Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
10381;~4
The invention relates to the fabrication of poly-
crystalline oxide fibers and includes the steps of 1) forming
elongate green fibers from a substantially non-aqueous s~p con-
sisting essentially of not more than 80 weight percent of
discrete temperature stable phase crystalline particles of
ceramic oxides selected from the group consisting of alumina,
zirconia, zircon, magnesia, chromia, iron oxide, spinel or com-
binations thereof, the said particles constituting substan-
tially all of the ceramic oxide content of said slip and having
a median diameter of 0.05 to 2.5 microns, dispersed in a
volatile organic solvent consisting essentially of a halogenated
hydrocarbon which solvent contains dissolved therein a linear
chain polyethylene oxide polymer binder having an average
molecular weight of at least about 400,000 and present in
amounts of 0.5 to 5~ of the solvent weight, said organic
solvent having a surface tension of not over 50 dynes per cen-
timeter and being selected as to dissolve said amount of poly-
ethylene oxide polymer, 2) displacing the green fibers sub-
stantially concurrent with their formation through an
evaporative environment to remove sufficient volatile fluid
therefrom to render the fibers substantially self-supporting,
while attenuating said green fibers to increase their length
and decrease their diameter by a factor of at least 4, said
attenuation being effected by exerting on said green fibers a
tensile force to effect movement of the green fibers at a
higher rate than their rate of formation, 3) and sintering
the green fibers to produce a coherent polycrystalline ceramic
oxide fiber in the said temperature stable phase having a
ceramic bond intermediate the particles thereof.
The invention further relates to the fabrication of
polycrystalline oxide fibers and includes the steps of 1)
forming a slip by: a) dispersing discrete temperature stable
1038124
phase particles of a ceramic oxide selected from the group
consisting of alumina, zirconia, zircon, magnesia, chromia,
iron oxide, spinel or combinations thereof and having a median
diameter of less than 2.5 microns in a chlorinated hydrocarbon
solvent along with an effective dispersant, present in the
amount of less than 5~ of the weight of solids, said dispersion
being accomplished by agitation employing substantial shear
effects to provide a first suspension, b) dissolving in an
additional quantity of said solvent 0.5 to 5.0% of a high
molecular weight linear chain polyethylene oxide polymer having
an average molecular weight of at least approximately 400,000
to provide a second suspension, c) adding to at least one of
the suspensions produced in the above said steps (la) and (lb)
an effective amount of a compatible plasticized resinous
binder as a slip strengthener along with sufficient additional
solvent compatible with both said chlorinated hydrocarbon
solvent and said additional resinous binder, d) combining
said first and second suspensions and mixing them to provide
said slip, 2) forming an elongate green fiber from said slip
by extruding said slip through at least one elongate extrusion
nozzle having a diameter of 200 to 700 microns and a length
of not more than 1 centimeter nor less than 0.1 centimeter,
the slip, prior to extrusion, having been filtered by passing
through a filter medium having openings therethrough of not
greater than one-half the size of the orifice openings, 3)
moving said green fiber through an evaporative environment
substantially concurrent with their formation to remove
sufficient volatile fluid therefrom to render said green
fiber substantially self-supporting while simultaneously
attenuating said green fiber to increase its length by a
factor of at least 16, the attenuated fiber being continuously
wound upon a take-up drum, 4) removing an accumulation of
~ - la -
1038~Z4
fibers from said take-up drum by slicing through said
accumulation along a line parallel to the axis of said drum
to provide multiplicities of fibers having lengths related to
the diameter of the drum and the fiber position thereupon, 5)
sintering said multiplicities of green fibers to produce
multiplicities of coherent polycrystalline alumina fibers
having ceramic bonds intermediate the particles thereof.
There is a continuing and growing need for ceramic
or refractory materials in fiber form especially where the
material has a hlgh melting temperature. Such fibers are use-
ful as high temperature insulation and even as strengtheners
in composites which may have a metal matrix. To date a number
of processes for producing fibers have been proposed but most
have some disadvantages with respect to the quality and con-
sistency of the fibers or in the economics of their production.
For instance, some of the fibers are short and of inconsistent
cross section and quality thus restricting their use as
strengtheners or high temperature insulation. Other processes
may be capable of producing longer lengths of more consistent
cross section but are marked by economic disadvantages which
seriously restrict their usefulness. Still other processes
rely on precursor fibers of material which undergoes undesira -
ble phase changes with resultant degradation upon heating to
useful temperatures. In accordance with the present improve-
ment long and very thin fibers or filaments of alumina or
other ceramic materials can be provided which in the green
(unfired) state possess sufficient strength that they can be
wound onto spools or otherwise handled with minimal degrada-
tion. Since these fibers are comprised of finely divided
particles of phase-stable ceramic material, they can be
heated to provide ceramic bonding without undesirable high-
temperature phase changes. This results in low cost
~ - lb -
10381Z4
refractory fiber having small diameter and sufficient strength
for use as high-temperature insulating material and other
applications.
Briefly, alumina or other refractory oxide or ceramic
material can be produced in continuous filament form by ext~usion
or other fiber forming techniques which may be coupled with atten-
uation or stretching, from a slip selectively composed as herein
described. The slip comprises the alumina or other ceramic
material suspended in a finely divided state, together with a
-- lc --
10381Z4
binder comprising a high molecular weight linear chain polyethy~ene
oxide polymer, in a suitable solvent, preferably an organic sol-
vent. A dispersing agent should be employed to facilitate a high
solids content where desired in the slip while maintaining fluid-
ity. Also, the slip may contain additional resinous binders to
improve the strength of the green fiber threads before firing.
Where such additional binders decrease the flexibility of the
green fiber a compatible plasticizer may be included as is the
known practice in formulating resin systems containing such bin-
ders. The additional binders and plasticizers may not be suffi-
ciently soluble in the principal solvent and additional solve~s
may be employed to advantage.
The invention is described with particular reference to
the production of alumina fibers although it should be understood
that the invention should find use in the production of other
ceramic refractory oxide fibers such as zirconia, titania, and
others. For example ferric oxide (Fe2O3), chromia (Cr2O3) and
zircon (ZrSiO4) have been produced as continuous green filaments
by the improved method. It is important that the ceramic material
be provided in a finely divided state to insure production of fine
fibers. It is preferred that the particles exhibit median diame-
ters of 3 microns or less, for instance 0.05 or 0.1 to 2.5 or 3
microns although still smaller particles may be even more desired
in some instances. The desired small particle sizes can be
achieved by grinding, milling or other known methods of subdivi-
sion. Referring to the particular embodiment em~oying alumina
it is desirable that the alumina be in the form of minute crystal-
lite particles having a median crystallite diameter of 0.05 to 0.1
to 2.5 microns or even still smaller sizes may be preferred in
some instances.
In addition to particle size the ceramic material
crystalline phase can be important. Where undesirable phase
10381Z4
changes would be encountered in firing the fiber, the ceramic
particles should be substantially in the phase condition desired
in the final fiber product. For instance referring to the case
of alumina, firing tends to impart the stable alpha phase. If
another phase dominated the starting material then firing would
cause the undesirable dimensional and grain size changes associ-
ated with the attendant phase changes. Thus, for alumina the
starting material should be primarily of the alpha phaseO
Where alumina forms the suspended phase it may be
desirable to include a small amount of magnesia or talc (magnesi-
um silicate) which perform the known function of inhibiting grain
growth in the fired refractory fibers. Larger quantities of
glass-forming fluxes may be added if desired but at a sacrifice
in refractorinessO For example~ 0.5 to 3.0% talc has been added
to an otherwise pure alumina composition with no noticeable effect
on green fiber productionO A talc level of 1.0% or less is pre-
ferred to improve tensile strength with minimum sacrifice in
refractoriness.
In accordance with the invention it is highly important
that a resinous binder be included in the slip and that this
binder be a linear chain polyethylene oxide polymerO The polymer
should have approximate or average molecular weight of at least
400,000 and preferably at least 700,000. One such suitable poly-
mer is marketed commercially under the Trade Mark "Polyox" by the
Union Carbide Corporation and is available in a number of average
or approximate molecular weight levelsO It has been found that
the use of the high molecular weight linear chain polyethylene
oxide polymer is essential to achieving the desired fiber forming
characteristics in accordance with the present inventionO This
polymer imparts to the slip a pituitous characteristic which
enables consistently forming fine and continuous filaments under
economically attractive conditions~ Generally, as the molecular
-- 3 --
10381Z4
weight of the linear polyethylene oxide polymer increases, solubil~y
decreases and pituitousness increases. Polymer with average molec-
ular weights of about 400,000 to above 6,000,000 at concentrations
of about 5 to 0.5% (solvent weight basis) have been successfully
used in this process. A preferred range of average molecular
weights is 600,000 to 1,500,000 at concentrations of 5 to 2%, the
higher concentrations corresponding to lower molecular weights.
Solvents found particularly suitable in formulating
slip suspensions to produce alumina fibers in accordance withthe
invention are trichloroethylene and ethylene dichloride which are
considered preferred; of these two the former is more preferred
because of its lower volatility. It was found that these two sol-
vents could dissolve substantial amounts of the polyethylene oxide
polymer while otherwise remaining compatible with the other
requisites for the slip suspension. Thus while a number of organic
and even aqueous type solvents may dissolve the polyethylene oxide
polymer to some extent the above-described trichloroethylene and
ethylene dichloride solvents are highly preferred solvents in
practicing the invention. ~owever, the invention is not neces-
sarily intended to be restricted to the use of these two solventswhich nonetheless are preferred since to date they have been
extremely successful, especially trichloroethylene. As described
below, mixtures of solvents may also be used to advantage in the
practice of this invention.
In order to obtain a relatively high solids content in
the slip a suitable dispersant may be employed. Although not
absolutely necessary, a high solid content favors the lowest cost
in making the fibers and is often highly desirable for this and
~her reasons. The requisites for a suitable dispersant are solu-
bility in the solvent, compatibility with the other system ingred-
ients and the ability to reduce the natural attraction between
suspended particles. In the case of alumina there are a number of
10381;~
known dispersants which can be employed with the organic solvents
described aboveO Polyunsaturated natural oil is quite suitable
for this purpose, fish oil serving quite well. Certain unsaturated
fatty acids are suitable as dispersants, oleic and ricinoleic acids
being examples. Another natural derivative, oil of peppermint,
has also proved effective. While the four dispersants just men-
tioned are highly satisfactory, it is expected that there are many
more which should also be useful. The dispersing agent, if used,
should be about 0.1 to 5.0%, weight % on sdids basis, and
preferably 1.0 to 2~0%o
In order to improve the strength of the green fiber
(before firing) additional binders may be employed as explained
more below; this, in turn, can necessitate additional solvents to
assure solution of the binder in the slipo The advantages in
increasing the strength of the fibers before firing are significant
in that the higher strength minimizes breakage and can permit fur-
ther stretching to form finer fiber diameters with less risk of
breakage and this becomes significant where a number of fibers are
being simultaneously extruded from a single source or reservoir
and fiber breakage produces undesirable interruptions or impaired
production levels. As is known, the use of some binders which may
be brittle resins at room temperature can result in increased stiff-
ness in the green fiber and accordingly the use of these binders
suggests the use of one or more plasticizers to offset the stiff-
ness. Hence the invention contemplates the use of such compatibly
plasticized resins whi~h is considered to mean in inherently plas-
ticized resin or a resin used in combination with a compatible
plasticizer so that the resin does not diminish the flexibility
of the green fiber to an excessive extent. As an example of a
suitable resin polyvinyl butyral has been employed but requires
the use of compatible plasticizers. The polyvinyl butyral cited
herein is available from Monsanto Chemical Co. under the Trade
103812~
Mark "Butvar". Two grades, B-98 and B-76, have been used
interchangeably in the improved process. In an alumina s~p con-
taining trichloroethylene and ethyl alcohol as solvents and Butvar
as a binder plasticizers which have proven effective and compati-
ble are polyethylene glycol, butyl phthalate, octyl phthalate and
castor oil. Although these specific plasticizers are cited here,
many others would likely be suitable as well, and it would be
extremely difficult to attempt to list even a large proportion of
such for instance, the manufacturer lists 20 plasticizers for
"Butvar" (Trade Mark) alone.
The use of an additional binder with or without a
plasticizer may necessitate the use of additional solvents. For
instance, considering the case of an alumina suspension in tri-
chloroethylene solvent, a polyvinyl butyral-polyethylene glycol
binder-plasticizer combination is difficult to dissolve. However,
the addition of a small amount of a solvent mutually compatible
with the major trichloroethylene solvent and the additional binder-
plasticizer combination such as ethyl alcohol, is advisable in
order to provide a true solution of organic components suspending
the alumina or other ceramic particlesO
The amount of suspended ceramic solids can vary from 5
to 80 weight % of the slip compositionO In general a lower solids
level favors the production of finer fibers but at increased cost
over higher solids levels. Also, the use of finer solids particles
favors finer fibers and involves lower solids content. Preferred
solids content ranges from 25 to 60 weight %.
The amount of dispersant present is typically around
Ool to 5% of the amount of alumina or other ceramic material
present. As indicated earlier it is essential that the solvent
or vehicle also contain the high molecular weight linear chain
polyethylene oxide polymer which should be present in amounts of
from O5 to 5% of the weight of the solvent present depending upon
1038124
the molecular weight of the polymer, larger amounts being
advisable for lower molecular weights. In a preferred embodi-
ment where the polymer molecular weight is at least 600,000,
the amount of polymer should vary from about 2 to 5% of the
amount of solvent present. Where resin binders are additionally
included they are typically present in amounts varying from
about 0.5 to 4~ of the weight of the solvent present. On a
weight basis the amount of plasticizer present is typically
similar to the amount of an additional resin binder but the
relationships between plasticizer and resin are already known
in the art and do not need extensive elaboration here. What
is generally done in adding plasticizer is to determine that
amount which produces best results in a given system. Where
additional resin binders are employed ethyl alcohol is a suita-
ble solvent addition and is typically present in amounts varying
from 2 to 6 times the amount of resin binder included. All the
foregoing formulations are intended as a guide but the invention
is not necessarily intended to be limited thereto. In the case
where the ceramic is alumina and the solvent is selected from
the group consisting of trichloroethylene, ethyl alcohol and
ethylene dichloride the examples below set forth preferred
compositions.
Example 1
Component Parts by Weight
Alumina 41.7
Fish Oil 0.2
Ethylene Dichloride 56.8
Polyethylene Oxide (average MW 1.3
1,000,000)
TOTAL 100.00
Example 2
Alumina 31.8
Fish Oil 0~5
Trichloroethylene 64.8
Polyethylene Oxide (average MW 2.9
830,000)
100. 00
-- 7 --
c~
.,
1038124
Example 3
Alumina 32.2
Fish Oil 0.5
Trichloroethylene 65.8
Polyethylene Oxide (average MW 1.5
1,550,000)100.00
Example 4
Alumina 30.3
Talc 0.3
Fish Oil 0.3
Trichloroethylene 63.2
Ethyl Alcohol 4.5
Polyethylene Oxide (average MW 1.4
900, 000)100. 00
Example 5
Alumina 41.2
Ricinoleic Acid 0.4
Trichloroethylene 50.3
Ethyl Alcohol 4.5
Polyvinyl Butyral B-98 1.2
Polyethylene Glycol 1.2
Castor Oil 0.6
Polyethylene Oxide (MM 1,000,000) 0.6
100. 00
Example 6
Alumina 44.2
Fish Oil 0-7
Ethyl Alcohol 6.7
Trichloroethylene 42.1
Polyvinyl Butyral B-76 0.9
Polyethylene Glycol 1.9
Butyl Phthalate 1.6
Polyethylene Oxide (MW 760,000)1.9
100. 00
The ingredients employed in the practice of the
invention in preparing the slip are advantageously combined
carefully in order to derive the maximum benefits from the
practice of the invention. It is generally preferable to com-
bine the solvent and the ceramic separately from combining
the solvent with the polyethylene oxide polymer. Thus the
total desired solvent is divided into two portions which are
separately combined with the ceramic and with the polymer
materials. One reason for this procedure is that getting
the ceramic material into suspension usually requires a
103~1Z4
substantial amount of milling or other severe agitation and this
agitation may degrade the polyethylene oxide polymer. Even get-
ting the polyethylene oxide polymer into solution can itself be a
problem. The polyethylene oxide polymer, a solid at room tempera-
ture, does not dissolve readily even with very good solvents. For
instance, employing the preferred trichloroethylene solvent cer-
tain care is advisable in introducing the polyethylene oxide
pdymer. One preferred method c~ntemplates subdividing that por-
tion of the solvent to be mixed with the polymer into two subpor-
tions. The first subportion is cooled to about 0C and the polymerpowder is introduced into this subportion with good agitation which
disperses it in the cool solvent which does not dissolve much of
the polymer. At this point the remaining solvent subportion, at
room temperature, is introduced into the cool suspension which of
course increases the temperature of the mixture. Since the solu-
tion rate is quite slow in the cool solvent the polymer particles
can be well dispersed in the solvent before significant dissolution
occurs. Adding the remaining solvent accelerates dissolution which
is further enhanced by the well dispersed distribution of the poly-
mer. If dry polymer were added to room temperature solvent, par-
tial dissolution can very rapidly occur which in turn can cause a
sharp increase in viscosity which in turn can increase the
difficulty of obtaining a good dispersion of the polymer.
If it is desired to include resin binders and
plasticizers such are normally introduced into the ceramic-
solvent suspension or into the polyethylene oxide-solvent solu-
tion before the two are combined. After the ceramic-solvent
suspension and the polymer-solvent solution are prepared they
are then mixed to produce a homogeneous slip.
The improved slips provided in accordance with the
invention exhibit a relatively unusual characteristic in slips,
that characteristic being a pituitous quality which can also be
10381Z4
expressed as the ability to ~ stretched. This permits the slip
to be extruded and attenuated into very thin fibers without the
excessive breakage occurrences which marked so many previous
attempts to extrude and attenuate slips into ceramic fibers.
The viscosity of the slip can be varied from 1,000 to 50,000 cp
with a range of 10,000 to 20,000 cp being preferred. Viscosity
in this case refers to readings obtained with a Brookfield vis-
cometer using spindle RVT 5 at a speed of 10 rpm.
As explained below fibers are formed by extruding
through orifices, a typical orifice diameter being 300 microns.
However, before extruding it is preferable to filter the slip in
order to remove any agglomeration of suspended or undissolved slip
constituents. It is highly preferred that the filter media have
openings considerably smaller than that of the extrusion orifice.
Preferably the filter cloth openings or mesh spacing should amount
to one-half or less than the size of the orifice, preferably one-
fourth or less. For instance employing an extrusion orifice of
340 microns it was found advisable to use a filter cloth with
openings of only 36 microns in order to minimize plugging of the
extrusion orifice. Filtration through a fine-mesh filter also
tends to improve uniformity of both the fiber surface and
extrusion-attenuation operation.
The invention contemplates extrusion of the slip to
produce fibers which are rendered self-supporting substantially
concurrently with the formation thereof by the evaporation of a
portion or substantially all of the volatile solvent. This is
accomplished by s~ply forcing the slip through one or more ori-
fices and moving the fibers so produced through a short distance
in contact with a drying media which can be room temperature air.
Obviously the drying media can be treated in order to increase
its drying effect as by heating the air. However, this can intro-
duce some problems with respect to retarding attenuation as
-- 10 --
10381Z4
discussed below. ~he size of the orifice openings can vary con-
siderably, typical orifice openings suitably having diameters of
up to 1000 microns although openings of 400 microns or less are
preferred. One preferred embodiment contemplates orifice open-
ings of 200 to 350 or 400 microns. Still smaller openings, for
example 100 microns or less, can also be employed but as orifice
size decreases flow rates are reduced, pressures must be in-
creased and plugging of the orifice becomes more frequent.
Smaller orifice sizes do favor production of slightly finer di-
ameter fiber and thus may be justified in some cases.
The extrusion can be effected through one or moreorifices provided in a plate which forms the bottom or end of an
extrusion container pressurized to force the slip through the
orifice openings. This general arrangement is known from the syn-
thetic fiber making art. However, in the practice of the inven-
tion it is highly preferred to depart from the use of a simple
orifice plate and employ, instead, one or more nozzle tubes having
very thin walls as it has been found that such tend to plug much
less than the perforated orifice plates employed in the synthetic
fiber making arts. The reason for this behavior seems to be re-
lated to a buildup of dried or semi-dried slip which starts to
accumulate around an orifice tending to obstruct it. The thin
tube nozzles tend to minimize the accumulation at the orifice
edges which in turn reduces the incidence of orifice outlet con-
striction and attendant fiber fracture. A very suitable source of
the orifice nozzles is to employ as the nozzles hypodermic needles
which have blunt ends and a length of 0.5 centimeters or less.
Longer lengths may be used but extrusion pressure must be corre-
spondingly increased which is a processing disadvantage. Usually
the application of relatively modest pressure to the extrusion
container is sufficient to extrude the slip through the orifice
openings, typical pressure levels ranging from 5 or 10 psi up to
-- 11 --
1038124
100 or more psi may be employed depending upon the size of the
orifice opening and the thickness or solids content of the slip,
in general accordance with the flow of viscous liquids.
Another technique useful in forming the fibers is
centrifugal extrusion in a spinning cha~er provided with
extrusion orifices around its circumference. This approach can
interfere with fiber attenuation.
In addition to the initial extrusion through an orifice
the fibers produced with the improved slip can be subjected to a
very substantial amount of attenuation which is simply a stretch-
ing which very substantially reduces the diameter of the fiber
while very greatly increasing its length. For instance, employ-
ing an orifice opening of 300 microns attenuation can reduce the
fiber diameter to near 20 microns, a diameter reduction by a
factor of 15 and an attendant fiber lengthening of some 225
times. A preferred practice in the invention contemplates an
attenuation of at least 4 times based on diameter and still rnore
preferred an attenuation of at least 10 times.
The fibers are suitably taken up on a reel or spool
because of their very considerable length. The spool by rotating
at a rate disproportionately higher than the linear rate at
which the fiber exits the extrusion orifice can produce the de-
sired tension which in turn produces the desired fiber attenua-
tion. The reel or take-up drurn can be situated a distance of
0.1 to 5 or more meters from the extrusion orifices where ambient
air is employed as the drying media. A preferred practice is to
locate the take-up drum 2 to 4 meters below the extrusion orifice.
The temperature of the drying media can exert some in-
fluence on the fiber forming and attenuation characteristics.
For instance, a heated gas especially if forced movement thereof
is employed can exert considerably increased drying rates over
ambient air. Forced movement of the drying media is always
1038124
preferred to provide a controlled drying atmosphere and collected
to remove evaporated volatile substances. The use of the heated
drying media while accelerating drying can retard attenuation by
excessively drying the fibers leaving them less stretchable. On
the other hand the use of a relatively cool ambient, for instance
10C air, may permit more fiber attenuation than room temperature,
27C, where which effect has been observed in making alumina
fibers. Employing the trichloroethylene solvent drying at 10C
proceeds at approximately one-half the rate as at 27C but at the
lower temperature the take-up speed on the drum has to be
increased to keep the system in ~ance and as a result of the
attendant attenuation the fiber reeled onto the drum is signifi-
cantly finer in diameter, by about 25%. The solvent content in
the vapor phase can also be used to control drying rate.
The fibers are removed from the take-up drum by cutting
through the fibers on the drum parallel to the axis of the drum
which produces a multitude of fibers having length corresponding
to the circumference of the drum, or at least the circumference
described by the fiber upon the drum. This fiber length which
can be 6 or 36 inches, or any length desired by selecting the drum
diameter appropriately, is quite useful and convenient in the
firing operation. The fibers are then heated to sintering tem-
perature which is typically from 1400 to 1600C, the higher tem-
peratures tending to produce better bonding in the fibers which
in turn, however, tends to reduce flexibility and tensile
strength of the fibers.
In accordance with the invention fibers of rather
substantial green length are provided at relatively high rates
of speed. For instance, a take-up speed of from 500 to 1,000
feet per minute can be employed for runs of several minutes and
green fiber lengths in excess of 200,000 feet have been produced.
Quite obviously sintering a fiber 200,000 feet in length and
- 13 -
~0381Z4
using such a fiber could be quite troublesome and this is why the
fibers are advantageously severed prior to removal from the drum
to provide convenient predetermined desired lengths. Upon firing,
slight bonding between fibers occurs which necessitates a mechan-
ical separation after firing. Due to this, the final product con-
sists of fiber which can be several centimeters in length but
more typically 1 cm or less. Typical fiber diameter can range
from a few microns up to 50 or 60 or even more if such would be
desired although the thicker fibers are, for the most part, less
desirable than the thinner fibers since the thinnest fibers pro-
vide the best thermal insulation. A very desirable fiber diameter
can be produced with a good degree of consistency the diameter
ranging from 20 to 25 microns which fiber offers great utility in
the thermal insulation field not to mention other fields of
potential application such as composite strengthening.
Various modifications may be made in the invention
without departing from the spirit thereof, or the scope of the
claims, and, therefore, the specific treatment described is to be
taken as illustrative only and not in a limiting sense, and it is
desired that only such limitations shall be placed thereon as are
imposed by the prior art, or are specifically set forth in the
appended claims.
