U.S. patent application number 15/664325 was filed with the patent office on 2017-11-30 for method of forming an asymmetric membrane.
The applicant listed for this patent is 3M INNOVATIVE PROPERTIES COMPANY. Invention is credited to Derek J. Dehn, Jonathan F. Hester, Daniel P. Meehan, Robin E. Wright, Jinsheng Zhou.
Application Number | 20170341031 15/664325 |
Document ID | / |
Family ID | 41466518 |
Filed Date | 2017-11-30 |
United States Patent
Application |
20170341031 |
Kind Code |
A1 |
Zhou; Jinsheng ; et
al. |
November 30, 2017 |
METHOD OF FORMING AN ASYMMETRIC MEMBRANE
Abstract
The present disclosure provides methods for forming asymmetric
membranes. More specifically, methods are provided for applying a
polymerizable species to a porous substrate for forming a coated
porous substrate. The coated porous substrate is exposed to an
ultraviolet radiation source having a peak emission wavelength less
than 340 nm to polymerize the polymerizable species forming a
polymerized material retained within the porous substrate so that
the concentration of polymerized material is greater at the first
major surface than at the second major surface.
Inventors: |
Zhou; Jinsheng; (Woodbury,
MN) ; Hester; Jonathan F.; (Hudson, WI) ;
Dehn; Derek J.; (Maplewood, MN) ; Meehan; Daniel
P.; (Newport, MN) ; Wright; Robin E.; (Hudson,
WI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
3M INNOVATIVE PROPERTIES COMPANY |
St. Paul |
MN |
US |
|
|
Family ID: |
41466518 |
Appl. No.: |
15/664325 |
Filed: |
July 31, 2017 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12997658 |
Dec 13, 2010 |
9751050 |
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PCT/US2009/043687 |
May 13, 2009 |
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15664325 |
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61076946 |
Jun 30, 2008 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 69/12 20130101;
Y10T 428/24942 20150115; B01D 2325/38 20130101; B01D 2325/36
20130101; B01D 2323/345 20130101; B01D 71/40 20130101; B01D 2325/14
20130101; B01D 69/02 20130101; B01D 69/10 20130101; B01D 69/125
20130101; B01D 2325/16 20130101; B01D 2323/30 20130101; B01D
67/0006 20130101; B01D 69/141 20130101; B01D 2325/022 20130101;
B01D 71/26 20130101 |
International
Class: |
B01D 69/02 20060101
B01D069/02; B01D 67/00 20060101 B01D067/00; B01D 71/26 20060101
B01D071/26; B01D 69/12 20060101 B01D069/12; B01D 69/14 20060101
B01D069/14; B01D 71/40 20060101 B01D071/40; B01D 69/10 20060101
B01D069/10 |
Claims
1. An asymmetric membrane formed by a method comprising: providing
a porous substrate having a first major surface and a second major
surface; applying a polymerizable composition to the porous
substrate providing a coated porous substrate, the polymerizable
composition comprising i) at least one polymerizable species; and
ii) at least one photoinitiator; and exposing the coated porous
substrate to an ultraviolet radiation source having a peak emission
wavelength less than 340 nm to polymerize the polymerizable
species, wherein the photoinitiator residing at the first major
surface is exposed to a greater peak irradiance of ultraviolet
radiation than the photoinitiator residing further into the
thickness of the porous substrate thus reducing the amount of
polymerization within the pores of the substrate through the
thickness of the substrate, thereby providing an asymmetric
membrane, the asymmetric membrane having a polymerized material
retained within the porous substrate, the polymerized material
having a concentration greater at the first major surface than at
the second major surface.
2. The asymmetric membrane of claim 1, wherein un-polymerized
material within the porous substrate is removed.
3. The asymmetric membrane of claim 1, wherein the porous substrate
is microporous.
4. The asymmetric membrane of claim 1, having a gradient
concentration of polymerized polymerizable material extending from
the first major surface to the second major surface.
5. The asymmetric membrane of claim 1, wherein the porous substrate
is microporous.
6. The asymmetric membrane of claim 1, wherein the porous substrate
comprises a microporous, thermally-induced phase separation
membrane.
7. The asymmetric membrane of claim 1, wherein the porous substrate
is hydrophilic.
8. The asymmetric membrane of claim 1, wherein the porous substrate
is hydrophobic.
9. The asymmetric membrane of claim 1, wherein the porous substrate
comprises a film, a nonwoven web, a woven web, a fiber, or
combinations thereof.
10. The asymmetric membrane of claim 9, wherein the porous
substrate further comprises a particulate.
11. The asymmetric membrane of claim 9, wherein the fiber is a
hollow fiber.
12. The asymmetric membrane of claim 1, wherein the porous
substrate comprises polyolefins, polyamides, fluorinated polymers,
poly(ether)sulfones, cellulosics, poly(ether)imides,
polyacrylonitriles, polyvinyl chlorides, ceramics, or combinations
thereof.
13. The asymmetric membrane of claim 12, wherein the porous
substrate comprises polyolefins.
14. The asymmetric membrane of claim 13, wherein the polyolefins
comprise polyethylene or polypropylene.
15. The asymmetric membrane of claim 12, wherein the porous
substrate comprises polyamides.
16. The asymmetric membrane of claim 1, wherein at least one of the
polymerizable species comprises acrylates, (meth)acrylates,
(meth)acrylamides, styrenics, allylics, vinyl ethers, or
combinations thereof.
17. The asymmetric membrane of claim 1, wherein at least one of the
polymerizable species comprises an ionic group.
18. The asymmetric membrane of claim 17, wherein the ionic group
comprises an amine or a quaternary ammonium salt.
19. The asymmetric membrane of claim 17, wherein the ionic group
comprises a carboxylic acid or a carboxylic acid salt.
20. The asymmetric membrane of claim 1, wherein the first major
surface of the asymmetric membrane is hydrophilic, and the second
major surface of the asymmetric membrane is hydrophobic.
Description
FIELD
[0001] The present disclosure relates to a method of forming an
asymmetric membrane.
BACKGROUND
[0002] Membranes can be used in separation processes where certain
species are retained and other species are allowed to pass through
the membrane. Some membrane applications include, for example, use
in food and beverage, pharmaceutical, medical, automotive,
electronic, chemical, biotechnology, and dairy industries.
[0003] Asymmetric membranes have been described. Asymmetric
membranes have been formed with the addition of photoblockers and
high photoinitiator concentrations under long wavelength
ultraviolet radiation sources.
SUMMARY
[0004] The present disclosure provides methods of forming
asymmetric membranes.
[0005] In one aspect, a method of forming an asymmetric membrane is
provided. The method includes providing a porous substrate having a
first major surface and a second major surface. The method includes
applying a polymerizable composition to the porous substrate
providing a coated porous substrate. The polymerizable composition
comprises at least one polymerizable species and at least one
photoinitiator. The method includes exposing the coated porous
substrate to an ultraviolet radiation source having a peak emission
wavelength less than 340 nm to polymerize the polymerizable species
providing an asymmetric membrane. The asymmetric membrane has a
polymerized material retained within the porous substrate. The
polymerized material has a concentration greater at the first major
surface than at the second major surface.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] FIG. 1 is a schematic representation of a porous substrate
irradiated with a long wavelength ultraviolet radiation source.
[0007] FIG. 2 is a schematic representation of a porous substrate
irradiated with an ultraviolet radiation source having a peak
emission wavelength less than 340 nm.
DETAILED DESCRIPTION
[0008] The recitation of numerical ranges by endpoints includes all
numbers subsumed within that range (e.g., 1 to 5 includes 1, 1.5,
2, 2.75, 3, 3.8, 4, and 5).
[0009] As included in this specification and the appended claims,
the singular forms "a", "an", and "the" include plural referents
unless the content clearly dictates otherwise. Thus, for example,
reference to a composition containing "a compound" includes a
mixture of two or more compounds. As used in this specification and
appended claims, the term "or" is generally employed in its sense
including "and/or" unless the content clearly dictates
otherwise.
[0010] Unless otherwise indicated, all numbers expressing
quantities or ingredients, measurement of properties and so forth
used in the specification and claims are to be understood as being
modified in all instances by the term "about."
[0011] Ultraviolet radiation sources are effective for initiating
and polymerizing polymerizable compositions to provide for
polymerized material retained within a porous substrate. The
polymerizable species of the polymerizable composition can
polymerize within the pores of the porous substrate. A gradient
concentration of polymerized material can be retained throughout at
least a portion of the thickness of the porous substrate providing
for an asymmetric membrane. In some embodiments, a low wavelength
ultraviolet radiation source can be selected for delivering
radiation to a coated porous substrate. The irradiance delivered to
a first major surface is greater than the irradiance delivered at
the second major surface. The irradiance can decrease as the
radiation travels and is absorbed progressing through the thickness
of the coated porous substrate. During exposure to the ultraviolet
radiation source, the polymerizable composition located at the
first major surface can receive a greater irradiance than the
polymerizable composition at the second major surface.
[0012] The method of the present disclosure provides for a
continuous process for forming high flux asymmetrical membranes
relative to symmetrical membranes of the same composition. The term
"asymmetric" refers to a membrane in which the pore size and
structure are not the same from one side of the membrane to the
other side. The pores of the asymmetric membranes are partially
filled (e.g., gel-filled) with polymerized material. Irradiating
one side of the coated porous substrate with an ultraviolet
radiation source having a peak emission wavelength less than 340 nm
under an oxygen (O.sub.2) free environment can result in an
asymmetrical distribution of polymerized material retained within
the porous substrate. The process can be accomplished without the
addition of 1) high concentrations of photoinitiator and/or 2)
photoblockers, and without the application of long wavelength
radiation sources. For example, the asymmetric membranes formed
herein have high flux and good salt rejections in water softening
applications.
[0013] Porous substrates are materials having a network of
interconnecting passages extending from one surface to the other.
These interconnecting passages provide tortuous passageways through
which liquids being filtered must pass.
[0014] In the method of the present disclosure, a porous substrate
having a first major surface, pores (e.g., interstitial), and a
second major surface can be selected from a variety of materials so
long as the porous substrate is coatable (e.g., capable of having a
polymerizable composition applied to at least a portion of the
thickness of the substrate) or can be adapted to be coatable, and
comprises openings or pores. The first major surface of the porous
substrate refers to the surface in close proximity to the
ultraviolet radiation source. The second major surface, or an
opposing surface to the first major surface, is located at a
distance greater to the ultraviolet radiation source than the
distance of the first major surface to the ultraviolet radiation
source.
[0015] Suitable porous substrates include, for example, films,
porous membranes, woven webs, nonwoven webs, hollow fibers, and the
like The porous substrate can be formed from polymeric materials,
ceramic materials, and the like, or combinations thereof. Some
suitable polymeric materials include, for example, polyolefins,
poly(isoprenes), poly(butadienes), fluorinated polymers, polyvinyl
chlorides, polyesters, polyamides, polyimides, polyethers,
poly(ether sulfones), poly(sulfones), poly(ether)sulfones,
polyphenylene oxides, polyphenylene sulfides, poly(vinyl acetates),
copolymers of vinyl acetate, poly (phosphazenes), poly(vinyl
esters), poly(vinyl ethers), poly(vinyl alcohols), poly(carbonates)
and the like, or combinations thereof. Suitable polyolefins
include, for example, poly(ethylene), poly(propylene),
poly(1-butene), copolymers of ethylene and propylene, alpha olefin
copolymers (such as copolymers of 1-butene, 1-hexene, 1-octene, and
1-decene), poly(ethylene-co-1-butene),
poly(ethylene-co-1-butene-co-1-hexene), and the like, or
combinations thereof. Suitable fluorinated polymers include, for
example, poly(vinyl fluoride), poly(vinylidene fluoride),
copolymers of vinylidene fluoride (such as poly(vinylidene
fluoride-co-hexafluoropropylene)), copolymers of
chlorotrifluoroethylene (such as
poly(ethylene-co-chlorotrifluoroethylene)), and the like, or
combinations thereof. Suitable polyamides include, for example,
poly(imino(1-oxohexamethylene)), poly(iminoadipoylimino
hexamethylene), poly(iminoadipoyliminodecamethylene),
polycaprolactam, and the like, or combinations thereof. Suitable
polyimides include, for example, poly(pyromellitimide), and the
like. Suitable poly(ether sulfone)s include, for example,
poly(diphenylether sulfone), poly(diphenylsulfone-co-diphenylene
oxide sulfone), and the like, or combinations thereof.
[0016] In some embodiments, the porous substrate can have an
average pore size less than about 10 micrometers. In other
embodiments, the average pore size of the porous substrate can be
less than about 5 micrometers, less than about 2 micrometers, or
less than about 1 micrometer. In other embodiments, the average
pore size of the porous substrate can be greater than about 10
nanometers. In some embodiments, the average pore size of the
porous substrate is greater than about 50 nanometers, greater than
about 100 nanometers, or greater than about 200 nanometers. In some
embodiments, the porous substrate can have an average pore size in
a range of about 10 nanometers to about 10 micrometers, in a range
of about 50 nanometers to about 5 micrometers, in a range of about
100 nanometers to about 2 micrometers, or in a range of about 200
nanometers to about 1 micrometer.
[0017] Some suitable porous substrates include, for example,
nanoporous membranes, microporous membranes, microporous nonwoven
webs, microporous woven webs, microporous fibers, and the like. In
some embodiments, the porous substrate can have a combination of
different pore sizes (e.g., micropores, nanopores, and the like).
In one embodiment, the porous substrate is microporous. In some
embodiments, the porous substrate can comprise a particulate or a
plurality of particulates.
[0018] The thickness of the porous substrate selected can depend on
the intended application of the membrane. Generally, the thickness
of the porous substrate can be greater than about 10 micrometers.
In some embodiments, the thickness of the porous substrate can be
greater than about 1,000 micrometers, or greater than about 10,000
micrometers.
[0019] In some embodiments, the porous substrate is hydrophobic. In
another embodiment, the porous substrate is hydrophilic. The porous
substrate either being hydrophobic or hydrophilic can be coated
with a polymerizable composition and exposed to an ultraviolet
radiation source as described below.
[0020] In some embodiments, the porous substrate comprises a
microporous, thermally-induced phase separation (TIPS) membrane.
TIPS membranes can be prepared by forming a solution of a
thermoplastic material and a second material above the melting
point of the thermoplastic material. Upon cooling, the
thermoplastic material crystallizes and phase separates from the
second material. The crystallized material can be stretched. The
second material can be optionally removed either before or after
stretching. TIPS membranes are disclosed in U.S. Pat. No. 1,529,256
(Kelley); U.S. Pat. No. 4,726,989 (Mrozinski); U.S. Pat. No.
4,867,881 (Kinzer); U.S. Pat. No. 5,120,594 (Mrozinski); U.S. Pat.
No. 5,260,360 (Mrozinski); U.S. Pat. No. 5,962,544 (Waller, Jr.);
and U.S. Pat. No. 4,539,256 (Shipman). In some embodiments, TIPS
membranes comprise polymeric materials such as poly(vinylidene
fluoride) (i.e., PVDF), polyolefins such as poly(ethylene) or
poly(propylene), vinyl-containing polymers or copolymers such as
ethylene-vinyl alcohol copolymers and butadiene-containing polymers
or copolymers, and acrylate-containing polymers or copolymers. TIPS
membranes comprising PVDF are further described in U.S. Patent
Application Publication No. 2005/0058821 (Smith et al.)
[0021] In some embodiments, the porous substrate can be a nonwoven
web having an average pore size that is typically greater than
about 10 micrometers. Suitable nonwoven webs include, for example,
melt-blown microfiber nonwoven webs described in Wente, V. A.,
"Superfine Thermoplastic Fibers"; Industrial Engineering Chemistry,
48, 1342-1346 (1956), and Wente, V. A., "Manufacture of Super Fine
Organic Fibers"; Naval Research Laboratories (Report No. 4364) May
25, 1954. In some embodiments, suitable nonwoven webs can be
prepared from nylon.
[0022] Some examples of suitable porous substrates include
commercially available materials such as hydrophilic and
hydrophobic microporous membranes known under the trade
designations DURAPORE and MILLIPORE EXPRESS MEMBRANE, available
from Millipore Corporation of Billerica, Mass. Other suitable
commercial microporous membranes known under the trade designations
NYLAFLO and SUPOR are available from Pall Corporation of East
Hills, N.Y.
[0023] In the method of the present disclosure, a polymerizable
species is applied to the porous substrate. The term "polymerizable
composition" generally refers to compositions having at least one
polymerizable species, and at least one photoinitiator. The
polymerizable species can be polymerized on the first major
surface, within the pores or at least a portion of the pores, or on
the second major surface of the porous substrate when exposed to an
ultraviolet radiation source having a peak emission wavelength of
less than 340 nm. The photoinitator selected for initiating the
polymerization of the polymeric species can selectively absorb
radiation from the ultraviolet radiation sources. In some
embodiments, the polymerizable composition applied to the porous
substrate doesn't require a photoinitiator as described in U.S.
Pat. No. 5,891,530 (Wright). The polymerizable composition can be
applied to at least a portion of the thickness of the porous
substrate. The polymerizable species of the polymerizable
composition, after exposure to the ultraviolet radiation source,
can form polymerized material extending through at least a portion
of the thickness of the porous substrate. The resulting polymerized
material can reside on the first major surface, the second major
surface, and within the porous substrate by chemical or physical
interactions. In some embodiments, the polymerized material can
graft onto the surfaces of the porous substrate. In another
embodiment, the polymerized material can reside within and on the
surfaces of the pores of the porous substrate through hydrogen
bonding, Van der Waals interactions, ionic bonding, and the
like.
[0024] The photoinitiator of the polymerizable composition can
initiate polymerization of the polymerizable species. The
polymerizable composition can comprise about 0.001 to about 5.0
weight percent photoinitiator. Some suitable photoinitiators can
include, for example, organic compounds, organometallic compounds,
inorganic compounds, and the like. Some examples of free radical
photoinitiators include, for example, benzoin and its derivatives,
benzyl ketals, acetophenone, acetophenone derivatives,
benzophenone, and benzophenone derivatives, acyl phosphine oxides,
and the like, or combinations thereof. In some embodiments, some
photoinitiators (e.g., acyl phosphine oxides) can absorb long
wavelength ultraviolet radiation, short wavelength ultraviolet
radiation, and the like or combinations thereof.
[0025] Exemplary photoinitiators for initiating free-radical
polymerization of (meth)acrylates, for example, include benzoin and
its derivatives such as alpha-methylbenzoin; alpha-phenylbenzoin;
alpha-allylbenzoin; alpha-benzylbenzoin; benzoin ethers such as
benzil dimethyl ketal (available, for example, under the trade
designation IRGACURE 651 from Ciba Specialty Chemicals, Tarrytown,
N.Y.), benzoin methyl ether, benzoin ethyl ether, benzoin n-butyl
ether; acetophenone and its derivatives such as
2-hydroxy-2-methyl-1-phenyl-1-propanone (available, for example,
under the trade designation DAROCUR 1173 from Ciba Specialty
Chemicals) and 1-hydroxycyclohexyl phenyl ketone (available, for
example, under the trade designation IRGACURE 184 from Ciba
Specialty Chemicals);
2-methyl-1-[4-(methylthio)phenyl]-2-(4-morpholinyl)-1-propanone
(available, for example, under the trade designation IRGACURE 907
from Ciba Specialty Chemicals);
2-benzyl-2-(dimethlamino)-1-[4-(4-morpholinyl)phenyl]-1-butanone
(available, for example, as IRGACURE 369 from Ciba Specialty
Chemicals). Other useful photoinitiators include pivaloin ethyl
ether, anisoin ethyl ether; anthraquinones, such as anthraquinone,
2-ethylanthraquinone, 1-chloroanthraquinone,
1,4-dimethylanthraquinone, 1-methoxyanthraquinone,
benzanthraquinonehalomethyltriazines; benzophenone and its
derivatives; iodonium salts and sulfonium salts as described
hereinabove; titanium complexes such as
bis(eta.sub.5-2,4-cyclopentadien-1-yl)bis[2,6-difluoro-3-(1H-pyrrol-1-yl)-
phenyl]titanium (obtained under the trade designation CGI 784 DC,
also from Ciba Specialty Chemicals); halomethylnitrobenzenes such
as, for example, 4-bromomethylnitrobenzene; mono- and
bis-acylphosphines (available, for example, from Ciba Specialty
Chemicals as IRGACURE 1700, IRGACURE 1800, IRGACURE 1850, and
DAROCUR 4265).
[0026] The photoiniator of the polymerizable composition can be
selected to initiate polymerization of the polymerizable species
throughout at least a portion of the thickness of the porous
substrate. The thickness of the porous substrate extends from the
first major surface to the second major surface. The photoinitiator
can initiate polymerization of the polymerizable species upon
exposure to the ultraviolet radiation source at the first major
surface, and can extend through a portion of the thickness of the
porous substrate. The initiation of polymerizable species for
forming polymerized material can decrease through the thickness to
the second major surface.
[0027] Polymerizable species (e.g., monomers) of the polymerizable
composition can polymerize by many polymerization routes. In
particular, the polymerizable species can attach to another
polymerizable species by chemical bonding (e.g., free radical
reaction) to form a covalent bond through known polymerization
procedures. Upon polymerizing the polymerizable species of the
coated porous substrate when contacted with an ultraviolet
radiation source can form an asymmetric membrane. The surface
properties of the porous substrate before being coated with the
polymerizable composition can be different than the surface
properties of the asymmetric membrane described herein. Similarly,
the asymmetric membrane having functional groups can have different
major surface properties than that of the porous substrate. For
example, the addition of polymerized material to the porous
substrate can provide for reactive surfaces when contacted by other
species, for example, by interactions including hydrogen bonding,
Van der Waals interactions, ionic bonding, and the like.
[0028] In some embodiments, the polymerizable species of the
polymerizable composition can be a monomer having a free-radically
polymerizable group. In some embodiments, the polymerizable species
may comprise a free-radically polymerizable group and an additional
functional group thereon. The free-radically polymerizable group
can be an ethylenically unsaturated group such as a (meth)acryloyl
group, an acryoyl group, or a vinyl group. The free-radically
polymerizable group, after initiation by a photoinitiator, can
polymerize within the porous substrate forming a polymerized
material upon exposure to the ultraviolet radiation source. The
reaction of the free-radically polymerizable groups of the
polymerizable species with other polymerizable species of the
coated porous substrate upon exposure to ultraviolet radiation can
result in the formation of a greater concentration of the
polymerized material at the first major surface and within the
openings or pores nearest the first major surface than at the
second major surface of the asymmetric membrane.
[0029] In addition to having a free-radically polymerizable group,
polymerizable species can contain a second or additional functional
group. In some embodiments, the second functional group is selected
from a second ethylenically unsaturated group, ring opening groups
(e.g., epoxy group, an azlactone group, and an aziridine group), an
isocyanato group, an ionic group, an alkylene oxide group, or
combinations thereof. The second or additional functional group of
the polymerizable species can provide for further reactivity or
affinity of the polymerized material retained within the porous
substrate. In some embodiments, the additional functional group can
react to form a linking group between the porous substrate and
other material such as other species or nucleophilic compounds
having at least one nucleophilic group.
[0030] The presence of an additional functional group can impart a
desired surface property to the asymmetric membrane such as an
affinity for a particular type of compound. In some embodiments,
the polymerizable species can contains an ionic group such that the
asymmetric membrane containing polymerized material can often have
an affinity for compounds having an opposite charge. That is,
compounds with negatively charged groups can be attracted to an
asymmetric membrane having polymerized material with a cationic
group and compounds with positively charged groups can be attracted
to a an asymmetric membrane having polymerized material with an
anionic group. Further, the choice of polymerized material can
impart a hydrophilic property to at least one major surface of the
asymmetric membrane that was hydrophobic prior to surface
modification by the polymerizable composition. In one embodiment,
the polymerized material containing an alkylene oxide group can
impart hydrophilic character to the asymmetric membrane.
[0031] In still other embodiments, suitable polymerizable species
of the polymerizable composition can have a free-radically
polymerizable group that is an ethylenically unsaturated group and
an additional functional group that is an ionic group. The ionic
group can have a positive charge, a negative charge, or a
combination thereof. With some suitable ionic species, the ionic
group can be neutral or charged depending on the pH conditions.
This class of species is typically used to impart a desired surface
affinity for one or more oppositely charged compounds or to
decrease the affinity for one or more similarly charged
compounds.
[0032] In still other embodiments, suitable ionic polymerizable
species having a negative charge include (meth)acrylamidosulfonic
acids of Formula I or salts thereof
##STR00001##
In Formula I, R.sup.1 is hydrogen or methyl; and Y is a straight or
branched alkylene (e.g., alkylenes having 1 to 10 carbon atoms, 1
to 6 carbon atoms, or 1 to 4 carbon atoms). Exemplary ionic species
according to Formula I include, but are not limited to,
N-acrylamidomethanesulfonic acid, 2-acrylamidoethanesulfonic acid,
2-acrylamido-2-methyl-1-propanesulfonic acid, and
2-methacrylamido-2-methyl-1-propanesulfonic acid. Salts of these
acidic species can also be used. Counter ions for the salts can be,
for example, ammonium ions, potassium ions, lithium ions, or sodium
ions.
[0033] Other suitable ionic polymerizable species having a negative
charge include sulfonic acids such as vinylsulfonic acid and
4-styrenesulfonic acid; (meth)acrylamidophosphonic acids such as
(meth)acrylamidoalkylphosphonic acids (e.g.,
2-acrylamidoethylphosphonic acid and
3-methacrylamidopropylphosphonic acid); acrylic acid and
methacrylic acid; and carboxyalkyl(meth)acrylates such as
2-carboxyethylacrylate, 2-carboxyethylmethacrylate,
3-carboxypropylacrylate, and 3-carboxypropylmethacrylate. Still
other suitable acidic species include (meth)acryloylamino as
described in U.S. Pat. No. 4,157,418 (Heilmann et al). Exemplary
(meth)acryloylamino acids include, but are not limited to,
N-acryloylglycine, N-acryloylaspartic acid,
N-acryloyl-.beta.-alanine, and 2-acrylamidoglycolic acid. Salts of
any of these acidic species can also be used.
[0034] Other ionic polymerizable species that are capable of
providing a positive charge are amino (meth)acrylates or amino
(meth)acrylamides of Formula II or quaternary ammonium salts
thereof. The counter ions of the quaternary ammonium salts are
often halides, sulfates, phosphates, nitrates, and the like.
##STR00002##
[0035] In Formula II, R.sup.1 is hydrogen or methyl; L is oxy or
--NH--; and Y is an alkylene (e.g., an alkylene having 1 to 10
carbon atoms, 1 to 6, or 1 to 4 carbon atoms). Each R.sup.2 is
independently hydrogen, alkyl, hydroxyalkyl (i.e., an alkyl
substituted with a hydroxy), or aminoalkyl (i.e., an alkyl
substituted with an amino). Alternatively, the two R.sup.2 groups
taken together with the nitrogen atom to which they are attached
can form a heterocyclic group that is aromatic, partially
unsaturated (i.e., unsaturated but not aromatic), or saturated,
wherein the heterocyclic group can optionally be fused to a second
ring that is aromatic (e.g., benzene), partially unsaturated (e.g.,
cyclohexene), or saturated (e.g., cyclohexane).
[0036] In some embodiments of Formula II, both R.sup.2 groups are
hydrogen. In other embodiments, one R.sup.2 group is hydrogen and
the other is an alkyl having 1 to 10, 1 to 6, or 1 to 4 carbon
atoms. In still other embodiments, at least one of R.sup.2 groups
is a hydroxy alkyl or an amino alkyl that have 1 to 10, 1 to 6, or
1 to 4 carbon atoms with the hydroxy or amino group being
positioned on any of the carbon atoms of the alkyl group. In yet
other embodiments, the R.sup.2 groups combine with the nitrogen
atom to which they are attached to form a heterocyclic group. The
heterocyclic group includes at least one nitrogen atom and can
contain other heteroatoms such as oxygen or sulfur. Exemplary
heterocyclic groups include, but are not limited to imidazolyl. The
heterocyclic group can be fused to an additional ring such as a
benzene, cyclohexene, or cyclohexane. Exemplary heterocyclic groups
fused to an additional ring include, but are not limited to,
benzoimidazolyl.
[0037] Exemplary amino (meth)acrylates (i.e., L in Formula II is
oxy) include, for example, N,N-dialkylaminoalkyl(meth)acrylates
such as, for example, N,N-dimethylaminoethylmethacrylate,
N,N-dimethylaminoethylacrylate, N,N-diethylaminoethylmethacylate,
N,N-diethylaminoethylacrylate, N,N-dimethylaminopropylmethacrylate,
N,N-dimethylaminopropylacrylate,
N-tert-butylaminopropylmethacrylate,
N-tert-butylaminopropylacrylate and the like.
[0038] Exemplary amino (meth)acrylamides (i.e., L in Formula II is
--NH--) include, for example, N-(3-aminopropyl)methacrylamide,
N-(3-aminopropyl)acrylamide,
N-[3-(dimethylamino)propyl]methacrylamide,
N-(3-imidazolylpropyl)methacrylamide,
N-(3-imidazolylpropyl)acrylamide,
N-(2-imidazolylethyl)methacrylamide,
N-(1,1-dimethyl-3-imidazoylpropyl)methacrylamide,
N-(1,1-dimethyl-3-imidazoylpropyl)acrylamide,
N-(3-benzoimidazolylpropyl)acrylamide, and
N-(3-benzoimidazolylpropyl)methacrylamide.
[0039] Exemplary quaternary salts of the ionic species of Formula
II include, but are not limited to,
(meth)acrylamidoalkyltrimethylammonium salts (e.g.,
3-methacrylamidopropyltrimethylammonium chloride and
3-acrylamidopropyltrimethylammonium chloride) and
(meth)acryloxyalkyltrimethylammonium salts (e.g.,
2-acryloxyethyltrimethylammonium chloride,
2-methacryloxyethyltrimethylammonium chloride,
3-methacryloxy-2-hydroxypropyltrimethylammonium chloride,
3-acryloxy-2-hydroxypropyltrimethylammonium chloride, and
2-acryloxyethyltrimethylammonium methyl sulfate).
[0040] Other polymerizable species can be selected from those known
to provide positively charged groups, for example, to an ion
exchange resin. Such polymerizable species include, for example,
the dialkylaminoalkylamine adducts of alkenylazlactones (e.g.,
2-(diethylamino)ethylamine, (2-aminoethyl)trimethylammonium
chloride, and 3-(dimethylamino)propylamine adducts of
vinyldimethylazlactone) and diallylamine species (e.g.,
diallylammonium chloride and diallyldimethylammonium chloride).
[0041] In some methods for making an asymmetric membrane, suitable
polymerizable species can have two free-radically polymerizable
groups as well as a hydrophilic group. For example, alkylene glycol
di(meth)acrylates can be used as polymerizable species to impart
hydrophilic character to a hydrophobic porous substrate. These
polymerizable species have two (meth)acryloyl groups and a
hydrophilic polyalkylene glycol (i.e., polyalkylene oxide)
group.
[0042] When the membrane has polymerizable species that contains an
epoxy group, an azlactone group, or an isocyanato group, the
asymmetric membrane can be further treated such that the functional
groups can react with a nucleophilic compound having a one or a
plurality of nucleophilic groups to impart a hydrophilic character
to a hydrophobic porous substrate. Unreacted nucleophilic groups
can contribute to forming a hydrophilic functionalized membrane.
Some exemplary nucleophilic compounds contain a hydrophilic group
such as a polyalkylene oxide group in addition to the nucleophilic
group. For example, the nucleophilic compound such as polyalkylene
glycol diamines and polyalkylene glycol triamines can include a
plurality of amino groups.
[0043] Polymerizable compositions of the present disclosure can be
prepared, for example, as a coatable solution, dispersion,
emulsion, and the like. The polymerizable compositions can be
applied to the first major surface, interstitial pores, and the
second major surface of the porous substrate. In some examples, the
porous substrate can be saturated or immersed with a polymerizable
composition comprising at least one polymerizable species and at
least one photoinitiator effective for coating the first major
surface, interstitial pores and the second major surface. The
concentration of the polymerizable species, for example, can vary
depending on a number of factors including, but not limited to, the
polymerizable species, the extent of polymerization or crosslinking
of the polymerizable species on and within the porous substrate,
the reactivity of the polymerizable species, the crosslinker
concentration, or the solvent used. In some embodiments, the
concentration of the polymerizable species of the polymerizable
composition can be in a range of about 2 weight percent to about
99.9 weight percent.
[0044] In some embodiments, the polymerizable composition further
comprises a solvent. In one aspect, the polymerizable composition
further comprises a crosslinker.
[0045] In one embodiment, the porous substrate can have a
hydrophilic surface prior to contacting the polymerizable
composition. After contacting the polymerizable composition with an
ultraviolet radiation source having a peak emission wavelength less
than 340 nm, the hydrophobic surface can impart a hydrophobic
property to at least one surface of the asymmetric membrane.
[0046] In some embodiments, the polymerizable species of the
polymerizable composition have a free-radically polymerizable group
that is a first ethylenically unsaturated group and a second
functional group that is a second ethylenically unsaturated group.
In one embodiment, the polymerizable species is a crosslinker
suitable for crosslinking the polymerizable species forming a
network or gelled polymerized material. Suitable polymerizable
species having two ethylenically unsaturated groups include, but
are not limited to, polyalkylene glycol di(meth)acrylates. The term
polyalkylene glycol di(meth)acrylate is used interchangeably with
the term polyalkylene oxide di(meth)acrylate. The term
"(meth)acryl" as in (meth)acrylate is used to encompass both acryl
groups as in acrylates and methacryl groups as in methacrylates.
Exemplary polyalkylene glycol di(meth)acrylates include
polyethylene glycol di(meth)acrylate species and polypropylene
glycol di(meth)acrylate species. Polyethylene glycol diacrylate
species having an average molecular weight of about 400 g/mole is
commercially available, for example, under the trade designation
SR344 and polyethylene glycol dimethacrylate species having an
average molecular weight of about 400 g/mole is commercially
available under the trade designation SR603 from Sartomer Company,
Incorporated of Exton, Pa.
[0047] In some embodiments, suitable polymerizable species have a
free-radically polymerizable group that is a first ethylenically
unsaturated group and an additional functional group that is an
epoxy group. Suitable polymerizable species within this class
include, but are not limited to, glycidyl (meth)acrylates. This
class of polymerizable species can provide a functionalized
asymmetric membrane having at least one epoxy group available for
further reactivity. The epoxy group can react with other reactants
such as with another species or with a nucleophilic compound to
impart a desired surface property to the porous substrate (e.g.,
affinity for a particular compound or functional group having
different reactivity). The reaction of the epoxy group with a
nucleophilic compound, for example, results in the opening of the
epoxy ring and the formation of a linkage group that functions to
tether the nucleophilic compound to the porous substrate. Suitable
nucleophilic groups for reacting with epoxy groups include, but are
not limited to, primary amino groups, secondary amino groups, and
carboxy groups. The nucleophilic compound can contain more than one
nucleophilic group that can crosslink multiple epoxy groups or more
than one optional groups that can impart hydrophilic character to
the functionalized membrane. The linkage group formed by
ring-opening of the epoxy group often contains the group
--C(OH)HCH.sub.2NH-- when the epoxy is reacted with a primary amino
group or --C(OH)HCH.sub.2O(CO)-- when the epoxy is reacted with a
carboxy group.
[0048] In some instances, the epoxy groups of the polymerized
material within the porous substrate can be reacted with a
multifunctional amine such as a diamine having two primary amino
groups or a triamine having three primary amino groups. One of the
amino groups can undergo a ring opening reaction with the epoxy
group and result in the formation of a linkage group that contains
the group --C(OH)HCH.sub.2NH-between the nucleophilic compound and
the porous substrate. The second amino group or the second and
third amino groups can impart a hydrophilic character to the
asymmetric membrane or can crosslink two or more polymerizable
species by reacting with one or more additional epoxy groups. In
some examples, the multifunctional amine is a polyalkylene glycol
diamine or polyalkylene glycol triamine and reaction with an epoxy
group results in the attachment of a polymerized material having a
polyalkylene glycol group (i.e., polyalkylene oxide group). The
polyalkylene glycol group as well as any terminal primary amino
group tends to impart hydrophilic character to the asymmetric
membrane.
[0049] In still other embodiments, suitable polymerizable species
have a free-radically polymerizable group that is an ethylenically
unsaturated group and an additional functional group that is an
azlactone group. Suitable polymerizable species include, but are
not limited to, vinyl azlactone such as
2-vinyl-4,4-dimethylazlactone. This class of polymerizable species
can provide an asymmetric membrane having at least one azlactone
group available for further reactivity. The azlactone group can
react with other reactants such as another species or with a
nucleophilic compound to impart a desired surface property to the
porous substrate (e.g., affinity for a particular compound or
functional group having different reactivity). The reaction of the
azlactone group with a nucleophilic compound, for example, results
in the opening of the azlactone ring and the formation of a linkage
group that functions to attach the nucleophilic compound to the
porous substrate. The nucleophilic compound typically contains at
least one nucleophilic group. Suitable nucleophilic groups for
reacting with an azlactone group include, but are not limited to,
primary amino groups, secondary amino groups and hydroxy groups.
The nucleophilic compound can contain additional nucleophilic
groups that can crosslink multiple azlactone groups or can contain
other optional groups that can impart a hydrophilic character to
the asymmetric membrane. The linkage group formed by ring-opening
of the azlactone group often contains the group
--(CO)NHCR.sub.2(CO)-- where R is an alkyl such as methyl and (CO)
denotes a carbonyl.
[0050] In some instances, the azlactone groups can be reacted with
a multifunctional amine such as a diamine having two primary amino
groups or a triamine having three primary amino groups.
[0051] One of the amino groups can undergo a ring opening reaction
with the azlactone group and result in the formation of a linkage
containing the group --(CO)NHCR.sub.2(CO)-- between the
nucleophilic compound and the porous substrate. The second amino
group or second and third amino groups can impart a hydrophilic
character to the asymmetric membrane or can crosslink multiple
polymerizable species. In some examples, the multifunctional amine
is a polyalkylene glycol diamine or a polyalkylene glycol triamine
and reaction with an azlactone group results in the attachment of a
polymerizable species having a polyalkylene glycol group (i.e.,
polyalkylene oxide group). The polyalkylene glycol group as well as
any terminal primary amino group tends to impart a hydrophilic
character to the asymmetric membrane.
[0052] In still other embodiments, suitable polymerizable species
can have a free-radically polymerizable group that is an
ethylenically unsaturated group and an additional functional group
that is an isocyanato group. Some suitable polymerizable species
include, but are not limited to an isocyanatoalkyl (meth)acrylate
such as 2-isocyanatoethyl methacrylate and 2-isocyanatoethyl
acrylate. This class of polymerizable species can provide an
asymmetric membrane having at least one isocyanato group available
for reactivity. The isocyanato group can react with other reactants
such as another species or with a nucleophilic compound to impart a
desired surface property to the asymmetric membrane (e.g., affinity
for a particular compound or functional group having different
reactivity). The reaction of an isocyanato group with a
nucleophilic compound can result in the formation of a urea linkage
if the nucleophilic group is a primary amino or secondary amino
group or in the formation of a urethane linkage if the nucleophilic
group is a hydroxy group. The nucleophilic compound can contain
additional nucleophilic groups that can crosslink multiple
isocyanato groups or can contain other optional groups that can
impart a hydrophilic character to the asymmetric membrane. The
linkage group formed by reaction of a nucleophilic compound with an
isocyanato group often contains the group --NH(CO)NH-- when the
nucleophilic group is a primary amino group or --NH(CO)O-- when the
nucleophilic group is a hydroxy.
[0053] In some embodiments, the polymerizable species can comprise
unreactive pendent groups. The polymerized material can be retained
within the pores due to physical entanglements.
[0054] In some embodiments, the polymerizable composition further
comprises a crosslinker to gel or form a network of polymerized
material within the porous substrate. The crosslinker can be added
to crosslink a portion of the polymerizable species, or to
substantially crosslink most or all of the polymerizable
species.
[0055] Crosslinkers (e.g., crosslinking materials) of the
polymerizable composition can include difunctional and
polyfunctional acrylate and methacrylate free radically
polymerizable monomers. Some examples of crosslinkers can include,
for example, ester derivatives of alkyl diols, triols, and tetrols
(e.g., 1,4-butanediol diacrylate, 1,6-hexanediol diacrylate,
trimethylolpropane triacrylate, and pentaerythritol triacrylate).
Some other difunctional and polyfunctional acrylate and
methacrylate monomers have been described in U.S. Pat. No.
4,379,201 (Heilmann et al.). In some embodiments, difunctional and
polyfunctional acrylate monomers include, for example
1,2-ethanediol diacrylate, 1,12-dodecanediol diacrylate,
pentaerythritol tetracrylate, and the like, or combinations
thereof. Difunctional and polyfunctional acrylates and
methacrylates can include acrylated epoxy oligomers, acrylated
aliphatic urethane oligomers, acrylated polyether oligomers, and
acrylated polyester oligomers such as those commercially available
under the trade designation EBECRYL from CYTEC SURFACE SPECIALTIES
of Smyrna, Ga. Examples of other commercially available monomers as
described above are available from Sartomer of Exton, Pa.
[0056] The polymerizable composition is applied to the porous
substrate so as to coat, soak, wet, or immerse the porous substrate
to provide a coated porous substrate. The polymerizable composition
can be applied to the porous substrate having a thickness extending
from a first major surface to a second major surface of the porous
substrate. The polymerizable composition can be applied to the
porous substrate to wet or penetrate into at least one micrometer
of the thickness extending from the first major surface. In some
embodiments, the polymerizable composition can wet or penetrate
through the entire thickness of the porous substrate. In some
embodiments, the porous substrate can be immersed with the
polymerizable species. "Immersed" or "saturated" generally refers
to the polymerizable composition being delivered to the first major
surface, to the interconnecting pores within the thickness of the
porous substrate, and to the second major surface. In some
embodiments, the polymerizable composition can wet the surfaces of
the pores throughout the thickness of the porous substrate to
include wetting the first and second major surfaces. Suitable
methods for applying the polymerizable species to the porous
substrate include, for example, saturation or immersion techniques,
spray coating, curtain coating, slide coating, flood coating, die
coating, roll coating, deposition, or by other known coating or
application methods. The polymerizable composition to be applied to
the porous substrate generally has a viscosity such that the first
major surface, the second major surface and the pores of the porous
substrate can be coated. The viscosity of the polymerizable species
can be altered dependent on the application method chosen to
receive the polymerizable composition.
[0057] After the polymerizable composition has been applied to the
porous substrate, the coated porous substrate can be exposed to an
ultraviolet radiation source having a peak emission wavelength less
than 340 nm to initiate and polymerize the polymerizable species of
the polymerizable composition. The ultraviolet radiation source
selected for forming asymmetric membranes can depend on the
intended processing conditions, and the appropriate energy source
required for activating the photoinitiator present in the
polymerizable composition for providing a gradient concentration of
polymerized material through the thickness of the porous substrate.
Similarly, other considerations for selecting the ultraviolet
radiation source can include the amount and type of polymerizable
species, crosslinker, and related materials present in the
polymerizable composition, the ultraviolet radiation source used
for activating the photoinitiator to polymerizing the polymerizable
species, the speed of the moving web (e.g., multilayer structure)
for a continuous process, the distance of the porous substrate from
the ultraviolet radiation source, and the thickness of the porous
substrate.
[0058] A variety of ultraviolet (UV) radiation sources can be used
to prepare the asymmetric membranes of the present disclosure.
Suitable sources include low and medium-pressure mercury arc lamps,
electrodeless mercury lamps, light emitting diodes, mercury-xenon
lamps, lasers and any other sources having some spectral output in
the region less than 340 nm. Available ultraviolet radiation
sources can be broadband, narrowband or monochromatic. When
broadband ultraviolet radiation sources are used, filters can be
applied to narrow the spectral output to a specific spectral region
such that the peak intensity (e.g., emission) occurs at a
wavelength less than 340 nm, thus eliminating longer wavelengths
that can be detrimental to the rewettable asymmetric membrane
forming process. Suitable ultraviolet radiation sources are not
restricted by power and can be pulsed or continuous sources. Some
of these radiation sources may or may not contain mercury.
Preferred ultraviolet radiation sources can be those that have
relatively low IR (infrared) emissions that generally require no
special cooling requirements. Dichroic reflectors (cold mirrors)
and/or dichroic front windows (hot mirrors), and/or water jackets
and other methods know to those skilled in the art can be used to
help control the IR emissions from the ultraviolet radiation
source.
[0059] In some embodiments, narrow bandwidth UV sources can be
selected for which the UV radiation output spans a range of no more
than about 50-100 nm. One example of a narrow bandwidth UV
radiation source includes, for example, fluorescent ultraviolet
lamps, which can operate without special filters and have low IR
emissions. In a preferred embodiment, monochromatic or
substantially monochromatic UV radiation sources such as excimer
lamps, lasers, light emitting diodes, and germicidal lamps are
used. These sources have greater than 95% of their spectral output
confined to a region spanning no more than about 20-30 nm. Some
examples of excimer lamps include a XeCl excimer lamp having a peak
emission at 308 nm, a KrCl excimer lamp having a peak emission at
222 nm, a Xe.sub.2 excimer lamp having a peak emission at 172 nm
and a germicidal lamp having a peak emission at 254 nm.
Substantially monochromatic lamps providing UV radiation output
within a narrow spectral range and having low IR output are
generally preferred. These lamps can allow for more control in
forming a gradient of polymerized material within the copolymer
retained within a rewettable asymmetric membrane and are
commercially available. Such sources are well known in the art. An
ultraviolet radiation source can be a single source or a plurality
of sources. Similarly, the plurality of ultraviolet radiation
sources can be of the same source or of a combination of different
ultraviolet radiation sources.
[0060] Low and high power ultraviolet radiation sources (e.g.,
lamps) can be useful for forming rewettable asymmetric membranes.
Lamp power can be expressed in watts/inch (W/in) based on the
length of the lamp. For example, a high power lamp such as a 600
W/in electrodeless "H" bulb (Fusion UV Systems, Inc., Gaithersburg,
Md.) is a 10-inch long medium-pressure mercury bulb that can be
excited by microwave energy. At full power, the 10 inch lamp
requires a power supply rated at 6000 W to deliver power of 600
W/in. Such high power lamps can generate copious amounts of UV
radiation, but operate at lamp surface temperatures exceeding
700.degree. C. such that the ultraviolet output is accompanied by
significant IR emissions. In contrast, a low power fluorescent UV
lamp can operate at a typical power of 1-2 W/in, and requires less
power to operate having a surface temperature of about 43.degree.
C. to 49.degree. C.
[0061] When exposing the coated porous substrate, the peak
irradiance is greater than 0 mW/cm.sup.2 and can extend up to about
100 mW/cm.sup.2 or greater in the spectral region of the peak
ultraviolet intensity and the spectral output must overlap with at
least a portion of the absorption spectrum of the
photoinitiator.
[0062] The UV spectrum is split into four primary spectral regions
known as UVA, UVB, UVC and VUV, which are commonly defined as
315-400 nm, 280-315 nm, 200-280 nm and 100-200 nm, respectively.
The wavelength ranges cited herein are somewhat arbitrarily
established, and may not correspond to the exact wavelength ranges
published by radiometer manufacturers for defining the four primary
spectral regions. Furthermore, some radiometer manufacturers
specify that a UVV range (395-445 nm) that spans the transition
from UV to visible radiation.
[0063] In some instances, high power UV radiation sources can be
employed These sources can have a peak irradiance of more than
about 1 W/cm.sup.2 accompanied by significant IR emissions. More
preferred ultraviolet radiation sources can comprise an array of
germicidal or fluorescent bulbs providing a peak UV irradiance in
the range from about 1-2 .mu.W/cm.sup.2 to 10-20 mW/cm.sup.2. The
peak irradiance from an array or a plurality of microwave-driven
fluorescent lamps commercially available from Quantum Technologies
of Irvine, Calif., can be as high as 50 mW/cm.sup.2. The actual
irradiance from an array of lamps can depend on a number of factors
which include the electrical voltage, the lamp's power rating, the
lamp spacing within an array or plurality of lamps, the
reflector(s) type (if present), the age of the individual lamps,
the transmission spectrum of any windows or films through which the
UV radiation must pass, the specific radiometer used and its
spectral responsivity, and the distance of the array of lamps from
the membrane.
[0064] The porous substrate can be exposed to the ultraviolet
radiation source for a period of time (e.g., exposure time) for
polymerizing the polymerizable composition to form the asymmetric
membrane. Some exposure times can range from less than a second at
high irradiance (>1 W/cm.sup.2) to several seconds up to several
minutes or longer at a low irradiance (<50 mW/cm.sup.2). The
total UV energy exposure to the porous substrate can be determined
by the UV source irradiance and the exposure time. For example, an
array of fluorescent or germicidal bulbs can be used to expose the
porous substrate to UV radiation. The total UV energy within the
spectral range associated with the peak lamp output can be from
about 100 mJ/cm.sup.2 to more than about 4000 mJ/cm.sup.2, from
about 200 mJ/cm.sup.2 to about 3000 mJ/cm.sup.2, from about 300
mJ/cm.sup.2 to about 2500 mJ/cm.sup.2, or from about 400
mJ/cm.sup.2 to about 2000 mJ/cm.sup.2.
[0065] The rewettable asymmetric membrane of the present disclosure
can be prepared such that a gradient concentration of polymerized
material extends from the first major surface through at least a
portion of the thickness of the porous substrate to the second
major surface. Upon exposure to the ultraviolet radiation source,
the photoinitiator residing at the first major surface can be
exposed to a greater peak irradiance of UV radiation. The higher
peak irradiance at the first major surface can result in a higher
rate of initiation at the first major surface and within the pores
at the first major surface. As the irradiance travels into the
thickness of the porous substrate, the peak irradiance decreases,
thus reducing the amount of photoinitiator decomposition and hence,
polymerization within the pores of the substrate. A gradient
concentration of polymerized material can be formed resulting from
inner filter effects. The inner filter effects can occur when
certain wavelengths are selectively filtered out by absorbing
species (e.g. photoinitiators, porous substrate, or combinations
thereof) as the ultraviolet radiation penetrates the thickness of
the porous substrate. These wavelengths are effectively removed or
diminished. As the UV radiation penetrates further into or through
the porous substrate, the wavelength distribution of the radiation
impinging on the surface can be changed resulting from the
absorption of certain wavelengths. At greater depths within the
porous substrate, insufficient ultraviolet radiation of the
prescribed wavelength region can be available to efficiently excite
the photoinitiator. The extent of polymerization of the
polymerizable species can decrease rapidly forming a gradient
concentration of polymerized material within the thickness of the
porous substrate.
[0066] The sharpness of the gradient concentration of polymerized
material can be determined by the absorbance of the porous
substrate at the wavelengths of the incident UV radiation. When
sources other than substantially monochromatic sources are
utilized, the absorbance is uncertain because absorbance is
wavelength dependent. However, when substantially monochromatic
sources are used, the absorbance (Beer-Lambert Law and measured
using a UV-Visible spectrophotometer) at the peak wavelength of the
radiation source through a film of the polymerizable composition at
a thickness comparable to the membrane thickness should be greater
than 0.3, greater than 0.4, greater than 0.5 or greater than 0.6.
In some embodiments, the absorbance can be greater than 1.0 or even
greater than 2.0 and as high as 10 or even 20.
[0067] The coated porous substrate selected for exposure to the
ultraviolet radiation source having a peak emission wavelength less
than 340 nm can have a thickness greater than about 10 micrometers.
In some embodiments, the thickness of the coated porous substrate
can be greater than about 1,000 micrometers, or greater than about
10,000 micrometers. The polymerizable composition can saturate or
immerse the porous substrate sufficient for wetting at least a
portion of the interconnected pores extending through the thickness
from the first major surface to the second major surface.
[0068] The irradiance of ultraviolet radiation received by a coated
porous substrate can affect the extent to which the polymerizable
species are polymerized. In some embodiments, at least 10 weight
percent of the polymerizable species can be polymerized. In other
embodiments, at least 20 weight percent, at least 30 weight
percent, or at least 40 weight percent of the polymerizable species
can be polymerized form polymerized material residing within the
thickness of the porous substrate.
[0069] The irradiance of the ultraviolet radiation delivered to the
coated porous substrate can be dependent upon, but not limited to,
processing parameters including the type of ultraviolet radiation
source selected, the line speed (e.g., continuous process line)
used, and the distance of the ultraviolet radiation source to the
first major surface of the coated porous substrate. In some
embodiments, the irradiance can be regulated by controlling the
line speed. For example, at the irradiance delivered to the first
major surface can be greater at lower line speeds, and the
irradiance delivered to the first major surface at the first major
surface can be reduced at faster line speeds.
[0070] The irradiance of the ultraviolet radiation source delivered
to a coated porous substrate can be dependent upon the residence
time as described above. The extent of polymerization of the
polymerizable species throughout the thickness of the porous
substrate can be controlled by the irradiance and can affect the
concentration of polymerized material distributed through the
thickness of the coated porous substrate. The peak irradiance
delivered through the thickness of the coated porous substrate can
be, for example, in a range of greater than 0 to about 100
mW/cm.sup.2.
[0071] In some embodiments, the irradiance at the coated porous
substrate upon exposure to the ultraviolet radiation source can be
at least about 0.5 micrometer extending into the thickness of the
substrate from the first major surface. In another embodiment, the
irradiance delivered to the coated porous substrate to polymerize
the polymerizable species can be at least about 1 micrometer from
the first major surface. In some embodiments, the irradiance
delivered to the coated porous substrate can affect the
polymerizable species to at least about 2 micrometers, to at least
about 5 micrometers, to at least about 10 micrometers, or to at
least about 25 micrometers extending into the thickness of the
porous substrate. While low irradiation and longer exposures are
preferred for using ultraviolet radiation sources, polymerizing
polymerizable species as a matter of practical operation may
necessitate speeds that can require higher irradiance and shorter
exposures.
[0072] FIG. 1 illustrates the application of a long wavelength
radiation source 40 having a peak emission wavelength greater than
340 nm to a coated porous substrate. FIG. 1 (comparative example)
illustrates a cross-section of a porous substrate 5 irradiated by
the long wavelength radiation source 40. Porous substrate 5
comprises a first major surface 10, a second major surface 20, and
a pore 35. The long wavelength radiation source 5 can irradiate the
polymerizable species to form polymerized material 30 which extends
through the thickness of the porous substrate from the first major
surface 10 to the second major surface 20. After irradiance of the
porous substrate 5, a symmetric membrane can be formed.
[0073] FIG. 2 illustrates the application of an ultraviolet
radiation source 80 having a peak emission wavelength less than 340
nm to a coated porous substrate is illustrated in FIG. 2. FIG. 2
illustrates a cross-section of a porous substrate 45 irradiated by
a low intensity radiation source 80. Porous substrate comprises a
first major surface 50, a second major surface 60, and a pore 95.
The ultraviolet radiation source 80 irradiates the polymerizable
species forming a first concentration of polymerized material 65
(first polymerized material) which extends through a portion of the
pore 95 in proximity of the first major surface 50 extending to an
average polymerized material concentration location 85. In pore 95,
a portion of pore 95 can contain a second concentration of
polymerized material 70 (second polymerized material) in proximity
of the second major surface 60. The first concentration and the
second concentrations of polymerized material recited in FIG. 2 are
merely for illustrative purposes, and do not define absolute
concentrations of polymerized materials retained within the pores
of the porous substrate. A first polymerized material-pore
interface 90 is formed where the first concentration of polymerized
material 65 contacts the pore 95. A second polymerized
material-pore interface 100 is formed where the second
concentration of polymerized material 70 contacts the pore 95.
Average polymerized material concentration location 85 can be
located from about 5 percent of the thickness extending from the
first major surface 50 to the second major surface 60. In some
embodiments, the polymerized material concentration location 85 can
be at least about 10 percent, at least about 25 percent, at least
50 percent or at least about 75 percent of the thickness of the
porous substrate extending from the first major surface 50 to the
second major surface 60.
[0074] In some embodiments, an asymmetric membrane can be formed
using a multilayer structure wherein the porous substrate is coated
with a polymerizable composition as previously described to provide
a coated porous substrate. A first layer can be positioned adjacent
to the first major surface of the coated porous substrate, and a
second layer can be positioned adjacent to the second major surface
of the coated porous substrate to thereby form a multilayer
structure. The first layer and the second layer may be discrete
pieces of materials or they may comprise continuous sheets of
materials. On a continuous process line, for example, the first
layer and the second layer may be unwound from rolls and brought
into contact with the coated porous substrate. In foregoing
embodiments wherein the coated porous substrate is positioned
(i.e., sandwiched) between a first layer and a second layer to form
a multilayer structure, a single roller or multiple rollers may be
used to meter or remove excess polymerizable composition and
entrapped air bubbles from the coated porous substrate. The first
layer and the second layer of the multilayer structure may comprise
any inert material that is capable of providing temporary
protection to the membrane from exposure to oxygen upon exiting the
ultraviolet radiation source having a peak emission wavelength of
less than 340 nm. Suitable materials for the first layer and the
second layer include, for example, sheet materials selected from
polyethylene terephthalate (PET), biaxially oriented polypropylene
(BOPP), fluorinate polyolefin available from 3M Company and Dupont,
other aromatic polymer film materials, and any other non-reactive
polymer film material. The first layer should be substantially
transparent to the peak emission wavelength of the ultraviolet
radiation source selected. Once assembled, the multilayer structure
typically proceeds to irradiation by the ultraviolet radiation
source. After irradiation, the first layer and the second layer can
be removed (i.e., eliminated) from the multilayer structure to
provide the asymmetric membrane.
[0075] The thickness of the first layer of the multilayer structure
can generally be in a range of 10 micrometers to 250 micrometers,
20 micrometers to 200 micrometers, 25 micrometers to 175
micrometers, or 25 micrometers to 150 micrometers. The second layer
may have the same or a different thickness than that of the first
layer. The first layer may be the same material or a different
material that that used for the second layer.
[0076] In some embodiments, a first layer is positioned adjacent to
the first major surface on the coated porous substrate to form a
bi-layer structure. The first layer can be positioned between the
ultraviolet radiation source and the coated porous substrate. After
irradiation by the ultraviolet radiation source, the first layer
can be removed (i.e., eliminated) from the bi-layer structure to
provide the asymmetric membrane.
[0077] In another embodiment, the coated porous substrate is free
of a first layer and a second layer. The coated porous substrate
may be subjected to an inert atmosphere (e.g., nitrogen, argon) to
reduce the penetration of oxygen (e.g., provide an oxygen free
environment) to the coated porous substrate.
[0078] In some embodiments, the penetration of the ultraviolet
radiation source can be limited by the selection of the ultraviolet
radiation source through the coated porous substrate to produce a
gradient of polymerized material within the asymmetric membrane
that can result in different polymerized material compositions on
the first major surface and the second major surface. In some
embodiments, polymerized material can reside on the first major
surface and within a portion of the thickness of the porous
substrate. The polymerized material residing within the thickness
of the porous substrate can have a gradient concentration of
polymerized material extending from the first major surface to the
second major surface. In one embodiment, an asymmetric membrane has
a hydrophilic surface and a hydrophobic surface.
[0079] The asymmetric membrane formed by the method of the present
disclosure can have a variety of surface properties and structural
characteristics depending on a number of factors. These factors
include without limitation the physical and chemical properties of
the porous substrate, the geometry of the pores of the porous
substrate (i.e., symmetric or asymmetric), the method of forming
the porous substrate, the polymeric species polymerized and
retained as polymerized material with the surfaces (i.e., first
major, interstitial pore, and second major) of the coated porous
substrate, optional post-polymerization treatments (e.g., a heating
step) administered to the asymmetric membrane, and optional
post-polymerization reactions (e.g., reactions of the additional
functional group of the polymerizable species species with a
compound such as a nucleophilic compound or a compound with an
ionic group).
[0080] Asymmetric membranes of the present disclosure can exhibit
various degrees of wettability upon exposure to various
polymerizable compositions. Wettability can often be correlated to
the hydrophilic or hydrophobic character of the asymmetric
membrane. As used herein, the term "instant wet" or "instant
wettability" refers to the penetration of droplets of water into a
given asymmetric membrane as soon as the water contacts the porous
substrate surface, typically within less than 1 second. For
example, a surface wetting energy of about 72 dynes/cm or larger
usually results in instant wetting. As used herein, the term "no
instant wet" refers to penetration of droplets of water into a
given substrate but not as soon as the water contacts the substrate
surface. As used herein, the term "no wetting" refers to the lack
of penetration of droplets of water into a given asymmetric
membrane. For example, a surface wetting energy of about 60
dynes/cm or less usually results in no wetting without applied
pressure.
[0081] Application of polymerizable compositions onto a hydrophobic
porous substrate and treating the coated hydrophobic porous
substrate to the ultraviolet radiation can result in a membrane
having first and second major surfaces having hydrophobic
character, a first major surface having hydrophilic character and a
second major surface having hydrophobic character, or first and
second major surfaces having hydrophilic character. Similarly,
applying polymerizable species onto a hydrophilic porous substrate
and treating the coated hydrophilic porous substrate to ultraviolet
radiation can result in an asymmetric membrane having first and
second major surfaces having hydrophilic character, a first major
surface having hydrophobic character and a second major surface
having hydrophilic character, or first and second major surfaces
having hydrophobic character.
[0082] In one embodiment, the porous substrate is hydrophobic or
hydrophilic. In another embodiment, an asymmetric membrane can be
formed comprising a hydrophobic surface and a hydrophilic surface.
A first major surface can be hydrophilic and a second major surface
can be hydrophobic.
[0083] In one embodiment, an asymmetric membrane can comprise a
symmetric porous substrate. The asymmetric membrane can comprise a
gradient concentration of polymerized material extending from the
first major surface to the second major surface, such that the
concentration of polymerized material is greater at the first major
surface than at the second major surface. In another embodiment, a
first major surface is hydrophilic and a second major surface is
hydrophobic.
[0084] In another embodiment, an asymmetric membrane can comprise
an asymmetric porous substrate. The asymmetric membrane can
comprise a gradient concentration of polymerized material extending
from the first major surface to the second major surface, such that
the concentration of polymerized material is greater at the first
major surface than at the second major surface. In another
embodiment, a first major surface is hydrophilic and a second major
surface is hydrophobic.
[0085] In one embodiment, the asymmetric membrane can be chemically
asymmetric. The asymmetric membrane comprises a symmetric porous
substrate having a first major surface and a second major surface,
wherein the major surfaces (e.g., being hydrophilic) can contain
polymerized material retained throughout at least a portion of the
thickness of the porous substrate. The asymmetric membrane can have
a greater concentration of polymerized material at the first major
surface than at the second major surface.
[0086] In another embodiment, the asymmetric membrane can be
physically asymmetric. For example, the physically asymmetric
porous substrate can have a greater concentration of the
polymerized material at the first major surface than at the second
major surface. In some embodiments, the gradient of polymerized
material can contribute to at least partially blocking of the pores
on at least one major surface and an increased pore size extending
through the thickness of the porous substrate to a second major
surface.
[0087] Asymmetric membranes formed having a greater concentration
of polymerized material at the first major surface than at the
second major surface are described. Asymmetric membranes can find
applications in water softening, filtration, and chromatography.
Asymmetric membranes formed by a continuous process provide for
producing membranes more efficiently and more economically.
[0088] The disclosure will be further clarified by the following
examples which are exemplary and not intended to limit the scope of
the disclosure.
EXAMPLES
[0089] The present disclosure is more particularly described in the
following non-limiting examples. Unless otherwise noted, all parts,
percentages, and ratios reported in the following examples are on a
weight basis.
Test Procedures
Water Flux Measurements and MgCl.sub.2 Rejection Measurements
[0090] Water flux and MgCl.sub.2 (magnesium chloride, salt)
rejection measurements of the asymmetric membrane prepared above
were measured with a stirred ultrafiltration cell (model 8400;
Millipore Corporation, Bedford, Mass.) having an active surface
area of 41.8 cm.sup.2. The trans-membrane pressure was set at 50
psi (pounds per square inch) under pressurized nitrogen gas. Water
flux was calculated based upon the amount of water passing through
the membrane as a function of time, asymmetric membrane area, and
the set pressure. The MgCl.sub.2 rejection (salt rejection) was
obtained from the conductivities of the permeate (C.sub.r) and the
feed (C.sub.f) (500 ppm MgCl.sub.2 aqueous solution) according to
the following equation;
R ( MgCl 2 ) = ( 1 - C p C f ) .times. 100 % ##EQU00001##
R=percent salt rejection.
[0091] The conductivity (C.sub.p and C.sub.f) was measured with a
conductivity meter (VWR Digital Conductivity Bench Meter; VWR
International, West Chester, Pa.), and the mass of permeate was
measured by an electronic balance (model TE3102S; Sartorius,
Edgewood, N.Y.). The conductivity and the mass of the permeate data
were collected as a function of time using Winwedge 32 computer
software (TAI Technologies, Philadelphia, Pa.). Measurements were
discontinued after the salt rejection measurements started to
decline after reaching a plateau. The salt rejection was adjusted
by the feed concentration at the end of testing.
Asymmetric Membrane Process
[0092] Asymmetric membranes were prepared by a continuous process.
A polypropylene thermally induced phase separation (TIPS) membrane
as described in U.S. Pat. No. 4,726,989 (Mrozinski) was die-coated
with a polymerizable composition to form a coated porous substrate.
The coated porous substrate was laminated between two liners in a
gap-controlled nip. One of the two liners (e.g., films) was
laminated to the first major surface and the other liner was
laminated to the second major surface forming a multilayer
structure. The biaxially oriented polypropylene liners ((BOPP)
films of 1.18 mil (30 micrometer) thickness; 3M Company, St. Paul,
Minn.) had a transmittance of about 78.5 percent (UVC) and 85.9
percent (UVA). The edges of the multilayer structure (i.e. edges of
the two liners) were sealed with a pressure sensitive adhesive tape
(Scotch ATG Tape 926; 3M, St. Paul, Minn.). The multilayer
structure was enveloped by the BOPP liners and the excess
polymerizable composition on the coated porous substrate was
minimized. The multilayer structure was irradiated with a Quantum
Microwave Multi-Lamp UV Curing System having a 47'' long UV window
(Model: Quant-23/48R, Quantum Technologies; Irvine, Calif.). The
Quantum UV System used either UVA lamps (26169-3, UV A 365 nm Peak
Lamps TL60/10R, Philips, Somerset, N.J.) or UVC lamps (23596-0,
Germicidal Sterlilamp 254 nm Lamps TUV115W, Philips, Somerset,
N.J.). The line speed was adjusted using the machine speed display.
The intensity of the ultraviolet radiation source was measured by a
PowerMap radiometer (EIT UV Power MAP Spectral Response, UV: A, B,
C, V, Range: Low, Head S/N 1408, Body S/N 1022; Sterling, Va.) as
the multilayer structure was carried through the UV tray. The
polymerizable species of the polymerizable composition were
polymerized forming polymerized material retained within the porous
substrate. The multilayer substrate was collected on a roll and the
liners were removed. An asymmetric membrane was recovered. The
asymmetric membrane was washed with distilled water prior to
further testing.
Example 1
[0093] A polypropylene microporous TIPS membrane (bubble point pore
diameter=0.58 .mu.m; thickness of about 3.5-3.6 mil (85-95
micrometers), water flux of 10521/m.sup.2hpsi) was die coated with
a polymerizable composition. The polymerizable composition
comprised 3-acrylamidopropyl trimethyl ammonium chloride ((APTAC),
75 wt. % in water; Sigma Aldrich, St. Louis, Mo.);
N,N'-methylenebisacylamide (97%; Alfa Aesar, Ward Hill, Mass.), and
1-[4-(2-Hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one
(Irgacure 2959; Ciba Specialty Chemicals, Tarrytown, N.Y.) in an
ethanol/water solvent mixture (60/40 volume:volume ratio). The
APTAC concentration was 0.55 mol/kg in the ethanol/water mixture.
N,N'-methylenebisacylamide in ethanol/water and Irgacure 2959 had a
concentration of 10 mole percent and 2 mole percent, respectively,
relative to APTAC of the polymerizable species. No pretreatment was
required for the porous substrate. The polymerizable composition
was applied to the polypropylene microporous TIPS membrane for
forming a coated porous substrate. The coated porous substrate was
prepared as described by Asymmetric Membrane Process prior to
forming a multilayer structure, and prior to irradiation by the UV
radiation source. The multilayer structure was conveyed by a
continuous process apparatus at a line speed of about 30.5
cm/minute. The first major surface (side A) of the coated membrane
was irradiated by a UVC radiation source (light intensity of 5.77
mW/cm.sup.2). The wet membrane was about 110-120 micrometers
(4.4-4.6 mils) in thickness. The separation performance of the
membrane is listed in Table 1.
Comparative Example 1
CE 1
[0094] Comparative Example 1 was prepared similarly to Example 1
except that the UV radiation source (UVA) used to irradiate one
side of the membrane. The first major surface (side A) of the
coated membrane was irradiated by UVA (light intensity of 28.55
mW/cm.sup.2). The wet membrane was about 110-120 micrometers
(4.4-4.6 mils) in thickness. The separation performance of the
membrane is listed in Table 1.
TABLE-US-00001 TABLE 1 Light Side Pure 500 ppm 500 ppm Irradiation
intensity facing water flux MgCl.sub.2 Flux MgCl.sub.2 Rejection
Membrane Source (mW/cm.sup.2) feed (kg/m.sup.2-h-psi)
(kg/m.sup.2-h-psi) (%) Example 1 UVC 5.77 A 0.84 0.76 93.3 B 0.80
0.71 49.9 CE 1 UVA 28.55 A 0.25 0.24 94.8 B 0.25 0.23 93.3
[0095] As illustrated in Table 1, Example 1 showed a 40% salt
rejection change from Side A to Side B suggesting a gradient
concentration of polymerized material extending from Side A to Side
B. When Side B faced the feed, the salt rejection was reduced
suggesting an asymmetric concentration of the polymerized material
in the membrane. Comparative Example 1 showed a negligible percent
salt rejection change from Side A to Side B suggesting a similar
concentration of polymerized material extending through the
thickness of the membrane from Side A to Side B.
[0096] Example 1 irradiated with a UVC radiation source shows about
a three fold improvement in pure water flux as compared to
Comparative Example 1 irradiated with a UVA radiation source.
Examples 2 and Comparative Example 2
CE 2
[0097] Example 2 (membrane of Example 1) and Comparative Example 2
(membrane of Comparative Example 1) were individually stained with
a negative charged dye under the trade designation METANIL YELLOW
commercially available form Alfa Aesar of Heysham, Lancashire,
England. The membranes of Example 2 and Comparative Example 2 were
immersed into an aqueous dye solution in a vial and stirred for
about 24 hours. The membranes were removed from the vials, rinsed
with deionized water and dried. Side A represented the first major
surface and Side B represented the second major surface of the
membranes of Example 2 and Comparative Example 2. Table 2 lists the
results.
TABLE-US-00002 TABLE 2 Side A (first major Side B (second major
Wettable Polymerizable Dye charge surface) dye binding surface) dye
binding Side Membrane Species (+/-) (yes or no/color) (yes or
no/color) (yes/no) Example 2 APTAC (+) METANIL Yes/dark yellow
No/organic yellow Side A (yes) YELLOW (-) Side B (no) CE 2 APTAC
(+) METANIL Yes/light orange Yes/light orange Side A (yes) YELLOW
(-) Side B (yes)
[0098] As illustrated in Table 2, Example 2 showed an affinity for
the yellow dye of the membrane at side A resulting in a dark yellow
color, whereas the yellow dye was washed away from Side B. The
surface at Side A has a greater concentration of polymerized
material than at Side B indicative of an asymmetric membrane.
Comparative Example 2 showed an affinity for the yellow dye of the
membrane nearly equivalent at both Side A and Side B. Each side of
Comparative Example 2 had a light orange color suggesting a similar
concentration of polymerized material at Side A and Side B
indicating the formation of a symmetric membrane.
Examples 3-4
[0099] A polypropylene microporous TIPS membrane (bubble point pore
diameter=0.72 .mu.m; thickness of 4.2-4.3 mil (120-130
micrometers), water flux of 1475 l/m.sup.2 h psi was die coated
with the polymerizable species of Example 1. No pretreatment of the
polypropylene membrane was required. The polymerizable species was
applied to the polypropylene microporous TIPS membrane to form a
coated porous substrate. The coated porous substrate was prepared
as described by Asymmetric Membrane Process prior to forming a
multilayer structure and irradiating by the UVC radiation source.
The multilayer structure was conveyed on by a continuous process
apparatus at a line speed of about 50 cm/minute. The first major
surface (side A) of the coated membrane was irradiated with
different UVC light intensities (light intensity as measured by a
PowerMap radiometer). The separation performance of the membranes
of Examples 3-4 is shown in Table 3.
TABLE-US-00003 TABLE 3 Wet Pure water 500 ppm 500 ppm Light
membrane flux MgCl.sub.2 Flux MgCl.sub.2 intensity thickness (kg/
(kg/ Rejection Membrane (mW/cm.sup.2) (.mu.m) m.sup.2-h-psi)
m.sup.2-h-psi) (%) Example 3 2.12 130-140 0.42 0.39 95.0% Example 4
5.77 120-128 0.95 0.84 91.9%
[0100] Table 3 illustrates the effects of UVC radiation at
different light intensity on coated porous substrates. A change in
pure water flux can be observed at higher UVC light
intensities.
Examples 5-13
[0101] A polypropylene microporous TIPS membrane as used in
Examples 3-4 was die coated with the polymerizable composition of
Example 1. No pretreatment of the PP membrane was required. The
polymerizable species was applied to the polypropylene microporous
TIPS membrane to form a coated porous substrate. The coated porous
substrate was prepared as described by the Asymmetric Membrane
Process prior to forming a multilayer structure, and prior to
irradiation by the UVC radiation source. The multilayer structure
was conveyed on a continuous process apparatus at variable line
speeds shown in Table 4. The first major surface (side A) of the
coated membrane was irradiated with different UVC light intensities
(light intensity as measured by a PowerMap radiometer). The
separation performance of the membranes of Examples 5-13 is listed
in Table 4.
TABLE-US-00004 TABLE 4 Line Pure water 500 ppm 500 ppm Light Speed
flux MgCl.sub.2 Flux MgCl.sub.2 intensity (cm/ (kg/ (kg/ Rejection
Membrane (mW/cm.sup.2) minute) m.sup.2-h-psi) m.sup.2-h-psi) (%)
Example 5 2.1 30.5 0.50 0.45 94.6 Example 6 2.1 152.4 0.61 0.56
93.9 Example 7 2.1 243.8 0.59 0.54 94.4 Example 8 3.9 30.5 0.76
0.68 92.8 Example 9 3.9 152.4 1.03 0.92 90.1 Example 10 3.9 243.8
1.17 1.06 89.3 Example 11 5.7 30.5 1.08 0.96 90.6 Example 12 5.7
152.4 1.50 1.32 85.8 Example 13 5.7 243.8 1.90 1.71 80.4
[0102] Table 4 illustrates the effect of line speed and UVC light
intensity on coated porous substrates for forming membranes.
[0103] Various modifications and alterations of this disclosure
will be apparent to those skilled in the art without departing from
the scope and spirit of this disclosure, and it should be
understood that this disclosure is not limited to the illustrative
elements set forth herein.
* * * * *