by as much as 80%, and reduce the differential and linear shrinkage of the green body during drying/firing by as much as 45%.

Claims

WHAT IS CLAIMED IS:

1. A process for forming noncementitious ceramic articles comprising:

(a) dispersing glass fibers having a surface moisture content of from about 0.5 to 30 percent by weight throughout a noncementitious ceramic-forming composition;

(b) forming the resulting glass fiber containing composition into a desired shape; and

(c) heating said ceramic-forming composition to a temperature sufficient to fuse said ceramic-forming composition and form a ceramic article.

2. The process of claim 1 , wherein said noncementitious ceramic-forming composition comprises an aqueous dispersion of particles of noncementitious ceramic-forming materials.

3. The process of claim 2, wherein said dispersion comprises from about 5 to 75 percent by weight water.

4. The process of claim 3, wherein said glass fibers comprise from about 50 to 85% by weight silica (Si0₂), 5 to 25% by weight alumina (Al₂0₃), 0 to 12% by weight magnesia (MgO), 0 to 25% by weight calcium oxide (CaO), 0 to 10% by weight boron oxide (B₂0₃), 0 to 15% by weight sodium oxide (Na₂0) and potassium oxide (K₂0).

5. The process of claim 4, wherein said glass fibers comprise chopped glass fibers having a length of from about 1 to 10 mm and a diameter of from about 5 to 20 microns.

6. The process of claim 5, wherein said glass fibers are added in an amount of from about 0.2 to 2.0 percent of the dry weight of the ceramic-forming composition.

7. The process of claim 6, wherein said glass fibers comprise from about 65 to 85 percent by weight silica, 15 to 25 percent by weight alumina, and 0 to 12 percent by weight magnesia.

8. The process of claim 6, wherein said glass fiber containing ceramic-forming composition is molded by introducing said composition into a porous mold and allowing the water to escape through the pores in the mold and deposit a layer of the ceramic-forming particles and glass fibers on the mold surface.

9. The process of claim 8, wherein said glass fibers have a moisture content of from about 8 to 12 percent by weight.

10. The process of claim 9, further comprising adding one or more defiocculants, selected from the group consisting of sodium silicate, sodium polyacrylate and sodium carbonate, to the glass fiber containing ceramic-forming composition.

11. The process of claim 10, wherein said glass fibers comprise from about 65 to 85 percent by weight silica, 15 to 25 percent by weight alumina, and 0 to 12 percent by weight magnesia.

12. The process of claim 11 , wherein said glass fibers are coated with a material selected from the group consisting of zirconia, titania, tin oxide, vanadia, silicon carboxynitride, phosphates of group I, II and III metals, and alumino silicates.

13. A noncementitious ceramic article made by the process of claim 1.

14. A noncementitious ceramic article made by the process of claim 4.

15. A noncementitious ceramic article made by the process of claim 6.

16. A noncementitious ceramic article made by the process of claim 11.

17. Glass fiber reinforced ceramic articles comprising glass fibers coated with a material selected from the group consisting of zirconia, titania, tin oxide, vanadia, silicon carboxynitride, phosphates of group I, II and III metals, and alumino silicates dispersed throughout a ceramic body.

18. The glass fiber reinforced ceramic articles of claim 17, wherein said glass fibers comprise 65 to 85 percent by weight silica, 15 to 25 percent by weight alumina, and 0 to 12 percent by weight magnesia.

19. The glass fiber reinforced ceramic articles of claim 18, wherein said fibers are coated with tin oxide.

20. A ceramic molding composition comprising chopped glass fibers having a moisture content of from about 0.5 to 30 percent by weight prior to mixing, dispersed throughout a colloidal aqueous dispersion of ceramic-forming particles.

21. The molding composition of claim 20, wherein said glass fibers comprise chopped glass fibers having a length of from about 1 to 10 mm and a diameter of from about 5 to 20 microns.

22. The molding composition of claim 21 , wherein said glass fibers are added at an amount of from about 0.2 to 2.0 percent of the dry weight of the molding composition.

23. The molding composition of claim 22, wherein said glass fibers comprise 65 to 85 percent by weight silica, 15 to 25 percent by weight alumina, and 0 to 12 percent by weight magnesia.

24. The molding composition of claim 24, further comprising one or more defiocculants selected from the group consisting of sodium silicate, sodium polyacrylate and sodium carbonate.

25. A method of forming a substantially uniform coating of metal, metal oxide or metal phosphate on the size coated glass filaments of a multi-filament glass strand comprising:

(1) applying a precursor of said metal, metal oxide or metal phosphate to the surface of said strand;

(2) heating said coated strand to a temperature sufficient to cause said precursor to permeate the strand and form a substantially uniform coating of metal, metal oxide or metal phosphate on the surface of said filaments.

26. The method of claim 25, wherein said strand is heated to a temperature of from about 300┬░C to 500┬░C for a time of from about 10 to 60 minutes.

27. The method of claim 26, wherein said precursor is selected from the group consisting of copper acetylacetonate, copper acetate, molybdenum carbonyl, aluminum acetylacetonate, nickel acetate, chromium acetylacetonate, zirconium acetylacetonate, magnesium acetate, titanium isopropoxide, polyurea silazane, tetra-n-butyltin, tetra-methyltin, di-n-butyltin diacetate, di-n-butyltin dichloride, monobutyltrichlorotin and dicalcium phosphate.

28. The method of claim 27, wherein said strand is chopped into segments and the segments are fed to a fluidized bed reactor where they are heated to said temperature.

29. The method of claim 28, wherein said precursor is applied to said strand segments in said fluidized bed reactor.

30. A method for increasing the thermal stability of size-coated or uncoated glass fibers, comprising:

(1) preparing a composition containing a material selected from the group consisting of zirconia, titania, tin oxide, vanadia, silicon carboxynitride, polyphosphate, pyrophosphate, and alumino silicates or a precursor thereof; and (2) depositing said material on the surface of said size coated or uncoated glass fibers.

31. The method of claim 30, wherein said material is tin oxide.

32. An apparatus for preparing and storing a glass fiber containing ceramic molding composition comprising:

(1) a tank of suitable volume to hold said molding composition;

(2) means for dispersing said glass fibers throughout said molding composition; and (3) means to stir said molding composition within said tank to maintain said glass fibers in a dispersed state.

33. The apparatus of claim 32, wherein said means for dispersing said glass fibers comprises:

(a) a hollow chamber having open top and bottom ends mounted within said tank such that said top end is above said molding composition within said tank and said bottom end is within said molding composition;

(b) a means for introducing chopped glass fibers into said top end of said chamber; and

(c) means within said chamber for shearing said glass fibers into said molding composition.

34. The apparatus of claim 33, wherein said means for shearing said glass fibers into said molding composition comprises a blade rotationally driven at from about 350 to 500 revolutions per minute.