U.S. patent application number 10/941351 was filed with the patent office on 2005-03-10 for hollow fiber membrane and braided tubular support therefor.
Invention is credited to Fabbricino, Luigi, Goodboy, Kenneth Paul, Mahendran, Mailvaganam.
Application Number | 20050051479 10/941351 |
Document ID | / |
Family ID | 23310155 |
Filed Date | 2005-03-10 |
United States Patent
Application |
20050051479 |
Kind Code |
A1 |
Mahendran, Mailvaganam ; et
al. |
March 10, 2005 |
Hollow fiber membrane and braided tubular support therefor
Abstract
An asymmetric membrane comprising a tubular polymer film in
combination with a tubular braid on which the film is supported,
requires the braid be macroporous and flexible, yet sufficiently
strong to withstand continuous flexing, stretching and abrasion
during use for microfiltration (MF) or ultrafiltration (UF). The
specifications for a braid of a long-lived membrane are provided. A
membrane is formed by supporting a polymer film in which particles
of calcined .alpha.-alumina are dispersed, on the defined tubular
braid.
Inventors: |
Mahendran, Mailvaganam;
(Hamilton, CA) ; Goodboy, Kenneth Paul; (Wexford,
PA) ; Fabbricino, Luigi; (Burlington, CA) |
Correspondence
Address: |
ANDREW ALEXANDER & ASSOCIATES
3124 KIPP AVENUE
P.O. BOX 2038
LOWER BURRELL
PA
15068
US
|
Family ID: |
23310155 |
Appl. No.: |
10/941351 |
Filed: |
September 14, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10941351 |
Sep 14, 2004 |
|
|
|
10037432 |
Jan 4, 2002 |
|
|
|
10037432 |
Jan 4, 2002 |
|
|
|
09335073 |
Jun 17, 1999 |
|
|
|
6354444 |
|
|
|
|
10037432 |
Jan 4, 2002 |
|
|
|
08886652 |
Jul 1, 1997 |
|
|
|
5914039 |
|
|
|
|
Current U.S.
Class: |
210/483 ;
210/488; 210/489; 210/490; 210/500.23 |
Current CPC
Class: |
C08K 9/02 20130101; B01D
69/141 20130101; B01D 2325/022 20130101; C08K 9/02 20130101; C08L
27/16 20130101; B01D 69/08 20130101; B01D 69/02 20130101; B01D
71/34 20130101; B01D 67/0011 20130101; B01D 2325/40 20130101; B01D
69/10 20130101; B01D 69/12 20130101; B01D 2325/04 20130101 |
Class at
Publication: |
210/483 ;
210/488; 210/489; 210/490; 210/500.23 |
International
Class: |
B01D 069/08; B01D
069/10 |
Claims
We claim:
1. A separation membrane comprising, (a) a tubular braid support
for a hollow fiber separation membrane made from 16 to 60 separate
yarns, each yarn being between 150 and 400 denier, the tubular
braid support having an outside diameter between 1.5 and 2.5 mm and
a wall thickness greater than 0.2 mm and less than 1.0 mm, the
tubular braid support having at least 30 picks per inch; and, (b) a
porous substance attached to the support, the porous substance
covering the outer circumferential surface of the support, the
porous substance being between 0.05 and 0.3 mm thick beyond the
outer surface of the support and having pores suitable for use as a
separation membrane.
2. The membrane of claim 1 wherein the yarns are shrunken to a
stable length before the porous substance is attached to the
support and the support with shrunken yarns has an extension at
break of at least 10%.
3. The membrane of claim 1 wherein the support has a pre-shrunk
length that is at least 1% less than an un-shrunk length of the
support.
4. The membrane of claim 3 wherein the support has a pre-shrunk
length that is between 1% and 20% less than an un-shrunk length of
the support.
5. The membrane of claim 4 wherein the support has a pre-shrunk
length that is between 1% and 8% less than an un-shrunk length of
the support.
6. The membrane of claim 1 wherein the support is flexible and
macroporous.
7. The membrane of claim 2 wherein the support has an extension at
break of at least 20%.
8. The membrane of claim 1 wherein the air permeability of the
support without the porous substance attached is at least 1
cc/sec/cm.sup.2 at 1.378 kPa.
9. The membrane of claim 8 wherein the air permeability of the
support without the porous substance attached is less than about 10
cc/sec/cm.sup.2 at 1.378 kPa.
10. The membrane of claim 1 wherein the support is not embedded in
the porous substance.
11. The membrane of claim 1 wherein the support is woven with from
1 to 3 multifilament ends.
12. The membrane of claim 11 wherein the support comprises a
multifilament made with from 40 to 100 filaments, each filament
being from 3 to 6 denier.
13. The membrane of claim 1 wherein the yarns are woven in a
pattern selected from Diamond, Regular or Hercules.
14. The membrane of claim 1 wherein the support comprises a tubular
braid woven with yarns having a plurality of multifilament ends,
and the ends are non-plied in each yarn but lie linearly adjacent
each other until taken up to form the braid.
15. The membrane of claim 1 wherein the porous substance has pores
of a size suitable for use as a microfiltration or ultrafiltration
membrane.
16. The membrane of claim 1 wherein the support has a moisture
regain of 0.2% to 7% by weight.
17. A separation membrane comprising, (a) a support for a hollow
fiber separation membrane made from yarns braided into a tube, each
yarn being between 200 and 400 denier, with from 16 to 60 carriers;
and, (b) a porous substance attached to and covering the outer
circumferential surface of the support and having pores suitable
for use as a separation membrane, wherein, the support has at least
36 crosses per inch, an outside diameter of between 1.5 mm and 2.5
mm and a wall thickness of more than 0.15 mm and less than 0.5
mm.
18. The membrane of claim 17 wherein the support comprises at least
16 separate yarns and is woven with from 1 to 3 multifilament
ends.
19. The membrane of claim 18 wherein the support comprises a
multifilament made with from 40 to 100 filaments, each filament
being from 3 to 6 denier.
20. The membrane of claim 17 wherein the yarns are woven in a
pattern selected from Diamond, Regular or Hercules.
21. The membrane of claim 17 wherein the support comprises a
tubular braid woven with yarns having from 1 to 3 non-plied
multifilament ends.
22. The membrane of claim 17 wherein the air permeability of the
support without the porous substance attached is less than about 10
cc/sec/cm.sup.2 at 1.378 kPa.
23. The membrane of claim 17 wherein the yarns are pre-shrunken to
a stable length.
24. The membrane of claim 17 wherein the support has an extension
at break of at least 10%.
25. The membrane of claim 17 wherein the support has a pre-shrunk
length that is at least 1% less than an un-shrunk length of the
support.
26. The membrane of claim 25 wherein the support has a pre-shrunk
length that is between 1% and 20% less than an un-shrunk length of
the support.
27. The membrane of claim 26 wherein the support has a pre-shrunk
length that is between 1% and 8% less than an un-shrunk length of
the support.
28. The membrane of claim 17 wherein the support is flexible and
macroporous.
29. The membrane of claim 17 wherein the support has an extension
at break of at least 20%.
30. The membrane of claim 17 wherein the porous substance is at
least 0.05 mm thick from the outer surface of the support to the
outside of the porous substance.
31. The membrane of claim 30 wherein the porous substance is
between 0.05 mm and 3.0 mm thick beyond the outer surface of the
support to the outside of the porous substance.
32. The membrane of claim 17 wherein the porous substance is less
than 2.0 mm thick beyond the outer surface of the support to the
outside of the porous substance.
33. The membrane of claim 17 wherein the support is not embedded in
the porous substance.
34. The membrane of claim 17 wherein the porous substance has pores
of a size suitable for use as a microfiltration or ultrafiltration
membrane.
35. The membrane of claim 17 wherein the support has a moisture
regain of 0.2% to 7% by weight.
36. The membrane of claim 1 wherein each yarn is on its own
carrier.
37. The membrane of claim 17 wherein the support has a rough and
uneven surface formed by the overlapping yarns.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. Ser. No.
10/037,432, filed Jan. 4, 2002, which is a continuation-in-part of
U.S. Ser. No. 09/335,073, filed Jun. 17, 1999, and a
continuation-in-part of U.S. Ser. No. 08/886,652, filed Jul. 1,
1997, now U.S. Pat. No. 5,914,039.
FIELD OF THE INVENTION
[0002] This invention relates to a braided tubular support for a
film of polymer which functions as an asymmetric semipermeable
membrane in microfiltration (MF) and ultrafiltration (UF)
applications. The braided tube is no more than about 3 mm in
outside diameter and relies on the polymer film to imbue the fiber
membrane product with sustainable high flux along with sufficient
abrasion resistance such that a skein of fibers (also referred to
as a "module") can operate in a commercial filtration application
for several months without the formation of pin-holes.
BACKGROUND OF THE INVENTION
[0003] U.S. Pat. No. 5,472,607 to Mailvaganam Mahendran et al
discloses a hollow fiber semipermeable membrane in which a tubular
macroporous support is superficially coated on its outer surface
with a thin film of polymer, most preferably of polyvinylidene
difluoride. The tubular braid is flaccid but other details of the
structure of the braid are not specified. For example, the effect
of characteristics of the material forming the braid were not
known; nor was the effect of a cross-section which was not truly
circular, i.e. having "cylindricity" substantially less than 1.0.
The term "cylindricity" (sometimes referred to as "roundness")
refers to how perfectly the circular cross-section of the tubular
support matches the geometry of a true circle drawn to correspond
to the mean diameter of the braid, a perfect match being 1.0. It
was therefore not known at that time, how critical the physical
characteristics of a preferred braid were to the performance of a
hollow fiber membrane using the braid.
[0004] In commercially available braid, made with conventional
braiding equipment from commercially available yarn, there were
numerous "breaks" in the fiber; also, accumulation of clumps of
broken filaments, referred to as "fuzz", braided into the
cylindrical wall of the braid, resulted in weak spots in the
polymer film coated onto the surface; and broken filaments,
referred to as "whiskers", protruding from the surface of the
tubular braid, resulted in too-thick domains of polymer which were
concentrated around the whiskers; and, when the domain was not
too-thick, whiskers have a proclivity to initiate pin-holes.
[0005] Further, if the open weave of the braid provided either too
high or too low a braid porosity as measured by resistance to air
flow, the fiber membrane formed was unusable in a commercial
installation. Too open a weave resulted in the braid being
embedded, that is, enclosed by and firmly fixed in the polymer
which also infiltrates into the bore of the braid; thus, too open a
weave results in greatly reduced permeability. Too tight a weave
results in the polymer not being anchored sufficiently well on the
surface; this increases the likelihood that, in service, the
polymer film will be peeled from the braid. When operating flux was
excellent, portions of the polymer film were sometimes found to
have been peeled away when the fibers were backwashed with clean
water or other fluid medium, whether water or permeate, under
pressure; or portions of the film were "blown off" the surface of
the fibers when their lumens were pulsed with air under pressure.
Even with the best braid produced under controlled conditions,
shrinkage during usage in an aqueous medium varied unpredictably.
This resulted in taut fibers which were prematurely fouled because
they were unable to move sufficiently to stay clean or rub against
each other. If too taut, the fibers are broken before they are
fouled, or torn from potting resin in the header. Particularly
because it is essential for best performance, and to shed
contaminants from the surfaces of the hollow fiber membranes, that
a skein of fibers operate with "slack" fibers, the structure of the
braid needs to survive repetitive twisting, and it was not known
what physical characteristic(s) of the braid was conducive to such
survival. A cylindricity less than 0.8 resulted in a resinous
filaments essentially insoluble in the solvent in which the
membrane-forming polymer is dissolved, the braid having a stable
heat-pre-shrunk length which is in the range from about 1% to 20%
less than its unshrunk length, preferably so that, irrespective of
the material forming the fibers, when the pre shrunk braid is
stretched longitudinally, it has "give", that is, the extension at
break is at least 10%, preferably in the range from 10% to 30%, and
more preferably about 20%.
[0006] It is a specific object of this invention to provide a
heat-pre-shrunk tubular braid made with specified patterns, using
carriers carrying yarn having defined number of filaments, ends,
denier, and picks, under conditions which control the porosity
(measured as permeability to air) of the braid, such controlled
porosity serving to anchor a polymer film non-removably on the
surface of the tubular braid.
[0007] It is another specific object of this invention to provide,
in a flexible macroporous tubular braid support for an outside-in
hollow fiber asymmetric membrane having a tubular film of synthetic
resinous material supported on the outer circumferential surface of
the braid without the support being embedded in a thin film having
a wall thickness of less than 0.2 mm, the improvement comprising,
16 to 60 separate yarns, each on its own carrier, each yarn being
multifilament 150 to 500 denier (g/9000 meters) yarn, each
multifilament being made with from 25 to 750 filaments, each
filament being from 0.5 to 7 denier. From 1 to 3 multifilament ends
constitute a yarn, and the individual ends are most preferably not
plied together, but lie linearly adjacent to each other until taken
up in the "fell" of the braid being woven. The braid being woven
has from 30 to 45 picks (crosses/inch). The higher the denier of
the filaments, the fewer the filaments used, but the braid wall
thickness is maintained in the range from about 0.2 mm but less
than three times the diameter of the yarn from which the braid is
woven, preferably less than 1.0 mm. The air permeability of the
braid of synthetic resinous yarn is in the range from about 1 to 10
cc/sec/cm.sup.2 at a differential pressure of 1.378 kPa (0.2 psi);
and the moisture regain is in the range from about 0.2% to 7% by
weight (wt). The finished fiber membrane is coated with a thin
polymer film having a thickness in the range from 0.05 mm to 0.3
mm, most preferably less than 0.1 mm thick. The film has an annular
peripheral barrier layer or "skin" circumferentially integral with
successive microporous layers in the film, each layer contiguous
with a preceding layer, the layers including an outer annular
layer, an intermediate transport layer, and an annular inner
layer.
BRIEF DESCRIPTION OF THE DRAWING
[0008] The foregoing and additional objects and advantages of the
invention will best be understood by reference to the following
detailed description, accompanied with schematic illustrations of
preferred embodiments of the invention, in which illustrations like
reference numerals refer to like elements, and in which:
[0009] FIG. 1A schematically illustrates a "diamond" pattern in a
tubular braid.
[0010] FIG. 1B schematically illustrates a "regular" pattern in a
tubular braid.
[0011] FIG. 1C schematically illustrates a "Hercules" pattern in a
tubularbraid.
[0012] FIG. 2 is a cross-sectional elevational view along a
longitudinal axis, of a coating nozzle used to form the thin
non-supporting film membrane on the braid.
[0013] FIG. 3 is a cross-sectional end view of a hollow fiber
membrane of this invention schematically illustrating the radially
disposed annular zones which extend longitudinally axially over the
length of the membrane, and showing how the tubular
non-self-supporting film is supported on the braid without being
embedded therein.
[0014] FIG. 4 is a cross-sectional view with greatly enlarged
dimensions, to illustrate the dimensional relationships of pores in
the component layers of the braid-supported membrane which pores
make the membrane so effective, particularly for microfiltration
and ultrafiltration.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0015] Details of a hollow fiber membrane are presented in the
aforementioned '607 patent and Ser. No. 08/886,652 application, the
disclosure of each of which is incorporated by reference thereto as
if fully set forth herein. A preferred tubular braid is woven with
yarn, the denier of which is chosen with consideration of the
outside diameter of the braid on which the polymer film is to be
coated, and whether the membrane is to be used for MF or UF. A
desirable air-permeability for a UF membrane to provide drinking
water, is in the range from about 5 to 25 LMH/kPa
(liters/m.sup.2/kPa/hr) or 20 to 100 GFD/psi
(gals/ft.sup.2/day/psi), preferably from about 7.4 to 18.5 LMH/kPa
(30 to 75 GFD/psi), measured with RO (reverse osmosis) water; a
desirable permeability for a MF membrane used to filter municipal
sewage and provide clean water is in the range from 10 to 50
LMH/kPa (40 to 200 GFD/psi), typically about 12.5 to 25 LMH/kPa (50
to 100 GFD/kPa), measured with RO water. A typical defect-free
fiber has a bubble point in the range from about 140 to 280 kPa (20
to 40 psi). For a UF membrane it is desirable to have a bubble
point in the range from 13 to 40 kPa (2 to 6 psi), preferably about
35 kPa (5 psi) to emphasize the importance of a defect in a fiber;
for a MF membrane it is desirable to have a bubble point in the
range from 6 to 20 kPa (1 to 3 psi), preferably about 13 kPa (2
psi), for the same reason.
[0016] The structure of the tubular braid is determined by the
machine used to weave the braid which is formed of intertwined,
spiral yarns so that its thickness is less than three yarn
diameters, and the yarn orientation is helical. The braided tube
may be woven on either vertical or horizontal tubular braiding
machines, the former being preferred. A machine includes a track
plate provided with intertwining tracks, plural tube or bobbin
carriers for the yarn capable of moving counterclockwise or
clockwise along the tracks for braiding, a former and a take-up
device. Bobbins are flanged tubes used for yarns which are
difficult to handle. Yarns from bobbins mounted on the bobbin
carriers are braided as they are guided to a gathering guide
disposed above the center of the disk. Each bobbin carrier is
rotated by a drive gear disposed under the track plate while it
moves along the tracks. The ratio between the moving speed of the
bobbin carriers and the braid drawing speed can be changed by
changing the gear ratio, so that the braids may differ from each
other in the angle of the strands. Different interlacings, or weave
patterns, can be achieved by controlling the motion of the yarn
carriers. By controlling the take-up rate, the angle of the braid
can be controlled. It is essential that the yarn tension be
controlled to provide uniform tension so as to form a uniform
braid. Machines for making the tubular braid and the method of
making it are well known and form no part of the invention. If
desired, axial reinforcements may be provided by using a third
system of yarns which can be inserted between the braiding yarns to
produce a triaxial braid. Such reinforcement is typically found
unnecessary.
[0017] A typical tubular braid is made from two sets of yarns or
ends which are intertwined. Preferred materials are polyesters and
nylons in yarn which is most preferably in the range from about 200
to 400 denier (g/9000 meters), with from 40 to 100 filaments having
a denier in the range from about 3 to 6. The braid is preferably
woven with from 16 to 28 carriers with from about 36 to 44 picks
(crosses/inch) to have an outside diameter in the range from about
1.5 mm to 2.5 mm and a wall thickness in the range from about 0.15
mm to about 0.50 mm, most preferably about 0.3 mm.
[0018] The pattern in which the braid is woven is not narrowly
critical provided the porosity is maintained within chosen limits,
and though a "Regular" or "Hercules" braid is usable, a "Diamond"
pattern is most preferred. Referring to FIG. 1A, there is
schematically illustrated a diamond braid having an alternation of
one yarn passing above and then below the other yarns (1/1). FIG.
1B illustrates the regular braid passes above two and below two in
a repeat (2/2). FIG. 1C illustrates the Hercules braid which has a
structure of 3 up, 3 down (3/3).
[0019] The load at break of a preferred heat-shrunk braid is at
least 50 lb-force, preferably from 444 to 888 Newtons (100 to 200
lb-force), recognizing that the heat-pre-shrunk synthetic resinous
braid has a stable length which is in the range from 1% to 16% less
than its unshrunk length.
[0020] The critical importance of providing a stable heat
pre-shrunk length is because the outer surfaces of taut fibers in a
skein (taut because of shrinkage during use) become so fouled that
they are ineffective to filter. Further, stresses on taut fiber
membranes stress not only the tubular braid but the overlying
polymer film. Undue stress on the braid results in breakage,
typically near the ends of the fiber membranes, where they are
potted in headers; and undue stress on the polymer film diminishes
its adherence and increases its susceptibility to peeling from, or
sloughing off the surface of the braid. Though a "shrink test" is
commonly conducted on yarns by heat shrinking in water at
98.degree. C. via a Texurmat boil off; or, in dry air at
177.degree. C. with 0.045 gf/dtex tension for 2 mm (DuPont); or, in
dry air at 190.degree. C. with 0.135 gf/d for 30 sec (Monsanto), to
date there has been no reason to heat pre-shrink any tubular braid
of synthetic resin, prior to its being coated with polymer for the
stated purpose of this invention, namely to make outside-in hollow
fiber microfiltration and ultrafiltration asymmetric membranes.
More particularly, since a braid woven with glass fiber is
essentially non-heat-shrinkable, there has been no reason to
provide a stable length of a polyester or nylon tubular braid by
pre-shrinking it so that its shrunk length is about 84% of its
pre-shrunk length at the same time ensuring that the braid retains
at least 95% of its tensile strength.
[0021] Heat-shrinking in dry air, referred to as Testrite tests, of
polyester and polyamide tubular braids to obtain most preferably
from about 16% to 18% shrinkage, may be achieved in an electric
furnace at 232.degree. C. for 29 sec.
[0022] The denier of the yarn and structural characteristics of the
braid determine the liquid and gas permeability. The liquid
permeability of the braid is at least one order of magnitude (that
is, more than 10 times) greater than the permeability of the
polymer film. Thus the weave of the braid is so open that it
presents an insubstantial barrier to gas flow.
[0023] Permeability to air of preferred polyester ("PE") and nylon
("NY") braids, determined by ASTM Standard "Air Permeability of
Textile Fabrics D 737-96" are measured for a differential pressure
of 1.38 kPa (0.2 psi). These are listed in the following Table 1
under "@0.2 kPa". Also listed are permeabilities "@0.02 kPa (0.029
psi) which are obtained by extrapolation of the data curve obtained
with measurements at 0.2 kPa, in the appropriate range:
1 TABLE 1 Permeability cc/sec/cm.sup.2 I.D. O.D. Length Area
Flowrate @1.38 Sample mm mm mm cm.sup.2 cc/sec kPa @0.2 kPa PE-1
0.76 1.55 30 1.461 10.36 7.09 1.03 PE-2 1.02 1.89 28 1.663 7.05
4.24 0.62 PE-3 1.13 2.04 26 1.666 5.00 3.00 0.44 PE-4 0.76 1.89 25
1.484 1.52 1.02 0.17 NY-1 1.00 2.10 27 1.781 3.94 2.21 0.32 NY-2
0.89 1.86 27 1.578 4.29 2.72 0.37 NY-3 1.28 2.04 23 1.474 3.93 2.67
0.37
[0024] The moisture regain values for polyester braids are in the
range from about 0.2% to 0.5% by wt., and for the above PE samples,
are from about 0.4% to 0.5% by wt. For nylon braids moisture regain
values are in the range from 4% to 7% by wt., and for the above NY
samples are from about 4% to 5% by wt. The structure of the tubular
braid provides an outer surface which is uniquely configured to
have a membrane's polymer film adhere to the surface sufficiently
so as not to be detached when the membrane is backwashed; the
polymer film is held by the upper portion of the wall of the braid
without having the wall embedded in the film. The degree of
adherence is affected to some extent by the affinity of the
chemical composition of the polymer for the material of the braid,
but to a greater extent by the structure of the braid. The polymer
film may be any polymer which provides a satisfactory asymmetric
membrane, and may be formed from a polyester, polyamide,
polyolefin, poly amine, polyurethane, polysulfone or cellulose
acetate, most preferably PVDF containing calcined a-alumina as
disclosed in U.S. Ser. No. 08/886,652, the disclosure of which is
incorporated by reference thereto as if fully set forth herein.
[0025] The test of establishing whether adherence is satisfactory
is determined by a Peel Test Procedure carried out on a Lloyd
Instruments "Materials Tester" (LR K5 with a 50 N load cell) having
a "German Wheel" (the "Tester" for brevity).
[0026] The "German Wheel" is used to execute a peel test of a
coating on a flexible substrate, at 90.degree. to the surface of
the substrate. Each sample is especially prepared according to the
standard being used. The German Wheel consists of a free running
axle mounted wheel and a yoke which receives the wheel and connects
it to the load to execute a test. The face of the wheel contains a
sharp angled slot into which one end of the coated substrate is
inserted and folded back against the sharp edge. This creates a
mechanical lock which holds the sample tight as its length is
drawn, coating side up, around the periphery of the wheel and
passed through a locking clamp. The clamp site is just beyond a
region where the coating tab length has been separated from the
substrate. Thus the flexible substrate is clamped and the coating
tab length hangs freely, in front of the clamp.
[0027] All tests are done on wet membranes, by slitting a six inch
(6") wet membrane longitudinally. One and one-half inch (1.5") of
membrane is peeled from the braid. A bare one inch (1") section of
braid is inserted into the angle slot and the rest of the braid is
bent around the wheel such that the longitudinal slot is facing
toward the wheel surface. The angled slot anchors one end and the
loose end is placed in the floating clamp arid tightened. Any slack
is removed by the sample tensioning screw. The loose end of the
peeled section of membrane is placed in the upper clamp of the
Tester. Four inches of membrane are pulled off the braid at a rate
of 100 mm/mm. The German wheel rotates freely to keep the angle of
peel constant. The material tester outputs a graph showing the
amount of force required to peel the membrane off the substrate.
The results of the samples are averaged together and plotted on a
graph. The average maximum force of approximately the two inch
section is recorded. Tensile Strength of Each Sample is conducted
as follows:
[0028] The wet samples of membrane obtained from the Peel Test are
placed in the clamps of the Tester. The clamps are placed one inch
apart. The membrane sample is pulled apart at a rate of 100 mm/mm.
The average maximum force for the samples is recorded along with
the standard deviation.
[0029] Cylindricity of the braid is determined by visual
examination under a microscope.
[0030] The asymmetric film comprises a very thin "skin" overlying a
more porous structure in which the pores are in open communication
with one another. Such a membrane may be used for filtering either
aqueous or non-aqueous solvents. For filtration of a solvent such
as a primary or secondary alcohol, a ketone or a hydrocarbon, the
polymer film is deposited from a solution of a solvent-resistant
polymer such as polyacrylonitrile (PAN) or polyetheretber ketone
(PEEK).
[0031] Referring to FIG. 2 there is shown a cross-sectional view of
the coating nozzle indicated generally by reference numeral 10,
which, in addition to limiting the amount of dope (polymer in
solution) passing through the nozzle, meters the correct amount of
dope over the surface, and distributes the metered amount uniformly
over the surface of the braid (not shown) as it is drawn
longitudinally axially through the nozzle. The nozzle 10 comprises
an inner barrel 12 having an internal bore 13 through which the
braid is advanced into an axial bore 14 of a nipple 15 which is
threadedly secured in the end 16 of the inner barrel 12. The bore
14 provides a rounding orifice to help the braid to acquire a
circular cross-section before it is coated with dope. The rounding
orifice 14 has a diameter in the range from about 1% to 10% less
than the nominal diameter of the braid. The barrel 12 with the
nipple 15 is inserted in an outer barrel member 20 having a
cylindrical base 25. The outer barrel 20 is provided with a stepped
inner axial chamber with a larger bore 22 and a smaller bore 23
provided with threads (not shown) near the end of the bore 23. A
top-hat bushing 27 having a stepped axial bore 27' is threaded into
the smaller bore 23 until it compresses an O-ring 27" in a groove
between the end of the barrel 20 and the lower portion of the
bushing. A sizing die 28 having a sizing orifice 24 is press-fitted
in the stepped axial bore 27'. The sizing orifice ensures the
circularity of the cross-section of finished hollow membrane, upon
leaving the rounding orifice. As the dope-coated braid is advanced
through the sizing orifice, it dresses the outside diameter of the
polymer-coated surface to provide the dope with a desired wall
thickness, which upon being coagulated, yields a thin film membrane
which is no more than 0.1 mm thick.
[0032] The base 25 is provided with a lower port 21 and an upper
port 26 each in open communication with the stepped bores 22 and
23, so that dope introduced into the port 21 can flow into the
reservoir formed around the inner barrel 12, by the stepped bores
22 and 23, and travel longitudinally axially in the direction in
which the braid is drawn through the larger bore 22, and the
smaller bore 23 displacing air as the reservoir fills. When the
dope having filled the reservoir flows out of the top port 26, it
is plugged. The base 25 is removably secured with through-bolts
(not shown) through the base 25 to a radially extending mounting
flange 29 having a longitudinal body portion 29'. The body portion
29' is provided with an internally threaded axial bore so that the
body portion 29' can be secured coaxially in position, aligning the
rounding orifice 14 and the sizing orifice 24. By increasing or
decreasing the number of turns of the body portion 29' the distance
between the mouth of the orifice 14 and the orifice 24 can be
varied. This distance is adjusted, depending upon the rate at which
the braid is pulled through, the viscosity of the dope, and the
thickness of the film of dope to be coated on the braid before it
is immersed in the coagulant. In all cases, the distance is
adjusted by trial and error, to provide a film of dope on the
circumferential outer surface of the braid only sufficient to coat
the braid superficially, and not enough to embed the braid in the
film.
[0033] To draw the braid through the orifice 24, a longitudinal
tension is maintained on the braid of at least I Newton but not
enough to distort the voids in the braid so badly that they cannot
return to an equilibrium state as they are being coated with dope.
Because the braid is not embedded in the viscous polymer solution,
only the outer surface of the braid is contacted with the dope so
as to provide the braid with a dope- and polymer-coated outer
surface.
[0034] It will now be evident that the coating nozzle 10 is a
special-purpose nozzle specifically designed to provide a
predetermined distance between the rounding orifice 14 and the
sizing orifice 24 while a dope coated braid no larger than about
2.5 mm (nominal O.D.) is advanced through both orifices
sequentially. The amount of dope metered into the coating nozzle
and the rate at which the braid is advanced through the rounding
orifice are determined by trial and error such as one skilled in
this art is accustomed to engage in under comparable
circumstances.
[0035] After the dope-coated braid leaves the sizing orifice, it is
led into a coagulating bath, typically under and over a series of
rolls, so that the liquid coagulant held in the bath contacts the
entire circumferential surface of the coated braid. Because the
polymer is insoluble in the coagulant it does not penetrate the
thin film formed and enter the lumen. Upon contacting the
coagulant, the dope coagulates, yielding the desired thin film
membrane. The bore of the fiber contains air at atmospheric
pressure.
[0036] Referring to FIG. 3 there is shown in a diametrical
cross-sectional view, much enlarged, a tubular braid indicated
generally by reference numeral 30 comprising a braid of woven yarn
31 having a "lumen" (inner bore) 32. A thin film membrane,
indicated generally by reference numeral 33, is self-adherently
secured to the circumferential outer surface 34 which is rough and
uneven because it is formed by the interwoven yarn which, in the
range of thickness used and the number of picks in which it is
woven, does not result in an even surface. The essential
characteristic of the thin film membrane 33 is that it is supported
superficially, on the circumferential surface of the tubular braid
without the braid becoming embedded in the thin film. This
characteristic is evident in a photomicrograph which clearly
illustrates that the circumferential inner surface of the tubular
braid's bore 32 is essentially free of polymer.
[0037] Referring to FIG. 4 there is schematically illustrated, more
greatly enlarged than in FIG. 3, the asymmetric thin film membrane
33, which when formed by being coagulated, is itself striated into
an overlying ultrathin barrier layer or "skin" 35 and three
distinctly identifiable layers of pores, an outer layer 36, an
inner layer 38 and an intermediate transport layer 37 between outer
layer 36 and inner layer 38, as schematically illustrated in
greater detail in FIG. 4. The skin is a very thin dense layer of
polymer formed as the dope contacts the coagulant. By reason of the
manner in which the skin and each layer is formed from the same
polymer, the layers have, in a radially inward direction from under
the skin to the braided yarn 39 which defines the bore 32,
progressively larger pores. As shown in FIG. 4, each "end" 39 or
yarn consists of a multiplicity of filaments 39', and the
circumferential surface of the interwoven strands of yarn does not
provide a smoothly cylindrical surface. The skin is generally
thinner and the pores for a MF membrane are larger than those of a
UF membrane made from the same polymer. The measured skin thickness
(by electron microscopy) for particular films made for the braided
membrane is given below to appreciate its thickness in relation to
the pores of the layers. The approximate ranges of sizes of the
pores for preferred MF and UF membranes are given below:
2 TABLE 2 MF, .mu.M UF, .mu.M Skin 35, thickness 0.1-1.5 1-4 Outer
layer 36, avg pore diam 0.5-1.0 0.5-2 Intermediate transport layer
37 2-6 5-10 (average pore diameter) Inner layer 38, avg pore diam
10-40 10-150
[0038] In membranes, in general, the thickness of the skin is small
relative to the thickness of the layers. The skin is thicker in a
UP membrane than in a MF membrane, and it would be even thicker in
a RO membrane (not measured). Though FIG. 4 is not to scale, by
reason of the manner in which the membrane is formed, the thickness
of the outer layer is generally smaller than that of the transport
layer, which in turn, is not as thick as the inner layer. The
approximate thickness of each layer in a MF and UP braided membrane
are given in the following Table 3.
3 TABLE 3 Thickness, average MF, .mu.M UF, .mu.M Skin 35 0.1-1.5
1-4 Outer layer 36 5-10 20-40 Intermediate transport layer 37 30-50
40-80 Inner layer 38 100-1000 100-1000
[0039] The following examples illustrate the invention, but should
not be construed as limiting the invention which is defined in the
appended claims.
EXAMPLE 1
Coating Braids with Different Properties with the Same Dope
[0040] In the following examples two tubular braids A and B, made
from yarns of nylon 6/6 fibers, and upon initial examination having
properties which are essentially the same except for the denier of
the filament, are each coated with a dope, of
poly(vinylidenefluoride) (PVDF) in N-methyl-2-pyrrolidone (NMP),
containing a polyhydroxy alcohol hydrophilic additive and having a
viscosity of 38,000 cps. The rate of flow of solution to the nozzle
is adjusted so that the solution is flowed upon and around the
periphery of the braid over a coating distance of 3 mm (0.125
inch). The braid, coated with the solution is then pulled through a
sizing die having a diameter of 2.5 mm, then led into a coagulation
tank where the polymer solution is coagulated in water to afford a
semi permeable membrane about 0.06 mm thick, supported on the
tubular braid which assumes an essentially circular cross-section.
It is then pulled through a glycerin bath, dried and taken up onto
the reel of a winder. The coating conditions for each braid are the
same, namely:
[0041] Bath Temperature 46.degree. C. (115.degree. F.)
[0042] WUS 12.19 meters/mm (40 ft/mm)
[0043] The braids differed as follows:
4 Braid A Braid B Yarn Denier 315 420 Filaments 68 68
Denier/filament 4.6 6.2 Ends 1 1 Picks 44 44 Cylindricity 0.9 0.9
Mean outside diam. 1.88 mm 2.01 mm Mean inside diam. 0.86 mm 1.06
mm Shrinkage 3.4% 3.4% Breaking strength, lb-f 5.93 7.68 Button
(the inside diameter of a 2.15 mm 2.53 mm finishing die through
which the coated braid is passed)
[0044] Upon being tested for filtration, coated Braid B provided a
permeability twice that of coated Braid A. Upon examination of the
coated braids, it is found that Braid A, made with lower denier
yarn, gave a looser" braid which allowed the dope to penetrate to
the inner wall of the braid, embedding it, and leaving little on
the outer surface, as is evident from the following:
5 Braid A Braid B Coated mean outer diam. 1.89 mm 2.15 mm Thickness
of coating 0.005 mm 0.070 mm Mean wall thickness 0.520 mm 0.475 mm
Flux @ 15 psi 171.9 usgfd 383 usgfd
[0045] A photograph of a cross-section of the braided MF membrane,
made with an electron microscope, shows the film membrane overlying
the braid to be about 0.05 mm thick and the braid is not embedded
in the film. The thickness of the skin 35 and each individual layer
36-38 will depend upon the conditions under which the film is made.
Measurements made in a vertical plane through the circumference,
across the wall of the film, provides the following data on pore
sizes:
6 Section Mm Skin thickness 0.8 Outer layer 36 0.781 Intermediate
layer 37 3 (average pore size) Inner layer 38 14-32
[0046] The braided membrane was used to form a MF filtration module
having a construction described in U.S. Pat. No. 5,783,083 to
Mahendran et al. The water permeability measured under 67 kPa (5
psi suction pressure) and 22.degree. C. is found to be 170 LMH (100
USgfd).
EXAMPLE 2
Comparison of Braids Made with Polyester and Glass Fiber Yarns and
an "ADC` Membrane
[0047] A dope, code ADC, is made up similar to the PVDF-in-NMP
solution used in Example 1 hereinabove, with 16 parts PVDF; 81
parts NMP; 2 parts HPVA; and 1 part LiCl; having a viscosity of
56,000 cps, and is fed to a nozzle through which tubular braids of
glass fiber and polyester are advanced to prepare fibers which are
substantially identical except for the material of the yarn from
which the braids are made. As before, the of flow of dope adjusted
so that the solution is flowed upon and around the periphery of
each braid over a coating distance of 3 mm (0.125 inch), pulled
through the same sizing die, coagulated in water to afford a thin
semipermeable membrane 0.05 mm thick, supported on the braid, then
pulled through a glycerin bath. Each braided MF membrane has an
O.D. of about 1.88-1.92 mm, and cylindricity of about 0.9, the I.D.
of each being about 0.9 mm. Each coated braid is taken up onto the
reel of a winder and used to make skeins.
[0048] The skeins, each having an area of 1302 ft are placed in MF
service in a reservoir of water contaminated with leachate from a
land-fill site. The COD of the leachate is in the range from 1000
to 1500 mg/L. Air in an amount in the range from 400 to 450
m.sup.3/hr is provided at the base of each skein. After six months
service under usual operating conditions and identical
back-flushing procedures, it was found that every skein made with
glass fiber braid bad suffered from 2 to 20 broken fibers.
[0049] When skeins made with fibers of polyester braid are placed
in the same service as the fibers of glass fiber braid above, under
identical operating conditions and the same back-flushing
procedures, it was found that after six months service, not a
single fiber of polyester braid was broken.
EXAMPLE 3
[0050] A dope is made up similar to the PVDF-in-NMP solution used
in Example 1 hereinabove, except that it is made up with the
following components in the relative amounts (parts by weight) set
forth: N-methyl-2-pyrrolidone (NMP) 82; polyvinylidene fluoride
(PVDF) 15; calcined a-alumina particles (".alpha.-Al") 2; 50%
hydrolyzed polyvinyl acetate (HPVA) 1; for a total of 100
parts.
[0051] 70 g of calcined a-Al particles having an average primary
particle size of about 0.4 .mu.m are weighted in a flask to which
2787 g of NMP is added and thoroughly mixed in a Sonicator.RTM. for
at least 1.5 hr, to ensure that agglomerates of primary particles
are broken up so as to form a suspension in which individual
primary particles are maintained in spaced apart relationship with
each other in the NMP. The suspension is milky white, the white
color being contributed by the white calcined .alpha.-Al. To this
suspension is slowly added 525 g of PVDF having a number average
mol wt of about 30,000 Daltons while stirring at high speed until
addition of the PVDF is complete. During the addition of the PVDF
the milky white color of the suspension changes first to pink, then
to yellowish brown, at the end to grey/brown. Since PVDF dissolved
in NMP produces no color change, and the milky white color of the
suspension is attributable to the .alpha.-Al particles, the changes
in color provide evidence of a reaction between the calcined
.alpha.-Al or a base present in the calcined alumina.
[0052] When the grey/brown color of the NMP/PVDF/.alpha.-Al complex
in suspension is stable and does not change upon standing for a
sustained period in the range from 4 hr to 24 hr, 118 g of a 30%
solution of 50% HPVA containing 1.6-1.7% sulfuric acid in NMP is
added to form a dope which is stirred overnight. The dope is then
degassed either by letting it stand for 24 hr, or by centrifuging
it. The viscosity of the degassed dope is about 14,500 centipoise
(cp).
[0053] The dope formed is fed to a nozzle through which Braid B
used in Example 1 above is advanced at about 12.2 meters/mm (40
ft/mm), and coated at a pressure of 274 kPa (25 psig) over a
coating distance of 3 mm (0.125 inch). The coated braid is sized in
a sizing die having a diameter of 2.55 mm, then led into a
coagulation tank where the polymer solution is coagulated in water
to afford a semipermeable membrane about 0.13 mm thick, supported
on the tubular braid which has a cylindricity of about 0.9. This
coated braid was then quenched by immersion in sequential first and
second coagulation baths of water, each at 47.degree. C.
(116.degree. F.), and finally through a glycerin bath before it is
taken up onto the reel of a winder. In tests, it is found that the
braided MF membrane provides excellent results.
[0054] After a section of the braided membrane was washed overnight
in cold water, its water permeability is determined by measuring
its flux which is found to be 6 LMH/kPa or, permeability of 25
GFD/psi measured at 5 psi. After another section of the braided
membrane, it is treated with an aqueous solution containing 2000
ppm of sodium hypochiorite (NaOCl). Water permeability of the
NaOCl-treated membrane was found to be 12 LMH/kPa measured at 35
kPa (50 GFD/psi measured at 5 psi). In each case, the pore size
measurements and molecular weight cut-off measurements provide
evidence that the pores in the film are suitable for
microfiltration.
* * * * *