Ceramic forming process

A novel process is disclosed for producing a fracture-free shaped ceramic green body which comprises casting an aqueous suspension of hydroxylapatite, whitlockite or a mixture of these having a high water content and high drying shrinkage in an impervious mold, the cavity surfaces of which are coated with an inert water-immiscible lubricant; drying the casting under controlled conditions of temperature and humidity and removing adsorbed lubricant from the resulting dried green body.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The subject matter of this invention resides in the field of ceramics, 
particularly in the area of forming methods for producing shaped ceramic 
articles. 
2. Description of the Prior Art 
The fabrication of shaped ceramic articles generally requires as a first 
step the formation of a green body, i.e. the molding of the mineral raw 
materials into a shape corresponding substantially to that of the 
ultimately desired ceramic article. The green body is then dried and fired 
to a hard, permanent shape. Many techniques known in the art can be 
employed to produce ceramic green bodies. A number of these, for example, 
powder pressing, extrusion, plastic forming and slip casting are described 
by Kingery (Introduction to Ceramics, John Wiley & Sons, Inc., New York, 
1960, pp. 33-77) and Norton (Elements of Ceramics, Second Edition, 
Addison-Wesley Publishing Company, Reading, Mass., 1974, pp. 92-153). 
Slip casting, a well-known and widely-used technique, is also described by 
Norton (Fine Ceramics, Technology and Applications, McGraw-Hill, Inc., New 
York, 1970, pp. 101-129). This method involves pouring a suspension or 
slip into a porous plaster mold. As the mold adsorbs water from the 
suspension a hard layer of clay is built up. The process is continued 
until the interior of the mold is filled. The molded clay body is then 
dried and fired. 
It is well recognized in the ceramics art that among the most important 
properties required in a casting slip are low drying shrinkage and good 
flow properties, that is, the slip must have as low a water content as 
possible so as to minimize shrinkage on drying and yet must contain 
sufficient water to flow readily. These conflicting requirements are 
usually met by employing deflocculating agents which permit the formation 
of a fluid suspension with a relatively small amount of water. Thus, 
conventional casting slips ordinarily contain no more than about 15-25 
percent water and on drying undergo a volume shrinkage of about 5-10 
percent and no more than about 15-20 percent. 
Other molding techniques have also been described. Thus, the E. G. Foster, 
et al. U.S. Pat. No. 3,526,685 filed Sept. 22, 1967, issued Sept. 1, 1970 
discloses a process for producing foamed gypsum castings which comprises 
casting a foaming slurry of calcium sulfate in a mold having its internal 
faces lined with non-porous polyolefin-coated material. 
Much current ceramic research has focused on producing a high-strength 
calcium phosphate ceramic, for example hydroxylapatite or whitlockite, in 
a form suitable for use as a surgical prosthetic device. This effort has 
primarily centered around powder pressing methods and has produced porous 
materials generally lacking the strength required in many implant devices. 
Recently, Jarcho (U.S. Pat. No. 4,097,935, filed Jan. 31, 1977, issued July 
4, 1978) has described a process for producing a novel, high-strength, 
non-porous ceramic form of hydroxylapatite which involves the 
precipitation of hydroxylapatite from aqueous solution in a gelatinous 
state, drying the gelatinous material and then sintering to produce the 
novel ceramic. However, the gelatinous precipitate which contains large 
amounts of occluded water undergoes substantial shrinkage upon drying 
resulting in separation, cracking or fracture, and conventional molding 
techniques have not been completely satisfactory when it is desired to 
produce large, defect-free, shaped bodies. Thus, there is need for a 
simple, economical and reproducible forming process for producing 
defect-free, shaped green bodies of hydroxylapatite and whitlockite which 
can be used to overcome this problem. 
SUMMARY OF THE INVENTION 
Despite the relatively high water content and the resultant high drying 
shrinkage of the gelatinous calcium phosphate precipitates, we have now 
discovered a method for slip casting these meterials to form defect-free, 
shaped, ceramic green bodies therefrom. 
It is therefore a primary object of this invention to provide a method of 
casting an aqueous suspension of a calcium phosphate having a water 
content of about 75-95 percent by weight and a volume drying shrinkage of 
about 80-95 percent to produce a fracture-free, calcium phosphate ceramic 
green body which can ultimately be fired to produce a polycrystalline, 
sintered ceramic article. 
It is a further object of the present invention to provide an improvement 
in the processes for producing shaped, polycrystalline, sintered calcium 
phosphate ceramic bodies. 
Accordingly, as more particularly described hereinbelow, the present 
invention provides a novel process for forming a defect-free, shaped, 
ceramic green body comprising a calcium phosphate selected from 
hydroxylapatite, whitlockite or a mixture of these which comprises casting 
an aqueous slurry of the appropriate calcium phosphate in an impervious 
mold, the cavity surfaces of which are coated with an inert, 
water-immiscible lubricant, drying the casting under conditions of 
temperature and humidity effective in producing a fracture-free green body 
and removing adsorbed lubricant from the resulting dried green body.

DETAILED DESCRIPTION OF THE INVENTION INCLUSIVE OF THE PREFERRED 
EMBODIMENTS 
The invention sought to be patented resides in a process for forming a 
shaped, ceramic green body comprising a calcium phosphate selected from 
hydroxylapatite, whitlockite or a mixture of hydroxylapatite and 
whitlockite which comprises the steps of: casting an aqueous slurry of 
said calcium phosphate in an impervious mold, the cavity surfaces of which 
are coated with an inert, water-immiscible lubricant having a viscosity 
such that a liquid film of lubricant is formed between the mold surface 
and the slurry at the drying temperature of the slurry; drying the 
resulting molded slurry at a temperature and relative humidity such that 
the rate of evaporation of water from the surface of the slurry is 
approximately equal to the rate of diffusion of water from the interior of 
the slurry to the surface thereof; and removing adsorbed lubricant from 
the resulting dried green body. 
In one of its preferred aspects the invention resides in a process for 
forming a fracture-free, shaped, ceramic green body comprising 
hydroxylapatite or a mixture of hydroxylapatite and whitlockite which can 
be sintered to produce a shaped article in the form of a substantially 
fully dense, polycrystalline ceramic as described in Jarcho U.S. Pat. No. 
4,097,935. 
In another of its preferred aspects the present invention resides in a 
process for forming a fracture-free, shaped, ceramic green body comprising 
whitlockite which can be sintered to produce a shaped article in the form 
of a substantially fully dense, polycrystalline, whitlockite ceramic. 
The mineral raw material for the present process, that is, a calcium 
phosphate either as hydroxylapatite, whitlockite or a mixture of these, is 
prepared as a gelatinous precipitate from aqueous solution in accordance 
with the procedures described hereinafter. The precipitate is isolated 
from the reaction medium, usually by centrifugation followed by 
decantation of the supernatant, and the water content of the residual 
gelatinous calcium phosphate is adjusted to afford a pourable slurry or 
casting slip. Such a slurry will ordinarily contain from about 5-25 
percent by weight, preferably about 10-15 percent by weight, of solids, 
the remainder, i.e. 85-90 percent, of course, being water. The pH of the 
slurry is not critical provided it is higher than about pH 7, below which 
the calcium phosphates are unstable. Oridinarily, a pH of about 9-10 is 
preferred. 
If desired, the slurry can be de-aired in order to remove entrained gases 
and thus prevent the formation of voids in the dried green body. Although 
this is ordinarily not necessary for the casting of simple shapes where 
any entrained air is expelled during the drying process, it may be 
advantageous in the casting of very large or complex forms where air 
bubbles may remain trapped in the interior of the mold. Furthermore, in 
the casting of complex shapes it may also be advantageous to vibrate or 
centrifuge the filled mold in order to drive air bubbles to the surface 
and to insure that the slurry completely fills the mold cavity. 
In contrast to conventional slip-casting methods in which the slip is cast 
in a porous mold in order that water might be absorbed from the slurry by 
the mold material, the present process requires that the calcium phosphate 
slurry be cast in a mold constructed of relatively impervious material 
such as polymethyl methacrylate, graphite, stainless steel, 
polytetrafluoroethylene and polyethylene, the last being preferred. 
However, any rigid material which is impervious and non-reactive with the 
calcium phosphate slurry can, of course, be used. 
Moreover, the cavity surfaces of the mold are coated with a 
water-immiscible lubricant having a viscosity such that it forms a liquid 
layer between the mold and the slurry at the drying temperature of the 
slurry. Since, as noted hereinabove, the slurry undergoes a volume 
shrinkage of about 80-95 percent on drying, the liquid layer of lubricant 
is important in reducing the frictional forces between the mold surface 
and the slurry, particularly when the latter has dried to a gel-like 
consistency. At this point, the cohesive strength of the casting is 
minimal, hence requiring an essentially zero coefficient of friction 
between the mold and the casting in order to avoid fracture of the latter. 
It will be apparent that the lubricant must also be non-reactive toward 
the calcium phosphate. Suitable lubricants are the various paraffin 
jellies and oils, i.e. semi-solid and liquid hydrocarbons having a density 
in the approximate range 0.820-0.905. Paraffin oil (density 0.875-0.905) 
is particularly preferred since it can be applied in aerosol form, thus 
insuring uniform coating of the cavity surfaces of the mold expecially 
where these are intricately shaped. 
Again due to its high drying shrinkage, the molded slurry must be dried 
under controlled conditions of temperature and humidity in order to 
produce an intact, defect-free green body. Thus, temperature and humidity 
are selected so that the rate of evaporation of water from the surface of 
the slurry is approximately equal to the rate of diffusion of water from 
the interior of the slurry to its surface, thereby avoiding the creation 
of density gradients which lead to stress development and cracking. Drying 
can be effected at approximately 30.degree.-60.degree. C. at ambient 
relative humidity or at about 70.degree.-90.degree. C. at high relative 
humidity. The latter conditions are conveniently achieved by heating the 
molded slurry at about 70.degree.-90.degree. C. in the presence of a 
reservoir of water. Specific drying conditions are generally selected on 
the basis of the shape of the casting. Thus, a casting having a large 
exposed surface area, for example a thin, flat shape is dried at low 
temperature and ambient relative humidity, whereas a casting having a 
small exposed surface area, for example a cylindrical rod or cone, is 
dried at high temperature and humidity. Drying time at a given temperature 
will depend upon the size or bulk of the casting. 
The dried green body will, of course, retain a certain amount of adsorbed 
lubricant which can be removed by extraction or volatilization. Extraction 
is effected by thoroughly washing the green body with a suitable solvent, 
such as heptane or hexane. Volatilization of the lubricant is conveniently 
accomplished by heating the green body. It is imperative that the heating 
be carried out slowly since rapid volatilization of the lubricant will 
cause the green body to rupture. Temperature, rate and time of heating 
will, of course, be dictated by the volatility and amount of adsorbed 
lubricant and to some extent, also by the volume of the green body since a 
large body could be subject to thermal shock and fracture upon rapid 
heating. Ordinarily, heating the green body from about 150.degree. C. to 
350.degree. C. at a rate of approximately 25.degree. C. per hour will 
completely remove all adsorbed lubricant. Of course, if the body is small 
and contains very little adsorbed lubricant, more rapid heating is 
possible, for example, from room temperature to 400.degree. C. in about 
one hour. 
The resulting dried green body, though opaque and porous, possesses 
considerable strength and can be easily handled. Thus, it can be further 
shaped, ground, machined or drilled prior to sintering. The green bodies 
comprising hydroxylapatite or a mixture of hydroxylapatite and whitlockite 
are sintered in accordance with the procedures described in Jarcho U.S. 
Pat. No. 4,097,935. Sintering of the green bodies comprising whitlockite 
can be effected by heating at about 1000.degree. C. to 1350.degree. C. for 
approximately 0.5 to 4 hours, preferably at about 1150.degree. C. to 
1200.degree. C. for approximately 1 hour. Sintering causes a further 
volume shrinkage of about 10-20 percent thus resulting in an overall 
volume shrinkage from molded slurry to sintered ceramic of approximately 
90-97 percent. 
The mineral raw materials themselves, i.e. hydroxylapatite, whitlockite or 
a mixture of these as well as the processes for the preparation thereof do 
not constitute any part of the present invention. These materials are 
either known or can be prepared according to the procedures described 
hereinbelow. 
Thus, the preparation of hydroxylapatite or a mixture of hydroxylapatite 
and whitlockite as a gelatinous precipitate is described in detail in 
Jarcho U.S. Pat. No. 4,097,935, the subject matter of which is 
incorporated herein by reference. As stated therein, the composition of 
the ultimately produced ceramic, that is, whether hydroxylapatite or a 
mixture of hydroxylapatite and whitlockite depends on the molar ratio of 
calcium to phosphorus in the gelatinous precipitate. Moreover, it is 
imperative that the hydroxylapatite or the mixed 
hydroxylapatite/whitlockite be prepared as a gelatinous precipitate from 
aqueous solution, for it is only in this cohesive gelatinous state that 
the product can be shaped or molded and then dried and sintered to produce 
a ceramic body. Accordingly, calcium ion is reacted with phosphate ion in 
the appropriate ratio in aqueous medium to give a gelatinous precipitate 
of a calcium phosphate. The gelatinous precipitate is separated from the 
reaction medium and the water content of said precipitate is adjusted to 
give a pourable slurry which is then cast in accordance with process of 
the present invention as above-described. 
Similarly, it is also necessary to prepare whitlockite as a gelatinous 
precipitate from aqueous solution in order to ultimately obtain a shaped 
ceramic body. In this instance, however, simply reacting calcium ion with 
phosphate ion in the appropriate whitlockite stoichiometry, i.e. Ca/P=1.5 
is ineffective in producing pure whitlockite and instead affords a mixture 
of whitlockite and hydroxylapatite. This problem has been overcome by the 
addition of a small amount of sulfate ion to the calcium phosphate 
precipitate which results in complete conversion of the latter to 
substantially pure whitlockite (containing within its crystal lattice 
about 0.1 to 2.2 percent by weight sulfate ion) containing no detectable 
trace of hydroxylapatite. Thus, calcium ion is reacted with phosphate ion 
in a molar ratio of about 1.2-1.5:1 in aqueous medium at a pH of about 
10-12 to produce a gelatinous precipitate of a calcium phosphate having a 
molar ratio of calcium to phosphorus in the approximate range 1.50-1.53:1, 
separating said gelatinous precipitate from the solution, washing said 
precipitate free of soluble salts with water, homogeneously suspending the 
washed precipitate in approximately 1-3 percent aqueous ammonium sulfate 
in the amount of about 10-20 ml. per gram of expected whitlockite and 
separating the precipitate from the ammonium sulfate solution. The water 
content of the precipitate is then adjusted to give an appropriate casting 
slip. 
Thus, whitlockite is precipitated from aqueous medium by reacting calcium 
ion with phosphate ion at a pH of about 10-12. Any calcium--or 
phosphate--containing compounds which provide calcium and phosphate ions 
in aqueous medium are suitable provided that the respective counter ions 
of said compounds are easily separated from the whitlockite product, are 
not themselves incorporated in the whitlockite lattice, or do not 
otherwise interfere with the precipitation or isolation of the 
whitlockite. Compounds which provide calcium ion are, for example calcium 
nitrate, calcium hydroxide, calcium acetate and the like. In the present 
method, calcium nitrate and diammonium hydrogen phosphate are the 
preferred sources of calcium and phosphate ions, respectively. 
First, calcium nitrate and diammonium hydrogen phosphate in a molar ratio 
of about 1.2-1.5:1 are interacted in aqueous solution at a pH of about 
10-12 to produce a gelatinous precipitate of calcium phosphate. 
Temperature is not critical and the precipitation can be carried out from 
about 0.degree. C. to 100.degree. C., but is preferably carried out at 
about room temperature. The gelatinous precipitate thus obtained is 
separated from the solution by suitable means, for example by 
centrifugation and decantation of the supernatant. The residual mineral 
sludge can be washed free of any remaining soluble salts by suspending in 
distilled water, centrifuging and decanting the supernatant. The residual 
product is then homogeneously suspended in 1-3 percent (w/w) aqueous 
ammonium sulfate. Ordinarily, 10-20 ml. of 1-3 percent (w/w) aqueous 
ammonium sulfate per gram of theoretically expected whitlockite ceramic is 
employed. The solid is then separated from the solution by centrifugation 
or vacuum filtration and the resulting wet precipitate is diluted with 
water to give a pourable slurry which is then cast in accordance with the 
process of the present invention as described hereinabove. 
It is important that the calcium to phosphorus ratio of the isolated 
precipitate correspond as closely as possible to the theoretical value for 
whitlockite, i.e. Ca/P=1.50, in order to minimize the hydroxylapatite 
content of said precipitate and thereby minimize the amount of ammonium 
sulfate required to produce substantially pure whitlockite. Thus, if the 
calcium to phosphorus ratio of the precipitate is substantially greater 
than about 1.53, exposure to 1-3 percent aqueous ammonium sulfate is 
inadequate to produce pure whitlockite and affords instead a mixture of 
whitlockite and hydroxylapatite. A calcium phosphate precipitate having a 
calcium to phosphorus ratio greater than 1.53 can be converted completely 
to whitlockite by employing a larger amount of ammonium sulfate. In fact, 
a precipitate of pure hydroxylapatite (Ca/P=1.67) can be converted to 
whitlockite by usig sufficiently large quantities of ammonium sulfate. 
However, the whitlockite produced thereby is contaminated with significant 
amounts of calcium sulfate. Accordingly, in order to ensure that the 
calcium to phosphorus ratio does not exceed about 1.53, the calcium and 
phosphate salts are mixed in a molar ratio of 1.5:1 or less, preferably 
1.2-1.4:1. The calcium phosphate precipitate so-produced has a calcium to 
phosphorus ratio of about 1.50-1.53, and following treatment with 1-3 
percent aqueous ammonium sulfate (about 10-20 ml. per gram of 
theoretically expected whitlockite) ultimately affords substantially pure 
whitlockite. 
The instant forming process can also be employed to cast a foamed calcium 
phosphate slurry to produce a shaped, ceramic green body as a stable dried 
foam which, upon sintering, produces a shaped, foamed ceramic article. 
This is conveniently accomplished by incorporating in the calcium 
phosphate slurry about 0.5-10 percent by weight of a blowing agent, for 
example azodicarbonamide, hydrogen peroxide or ammonium carbonate, and 
about 0.5-10 percent by weight of a foam stabilizer such as albumen, 
polyvinyl alcohol or polyethylene glycol. Upon heating, the blowing agent 
releases gaseous decomposition products which are trapped by the foam 
stabilizer thereby creating a stable foam. 
Thus, to the gelatinous calcium phosphate precipitate are added about 
10-100 mg., preferably about 15-20 mg., of spray-dried egg white per gram 
of theoretically expected ceramic and at least an equal amount, i.e. about 
10-200 mg., preferably about 15-30 mg. of azodicarbonamide. The water 
content of the mixture is then adjusted to give a pourable slurry which is 
cast in a suitable mold in accordance with the above-described process and 
heated at about 70.degree.-90.degree. C. under conditions of high relative 
humidity for approximately 8 to 20 hours, or until decomposition of the 
blowing agent and drying of the resulting foam are substantially complete. 
The resulting shaped, dried foam is heated at about 
1000.degree.-1350.degree. C. until volatilization of the foam stabilizer 
and sintering of the resulting product are substantially complete. 
Alternatively, the blowing agent can be omitted and the foam created 
mechanically by whipping air into the mixture. It is also possible to 
incorporate in the casting slurry volatile, pore-forming agents such as 
starch, collagen, cellulose and organic compounds such as naphthalene. 
Thus, to the gelatinous calcium phosphate precipitate are added about 
0.5-10 percent by weight of a foam stabilizer such as egg albumen and 
0.5-10 percent by weight of a volatile pore-forming agent such as 
cellulose. The water content of the mixture is then adjusted to give a 
pourable slurry which is whipped to a foam by vigorous stirring. The foam 
is poured into a suitable mold and dried in accordance with the process of 
the present invention. Heating the resultant shaped, dried foam at about 
1000.degree.-1350.degree. C. until volatilization of the foam stabilizer 
and pore-forming agent and the sintering of the resulting product are 
substantially complete produces a polycrystalline foamed ceramic body. 
The invention is further illustrated by the following examples, without 
however, being limited thereto. 
EXAMPLE 1 
Hydroxylapatite in the form of a gelatinous precipitate of a phosphate of 
calcium having a molar ratio of calcium to phosphorus of about 1.67 
prepared according to the procedures described in Jarcho U.S. Pat. No. 
4,097,935, was treated with sufficient water to give a pourable slurry 
(specific gravity=1.12). The slurry was poured into a polyethylene mold 
(38.times.38.times.38 mm.) the cavity surfaces of which were coated with 
paraffin jelly (sold by Chesebrough-Ponds, Inc. under the trademark 
Vaseline). The molded slurry was dried at 50.degree. C. overnight at 
ambient humidity. The resulting dried body (a block 22.times.22.times.12 
mm.) was cut in half for visual examination to confirm homogeneity. Both 
halves were heated to 350.degree. C. over a period of about 2 hrs. to 
remove adsorbed petrolatum. Sintering at 1100.degree. C. for 1 hour 
afforded blocks (18.times.9.times.9 mm.) of hydroxylapatite ceramic. 
EXAMPLE 2 
A wet gelatinous precipitate of a phosphate of calcium prepared according 
to the procedure described in Jarcho U.S. Pat. No. 4,097,935 and having a 
molar ratio of calcium to phosphorus of about 1.53 and containing 
approximately 8 percent hydroxylapatite and about 92 percent whitlockite 
was diluted with sufficient water to give a pourable slurry. The slurry 
was de-aired and poured into a polyethylene mold (38.times.38.times.38 
mm.) the cavity surfaces of which were coated with paraffin jelly. The 
molded slurry was dried at 80.degree. C. overnight in a circulating air 
dryer containing a reservoir of water so as to maintain a relative 
humidity slightly less than 100 percent. The dried body 
(16.times.16.times.4 mm.) was then heated slowly over approximately 2 
hours to 1100.degree. C. and maintained at that temperature for about 1 
hour to give a shaped ceramic piece 12.times.12.times.3 mm. 
EXAMPLE 3 
A solution containing 264.12 g. of diammonium hydrogen phosphate in 4.5 
liters of water was adjusted to pH 11-11.5 with approximately 2.2 liters 
of concentrated ammonium hydroxide. Additional distilled water was added 
to dissolve precipitated ammonium phosphate giving a total volume of 9.6 
liters. The pH of the solution was readjusted to 11-11.5 with 800 ml. of 
concentrated ammonium hydroxide. This solution was added dropwise over 
about 0.75 hour to a vigorously stirred solution containing 3 moles of 
calcium nitrate in 5.4 liters of distilled water previously adjusted to pH 
11-11.5 with 90 ml. of concentrated ammonium hydroxide. When the addition 
was complete, the resulting gelatinous suspension was stirred at room 
temperature for 24 hours. A 200-ml. aliquot was centrifuged and the 
supernatant decanted. The residual sludge was diluted with 100 ml of 1 
percent aqueous ammonium sulfate and stirred vigorously to give a 
homogeneous suspension. After centrifugation and decantation of the 
supernatant, the residue was diluted with 10 ml. of distilled water and 
the resultant slurry poured into a rectangular polyethylene mold, the 
cavity surfaces of which were coated with paraffin jelly. The molded 
slurry was dried at 90.degree. C. overnight in a circulating air dryer 
containing a reservoir of water so as to maintain a relative humidity 
slightly less than 100 percent. The resulting partially dried slurry was 
further dried at 85.degree. C. at ambient relative humidity for an 
additional 20 hours, after which it was sintered 1 hour at 1000.degree. C. 
to afford a shaped body of fully dense whitlockite ceramic. 
EXAMPLE 4 
To a wet gelatinous precipitate containing 24 g. of a phosphate of calcium 
prepared according to the procedure described in Jarcho U.S. Pat. No. 
4,097,935 and having a molar ratio of calcium to phosphorus of about 1.54 
there was added 1.2 g. of powdered cellulose and 2 g. of spray-dried egg 
white previously reconstituted in 88 ml. of distilled water. The water 
content of the mixture was adjusted to give a pourable slurry and then 
whipped into a foam by stirring at high speed for about 1 hour. The foamed 
slurry was then poured into a rectangular polyethylene mold, the cavity 
surfaces of which were coated with paraffin jelly, and dried at 70.degree. 
C. and high relative humidity. The resultant dried foam was then heated 
slowly over approximately 2 hours to 1100.degree. C. and maintained at 
that temperature for about 1 hour to give a rectangular block of foamed 
ceramic comprising 12 percent hydroxylapatite and 88 percent whitlockite.