Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1~8735~3 ~
The present invention resides in normally
hydrophobic, microporous hollow fibers which have high
gas permeabilities and which can be prepared from a
strong, inert, inexpensive and readily processed polymer,
i.e., from polyethylene. The present invention also
resides in a flexible, practical process for making
~ such fibers. The hollow fibers have particular utility
; as separatory membranes in so-called artificial lungs
or oxygenators.
The advantages of permeable hollow fibers as
separatory membranes are now well known. For èxample,
~`' the large membrane areas per unit volume of device
attained with this membrane configuration are of parti-
cular importance in minimizing priming blood requirements
$ 15 for devices such as so-called artificial kidney or arti- -~
ficial lung units. Additionally, a simplification over ~-
~d flat membrane devices is realized in that hollow fibers
by reason of their cross-sectional shape, are essentially ;
,. ~ ..
self-supporting and do not collapse under the trans-
membrane pressures required for processes such as reverse
o~mosis of sea water.
; Polyethylene is a particularly attractive
material for hollow fiber manufacture because it is
inexpensive, relatively inert, non-toxic, readily processed
and strong. Polyethylene hollow fibers would be
expected to lack the hydrophilicity which is character-
istic of the types of membranes which have been most
widely used; i.e., in such processes as recovery of
water from brines by reverse osmosis. However, hydro-
philicity is not essential for permeability separations
~,, '
1~87358
such as removal of dissolved gases from aqueous or
non-aqueous solutions or ultrafiltration of organic
solutions containing relatively large solute molecules.
The only process known to Applicants for making
polyolefin hollow fibers is that disclosed in U.S. Patent
3,423,491, which teaches the preparation of permselective ,
hollow fibers by melt spinning a mixture of a thermo-
plastic polymer and a plasticizer, cooling and leaching
out the plasticizer. The following plasticizers, disclosed
as leachable with alcohols or aromatics, are listed as ;;
suitable for the fabrication of semi-permeable hollow
fibers from polyolefins: dioctyl phthalate, polyethylene
wax, tetrahydronap~thalene and chlorinated biphenyls.
It is apparent from the data given in the
'491 patent (for cellulose triacetate fibers) that the
membrane structures obtained by the disclosed process
are much "tighter" than is required for the types of
separations for which polyolefin hollow fibers appear
to be most suitable. A microporous structure, in which
selectivity results primarily from pore size rather than
from the chemical nature of the membrane material,
would appear to be preferable.
U.S. Patent 3,745,202 is directed to the
preparation of porous hollow fibers having a semipermeable
outer layer or "skin". A mixture of a cellulose ester
ether with a plasticizer is melt spun and the resulting
molten hollow fiber is drawn, gelled by cooling an~
leached to remove the plasticizer. At this stage, the
fiber structure is said to have a graded porosity but
to lack the outer skin required for selectivity. After
17,803-F -2-
1~873~3
a following treatment with hot water, the fibers are
s~ said to be more crystaline and more permeab~e but ~
require ~h*~er treatment for attainment of selectivity.
U.S. Patent 3,093,612 (corrected to 3,092,612)
S is directed to polyolefin/alkoxyalkyl ester compositions
which are'suitable for the preparation of solid polyolefin ' '~
filaments comprising "small pores" which are not '
contiguous. A polyolefin/ester mixture is heated to ;
form a solution, forced through a spinnerette, solidified
` 10 by cooling and immersed in a wash bath, such as isopropanol.
The resultant fiber is air dried and then stretched (drawn) ~'-'"'`
to achieve a maximum degree of orientation. Most of
i the solvent (ester) is said to be removed during coagula- ' ~
tion of the fiber and the rest (except for a residual ~' '
solvent content) by washing. ';~
The holl~w fibers of the present invention have
,::
' gas permeabilities up to an order of magnitude greater
than hollow fibers of the same size and composition which ~''-
are made according to the teachings of the prior art.
Additionally, the microporous, oleophilic hollow fibers ' '
whi~h, when pore wetted, have utility as separatory
membranes in ultrafilters and dialyzers. The permeability
of the fibers can be controlled simply by choice of a
leaching medium and contact time.
It has now been discovered that microporous
normally hydrophobic hollow fibers can be prepared by
spinning a'homogeneous solution of polyethylene and an
alkoxyalkyl ester in hollow fiber form, gelling the
forming fibers, drawing the fibers in a solidified gel
state and then contacting the drawn fibers with a liquid
17,803-F ~3~
~87358
ester-removal medium and removing at least a major
proportion of the ester. The pores in the resultant
fibers are contiguous between the inner and outer
fiber surfaces, are of a size suitable for applications
such as blood oxygenation, ultrafiltration, dialysis,
etc. The fibers have 2 permeabilities of from about -
2xlO S to about lxlO 2c.c. per cm2 per second per cm
Hg. transmembrane pressure.
It has also been discovered that other types
of esters which form homogeneous melts with polyethylene
can be employed in place of the preceding alkoxyalkyl
esters.
More precisely, the fiber and process of the
present invention, may be defined as follows: -
A microporous hollow fiber in which the
; porosity is contiguous~ the 2 gas permeability is at
least 2 x 10 5 cc (STP) per cm2 per second per cm Hg
transmembrane pressure differential and said fiber ~-
comprising at least 85% by weight polyethylene.
A process for preparing a microporous hollow
fiber comprising the steps of:
(a) providing a polyethylene/ester mixture of which
20 to 80 weight percent comprises polyethylene
and the balance comprises an ester or a mLxture
of esters, of the formula Rl(COOR )n' wherein
n is 1 or 2, Rl is a mono- or divalent hydrocarbon
radical, optionally interrupted by one or two
-O- or -S- links, and containing frDm 1 to 32
carbon atoms, each R independently is an aliphatic
hydrocarbon radical of 2-26 carbon atoms or
17,803-F -4-
, ...
, . , .
1(3873~
consists of two aliphatic hydrocarbon moieties
joined by an -0- or -S- link and containing a
total of from 3 to 20 carbon atoms. --
(b) heating said polyethylene/ester mixture to a
temperature at which it is a homogeneous liquid
^ but below the boiling or decomposition tempera-
ture thereof, and extruding it from a spinnerette
in the form of a hollow fiber;
(c) cooling the issuing extrudate to solidify it -
while maintaining sufficient tension on the
forming fiber to achieve a draw ratio of up to 40/1,
(d) at a temperature above the melting point of the
ester, but at least ten degrees below the cloud ~-
point of said homogeneous liquid, post-spin
drawing the solidified fiber under sufficient
tension to achieve an overall draw ratio of
from 1.5/1 to 800/1: and
(e) contacting the post-spin fiber with a liquid
ester-removal medium, at a temperature inter-
mediate of the freezing point of the medium
and the flow point of the polyethylene, until
at least a major proportion of the ester is
removed from the fiber.
DETAILED DESCRIPTION OF THE I~VENTION
:
Suitable polyethylenes for the practice of the
present inventlon are those consisting essentially of
; units formed by polymerization of ethylene. Polymer
units derived fr~m other monomers preferab~y are not
included but may be present in such minor amounts that
17~803-F -5-
,, .
1~87358
.
. : ~
;; the essential character of the polyethylene is not lost.
~ Said amounts will generally not exceed about 15 weight
- percent and desirably are less than about 10 weight percent
of the polyethylene. Generally, said fiber contains at
least about 85 weight percent polyethylene. Prefer~ably,
said fibers contain at least about 90 weight percent
~ polyethylene.
; Preferably, any nonethylenic units present are
- derived from olefinic monomers. It is also preferred
.
17,803-F -Sa-
1087358
.. .
that any nonethylenic units be incorporated as one or more
copolymeric components with ethylene. However, discrete
homo- or copolymers which are compatible with the poly-
, .~;
-~ ethylene may be incorporated by known techniques, such as
;~ 5 melt blending or solution blending. This, of course,
., .
is with the provision that the incorporated material will
~ be retained in the finished microporous fiber as part of
-~ a substantially homogeneous microstructure, i.e., that
it will not be phase separated or lost during post-spinning
processing.
It should be noted that the hollow fibers of the
present invention may include residual ester contents
which are not so high as to result in loss of their essential
polyethylene character. Preferably, the process of the
present invention is carried out in such manner that
.,
the resulting fibers contain 5~ or less of the ester by
weight.
Suitable polyethylenes include those which are
more branched and less regular, i.e., the so-called "low
density" polyethylenes, as well as the more linear, more
highly crystalline polyethylenes of the Ziegler and
Phillips types, commonly referred to as "high density" -
polyethylenes.
Such parameters as linearity, crystallinity,
average molecular weight and molecular weight distribution -
are significant largely to the extent to which they are
determinative of properties such as tensile strength, ;
flexibility, solubility and melting (or flow) points.
The prior art knowledge of these types of dependencies
is such as to provide considerable guidance in the
- 17,803-F -6-
1~873~
;;``` ~ :
selection or design of a polyethylene composition from
which to make hollow fibers having the properties (other
;-~ than porosity) suitable for a particular application.
Furthermore, the applications for which fibers of a
given polyethylene are most suitable can readily be
`` checked by a combination of standard fiber evaluation
tests (tensile strength per denier, for example) and the -~
permeability determination procedures described in the
examples herein.
Suitable monomers co-polymerizeable with ethylene
are exemplified by olefins, styrene, vinylpyridines,
- butadiene, acetylene, cyclopentadiene, acrylonitrile,
vinyl chloride, vi~ id~ chloride, allyl alcohol,
diallyl ether, maleic anhydride, tetrafluoroethylene,
divinylbenzene monoxide, N-vinylpyrollidone, vinylisobutyl
ether, vinyl acetate, vinyl dimethyl boron, cyclohexene,
phenylpentenyl thioether, N-methyl-N-butenyl aniline,
styrene sulfonic acid and alpha, alpha'-dichloro-p-xylene.
Exemplary suitable olefins co-polymerizeable with
ethylene are propylene, l-butene, isobutylene, l-pentene,
4-methylpentene-1, 2-butene, 2-pentene, 2-methylbutene-1,
2-methylbutene-2, 3-ethylbutene-1, l-hexene, 2-hexene,
l-heptene, l-octene, l-tridecene, l-hexadecene and
3-ethyloctadecene. Polymers of olefins containing up to
20 carbon atoms and methods of preparing such polymers
are disclosed in numerous publications, including an
extensive patent literature. Reference may also be had
to compendiums, such as the Encyclopedia of Polymer
Science and Technolgoy; Interscience, 1968, Volume 9.
17,803-F -7-
1~8~358
.
! ~ Suitable types of non-olefinic polymers which
may be incorporated in minor amounts are exemplified by
` homo- and copolymers of the above listed non-olefin monomers
and by nylons, polyesters, polyurethanes, polyalkylene
oxides, polyalkylenimines, polyimides, urea/formaldehyde
resins, phenolics, and so on. Preferably, such polymers
are thermoplastic or but lightly crosslinked. In any
event, they are employed only in such amounts as to be
-~ miscible with melts of the polyethylenes with which they
are to be blended, independently of the presence of any
esters. The more polar a polymer of the preceeding type
is, the lower its solubility in a molten polyethylene will
tend to be. The latter tendency can be overcome to a limited
extent by co-polymerizing minor amounts of unsaturated
monomers containing polar groups with the ethylene employed.
That is, the resulting co-polymers will be somewhat more
compatible with polar polymers. In general, the more
polar polymers are not incorporated in amounts greater than
about 10 weight percent and preferably constitute from
0 to abou~ 5 weight percent of the material in the fibers
produced by the process of the present invention.
Suitable esters for the practice of the present "^
invention are any mono- and diesters, as above defined,
capable of forming homogeneous melts with at least one -
polyethylene as above defined.
U.S. Patent 3,092,612 describes a number of
alkoxyalkyl esters of aliphatic monocarboxylic acids which
w111 form homogeneous melts with polyethylene (and other
polyolefins). These esters are represented in the patent
by the formula R-COO-~'-O-R', wherein R is an aliphatic
-
.,.
17,803-F -8-
~0873`58
hydrocarbon chain containing from 1 to 32 carbon atoms
and R' is a saturated aliphatic hydrocarbon chain of
from 1 to 8 carbon atoms. However, the defining formula,
Rl~COO-R2)n, used herein encompasses not only alkoxyalkyl
but also alkoxyalkenyl, alkenyloxyalkyl and alkenyloxyalkenyl
R2 groups. Further, analogues of the preceding groups
in which the oxygen is either omitted of replaced by a
sulfur are also included. -~
Diesters of aliphatic or heteroaliphatic
dicarboxylic esters, Rl(COOR2)2, in which Rl and R2 are
as above defined, are also suitable. Such esters in
which R is a hydrocarbylene moiety of at least 5 carbon
atoms are preferred. It is also preferred that each of `
~ the R groups contains at least 4 carbon atoms, particularly ;`
; 15 when Rl contains less than 5 carbon atoms.
Similarly, mono- and di-esters as above defined -
in which Rl is a mono- or divalent hydrocarbyl radical
consisting of or containlng carbocyclic rings are suitable.
Necessarily, R in such esters comprising an aromatic
ring will contain at least six carbon atoms (at least one ~` ;
benzene ring). Additionally, however, Rl in the latter
esters may contain one or more aliphatic moieties, each
optionally interrupted by an -O- or -S- link, and having
a total of from 1 to 26 carbon atoms. The ester group,
or groups (-CO-O-R2) may be attached either to aliphatic
or carbocyclic carbon atoms in Rl.
Specific exemplary Rl groups are included in
the acyl(R-CO-) portions of the monocarboxylate esters
listed in U.S. Patent 3,092,612, including those derived
from unsaturated acids, such as acrylic, crotonic,
~87358
isocrotonic, oleic, erucic and elaidic acids. Other
specific, exemplary R groups, which may be introduced ~ ;
in esters of the above formula by esterification of
various alcohols, constitute the R1 group in the following
mono- and dicarboxylic acids of the formula Rl~COOH)n,
''withjn being 1 or 2: propiolic acid, benzoic acid, the
isomeric cinnamic acids,~2-decalincarboxylic acid, phenylacetic''
.. . . . ... .. .
acid, hexacosanoic acid, the toluic acids, 4-t-butyl-benzoic
acid, the biphenylcarboxylic acids, the naphthioc acids,
cyclobutanecarboxylic acid, the oxtadecatrienoic acids,
abietic acid, 9- or -10- phenyloctadecanoic acid, 3,7,11- `
-trimethyldodeca-2,4,11-trienoic acid, chrysanthemic acid,
prostanoic acid,' naphthalenepropionic acids, pimaric acid
and diphenyl acetic acid; phthallic, isophthallic and
terephthallic acids, the cyclopropane dicarboxylic acids,
2,4-cyclohexadiene-1,2-dicarboxylic acid, malonic acid,
decanedioc acid, epitruxillic acid, isocamphoric acid,
fumaric acid, cinnamylidenemalonic acid, cetylmalonic acid,
muconic acid, the naphthalene dicarboxylic acids,
norcamphane dicarboxylic acid, 2-phenylpentanedioic acid,
roccelic acid, l-phenyl-1,4-tetralindicarboxylic acid,
acetylene dicarboxylic acid and 1,2-diphenoxyethane-p,p'-
dicarboxylic acid.
Specific exemplary alkanol esters suitable for
the practice of the present invention ar~ corresponding
'~ mono- or di-esters of the foregoing acids with any of the
following monohydroxy alkanes: ethanol, isopropanol,
t-butanol, neo-pentanol, 2,3-dimethylbutanol-1, penta- '
methylethanol, n-heptanol, 2-ethyl-hexanol-1, capryl
alcohol, lauryl alcohol, pentadecanol-l and ceryl alcohol.
17 803-F -10-
1~873~8 :-;
-.
Specific exemplary _oR2 groups in which R is
an alkenoxyalkyl, alkoxyalkenyl or alkenoxyalkenyl radical
are derivable from the following alcohols, which may be
prepared as indicated and reacted with an anhydride,
RlCO-O-CO-Rl or aceyl chloride, RlCO-Cl to produce
corresponding exemplary esters Rl-CO-O-R2:
CH2 = CH - CH2 - O - CH2 - CH(CH3) - OH,
by the reaction of allyl chloride with the monosodium
salt of propylene glycol;
CH3 - CH(CH = CH2) - O - CH2 - CH2OH,
by adduction of methyl vinyl carbinol with ethylene oxide;
H2C = CH - CH2 - CH2 - CH(CH3) - O - CH2 - ~
2 - ;
by adduction of 5-hexene-2-ol with butadiene monoxide;
H2C = C(CH3) - CH2 - O ~ CH2 - CH = CH - CH2OH,
by reaction of methallyl chloride with the monopotassium
~.
salt of 2-butene-1,4-diol and
~. ..
CH3(CH2)lO ~ CH2 ~ CH2 = CH ~ CH2OH,
by reaction of lauryl iodide and the monosodium salt of ~-
2-butene-1,4-diol. `
A wide variety of unsaturated, acylic alcohols
~ , " ,, ~.
are known. Exemplary -OR groups in which R is an alkenyl
or alkadienyl radical of from 3 to 20 carbon atoms are
derivable from the following alcohols by standard esterifi- -
cation procedures: allyl alcohol, crotyl alcohol,
4-pentene-1-ol, methyl vinyl carbinol, 5-hexen-2-ol,
geraniol and oleyl alcohol. `
Exemplary _oR2 groups in which the main chain
in the _R2 radical is interrupted by an -S- moiety are
derivable from the following alcohols, which may be
17,803-F -11-
- , . . . . ..
~ 1~873~8 :~-
prepared as indicated: .
CH3 - S - CH2 - CH2OH,
by adduction of methyl mercaptan with ethylene oxide;
CH3 - (CH2)4 - CH2 - S - CH2 - CH = CH - CH2OH,
by reaction of sodium hexyl mercaptide with Br-CH2-CH=CH-CH2OH;
2 2 (C 2)2 CH2OH,
by reaction of allyl chloride and the sodium salt of
alpha-hydroxypropyl mercaptan;
(CH ) CH - S - CH(CH3) - CH2 (CH2)3 2
by addition of isopropyl mercaptan to 6-hepten-1-ol in .-.
the presence of sulfur and absence of peroxides; .
CH3 - C(CH3) = CH - S - CH2 - CH(OH) - CH = CH2
by adduction of isobutenyl mercaptan and butadiene monoxide
and
CH3 - C(CH3) = CH - S - CH2 - CH = CH - C~2OH,
by reaction of sodium isobutenyl mercaptide with Br-CH2-
CH = CH-CH2H-
The preferred esters (or ester mixtures) for
the practice of the present invention are those, as above ~ -
defined, which:
a. form homogeneous melts with a polyethylene
at a temperature less than 250C., when
admixed with the polymer in at least one
weight ratio within a range of from 20/80
to 80/20; and/or
b. are liquids at ordinary ambient temperatures;
and/or
c. are readily soluble in a relatively low
boiling or water-miscible liquid which is
a poor or non-solvent for the polyethylene;
and/or
17,803-F -12-
1~873~i;B ~ ~:
d. are readily made or commercially available.
Alkoxy-alkyl esters, such as, for example, those
described in U.S.P. 3,092,612 have all of the foregoing
advantages a-d, and accordingly are a preferred group of
S esters. Among this group, butoxyethyl laurate and butoxy-
ethyl oleate, particularly the latter, are highly preferred.
Similarly, di-alkyl esters of aromatic dicarboxylic - -
acids constitute another preferred group of esters.
Particularly preferred esters of this type are dialkyl
phthalales such as, for example, dicaprylphthalates,
di(2-ethylhexyl) phthalate ("dioctyl phthalate"),
di(2-ethylbutyl) phthalate ("dihexyl phthalate"),
diisobutyl phthalate, diisodecyl phthalate and di(n-octyl,
n-decyl) phthalate, all of which have the foregoing
advantages a-d.
Suitable polyethy-lene/ester weight ratios are
within the specified ratio of from 20:80 to 80:20. In
general, membrane permeability will decrease and selectivit~
will increase as the proportion of polymer in the spinning
solution is increased. The former effect is particulariy
evident for processes requiring separation of gases from
liquids, as it is the case if gases are introduced into
liquids or removed from liquids. In general, the viscosity
of the spinning solution and the strength of the fibers
produced will increase as the proportion of pol~mer goes up.
What constitutes an ideal combination of fiber
properties of course depends on the contemplated use.
However, from the standpoints of ease of fiber preparation
and of obtaining a good compromise set of fiber properties,
a range of from 30 to 70 weight percent polymer in the
polv~er/ester mix~ure is prefe-rcd.
17,803-F -13-
1~8735~3
Method of mixing the polymer and ester(s). The
polymer and ester(s) may be mixed by conventional methods, ;
Polymer particles or strands may be "dry" mixed with the
liquid or particulated ester(s), heated to form a solution
or fluid slurry and then stirred or co-extruded through
a heated ram or screw extruder. A particularly effective
method for mixing other than in situ is to premix as a
melt, re-solidify, fragment and then re-melt and extrude
through a hollow fiber spinnerette.
Suitable spinning temperatures range from the
lowest temperature at which the polymer/ester mixture
is a homogeneous liquid up to the lowest temperature at
., ~ ..
which boiling or detrimental decomposition occurs. In
general, temperatures at least 10 above the cloud point ~ -~
.~. ; . .. ~ i
of the solution and at least ten degrees below the
temperature at which any substantial rate of evaporation
or decomposition results will be preferred. Considerations ~`~
such as the dependency of viscosity on temperature and
the dependency upon viscosity in turn of spinning solu-
tion behavior during and subsequent to issuance from
the spinnerette are familiar to those skilled in the æ t
of spinning fibers and will not be dwelt upon here. The -
optimum spinning temperature for any particular mixture
::
can be determined empirically and this is readily done
by carrying out a relatively small number of experimental ~-
extrusions with a laboratory type, single orifice spinnerette.
In some instance, the minimum useable spinning
temperature may be determined by the safe working pressure
of the equipment used to force the polymer solution
; 30 through the spinnerette at the required rate. That is,
' ~
-':
17,803-F -14-
1~873~8
the viscosity of the mixture must be low enough to
provide for an adequate rate of spinning under a pressure
head equal to or less than the maximum safe working pressure
of the ram, pump or screw employed. Adjustments in the
proportion or composition of the polymer in the mixture
can also be made in order to increase or decrease viscosity.
Spin temperatures within the range of 100 to
250C. will usually be suitable for the polyethylene/
ester mixtures (solutions) employed in the present process.
Temperatures of from 180 to 220 are preferred, however.
Cooling and initial drawing. ;~
The initial draw is made, to a ratio of from
1.0:1 to 40:1, while the extrudate is being cooled and
gelled. The fiber structure must be "set", i.e.,
solidified or gelled as an interdispersion of two discrete
phases (polymer and ester(s) ), before the post-spin, or
following draw is made. It is therefore desirable to
spool or drum the fiber skein as it is withdrawn from the
spinnerette, or otherwise operate, so that the tension
exerted during the subsequent draw will not be transmitted
back to as yet ungelled fiber portions adjacent the -
orifice(s). Since some tension will ordinarily be exerted
during spooling, it is usually difficult to avoid some -
drawing, on the order of 1.01 to 1.1:1 for example,
during cooling and take-up of the newly formed fibers.
It is also difficult to make spinnerettes having the
small dimensions required for production of thin-walled
fibers at lower draw ratios and relatively high overall
ratios will therefore often be necessary. The attainment
of higher overall ratios is a much more practical opera-
'
17,803-F -15-
1~873~8
tion if it can be effected in two stages. Fortunately,
the initial draw can be as high as about 40/1 without
markedly reducing the beneficial effects on microporosity
attained during the subsequent draw. It has been found
that a substantially more permeable structure results ~ !
if the final dimensions of the fiber are not attained
while the fiber is being cooled and solidified.
Cooling can be done by contacting the newly
formed fibers with a cool fluid, such as a quench gas
or liquid. If desired, close control of the temperature
profile in the cooling zone can be attained by flow of
a cooling gas through a conduit surrounding the fiber
skein. Alternatively, the fiber may be spooled in or
passed through a body of a quench liquid; optionally,
in counter-current flow. When a quench bath is employed,
it may or may not be preceded by a flow of gas, for
cooling or for any other desired effect.
The length of the cooling zone and the lowest
temperature therein will depend on the solidification
temperature for the polymer solution and on how rapidly -
it is desired to establish this temperature throughout
the entire thic~ness of the fiber wall. The temperature
needed to cause rapid solidification will be at least
as low as the cloud point of the polymer solution and the
temperature employed will usually be ten degrees or more
below this point.
~ wide variety of fluids are suitable for
cooling, since temperatures outside the range of about
0C. to 100C., will usually not be employed. Such
fluids of course should be poor solvents or non-solvents
1~873~B
for the extruded fiber materials. Cooling fluid tempera-
tures within the range of about 10C. to 50C. are
preferred.
In the event that it is desired to employ -
as the quench bath a liquid in which the ester(s) to be
subsequently removed is undesirably soluble, sufficient -~;
of the ester(s) to suppress solubility losses may be
; dissolved in the quench liquid prior to use. Since the
residence time of the fibers in the quench bath will
usually be quite low, only a few percent of the ester(s)
will ordinarily be enough. However, greater amounts, up
to the saturation level, may be employed.
Residence times in contact with the cooling
medium should be minimized when the medium is a liquid,
since ester loss will tend to occur (particularly at
higher temperatures) even when the medium is not miscible
with the ester. Preferably, contact with a liquid quench
medium is terminated as rapidly as is practicable after
the fiber has solidified to the desired extent (usually
throughout the entire wall structure). Contact with
gaseous cooling media can be extended indefinitely and
is limited only by considerations of process efficiency.
Post-Spin draw (PSD) conditions.
.
~ The cooled, solid fibers are drawn to provide
an overall draw ratio of from about 1.5:1 to about 800:1.
Overall draw ratios of about 10:1 to about 500:1
- are generally preferred over higher ratios, since the
PSD step tends to "close up" the internal structure of
the fiber at such higher ratios. However some polymer
solutions, particularly at low polymer contents, tend
~' .
1~87358
initially to be very open and some "closing down" may be
....-
essential to attaining adequate strength and limiting
pore size. Where higher overall ratios, i.e., from about
500 to 800, are to be employed, it will usually be
desirable to take a relatively high initial draw ~up
~h~
to 40:1), while the polymer and ester mixture is ~IEsing
and gelling. The porosity of the final fiber is not
highly dependent upon the initial draw, which is important
primarily to achieving adequate size reduction without
1 10 the degree of porosity reduction which will result if ~-
essentially all of the draw is taken in the PSD step, -
i.e., after the polymer and ester(s) are present as
discrete phase regions. In general, the overall ratio
for both draws will not exceed that required to provide
a final wall thickness as low as about 10 microns. The
fibers are maintained at or heated to a temperature above
the melting point of the ester or ester mixture but at
~ least ten degrees below the cloud point of the polymer/
; ester solution from which the fibers were spun.
.; .
~ 20 Temperatures of from 50C. to 110C. will
~.
generally be preferred for post-spin drawing, particularly
when the spin mix contains substantially less than about
40 weight percent of the ester(s). On the other hand,
ambient temperatures (about 25-30C.) will often be
satisfactory when normally liquid esters or ester mixtures
are employed, particularly at ester levels greater than
~ about 60 weight percent. The PSD fibers are spooled
; or drummed under sufficient tension (taken up at a
sufficient rate) to attain the desired overall draw
ratio. Any requisite heat input may be provided by
irradiation or by contact with a heating medium, such
~' .
~ 1 Q _
~ 108735B
as a hot metal roll or a hot gaseous or liquid fluid.
If a liquid heating medium is employed, it can
also function, in an alternative mode of operation, as
the ester removal medium. In this case, contact with the
liquid is prolonged after drawing, usually in the sub-
stantial absence of tension. This can be done, for example,
by passing the fibers through the hot liquid under draw
tension and then taking them up on a spool, drum or frame
immersed in the liquid. Immersion is continued until
any desired proportion of the ester(s) present in the
fibers is removed, as by dissolution and/or by being
squeezed out. The removed ester(s), as such or in solution,
are separated by flushing or by gravity separation. -
In the PSD operation, it is essential that the
fibers be brought to temperature and drawn before the
proportion of ester they contain drops below the requisite
level. An excess of the ester sufficient to compensate
for losses during the initial stage of the PSD operation,
i.e., before drawing, can be included in the original spin
mixture. In this event, a somewhat lower spin temperature
may be required in deference to viscosity requirements for
spinning. In general, it is preferred that no substantial
ester loss or removal occur prior to the leaching (ester
removal( step.
Heating may be carried out in any manner described
above. Preferably, the coolèd, solid fibers may be heated
by irradiation or by contact with a hot gas or liquid to
the selected draw temperature while being subjected to the
requisite draw tension, taken up on a drum which is not
.
immersed in the
~,:
. -,,' ' . .
17,8G3-F - 19 -
1(~87358 : - ~
heating medium and then placed in or passed through a
separate ester removal bath. Optionally, the solid fibers
may be preheated, as by brief immersion in a heating bath,
before being subjected to the drawing tension.
The heating medium should be a poor solvent and
preferably is a non-solvent for the polymer. A particularly
preferred medium of the latter type for draw temperatures
up to 100C. is water.
Any non-hazardous fluid which is not detrimentally ~-
reactive or instable and is not a good solvent for the `~
polymer can be used as the heating medium, as the drawing ~
medium, or both. Such liquids as are miscible with the ;
ester to be removed are preferred for ester removal
but are less desirable as heating or drawing media. The
suitability of a given liquid as a medium can readily be
determined by test. Also, the literature includes a -~
- ,
considerable amount of information on the solubilities -
of ethylenic polymers in the common solvents.
The fibers do not necessarity have to be in ~;
contact with any medium during heating and/or drawing.
If desired, the fiber (Lwu) may be heated by irradiation
and drawn while passing through or being taken up in
an evacuated zone. ~ -
Ester removal.
~ . : -- ,:,
~25 The ester(s) present in the drawn fibers are
removed by dissolution in the ester removal medium or
by separation as a solid or liquid phase which is not P
miscible with the medium. The medium preferably is
capable of dissolving the ester but should be a non-
-solvent ~or the polymer component of the fibers, since
.`' ' ' ~
17,803-F -20-
7358
ester removal will generally require substantial contact
duration.
Exemplary suitable ester removal media are
water, alcohols, ketones, dimethylacetamide, carbon
tetrachloride, methylene chloride and acetonitrile.
Methylene chloride and ethanol are preferred for this
purpose. In general, any non-hazardous liquid which is -;~
a non-solvent for the polyethylene and is not unstable ~
or detrimentally reactive may be employed to effect -
ester removal. The suitability of any such liquid for
removal of a particular ester is of course readily -
determinable by test.
Suitable temperatures for the ester removal `
operation range from just above the freezing point of
b~o~ ~-
15 ~ the medium to within about 10 degrees ~3 the flow point
of the polyethylene. Temperatures of from 20C. to
50C. are preferred. Temperatures above 50C. will `
generally not be employed if the fibers are to be
subjected to any substantial amount of tension while the
ester is being removed. As a practical matter, temperatures
above the normal boiling point of the medium employed
(which require superatmospheric pressures) will usually
not be employed.
Contact between the fibers and the ester removal
medium should be continued until at least a major portion
,. ~ .
(more than 50~) of the ester or ester mixture has been
removed. Since attainment of higher permeabilities in the
product fibers is dependent on reduction of the ester
content to a low level, it will generally be desirable
to remove as much ester as is possible in an economic
' ':
17,803-F -21-
11~87358
period of time. However, the rate of removal generally
decreases as the concentration of the ester in the fiber
decrease and it will thus often be impractical to effect
complete removal.
In general, the rate of ester removal will be
higher at more elevated temperatures and this fact can
be taken advantage of when using higher boiling removal
media, such as alcohols. ;~
Optionally, the leached fibers may be left in
contact with the ester removal medium until the micropores
are wet with or even filled with the medium. When this
is done with a water miscible medium, such as ethanol,
for example, the medium may then be displaced with water.
This is a convenient method by which the normally
hydrophobic membrane (fibers) may be pore wet and rendered
suitable for carrying out permeability separatory processes ~ ~-
with aqueous solutions or suspensions.
The ester will usually be removed immediately ~
- after post-spin drawing. However, ester removal may be ;
delayed indefinitely. If the ester content of the
unleached fibers does not interfere with "potting",
ester removal may even be delayed until an end device
-incorporating the fibers is put into use. Often, however,
~ it will be preferable to complete device manufacture, i.e., ~ ;
; 25 to remove the ester, before the device is shipped or put ~
i,:, ~ :
into use.
EX~LES
The following examples illustrate specific
embodiments in the practice of the present invention.
, ;~ ,
17,803-F -22-
11~873~8
Typical Procedure
Stage 1. A solution of polyethylene and ester is formed
in a stirred and heated vessel, solidified by cooling ~
and cut into pieces. The particulated mixture is placed -
in a heated tube, melted and ram-extruded through a hollow
fiber spinnerette, air being pumped through the center
of the spinnerette to keep the forming fiber continuously
hollow. The nascent fiber is passed through an 8-inch
air cooling zone and then, at room temperature, through a
quench bath consisting of two vessels of isopropanol in
series, the path length in each vessel being 40 cm. Upon
exiting from the quench bath, the fiber is taken up at
a preselected rate (depending on the purge rate at the
spinnerette and on the initial draw ratio desired) on a
rotating spool.
Stage 2. The hollow fiber is unspooled, and drawn at a
preselected temperature to a preselected overall draw
ratio. The drawing is done manually, as by immersing the -
~ fiber portion to be drawn in hot water, waiting about
,~, . . ...
- 20 10 seconds, drawing over another 10 second interval to
the desired length, waiting about ten seconds and then
.
,~ . - . .
~ removing the drawn fiber from the water. The same
:. ; '
~-- sequence of steps can be carried out in a continuous,
mechanical mode of operation by methods readily apparent
to those skilled in the art of fiber manufacture.
.
Stage 3. The hollow fiber is leached in a preselected
solvent for a period of from 1/4 hour to 24 hours.
~ .
.
1~87358 -~
Stage ommission or se~ence alteration.
In two of the following experiments, a fiber ;(or a portion of a fiber) is leached and then drawn, the
sequence of stages 2 and 3 being reversed for purposes `
of comparison. Leaching is done in most tests after a
fiber portion has been potted in a permeability testing
assembly.
Permeability measurements.
The permeability of a fiber portion is determined
after any stage by cutting off a number of lO cm. lengths,
assembling them with other components as a so-called ~- -
beaker ultrafilter and measuring the rate of permeation
of a gaseous or liquid test fluid. The lengths are
arranged in parallel, the resulting bundle is bent in an
inverted U-shape and the ends of the bundle potted in
short sleeves extending side by side from the bottom
of a plastic beaker and fitted with tubing connectors
`to permit passage of a test fluid or permeate through
` the fiber lumen. The bottom of the beaker is also pro-'
vided with dialysate inlet and/or outlet connections.
`The beaker is inverted and joined by mating threads at ;~-
its open end to a shallow cup. The effective membrane
~- area of the fiber bundle is calculated from the fiber
~ àverage diameter, the unpotted length of the fibers and
their number (about 10-50, depending on the amount of
fiber available). Gas permeation rates are determined
by pressurizing the exteriors of the fiber lengths with
the test gas and measuring the volume of a liquid displaced
by the permeated gas issuing from the fiber lumen in a
given time period. For liquids, the permeation rate is
:'
1(~8735~ :
determined by measuring the volume of liquid permeate
directly. Solute contents in both the feed and permeate
are determined by conventional analytical methods.
In the tabulations of data given in the examples -below, the symbols used have the following meanings:
UFR - ultrafiltration rate - in ml of ultrafiltrate per
minute per square meter of membrane surface, adjusted
for a standard transmembrane pressure differential
of 300 mm Hg; GPR - gas permeation rate - in cc. per
- 10 cm of membrane area per second per cm Hg of transmembrane
pressure differential, % Rej. = weight percent of the
indicated solute not passed by the membrane. ;
Example 1
Seven series of microporous hollow fibers were
.j ... .
prepared from polyethylene (Dow high density P.E. resin -
70065, melt index 0.70, density 0.965) having a tensile -
yield of 4550 lb.ft/in2, a shore D hardness of 65, an ~-~
' IZOD impact of 2.0, and a Vicat softening pt. of 127C.)
and 2-butoxyethyl oleate, butyl stearate or dioctyl -
phthalate to demonstrate the importance of post-spin drawing
; prior to leaching.
These preparations and the properties of the
resultant fibers are summarized in Table 1 below. In the
column headed "Stage Sequence", stage numbers and commas 1~;
are used to show which of the stages (in the basic procedure
,
described above) the fibers were subjected to and in
~; what order. Thus, an entry 1, 2, 3, means stages 1-3
were employed in their ordinary sequence. The entry
~,,
1~87358 :::
1, 3, means step 2 was omitted and 1, 3, 2 means that ~-
the sequence of stages 2 and 3 was inverted.
All fibers leached in this example were leached :
1/2 hour in ethanol. .
;,~
:' ., :.
.~
. :
','i~ '' -; " ~ ' ''
;
.~; . , ,:
" . ., ' ' ' - ~ '' '` ~ ' :, ' ' ,,
. ~ .
: . ' ' ,
;' ' , , ~. ' i.
". , :. ~ .
~ '
,
: ~
~ ,,, ~ ` ' ' `
17,803-F . -26-
:, , , ~,
~C~873S~
-' oO _
. ~ . . U~ ,, .
~u ~ ~ a~ ~ ~ o ~D tU ~ ~
.. ,,. ~ ) ,~ r~ ~D' O O O O
". ~ ~ i oo~ o~ CU .
~ ~ ~O ~ 0 ~ ~ D ~ ~ ~
I ~ 0~ O OC~JO O C~ C~ O O O
F9 . . . '
I ~olo ~o ~o ~o-lolo ~o ~o . .
P~ ~ ~ ~ ~X'~ ~ ~. ,,
. ~ ~ ~
.- ~ H . . ¦ ~N zN _ ~ CU
. l 000 00 0 ~ O, O O _ ' ' .:
~ I o ,~ ,~ 1~ o It'~ a~ D CO
. ~ ~ ~ t- ~ CU C~ I CU
~ .~ ~ o ~ cs~ ~ ~ :t ~
el~ I~ . ~ ~ ~ . t- O O O . , ::
~7 tQ` ,~ O ~ ' CU u~ ~ ) , . '
H Oæ "~ ~ ~ O ~ ~ J >
~: H Fc~ . ~) O~ O~ ~ ~ ,~
H H O ~ ~ .
. - :
1 PO ~ ~ C~ CC~
.. ' ' ''
~ H S~ O ~ Jl~ t-- .
;`'~'' I_ l ~ O~' ~ ' ~
~ ~ ~1 I ~ , ~ ~r),, .~ ~ ~ O .~ ., , ' ' ' , '~' ~
I E~ ~ 0 0O O 0 ~ o C~
,.;,, . ~ 1~ I~ . . . .. ~ ''.. 1:.::
- æ ¦ H N N N D ~ ~D ~ t--
H ~ r1 O ~ g
! I I C~ E~ N N N
H ~ a ~ N
U~ ~ ~ N N ~ ~7 N ~ N N N
:~ ~n ~ ~ 1 ~ ~ , ~1 ~ ~ ~ ' i
A _ ~ O
H ~ ~ r~
HO ~ m ~ m . .
P~ ~; N O O
c ~ P I ~ u ,
c ~q ~
I . ,¢ cq ,1, - ~1~ cl~ I I I I . -
1 3 N
17, 803-F -27-
,, ` . .
1~87358
,~ -1 ¦ N ~
` I ~ oo I
~ ~ 0 ~ ~ ~ ~ ~ ~ .. . ~ .
~ I ~ ~ ~X ;~X ~ ~ . . . ..~.
t.~ ,-~ t~ N cr. 11~ 0~ ~ ~i ~
l D~ . . . . . . ' , " .' . ,~
¦ . ¦ h ~ O oNo oN . .
l I ~D O~ dC~0~0 ~10 .
E-l I ~ . ~_ ~ ~ ~ C~ 0 . ~'' ~ . ;;
~ . ~ ~ . ~D ~ t--~ 0~ N
~ ~ ~ O, ~D ~0 0~ ~ . '. ';
I :Z ~: I t- 0 ' ~ 0 t- ~
~: H ;:~ . ~1 N ~7 11~ ~) 0~ t--
a H ~ ~. t~ N N C~ l
E!~ H H N tr~ 0 --1 ~1 0
iI a I ~ ,~ c~ o~ ~ 0
O H ~ ~ t_ -~ . o a~ a~ CU
'~' ~ ~ O ~1 O u~ ,J ~ u~
.. . .~ a U~ ~o. ~ o~ ~ ~
H H t_ ~ N N ~
G~ 3: O :~ J- O O ~t ~ O O O . ., ' ' .
. ' ~ io d ~J H ~ri ~ ~ ~ ~D 0 0 O ~ N~
EH . . . ~ . :
~ a l l 0 0~ I I CD 0~ 0~ p0 0 . ~ .
'i a C~ ~li o ~ . .. ~ '
~:1 H . ~ ~ ~ ~ ~. ~ t . . . . . . . ~ .- .
H ,i ~i ~i ~_i ~i ~i ~i ~i ~ O~ ~ t-- . - . . .. .
. ~ N . . . . .
H a
~ ~i rl ¦ N . N O N N N
: ~ r ~ ~ g .
P~ t~) ¦ ~ ~1 N t~ .
ta d ~r) N N ~ N N~) ~ N ~) N
U~ ¦ r~l r l r-l r-l ~1 ~1 ~I r~l H r-l ~t r-l r~l r-l
1...... .
~ ¦ O O ~ r~l
~ c . I W W ~D rO
.~ o ~ m m m r~
o ~ r I o . o o . o
:~ ~ U~ U~ U~ U~
H N ~ ~ ~1 iN t~) J
I I I I ~ r~ r-l N r-l ~.
I m ~' C~ l l l ~ ~i ~c Fq
¦ rl~ P~ q P~ ~
.- I
17, 803-F -28-
1(1 87358
:~;
Notes for Table I:
1. Polethylene
2. Temperature of spin block, C.
3. Butoxyethyl oleate
4. After initial draw and before leaching
I.D. = Internal Diameter, O.D. = Outside
Diameter, Wall = Wall thickness.
5. After last stage listed hot-draw (2) or
leach (3)
6. Distllled water
7. 2~ aqueous albumin
:;:
..
8. No quench bath employed. Air cooled only. ~
.; -:.~ . .
9. Butyl stearate
...
,,.
10. "Dioctyl" phthalate (Di-(2-ehtylhexyl)phthalate)
The relative effects, on fiber permeability,
of leaching alone, post-spin drawing alone and post-spin
~; drawing and then leaching are apparent from the date in
Table I for fibers PE-l, PE-2, PE-5A, -5B and -5C. The -
-~ effect of leaching before drawing, rather than afterwards,
is evident from the results for fiber PE-7. It will be
seen that post-spin drawing, and then leaching, is essential
to the attainment of permeabilities above about 1.7x10 5
:: .
c.c. per cm2 per second per cm. Hg transmembrane pressure.
It is also apparent, from the results given
in Table I, for fibers PE-17 A-l, 2 and B-l, that a
-
significant improvement in permeability results when
hollow fibers prepared from polyethylene and dioctyl
phthalate, in accordance with the teachings of U.S.P.
3,423,491, are post-spin drawn before being leached to
remove the "plasticizer".
Z~
1~87358
.; .
Example 2
Hollow fibers were spun at 200C. from a mixture`
of BEO (47 wt. percent) and Dow high density polyethylene
resin B5965 (53 wt. percent; Melt Index 0.85, Density 0.965,
Tensile Strength 4200 psi, Flexural Modules 24,500 psi).
The relative rates of take-up and extrusion were such that ~-
a draw ratio of 14.3/1 resulted. The nascent fibers were
quenched with air only. The dimensions and mechanical -
properties (averaged for three replicate specimens each)
,. . . of the quenched fibers were as follows: I.D. 380/~ O.D.
480~, wall thickness 50~; Tensile Strength 63 gmf (1171
psi), Tensile Yield Stress 34 gmf (648 psi), Ultimate
Elongation 886%, Tensile Yield Strain 7.5~ and Elastic
Modules 645 gmf (8600 psi).
.,~ .
Some of these fibers were immersed in hot
(83C.) water for ten seconds, drawn over a period of
~about 2 seconds to a wall thickness of 23 microns (post-
-spin draw ratio 3.4/1), allowed to soak in the hot
water for about 10 seconds more and then placed on paper
to cool and dry.
The rates of BEO leaching (at 25C.) from both
the drawn and undrawn fibers with me~hylene chloride,
l,l,l-trichloroethane and ethanol were determined and
- are given in Table II. The observed dependency of
leaching rate on fiber dimensions includes any effect
of drawing per se.
- : .
'
17,803-F 30_
. . : : : : ....
; ~
11)87358
':
....
~..
0
o s
., ~ 0 ~ ~ 'U U~ ~ .`,, -
~ : ~ ~r ~ ~ ~1 ~ ',
: s ~ o.~
.~ ~^ oq ~ ", :,,
U~ . dP h ~ ~ :
3 ~3 5 e~ 0 ~
a ~F4 u~ s o
x ~ ~ _~ ,,~ uS ~:
H ~.) S u~ o .~ ~ 0 .`
m ~ O 1 3 ~ ~
' 0 ~ ~ S ~ S~ ''"~" ,:' '
H ~ ~ ,t
~ Z ~ l
H ~ 1~ ~. Ql
' ~' ' : ~ ~ O ~ ~ . U~ O o
1 u
. ~ O~1' 1 ~ , 0 ~u ID ;
: .:.
a -I ~ 3 s ` .
Z
.s 0~ 0` 8
a 1l ~ ~o $ ~ ~ :
S ~ ~0 ::
O ~
N _I N
S N ~ ~ Ui
'
,; :
~37358 :
Example 3
Replicate lengths of fibers designated in
Table I as 5A-1, 5B-1 and 5C-l (none of which were -
drawn after spinning), 5B-3 and 5C-3 (both post-spin
drawn to 5:1 ratio) were leached with ethanol for 24 - -
hours or with methylene chloride for 4 hours. The
residual BEO contents of the A and B series leached fibers
- were determined by gas phase chromatography. Permeabili- ;
ties of all the leached fibers were determined as in
I0 Example 1, the fiber lengths having been mounted in
,:, :
~ beaker U.F. units. The dimensions of the fibers before
J, leaching are included with the BEO residuals and
permeabilities in Table III-A.
.. :.
, .
~, . . .
.: ,:
-
` ' ' '
..
,~ . ', :'.
~ ' .' ;
- : ., ,
' ~
17,803-F -32-
1~87358 ~ ~
, ' :
:. ..
,
. o o o o o o o o
., P; X X X X X X X X -~
," ~ ~ ~D r~ O 1~
~ _~ r~ _i r,~l r~
o ~ .,.
~o m ~ ~:
dP -~
~1 ~ D t~ r~
o~ ,
~ ~ o ..
o o
.:
H ~ ~ I r~ `I =
H U~
3 ~ a I r o D ~ l ~
r.l ~ ~ I~ ~ ~ : rJo 1~ - a~
O u~ r o o
a m a l ,ol
¢ ~ Z H
I H
H ~3: E~
l -
H C)
.' 0~ 0 ' '
E~ m a~
2-
O H ~ ~_I ~1 ~ _I ~ ~1 --I --I . .
E l . ~.' '
~ ~ m ~ ~
~
h P~ ~ P,
"
',`
U~ - ,
. ~ .
r,~ r~ N
~: ~1
~ ~`1 N ~ ~ ~
rl ~ m o ~ ~
.. ~ o ~ o ~ ~ o ~ ~
. :.
_l ~ rr~
, :
17,803-F -33-
1~87358
:;:
To facilitate comparisons, the data of Table
III-A are recapitulated in Table III-B. For example, ~-
leach numbers 6 and 7 (in Table III-A) may be compared
(Comparison A in Table III-B) as differing with respect -.
to both the leach medium employed and duration of
leaching and as being the same with respect to fiber ;
aize and in not having been post-spin drawn.
~ '
'' ' ''
17,803-F _34_
.; 1~87358
`
TABLE III-B
Factors Residual Ratio
~ Compar- Held Leach BE0 ~ -
- ison 'Constant ~aried # Le~elGPR GPR's
A Fiber Leach 7 3.81.40xlO 5 1.71
size. medium 5
Not and time. 6 3.7 0.82xlO
drawn.
B Fiber Leach 2 2.4 1.60xlO 2.~3
- size. medium 5
Not and time. 1 5.1 0.66xlO
drawn. -
C Fiber Leach 4 1.6l.OxlO 3 2.50
size. medium 3 -
Not and time. 3 3.70.4xlO
drawn.
Leach Drawn 3 3-7 39.00xlO 5 5g.1
medium -
ana time. - ~5
Fiber Not drawn 1 5.1 0.66xlO -;~
- size.
E Leach Drawn 4 1.6 lOxlO 62.5
~eai~m . -
and time. 4
Fiber Not drawn ? 2.4 0.16xlO
size. - -6
F Leach Fiber 6 3~7 ô.2x10 1.24
medium size
and ti~e. -6
Not drawn. 1 5.1 6.6xlo
G Leach Fiber 7 3.ô1.4xlO 5 o.88
medium size
snd time. 5
Not drawn. 2 2.4 1.6xlO
LeachDrawn to 3 3.7 39.0xlO 5 47.6 -
medium 76~ - - -
~' and time. _5
Not drawn 6 3.7 0.82xlO
Leach Drawn to ~,5 est.- 185xlO 5 200. - ~ ~,
medium 77. ~ avg. 1.6 a~erage
and time. 5
Not dra~n 7,8 est. 0.925xlO
162. ~ avg. 3.8 a~erage
~ ... , .' : ~'
.
17,803-F ~35~
- 1~873~8
Taking comparisons A, B, C, F and G as one
group and contrasting D, E, ~ and I as another group,
it is clearly evident that post-spin drawing before
leaching has a much more beneficial effect on permea-
bility than can be attributed simply to facilitating
more complete solvent (BEO) removal (by fiber wall
thickness reduction). It should be noted that, in fact,
the effect of wall thickness reduction may even be
countered to some extent by the concurrent reduction in
1~ fiber inner diameter (making interior wetting by the
leach medium more difficult).
Example 4
Hollow fibers are prepared from a molten
mixture consisting of polyethylene resin 70065 (80 wt. %)
and 2-butoxyethyl oleate (20 wt. %), in the manner in
which fibers PE-5A-6 (Example 1 herein) were prepared -
are of comparable dimensions and are found to have 2
gas permeabilities, as above defined, of at least
2xlO 5.
- Scanning electron micrographs of membranes of
this invention reveal an internal structure of inter-
connected porous domains. The size of these porous
' domains varies from a maximum of ~1 micron to the
smallest resolveable by the microscope of ~0.1~. The
inability of some dissolved solutes to pass through the
membrane indicates the minimum pore size is many times
smaller than can be seen with the scanning electron
~ microscope. The domains of porosity are distributed
; quite uniformly throughout the membrane. The photo-
graphs revealed that very few, if any, of these porous
17,803-F -36-
1~873~8 ;
regions are completely encapsulated by the polyethylene.
Instead~ the porous regions tend to be connected to the
bordering porous regions. This effect of contiguous
pores being connected to each other, leads to the
usefulness of the membrane. Being connected, the adjacent
porous regions provide a continuous, if somewhat
tortuous, path from one side of the membrane to the other.
On the basis of the following considerations, -
maximum pore radii of the fibers produced by the present
method are estimated to range up to about 50A.
The most rapid and practical method of esti-
mating the efective pore size of a membrane is by
ultrafiltration experiments. A solution of a solute
of known molecular size is pressurized against one side
~ 15 of the membrane. The permeate which consequently passes
; through the pores of the membrane is collected and
analyzed. The rejection coefficient (R) of the membrane
C . . ~ ~
is: R = 100 (lCP)%, where Cp is the concentration of
solute in the permeate, and CB is the concentration of
solute in the initial solution. In this process, the
pores of the membrane behave as a molecular sieve.
Solute molecules larger than the pore diameter are
excluded from the interior of the pore. The pores then
preferentially transport the very small solvent molecules.
Hollow fiber membrane PE-5A-5 (Table I) rejected >99.g%
of the albumin molecules, allowing only water to pass
through the membrane. The albumin molecule in aqueous
solution exhibits a radius of gyration of approximately ;
30A. Since the pores of the microporous polyethylene
do not allow the passage of the solute albumin, the
17,803-F _37_
:
:``
~87358
.
radius of these pores must, at some point be smaller
than the effective radius of the solute albumin (30A).
Other fibers, such as PE-5A-3, exhibit albumin rejections
of only about 95% and accordingly must include some
pores having radii substantially larger than 30A.
Assuming a normal Gausian distribution of pore sizes,
.. . O 1~
~ a radius of about 50A appears to be a reasonable maximum
~ . .A~,
for the latter type of fibers.
The foregoing examples are illustrative only
and are not to be construed as restricting the scope of
the present invention, which is~limited only according -
to the appended claims.
~'
.
.. .. .
' , ,'.
~ 7 0~ 38-
