WO1988007505A1

WO1988007505A1 - Polymerizable binder solution for low viscosity, highly loaded particulate slurries and methods for making green articles therefrom

Info

Publication number: WO1988007505A1
Application number: PCT/US1988/001232
Authority: WO
Inventors: Richard Waack; Krishna Venkataswamy; Bruce E. Novich; John W. Halloran; Anna R. Egozy; James D Hodge; Ellen S. Tormey
Original assignee: Ceramics Process Systems
Priority date: 1987-04-01
Filing date: 1988-04-01
Publication date: 1988-10-06

Abstract

Slurries for powder forming include inorganic powders (e.g., ceramic particles) and a binder solution composed of organic solvents and at least one species of monomer. The slurries are substantially devoid of an already polymerized binder, and thus have a sufficiently low, pourable viscosity that they can be formed under low pressure, such as tape casting or low pressure injection molding. In addition, the slurries can have a high solids loading, such as 60 vol.%, and still remain sufficiently pourable for low pressure forming applications.

Description

POLYMERIZABLE BINDER SOLUTION FOR

LOW VISCOSITY, HIGHLY LOADED PARTICULATE SLURRIES

AND METHODS FOR MAKING GREEN ARTICLES THEREFROM Field of the Invention

This invention relates to novel processes for preparing ceramic compounds and to novel precursor compositions suitable for these processes. In general, this invention relates to a new in situ polymerization process including a new binder composition utilized with high concentrations of ceramic powder, in one aspect directed specifically to a ta casting process including such compositions, and, in anothe particular aspect, directed to the composition including whiskers. Background Art

In the manufacture of ceramic articles, it is common to employ a variety of polymers to act as binders to impart strength to the unfired or "green" ceramic article. This often accomplished by blending the ceramic powder with a binder solution containing solvent and polymers, or by directly compounding the ceramic powder with molten polymer The former method is generally used for tape casting, slip casting, and extruding, whereas the latter is typically use for injection molding. Both methods suffer from a number of problems. First, th extremely high viscosity of the binder solution restricts th concentration of ceramic powder which can be conveniently handled. This restriction on viscosity is particularly exacerbated with submicron-sized ceramic powders, as well as those having a narrow particle size distribution. Secondly, when using the binder solution technique, the ceramic articl has to be dried to evaporate the solvent. Thirdly, it is difficult to completely remove the binder of either techniqu in the "binder burnout" stage prior to high temperature sintering.

One class of ceramic articles are suitable for use as substrates. Electronic components are made by applying circuitry to the thin ceramic substrates. Substrates are generally formed by mixing together ceramic powders of various compositions with suitable amounts of solvents, an organic binder soluble in the solvent, and a plasticizer to form a slurry, which is cast onto a thin film (usually made from a material such as cellulose acetate, TEFLON, or MYLAR) in the shape of a tape. The cast slip is allowed to dry, producing a "leather hard" green ceramic tape. The tape can be cut, and via holes (which allow for three-dimensional circuitry when layers of tape are laminated) may be punched in it. It is then ready for firing to produce a rigid substrate upon which circuitry can be applied.

The recent United States Patent No. 4,587,068, issued for an invention of Borase et al., teaches a tape casting technique wherein the ceramic powder is mixed with a solution of monomers having a low vapor pressure. No solvents, either aqueous or organic, are present in the mixture. The tape is cast, then the monomer is polymerized. While there are some advantages to this method, the viscosity of the solventless fluid limits the applicability of the technique to tape casting or other molding methods involving a non-pourable paste; for example, at 56 wt.% solids the viscosity is 50 cps (after 41 hours of milling) and at 84 wt.% solids the viscosity is 1180 cps (after 90 hours of milling) . Further, due to the relatively high percentage of polymer in the article, a clean binder burnout may be difficult to achieve, particularly where the surface-to-volume ratio of the article is relatively small. Indeed, this patent itself notes that "some deformation of the ceramic material occurs during sintering."

United States Patent No. 3,962,162, issued for an invention of Schmank, discloses green refractory compositions including refractory powder, 1-30% unsaturated polyester, 1/2-12% unsaturated vinyl monomer, 0.2-0.5% catalyst for vinyl polymerization, 0.5-5% internal mold release compounds, and 0.5-7% volatile mold release and lubricant. The volatile mold release compounds described include butyl stearate, and other oily materials that volatilize without changing from about 120°C to 200°C, including methyl stearate and dioctylphthalate. The mold release is then said to "be expelled from solution during polymerization and appears to provide a reticulation of capillaries during volatilization so that subsequent firing can proceed without danger of rupture of the green ceramic during decomposition of polymerized binder." These green compositions, prior to polymerization, are variously described as "a fairly stif plastic mass," "a very plastic dough," "a plastic dough," an "a stiff dough."

Accordingly, these are not suitable for molding technique requiring pourable slurries, for example, a low pressure injection molding system. Such a system would require the use of slurry which has a low viscosity, essentially pourable, yet also having a high solids loading in order for the green piece to meet the requirements of sinterability. Further, it would be desirable to form a solid structure without having to drain away excess fluids after the slurry has been injected into the mold. The volume coefficients of expansion for typical polymers used in conjunction with ceramics are in the range of 1-3 x 10~⁴/°K; for example, pure poly(methyl methacrylate) is 2.6 10"^*4/^oK below the glass transition temperature (Tg) and abov

T,q it equals 5.6 x 10^~V°K. For- pure polystyrene it is 1.7- x 10~⁴/°K below T_g and 5.6-6 x 10^*"⁴/°K above T_g. This means that a length of polymer would experience a 2.5% change in length in the event of a temperature increase of only 100^°K. While these figures are typical of pure polymers, the presence of ceramic particles mixed with the polymer are believed to cause some reduction in these numbers. However, the effects of increased temperature on expansion of the polymer/ceramic may be substantial in many instances. Also, both expansion and contraction of the green piece prior to sintering can disrupt the green microstructure, which would thereby result in defects in the sintered piece.

Accordingly, it would be desirable to reduce the thermal stress to which a green article is subjected prior to firing. The addition of whiskers to a material is one method of imparting greater fracture toughness, stiffness, and tensile strength to ceramic products. Whiskers are monocrystalline materials (as opposed to fibers, which are-polycrystalline) with diameters that are typically less than 1 micrometer and lengths ranging from approximately 10-80 micrometers. Whiskers have been made from a number of materials including silicon carbide,- silicon nitride, and aluminum oxide. In order to impart uniformly the desired properties, the whiskers should be uniformly dispersed throughout the suspension. However, due to particle-to-particle interactions, this has been difficult to achieve. Accordingly, it would be desirable to overcome these interactions and achieve a slurry having a high volume fraction of solids including both particles and whiskers.

Disclosure of the Invention The present invention overcomes problems of the prior art by providing a polymerizable binder solution containing organic solvent and monomers, and which is capable of dispersing a large volume of ceramic solids while still retaining a low, pourable viscosity; the composition is essentially devoid of an already polymerized binder. This composition, in the form of a polymerizable dispersion, can then be poured or pumped under low pressure into a desired mold, and then the monomers are polymerized in situ. The result is a rigid green body having a high solids content, high strength, and requiring no drying prior to binder burnout and firing. The make up of the original binder is such that, during the firing step, binder burnout is substantially complete.

We have discovered that the problems connected with the prior art tapes, i.e., cracking and incomplete binder burnout, can be avoided by using a binder solution composed of acrylic monomers and a solvent solution, including a dispersant and, if desired, a plasticizer and/or an initiator. We have found that the use of such components significantly enhances the degree of binder burnout and tends to reduce problems such as cracking and warping of the material being fired. In a preferred embodiment, all components of the slip, i.e., monomers, solvent, dispersants and plasticizers, if used, should have a sufficiently high boiling point such that the vapor pressure of these components at the cure temperature is not high enough to for gas bubbles in the polymerizing tape. When the tape is cast, it is cured by elevating the temperature to a level sufficient to initiate free radical polymerization of the monomers. The supporting tape is then removed and the article is fired.

Further, it has been found that by continuously maintaining the molded object at a temperature equal to at least that at which in situ polymerization occurs until firing has commenced, a substantially better product is produced than if the object is permitted to cool to ambient temperature after polymerization.

The present invention overcomes still other problems of the prior art by providing an in situ polymerizable composition which, in addition to monomers, solvent, and ceramic particulates, also contains whiskers. The binder solution is capable of dispersing a large volume of ceramic powder and of uniformly dispersing whiskers while still retaining a low, pourable viscosity. The dispersion can then be poured or injected at low pressure into a mold of the desired shape, and then the monomers polymerized in situ. The result is a rigid green body having a high solids content, high strength, and requiring no drying prior to binder burnout and firing. Again, the make up of the original binder is such that during the firing step binder burnout is substantially complete.

Description of Specific Embodiments In one preferred embodiment, the ceramic material is alumina, silicon nitride, SiAlON, or other powder which may have a small size range, i.e., a mean particle size in the range of approximately 0.3-2.5 micrometers with a standard deviation not exceeding about 50% of the mean particle size. Alternatively, the powder may have conventional size ranges. Submicroh-sized powders are also quite useful. The powders can be provided by a variety of techniques, including precipitation and by utilizing a centrifugation classification system as more fully described in applicant's PCT International Application filed March 23, 1988, which is hereby incorporated herein by reference.

It is important to note that while the present invention will be illustrated by ceramic powders, the invention is equally applicable to metal powders (which may have conventional particle sizes of up to about 50 microns in diameter) . That is, the present invention is directed primarily to forming green articles, and to overcoming problems in the formation thereof, which problems are manifest as defects in the final, densified article. Of course, it will be appreciated that the densification process is different for ceramics and metals (and for combinations thereof) , and that such are readily determinable by the artisan. Nevertheless, the aspects addressed herein, such as high solids loading for pourable slurries, are applicable to forming green articles composed of either metallic or ceramic particles, or combinations thereof.

The powders are typically used at a high concentration, up to and including approximately 80-95 wt%, and generally greater than about 50 vol.%. The useful weight percent varies with the density of the powder since maximum concentration is primarily a function of volume of the particles being dispersed.

The binder solution contains monomers, plasticizer, and a solvent, each component described in more detail below. The dispersant solution typically is approximately 15% of the entire slurry, when the powder density is approximately 3-4 g/cm³.

An important feature of this invention is that despite the high solids loading, the slurry is pourable. Typically, the viscosity ranges from 50 to 50,000 with 50 to 5,000 being preferred. However, it is to be understood that the preferred viscosity will depend on the fabrication equipment and the shape of the piece being formed. It is of primary importance that the slurry can flow readily without the need to use high pressure to induce flow or to overcome a yield stress to flow. Furthermore, as illustrated below, the present invention can provide slurry viscosities of less th 1000 cps (at 100 s"¹) at solids loadings of at least about vol.%. Such slurries are not only pourable, but are injectable under extremely low, virtually zero pressure (i.e., gauge pressure as opposed to absolute).

The monomers used in the dispersant solution can be any vinyl or acrylic monomer or mixtures thereof, or they may also include oligomers with vinyl (e.g., acrylic) functionalities. They may also be multi-functional or can contain other reactive moieties, such as hydroxyl, epoxy, o urethane groups. In preferred embodiments, the monomer may be chosen from among the acrylates, styrenes, vinyl pyridines, other vinyl compounds, or a mixture of these or their derivatives. After polymerization these yield polyacrylates, polystyrenes, poly-(vinyl pyridines), polyvinyls, or a mixture of these polymers or copolymers or their derivatives. The term "monomer" is used herein to connote both monomers and oligomers, components that are essentially unpolymerized with respect to the final polymerization product.

The monomers may make up about 50 wt.% of the binder solution, or about 7 wt.% to about 10 wt.% of the entire slurry, but higher or lower levels of monomers may be usefu depending on the shape of the piece being formed.

The remaining portion of the binder solution (or about 7%-10% by weight of the slurry) is made up of various volatile organic solvents. This additional component of th dispersant solution may contain plasticizers, diluents, and dispersants. Typical plasticizers are dibutylphthalate and other phthalate esters. Examples of diluents include decal and volatile fatty acids or esters, such as oleic acid. Commonly used dispersants include GAFAC RE-610 (an anionic polyoxyethylene nonylphenyl ether phosphate, available from the GAF Corp., Wayne, NJ) , AEROSOL OT (a dioctyl ester of sodium sulfosuccinic acid, available from American Cyanamid, Danbury, CT) , organic titanates such as KR TTS or KR-7 (bot available from Kenrick Petrochemicals, Inc., Bayonne, NJ) , SPAN 85 (a nonionic sorbitan monolaurate, available from ICI Americas, Wilmington, DE) , or EMCOL CC-55 (a cationic polypropoxy quaternary ammonium acetate, available from Witco Chemical Co., Perth A boy, NJ) . These are important for a number of reasons. First, they facilitate the ability to achieve a pourable slurry which has a high solids loading by decreasing particle-to-particle interactions (both agglo erative and repulsive) . Further, we have found that the suspension with the additional organic components exhibits better isotropic properties in both the green piece and the sintered piece. The particles seem to have a more random orientation, resulting in less internal stress and ultimately fewer defects. In accordance with a preferred embodiment of the present invention, the solvent is substantially retained in the slurry until after polymerization of the monomer has been substantially completed. The solvent is thereafter substantially removed from the formed article by time of the early portion of the binder burnout stage. We have found that use of a solvent in this manner significantly enhances binder burnout and tends to reduce problems encountered such as cracking and warping of the material being fired. One way of achieving this result is to employ a volatile organic solvent having a boiling point that is higher than the temperature used to effect polymerization but lower than the temperature at which the polymer burns out. However, in some cases it may be desirable to employ a solvent having a boiling point below the polymerization temperature and to suppress volatilization of the solvent during polymerization by an alternative method, such as by keeping the formed article under pressure during polymerization. When this approach is followed, however, care must be taken with respect to the effect of the solvent vapor pressure in whatever environment the formed article is placed following polymerization; a sudden release of a large solvent vapor pressure could cause cracking or deformation of the formed article. In another embodiment, the in situ polymerization occurs without the need for elevating the temperature above room temperature (approximately 20-35°c) although the reaction ma occur at elevated temperatures. In this embodiment, two slurries are mixed. Both slurries contain ceramic particles monomers, and organic solvents as described above. In addition, one slurry contains a catalyzable initiator, such as behzoyl peroxide, and the second contains a compound whic catalyzes the initiator. Examples of such catalysts can be found in U.S. Patents Nos. 3,991,008 and 3,591,438.

Preferred compounds of this type include dimethyl aniline, dimethyl toluidine, and thioureas. As the two slurries are mixed, the reaction of the catalyst with the initiator triggers in situ polymerization. Alternatively, the catalys may, of course, be added directly to a slurry containing the initiator. Yet another alternative, depending on the shape produced, is to photoinitiate polymerization.

In another embodiment, the slurry can be injected 'into a heated mold, whereby forming both by shaping and by polymerizing occurs concurrently.

The following examples are presented to illustrate aspect of the invention.

Example 1

Alumina powder (designated AKP 20, available from Sumitom Chem. Co., Tokyo, Japan) and the binder solution (containing a mixture of monomers, diluent, plasticizers, and dispersant were roller milled for 18 hrs at ambient temperature.

Benzoyl peroxide was added as an initiator at 0.45 wt.% base on the dispersion. The mixture was agitated mildly for 10 minutes and then was poured into a mold. Just prior to molding the mixture had a viscosity of approximately 200 cps at 100 s^_1.

Table 1 Weight Percent alumina 86.0 (-60 vol monomers* 8.5 plasticizer (dibutylphthalate) 2.5 diluent (decalin) 1.5 dispersant (GAFAC RE-610) 1.5

100.0 *90/5/5 n-butylmethacrylate/methacrylie acid/diethyleneglycol dimethacrylate (the ratio of 90/5/5 being on a weight basis) .

Example 2

A mixture was made according to the procedure of Example 1, but the following reactants were used:

Table 2 Weight Percent alumina 85.0 (-59 vol.%) monomers* 8..5 plasticizer (dibutylphthalate "DBP") 3.5 diluent (decalin) 1.5 dispersant (GAFAC RE-610) 1.5

100.0

*90/5/5 methylmethacrylate/methacrylic acid/diethylene glycol dimethacrylate. Benzoyl peroxide was added as an initiator at 0.45 wt.%.

Example 3

Monomer Variations. The mixture was made according to the procedure of Example 1, but the following reactants were used:

Weight Percent SiAlON 83.0 (-60 vol.%)

DBP 5.0

Decalin 2.5 RE-610 1.5

Monomers 8.0

100.0

In a series of variations of this example, each of the following indicated combinations of monomers was used in the formulation listed above:

A) 95/5 2-ethylhexylacrylate (2-EHA)/l,6- hexanedioldiacrylate (#238) (available from Sartomer Co., Westchester, PA)

B) 90/10 2-EHA/#238 C) 75/25 2-EHA/Tripropyleneglycoldiacrylate

D) 75/25 2-EHA/CELRAD 3702⁺

E) 50/50 Tetrahydrofurfurylmethacrylate/ 1,6-hexadioldimethacrylate

F) 90/5/5 n-butylmethacrylate/ Diethyleneglycol dimethacrylate/ methacrylic acid

G) 90/5/5 methylmethacrylate/methacrylic acid/diethylene glycoldimethacrylate ⁺CELRAD 3702 is a commercially available bis-phenol A diacrylate solution available from Interez Co., Louisville, KY.

For the above formulations A-C and E-G, the slip viscosities were about 500-600 cps at 100 s^*"1 (and would be expected to be in the range of about 300 to about 1000 cps at the same shear rate) ; formulation D would be expected to have a viscosity of about 1000 - 2000 cps at 100 s"¹.

Example 4 The following compositions using stearic acid as an internal mold release agent were made:

Weight Percent

1 2 3 4

SiAlON 83.0 83.0 83.0 83.0 monomers* 8.0 8.0 8.0 8.0

DBP 5.0 5.0 2.5 4.0

Decalin 0.0 1.5 2.5 2.5

Stearic Acid 1.5 1.0 2.5 1.0

RE-610 1.5 1.5 1.5 1.5 100.0 100.0 100.0 100.0

*90/5/5 n-butylmethacrylate/ diethyleneglycol dimethacrylate/methacrylic acid.

The viscosities of these formulations was increased, due to the presence of the stearic acid, to about 500-1000 cps at 100 s"¹.

Example 5

The following composition using oleic acid was made: weight percent alumina 86.0 monomer* 8.5 oleic acid 4.0 RE-610 1.5

100.0 *Monomers used were those in both Example 1 and those designated A through G in Example 3; the slurry having the monomers of Example 1 had a viscosity of about 1000 cps at 100 s"¹.

Example 6 Two slurries were made in accordance with Example 1 above, except benzoyl peroxide was added to the first slurry at 0.45 wt.% of slurry and dimethyl toluidine was added at approximately 0.1 wt.% to the second slurry. The two slurries were brought into contact and polymerization proceeded, resulting in a solid green piece ready to be fired.

Example 7 All the compositions listed in example 1-5 were cured at either 85°C or 120°C for 1 hour. They were de-molded and all produced solid green bodies of acceptable or better qualify.

Example 8 . The green bodies of example 7 were fired under sintering conditions convention for alumina. The resultant pieces were of very good quality.

All of the viscosity measurements (including both the foregoing and the following examples) : for alumina slips, measurements were not taken until after the initiator had been added; measurements were randomly taken both before and after the initiator was added for SiAlON slips. Tape Casting

The slip containing the ceramic powder, monomers, solvent, and optionally an initiator and/or a plasticizer is mixed by ball milling or by another conventional method. It is then vacuum de-aired, filtered, and cast onto a carrier film for which MYLAR is preferred. An additional step which we have found advantageous is to apply a second film sheet to the top of the cast tape. This second sheet helps prevent loss of the monomer via evaporation from the surface of the tape prior to curing.

The tape is then cured as described above, such as by elevating the temperature for a given time. In an embodiment using methacrylates and acrylates as monomers, the tape is cured at 80^oC for 2-3 hours. During this step, the monomers are converted to polymers. in accordance with a preferred embodiment of the present invention, the solvent is substantially retained in the slurry until after polymerization of the monomer has been substantially completed.

The solvent is thereafter substantially removed from the formed article by the time of the early portion of the binder burnout stage. We have found that use of a solvent this manner significantly enhances binder burnout and tends to reduce problems encountered such as cracking and warping of the material being fired. One method for achieving this result is to employ a volatile organic solvent having a boiling point that is higher than the temperature used to effect polymerization, but lower than the temperature at which the polymer burns out. However, in some cases, it ma be desirable to employ a solvent having a boiling point bel the polymerization temperature and to suppress volatilizati of the solvent during polymerization by an alternative method, such as by keeping the tape under pressure during polymerization. When this approach is followed, however, care must be taken with respect to the effect of the solven vapor pressure in whatever environment the tape is placed following polymerization; a sudden release of a large solve vapor pressure could cause cracking or deformation of the tape. Another embodiment of tape casting is analogous to t casting method discussed above, where two slurries, one including a catalyzable initiator and another including a catalyst therefor, are mixed to initiate polymerization whi being shaped, in this embodiment into a tape.

After curing, the sheets are removed and the green ceramic tape can be further processed by conventional techniques, i.e., laminating, cutting, punching, etc. The organic solvent is removed (such as by evaporation) , after which the polymer is burnt out during firing. The exact temperature and time will depend on the composition of the ceramic as well as the particle size, as such factors are readily determined by an artisan.

The system as just described regarding tape casting provides many surprising beneficial results over the system found in the prior art. These include:

1. Monomers are not lost due to evaporation; rather, they are converted into a polymer that does not evaporate.

2. Curing can be initiated at a relatively low temperature and for a short duration. This makes the entir process more efficient. 3. When the solvent has a boiling point higher than the polymerization temperature, it is not lost during curing, and the film remains homogeneous.

4. Fired pieces exhibit much lower cambers than those made using conventional techniques.

5. The fired pieces have equally good surface finishes on both sides. One microinch surface finishes on both sides are readily obtainable.

6. The fired pieces have few defects on both sides. Example 9

The following components of the slip formulation were ball milled:

Component Function Wt%

A1₂0₃ ceramic 85.7 n-butyl methacrylate solvent/binder 6.8

2-ethylhexyl aerylate so1vent/binder 4.5

GAFAC RE-610 dispersant 2.0 butyl acetate solvent 0.7 benzoyl peroxide initiator 0.3 100.0

GAFAC RE-610 was obtained from the GAF Corp., Wayne, NJ.

The resulting slip was vacuum de-aired, filtered and tape cast onto a MYLAR film in the conventional manner. A second Mylar sheet was then applied to the top, and the sandwiched layer of slip was calendared into a uniform thickness. The tape was cured for approximately 2 hours at approximately 80°C. Mylar sheets were the removed and the green ceramic was fired. Example 10

Tapes were made in accordance with the procedure of

Example 1, but substituting the following formulations. The resulting tapes were fired at 1470"C for 4-6 hours. wt.% Gafac RE-610 dispersant 1.7%

A1₂0₃ ceramic 83.9 n-butyl methacrylate soIvent/binder 7.4

2-ethylhexylacrylate solvent/binder 4.9 magnesium stearate sintering aid 0.75 benzoyl peroxide initiator 0.35 butyl acetate solvent 1.0

100.00 Example 11

Tapes were made in accordance with the procedure of

Example 1, but substituting the following formulations. The resulting tapes were fired at 1470°C for 4-6 hours. EMCOL CC-55* dispersant 2.5 wt. A1₂0₃ ceramic 83.9 n-butyl methacrylate solvent/binder 8.0 2-ethylhexylacrylate solvent/binder 3.5 magnesium stearate sintering aid 0.75 benzoyl peroxide initiator 0.35 butyl acetate solvent 1.0

100.00

*Emcol CC-55 is from Witco Chemical Co., Perth Amboy, NJ Example 12

Tapes were made in accordance with the procedure of

Example 1, but substituting the following formulations. The resulting tapes were fired at 1470°C for 4-6 hours.

GAFAC RE-610 dispersant 0.4 wt.% Methacrylic acid dispersant 0.7

AI₂O₃ ceramic 84.1 n-butyl methacrylate solvent/binder 8.1 2-ethylhexylacrylate solvent/binder 5.3 benzoyl peroxide initiator 0.4 butyl acetate solvent 1.0

100.0

All tapes made in Examples 9-12 exhibited excellent quality.

Example 13 Two slurries were made in accordance with Example 1, above except benzoyl peroxide was added to the first slurry 0.45 wt.% slurry and dimethyl toluidine at approximately 0.1 wt.% was added to the second slurry. The two slurries were brought into contact and polymerization proceeded while the tapes were cast resulting in a green tape ready to be fired. For all of the tape casting examples, the alumina was commercially available under the designation A-16 from Aluminum Co. of America, Pittsburgh, PA, and the slip viscosities were all about 1000 - 1200 cps at 10 s"¹ (a lower shear rate is used because of the characteristics of tape casting) . Thermal Control After the polymerization reaction is complete, it is another aspect of this invention to keep the green article in an environment similar to that of the polymerization step. Generally, this means that the green article will be removed from the mold, but maintained at desired elevated temperature, preferably approximately at or above the polymerization temperature. In many instances, this procedure results in maintenance of temperatures ranging generally between approximately 80 and 200°C. In other words, under one embodiment of the invention, the green article is continuously maintained at the polymerization temperature after removal from the mold until firing has commenced. Under another embodiment, the green article is further heated until its temperature rises to a desired level, and this new temperature is continuously maintained until commencement of firing. The new temperature is not even limited by the melting point of the pure polymer since the ceramic/polymer may maintain its shape above this temperature. The sole upper limit of the temperature is the point at which polymer decomposition occurs, a process typically associated with binder burnout. Use of an inert atmosphere (such as nitrogen or argon) can increase the upper limit of the temperature range. It will be apparent that other embodiments are possible; for example, the temperature may be maintained, until commencement of firing, at some other desired temperature that is elevated in comparison to ambient temperature. It will be apparent that the invention may readily by employed with respect to tape-cast articles and articles that have been formed by other means, including the embodiment previously described wherein two slurries, respectfully containing a catalyzable initiator and a catalyst therefor, are admixed.

Although the present theory of operation is not intended as a limitation on the scope of the invention, it is believed that this step of continuously maintaining the temperature of the green article produces a better finished article because the green piece is not subject to stresses due to temperature changes between polymerization and the early stages of firing. As a molded piece cools and heats up, the temperature change induces expansions and contractions of the piece as a whole, and particularly on a microscopic scale. The polymeric binder is especially susceptible to volume changes when the temperature varies between room temperature and the melting point of the polymer. As the volume changes, the result is micro and macro defects in the structure of the green article. Retaining the piece at a temperature near the polymerization temperature avoids the fluctuation in volume and the subsequent occurrence of defects.

In situations where more than one type of monome is employed as a binder precursor, the polymerized binder ma not melt at high temperatures, but may instead exhibit a gel-like behavior due to its three-dimensional bond structure. However, certain bonds will contain discrete regions that will experience temperature induced behavior similar to that of a non-crosslinked system. Thus, this invention is particularly well suited to cross-linked system since the origin of defects is microstructural related.

Example 14

Compositions containing the following components were formulated: ceramic particles diluent plasticizer monomers

The compositions were molded, then cured at 85"C for hr. After polymerization had occurred, the piece was remove from the mold while it was still hot, and kept in an oven at

85°C for approximately 18 hours before it was fired. The resultant fired piece showed no defects.

It will be appreciated that the post-polymerization process described above may be applied to a wide variety of polymerized green bodies, including those described in the following examples. Example 15

Alumina powder (designated A-16, described above) and the dispersant solution (containing a mixture of monomers, diluent, plasticizers, and dispersant were roller milled for 18 hrs at ambient temperature. Benzoyl peroxide was added as an initiator at 0.45 g/100 g dispersion, followed by 10 minutes of mild agitation, and the mixture was then poured into a mold. Just prior to molding the mixture had a viscosity of approximately 200 cps.

Table 1 Weight Percent alumina 86.0 monomers* 8.5 plasticizer (dibutylphthalate) 2.5 diluent (decalin) 1.5 dispersant (Gafac RE-610) 1.5

100.0

*90/5/5 n-butylmethacrylate/methacrylie acid/diethylene glycol dimethacrylate. Example 16

The mixture was made according to the procedure of

Example 1, but the following reactants were used:

Table 2 Weight Percent alumina 85.0 monomers* 8.5 plasticizer (dibutylphthalate, "DBP") 3.5 diluent (decalin) 1.5 dispersant (GAFAC RE-610) 1.5

100.0 *90/5/5 methylmethacrylate/methacrylic acid/diethyleneglycol dimethacrylate. Benzoyl peroxide was added as an initiator at 0.45 wt.%.

Example 17

Monomer Variations. The mixture was made according t the procedure of Example 7, but the following reactants were used to produce:

SiAlON 83 Wt.%

DBP 5

Decalin 2.5 RE-610 1.5

Monomers 8

In a series of variations of this example, each of th following indicated combinations of monomers was used in the formulation listed above in Example 3.

Example 18 The following compositions were made according to the general procedure of Example 15 and using additionally stearic acid as an internal mold release agent: Weight Percent

1 2 3 4

SiAlON 83.0 83.0 83.0 83.0 monomers* 8.0 8.0 8.0 8.0

DBP 5.0 5.0 2.5 4.0

Decalin 0.0 1.5 2.5 2.5

Stearic Acid 1.5 1.0 2.5 1.0

RE-610 1.5 1.5 1.5 1.5

methacrylic acid.

Example 19

The following composition using oleic acid was made according to the general procedure of Example 14: wt. alumina

86.0 monomers* 8, .5 oleic acid 4, .0 RE-610 1. .5 100.0

*Monomers were the same as in Example 14.

Example 20

All the compositions listed in examples 14-19 were cured at either 85°C or 120°C for 1 hour. All produced solid green bodies of good quality. The pieces were more crack-free and exhibited fewer micro-defects than pieces that were allowed to cool.

Example 21

The green bodies of Example 20 were fired at conventional temperatures and times. The resultant pieces were of very good quality.

Example 22

Two slurries were made in accordance with Example 14 above, except benzoyl peroxide was added to the first slurry at 0.45 wt.% slurry and dimethyl toluidine at approximately

0.1 wt.% was added to the second slurry. The two slurries were brought into contact and polymerization proceeded at room temperature, resulting in a solid green piece ready to be fired. Low Pressure Injection Molding

The dispersions described above are suitable for injected at low pressure into a closed non-draining mold. This may be accomplished by a hand held syringe (ultralow or essentially zero gauge pressure) or through automated equipment. After or during injection, the mold is heated to initiate thermal polymerization of the monomers, resulting in a cured rigid body. The specific temperature and time needed will depend on the particular monomer and initiator system used, but will typically be from 50°C - 150"C for at least 2-3 hours, with approximately 85°C being preferred for many embodiments. After curing, the rigid, high strength green piece is removed from the mold and is ready for firing by any conventional process. The following examples are illustrative of the process.

Example 23 The following components were sequentially added to a ball milling container and milled for 18 hours at ambient temperature. wt

A Alluummiinnaa 86.0

Monomers* 8.5

DBP 2.5

Decalin 1.5

GAFAC RE-610 1.5

100.0 * 9.0/5/5 n-butylmethacrylate/ methacrylic acid/diethyleneglycol dimethacrylate.

An initiator, benzoyl peroxide at 0.45 wt.% slurry, was added, followed by 10 minutes of mild agitation. The slurry was injected into a cube cross hole widget mold and a rotor mold. Then the molds were heated to 85°C for 2 hours to cure. After cooling, the green pieces were removed. They had smooth surfaces and high green strengths. Particle packing was greater than 55 vol%. Upon firing, the pieces remained intact without distortion and without cracking.

Example 24

The same procedure as in Example 23 was followed and using the same components, but in the following formulation amounts: Alumina 85.0 wt.%

Monomers 8.5 Plasticizer 3.5

Diluent 1.5

Dispersant 1.5

100.0 Like the articles formed in Example 23, these pieces exhibited a smooth surface and high green strength. Partic packing was greater than 55 vol%. Upon firing, the pieces remained intact without distortion and without cracking.

Example 25 Two slurries were made in accordance with Example 23 above, except benzoyl peroxide was added to the first slurr at 0.45 wt.%, and dimethyl toluidine at approximately 0.1 wt.% was added to the second slurry. The two slurries were brought into contact and polymerization proceeded, resulting in a solid green piece ready to be fired.

Example 26

Monomer Variations. The mixture was made according t the procedure of Example 23, but the following reactants wer used to produce:

Weight Percent

SiAlON 83

DBP 5

Decalin 2.5

RE-610 1.5 Monomers 8.0

100.0

In a series of variations of this example, each of th monomer compositions A - G of Example 3 were used in the above formulation. Example 27

The following compositions using stearic acid as an internal mold release agent were made:

Weight Percent

1 2 3 4

SiAlON 83.0 83.0 83.0 83.0 monomers* 8.0 8.0 8.0 8.0

DBP 5.0 5.0 2.5 4.0

Decalin 0.0 1.5 2.5 2.5

Stearic Acid 1.5 1.0 2.5 1.0

RE-610 1.5 1.5 1.5 1.5

100.0 100.0 100.0 100.0 *90/5/5 n-butylmethacrylate/diethyleneglycol dimethacrylate/ methacrylic acid. Example 28 The following composition using oleic acid was made: alumina

86.0 Wt.% monomers* 8.5 oleic acid 4.0

RE-610 1.5

*As in Example 23.

Example 29 All the compositions listed in Examples 23, 24 and 26 28 were cured at either 85°C or 120°C for 1 hour. They wer de-molded and all produced solid green bodies of acceptable or better qualify.

Example 30 The green bodies of Examples 25 and 9 were fired unde conventional conditions. The resultant pieces were of very good quality. Whisker reinforced composites

The ceramic or metallic powders are typically used at a high concentration, up to and including approximately 80-9 wt%. The useful weight percent varies with the density of the powder since maximum concentration is primarily a function of volume of the particles being dispersed. Particle surface area is also an important criteria. The large surface area associated with fine particles usually requires a lower concentration of dispersed phase due to the increased double layer. But it is a feature of the present invention that it can accommodate high surface area material while maintaining fluidity of high solids. It is of primary importance that the slurry can flow readily without the need to use high pressure to induce flow or to overcome a yield stress to flow.

The addition of whiskers to the slurry is achieved by the process set forth in United States patent application serial number 036,377, filed April 9, 1987, incorporated herein by reference. The whiskers utilized may be virtually any commercially available whisker, including those made fro silicon carbide, silicon nitride and aluminum oxide. The whiskers can be mixed into the highly loaded monomer solutio described above. A preferred concentration will vary with the intended use, but can range up to about 30-40 volume percent.

The following examples are presented to illustrate th invention.

Example 31

The following components were milled together:

AKP 20 Alumina 55.75g (Sumitomo Chemical Co., Japan) yttria 6.20

Monomers* 8.5 dibutylphthalate 4.5 decalin 2.5

GAFAC RE-610 1.5 *90/5/5 n-butyl methacrylate/methacrylic acid/diethylene glycol dimethacrylate.

To this was added 21.05 g silicon carbide whiskers (A

SiC-3 or AK Sic, available from Tatcho Chemicals, Japan) . A initiator, benzoyl peroxide, was then added at 0.45 wt.% based on the slip. The desired shape was formed in an injection mold (rotor or widget) under low pressure, as viscosity is low. The shape was heated to approximately

85"C, whereupon the monomers polymerized, yielding a solidified shape which was near net shape. The pieces were fired at 1720"C for 2 hours.

Example 32

The same general procedure as detailed in Example 31 was followed, except the monomers used were 90/5/5 n-butyl methacrylate/methacrylic acid, 2-ethylhexylacrylate/ diethylene glycol dimethacrylate.

Example 33

The same general procedure was followed as detailed in

Example 31, except the following components were used:

AKP 20 Alumina 65.54 g yttria 7.28

SiC whiskers 10.18 monomers* 8.50

DBP 4.50

Decalin 2.50 RE-610 1.50

*90/5/5 n-butylmethacrylate/methacrylie acid/diethylene glycol dimethacrylate. Example 34 The same formulation as in Example 33 was made, except that monomers used were 90/5/5 n-butyl methacrylate/2-ethylhexylacrylate/diethylene glycol dimethacrylate.

Example 35 The following formulation was made in general accordance with the procedures of Example 31:

AKP 20 Alumina 64.80 g yttria 7.10

SiC whiskers 10.00 monomers* 9.70

DBP 4.40

Decalin 2.50 RE-610 1.50

*90/5/5 n-butyl methacrylate/methacrylic acid/diethylene glycol dimethacrylate.

Example 36

The following formulation was made in general accordance with the procedures of Example 31:

AKP 20 Alumina 63.17g yttria 7.01

SiC whiskers 9.82 monomers* 11.00 DBP 5.00

Decalin 2.50

RE-610 1.50

*90/5/5 n-butyl methacrylate/methacrylic acid/ diethylene glycol dimethacrylate. In addition, 10 g of 85/5/5/5 isobutyl methacrylate/ methacrylic acid/diethyleneglycol dimethacrylate/2-ethylhexylacrylate, along with 6 g of DBP, were substituted in the above formulation.

The present invention has been illustrated by the foregoing examples and descriptions. Various modifications, additions, and the like may be apparent to one of ordinary skill in the art upon reading the present specification, and such changes are intented to be included within the scope and spirit of the invention as defined by the claims.

Claims

What is claimed is:

1. A slurry, for forming a green article, consisting essentially of: (a) a binder solution, composed at least one species of vinyl monomer and organic solvent, and (b) sinterable particles, the slurry being essentially devoid of polymeric binder and having a pourable viscosity.

2. A slurry according to claim 1, wherein the organic solvent includes at least one plasticizer, at least one diluent, and at least one dispersant.

3. A slurry according to claim 1, wherein the particles a selected from the group consisting of ceramics, metals, and mixtures thereof.

4. A slurry according to claim 1, wherein the ratio of th weight percent of monomers to the weight percent of organic solvent ranges from approximately 1:10 to 10:1.

5. A slurry according to claim 3, wherein the ceramics ar selected from the group consisting of alumina, silicon nitrid and SiAlON.

6. A slurry according to claim 1, wherein the particles have a mean particle size ranging from approximately 0.3-2.5 microns with a standard deviation not exceeding 50% of the mean.

7. A slurry according to claim 1, wherein the monomer species are selected from the group consisting of acrylates, styrenes, vinyl pyridines, and mixtures thereof.

8. A slurry according to claim 1, wherein the dispersant i selected from the group consisting of quaternary ammonium acetates, phosphate ethers, sorbitan esters, sulfosuccinate esters, and mixtures thereof.

9. A slurry according to claim 2, wherein the diluent has a boiling point above that temperature needed for the monomers to polymerize.

10. A ceramic slurry according to claim 1 further comprising a polymerization initiator.

11. A slurry according to claim 10, wherein the initiator is a thermal initiator.

12. A slurry according to claim 10, wherein the initiator is selected from the group consisting of benzoyl peroxide, dimethyl aniline, dimethyl toluidine, thioureas, and mixtures thereof.

13. A slurry according to claim 1, further including whiskers.

14. A slurry according to claim 13, wherein the whiskers are carbides selected from the group consisting of silicon and tungsten.

15. A slurry according to claim 2, wherein the whiskers are present in an amount of about 10-40 vol.%.

16. A slurry according to claim 1, wherein the particles are present in an amount of at least about 50 vol.% and the slurry viscosity is less than about 400 cps at 100 s""¹.

17. A slurry according to claim 16, wherein the particles are present in an amount of at least about 60 vol.% and the slurry viscosity is less than about 1000 cps at 100 s^-1.

18. A slurry, for forming a green article, consisting essentially of: (a) 80-95 wt.% sinterable particles;

(b) 5-15 wt.% of at least one species of vinyl monomer; (c) 1-7 wt.% plasticizer; (d) 1-5 wt.% diluent; and (e) 0.5-5 wt.% dispersant; the slurry being essentially devoid of a polymeri binder and having a pourable viscosity.

19. A process of producing a green article, comprising: (a admixing sinterable particles with a binder solution consisti essentially of at least one species of vinyl monomer and organic solvent to form a pourable slurry; (b) forming the slurry into a desired shape; and (c) polymerizing the monomer so that a rigid green body is formed.

20. A process according to claim 19, further comprising adding a thermally activated initiator to the slurry prior to molding the slurry.

21. A process according to claim 19, wherein step (a) includes admixing a sufficient quantity of particles so that the resulting slurry is at least 40 volume percent particles.

22. A process according to claim 19, wherein step (a) includes admixing a diluent which has a boiling point above that temperature needed for the monomers to polymerize.

23. A process for forming a dense ceramic article by the process as defined by claim 19, wherein the particles are ceramic and the process further comprises the step of firing the green article.

24. A process according to claim 19, wherein the admixing further includes admixing whiskers.

25. A process, for producing a green article, comprising: (a) admixing sinterable particles with a binder solution consisting essentially of at least one species of vinyl monomer, organic solvent, and a catalyzable initiator to form a second pourable slurry; (b) admixing sinterable particles with a solution consisting essentially of at least one species of vinyl monomer, organic solvent, and a catalyst for the initiator to form a second pourable slurry; (c) contacting the first and second slurries while forming the resultant slurry into a desired shape, resulting i the polymerization of the monomers and the formation of a soli green article.

26. A process according to claim 25, wherein step (a) includes admixing benzoyl peroxide as the catalyzable initiator.

27. A process according to claim 25, wherein step (b) includes admixing an catalyst selected from the group consisting of dimethyl aniline, dimethyl toluidine, thioureas, and mixtures thereof.

28. A ceramic article produced by the process of claim 25, wherein the particles comprise ceramics and the process furthe comprises the step of firing the green article.

29. A process according to claim 19, wherein the forming includes casting the slurry onto a first film, and thereby creating an exposed top layer of slurry.

30. A process according to claim 29, further comprising applying a second film to the exposed top layer.

31. A process according to claim 19, further comprising the steps of firing the green article and of continuously maintaining the temperature of the green article at or above the polymerization temperature from immediately following polymerization until the commencement of firing.

32. A process according to claim 31, wherein step of maintaining includes maintaining the green piece at a temperature in the range of from about 60°C to about 200°C.

33. A process according to claim 19, wherein the step of forming includes injecting the slurry under low pressure into closed mold of a desired shape.

34. A process according to claim 33, wherein the injection pressure is less than about 50 psig.

35. A process according to claim 34, wherein the injection pressure is less than about 10 psig.