Process to produce inorganic hollow fibers

Process for the production of small tubing, e.g., hollow fibers comprising PA1 (a) preparing a solution of an organic fiber-forming polymer, containing, in a uniformly dispersed form, a sinterable inorganic material; PA1 (b) extruding the inorganic material-containing polymer solution through a hollow fiber spinneret; PA1 (c) forming a polymeric precursor hollow fiber, laden with the inorganic material; PA1 (d) treating the polymeric precursor hollow fiber to remove the organic polymer; and PA1 (e) sintering the resulting inorganic material in hollow fiber form.

This invention relates to a process to produce small tubing, e.g., hollow 
fibers. Metal tube drawing procedures to make small tubing are expensive. 
Such procedures to make extremely small tubing, i.e., with fiber size 
outer diameters, are particularly expensive and may not be technically 
viable. This invention provides a process that readily and economically 
produces metal tubing of extremely small size. The process has also been 
found to be useful to produce small tubing of other inorganic materials. 
The value of the process of this invention varies, generally, in inverse 
proportion with the outer diameter of the small tubing. That is, the 
smaller the tubing desired the move valuable the process. For very small 
outer diameter tubing, the costs of the process of the present invention 
do not apparently increase per unit length which contrasts with the costs 
of tube drawing procedures which generally accelerate when producing such 
small outer diameters. 
In the description of the present invention, the following definitions are 
used. 
The term "hollow fiber" as used in this application means a fiber (or 
monofilament) which has a length which is very large as compared to its 
diameter and has an axially disposed continuous channel which is devoid of 
the material that forms the fiber (more commonly referred to as the 
"bore"). Such fibers can be provided in virtually any length desired for 
the use intended. 
The phrase "essentially inorganic materials" denotes a sinterable inorganic 
material that is substantially free of organic polymeric material. 
The term "monolithic" means that the material of the fiber has the same 
composition throughout its structure with the fiber maintaining its 
physical configurations due to the presence of sintered particles. 
The term "porous" refers to that characteristic of the fiber wall which, 
although otherwise being continuously relatively dense, has very small, 
often tortuous, passageways that permit the passage of fluid through the 
fiber wall other than by diffusion. 
SUMMARY OF THE INVENTION 
The present invention provides a process to produce essentially inorganic, 
monolithic hollow fibers (i.e., small tubing). Such hollow fibers 
comprising metal are particularly preferred. The process for producing 
such fibers comprises (a) preparing a solution of an organic fiber-forming 
polymer, containing, in uniformly dispersed form, a sinterable inorganic 
material; (b) extruding the inorganic material-containing polymer solution 
through a hollow fiber spinneret; (c) forming a polymeric precursor hollow 
fiber laden with the inorganic material; (d) treating the polymeric 
precursor hollow fiber to remove the organic polymer; and (e) sintering 
the resulting inorganic material in hollow fiber form. The essentially 
inorganic hollow fiber produced will be similar to the polymeric precursor 
hollow fiber but on a reduced scale.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The hollows fibers provided by the present invention will be very useful to 
workers in numerous fields. These hollow fibers can be prepared relatively 
economically with widely varying physical configurations while utilizing 
many types of inorganic materials. Furthermore, it has been found that 
large amounts of these fibers can be produced with only nominal losses due 
to flaws and imperfections. 
The hollow fibers produced by the process of this invention comprise 
essentially inorganic materials which are sintered in hollow fiber form. 
The sinterable inorganic materials comprise a very large group of 
materials. The preferred sinterable inorganic materials are metals. 
Nickel, iron and their alloys are particularly useful. The sinterable 
inorganic materials can be ceramics, such as aluminum oxide, beta-alumina, 
etc. The sinterable inorganic materials can be cermets or metcers, such as 
iron metal/aluminum oxide, titanium carbide/nickel, etc. 
The hollow fibers produced can have an outer diameter of up to about 2,000 
microns. However, production of fibers of larger outer diameters such as 
3,000 or 4,000, up to about 6,000 microns, is also contemplated. 
Generally, the more economically advantageous hollow fibers have an outer 
diameter of from about 50 to about 700, most preferably from 100 to 550, 
microns. The fibers often have wall thicknesses of from about 20 to about 
300 microns. More particularly preferred are fibers having wall 
thicknesses of from about 50 to about 200 microns. The fibers generally 
have a wall thickness to outer diameter ratio of from about 0.5 to about 
0.03, particularly preferred of from about 0.5 to about 0.1 
An extremely important contribution of the present invention is the ability 
to provide inorganic hollow fibers with varying sizes and configurations. 
The size of the fiber can be influenced by the simple expedient of 
changing spinnerets as is well known in the synthetic fiber field. By 
varying the extrusion and fiber-forming conditions the fiber wall 
thickness can also be varied over wide ranges. These characteristics 
provide those skilled in the art with a unique ability to produce hollow 
fibers tailored for the application of interest. 
These features are provided by the process of this invention which is 
described more particularly below. 
Preparation of Polymer Solution Containing Inorganic Material 
A mixture which comprises an inorganic material in uniformly dispersed form 
in a polymer solution is prepared. The polymer solution comprises a 
fiber-forming organic polymer dissolved in a suitable solvent. In general 
the concentration of the organic polymer in the solution is sufficient to 
form, when the solution contains the inorganic material, the precursor 
polymeric hollow fibers by dry and/or wet spinning techniques. The polymer 
concentration can vary over a wide range and depends on the 
characteristics desired in the resultant hollow fiber. For instance, if 
hollow fibers having relatively dense walls are desired the concentration 
can be on the low side. On the other hand, if hollow fibers having less 
dense walls are desired (all other variables remaining constant) the 
concentration must be somewhat higher. The maximum concentration is, of 
course, limited to that where the polymer solution containing the 
inorganic material is not amenable to extrusion through a spinneret. 
Correspondingly, the lower limit is where the resultant polymeric 
precursor hollow fiber does not have sufficient polymer to maintain its 
structure. In general, the polymer concentrations will be from about 5 to 
about 35% by weight of the polymer solution. Particularly preferred 
polymer concentrations are from about 10 to about 30%, more particularly 
preferred 15% to 30%, by weight of the polymer solution. 
The nature of the organic polymer employed in the preparation of the 
polymeric precursor hollow fiber according to this invention is not 
critical; for example, polyacrylonitrile, polymers of acrylonitrile with 
one or more other monomers polymerizable therewith such as vinyl acetate, 
methyl methacrylate, polyurethanes and polyvinyl chloride may be used. 
Both addition and condensation polymers which can be cast, extruded or 
otherwise fabricated to provide hollow fibers by dry or wet spinning 
techniques are included. Typical polymers suitable for use in the process 
of the present invention can be substituted or unsubstituted polymers and 
may be selected from polysulfones; poly(styrenes), including 
styrene-containing copolymers such as acrylonitrile-styrene copolymers, 
styrene-butadiene copolymers and styrenevinylbenzylhalide copolymers; 
polycarbonates; cellulosic polymers, such as cellulose acetate-butyrate, 
cellulose propionate, ethyl cellulose, methyl cellulose, nitrocellulose, 
etc.; polyamides and polyimides, including aryl polyamides and aryl 
polyimides; polyethers; poly (arylene oxides) such as poly(phenylene 
oxide) and poly(xylylene oxide); poly(esteramidediisocyanate); 
polyurethanes; polyesters (including polyarylates), such as poly(ethylene 
terephthalate), poly(alkyl methacrylates), poly(alkyl acrylates), 
poly(phenylene terephthalate), etc.; polysulfides; polymers from monomers 
having alphaolefinic unsaturation other than mentioned above such as 
poly(ethylene), poly(propylene), poly(butene-1), poly(4-methyl pentene-1), 
polyvinyls, e.g., poly(vinyl chloride), poly(vinyl fluoride), 
poly(vinylidene chloride), poly(vinylidene fluoride), poly(vinyl alcohol), 
poly (vinyl esters) such as poly(vinyl acetate) and poly (vinyl 
propionate), poly(vinyl pyridines), poly(vinyl pyrrolidones), poly(vinyl 
ethers), poly(vinyl ketones), poly(vinyl aldehydes) such as poly(vinyl 
formal) and poly(vinyl butyral), poly(vinyl amines), poly(vinyl 
phosphates), and poly(vinyl sulfates); polyallyls; 
poly(benzobenzimidazole), polyhydrazides; polyoxadiazoles; polytriazoles; 
poly(benzimidazole); polycarbodiimides; polyphosphazines, etc., and 
interpolymers, including block interpolymers containing repeating units 
from the above such as terpolymers of acrylonitrile-vinyl bromide-sodium 
salt of parasulfophenylmethallyl ethers; and grafts and blends containing 
any of the foregoing. Typical substituents providing substituted polymers 
include halogens such as fluorine, chlorine and bromine; hydroxyl groups; 
lower alkyl groups; lower alkoxy groups; monocyclic aryl; lower acyl 
groups and the like. 
Furthermore, since the organic polymer is to be treated to remove it in 
subsequent steps of the process, it should be amenable to this treatment. 
For instance, a more preferred polymer would be one that readily 
decomposes and/or reacts, but not at an excessively rapid rate, to effect 
its removal. Still further, such polymers should not form reaction 
products that will adversely interact with the inorganic materials or 
interfere with the subsequent steps in the process. 
Obviously the cheapest and most readily available polymers are preferred. 
Polymers and polymers of acrylonitrile with one or more monomers 
polymerizable therewith or particularly amenable to the process of this 
invention. 
The solvents to be used in the preparation of the polymer solution can be 
any number of those well known to one skilled in the art. For instance, 
such solvents as dimethylacetamide, dimethylformamide, dimethyl sulfoxide, 
etc., are particularly useful with polymers of acrylonitrile with one or 
more monomers polymerizable therewith. Obviously the solvent selected 
should be a good solvent for the organic polymer and should be amenable to 
the dry or wet spinning techniques contemplated in the subsequent steps of 
the process. 
The polymer solution containing an inorganic material can be prepared by 
dispersing the inorganic material in the solvent followed by the addition 
and dissolution of the polymer in the solvent. Any other suitable means of 
preparing the polymer solution containing an inorganic material is 
acceptable, for instance, by concurrently mixing polymer, inorganic 
material and solvent or by mixing the polymer and the solvent followed by 
addition and dispersion of the inorganic material, etc. It is preferred to 
disperse the inorganic material in the solvent prior to polymer addition. 
Ambient or somewhat higher temperatures are usually quite adequate for the 
preparation of the polymer solution containing an inorganic material. 
Dependent on polymer, solvent and/or inorganic material utilized higher or 
lower temperatures may aid the preparation but are not considered 
critical. 
The amount of the inorganic material is inversely related to the same 
general considerations discussed above concerning the polymer 
concentration in the polymer solution. The maximum amount is limited to 
that where the precursor fiber structure can not be maintained because 
sufficient polymer is not present. The minimum amount is where the 
inorganic material particles are so widely dispersed that they do not 
sufficiently fuse or bond during sintering. Normal ratios, by weight, of 
inorganic material to polymer will range from about 3.5 to about 15. 
Preferred ratios of inorganic material to polymer are from about 4 to 
about 12, more preferably from 4.5 to 10. 
The inorganic material must be uniformly dispersed as, e.g., small 
particles, throughout the polymer solution. Sufficient mixing must be 
carried out to achieve such a uniform dispersion. Although some amount of 
inorganic material may be dissolved, and this may be helpful in achieving 
a uniform dispersion, this is not critical to achieving the objectives of 
the present invention. 
The inorganic material incorporated into the polymer solution is a 
sinterable inorganic material (this phrase includes materials from which a 
sinterable material can be prepared). Such materials constitute an 
extraordinarily large group of materials that either are suitable as such 
or that can be converted to the desired sinterable inorganic material. For 
instance, if the desired fiber is to comprise a metal, such as nickel or 
its alloy, either the metal, its oxide or other compounds that can be 
ultimately converted to the metal can be used. 
Although the process of the present invention is particularly useful in 
producing hollow fibers or metals, such as by the reduction of metal 
oxides to metal and sintering of the metal, it may be utilized to produce 
hollow fibers of any inorganic materials that are sinterable or that can 
be converted to a sinterable material. Such inorganic materials are 
discussed above. For purposes of illustration, the following detailed 
description will be limited to metal compounds which are reducible to 
metals and which are sinterable. 
Since the reduction temperatures must, of course, be below the melting and 
vaporization point of the compounds being reduced and of the elemental 
metal formed, the metal compounds which vaporize or sublime excessively at 
temperatures below that at which they will react with hydrogen or carbon, 
the metal component of which has such a low temperature of vaporization of 
sublimation (e.g., K, Na, Li, etc.), may not be satisfactorily used in 
accordance with the present process without special consideration. 
Although the use of hydrogen to provide the environment for reducing the 
metal compound particles to elemental metal is a preferred embodiment of 
the present invention, other reducing materials may be employed. For 
example, the metal compounds and particularly nickel and iron oxides can 
be reduced by partially or wholly substituting carbon monoxide for the 
hydrogen reducing environment. Obviously the constituents of the polymer 
and traces of solvent will also contribute to such a reducing environment. 
Additionally the metal compound itself is limited to those materials 
wherein the reaction products, other than the elemental metal, will leave 
the reaction zone prior to or during sintering of the hollow fiber. 
The most significant metal compounds are, of course, the oxides since these 
compounds are the most plentiful; and, in fact, are the state in which 
metals are most commonly found as by-products of manufacturing and in 
natural ore concentrates. Other compounds which may be utilized include 
metal halides, hydroxides, carbonates, oxalates, acetates, etc. 
Particle size is an important factor for producing the desired hollow 
fibers regardless of the inorganic material utilized. Small particles 
utilized for dispersion in the polymer solution usually range in size from 
less than 15 microns, preferably 10 microns, most preferably 5 microns or 
less. Generally such particles will range in size distribution from one 
end of the scale to the other. Obviously the smaller particle sizes, i.e., 
less than 10 microns, are preferred in order to obtain a uniform 
dispersion. To obtain metal fibers of desired characteristics it may be 
necessary to use very small particles, i.e., 5 microns or less. This may 
require particle size comminution and/or classification to achieve desired 
sizes. 
A generally smaller diameter particle would be expected to intensify 
"outgassing" cracking and surface problems observed with compaction 
procedures since the smaller particles are closer together leaving less 
room for the evolved reaction gases to escape. However, it has been found 
that where the smaller diameter particles are utilized a more flaw-free 
hollow fiber can be produced. 
A still further difficulty in using very fine metal particles relates to 
the tendency of many metals to oxidize when exposed to air in small 
particle form. For example, fine iron particles (40 microns or less) tend 
to react exothermically when exposed to air to form iron oxide particles. 
Thus, it is difficult to handle such materials while the oxide particles 
can be freely shipped and easily handled without providing air tight 
protective envelopes or making special provisions to avoid spontaneous 
reactions. The process of this invention is particularly amenable to use 
of oxides since oxide particles are often by-products of metal treating, 
and, consequently, are readily available at low prices. For example, iron 
oxide particles obtained as a by-product from hydrochloric acid pickling 
is readily available. Other sources of iron oxide particles include dust 
from basic oxygen converters, rust, mill scale, and high-grade iron ore. 
Nickel oxide is available at nominal prices. 
Metal compound particles of any general shape (i.e., spherical, oblong, 
needles, or rods, etc.) may be employed in accordance with the present 
invention. Metal oxide particles obtained by the process of spray drying a 
dissolved metal compound can provide superior hollow fibers. 
Accurate particle size determinations of fine-grained particles are 
difficult to obtain, particularly where the size includes particles less 
than 10 microns in diameter (or smallest dimension). Such determinations 
are most difficult where the particles are of non-uniform shape. For 
example, many of the particles are likely to be of a relatively elongated 
configuration so that it is difficult to determine the smallest dimension 
of the particle. Elongated particles will not pass through a screen having 
a mesh that is designed to accommodate a relatively symmetrically shaped 
particle of equivalent mass. As a result particle size and particle size 
distribution measurements vary to a considerable degree for a given 
material between the known methods and procedures for making such 
determinations. 
Relatively accurate fine-grained particle size determinations may be made 
through the use of Coulter counter procedure. In this procedure the 
particles are suspended in an electrically conductive liquid and are 
caused to flow through a small orifice. A current is caused to flow 
through the orifice by means of two immersed electrodes, one on each side 
of the orifice. As the particles flow through the orifice, the change of 
electrical resistance between the electrodes is measured to determine 
particle size. Thus, the measure primarily is interpreted on particle mass 
and is not affected by shape. 
A particularly desirable feature of the process of the present invention 
when using metal compounds relates to the "active" state of the metal 
fiber reduction of the metal compound particles and prior to sintering. 
Metal particles tend to acquire a thin oxide coating or film and in fact 
nearly all metal powders of fine particle size must acquire or be provided 
with such a film to prevent rapid oxidation or defeat the pyrophoric 
nature of such materials. Such a film renders the particles "passive" so 
that they may be handled in ordinary atmosphere. However, such a film is 
difficult to reduce and retards sintering. When metal compound particles 
are reduced in accordance with the process of the present invention and 
are sintered subsequent to reduction without being exposed to an oxidizing 
environment hollow fibers having excellent properties may be obtained due 
to the "active" nature of the reduced particles. This feature further 
enhances the value of this invention. 
Metal alloys can be provided as the inorganic material of the fiber of this 
invention by the simple expedient of mixing particles of metal compounds, 
e.g., metal oxides, and dispersing this mixture in the polymer solution. 
Such alloys can provide useful strength and other characteristics. 
Exemplary of such alloys are those formed using nickel and iron oxides. 
Another acceptable procedure for making metal hollow fibers by the practice 
of the process of the present invention is to incorporate metal particles 
with the particulate metal compounds. Preferably the metal particles will 
be blended with the metal compounds prior to dispersion in the polymer 
solution. Reducing and sintering may be accomplished at the usual 
temperatures and in the presence of the usual atmospheres (in accordance 
with the process of the present invention). The sintering temperature may 
be high enough to effect diffusion of the elemental metal into the reduced 
base metal to effect alloying. Consequently, it may be necessary or 
desirable to employ a somewhat higher sintering temperature where the 
elemental metal has a low diffusion rate. If the sintering temperature of 
the elemental metal (or temperature at which diffusion of the elemental 
metal into the base metal will occur) is higher than the melting point of 
the base metal then alloying may not be accomplished. However, in the 
latter eventuality the elemental metal or its oxide may dispersion 
strengthen the base metal. 
An additional use of metal particles is to reduce shrinkage of the sintered 
product. In any sintering process, the metal article shrinks in its outer 
dimensions due to the elimination of the void spaces between the particles 
when the particles fuse to form a solid mass. When the inorganic material 
comprises metal compounds such as metal oxides that are first reduced and 
then sintered in accordance with the method of the present invention such 
shrinkage is accentuated due to the fact that the reduced particles are 
smaller than the metal compound particles and thus provide greater void 
spaces between particles. Such shrinkage can be reduced or minimized by 
adding elemental metal particles to the metal compound particles for 
incorporation in the polymer solutions. For example, it may be desirable 
to add up to 50 percent, by weight, nickel powder to nickel oxide powder 
to reduce shrinkage of the resultant hollow fiber. The particle size of 
the elemental metal particles will preferably be very small since such 
dispersed particles will diffuse into a matrix metal quickly and evenly. 
Further, by including with the metal compound a proportion of dispersed, 
non-reducible (or diffusible) materials of controlled particle size, it is 
possible to effect a dispersion strengthened sintered product. The 
particles may consist of elemental metals that sinter at a higher 
temperature than the sintered product. 
As mentioned above, the sinterable inorganic material can be a material 
that comprises the fiber material without chemical modification or a 
material that is converted to a desired form by chemical modification. As 
extensively discussed above, metal compounds particularly metal oxides, 
are illustrative of the latter materials. If metal fibers are desired 
these oxides require reduction to the elemental metal prior to or during 
sintering. Other materials that are amenable to the process of the present 
invention are those that may require oxidation or both oxidation and 
reduction to form the material comprising the resultant hollow fiber. 
Although these procedures will not be discussed in the detail provided for 
metal compounds, these materials, such as aluminum, are also useful with 
the process of this invention. Other inorganic materials which can be 
provided by simultaneous oxidation and reduction are also useful in the 
process of this invention. Illustrative of these materials is the 
simultaneous oxidation and reduction of aluminum or titanium and iron 
oxide or nickel oxide. The following materials illustrative of those 
materials which can comprise the final fibers without chemical 
modification (i.e., without reduction and/or oxidation) are metals, 
ceramics such as alumina, beta-alumina, glass, mullite, silica, etc. 
The polymer solution containing an inorganic material can also contain 
other additives to assist in this and subsequent steps in the process, 
particularly for instance, in the extrusion and fiber-forming steps. 
Surfactants such as sorbitan monopalmitate, etc., are useful to wet the 
inorganic material by the solvent of the polymer solution. Plasticizers 
such as N,N-dimethyl lauramide, etc., are useful to provide polymeric 
fiber flexibility. 
Extrusion of Polymer Solution Containing Inorganic Material 
In making hollow fibers by the process of the present invention, a wide 
variety of extrusion conditions may be employed. As previously discussed, 
the weight percent polymer in the solution may vary widely but is 
sufficient to provide a hollow fiber under the extrusion and fiber-forming 
conditions. If the inorganic material, polymer and/or solvent contain 
contaminants, such as water, particulates, etc., the amount of 
contaminants should be sufficiently low to permit extrusion and/or not 
interfere with or adversely affect subsequent steps in the process or the 
resultant fiber. If necessary, contaminants can be removed from the 
polymer solution by filtration procedures. Obviously filtration must be 
appropriate to remove contaminant particles while passing the particles of 
inorganic material. Such filtration may also remove particles of inorganic 
material which are above the desired particle size. The presence of 
excessive amounts of gas in the polymer solution containing inorganic 
material may result in the formation of large voids and undesirable 
formation of porosity in the precursor polymeric hollow fiber. 
Accordingly, degassing procedures are also appropriate. Such degassing 
and/or filtration procedures can be carried out immediately after or 
during preparation of the polymer solution containing an inorganic 
material or can be carried out immediately prior to or during the 
extrusion step. 
The size of the hollow fiber spinnerets will vary with the desired inside 
and outside diameters of the resultant polymeric precursor hollow fiber. 
The spinnerets may also vary in shape, i.e., hexagonal, oblong, star, etc. 
The spinnerets are generally circular in shape and may have outer 
diameters of, for instance, about 75 to about 6000 microns with center pin 
outer diameters of about 50 to about 5900 microns with an injection 
capillary within the center pin. The diameter of injection capillary may 
vary within the limits established by the pin. The polymer solution 
containing the inorganic material is frequently maintained under a 
substantially inert atmosphere to prevent contamination and/or coagulation 
of the polymer prior to extrusion and to avoid undue fire risks with 
volatile and flammable solvents. A convenient atmosphere is dry nitrogen. 
The temperature preparatory for extrusion of the polymer solution 
containing inorganic material can vary over a wide temperature range. In 
general the temperature is sufficient to prevent undesirable coagulation 
or precipitation prior to extrusion. The temperature generally can range 
from about 15.degree. C. to about 100.degree. C. preferably from about 
20.degree. C. to about 75.degree. C. 
The pressure to accomplish the extrusion is normally those within the 
ranges understood by those skilled in the fiber spinning arts. The 
pressure depends on, for instance, the desired extrusion rates, the 
orifice size and the viscosity of the polymer solution containing the 
inorganic material. Of particular note is the fact that relatively low 
pressures can be utilized with the process of the present invention. This 
contrasts with compaction procedures which often require hundreds of 
atmospheres of pressure to provide compacted and sintered articles. The 
pressure useful with the present invention normally range from about 1 
atmosphere up to about 5 atmospheres or higher. 
Obviously the fibers can be extruded through a plurality of spinnerets. 
This will enable the concurrent formation of multiple fibers while, for 
instance, using the same coagulating bath. 
Formation of the Polymeric Precursor Hollow Fiber 
In general, fiber-forming spinning techniques are known to those skilled in 
the synthetic fiber-forming industries. These skills can be advantageously 
applied to the fiber-forming step of the process of this invention. The 
fiber-forming step may be conducted using wet or dry spinning techniques, 
i.e., the spinneret may be in or removed from the coagulating bath. The 
wet technique is often preferred and may be used for the sake of 
convenience. That is, the fiber coagulation can be effected by bringing 
the fiber which is being formed by extrusion into contact with a 
coagulating bath. It suffices to pass the fiber which is being formed into 
the coagulating bath. A fluid which coagulates the polymer of polymer 
solution is usually injected into the bore of the fiber being formed. The 
fluid may comprise, e.g., air, isopropanol, water, or the like. 
Any essentially non-solvent for the polymer can be employed as the 
coagulating agent in the coagulating bath. The coagulating agent may be 
miscible with the solvent. The nature of the coagulating agent selected 
depends on the solvents used for the organic polymer and the choice 
depends on criteria known in the field of fiber spinning. It is important 
to use mild coagulating agents for both the bore injection fluid and in 
the coagulating bath to obtain uniform density fiber walls. By a "mild 
coagulating agent" is meant a medium in which the organic polymer will 
precipitate slowly so that coagulation does not occur rapidly. 
Conveniently, water is employed as a coagulating agent at low 
concentrations in the coagulating bath. Other coagulating agents are: 
ethylene glycol, polyethylene glycol, propylene glycol, methanol, ethanol 
and propanol, etc. Ethylene glycol is a particularly preferred coagulating 
agent. The residence time for the extruded fiber in the coagulating bath 
is at least sufficient to ensure reasonable solidification of the fiber. 
The fiber wall is formed due to interaction with the coagulating agents 
and/or cooling. (Cooling may also be achieved by bringing the extruded 
polymer solution containing inorganic material into contact with a gas at 
a temperature below the gelling temperature of the polymer solution. Where 
gelling is accomplished in this manner, the cooling gas can be subjected 
to a relatively rapid translatory movement which can be oriented in a 
direction parallel to that of the hollow fiber. This gas may additionally 
be charged with water vapor or the vapor of some other non-solvent). Where 
gelling is also accomplished in the coagulating bath the bath may, in 
addition to its gelling effect, also impart a coagulating effect. 
The temperature of the coagulating bath may also vary widely, e.g., from 
-15.degree. to 95.degree. C. or more, and is most often about 1.degree. to 
35.degree. C., say, about 2.degree. to 25.degree. C. The temperature of 
the fluid injected into the bore is generally within the same ranges. 
After coagulating the fiber it may be washed to remove solvent by, for 
instance, washing with the coagulating bath solution or with other 
non-solvents that are miscible with the solvent of the polymer solution. 
Washing may cause further coagulation. The precursor hollow fiber may also 
be stored in a water or other liquid bath. 
The extrusion and fiber-forming conditions are preferably such that the 
fiber is not unduly stretched. Although not necessary, stretching can be 
used say, about 1 to about 5 fold. Frequently, extrusion and fiber-forming 
speeds are within the range of about 5 to 100 meters per minute although 
higher speeds can be employed providing the fiber is not unduly stretched 
and sufficient residence time is provided in the coagulating bath. 
Stretching generally strengthens the polymeric precursor hollow fiber. 
Stretching also allows increased linear productivity and smaller fiber 
diameters with a given spinneret. 
An annealing procedure may also be carried out to toughen the polymeric 
precursor hollow fiber. Both the stretching and annealing procedures can 
be conducted by, for instance, passing the fiber through boiling water. 
The precursor hollow fibers of polymer laden with an inorganic material can 
be subjected to the subsequent steps in the process or can be taken up and 
stored in precursor form on, for instance, bobbins. The precursor fibers 
are flexible and have reasonable degree of strength and can therefore be 
handled without undue concern for damage. 
After obtaining the precursor fiber by the process of the invention drying 
may be carried out in a known manner. The fibers are generally, but not 
necessarily, dried prior to treatment to remove the organic polymer. The 
drying may be conducted at about 0.degree. to 90.degree. C., conveniently 
about room temperature, e.g., about 15.degree. to 35.degree. C., and at 
about 5 to 95, conveniently about 40 to 60, percent relative humidity. 
The precursor hollow fiber comprises the polymer in minor amount acting as 
the carrier for the inorganic material which is uniformly dispersed 
throughout the polymer. Generally, the polymer is present in the precursor 
hollow fiber in concentrations substantially less than 50% and often as 
low as 25%, 15%, or 5% by weight. The major component in the precursor 
fiber being, of course, the inorganic material. Other materials may be 
present in the precursor fiber but generally only in small amounts. 
Treatment to Remove Organic Polymer 
After formation of the polymeric precursor hollow fibers laden with 
inorganic material the fiber can be preferably dried or stored and dried 
as discussed above, or transferred directly to a treatment to remove the 
organic polymer from the fiber. This can be accomplished by heating to 
decompose and/or react the organic polymer. This may be accomplished in an 
inert or reducing atmosphere to aid in reduction of the inorganic 
material, although this is not always necessary. 
As mentioned above, the reaction products formed from the organic polymer 
may serve to enhance the other steps of the process. For instance, the 
hydrogen and carbon present in the polymer serve as an excellent source of 
a reducing environment. This environment helps to reduce metal compounds, 
e.g., oxides, to the elemental metal. 
The fiber containing inorganic material may, optionally, be subjected to 
reduction and/or oxidation. (It is, of course, recognized that neither 
reduction or oxidation may be necessary if the inorganic material 
dispersed into the polymer solution is in the chemical form desired for 
sintering.) Preferably an appropriate atmosphere will be provided just 
prior to the fiber being subjected to the reduction and/or oxidation 
temperature. For instance, with reduction, this may be accomplished by 
continuously passing the polymeric precursor hollow fiber laden with a 
reducible inorganic material through a commercially available oven. An 
atmosphere comprising, for instance, hydrogen may be caused to flow 
countercurrently and in contact therewith. As the fiber first contacts the 
heat of the oven, the remaining volatile components will outgas. As the 
temperature approaches reducing temperatures, the reducible inorganic 
material, for instance, metal compounds are converted to elemental metal 
and the reaction products outgas. 
For the purposes of the present invention and this specification, it will 
be understood that the temperature range at which polymer removal and 
reduction and/or oxidation will occur and the sintering temperatures may 
overlap to some extent. In other words, some sintering may occur at the 
temperatures at which polymer removal and reduction and/or oxidation is 
carried out, although it is preferable that the temperature be such that 
reduction takes place immediately preceding sintering. The preferred 
temperatures at which reducible inorganic materials, i.e., metal compounds 
will reduce are well-known to those skilled in the art or their 
determination is well within the skill of those of ordinary competency. 
The preferred reducing environment may be provided by any atmosphere which 
provides a source of hydrogen. For example, such an atmosphere may 
comprise pure hydrogen, cracked hydrocarbons, dissociated ammonia, 
combinations of each, combinations of one or more of such gases and other 
gases or vapors which will not materially interfere with the reduction 
reaction. The reaction products from the decomposition and/or oxidizing of 
the polymer are valuable aids in providing the reducing atmosphere. 
Solid reducing materials, carbon for example, may be employed in 
combination with the hydrogen yielding gas only where the reactants (e.g., 
CO and CO.sub.2) appropriately "outgas" and will not leave residual 
elements in the sintered fiber that will interfere with the desired fiber 
properties. For example, carbon may be a desired addition to the oxide 
powder as set forth above where the ultimate product is a steel 
composition and the residual carbon is a necessary element for the 
finished fiber. 
Oxidation of the inorganic material can be conducted at the appropriate 
temperatures under suitable pressures and atmospheres. Air is the 
preferred atmosphere. The oxidation temperatures are generally well-known 
or readily ascertainable. Simultaneous oxidation and reduction can occur, 
say, for instance, in the formation of cermets. The resulting fiber 
comprising a sinterable inorganic material may then be conducted directly 
into a sintering zone. 
Sintering to Form to Inorganic Fiber 
The term "sintering" is meant to include an agglomeration by fusion and 
bonding of the sinterable inorganic material to at least that point at 
which the particulate material forms a monolithic structure. Sintering 
should provide a fiber having substantial strength as compared to a fiber 
which has undergone the previous steps and has not been sintered. The 
sintering must be conducted under conditions that assure that the valence 
state desired is achieved or maintained under sufficient temperatures and 
times to allow the fusion and bonding to occur. 
In the production of the hollow fibers of this invention there are little 
or no limitations on the heating rate for sintering. For instance, the 
sintering of a nickel-iron alloy fiber can be at from about 950.degree. C. 
to about 1200.degree. C. for from 15 to 5 minutes, respectively. A 
nickel-iron alloy fiber produced under these conditions is excellent. In 
general, similar to the reduction and oxidation temperatures, the 
preferred sintering temperatures of the inorganic materials are well-known 
or readily ascertainable. 
During the organic polymer removal, optional reduction and/or oxidation of 
the inorganic material and sintering steps, suitable conditions must be 
maintained to avoid damage or destruction to the fiber wall structure and 
integrity. A shrinkage ratio (final fiber to precursor fiber) of from 
about 0.2 to about 0.9 can be expected, usually 0.3 to 0.6. That is, the 
precursor hollow fiber is often transformed to the final hollow fiber with 
substantial size reduction. This is expected during these process steps. 
For instance, the fiber is substantially reduced in length and the fiber 
outer diameter and wall, although remaining in relative relationships, are 
also reduced in size. During these steps means must be provided to handle 
the fiber as it shrinks. Particularly critical is the point immediately 
prior to sintering where the fiber is fairly fragile. At this point, 
particular care must be taken to provide means to afford such shrinkage 
without damage to the fiber. For instance, if the fiber is allowed to 
adhere to a conveying surface at this point it may break as it shrinks. 
One method of handling the fiber at this point is to feed a precursor 
fiber, which may be pretreated, e.g., with water, to provide better 
handling characteristics, into the furnace by means of a conveyor belt 
which is fabricated of material which does not adhere to the fiber under 
the operating conditions of the furnace. This conveyor belt can be 
transporting the fiber at the speed of the final fiber as it exits the 
furnace. The precursor fiber feed speed is faster than the final fiber 
speed. The precursor feed speed can be adjusted to account for the 
shrinkage that occurs. 
A particularly important feature of the process of this invention is the 
ability to produce fibers having relative strong and dense walls. This 
feature is surprising since the polymer of the polymeric precursor fiber 
is the continuous phase which is removed as discussed above. It has been 
found that, although the polymer is removed from the fiber wall of a 
precursor fiber, the final fiber, after sintering, is usually quite strong 
and dense. Although it might be expected that shrinkage and reduction of 
interstices between particles of inorganic materials might occur when the 
inorganic material undergoes reduction, oxidation and/or sintering, the 
formation of a fiber wall that is strong and dense, i.e., inhibits passage 
of fluids, is both desirable and unexpected. This phenomena appears to 
occur throughout the fiber wall where ever polymer is removed. It has been 
observed particularly when using metal compounds, e.g., oxides, to convert 
to elemental metal. 
The process of this invention can also produce hollow fibers having a 
porous wall. This can be achieved by, for instance, treating the fiber 
wall with a fluid that has some interaction with the material of the wall 
to produce a porous wall. For instance, a polymeric precursor fiber 
containing nickel oxide can result in a uniformly porous wall surface by 
introducing ammonia gas in the atmosphere in the furnace. 
An alternate means to obtain a porous fiber wall is to introduce a 
relatively small amount of fine particulate material which does not 
participate in the sintering or participates in the sintering to a lesser 
degree. Incorporation of such fine particulate materials in the polymer 
solution containing an inorganic material during its preparation can 
result in a porous fiber wall in the final inorganic fiber. 
The hollow fiber resulting from the process is strong compared to precursor 
fiber and fibers from the intervening steps. The final fibers may be 
flexible enough to be stored on bobbins.