Process for preparing microporous polyethylene hollow fibers

Normally hydrophobic, polyethylene hollow fibers having contiguous microporosity are prepared by extruding a heated solution of a polyethylene and an ester from a hollow fiber spinnerette, simultaneously cooling and drawing the forming fibers to a ratio of up to 40 to 1, drawing the gelled fibers to an overall ratio of from about 1.5/1 to 800/1 and then removing the ester. The maximum pore radius ranges up to about 50A, and the fibers have gas permeabilities approaching 10.sup.-2 cc (STP) per cm.sup.2 per second per cm. Hg. transmembrane pressure differential. A significant increase in permeability over polyethylene hollow fibers made according to the teachings of the prior art is achieved.

BACKGROUND OF THE INVENTION 
The advantages of permeable hollow fibers as separatory membranes are now 
well known. For example, the large membrane areas per unit volume of 
device attained with this membrane configuration are of particular 
importance in minimizing priming blood requirements for devices such as 
so-called artificial kidney or artificial lung units. Additionally, a 
simplification over 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 transmembrane pressures 
required for processes such as reverse osmosis 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 characteristic 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, hydrophilicity 
is not essential for permeability separations 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. Pat. No. 3,423,491, which teaches the preparation 
of permselective hollow fibers by melt spinning a mixture of a 
thermoplastic 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, tetrahydronapthalene and chlorinated biphenyls. 
It is apparent from the data given in the U.S. Pat. No. 3,423,491 (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. Pat. No. 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 or ether with a plasticizer is melt spun and the resulting 
molten hollow fiber is drawn, gelled by cooling and 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 selectively. After a 
following treatment with hot water, the fibers are said to be more 
crystaline and more permeable but require further treatment for attainment 
of selectivity. 
U.S. Pat. No. 3,093,612 (corrected to 3,092,612) 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 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 
the solvent (ester) is said to be removed during coagulation of the fiber 
and the rest (except for a residual solvent content) by washing. 
OBJECTS OF THE INVENTION 
A primary object of the present invention is to provide normally 
hydrophobic, microporous hollow fibers which have high gas permeabilities 
and can be prepared from a strong, inert, inexpensive and readily 
processed polymer, i.e., from polyethylene. 
A further object is to provide a flexible, practical process for making 
such fibers. 
An additional object is to provide microporous, polyethylene hollow fibers 
having particular utility as separatory membranes in so-called artificial 
lungs or oxygenators. 
Another object is to provide novel hollow fibers having 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. 
Yet another object is to provide microporous, oleophilic hollow fibers 
which, when pore wetted, have utility as separatory membranes in 
ultrafilters and dialyzers. 
It is also an object to provide a process for making microporous, 
polyethylene hollow fibers in which the permeability of the fibers can be 
controlled simply by choice of a leaching medium and contact time. 
SUMMARY DESCRIPTION OF THE INVENTION 
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 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, and are of a size suitable for 
applications such as blood oxygenation, ultrafiltration, dialysis, etc. 
The fibers have O.sub.2 permeabilities of from about 2 .times. 10.sup.-5 
to about 1 .times. 10.sup.-2 c.c. per cm.sup.2 per second per cm Hg. 
transmembrane pressure. 
It has also been discovered that other types of esters which form 
homogeneous melts with polyethylenes can be employed in place of the 
preceding alkoxyalkyl esters. 
More precisely, the fiber and process of the present invention, and 
preferred embodiments thereof, may be defined as follows: 
(1) A microporous hollow fiber in which the porosity is contiguous, the 
O.sub.2 gas permeability is at least 2 .times. 10.sup.-5 c.c. (STP) per 
cm.sup.2 per second per cm Hg. transmembrane pressure differential and 
said fiber consists essentially of polyethylene. 
(2) A fiber as in embodiment 1 in which said oxygen gas permeability is at 
least 1 .times. 10.sup.-3. 
(3) A fiber as in embodiment 1 having an oxygen gas permeability, as 
therein defined, of at least 2 .times. 10.sup.-3. 
(4) A fiber as in embodiment 1, of which 10 weight percent or less is 
derived from olefins other than ethylene. 
(5) A process for preparing a hollow fiber according to embodiment 1 which 
comprises: 
(a) providing a polyethylene/ester mixture of which about 20 to about 80 
weight percent consists essentially of polyethylene and the balance 
consists essentially of an ester, or a mixture of esters, of the formula 
R.sup.1 (COOR.sup.2).sub.n, wherein n is 1 or 2 R.sub.1 is a mono- or 
divalent hydrocarbon radical, optionally interrupted by one or two --O-- 
or --S-- links, and containing from 1 to 32 carbons, each R.sub.2 
independently is an aliphatic hydrocarbon radical of 2-26 carbon atoms or 
consists of two aliphatic hydrocarbon moieties joined by an --O-- or --S-- 
link and containing a total of from 3 to 20 carbons, 
(b) heating said polyethylene/ester mixture to a temperature at which it is 
a homogeneous liquid but below the boiling or decomposition temperature 
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 from 
about 1.0/1 to about 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 about 1.5/1 to about 800/1, 
(e) contacting the post-spin drawn fiber with a liquid ester-removal 
medium, at a temperature intermediate of the freezing point of the medium 
and the flow point of the polyethylene, until at least a major proportion 
of the ester or ester mixture is removed from the fiber. 
(6) The process of embodiment 5 in which said ester has the formula R.sup.1 
--COOR.sup.2, in which R.sub.1 is a monovalent radical defined as in 
embodiment 5 and R.sup.2 consists of two aliphatic hydrocarbon moieties 
joined by an --O-- link and containing a total of from 3 to 20 carbons. 
(7) The process of embodiment 5 in which R.sup.1 is an aliphatic radical 
containing from 10 to 24 carbons. 
(8) The process of embodiment 5 in which said ester has the formula R.sup.1 
--COOR.sup.2, in which R.sup.1 is a monovalent radical defined as in 
embodiment 5 and R.sup.2 is an aliphatic hydrocarbon radical of 2-20 
carbon atoms. 
(9) The process of embodiment 6 in which said ester is butyl stearate or 
2-butoxyethyl oleate. 
(10) The process of embodiment 6 in which said ester is 2-butoxyethyl 
oleate. 
(11) The process of embodiment 5 in which said post-spin drawn fiber is 
contacted with methylene chloride or ethanol until the residual content of 
said ester therein is less than 5 weight percent. 
(12) The process of embodiment 5 in which said overall draw ratio is within 
the range of from about 20/1 to about 500/1. 
DETAILED DESCRIPTION OF THE INVENTION 
Suitable polyethylenes for the practice of the present invention are those 
consisting essentially of units formed by polymerization of ethylene. 
Polymer units derived from other monomers preferably are not included but 
may be present in such minor amounts that 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. 
Generally, said fiber contains at least about 85 weight percent 
polyethylene. Preferably, said fibers contain at least about 90 weight 
percent polyethylene. 
Preferably, any non-ethylenic units present are derived from olefinic 
monomers. It is also preferred that any non-ethylenic units be 
incorporated as one or more copolymeric components with ethylene. However, 
discrete homo- or copolymers which are compatible with the polyethylene 
may be incorporated by known techniques, such as 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 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, vinyllidine 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, 
1-butene, isobutylene, 1-pentene, 4-methylpentene-1, 2-butene, 2-pentene, 
2-methylbutene-1, 2-methylbutene-2, 3-ethylbutene-1, 1-hexene, 2-hexene, 
1-heptene, 1-octene, 1-tridecene, 1-hexadecene and 3-ethyloctadecene. 
Polymers of olefins containing up to 20 carbons 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 Technology; Interscience, 
1968, volume 9. 
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 preceding 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 about 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. Pat. No. 3,092,612 describes a number of alkoxyalkyl esters of 
aliphatic monocarboxylic acids which will form homogeneous melts with 
polyethylene (and other polyolefins). These esters are represented in the 
patent by the formula R--COO--R'--O--R', wherein R is an aliphatic 
hydrocarbon chain containing from 1 to 32 carbon atoms and R' is a 
saturated aliphatic hydrocarbon chain of from 1 to 8 carbons. However, the 
defining formula, R.sup.1 --(COO--R.sup.2).sub.n, used herein encompasses 
not only alkoxyalkyl but also alkoxyalkenyl, alkenyloxyalkyl and 
alkenyloxyalkenyl R.sup.2 groups. Further, analogues of the preceding 
groups in which the oxygen is either omitted or replaced by a sulfur are 
also included. 
Diesters of aliphatic or heteroaliphatic dicarboxylic esters, R.sup.1 
(COOR.sup.2).sub.2, in which R.sup.1 and R.sup.2 are as above defined, are 
also suitable. Such esters in which R.sup.1 is a hydrocarbylene moiety of 
at least 5 carbons are preferred. It is also preferred that each of the 
R.sup.2 groups contains at least 4 carbons, particularly when R.sup.1 
contains less than 5 carbons. 
Similarly, mono- and di-esters as above defined in which R.sup.1 is a mono- 
or divalent hydrocarbyl radical consisting of or containing carbocyclic 
rings are suitable. Necessarily, R.sup.1 in such esters comprising an 
aromatic ring will contain at least six carbons (at least one benzene 
ring). Additionally, however, R.sup.1 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 carbons. The ester group, 
or groups (--CO--O--R.sup.2) may be attached either to aliphatic or 
carbocyclic carbons in R.sup.1. 
Specific exemplary R.sup.1 groups are included in the acyl(R--CO--) 
portions of the monocarboxylate esters listed in U.S. Pat. No. 3,092,612, 
including those derived from unsaturated acids, such as acrylic, crotonic, 
isocrotonic, oleic, erucic and elaidic acids. Other specific, exemplary 
R.sup.1 groups, which may be introduced in esters of the above formula by 
esterification of various alcohols, constitute the R.sup.1 group in the 
following mono- and dicarboxylic acids of the formula R.sup.1 
(COOH).sub.n, 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 napthoic acids, cyclobutanecarboxylic acid, the 
octadecatrienoic acids, abietic acid, 9- or -10- phenyloctadecanoic acid, 
3,7,11-trimethyldodeaca-2,4,11-trienoic acid, chrysanthemic acid, 
prostanoic acid, napthalenepropionic 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, 
1-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 are 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, pentamethylethanol, n-heptanol, 
2-ethyl-hexanol-1, capryl alcohol, lauryl alcohol, pentadecanol-1 and 
ceryl alcohol. 
Specific exemplary --OR.sup.2 groups in which R.sup.2 is an alkenoxyalkyl, 
alkoxyalkenyl or alkenoxyalkenyl radical are derivable from the following 
alcohols, which may be prepared as indicated and reacted with an 
anhydride, R.sup.1 CO--O--CO--R.sup.1 or acyl chloride, R.sup.1 CO--Cl to 
produce corresponding exemplary esters R.sup.1 --CO--O--R.sup.2 : 
EQU ch.sub.2 .dbd.ch--ch.sub.2 --o--ch.sub.2 --ch(ch.sub.3)--oh, 
by the reaction of allyl chloride with the monosodium salt of propylene 
glycol; 
EQU CH.sub.3 --CH(CH.dbd.CH.sub.2)--O--CH.sub.2 --CH.sub.2 OH, 
by adduction of methyl vinyl carbinol with ethylene oxide; 
EQU H.sub.2 C.dbd.CH--CH.sub.2 --CH.sub.2 --CH(CH.sub.3)--O--CH.sub.2 
--CH(OH)--CH.dbd.CH.sub.2, 
by adduction of 5-hexene-2-ol with butadiene monoxide; 
EQU H.sub.2 C.dbd.C(CH.sub.3)--CH.sub.2 --O--CH.sub.2 --CH.dbd.CH--CH.sub.2 OH, 
by reaction of methallyl chloride with the monopotassium salt of 
2-butene-1,4-diol and 
EQU CH.sub.3 (CH.sub.2).sub.10 --CH.sub.2 --O--CH.sub.2 --CH.dbd.CH--CH.sub.2 
OH, 
by reaction of lauryl iodide and the monosodium salt of 2-butene-1,4-diol. 
A wide variety of unsaturated, acyclic alcohols are known. Exemplary 
--OR.sup.2 grous in which R.sup.2 is an alkenyl or alkadienyl radical of 
from 3 to 20 carbon atoms are derivable from the following alcohols by 
standard esterification procedures: allyl alcohol, crotyl alcohol, 
4-pentene-1-ol, methyl vinyl carbinol, 5-hexen-2-ol, geraniol and oleyl 
alcohol. 
Exemplary --OR.sup.2 groups in which the main chain in the --R.sup.2 
radical is interrupted by an -S- moiety are derivable from the following 
alcohols, which may be prepared as indicated: 
EQU CH.sub.3 --S--CH.sub.2 --CH.sub.2 OH, 
by adduction of methyl mercaptan with ethylene oxide; 
EQU CH.sub.3 --(CH.sub.2).sub.4 --CH.sub.2 --S--CH.sub.2 --CH.dbd.CH--CH.sub.2 
OH, 
by reaction of sodium hexyl mercaptide with Br--CH.sub.2 
--CH.dbd.CH--CH.sub.2 OH; 
EQU ch.sub.2 .dbd.ch--ch.sub.2 --s--(ch.sub.2).sub.2 --ch.sub.2 oh, 
by reaction of allyl chloride and the sodium salt of alpha-hydroxypropyl 
mercaptan; 
EQU (CH.sub.3).sub.2 --CH--S--CH(CH.sub.3)--CH.sub.2 --(CH.sub.2).sub.3 
--CH.sub.2 --OH, 
by addition of isopropyl mercaptan to 6-hepten-1-ol in the presence of 
sulfur and absence of peroxides; 
EQU CH.sub.3 --C(CH.sub.3).dbd.CH--S--CH.sub.2 --CH(OH)--CH.dbd.CH.sub.2 
by adduction of isobutenyl mercaptan and butadiene monoxide and 
EQU CH.sub.3 --C(CH.sub.3).dbd.CH--S--CH.sub.2 --CH.dbd.CH--CH.sub.2 OH, 
by reaction of sodium isobutenyl mercaptide with Br--CH.sub.2 
--CH.dbd.CH--CH.sub.2 OH. 
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 
250.degree. C., when admixed with the polymer in at least one weight ratio 
within the specified 20/80 to 80/20 range; 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 
d. are readily made or commercially available. 
Alkoxy-alkyl esters, such as, for example, those described in U.S. Pat. No. 
3,092,612 have all of the foregoing advantages a-d, and accordingly are a 
preferred group of esters. Among this group, butoxyethyl laurate and 
butoxyethyl 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 ("dioctylphthalate"), 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 polyethylene/ester weight ratios are from about 20/80 to about 
80/20. In general, membrane permeability will decrease and selectivity 
will increase as the proportion of polymer in the spinning solution is 
increased. The former effect is particularly evident for processes 
requiring separation of gases from (or introduction of gases to) liquids. 
In general, the viscosity of the spinning solution and the strength of the 
fibers produced will increase as the proportion of polymer 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 about 30 to about 70 weight percent polymer in the 
polymer/ester mixture is preferred. 
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.degree. above the cloud point of the 
solution and at least 10.degree. 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 solution 
behavior during and subsequent to issuance from the spinnerette are 
familiar to those skilled in the art 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 instances, the minimum useable spinning temperature may be 
determined by the safe working pressure of the equipment used to force the 
polymer solution through the spinnerette at the required rate. That is, 
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 about 100.degree. to 250.degree. C. 
will usually be suitable for the polyethylene/ester mixtures (solutions) 
employed in the present process. Temperatures of from about 180.degree. to 
about 220.degree. are preferred, however, 
Cooling and Initial Drawing 
The initial draw is made, to a ratio of from about 1.0/1 to about 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 about 1.01 to about 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 operation 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 fibers 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 thickness 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. 
A wide variety of fluids are suitable for cooling, since temperatures 
outside the range of about 0.degree. to 100.degree. C., will usually not 
be employed. Such fluids of course should be poor solvents or non-solvents 
for the extruded fiber materials. Cooling fluid temperatures within the 
range of about 10.degree. to 50.degree. C. 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 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 to 40/1), while the polymer and ester 
mixture is phasing 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 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. 
Temperatures of from about 50.degree. to about 110.degree. 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.degree.-30.degree. C.) 
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 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 
substantial 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 occurs prior to the leaching (ester removal) step. 
Heating may be carried out in the preceding manner or, preferably, in other 
ways. For example, the cooled, 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 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 100.degree. C. 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 necessarily have to be in contact with any medium during 
heating and/or drawing. If desired, the fiber two may be heated by 
irradiation and drawn while passing through or being taken up in an 
evacuated zone. 
Ester Removal 
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 miscible with the medium. The medium preferably is capable of 
dissolving the ester but should be a non-solvent for the polymer component 
of the fibers, since ester removal will generally require substantial 
contact duration. 
Examplary 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 instable or detrimentally reactive may be 
employed to effect ester removal. The suitablity 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 the medium to within about 10 degrees of the flow 
point of the polyethylene. Temperatures of from about 20.degree. to about 
50.degree. C. are preferred. Temperatures above 50.degree. C. 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 period of time. However, the rate of removal 
generally decreases as the concentration of the ester in the fiber 
decreases 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., to remove the ester, before the device is shipped or 
put into use.

EXAMPLES 
The following examples illustrate specific embodiments in the practice of 
the present invention. 
Typical Procedure 
Stage 1 
A solution of the polyethylene and the 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 10 
seconds, drawing over another 10 second interval to the desired length, 
waiting about 10 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. 
Stage Ommission or Sequence 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 10 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 provided 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 average 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 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 cm.sup.2 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 prepared from polyethylene 
(Dow high density P.E. resin 70065, melt index 0.70, density 0.965) having 
a tensile yield of 4550 lb.f/in.sup.2, a shore D hardness of 65, an IZOD 
impact of 2.0, and a Vicat softening pt. of 127.degree. C.) 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 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, 3 means step 2 was omitted and1, 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. 
3 TABLE I 
DEPENDENCY OF PERMEABILITY ON HOT-DRAWING AND THEN LEACHING SPINNING & 
DRAWING CONDITIONS COMPOSITION Initial Overall FIBER DIMENSIONS(.mu.) P 
ERMEABILITIES.sup.5 Wt.% Stage Spin Draw PSD Draw Initial.sup.4 
Final.sup.5 % Fiber P.E..sup.1 Ester Sequence Temp..sup.2 Ratio Temp. 
Ratio I.D. O.D. Wall I.D. O.D. Wall Gas GPR Liquid UFR Rej. 
PE-1 26.2 B.E.O..sup.3 1,3 280 2.16 -- 2.16 317 537 110 331 531 100 
O.sub.2 nil 1-A 1,2 2.16 100 13 197 279 41 O.sub.2 /N.sub.2 
1.24.times.10.sup.-4 1-B 1,2,3 2.16 100 13 197 279 41 O.sub.2 
/N.sub.2 7.22.times.10.sup.-4 H.sub.2 O.sup.6 1.82 PE-2-1 50.0 B.E.O. 
1,3 270 6 6 390 556 83 360 510 75 O.sub.2 nil 
1A 1,3,2 6 100 33 166 227 30 O.sub.2 /N.sub.2 1.7.times.10.sup.-5 
H.sub.2 O.sup.6 0.78 H.sub.2 
O.sup.7 0.67 1B 1,2,3 6 100 33 179 229 25 O.sub.2 
/N.sub.2 1.0.times.10.sup.-3 H.sub.2 O.sup.6 3.59 
H.sub.2 O.sup.7 2.47 PE-5A-1 50.0 B.E.O. 1 200 7 -- 7 401 577 88 401 577 
88 Air 1.9.times.10.sup.-7 
2 1,3 7 7 1.3.times.10.sup.-6 
3 1,2,3 7 100 35 248 304 28 O.sub.2 7.0.times.10.sup.-3 H.sub.2 
O.sup.6 83.4 H.sub.2 
O.sup.7 36.0 95.1 4 1,2,3 7 100 105 172 204 16 
O.sub.2 5.2.times.10.sup.-4 H.sub.2 O.sup.6 0.6 H.sub.2 
O.sup.7 0.2 
5 1,2,3 7 83 35 248 304 28 O.sub.2 5.0.times.10.sup.-3 H.sub.2 
O.sup.6 50.7 H.sub.2 O.sup.7 20.3.about.100 PE-5B-1 50.0 
B.E.O. 1 200 1.4 -- 1.4 755 1079 162 Air 1.4.times.10.sup.-7 
2 1,3 1.4 -- 1.4 5.0.times.10.sup.-6 
3 1,2 1.4 83 7.0 317 469 76 7.2.times.10.sup.-7 
4 1,2,3 1.4 83 7.0 7.6.times.10.sup.-3 H.sub.2 O.sup.6 147. 
PE-5C-1 50.0 B.E.O. 1 200 1.4 -- 1.4 765 1091 163 
Air 7.6.times.10.sup.-8 See-2 1,3 1.4 -- 1.4 4.8.times.10. 
sup.-8 Note-3 1,2 1.4 83 7.0 328 486 79 5.1.times.10.sup.-7 B -4 
1,2,3 1.4 83 7.0 2.7.times.10.sup.-3 PE-7-1 50.0 B.S..sup.9 1,3 
200 6.0 83 6.0 331 507 88 331 507 88 O.sub.2 9.2.times.10.sup.-7 H.sub.2 
O 0.332 
2 1,3,2 83 18 259 357 49 5.3.times.10.sup.-6 1.75 
3 1,2,3 83 18 238 334 48 9.0.times.10.sup.-3 148. PE-17 
A-1 50.0 DOP.sup.10 1 200 10 -- 10 297 479 91 O.sub.2 nil 
A-2 1,3 200 10 -- 10 297 479 91 297 479 91 O.sub.2 1.3.times.10.sup 
.-5 H.sub.2 
O.sup.6 1.6 B-1 
1,2,3 200 7 83 32 376 552 88 376 552 88 O.sub.2 4.4.times.10.sup.-4 
H.sub.2 
Notes for Table I: 
.sup.1 Polyethylene 
.sup.2 Temperature of spin block, .degree. C. 
.sup.3 Butoxyethyl oleate 
.sup.4 After initial draw and before leaching; I.D. = Internal Diameter, 
O.D. = Outside Diameter. 
.sup.5 After last stage listed hot-draw (2) or leach (3) 
.sup.6 Distilled water 
.sup.7 2% aqueous albumin 
.sup.8 No quench bath employed. Air cooled only. 
.sup.9 Butyl stearate 
.sup.10 "Dioctyl" phthalate (Di-(2-ethylhexyl)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-1, 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 
1.7.times.10.sup.-5 c.c. per cm.sup.2 per second per cm. Hg. transmembrane 
pressure. 
It is also apparent, from the results given in Table I, for fibers PE-17 
A-1, 2 and B-1, that a significant improvement in permeability results 
when hollow fibers prepared from polyethylene and dioctyl phthalate, in 
accordance with the teachings of U.S. Pat. No. 3,423,491, are post-spin 
drawn before being leached to remove the "plasticizer". 
EXAMPLE 2 
Hollow fibers were spun at 200.degree. c. from a mixture of BEO (47 wt. 
percent) and Dow high density polyethylene resin 85965 (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.mu., O.D. 480.mu., wall thickness 50.mu.; 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 (83.degree. C.) water for 10 
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 25.degree. C.) from both the drawn and 
undrawn fibers with methylene chloride, 1,1,1-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. 
TABLE II 
______________________________________ 
DEPENDENCY OF LEACHING RATES ON FIBER 
DIMENSIONS AND LEACHING MEDIUM 
Percent of Original BEO Content Left in Fibers 
Undrawn Drawn 
(50.mu.W.T..sup.1) 
(23.mu.W.T.) 
Fibers Fibers 
Leach Time 
1/4 hr. 1 hr. 16 hrs. 
1/4 hr. 
1 hr. 16 hrs. 
______________________________________ 
CH.sub.2 Cl.sub.2 
10.0% 6.5% 5.5% 5.5% 5.4% 5.4% 
CH.sub.3 -CCl.sub.3 
24.0 17.0 12.1.sup.2 
10.0 8.6 7.3.sup.2 
CH.sub.3 -CH.sub.2 OH 
-- 22.0 -- -- 13.5 -- 
______________________________________ 
NOTES: 
.sup.1 W.T. = Wall Thickness 
.sup.2 When the fibers which had been leached with trichloroethane for 16 
hours were further leached with methylene chloride for 15 minutes, the 
residual BEO levels were reduced to .about.5.6% in each instance. 
EXAMPLE 3 
Replicate lengths of fibers designated in Table I as 5-A-1, 5-B-1 and 5-C-1 
(none of which were drawn after spinning), 5-B-3 and 5-C-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. 
Permeabilities of all the leached fibers were determined as in Example 1, 
the fiber lengths having been mounted in beaker U.F. units. The dimensions 
of the fibers before leaching are included with the BEO residuals and 
permeabilities in Table III-A. 
TABLE III-A 
__________________________________________________________________________ 
EFFECTS OF POST-SPIN DRAWING AND LEACH MEDIUM 
ON RESIDUAL BEO CONTENT AND PERMEABILITY 
Post 
Spin 
Dimensions Percent of 
Leach Draw 
Before Leach 
Original BEO 
# Medium 
Time Fiber Ratio 
I.D. 
O.D. 
W.T. 
Content Left 
G.P.R. 
__________________________________________________________________________ 
1 ETOH 24 hrs. 
PE-5A-1 
-- 401 577 
88 5.1% 6.6.times. 10.sup.-6 
2 CH.sub.2 Cl.sub.2 
4 " -- " " " 2.4 1.6.times.10.sup.-5 
3 EtOH 24 PE-5B-3 
5:1 317 469 
76 3.7 3.9.times.10.sup.-4 
4 CH.sub.2 Cl.sub.2 
4 " " " " " 1.6 1.0.times.10.sup.-3 
5 CH.sub.2 Cl.sub.2 
4 PE-5C-3 
" 328 486 
79 -- 2.7.times.10.sup.-3 
6 EtOH 24 Pe-5B-1 
-- 755 1079 
162 3.7 8.2.times.10.sup.-6 
7 CH.sub.2 Cl.sub.2 
4 " -- " " " 3.8 1.4.times.10.sup.-5 
8 CH.sub.2 Cl.sub.2 
4 PE-5C-1 
-- 765 1091 
163 -- 4.5.times.10.sup.-6 
__________________________________________________________________________ 
To facilitate comparisons, the date 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 size and in not having been post-spin drawn. 
TABLE III-B 
__________________________________________________________________________ 
Factors Residual Ratio 
Compar- 
Held Leach 
BEO of 
ison Constant 
Varied # Level 
GPR GPR's 
__________________________________________________________________________ 
A Fiber Leach 7 3.8 1.40.times.10.sup.-5 
1.71 
size. medium 
Not and time. 
6 3.7 0.82.times.10.sup.-5 
drawn. 
B Fiber Leach 2 2.4 1.60.times.10.sup.-5 
2.43 
size. mdium 
Not and time. 
1 5.1 0.66.times.10.sup.-5 
drawn. 
C Fiber Leach 4 1.6 1.0.times.10.sup.-3 
2.50 
size. medium 
Not and time. 
3 3.7 0.4.times.10.sup.-3 
drawn. 
D Leach Drawn 3 3.7 39.00.times.10.sup.-5 
59.1 
medium 
and time. 
Fiber Not drawn 
1 5.1 0.66.times.10.sup.-5 
size. 
E Leach Drawn 4 1.6 10.times.10.sup.-4 
62.5 
medium 
and time. 
Fiber Not drawn 
2 2.4 0.16.times.10.sup.-4 
size. 
F Leach Fiber 6 3.7 8.2.times.10.sup.-6 
1.24 
medium 
size 
and time. 
Not drawn. 1 5.1 6.6.times.10.sup.- 6 
G Leach Fiber 7 3.8 1.4.times.10.sup.-5 
0.88 
medium 
size 
and time. 
Not drawn. 2 2.4 1.6.times.10.sup.-5 
H Leach Drawn to 
3 3.7 39.0.times.10.sup.-5 
47.6 
medium 
76.mu. 
and time. 
Not drawn 
6 3.7 0.82.times.10.sup.-5 
162.mu. 
I Leach Drawn to 
4,5 est. 185.times.10.sup.-5 
200. 
medim 77.5.mu. avg. 
1.6 average 
and time. 
Not drawn 
7,8 est. 0.925.times.10.sup.-5 
162.5.mu. avg. 
3.8 average 
__________________________________________________________________________ 
Taking comparison A, B, C, F and G as one group and contrasting D, E, H and 
I as another group, it is clearly evident that post-spin drawing before 
leaching has a much more beneficial effect on permeability 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 by countered to some extent by 
the concurrent reduction in 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 O.sub.2 gas permeabilities, as above 
defined, of at least 2.times.10.sup.-5. 
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. 
Scanning electron micrographs of membranes of this invention reveal an 
internal structure of interconnected porous domains. The size of these 
porous domains varies from a maximum of .about.1 micron to the smallest 
resolveable by the microscope of .about.0.1.mu.. 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. These photographs reveal that very few, if any, 
of these porous 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 estimating the effective pore size 
of a membrane is by ultrafiltration experiments. A solution of a solute of 
know molecular size is pressurized against one side 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 is: 
##EQU1## 
where C.sub.P is the concentration of solute in the permeate, and C.sub.B 
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) reject &gt;99.9% 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 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, a radius of about 50A appears to be reasonable maximum for the 
latter type of fibers.