Note: Descriptions are shown in the official language in which they were submitted.
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Back~round of the Invention
The present invention relates to microporous
hollow polypropylene fibers and a method for preparing the
same.
Hollow porous fibers are well known in the art.
The advantages of perm~able hollow ibers over permeable
films are also well known. For example, porous hollow
fibers possess a larger surface area per unit volume than
a flat film of similar porous configuration. Consequently,
there has been a growing tendancy to employ, if possible,
hollow microporous fibers in those applications typically
reserved for permeable films.
While the technology for preparing and imparting
permeability to hollow fibers and films may at first glance
appear to be similar in many respects, there are processing
differences peculiar to each technology which lead ~o signi-
ficant unpredictable results in permeability performance so
as to preclude the wholesale application of film technology
to hollow fibers.
For example, U.S. Patent No. 3,80l,404 dascribes a
cold stretch/hot stretch process for praparing polypropylene
microporous ~ilms which includes the steps of extruding a
precursor film by a blown film method at a temperature of
about 180 to 270C and taking the film up at take-up speeds
of about 30 to 700 ft/min (i.e., 9 to 213 meters/min1 and at
a drawdown ratio of 20:l to 200:l. The precursor film is
then optionally annealed, cold stretched at a temperature
below abou~ 120C e.~., 25~C, hot stretched at a temperature
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above 120C and below the polymer fusion temperature e.g.,
between 130 and 150C, and finally heat set at a temperature
in the range of about 125C up to less than the polymer
fusion temperature e.g~, abou~ 130 to 160C.
If extrusion or melt spinning ~emperatures of
increasingly less than 230C are employed in preparing the
hollow microporous polypropylene fibers o~ the present
invention the structural uniformity of the hollow fibers
with respect to inner and outer diameters as well as wall
thickness is substantially reduced. Thus, the melt spinning
temperature of the hollow fibers represents a process para-
meter which is not readily translated from film to fiber
technology due to the inherent differences between fiber and
film configuration, i.e., the concept of uniformity of inner
and outer diameter fiber dimensions does not exist in film
technology. The melt index of the polypropylene employed to
prepare the subject hollow fibers must also be controlled to
preserve the ~tructural and dimensional uniformity of the
hollow fiber.
Similarly, while the preparation of a film by the
blown film method resembles the preparation of a hollow film
by the air injection method there is an important difference~
The take-up speed in preparing blown presursor films is
, limited in that at increasingly higher take-up speeds (other
i~ processing variables being held constant) the uniformity o~
the microporous film structure subsequently imparted to the
precursor film decreases. Consequently, the process variable
of take-up speed cannot be controlled (i.e., increased) in a
manner sufficient to impart increasin~ly higher levels of
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orientation to the precursor film structure. Higher degrees
of orientation up to a certain threshold have a beneficial
effect on the ultimate permeability imparted to the film but
because of problems of uni~ormity of pore structure a limit
is placed on the degree of orientation which can be imparted
via take-up speed control to the precursor film.
When hollow fibers are prepaxed by the process of
the present invention the problems of microporous structural
uniformity are substantially reduced and higher take-up
speeds can be employed to impart a higher degree of orien-
tation, as determined by the wide angle x-ray dif~raction of
a (110) plane, to the precursor hollow fibers, thereby
imparting a higher permeability potential therëto. However,
this increase in permeability potential is not readily
realized in the absence of controlling the inner diameter
(I.D.) of the hollow precursor fiber which in turn controls
the inner diameter of the subsequently formed hollow micro-
porous fiber. Control of the dimensions of the hollow
~microporous fiber is a further process variable which
cannot be translated from film technology.
:
U.S. Patent No. 3 558,764 describes a cold stretch
process for preparing microp3rous films which includes the
steps of extruding a polymer at specifically defined tempera-
tures to form a precursor film, cooling the precursor ~ilm,
annealing the precursor ilm at specifically defined tempera-
tures (i.e., 5 to 100C below the melting point of the
polymer whîch is about 16SC for polypropylene), cold drawing
the resulting film at a specifically defined draw ratio and
temperature, and heat setting the cold drawn fîlm at a
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temperature of about 100 to about 150C while under tension.
The primary difference in this process from the former cold
stretch/hot stretch process is the absence of a hot stretch-
ing step. The cold stretch/hot stretch process described
above represents an improvement over the cold stretch process
of this patent with respect to nitrogen flux.
In contrast, when hollow precursor fibers are
annealed, cold stretched, and heat set generally in accord-
ance with the above cold stretch procedures, particularly
when the heat set temperature is at or below the initial
annealing temperature, the resulting hollow microporous
fibers exhibit varying degrees of shrinkage and tend to curl
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up which is disadvantageous depending on the use for which
the hollow fib r is employed.
,. :
.S. Patent No. 4,055,696 describes a similar cold
stretch process which is employed to prepare hollow polypro-
pylene microporous fibers rather than films. This process
requires that the size of the pores be kept within a specified
range by limiting the degree and temperature of cold stretch
to 30 to 200% of the originaI fiber len~th and less than
100C respectively. The resulting cold stretched fibers
whlch have been previously annealed are heat set at a tempera-
ture at or above the initial annealing temperature, employed
prior to stretching as described above. A separate hot
stretching step as employed in the present invention is not
incladed in the preparation of these hollow fibers. Annea~edf
cold stretched, heat set, hollow fibers prepared in accordance
with this patent tend to exhibit varying degrees of shrinkage
depending on the relationship of the prior annealing tempera-
J~
ture an~ duration to the heat setting temperature andduration. Moreo~er, there is no control of the inner diameter
of the hollow fibers of this patent to improve oxygen gas
permeability thereof.
Japanese Kokai Patent No. Sho 53 ~1978] - 38715
published April 10, 1978 is directed to an improvement in
the method for preparing porous polypropylene hollow ~ibers
disclosed in U.S. Patent No. 4,055,696. The improvement
comprises controlling the annealing temperature to be below
155C and controlling the heat setting temperature after
cold stretching to be from lS5 to 175C for from 3 seconds
to ~0 minutes. This process also fails to employ either a
hot stretchin~ step in addition to the cold stretching step
as required by the present invention or control o the inner
diameter of the hollow microporous fibers to improve oxygen
gas permeability.
One particular important use for hollow microporous
fibers is as a blood oxy~enator as illustrated by V.S.
Patent No. 4,020,230 which discloses hollow microporous
flbers prepared from polyethylene. As is well known the
properties required in a blood oxygenator membrane include
good gas permeability with respect to gaseous oxygen and
carbon dioxide, chemical stability, blood compatibili~y or
substantially non-thrombogenic behavior in blood containing
environments, suficiently hydrophobic character to serve as
a water vapor barrier, ease in manufacture, non-toxicity,
relative inertness to body fluids, and mechanical strength
and handling properties adequate for facilitating the assembly
and use of blood oxygenation devices.
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Microporous polypropylene films have previously
been employed as blood oxygenation membranes and such films
ha~e been found to meet all of th~ ~bove requirements. How-
ever~ because of the relatively low surface area of such
films, relatively large volumes of blood must be removed
from the body to achieve the required oxygen and carbon
dioxide gas transfer. In contrast, hollow polypropylene
microporous fibers offer the advantage of being able to
~ achieve the same gas transfer using much lower volumes of
~ blood.
~ There has therefore been a continuing search for
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hollow polypropylene microporous fibers and a process for
preparing the same which exhibit a hiyh oxyqen-gas perme-
: ability. ~he present invention is a result of this search.
~ ~ It is therefore an object of the present invention
:~,
to pro~ide microporous hollow polypropylene fibers which
have high oxygen ga:s pexmeabilities.
: . ~
: It is a further object of the present invention to
provlde a process:for preparing hollow polypropylene micropor-
ous flbers having high oxygen gas permeabilities.
These and other objects and features of the inven-
tion will become apparent from the claims and from the
, :
~ following description when read in conjunction with the
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accompanying drawing.
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Summary of the Invention
In one aspect of the present invention there is
provided a pxocess for preparing hollow open celled poly-
propylene misroporous fibers having an oxygen flux of at
least 3S cc/cm2.min at 10 psi comprising:
(A) melt spinning at a temperature of at least
230C isotactic polypropylene having a melt index of at
least 1 in a manner sufficient to obtain hollow non-porous
polypropylene precursor fibers, takin~ up said precursor
fibers at a drawdown ratio of at least about 40, said melt
s~inning being conducted in a manner sufficient to impart to
said precursor fibers after take-up an avarage inner diameter
of at least about 140 microns, an average inner diameter to
average wall thickne~s ratio of from about 8:1 to about
40:1, and a degree of orientation as determined from the
:half width of the wide angle::~llO) X-ray diffraction arc, o~
not greater than 25, and an elastic recovery from 50%
;extension at 25C, 65~ relative humidity, and zero recovery
time, of at least 50~;
(B) :annealing the precursor fibers at a temperature
between about 50C and less than 165C for a period of about
.5~ second to about 24 hour~s;
; (C) cold stretching the non-porous precursor
hollow fibers ln the direation of their length at a tempera-
ture greater than the glass transition temperature of the
precursor fiber and not greater than about 100C to impart
porous surface regions to the walls of the fiber which are
perpendicular to the cold stretching direction;
(D) hot stretching the annealed cold stretched
hollow fibers of (C) in the same direction of cold stretch
at a temperature above the cold stretching temperature and
below the melting point o the polypropylene to impart an
open celled microporous configuration to the hollow fiber
walls, said cold stretching and hot stretching being conducted
in a manner sufficient to control the average inner diameter
of the resulting hot stretched hol~ow microporous fibers to
: be at least 100 microns and to achieve a total degree of
combined stretching of rom a~out 80 to about 200%, an
extension ratio of from about 1:3 to about 1:20, and a
strain rate of from about 10 to about 200%/minute;
(E) heat setting the resulting hot:stretched
: : ~ fibers of (D) under tension to produce dimensionally stable
open celled hollow microporous fibers having an average
; ~ ~ inner diameter of at least 100 microns.
:: In another aspect of the present invention there
: lS provided hollow polypropylene open celled microporous
fibers ha~ing an oxygen flux of at least 35 cc/cm2.min at 10
: psi prepared by the above process.
Brief Descri~tion of the_Drawing
The Figure is a schematic representation of a
means for achieving hot stretchins in multiple stages.
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Description of the Preferred Embodiments
The hollow microporous fibers of the present
inYention are prepared from isotactic polypropylene having a
weight avera~e molecular weight ranging from about 100,000
to about 750,000, and a melt index of not less than about 1
(e.g., not less than about 53, typically from about 1 to
about 30 or higher, preferably from about 3 to about 15 and
most preferably from about 5 to about 10.
The term melt index as used herein is defined as
the value obtained by performing ASTM D-1238 under conditions
of temperature, applied load, timing interval and other
operative variables which are specified therein for the
particular polymer being tested, i.e., polypropylene.
If a fiber is prepared from polypropylene having a
melt index increasingly below about 1, e.g., 0.5, the fiber
exhibits an increasiny tendency to break or split and an
increasingly greater fluc~uation in the uniformity of the inner
diameter and cross section of the fiber.
The density of the polypropylene should be about
0.90 ym/cc.
The isotactic polypropylene is converted to a
hollow precursor iber by melt spinning. The molten polymer
is caused ta flow through one or more orifices (i.e., jets)
of a spinneret which is capable of imparting the desired
continuous hollow configuration to the ~iber. For instance,
in the preferred embodiment the melt is caused to flow
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throuyh one or more annular dies having a needle extending
into each central portion thereof. A gaseous stream is then
passed through the needle as the melt is pumped through the
annular die thereby imparting a hollow configuration to the
fiber. Alternatively, the hollow lumen o~ the fiber may be
formed by passing the molten polymeric material through an
annular orifice or a solid core capable of causing the
desired hollow structure to be formed.
Th~ ~emperature at which the polypropylene is ex-
truded, i.e., melt spun (assuming other spin variables as
described herein are employed) should be at least 230C
preferably from about 240 to about 280C, and most preferably
from about 240 to about 250C.
If an extrusion temperature increasingly below
about 230C is employed the uniformity of the ~iber with
respect to inner and outer diameters increasingly deteriorates.
In contrast precursor films employed in the cold stretch~hot
stretch process of U.S~ Pa ent No. 3,801,404 can be prepared
at extrusion temperatures as low as 180C. At extrusion
temperatures increasin~ly higher than about 280C the spin
stress~applied to the extruding polymer must be substantially
increased, and there is a danger of polymer degradation.
When an air injection hollow fiber spinneret is
employed the jet diameter, air flow rate, take-up speed,
:,
-~ extrusion velocity, and drawdown ratio are controlled in a
manner sufficient to achieve a precursor hollow polypropylene
~::
fiber having an average inner diameter and average wall
: thickness with dimensions as described herein and a degree
o orientation of not greater than about 25 as determined
~ ~ f_
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by the half width of a tllO) wide angle X-ray diffraction
arc.
The degree of fiber molecular orientation is
determined by superimposing the fibers in alignment to a
thickness of 50 mg/cm . X-rays are then irradiated to ~he
fibers in a direction perpendicular to the axial direction
of the fibers and the half-width of a (110) wide angle
diffraction arc is recorded on film. The angular spread of
this (110) diffraction arc is then determined and should not
he greater than 25~.
The dimensions (i.e., inner and outer diameters)
and wall thickness of the hollow fibers produced can be
i~ controlled in several ways. Initially, the diameter of the
die and inert gas pressure selected will govern the inner
and outer dimensions respectively of fibers produced, as
modified by the degree of enlargement of fiber dimensions by
release from the metered pressure of extrusion through the
spinneret~ Diameter and wall thickness can also be varied
by varying the pressure of extrusion through the spinneret
and the take-up speed at which the fibers are drawn away
from the spinneret. Changes in one of these values can be
compensated for by changes in the other to achieve the
desired results.
While the above processing parameters are controlled
;~ with a view toward achieving an inner diameter within a
limited range they are also controlled to impart the proper
morphology to the precursor fiber to insure that subseguent
processing achieves a microporous structure having a suitable
gas pe~meability.
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Accordingly, the melt spinning or melt extrusion
step of the process is conducted at a relatively high "draw-
down" or "spin draw" ratio so that the hollow fihers are
spin-oriented as they are formed. Drawdown ratio is defined
as the ratio of the velocity of initial take-up speed of the
hollow fibers to the linear velocity of extrusion of the
polymer through ~he spinneret orifice. The drawdown ratio
used in the process of the present invention is at least 30,
perfexably at least 40, (e.g., from about 40 to about lO0~
and may be as high as about 700. Take-up speeds employed to
accomplish the requisite drawdown ratios are generally at
least about 200 meters/minute, typically from about 200 to
about 1000 meters/minute and preferably about 200 to 500
meters/minute. Typically high shear forces are developed in
the polymeric material which are not relaxed prior to fiber
solidification.
The air flow rate i.e~/ the rate at which the air
is passed throuyh the needle in the central portion of the
et hole will vary depending on the number of jet holes in
the spinneret and is typically controlled to be from about
5 to about 70 cc~min/jethole, and preferably from about 10
to about 50 cc/min/~ethole.
The temperature o the air as it exits the air
injection spinneret is typically the same temperature as the
melt spinning temperature of the polymer.
The hollow spin oriented precursor fibers may
optionally be quenched by passing them through a current of
gas r such as ordinary air at room temperature or through an
inert liquid such as water so that rapid cooling of the
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just-spun hollow fiher results. The temperature of the
quenchin~ medium can be as high as 80C and as low as 0C
(e.g., 0 to 40C) depending on other spinning parameters.
However, the preferred quench temperature is 25~C and passage
of the just-spun fibers through ambient air results ln an
adequate quench when the take-up roll is located about 5 to
about lO feet or more from the spinneret.
The resultin~ precursor hollow polypropylene fiber
is non-porous and exhibits a crystallinity of at least 30%,
preferably at least 40%, and most preferably at least 50%
le.g., about 50 to about 60~ or more). Percent crystal-
linity i5 determined from the relatiGnship:
... .
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% crys~allinity = Va - V x 100
;~ Va - Vc
wherein Va is the specific volume of the 100~ amorphous
polymer, Vc is specific volume of the 100% crystalline
polymer and V is the specific volume o~ the sample o~ interest.
The specific volume of a polymer is 1/D where D is the
density of the polymer. The den~ity of the polymer is
~ ~ measured by means of a density gradient column as described
;~ ; in ASTM D-1505-68. The precursor hollow polypropylene
fibers should also exhibit an elastic recovery at zero
~` recovery time when subjected to a standard strain (exten-
sion~ of 50% at 25C, and 65~ relative humidity, of at least
about 50%, preferably at laast about 60%, and most prefer-
ably at least about 65%.
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Elastic recovery as used herein is a measure of
the ability of structured or shaped articles, such as hollow
fibers, to return to their original siæe after heing stretched.
The elas~ic recovery value is determined with an
Instron Tensile Te~ter operating at a strain rate of 100%/minute.
After the fiber is extended to the desired strain value, the
jaws of the apparatus are reversed at the same speed until
the distance between them is the same as at the start of the
test, i.e. the original gau~e length. The jaws are again
immediately reversed, and are stopped as soon as the stress
begins to increase from the zero point. The elastic recovery
is then calculated as follows:
~:
Elastic Recovery = (Total length Final Distance) x 100
When Extended - Between Jaws
Length Added When Extended
Measurements with the Instron Tensile Tester are conducted
at room temperature, e.g. 25C., in air at 65 per cent
relative humidity~
a~ 4rk
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Althou~h a standard strain of 50% is used to
identify the elastic properties of the percursor fib~rs,
such strain is merely exemplary. In general, such precursor
fibers will have elastic recoveries higher at strains less
than ~0% and somewhat lower at strains substantially higher
than 50%, as compared to their elastic recovery at a 50
strain.
The above processing conditions are controlled to
provide precursor hollow polypropylene fibers having an
average inner diameter (I.D.~ of at least 140 microns,
preferably from about 140 to a~out 400 microns or higher
most preferably from about 200 to about 30~ microns. The
above averase inner diameters have been found to b~ necessary
for impartin~ a high gas permeability potential to the
precursor hollow fibers. Where high gas permeability is not
a controllin~ factor in the desired end use of hollow fibers
the inner diameters may be reduced to below 140 micr~ns.
he dimensions of the hollow fibers are expressed
as an average value since such dimensions will Yary to some
extent depending on where, along the fiber length, the
~:
dimensions are determined. ~onsequently the average inner
and outer diameters are determined by cutting cross sections
; of the fiber at 6 inch intervals for a total of 5 intervals
along the fiber length and determining the fiber dimensions
~: ~
at each of these intervals. The fiber sections are then
immersed in standard optical immersion oil and the dimension
:
~; ~ at each interval is determined using an optical microscope
and optical scaling. The results are then averaged to
determine the average inner and outer diameters.
.
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The minimum wall thickness of the precursor hollow
fibers should be sufficient so as to not be readily ruptured
or otherwise undergo physical deterioration at a rate that
would make their use unattractive after they have been
rendered microporous by the procedures described hereinO
The maximum wall thickness of the hollow fibers is limited
by the degree of permeability sought to be imparted to the
final product.
The measurement of average wall thickness is
accomplished by determining the average outer diameter and
average inner diameter of the fiber and taking as the wall
thickness one half of the difference in these average diameters.
Furthermore, the average wall thickness may be
expressed as a function of ~he average inner diameter of the
hollow fiber. The ratio of the average inner diameter of
the hollow precursor fiber to its average wall thickness can
ay from about 8:1 to about 40:1, preferably from about
10:1 to about 30:1 and most preferably from about lO:l to
about 20:1. More specifically an average precursor fiber
wall thickness of at least lO micxons and typically from
about 10 to about 25 microns is preferred.
While it is the inner diameter, and associated
wall thickness, of the final microporous hollow fiber product
::`:
~ which are believed to be a primary controlling factors of
~ ~ :
~ gas permeabiIity it is the inner diameter and wall thickness
~. ~
of the precursor hollow fiber which predetermines the maximum
obtainable inner diameter and wall thickness of the final
product. ~his results from the fact that the inner diameter
of the precursor fiber shrinks about 25% when subjected to
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~;72~
the two-stage stretching process described herein. The
average wall thickness of the hollow microporous fibers
remains substantially unchanged by processing in comparison
to the precursor fiber although it may be reduced to a small
extent.
The hollow precursor fibers are next subjected to
a heat treatment or annealing step in which the amount of
crystallinity and/or their crystal structure is improved.
More specifically, this step of the process increases crystal-
lite size and removes imperfections in the molecular align-
ment. The annealing is conducted for a balance of time and
temperature so as to achieve the desired improvements as
described above and yet sufficient to avoid destroying or
adversely affecting the precursor polymer structure (e.g.
orientation and/or crystallinity).
The preferred annealing temperatures can vary from
about 130 to about 145C, for a period of about 30 minutes.
As the annealing temperature is increased above about 145C
the time during which the precursor fiber is annealed i5
accordingly reduced. Conversely, as the annealing tempera-
ture decreases below 130C., increasingly longer annealing
i ,
times axe employed.
If the annealing temperature increasingly exceeds
145C. at an annealing time of 30 minutes, the precursor
poly~er fiber structure will be adversely affected and the
gas permeability potential of the precursor fiber will be
increasingly reduced. If the annealing temperature is
increasingly less than 130C. for 30 minutes, the gas perme-
ability potential of the precursor fiber will also be increas-
ingly reduced. ~
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In view of the above, the annealing is conductedfor periods of about 0.5 second to about 24 hours at a
temperature of from about 50 C. to less than the melting
point of the polypropylene (i.e., 165C based on differential
scanning calorimetry).
The annealing step may be conducted in a tensioned
or tensionless state by depositing the precursor fiber in a
static conditon in a heating zone which is maintained at
the requisite elevated temperature, or by continuously
passing the precursor fiber throu~h the heating zone. For
example, the elevated temperature may be accomplished by the
use of a conventional circulating air oven, infra-red
heating, dielectric heating, or by direct cont~ct of the
:,
; ~ running fiber with a heated surface which is preferably
curved to promote good contact. The precursor fiber may be
continuously passed thorugh a jacketed tube or shroud which
: .
radiates heat at the desired temperature. Alternatively,
the precursor fiber may be wound under substantially no
stress on a bobbin while undergoing annealing,or simply
placed in the heatin~ zone in a loose state, such as a skein
of continuous fibers. For best results it is recommended
that the hollow fiber be maintained at constant length
~ :~
during the annealing step, i.e., under tension sufEicient to
prevent a longitudinal extension or shrinkage of greater
than abou~ 5%. This can he achi~ved by passing the fibers
~: ~
in the direction of theîr length over and about a first
stress isolation device through a heating zone maintained at
.,
~; the appropriate temperature and than over and about a second
stress isolation device. Each stress isolation device may
~:
convently take the form of a pair of skewed rolls. Control
of the ratio of the surface speeds of the two sets of rolls
permits isolation and control of the stress of the fibers
between the rolls as they undergo annealing.
The resulting non-porous precursor hollow fiber is
then subjected to a two-stage stretching process and subse-
quently heat set.
In the first stretching stage referred to herein
as "cold stretching", the precursor hollow fibers are stretched
at a temperature above the glass transition temperature (Tg)
of the precursor fiber and not greater than about 100C.
Typical cold stretching temperatures can vary from a~out 0
to about lOO~C, preferably from about 15 to ab,o,ut 70C, and
conveniently at room temperature, e.g., 25C. The ~-emperature
of the fiber itself is referred to as the stretch temperature.
It is recognized by those skilled in polymer
technology that the glass transistion temperature (Tg) is
the temperature at which the structure'of a wholly or partially
amorphous polymeric material changes from a vitreous state
to a viscoelastic state. The glass transition temperature
of polypropylene is measured by plotting its specific heat
ayainst temperature and noting ~he temperature at which
there is a change in the slope of the curve. This measure-
ment is commonly termed thermomechanical analysis and can be
carried out with commercially available instruments such as
a Thermomechanical Analyæer Model No~ 990 manufactured by
~u Pont. The glass ~ransition temperature is also referred
to as the second~order transition temperature.
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Cold stretching imparts porous surface regions or
areas to ths fiber wall which are elongated p~rpendicular to
the stretch direction.
The second stretching stage, referred to herein as
hct stretching, is conducted at a temperature above the cold
stretching temperature but less t}lan melting point of the
precursor fiber before or after cold stretching i.e. the
; first-order transition temperature, as determined by differ-
ential scanning calorimeter analysis.
Typical hot stretching temperatures will be greater
than about 100C and can vary from about 105 to about 145C,
preferably from about 130 to about 145C, and most preferably
from about 135 to about 145C. Again the temperature of the
fiber itself being stretched i5 referred ~o herein as the
hot stretch temperature.
When the hot stretching temperature employed is
increasingly less than 130C increasingly higher degrees of
~ shrinkage in the final fiber product are observed.
`~ ~ Hot stretching opens the porous surface regions
imparted by the cold stretching to form an opPn celled
microporous structure.
The stretching in the two stretching stages must
be consecutive, in khe same direction and in that order,
i.e., cold stretched and then hot stretched but may be done
in continuous, semi-continuous, or batch process as long as
p~ the cold stretched fiber is not allowed to shrink to any
significant degree (e.g., not greater than about 10% based
on the initial precursor fiber length).
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The sum total degree of stretching in the above
cold and hot stretching stages can vary from about 80 to
about 200~ (e.g~, about 80 to about 155%), and preferably
from about 85 to about 120% (e.gO about 90%), based on the
initial length of the precursor fibers. When the total
degree of stretch is increasingly less than about 80%, the
resulting oxygen gas permeability at 10 psi is increasingly
less than about 35 cc/cm2. min. The ratio of the degree of
stretch of the first ~cold~ to second (hot) stretching
stages, is referred to herein as the extension ratio. The
extension ratio can vary from about 1:3 to about 1:20 prefer-
ably about 1:3 to about 1:10 (e.g., 1:3 to about 1:5).
It i9 to be understood that the particular total
degree of stretch and extension ratio are selected from the
above ranges in a manner sufficient to control the final
average inner diameter of the hot stretched microporous
~:
fibers within the limits~described herein.
The strain rate, i.e., the degree of stretch per
unit time at whlch the precursor fibers are stretched during
both stretching stages is preferably the same for each stage
and can vary from about 10 to about 200~/minute, preferably
from about I0 to about 100%/minute and most preferably from
about 15 to about 30~/minute ~e.g., about 20~/minute).
At the preferred total degree of stretch of about
80 to about 120% the preferred extension ratio is 1:3 to
about 1:5, e.g., 20~ cold stretched and ~rom about 60 to
about 100% hot stretched.
The cold and hot stretching of the precursor
fibers may be performed in any convenient manner using known
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techniques. For example, the hollow fibers can be stretched
on a conventional draw frame located in a heating zone which
controls the temperature of the fibers during stretching.
Alternatively the fibers may be cold and hot stretched in a
continuous fashion by means of two sets of stress isolation
devices (one set for each stage) similar to those described
in connection with the annealing step.
Accordingly, precursor fibers may be wound several
times about a first pair of skewed rolls, passed throuqh a
heating zone, wherein for example they are contacted with a
suitable heating device or medium and maintained at a suitable
cold stretch temperature and wound several times about a
second pair of skewed rolls. This arrangemen~ permits
solation and control of the longitudinal stress of the
fibers between the two pairs of rolls during cold stretching.
The fibers are ~hen passed through a similar set of paired
skewed rolls while heated to the appropriate hot stretch
temperature. The differential ratio of ~he surface speed of
each of the second pair~of rolls to the surface speed of
each of the first pair of rolls determines the stretch ratio
and strain rate which are adjusted accordingly.
~' 1 :
It is to be understood that in a continuous process
the cOla stretch fibers may undergo shrinkage as they pass
from the cold stretching stage ~o the hot stretching stage.
This can occur as a result of warm-up of the cold stretched
fibers as they enter the hot stretching zone~ such as a
forced hot air oven, but beor~ ~hey are actually hot stretched.
Consequently, it is preferred to insert a tensioning aevice
between the cold and hot stretching stages to prevent shrink-
-23-
age of greater than about 5~ based on the cold stretch fiber
length. Such tensioning device may conveniently take the
form of a single pair of skewed rolls.
The heating zones which heat the precursor fibers
to the a~propriate cold stretch or hot stretch temperature
are the same as described for annealing prior to cold stretch-
ing and may conveniently take the form of a gas such as air,
heated plate, heated liquid and the like. The preferred
heating device is a forced hot air oven ~hich houses the
stretching means.
After the above described cold and hot stretching
operations, the stretched fibers are heat set or annealed
while in the stretched conditon at a temperature from about
125C up to less than the fusion temperature of the polymer~
As is known to those skilled in the art, the fusion temperature
may be determined by a standard diferential scanning calori-
meter or by other known apparatus which can detect thermal
transitions of a polymer. The preferred heat setting tempera-
tures can vary from a'oout 130 to about ~45C. The most
preferred heat setting temperature is the same as the tempera-
.
ture employed during hot stretching. Heat setting does notchange the fiber dimensions as they exist subsequent to hot
stretching.
The heat setting step may be conducted in a batch
process as in an oven or autoclave, or in a continuous
manner. Fox instance, the hollow microporous fibers may be
rewound on a bobbin after hot stretching, and subjected to
the annealing operation in that form. Alternatively, the
hollow fibers may be stretched and heat set in a continuous
; `
~; -24-
.
2~.
pxocedure by means of two pairs of driven rolls downstream
to the stretching rolls traveling at the same speeds with
the material between the rolls continuously passing at
constant length after hot stretching through the heating
zone. Consequently, the stretching and heat setting steps
of the process may be carried out seguentially or they may
be combined in a single in line operation.
The heat setting ~reatment should be carried out
while the fibers are being held under tension, i.e., such
that th2 fibers are not free to shrink or can shrink only to
a controlled extent not greater than about 15% of their
stretched length, but not so great a tension as to stretch
the fibers more than an additional 15%. Preferably, the
tension is such that substantially no shrinkage or stretching
occurs, e.g., less than 5~ change in stretched length.
The period of heat setting treatment which is
.: ~
preferably carried out se~uentially with and after the hot
stre~ching operation, should not be longer than 0.1 second at
the higher heat setting temperatures and, in general, may be
wlthLn the range of about 5 seconds to l hour and preferably
about 1 to 30 minut.es.
Since the most preferred heat set temperature is
the same as the hot stretching tempexature, it is preferred
to conduct botll hot stretching and heat setting in the same
hea~ing means, such as a hot air oven, in which case the
total residence time in the oven for hot stretching and heat
setting steps can vary from about lO to about 45 minutes,
and preferably from about 25 to about 35 minutes (e.g., 35
minutes) for hot stretching temperatures of about 130 to
about 145C.
The function of the heat setting step is to improve
the thermal stability of the microporous structure and
reduce shrinkage of the fibers.
In an alternative embodiment the hot stretching
and heat setting steps may be combined into a single step.
In this embodiment hot stretching is achieved
using a plurality of discrete sequential hot stretching
operations at the appropriate hot stretch temperature. For
example, after the fibers have been cold stretched they are
~` guided into a means capable of stretching the fibers in an
incremental fashion while maintained at the appropriate hot
stretch temperature so that the total degree of stretch of
each increment add,; up to the desirad degree of total hot
stretch.
!
The multiple stage hot stretching means may con-
veniently take the form of a plurality of rolls disposed in
~' an oven. Preferably the rolls are disposed in a festoon
,~ .
configuration similar to that described in United States
Patent No. 3,843,761. The employment of a festoon arrange- -
ment is preferred in that it provides extended exposure time
, in the oven containing the multiple stage hot stretching means
and thereby eliminates the need for any heat setting step
after hot stretching is concluded.
To illustrate a preferred method for achieving
.,i:
multiple stage hot stretching and combined heat setting,
reference is made to the Figure. Non-porous precursor
- 26 -
~ ::
r ,~ ~
fibers 5 which have been annealed are unrolled from a supply
roll 4, over idler rolls 6 and 7 into a cold stretching zone
generally deno~ed at 2. The cold stretching apparatus
includes two pairs of shewed rolls 8-9 and 11-12 which are
driven at peripheral speeds Sl, S2, S3 and S4, respectively,
by suitable driving means 10 and 13 to achieve the desired
degree of cold stretch as described herein. For purposes of
illustration the cold stretch temperature is at room tempera-
ture and no heating or cooling means is required for this
stage. If desired, however, suitable temperature control
means may be supplied as described herein. The cold stretched
fibers~ now denoted 15, are guided into hot stretching means
generally indicated at 3 over one or more idle~ rolls 14.
Nip rolls are not employed since they tend to crush the
hollow fibers which is disadvantageous to the final product.
The hot stretching means 3 comprise a single set of skewed
rolls 16 and 17 and a plurality of additional multiple hot
stretcbing rolls disposed in an oven in a festoon configuration.
In order to minimize the unsupported fiber length between
:
adjoining hot stretch rolls, which is relatively long in the
; preferred festoon arrangement, at least one idler roll is
provided between adjoining hot stretch rolls.
: .
`~ ~ The single set of skewed rolls 16 and 17 helps to
maintain tensioning of the cold stretched fibers by control-
ling the peripheral speeds S5 and S6 respectively thereof.
Tensioning prevents shrinkage, sag and the like caused by
any preheating of the fibers as they pass into the oven but
~;~ before they are hot stretched. Such tensioning helps to
:
:
~ -27-
:
.
~7~
avoid any decrease in cold stretched fiber properties caused
by preheating. Although this tensioning step, to prevent
fiber relaxation, may result in a small amount of stretching,
the primary effect of this procedure is tensioning and the
peripheral speeds S5 and S6 are controlled accordingly by
drive means 18 to maintain constant length between cold
stretching and hot stretching zones. Thus this procedure is
but a preferred embodiment of a means for maintaining fiber
tension prior to hot stretching. Other methods that prevent
fiber relaxation during warm up of the fibers prior to hot
stretching may be employed.
The tensioned cold stretched fibers 15 are then
conveyed downstream over idler rolls l9 and 20 on~o a first
hot stretch ro~l 21. The fibers are hot stretched for the
first time between roll 21 and the second tensioning roll
16. This occurs because the downstream first hot stretch
roll 21 is rotated at a peripheral speed S7 which exceeds
the peripheral speed S5 imparted to the fibers by roll 16.
It should be noted that an idler roll l9 is disposed between
rolls l6 and 21 in order to decrease the unsupported fiber
length during the hot stretching step.
This procedure is continued for as many discrete
steps as may be preferred. For example, the fibers are
stretched for a second time between the first hot stretch
roll 21 and a second hot stretch roll 23. In this second
hot stretch step, the peripheral speed of the second hot
stretch roll 23 is S8. Peripheral speed S8 is greater than
the peripheral speed of S7 of the first hot stretch roll 21.
Thusl the fibers are hot stretched in the second hot stretch
.
. -28-
~ ~ ~t7~
step at a hot stretch ratio of S8/S7. Again/ in order to
minimize the unsupported fiber length at least o~e idler
roll 24 is disposed between the second and third hot ~tretch
rolls 23 and 25. In a preferred embodiment, illustrated in
the Figure, the idler rolls are disposed approximately
midway between adjoining hot stretch rolls.
In the embodiment illustrated in the Figure twenty
stretching steps, which occur sequentially, are provided.
As illustrated in the Figure, in order to provide 20 stretching
steps, 21 hot stretch rolls are required. It should be
noted that the second tensioning roll 16 is equivalent to
the first hot stretching roll. In general, in the hot
stretching apparatus of the preferred embodiment, (n ~ 1) hot
stretch rolls are required to provide n sequential hot
stretch steps. Preferably 2 to 40 stretching steps are
preferred in the multiple stage hot stretching operation.
~ Two preferred methods may be employed to provide
;~ continuously increasing peripheral speed with each additional
downstream hot stretch roll. In one preferred embodiment,
, . ,
all the rolls are driven by one common drive mechanism.
Thus, each hot stretch roll is driven at the same rotational
speed. However, each hot stretch roll is of different
diameter. More specifically, each additional downstream hot
stretch roll has a greater diameter than the upstream roll
adjacent to it. Thus, roll 23 is of greater diameter than
roll 21 and roll 57, the down~tream m~st hot stretch roll
has a diameter greater than the diameter of the next to last
downstream roll 55. ~s those skilled in the art are aware,
the peripheral or surface speed of a larger diameter roll
': .
~ ~'6~
rotating at its center at the same speed as a roll of smaller
diameter is greater than the smaller roll. Th~refore, the
employment of increasingly greater diameter rolls serves the
purpose of providing differential peripheral speeds between
adjoining hot stretch rolls.
A second preferred method for providing a differ-
ential increasing peripheral speeds between adjoining hot
stretch rolls is to provide separate driving means for each
roll. In this preferred embodiment, each roll may be of the
same diameter. The increasing speed of adjoining downstream
hot stretch rolls then becomes a function of the power
imparted to each roll.
It will be understood that the proce s variables
described above in connection with the single increment hot
stretching procedure are applicable to the multi-stage hot
stretching operation with the exception that obvious modifi-
cations may be necessary in going from the former to the
latter. For example, as described a~ove, the total degree of
: ::
hot stretch in both stretching embodiments is the same with
the exception that in multiple stage hot stretching the
total degree of stretch is achieved in several, preferably
equal, increments. Also the strain rate for each hot stretch-
ing increment is preferably controlled to provide a total
residence time in the multiple hot stretching zone approximately
:
~ ~ equal to tha combined resistence time for heat setting
; ; employed in connection with single increment hot stretching
and that obtained when the strain rate is within the ranges
dsscribed herein for hot stretching in a single
stage.
-30-
The resulting hollow microporous fibers posses an
average inner diameter as defined herein of about 100 to
about 300 microns or higher, and preferably from about 200
to about 300 microns (e.g., 250 microns).
The average wall thickness of ~he hollow microporous
fibers does not change substantially from that of the corre-
sponding precursor fiber and the change in the average inner
diameter to average wall thickness ra~io of the microporous
fibers from the precursor fibers is due to the reduction in
the precursor fiber average diameter caused by stretching.
The average inner diameter to average wall thickness
ratio of the hollow microporous fibers will vary from about
7:1 to about 35:1, and preferably from about 10:1 about
30:1. The particular wall thickness achieved is predetermined
by the precursor ~iber wall thickness which as described
a~ove will depend on the end use for which the fibers will
be employed and the pressure to which they will be subjected.
Preferably, the particular wall thickness selected is the
minimum which will withstand normal operating conditions for
":
a particular end use without undergoing physical deterioration
at an unacceptable rate.
When the hollow microporous fibers are employed
or ~lood oxygenation the wall thickness can vary from about
10 to about 30 microns and the average inner diameter can
: ~
~'
~ -31-
i7~
vary from about 200 to about 400 microns and still exhibit
high gas permeabilities and structural integrity.
When the average inner diameter of the hollow
microporous fiber is decreased below 100 microns for a given
wall thickness the gas perm~ability at 10 psi decreases
substantially.
When the average inner diameter of the microporous
hollow fibers of the present invention is at least 100
microns, and the inner diameter to wall thickness ratio is
not less than about 7:1, such hollow fibers will exhibit an
oxygen flux at 10 psi of at lea t 35 cc/cm2.min, typically
from about 35 to about 85 cc/cm .min, and preferably from
about 40 to about 60 cc/cm2.min.
The oxygen flux Jg is determined by passing oxygen
gas through a hollow iber module which is discussed in
greater detail in the examples. The hollow fiber module
permits gas to be passed under pressure ~e.g., 10 psi)
through the interior of the hollow fibers, through the
microporous hollow fiber wall and collected. The volume of
the~gas collected over a period of time is then used to
calculate the gas flux in cc/cm~Dmin of the hollow fibers
according to the equation:
= v
9 ( A ) ( T ~
wherein V is th volume of gas collected, A is the internal
surface area of the hollow fibers determined from the equa-
tion A = n ~dl wherein n is the number of hollow fibers, d is
the inner diameter of the hollow fibers in cen-timeters, and 1
is the fiber length in centimeters; and T is the time in
minutes it takes to collect the gas.
3~ ~
; '
The pores of the hollow microporous fibers are
essentially interconnected through tortuous paths which may
extended from one exterior surface or surface regions to
another, i.8., open-celled. This term "open-celled structure"
signifies that the major portion of the void or pore space
within the geometric confines of the walls of the hollow
fiber is accessible to the surfaces of the fiber walls.
Further, the porous hollow fibers of the present
invention are microscopic, i.e., the details of their pore
configuration or arrangement are described only by microscopic
examination. In fact, the open cells or pores in the fibers
are smaller than those which can be measured using an ordi-
nary light microscope, because ~he wavelength of visibl~
light, which is about 5,000 Angstroms (an Angstrom is one
ten-billionth of a meter), is longer than the longest planar
or surface dimension of the open cell or pore~ The micropor-
OU5 hollow fibers of the present invention may be identified,
however, by using electron microscopy techniques which are
capable of resolving details of pore structure below 5,000
Angstroms.
The open-celled microporous hollow fibers prepared
in accordance with the present invention have an average
pore size of 100 to 5,000 Angstroms, and more usually 150 to
3,000 Angstroms. These values axe determined by mercury
porosimetry, as described in an article by R. G. Quynn, on
pages ~1-34 of Textile Research Journal, January, 1963.
Alternati~ely, an electron micrograph of the fibers can be
taken and pore length and width measurements are obtained by
using an image analyzer or ruler to directly measure the
-33-
7~
length and width of the pores thereoE, usually at 5,000 to
12,000 magnification and scaling down to approrpriate size.
Generally, the pore length values obtainable by electron
microscopy are approximately equal to the pore size values
obtained by mercury porosimetry.
The hollow microporous fibers of the present
invention are also characterized by a reduced bulk density,
sometimes hereinafter referred to simply as a "low" density.
The bulk density is also a measure of the increase in porosity
of the fibers. That is, these microporous hollow fibers
have a bulk or overall density lower than the bulk density
of corresponding precursor hollow fibers composed of identical
polymeric material, but having no open-celled or other voidy
structure. The ferm "bulk density" as used herein means the
weight per unit of gross or geometric volume of the fiber,
where gross volume is determined by immersing a known weight
of the fiber in a vessel partly filled with mercury at 25C.
:~ :
~ and atmospheric pressure. The volumetric rise in the level
,. ~
of mercury is a direct measure o~ the gross volume. This
; method is known as the mercury volumenometer method, and is
described in Encyclopedia of Chemical Technoloyy, Vol. 4,
page 892 (Interscience, 1949)r
;:
Thus the hollow microporous fibers have a bulk
density no greater than 95~ and preferably about 40 to about
85% of the precursor fibers. Stated another way, the bulk
density has been reduced by at least 5% and preferably about
15 to about 60~. The bulk density i5 also a measure of
porosity in that, where the bulk density is about 40 to 85
of the precursor fiber, the porosi~y has been increased by
60 to 15~ because of the pores or holes.
3 ~f_
The final crystallinity of the microporous hollow
fibers is preferably at least 35~, more preferably at least
45% and more suitably about ~0 to 100% as determined by the
aforementioned density method.
The hollow microporous fibers also have a breaking
elongation (ASTM D123-70~ of not less than about 50~, and
preferably not less than about 100%.
The surface area of the hollow microporous fibers
described herein will exhibit a surface area of at least 15
m2/gm and preferably from about 20 to about 60 m2/gm.
Suxface area may be determined from nitrogen or
krypton gas adsorption isotherms using a method and apparatus
: described in U.S. Patent No. 3,262,319. The surface area
obtained by this method is usually expressed as square
meters per gramO
:~ In order to facilitate comparison of various
materials, this value can be multiplied by the bulk density
of the material in grams per cc. resulting in a surface area
quantity expressed as square meters per cc.
The microporous hollow polypropylene fibers of the
~;: present invention in addition to having good gas permeability
also exhibit good liquid flux and are suitable for a number
~ of applications including blood oxygenation, ultra filtration,
; ~ dialysis, separation of gamma globulin from blood, for ascites
~ treatment, as well as a variety of other applications which
', :~:
,;
:;
. -35-
.
. - .
employ hollow microporous fibers. For certain uses it may
be desired to render the normally hydrophobic hollow micro-
porous fibers of the present invention hydrophilic. This
can be achieved by any means known to those skilled in the
art such as by the impregnation o the pores of ths fibers
with a suitable surfactant such as high molecular-weight,
non~ionic surfactants available under the trade name Pluronics
from Wyandotte Chemicals Corp. which are prepared by condensing
ethylene oxide with a hydrophobic base formed by the condensa-
tion of propylene oxide with propylene glycol. Other surfac-
tants include the series of non-ionic surace-active agents
available under the trade name TweenTM which are polyoxyalky-
lene derivatives of hexitol anhydride partial iong chain
fatty acid esters. Alternately, the fibers may b~ treated
with sulfuric acid, chlorosulfonic acid or other such agents
to render the fibers hydrophilic.
To employ the hollow fibers for blood oxygenation,
~undles of hollow fibers containing the desir~d number of
fibers can be prepared by applying an adhesive to each end
of a group of prearranged parallel hollow fibers. The
bundled fibers are then preferably inserted into an elongated
fluid-ti~ht tubular casing assembly formed of a suitable
material such as steelO Each end of the bundled fibers
communicates to the outside of the casing while at either
end o the casing a means for sealing each end of the fiber
bundle to the ends of the casing is provided. Thus, blood
can be pumped through the hollow ibers. The tubular casing
is further provided with valves which open into the interior
of the casing and to the outer surface of each of the fibers
- 3
i~6~7~
in the bundles, so as to provide a means for circulating
oxygen gas about the hollow fibers. Although the fiber
bundle should be packed as tightly as possible, it should be
packed loosely enough to allow a gas -to pass between the
individua] fibers and effectively surround each hollow
fiber.
The oxygen gas can then pass through the external
walls of the hollow fibers and oxygenate the blood passing
within the fiber while carbon dioxide is passed out of the
blood through the hollow fiber.
Alternatively, the oxygen gas may be passed into
the center of the hollow fibers and the blood circulated
through the casing thereby contacting the external surface
of the hollow fibers.
Rather than utilizing a dual-ended tubular casing
in which both ends are open to allow the passage of blood,
it is possible to utilize a permeator in which hollow fiber
; bundles have been formed into a loop so that the ends of
each of the fibers both exit through the same opening in the
tubular casing.
For a further illustration of devices which can
employ hollow fibers for blood oxygenation, see United States
Patent Nos. 2,972,349; 3,373,876; and 4,031,012.
The invention is additionally illustrated in
connection with the following Examples which are to be .
considered as illustrative of the present invention. It
should be understood, however, that the invention is not
limited to the specific details of the Examples. All parts
and percentages in the claims and the remainder of the
specification are by weight unless otherwise specified.
- 37 -
.
EXAMPLE_l
Isotactic polypropylene having a melt index of 5,
a weight average molecular weight of 380,000 and a density
of 0.90 gm/cc,~is melt spun through a ive-hole concentric
hollow jet spinneret. Each jet hole of the spinneret is of
the standard tube-in-orifice type with the tube supplied
with a source of low pressure air, the pressure being control-
led with an air flow metering device set at 3.8 which indi-
cates a flow rate of 120 cc~min. The outer diameter of each
extrusion orifice (jet hole) of the spinneret is 1.391 mm,
and the inner diameter of each extrusion orifice is 0.772
mm. The diameter of ~he air tube within each extrusion
orifice is 0.332 mm. Pellets of the polypropylene are
placed in a ~arbender 3/4 inch extruder and fed into the
feed zone of the extruder by gravity. The extruder is
prsvided with a me ering pump to control the melt pressure
of the spinneret assembly to provide a throughput through
the spinneret of 23 gms/minute. The temperatures of the
feed zone, metering and melt zones of the extrud~r are
controlled by separate jacket sections. The temperature of
~ .
the spinneret assembly is controlled by a separate elec-
trically heated ~acket and a constant extrusion, i.e., spin
temperature of 245C is maintained as indicated by a thermo-
couple in the spinneret assembly. An adjustable feed take-
up device collects the extruded fibers at a take-up speed(TUSt
:`: ~`~ :: :
,~
-38-
~:'
of 500 me~er~/minute. The hollow precursor fibers are
accordingly drawn at a drawdown or spin ratio of 100:1. The
take-up roll is located 10 feet from the spinneret and the
extruded fibers are quenched by passage thxough air at room
temperature i.e., 25C. The degree of orientation as deter-
mined by X-ray difraction analysis as described herein is
16. The precursor fibers exhibit an elastic recovery from
5~ extension at zero recovery time, 25C, and 65% relative
humidity, of 70%, an average inner diameter of 223 microns,
an average outer diameter of 257 microns and an average wall
thickness of 17 microns. The resulting fibers are then
annealed at constant length while still wrapped around the
take-up roll by placing the take-up roll in an oven and
l heating them to 140C for 30 minutes.
`~ Samples of the annealed precursor fiber are then
~` :
subjected to varying degrees of cold stretch at ambient
temperatures as shown in Table I runs 1 to 6 and then to
varying degrees of hot stretch at 140C also as shown in
Table I runs 1 to 6. The strain rate for both hot stretch
and cold stretch is also shown at Table Io Cold and hot
stretching is achieved using a conventional Bruckner stretch
frame and the elevated temperatures during hot stretching
are achieved by placing the stretch frame in a forced hot
air oven. The hot stretched fibers are left in the oven for
! ~ ~
30 minutes to achieve heat setting at ~he same temperature
employed for hot stretching i.e., 140C. The fibers are
~ ~ :
-39-
:::
maintained at constant length during heat setting by the
stretch frame.
For runs 7 to 10 the precursor fiber sample pre-
paration is varied with respect to spin temperature, take-up
speed, draw ratio, throughput rate, and air flow meter sett
ing as shown at Table I. The degree of orientation (as
determined by x-ray diffraction analysis as described herein)
of the precursor ibers as prepared in accordance with runs
7 and 8 is 16 and for runs 9 and 10 is 22~ The elastic
recovery (ER) from 50% extension at ~ero recovery time for
runs 9 and 10 is 64%. The ER for runs 7 and 8 was not
determined. The degree of cold stretch and hot stretch as
well as the strain rate is also shown at Table I.
The resulting heat set microporous hollow fihers
are then tested for surface area by nitrogen absorption as
described herein and also for oxygen flux. The oxygen flux
is determined in the following manner.
~:~
~ Twenty of the hollow microporous fibers from each
, ~
run 16 inches in length are prearranyed in a parallel fiber
bundle configuration and then looped so that the 40 open
ends of the fibers are contiguous and lie flush in a single
::::
plane. The open ends of the fiber loop are then inserted
through a short length (1.25 inches) hard plastic tubing
:~, :
having a l/8 inch inner diameter. The fibers are then
coated with epoxy resin 5 to 6 inches from the open looped
fiber ends. The plastic tubing is then slipped down over
the resin coated section so that about two inches of the
uncoated fiber bundle protrude out of the tubing leaving the
open ends of the looped fiber bundle extending out of the
-40-
tubing. When the resin has hardened the open ends of the
looped fiber bundle are trimmed flush with the plastic
tubing. However, to preserve the open circularity of the
open fiber ends trimming is achieved by first immersing the
fibers in liquid nitrogen, then dipping them in isopropyl
alcohol to fill the lumens with liquid, re-immersing the
fibers in liquid nitrogen for about 1.5 minutes to freeze
the alcohol, and then laying them across a small wood block
also immersed in the liquid nitrogen. The open ends of the
fibers can then be easily trimmed with a razor against the
wooden block wit'nout damage. The tubing-fiber assem~ly is
then sealed in a l/4 or 3/5 inch dia~eter SweglokTM adapter
with epoxy resin leaving a 3/4 inch extension of the tubing
exposed above the adaptors. The epoxy potted fiber assembly
is then inserted into a 7-inch length of 3/8 inch diameter
copper tubing and the SweglokTM adaptor sealed with appro-
priate fittings. For access convenience, a 3-way T fitting
is attached to the distal end of the copper tubing twith
respect to the Sweglok fitting) and one of the exits of the
T-fitting is sealed. One end of a rubber hose is attached
to the open orifice of the T-fitting and the other end is
inserted into a inverted graduated cylinder filled with water
and immersed in a water bath. Oxygen gas is then passed
throuqh the open ~iber ends through the fiber walls and
collected in the graduate cylinder~ The gas pressure is
maintained first at 5 psi and then at 10 psi as shown in
Table I. The gas flux (Jg) in cc/cm2.min is determined from
the equation described herein.
-41
, ~
As may be seen fro~ the results of Table I, oxygen
permeabilities or fluxes higher than about 80 cc/cm2.min can
be obtained from hollow microporous fibers prepared in
accordance with the process of the present invention. Such
permeabilities when normaliæed to flux per micron of fiber
wall thickness represent a substantial improvement over the
normalized gas permeabilities of microporous films in flux
per micron of film thickness, when such films are prepared in
accordance with the cold stretch/hot stretch process of ~.S.
Patent No. 3,801,404.
~ For example, the normalized flux of the hollow
:~ microporous fibers having an oxygen gas permeability at 10
psi of 82.9 cc/cm2.min is obtained by dividing thls permeability
by the average fiber wall thickness of 15 microns to give a
normalized flux per micron of fiber wall ~hickness of 5.5.
Similarly, a microporous film prepared by the cold
,:
stretch/hot stretch process of U.S. Patent No. 3,801,404 and
having a film thickness of about 25.5 microns exhibits an
oxygen ~as flux of about 44 cc/cm2.min When this gas flux
is normalized for comparison with the normalized gas flux of
the hollow microporous fibers of the subject invention a flux
per micron of ilm thickness of 1.73 is obtained. Similar
comparisons can be made wi-th runs 1 to 9 of Example I.
~ I :
.: Thus, hollow microporous fibers can be prepared in
, ~
accordance with the process of the present inven~ion which
:
~: : exhibit a normalized flux about 3 ~imes the normalized flux
of microporous films prepared by the process of the above
described patent.
: 4
.
:
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o
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a ~o K .~ o ~,1
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Comparative Example I
Example I is repeated with the exception that the
inner diameter of the precursor fiber is reduced to below 140
microns as shown at Table II. The degree and strain rate of
the cold stretching and hot stretching as well as the processing
variables are also summarized at Table II. Note that the
microporous hollow fiber wall thickness is assumed to remain
substantially unchanged and has not been measured empirically.
Runs 1 to 10 illustrate the reduced oxygen perme-
ability obtained when the precursor fiber inner diameter
averages substantially below 140 microns, e.g., about 86
~ microns, as compared to the gas permeability obtained from
;~ the runs of Example I employing precursor fiber inner dia-
meters in excess of 140 microns. The highest oxygen flux
obtained was only 10.1 cclcm2.min.
Runs 7 to 10 illustrate a substantial reduction in
gas permeability when the hot stretch step is eliminated or
the extension ratio is selected so that the degree of cold
stretch is greater than the degree of hot stretch.
Runs 11 to 14 illustrate unsuccessful attempts to
:' :
;~ improve the gas pzrmeability of cold stretched fibers by
allowing them ~o relax 10~ (i.e., Runs 11 & 12) and by
: .;
allowing the cold stretched fibers to relax 10~ at a tempera-
ture of 130C (i.e., runs 13 & 14).
Runs 15 to 26 illustrate the gas permeabilities
~:
achieved at varying process conditions by employing average
~ inner precursor fiber diameters of 110 microns. As may be
; ~
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~ .
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seen therefrom the gas permeability is substantially reduced
.in comparison to the gas ~lux of precursor fibers having
average inner diameters employed in the runs of Example 1.
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Comparative Example II
Example I Run 1 is repeated with respect to the
preparation of the precursor fiber. Precursor fiber samples
are then annealed at 140~C, for 30 minutes at constant length,
cold stretched 100% at a strain rate o~ 20%/min and a tempera-
ture o~ 25C and then heat set at different temperatures of
140C, 145C, 150C, and 155C for 30 minutes. When the heat
set temperature is the same as the annealing temperature, i.e.,
140C, the fibers shrink and curl up. At a heat set temperature
slightly above ~he annealing temperature, i.e., 145C the fibers
shrink but to a lesser extent. When the heat set temper~ture
is lS0 or 155C no shrinkage is observed. If the heat set
time employed at 150C and 155C is lowered substantially
be}ow 30 minutes, shrinka~e will again be o~served.
.
The principles, preferred embodiments and modes of
operation of the present invention have been described in the
foregoing specification. The invention which is intended to
.
be protected herein, however, is not to be construed as
~; limited to the particular forms disclosed, since these are to
be regarded as illustrative rather than restrictive. Variations
.
and changes may be made by those skilled in the art without
departing from the spirit of the invention.
:~ :
t`~: ~
~ -48-
.
~ .
