Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVED CELLULOSE SEMIPERMEABLE HOLLOW FIBERS ~`
AND METHOD FOR MAKING SAME
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BACKGROUND OF THE INVENTION
This invention relates to cellulose semipermeable
S hollow fibers of the type useful in dialysis, osmosis or
ultrafiltration type separatory cells, and more particularly
cells useful in the detoxification of blood by hemodialysis j~
or hemofiltration. The invention also relates to an r
improved method for making the new fibers.
In the past the major quantity of cellulose hollow
fibers used in artificial kidneys in hemodialysis was made
by melt extrusion of a cellulose ester, such as cellulose
triacetate, in a continuous process such as the process of
U.S. Patent 3,546,209. Another portion of the cellulose
hollow fibers were made by the cuprammonium regenerated
cellulose process of the type disclosed in an improved
form in U.S. Patent 3,888,771. Whereas these basically
dissimiliar processes produce cellulose fibers possessing
commercially acceptable water permeability (ultrafiltration) ¦
and urea permeability (clearance) characteristics for use
in artificial kidneys, the fibers nevertherless fail to
possess optimum combination permeabilities. For example,
cellulose fibers made by the process of U.S. Patent 3,546,209
have lower water permeability than is desirable in fibers
that have acceptable clearance characteristics for urea,
creatinine, vitamin B12 and other low molecular weight r
blood impurities.
Moreover, continous manufacture of cellulose fibers
from melt extruded cellulose acétate fibers involves chemical
conversion from the thermoplastic polymer cellulose acetate L
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to the non-thermoplastic polymer cellulose by hydrolysis
or saponification in an aqueous alkali bath. During this
hydrolysis the thin wall, small size fibers are especially
sensitive to contact and fragile. Successful manufacture
at minimum efficiency to be commercial depends upon main-
taining sufficient tensile strength in the fibers throughout
the wet processing steps to avoid breakage or damage. Thus,
it would be highly desirable to improve the tensile strength
characteristics of the fiber, particularly its wet strength
during hydrolysis, or conversion from cellulose ester to
cellulose, prior to drying and storage and assembly into
hemodialyzers or hemofilters.
The concept of preparing semipermeable hollow fibers
by melt spinning a placticized polymer composition was
developed in the early 1960's and was first described in
U.S. Patent 3,423,491; various types of polymers are there
described, including cellulose esters, and suitable plasti-
cizers are discussed for use in forming the melt spin
compositions with different types of thermoplastic polymers.
The cellulose ester class of polymer developed into the
favorite commercial polymer, particularly cellulose acetates,
and tetramethylene sulfone, commonly called sulfolane, was
normally employed as the plasticizer to make the melt spin
composition for use in melt spinning cellulose acetate
fibers. U.S. Patents 3,494,780 and 3,532,527 disclosed
improvements in the sulfolane - cellulose acetate melt
spinning process of extruding cellulose acetate fibers
involving either an after-spin immersing of the spun fiber
in a bath containing a mixture of sulfolane and a polyol
having a molecular weight below 4,000, or prior to spinning
the fiber modifying the sulfolane plasticizer to include
a minor amount of a polyol having a molecular weight
below a~out 20,000. These patents also disclosed that
polyols were considered to be unsatisfactory for use
alone as the plasticizer to form melt spin compositions
with cellulose esters, particularly the cellulose acetates.
This invention is based on the unexpected discovery
that cellulose ester melt spin compositions which are
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sulfolane free and which include only certain low molecu-
lar ~eight polyols, or mixtures thereof can be melt spun ~,
into fibers that can be hydrolyzed into cellulose fibers _
which unexpectedly possess greatly improved wet strengths
du~ g the conversion from cellulose ester to cellulose.
Omitting the sulfolane, previously considered to be neces-
sary, is the key change which enables production of the
greatly improved fibers of this invention. The resulting
cellulose ester fibers of this invention possess satisfactorily
high intrinsic tensile strengths in their as spun form and
moreover the spun fibers retain, and in certain instances
increase, their intrinsic tensile strengths during the
polyol leaching and hydrolyzing, or deacetylation, steps
which convert the cellulose ester fiber into a cellulose
fiber. r
SUMMARY DESCRIPTION OF THE INV~NTION
This invention provides an improved cellulose semi-
permeable hollow fiber which is melt extruded from a
cellulose ester melt spin composition which is sulfolane
free and contains only low molecular weight polyols and
the melt spun fibers possess substantially improved
intrinsic tensile strength during wet processing steps
which remove the polyol and hydrolyze the fibers to
cellulose; the resulting cellulose fibers are charac-
terized by substantially increased water permeability
and improved clearance capabilities for separating low
molecular weight impurities from blood such as urea,
creatinine and the like. The improved cellulose hollow
fiber of this invention is characterized by possessing
an intrinsic wet tensile strength of about 2 X 104 to
about 11 X 104 grams intrinsic fiber tensile strength per
gram of cellulose polymer, a water permeability or ultra-
filtration coefficient KUFR in the range of about 2 to ~ F
about 200 millimeters per hour per square meter per milli-
35 meter of mercury pressure across the semipermeable wall
of the fiber, and a urea clearance coefficient KUREA in L
the range of about 15 X 10 3 to about 45 X 10 3 centi-
meters per minute at 37 C. These functional characteristics
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qualify such fibers for use in blood detoxification processes
including hemodialysis and hemofiltration. This invention
also provides an improved method for making the new fibers of
this invention, which comprises: -
(a) melt spinning a hollow cellulose ester fiber
through a nozzle from a melt spin composition
consisting essentially of about 35 to about
80 weight percent cellulose ester and the
balance at least one polyol having an average
molecular weight between about 106 and about
900;
(b) hydrolyzing said cellulose ester fiber sub-
stantially into a cellulose hollow fiber;
(c) replasticizing said cellulose fiber while
said fiber is still wet with a water soluble,.
essentially non-volatile plasticizer; and
(d) drying said plasticized fiber.
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DETAILED DESCRIPTION OF T~E_INVENTION
The improved cellulose semipermeable hollow fibers
of this invention are made by melt spinning a cellulose
ester melt spin composition consisting of about 35~ to
about 80%, by weight, of a selected cellulose ester, or
mixtures thereof, and a polyol, or mixtures thereof,
having an average molecular weight in the range of about
106 to about 900.
The cellulose esters that are suitable include the
cellulose mono-, di- and triacetates and mixtures thereof,
cellulose acetate propionate, cellulose acetate butyrate,
cellulose propionate and cellulose butyrate and mixtures
of any two or more thereof~ The acetates are preferred,
particularly cellulose diacetate and mixtures advantageously
include at least a minor proportion of one or more of
the other cellulose acetates.
Polyols are not suitable as a class for mixture
with the selected cellulose ester to make an acceptable
melt spin composition, but rather the polyols that are
satisfactory are limited to those low molecular weight
polyols having an average molecular weight in the range
of about 106 to about 900. Attempts to use a single polyol
having a molecular weight above about ~00 with cellulose
acetate have failed because the composition could not be
melt spun. However, satisfactory melt spin compositions
have been prepared using mixtures of polyols in which one
of the polyols in the mixture has an average molecular
weight substantially higher than 900, for example, a
molecular weight of 1450; a mixture of two polyethylene
30 glycols one of molecular weight 200 and the other 1450 to
produce an average molecular weight of 902 was successfully
used to make a cellulose acetate melt spin composition that
was spun into hollow fibers which exhibited the improved
te.nsile stengths that characterize the fibers of this
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invention. Mixtures of polyethylene glycols with polyols _
for example a mixture of polypropylene glycol having an
average molecular weight of 400 a`nd glycerine produce a
spinnable composition whereas polypropylene glycol, alone ,
having an average molecular weight of 400 could not be
successfully spun. Mixtures of polyethylene glycols
and ethylene glycol are satisfactory and mixtures of two _
or more low molecular weight polyethylene glycols may
be used with or without glycerine.
It has also been found that pure polyethylene glycols, ~F
or pure polypropylene glycols which have too high a molecular
weight to form a spinnable cellulose ester composition
can be modified by adding glycerine, a recognized non- _
solvent for cellulose esters, to thereby form a composition ~;,
that can be successfully spun into the improved fibers of
this invention. The proportion of glycerine needed for
this purpose varies with the molecular weight of the pure
polyethylene or propylene glycol selected and also with
the cellulose ester or mixture thereof that is present.
In general, the quantity of glycerine that is needed in-
creases as the average molecular weight of the pure glycol
increases above about 600; glycerine concentration should r
also increase when the proportion of cellulose ester that
is mixed with cellulose diacetate increases or for melt
25 spin compositions made from cellulose propionate or cellulose
butyrate or mixtures thereof. As a general guide~, amounts
of glycerine in the range of about 5% to about 35%, by
weight, of the cellulose ester melt spin composition may
be satisfactorily employed. Polyols having at least two
30 hydroxyl groups in the molecule that are satisfactory for
use include diethylene glycol, triethylene glycol, tetra-
ethylene glycol, the mono-, di- and tri-propylene glycols
and mixtures of one or more of the propylene and ethylene
glycols or glycols having ethylene-propylene chains in the r
35 glycol molecule, and mixtures of any one or more polyethylene,
or polypropylene glycol with glycerine in an amount less
than 50~, by volume, of the glycol-glycerine mixture.
The process of this invention comprises the steps
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of mixing the selected cellulose ester and selected polyol h
to form a melt spin composition, melt spinning hollow ~;
fibers and cooling same to a gelled self-supporting state, _
hydrolyzing, or deacetylating, the cellulose ester hollow
fiber to substantially a cellulose fiber, that is, hydro-
lyzing a substantial portion of the ester groups back to r
the cellulose hydroxyl group. Complete hydrolysis is
usually not obtained and is not necessary but it is de-
sirable for best overall permeability characteristics and
the maintenance thereof during storage and shipment, to
bring about or achieve substantially complete hydrolysis,
for example, above about 90%. The polyol which is present
in the as-spun cellulose ester fiber is normally leached _
from the fiber during hydrolysis or deacetylation; alterna-
tively the polyol may be removed in a separate step pre-
ceding the hydrolysis.
Cellulose semipermeable hollow fibers made from a
sulfolane-acetate melt spin composition by the process of
U.S. Patent 3,546,209 on a commercial basis and as used in -
artificial kidneys extensively after about 1972 have
typically possessed a water permeability ceofficient
KUFR of about 1.0 - 1.2 milliliters per hour per square r
meter of fiber surface area per millimeter of mercury
pressure across the fiber semipermeable wall at 37C, a
urea coefficient KUREA of about 28 to about 30 X 10 3
centimeters per minute and a wet cellulose fiber strength
after deacetylation of about 1.4 to about 1.8 grams
intrinsic fiber tensile strength per gram of cellulose
polymer. The improved cellulose fibers of this invention
are substantially improved in each of these three important
functional characteristics; the most unexpected and
significantly improved property is the increased intrinsic wet E
tensile strength. As above indicated the wet tensile F
strength of the fibers is critically important to successful
continuous production in a production train or line.
Typically such a line employs a tow of 16 to 30 fibers, or
a plurality of such tows, which passes from the spinnerettes _
through air to gel to a self-supporting fiber and thence
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~hrough a series of liquids in treatment tanks that sequen-
tially leach the polyol from the cellulose ester fiber,
hydrolyze the ester to cellulose, rinse the hydrolysis
products and excess hydrolyzing agent from the cellulose
S fiber and replasticize the cellulose fiber. The replasti- -
cized product fibers are then taken up on receiving rolls.
During the wet treatmenY steps the fibers are continually
subjected to a pulling, or longitudinally applied, force
while undergoing drastic internal molecular rearrangements,
particularly those resulting from polyol removal and
chemical changes during deacetylation from a cellulose ester
to substantially a cellulose fiber. Fiber breakage, or
damage, due to the inability of the polymer skeleton in
each fiber to withstand tensional or frictional forces
during such continous processing is disruptive to continu-
ous operation and is highly undesirable.
It has been observed that cellulose acetate fibers
made from a melt spin composition of this invention, which
is free of sulfolane, tend to retain a higher proportion
of their as-spun tensile strength during travel through
the successive wet processing steps than those made from
the sulfolane-containing melt spin compositions that have
long been in commercial use. Moreover, certain of the
improved fibers experience an unexpected increased in
tensile strength from their as-spun state and this increase
occurs during the hydrolyzing steps of the process. The
cellulose fibers of this invention exhibit average intrinsic
tensile strengths, in the wet state after hydrolysis, that
are at least twice and up to about seven times higher than
those of cellulose hollow fibers made by the process of
U.S. Patent 3,546,209. As used in this specification, and
in the claims, the expression "intrinsic fiber tensile
strength" as applied to wet cellulose fiber tensile strength
measurements refers to the ultimate, or breaking, strength
in grams per gram of polymer in a two inch length of a
single wet fiber required to break that fiber when it is
vertically suspended between gripping jaws in an Instron*
* Trademark.
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machine. In the tests made to obtain the wet intrinsic _
tensile strengths referred to in Table I, the grams of
polymer în a selected two inch length of fiber represent r~
an average weight in grams which was determined for each _
5 particular two-inch sample of cellulose fiber by drying the :
adjacent one thousand inches of fiber from the same strand
to a constant weight and then weighing the 1000 inch long .
strand and dividing the total weight by 500 to thereby
obtain an average weight which is then used for the parti-
cular 2" portion subjected to the breaking strength test. ¢~
This procedure effectively increases the accuracy of the
thus determined tensile strength by substantially elimin-
ating the potential error due to fiber wall thickness
variation along the continuous fiber. Additionally, each
tensile strength value represents the average of six de- ~
terminations on separate two-inch long specimens. The F
thus determined increases in intrinsic tensile strength
that are achieved are commercially significant in~that
they greatly increase overall manufacturing efficiency
in the continuous manufacture of the fibers of this
invention. ~;
Whereas the above described effects of increased wet r
tensile strength during fiber manufacture has high commer
cial value, the increase in water permeability capability
attained in certain of the fibers made by the process of
this invention is also highly-important; certain fibers
have reached up to 80 times increase in water permeability
relative to heretofore available cellulose fibers made
from cellulose acetate melt spin compositions. Such in-
creases in water permeability means that the fiber capa-
bility of separating water from a water containing fluid, !
such as blood, is drastically increased and the practical
advantage which results is significant in that it enables
substantial reductions in the required time per hemodialysis F
treatment, as is well understood by those skilled in this
art.
Formulating the melt spin composition may be accom-
plished in any convenient manner with conventional mixing t
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1~5.;i~71
equipment, the important feature being to insure sufficient
mixing to obtain an intimate uniform mixture. For example,
dry cellulose acetate powder is blended with a weighed
amount of selected polyol in a shearing Hobart*mi~er; the
mixed material is further homogenized and blended by feeding
the same into a heated counter-rotating twin screw extruder
and the molten extrudate then forced through a spinnerette,
for example, a 16-30 hole spinnerette of the type including
conventional gas supply means for injecting gas into the
core of the extrudate. A preferred gas for this purpose is
nitrogen but other gases may be satisfactorily employed,
including carbon dioxide, air, or other innocuous gases.
If desired, a spinnerette equipped with means for injecting
a liquid into the extrudate core which is a non-solvent
for the cellulose ester and the polyol may be used, for
example by using a spinnerette ~f the type disclosed in
U.S. Patent 3,888,771. The extrudate exiting from the
spinnerette is subjected to cooling, such as forced air
cooling of varying force and/or temperature, to cause
gellation and solidification of the extrudate into solid,
self-supporting fibers.
The cellulose ester fiber may be hydrolyzed satis-
factorily by any of the now well known deacetylation tech-
~iques. The preferred procedure is to use an aqueous
sodium hydroxide bath. Suitable techniques are described
in a variety of books and technical papers including, for
example, Laidler, Chemical Kinetics, McGraw Hill Book Co.,
New York ~1950), pp. 282-290; Howlett, et al., Technical
Inst. J. 38, 212 (1947); Hiller, Jour. Polymer Science 10,
385 (1953) etc. After rinsing to remove the products of
hydrolysis and excess hydrolyzing agent, or neutralizing
same, the fibers while still wet are replasticized with a
water soluble, essentially non-volatile plasticizer in
accordance with the teachings of Lipps U.S. Patent 3,546,209.
3S By the expression "essentially non-volatile" as used in this
specification and in the claims is meant that the plasticizer
is essentially retained by the cellulose fiber during the
subsequent drying step and ambient temperature storage.
*Trademark.
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Suitable plasticizers for the deacetylated cellulose
fibers include those which are capable of swelling the
fiber presumably by such interactions as hydrogen bonding
or dipole-dipole reactions. Preferably the water soluble,
essentially non-volatile plasticizer is a hydroxyl contain-
ing compound and more preferably a polyol such as polyalky-
lene oxides; glycols such as ethylene glycol, diethylene
glycol, dipropylene glycol, tripropylene glycol and the like:
glycerine and the like.l Glycerine is a plasticizer of
choice.
Following plasticization the fiber is then dried by
any convenient method such as vacuum drying, forced air
drying at ambient or elevated temperatures, microwave
drying and the like. Elevated temperatures may be used
so long as the temperature does not cause any substantial
loss of plasticizer. Consequently, an elevated drying
temperature is dependent on the particular plasticizer used
and a suitable temperature is readily determined.
The dried final product cellulose fibers of this inven-
tion are of capillary size and are in the range of about 200
to about 400 microns outside diameter and have a wall thick-
ness in the range of about 10 to about 80 microns. For
separatory cells used in hemodialysis the fibers preferably
have a wall thickness in the range of about 10 to 50 microns
and a maximum outside diameter of about 230-320 microns.
The following examples illustrate the new process and
the improved cellulose hollow fibers of this invention and
contain the best mode contemplated to be employed. As used
in this specification and claims, unless otherwise speci-
fically indicated, all compositional percentages are by
weight. Each of the specifically formulated melt spin
compositions was made by using the above identified mixing
equipment and steps described and the fibers were spun from
a 16 opening spinnerette using nitrogen injected into the
fiber core. After gelling in air and samples were removed
for determination of the as-spun grams intrinsic fiber
tensile strength described procedurel the fibers were de-
acetylated in a caustic solution, typically aqueous sodium
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hydroxide in the range of 0.2-1.2~, by weight, at a temper- _
ature in the range of 20C - 60C.
As spun intrinsic tensile strengths were established
by weighing 600 inches of the adjacent fiber to establish
the average weight for the selected 2" portion of each
single fiber subjected to the breaking test.
The resulting cellulose fibers were thereafter washed
thoroughly in water and samples taken for determination of
wet tensile fiber strength by the test proceaure above des-
cribed. The product fibers were also tested for water perm-
eability KUFR, and urea clearance, or transport, KUREA in a
laboratory test apparatus. The test apparatus consisted of
a fluid reservoir equipped with a magnetic stirrer, and a
dialyzer test beaker fitted with a magnetic stirrer, a top
closure plate having pressure fittings and connectors for
receiving the ends of the potting sleeves attached to each
end of a bundle of fibers containing between 128 and 178
fibers per bundle. The fiber bundle was bent into a U-shape
and inserted into the beaker and connected to the closure
plate; one sleeve was connected by a fluid line to a pump t~
connected with a line to the reservoir and the other sleeve
was connected by a return line to the reservoir to thereby r
enable fluid from the reservoir to be pumped under controllable
pressure through the lumens of the fibers located in the
2~ dialysis beaker. The beaker was also provided with dialysate
inlet and outlet connections and during testing the fibers
were immersed in a surrounding stirred pool of water for
e UFR
The water transport coefficient, KUFR, was determined
by pumping water under pressure through the fibers and
measuring the increase in water volume external to the
fibers in the dialyzer beaker, the tests being run at
37C. KUFR was then calculated for each test in milli-
liters per square meter per hour per millimeter of mercury F
pressure differential as shown in Table I.
The urea coefficient, KUrea~ was determined by pro- _
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viding a water-urea solution in the supply reservior and
pumping same through the fiber lumens, the pool surrounding
the fibers in the dialysis beaker being initially pure
water. Measurements were made to determine the urea con-
centration in the dialysate fluid at time intervals.
The tests were conducted at 37C and there was no
pressure differential across the fiber wall surface during
the tests.
The urea coefficient, KUrea, was calculated by taking
into account the difference in the concentrations of urea
in the supply reservoir and in the dialysis beaker on the
outside of the fibers as a function of time and the fiber
area in accordance with the equation:
N = KUREA A (Cl - C2) wherein N represents the flux
across the membrane in moles per minute, Cl is the initial
urea concentration, C2 is the final, or measured, concen-
tration and A is the area of the fiber wall or membrane
between the two solutions.
In a two-chamber system without a pressure differential
or resultant ultrafiltration the transfer of urea across
the membrane wall may be integrated over a time interval,
t, to yield the further equation:
Cl C2) 1 ~Vl + 2 A 1 KUREA t
l(Cl _ C2)t ¦ l Vl V2
wherein Vl is the volume of supply reservoir solution, and
V2 is the volume of the solution in the dialysis beaker.
In the tests, the volumes, Vl and V2 and the area
A are measured separately so that a plot of the values on
each side of the inte~grated equation produced a straight
line, the slope of which allows KUrea in units of centimeter
per minute to be calculated.
EXAMPL~ 1
An intimate blend of cellulose diacetate polymer and
plasticizer was prepared as previously described. The blend
consisted of a mixture of 80~ cellulose diacetate and a 20%
of a mixture of polyethylene glycols of molecular weights
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200 and 1450 Daltons to produce an average molecular weightof the mixed polyethylene glycol plasticizer of 902 Daltons.
The cellulose diacetate hollow fibers were subsequently .
deacetylated to cellulose hollow fiber membranes in a 0.8%
sodium hydroxide aqueous solution at 50C. The hollow fiber
membrane intrinsic tensile strengths, water permeabilities
(~UFR)' and urea transport rates (KUr~a) are summarized
in Table I, Column B.
As may be seen from Table I, the wet cellulose fiber ~~-
of this example has over 2.5 times the wet intrinsic tensile
strength of the reference fiber. The product cellulose fiber
produced from this relatively high molecular weight polyol
melt spin composition also exhibits superior performance
characteristics for blood purificiation and higher water
permeability. The water permeability (KUFR) is 1.5 times
that of the reference fiber. The rate of urea transfer is
also higher: KUrea = 38 X 10 min as compared to 30 X 10 3
cm of the reference cellulose fiber membrane.
mln ,
A similar blend was made by mixing cellulose diacetate
and the same mixture of polyethylene glycols having an
average molecular weight of 902 except 43% of cellulose r
diacetate was used instead of 80%. After melt spinning,
the cellulose acetate hollow fibers were hydrolyzed, or
saponified, in a 0.4% NaOH aqueous solution at 50 C.
After testing, as above, the intrinsic wet tensile strength
was 1.3 times the reference fiber. KUrea was 26 as com-
pared to 30 for the reference fiber but KUFR was increased
drastically to 86 times the KUF~ of the reference fiber,
or a value of 104 millimeters per hour per square meter
per millimeter of mercury. From a comparison of these two
melt spin compositions it will be seen that for a given
polyol plasticizer the decrease in cellulose ester concen- !
tration causes a substantial increase in water permeability F
at acceptable urea clearance values. It should also be
noted that a weaker hydrolyzin~ solution was employed and
that stronger saponifying solutions usually increase water
permeability. Thus, the desired combination of intrinsic
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wet strength, KUFR and KUrea properties can be modified _
to those specifically desired by similar melt spin com-
position changes, ~or by modifying~the average molecular
weight of the polyol as may be seen from comparisons that
may be made with the examples which follow.
EXAMPLE 2
An intimate blend of cellulose diacetate polymer and _
plasticizer was prepared using the above described proce- ~
dures. The blend consisted of 43% of a mixture of cellulose ~:
diacetate and 57% of polyethylene glycol having an average
molecular weight of 400 Daltons. The cellulose diacetate
hollow fibers after being deacetylated to cellulose hollow
fiber membranes in an aqueous 0.4~ NaOH solution at 50 C
were tested for intrinsic tensile strengths, water perm-
eabilitY (K FR) ~ and urea transport (Kurea) and the results
are summarized in Table I, Column C.
The wet intrinsic tensile strength of the product fiber
is 2.4 times the intrinsic strength of the reference fiber,
KUFR is 2.1 times higher and KUrea is 32 X 10 relative to
30 X 10 for the reference fiber. It should be noted that
during saponification from cellulose acetate to cellulose
the wet intrinsic tensile strength increased to a value r
nearly double that of its as spun intrinsic tensile
strength.
Another blend having the same proportions of cellulose
diacetate and polyethylene glycol was prepared except the
glycol average molecular weight of 400 Daltons was attained
by mixing polyethylene glycol of average molecular weight
of 600 with glycerine having an average molecular weight
of 92 Daltons. This change caused an improvement in all
of the fiber properties to a wet cellulose fiber intrinsic
tensile strength of 3.7 times the reference, a KUFR of
3.4 times the reference and a KUrea of 33 X 10 compared
to 30 X 10 3 min for the reference.
EXAMPLE 3
An intimate mixture of cellulose diacetate polymer l l
and plasticizer was prepared as previously described. The L 1`
blend consisted of a 43% mixture of cellulose diacetate
and 57~ of polyethylene glycol of molecular weight 108
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Daltons. The cellulose diacetate hollow fibers were sub-
sequently deacetylated to cellulose hollow fiber membranes
in an 0.8% NaOH solution at 50C.` The hollow fiber membrane
intrinsic tensile strengths, water permeabilities and urea _
transport rates are summarized in Table I, Column D.
As can be seen from Table I, the wet cellulose fiber
of this example has 1.9 times the intrinsic tensile _
strength of the reference fiber. The water permeability ~-
KUFR is 1.8 times that of the reference fiber and KUrea
is 32 X 10 3 cmn as compared to 30 X 10 3 min for the
reference cellulose membrane.
A similar blend was made in the same manner except
that the polyethylene glycol had a molecular weight of 150.
Saponification of the cellulose acetate to cellulose was
done in a 0.4% NaOH aqueous solution at 50C. The same
fiber properties were measured, as above described, and r
the wet intrinsic tensile st ength was 6.6 times the
ference fiber while KUFR and KUrea were substantially
the same as the reference fiber. By comparing the wet L
intrinsic tensile strengths of the cellulose fibers of
Example 2 with those of Example 3 it will be seen that
best wet intrinsic tensile strength, for a given cellulose r
ester concentration, 43% cellulose diacetate, is obtained
' at a polyglycol average molecular weight between 106
and 400 and appears to peak at about 150.
EXAMPI.E 4
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An intimate mixture of cellulose diacetate polymer
and plasticizer was prepared as previously described. The
blend consisted of 43% (by weight) of a mixture of
cellulose diacetate and 57% (by weight) of a mixture of
polyethylene gycols of molecular weights 400 and 1450
Daltons. The average molecular weight of the polyethylene
glycol plasticizer was 713 Daltons. The cellulose di- F
acetate hollow fibers were subsequently deacetylated to
cellulose hollow fiber membranes by the procedure previously
described. The fiber properties are summarized in Table I, L
Column E. I
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~ From Table I it will be seen that wet intrinsic tensile !.
strength increased to a value more than twice that of the as
spun intrinsic tensile strength and to a wet cellulose intrinsic _
tensile strength 3.2 times that of the reference fiber, with
a KUrea of 34 X 10 versus 30 X 10 3 mln for the reference
fiber, and a KUFR of 2.4 times that of the reference fiber.
EXAMPLE 5
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An intimate mixture of cellulose diacetate polymer ,
and plasticizer was prepared as previously described.
The blend consisted of 43% of a mixture of cellulose di-
acetate and 57% of a mixture of polyethylene glycol of
molecular weight 400 Daltons and glycerine of molecular
weight 92 Daltons to give a polyol blend having an average
molecular weight of 362 Daltons. The cellulose diacetate k~
15 hollow fibers were subsequently deacetylated to cellulose F
hollow fiber membranes in a 0.4~ NaOH aqueous solution at
50C. The cellulose hollow fiber membrane intrinsic tensile
' UFR and KUrea~ are su~marized in Table I
Column F.
From Table I it will be seen that wet intrinsic r~
tensile strength increased 242% during saponification r
to give a cellulose fiber wet intrinsic tensile strength !
5.8 times that of the reference fiber. The urea clearance
rate was substantially improved to 42 X 10 3 cmn or 1.3
25 times that of the reference fiber while KUFR was 2.1 times
as high as that of the reference fiber.
Another melt spin composition was made identical to
the above described polyethylene glycol-glycerine blend
and cellulose diacetate except that the glycerine was
30 replaced with a like quantity, 7% by volume, of the ethylene ,
glycol to thereby form a polyol blend having an average
molecular weight of 358. Fibers were spun and deacetylated
under the same conditions; comparable properties of the F
resultant cellulose fiber were determined in the same manner;
35 the cellulose fibers had a wet intrinsic tensile strength
that was 5.8 times that of the reference fiber, a K L
urea
_
:
::
7~
of 21 X 10 3 mmn and a KUFR that was 1.7 times that of the :
reference fiber.
EXAMPLE 6
A melt spin composition was formed by uniformly
blending 36% cellulose diacetate and 64~ of a polyol blend
consisting of a mixture of polyethylene glycol having an _
average molecular weight of 600 Daltons and glycerine in r
amounts to produce an average molecular weight of 421 for F
the blend. Fibers were melt spun and deacetylated in a F
0.4% NaOH aqueous solution at 50 C and the properties were
determined by the above described procedures and they appear
in Table I, Column 9. These cellulose fibers have a com-
bination of high wet tensile strength, high KUFR and
high KUrea and represent a preferred form of the invention
in that such fibers are satisfactory for use in hemodialysis n~
or hemofilters and are particularly desireable for hemofilter
use. The wet intrinsic tensile strength is 3.4 times that
of the reference fiber, the KUFR is 32 times higher than
the reference fiber and KUrea is 49 X 10 mln
EXAMPLE 7
r~ .
An intimate mixture (blend) of cellulose di-ester
(propionate/acetate) and plasticizer was prepared as pre-
viously described. The cellulose ester of this example
may be generally considered to be cellulose propionate,
as 96% of the ester groups are propionate, and only 4
are acetate.
The blend consisted of 43% of a mixture of cellulose
propionate and 57% of a mixture Qf polyethylene glycol of
average molecular weight 400 Dalt~ns and glycerine of
molecular weight 92 Daltons for form a polyol blend having
an average molecular weight of 362 Daltons. The cellulose
propionate hollow fibers were subsequently deacetylated
to cellulose hollow fiber membranes in a 0.4~ NaOH aqueous F
solution at 50 C. The hollow fiber membrane intrinsic
tensile strengths, water permeabilities and urea transport
rates are summarized in Table I, Column H.
As can be seen from Table I, the wet cellulose fiber
of this example has 3.5 times the intrinsic tensile r
I I
. _ .. .. _..... . _ . I
:
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-18-
strength of the reference fiber. The water permeability
KUFR is 2.4 times that of reference fiber, and the rate
of urea transfer, KUrea is 33 X mln.
S EXAMPLE 8
A melt spin composition was formed by intimately
blending 43% cellulose diacetate and 57% of a polyol blend
consisting of a mixture of polypropylene glycol having an
average molecular weight of 400 Daltons with glycerine
to form an average molecular weight for the polypropylene
glycol/glycerine blend of 297. Cellulose fibers were made
and tested by the procedures described above with deacety-
lation being effected in a 0.4% NaOH solution at 50C.
The properties appear in Table I, Column I.
EXAMPLE 9
Artificial kidneys of the type commercially available from
Cordis Dow Corp. under the trademark C-D~K artificial kidneys
were manufactured using cellulose fibers made in commercial
quantities by the process described in Example 5, above,
using the melt spin composition consisting of 43% cellulose
diacetate and 57% of a mixture of polyethylene glycol
having a molecular weight of 400 Daltons and glycerine to
give a polyol blend having an average molecular weight of
362 Daltons. The wet intrinsic tensile strength was
determined to be 11 X 104 grams per gram cellulose fiber.
After the cellulose fibers were water rinsed and re-
plasticized in an approximately 10~ glycerine-water solution
and then dried the fibers contained approximately 20%
glycerine. The intrinsic tensile strength of these fibers
was determined by the single fiber breaking test on the
Instron*machine described above in connection with the tests
on dry cellulose acetate fibers with the following variations.
A tow consisting of 360 fibers one meter long was weighed,
the glycerine was extracted and the pure cellulose fiber
was weighed. The average weight of a two-inch long section
was then calculated on a 100% polymer fiber basis. The
tensile test results represent the average of six separate
*Trademark.
.
. ~ :
7i
tests on six two-inch long fibers from the glycerine-free
tow. The dry intrinsic tensile weight was 20 X 104 grams
per gram of cellulose fiber.
One of the artificial kidneys containing 1.5 square
meters of cellulose fiber area was sterilized, dry, by the
application of 2.5 MRAD of gamma rays. After sterilization
the kidney was opened and fiber samples removed and subjected
to the wet intrinsic tensile test, and the fibers were
found to have an average wet intrinsic tensile strength of
8.6 X 104 grams per gram of cellulose fiber. Another 1.5
square meter kidney manufactured to contain the cellulose
fibers of this example was sterilized while the kidney was
filled with physiological saline solution by the application
of 2.5 MRAD of gamma rays. Fibers from this wet gamma
ray sterilized kidney had a wet intrinsic tensile strength
of 5.2 X 104 grams per gram of cellulose fiber.
In comparison artificial kidneys from commercial
production at Cordis Dow Corp. containing 1.5 square meters
of cellulose fibers made by the process of ~.S. Patent
3,546,209 were tested in the same manner as were the above
described fibers and were found to have the following
properties:
Dry intrinsic cellulose fiber tensile strength - 7.5
X 10 grams/gram
Wet intrinsic cellulose fiber tensile strength - 1.6
104 grams/gram
Wet cellulose fiber after dry gamma ray sterilization -
intrinsic tensile strength 1.53 X 104 grams/gram
Wet cellulose fiber after wet (physiological saline~
gamma ray sterilization - intrinsic tensile strength -
1.28 X 10 grams/gram
Clinical evaluations of two 1.5 square meter artificial
kidneys containing the improved cellulose fibers of this
example were run on two intermittent hemodialysis patients
at an average blood flow rate of 200 ml/min and a dialysate
flow rate of 500 ml/min for times of 3.5 hours and 4.1 hours.
UFR was 2.1 h 2 at 37 C. The Xurea was 31.6
i~
. .
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1~b~ 71 ~_
-20-
mmln at 37C. _
For comparison, commercial Cordis Dow artificial
kidneys containing 1.5 square meters of cellulose fibers _
made by the process of U.S. Patent 3,546,209 used on
three intermittent hemodialysis patients gave average
values for K of 0.89 ml at 37C and a K
UFR hr-M2-mmHg urea
of 29.6 mmn at 37C. ~;
The artificial kidneys used in this example and
above referred to as the type commercially available
from Cordis Dow Corp. are devices having a pair of blood
chambers spaced apart by an intervening dialysate chamber
that is integrally connected to the blood chambers. A 2
bundle of hollow fibers consisting normally of multi- r-
thousands of individual fibers, for example six thousand
to fifteen thousand fibers, terminate at their opposite .
ends in a plastic tubesheet, typically a polyurethane.
The tubesheet binds the fibers to each other and also -
provides an annular portion lying outside the periphery
of the fibers in the bundle which serves to join the
tubesheets to the end portions of the dialysate chamber r
and to the blood chambers to thereby seal the blood
chambers and the dialysate chambers into a unit, with
the fibers located within the dialysate chambers, such ~
that the chambers are isolated from each other in fluid-
tight sealing relationship. The open ends of the hollow
fibers terminate in the plane of the outer end of each
tubesheet and the passageways in the hollow fibers provide
communications between the interiors of the spaced apart-
blood cham~ers.
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