U.S. patent application number 10/032234 was filed with the patent office on 2002-10-03 for oleophobic filter materials for filter venting applications.
Invention is credited to McDonogh, Richard, Wang, I-Fan.
Application Number | 20020139095 10/032234 |
Document ID | / |
Family ID | 23260381 |
Filed Date | 2002-10-03 |
United States Patent
Application |
20020139095 |
Kind Code |
A1 |
Wang, I-Fan ; et
al. |
October 3, 2002 |
Oleophobic filter materials for filter venting applications
Abstract
Oleophobic and hydrophobic filters for filter venting
applications are made by forming a polydimethylsiloxane coating on
the surface of a filtration substrate. The filters have high water
penetration pressures and high air permeabilities. The coatings are
formed by polymerizing vinyl terminated siloxane with a crosslinker
such as hydrosilicone in the presence of a catalyst. Alternatively,
the coatings can be formed by heat curing after exposing the
substrate to methyl silicone and crosslinking at high
temperature.
Inventors: |
Wang, I-Fan; (San Diego,
CA) ; McDonogh, Richard; (San Diego, CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
92660
US
|
Family ID: |
23260381 |
Appl. No.: |
10/032234 |
Filed: |
December 20, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10032234 |
Dec 20, 2001 |
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09323709 |
Jun 1, 1999 |
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6355081 |
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Current U.S.
Class: |
55/385.4 |
Current CPC
Class: |
Y10T 428/31663 20150401;
B01D 2323/30 20130101; B01D 67/0093 20130101; B01D 39/083 20130101;
B01D 67/0088 20130101; Y10T 442/2172 20150401; B01D 2239/0478
20130101; B01D 2239/10 20130101; B01D 2239/0464 20130101; B01D
2239/0622 20130101; B01D 2239/0654 20130101; A61M 2205/7536
20130101; B01D 2239/0428 20130101; Y10S 55/05 20130101; D06M 14/18
20130101; D06M 15/3568 20130101; B01D 2239/083 20130101; D06M
15/643 20130101; B01D 2239/065 20130101; B01D 2239/0668 20130101;
B01D 39/1623 20130101; Y10T 428/2962 20150115 |
Class at
Publication: |
55/385.4 |
International
Class: |
B01D 046/00 |
Claims
What is claimed is:
1. A vent filter for a medical device, the vent filter comprising
an oleophobic, hydrophobic, coated porous polymeric membrane, the
membrane having an air permeability, the coated filter further
comprising a coating derived from a coating formulation, wherein
the coating is permanently crosslinked to the membrane, and wherein
the coating formulation comprises a vinyl-terminated siloxane
polymer, wherein the vent filter may be steam sterilized or
chemically sterilized without substantial loss of air
permeability.
2. The filter of claim 1, wherein the porous polymeric membrane
comprises an asymmetric membrane.
3. The filter of claim 1, wherein the porous polymeric membrane
comprises an isotropic membrane.
4. The filter of claim 1, wherein the vinyl-terminated siloxane
polymer comprises a vinyl-terminated fluorosiloxane polymer.
5. The filter of claim 1, wherein the siloxane polymer comprises a
vinyldimethyl-terminated siloxane.
6. A method of producing a vent filter for a medical device
comprising a hydrophobic, oleophobic filter, comprising the steps
of: providing a porous polymeric membrane; contacting the membrane
with a coating formulation comprising a vinyl-terminated siloxane
polymer to produce a coated filter; crosslinking the coating
formulation to the filter; recovering an oleophobic, hydrophobic,
permanently coated filter having an air permeability; and
incorporating the oleophobic, hydrophobic permanently coated filter
into a vent filter for a medical device, wherein the vent filter
may be steam sterilized or chemically sterilized without
substantial loss of air permeability.
7. The method of claim 6, wherein the porous polymeric membrane
comprises an asymmetric membrane.
8. The method of claim 6, wherein the porous polymeric membrane
comprises an isotropic membrane.
9. The method of claim 6, wherein the vinyl-terminated siloxane
polymer comprises a vinyl-terminated fluorosiloxane polymer.
10. The method of claim 6, wherein the siloxane polymer comprises a
vinyldimethyl-terminated siloxane.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to filters having both
hydrophobic (water repellent) and oleophobic (oil repellent)
properties. The properties are produced by forming a
dimethylsiloxane coating on a substrate such as a hydrophobic or
hydrophilic membrane or other filtration medium.
[0003] 2. Background of the Invention
[0004] Hydrophobic filters are used in filtration of gases, in
venting filters, and as gas vents. These hydrophobic filters allow
gases and vapors to pass through the filter while liquid water is
repelled by the filter.
[0005] Polytetrafluoroethylene (PTFE) has been the most common
material in filters for gas venting. PTFE is chemically and
biologically inert, has high stability, and is hydrophobic. PTFE
filters therefore allow gases to be selectively vented while being
impervious to liquid water.
[0006] Hydrophobic membranes are used as filters in healthcare and
related industries, for example, as vent filters for intravenous
(IV) fluids and other medical devices. In the health industry, the
membrane must be sterilized before use. PTFE membranes can be
sterilized for these health-related applications with steam or by
chemical sterilization without losing integrity.
[0007] Treating PTFE membranes with steam can cause pore blockage
due to condensation of oil from the machinery used to generate the
steam. The resulting loss of air permeability reduces the
membrane's ability to serve as an air vent. Although chemical
sterilization minimizes exposure of the membrane to oil, chemical
sterilization uses toxic chemicals and can generate byproducts
which cause waste disposal problems. Ionizing radiation has also
been used for sterilization of materials used in medical and
biological devices. PTFE is unstable toward ionizing radiation.
Irradiated PTFE membranes have greatly reduced mechanical strength
and cannot be used in applications where they are subjected to even
moderate pressures.
[0008] Perhaps the two biggest drawbacks to PTFE as a filter for
venting gases are the high cost and the low air permeability of
PTFE membranes. PTFE membranes are formed by extruding and
stretching PTFE. Both the PTFE raw material and the processing to
form the PTFE membranes are expensive. Further, the extruding and
stretching processes used to form PTFE membranes create a membrane
which has relatively low air permeability.
[0009] The oleophobicity of PTFE can be improved by impregnating or
coextruding the PTFE with siloxanes (for example, U.S. Pat. No.
4,764,560), fluorinated urethane (U.S. Pat. No. 5,286,279), or
perfluoro-2,2-dimethyl-1,3-dioxole (U.S. Pat. No. 5,116,650).
Although the oil resistance of the PTFE is improved, the treated
PTFE membranes are expensive, and air permeability remains fairly
low.
[0010] As a result, efforts have been made to identify alternative
substrates which are less expensive and have higher air
permeability than PTFE and which can be modified to be hydrophobic
and oleophobic.
[0011] Coating filtration substrates allows one to retain the
desirable bulk properties of the substrate while only altering the
surface and interfacial properties of the substrate. Coating
substrates to increase their hydrophobic and oleophobic properties
has not been very practical, because the coatings can reduce
permeability. Furthermore, many of the coating materials are
expensive.
[0012] Scarmoutzos (U.S. Pat. No. 5,217,802) modified the surface
of substrates made of nylon, polyvinylidene difluoride (PVDF), and
cellulose by treating the substrate with a fluorinated acrylate
monomer. The substrate was sandwiched between two sheets of
polyethylene, and the monomer was polymerized by exposing to
ultraviolet light. The resulting composite filters had hydrophobic
and oleophobic surfaces. The air permeability of the filters
decreases with time.
[0013] Moya (U.S. Pat. No. 5,554,414) formed composite filters from
polyethersulfone and PVDF membranes with a method similar to that
of Scarmoutzos. The resulting filters did not wet with water or
hexane. The disadvantage of the Moya filters is that air
permeability of the treated filters was lower than the untreated
substrates, and the fluorinated monomer is expensive.
[0014] Sugiyama et al. (U.S. Pat. No. 5,462,5856) treated nylon
fabric and PTFE membranes with solutions containing two different
preformed fluoropolymers. The treated filters were resistant to
water and oils. The durability of filters coated with preformed
polymers is often less than that for filters where the coating is
formed by polymerizing a monomer on the surface of the
substrate.
[0015] Kenigsberg et al. (U.S. Pat. No. 5,156,780) treated a
variety of membranes and fabrics with solutions of fluoroacrylate
monomers and formed coatings on the substrate by polymerizing the
monomer. The coating conferred oil and water repellency onto the
substrate. However, the air flow through the treated membrane was
reduced, compared to the untreated membrane.
[0016] Hydrophobic media suitable for garments have been made by
extruding mixtures of polypropylene or PTFE and the fluorochemical
oxazolidinone as disclosed in U.S. Pat. No. 5,260,360. The media
made by extruding tend to have relatively low air permeability.
[0017] There is a need for an oleophobic and hydrophobic filter
which is inexpensive and has high air permeability. Specifically,
there is a need for a coating for filter medium substrates that
makes the substrate oleophobic and hydrophobic, and for a more cost
effective process of making oleophobic filters.
SUMMARY OF THE INVENTION
[0018] The present invention provides an oleophobic, hydrophobic,
coated filter, including a substrate, the substrate having a
pressure of water penetration, the coated filter further including
a coating derived from a coating formulation, wherein the coating
is permanently crosslinked to the substrate, and wherein the
coating formulation includes a vinyl-terminated siloxane polymer,
and wherein the coated filter has a pressure of water penetration
at least 10 percent greater than the pressure of water penetration
of the substrate without the coating. The substrate may include a
porous polymeric membrane, a nonwoven material, or a woven
material. The substrate may include a polymer such as polysulfone,
polyethersulfone, polyarylsulfone, polyvinylidene fluoride,
polypropylene, polyethylene, poly(tetrafluoroethylene),
poly(tetrafluoroethylene-co-ethylene), nylon, or cellulosic esters.
The siloxane polymer may include a vinyldimethyl-terminated
siloxane. The coating formulation may further include a
crosslinker, such as, for example, methylhydro,
cyanopropylmethylsiloxane; methylhydro, phenylmethylsiloxane;
methylhydro, methyl-octylsiloxane; methyltriacetoxy silane; or
methyl silicone. The coating formulation may further include a
crosslinker catalyst. The filter of the invention may also be
bonded to a fabric.
[0019] In another aspect of the invention, there is provided a
method of producing a hydrophobic, oleophobic filter, including the
steps of providing a substrate having a first pressure of water
penetration; contacting the substrate with a coating formulation
including a vinyl-terminated siloxane polymer to produce a coated
filter; crosslinking the coating formulation to the filter; and
recovering an oleophobic, hydrophobic, permanently coated filter
having a second pressure of water penetration, wherein the second
pressure of water penetration is at least 10 percent greater than
the first pressure of water penetration. In this method, the
substrate may include a porous polymeric membrane, a nonwoven
material, or a woven material. The substrate may include a polymer
such as polysulfone, polyethersulfone, polyarylsulfone,
polyvinylidene fluoride, polypropylene, polyethylene,
poly(tetrafluoroethylene), poly(tetrafluoroethylene-co-ethylene),
nylon, or cellulosic esters. The siloxane polymer may include a
vinyldimethyl-terminated siloxane. The coating formulation may
further include a crosslinker, such as, for example, methylhydro,
cyanopropylmethylsiloxane; methylhydro, phenylmethylsiloxane;
methylhydro, methyl-octylsiloxane; methyltriacetoxy silane; or
methyl silicone. The coating formulation may further include a
crosslinker catalyst. The filter of the invention may also be
bonded to a fabric. The crosslinking step may include exposing the
coated filter to a temperature sufficient to facilitate a
crosslinking activity of the crosslinker. The coating formulation
further may include a crosslinker catalyst. The crosslinking step
may also include exposing the coated filter to water or water
vapor, or exposing the coated filter to ultraviolet radiation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] The present invention relates to hydrophobic and oleophobic
filters that have high gas permeabilities and that repel water and
other liquids. The invention also relates to methods of preparing
such filters.
[0021] The filter medium substrate is treated with a coating
material comprising crosslinked vinyldimethyl terminated siloxane,
which treatment coats the surface of the substrate. Coating the
substrate with a material comprising crosslinked vinyldimethyl
terminated siloxane gives permanent oleophobicity and
hydrophobicity to the filter. The treated filters have high
permeabilities for air flow and reject liquid water, as evidenced
by high water penetration pressures. The filters are useful, for
example, as air filters or vent filters for intravenous (IV) or
other medical devices. The critical surface tension for spreading
(.gamma..sub.c), which is defined as the wettability of a solid
surface by noting the lowest surface tension a liquid can have and
still exhibit a contact angle (.theta.) greater than zero degrees
on that solid, was dramatically reduced after treatment of the
substrates according to the process of the invention.
[0022] The process can be used to coat substrates made from sulfone
polymers such as polysulfone, polyethersuflone, or polyarylsulfone,
as well as other polymers, such as polyvinylidene fluoride (PVDF),
polytetrafluoroethylene (PTFE), polyolefins including polyethylene
and polypropylene, poly(tetrafluoroethylene-co-ethylene) (ECTFE),
acrylic copolymers, polyamides, polyesters, and polyurethanes.
[0023] For the present invention, substrates may include not only
flat sheet polymer membranes made from casting a
polymer-solvent-nonsolvent dope mix, but substrates may also refer
to any other suitable filtration or exclusion medium. Non-limiting
examples of other media within the meaning of substrate include
hollow fibers, melt blown or other nonwoven media, woven media, or
sedimented structures. In addition, filters of the present
invention may be composites, such as, for example, composites
having different layers of any of the foregoing media, composites
having multiple layers of the same medium, or composites having
layers of the same medium, but of different pore sizes, porosities,
geometries, orientations, and the like.
[0024] In accordance with the invention, the substrate can be
coated by any workable method. A few examples of approaches to coat
formation are provided herein. However, the possible useful coating
methods are not limited to the methods listed below:
[0025] 1. Crosslinking the coat formulation to the substrate by
moisture curing polydimethyl siloxane with the crosslinker
methyltriacetoxy silane.
[0026] 2. Polymerizing vinyl terminated siloxane (structure I,
below) with a crosslinker such as hydrosilicone in the presence of
a catalyst. The reaction is shown below.
[0027] 3. Crosslinking a coating to a substrate by curing a
siloxane coating on a substrate by exposure to ultraviolet (UV)
radiation.
[0028] 4. Heat crosslinking with methyl silicone at a temperature
above 100.degree. C.
[0029] In the first method, the substrate is impregnated with
polydimethyl siloxane and the crosslinker methyltriacetoxy silane
to form a coat on the substrate. The coat is then cured with
moisture to further bond the coat to the substrate. The moisture in
the air slowly cures the siloxane polymers in a process that may
require more than 12 hours. The moisture cure systems can employ
the most common crosslinkers such as, for example, acyloxy, enoxy,
and oxime crosslinkers.
[0030] In the second method of forming the coating, a vinyl
terminated siloxane is reacted with a crosslinker such as
hydrosilicone in the presence of a catalyst. The structure of the
vinyl terminated siloxane is shown below as I: 1
[0031] The reaction which occurs between the vinyl terminated
siloxane and the hydrosilicone is: 2
[0032] The vinyl terminated siloxane can have one or more vinyl
groups. The weight % of vinyl groups can range from 0.1 to 0.4 wt
%. The viscosity of the vinyl terminated siloxane can be from 500
to 70,000 centipoise (cps), more preferably from 800 to 10,000 cps,
and most preferably from 1,000 to 5,000 cps
[0033] The crosslinker is a compound which contains one or more
silicon-hydrogen bonds. Hydrosilicone is an exemplary crosslinker.
Methylhydro, trimethylsilyl-terminated dimethylsiloxane is another
preferred crosslinker. In a preferred embodiment, the crosslinker,
hydrosilicone, has viscosity of 25-35 cps and a molecular weight of
500 to 10,000 Daltons. The weight % of the methylhydro units in the
polymer is about 15 to 50%. Other crosslinkers, such as
methylhydro, cyanopropylmethylsiloxane, methylhydro, methyloctyl
siloxane, and dimethylsiloxy-terminated methylhydro, phenylmethyl
siloxane also can be used. The silicon-hydrogen bond of the
crosslinker reacts with the double bond of the vinyl group of the
vinyl terminated siloxane. The weight ratio of vinyl terminated
siloxane to crosslinker is between 20 and 0.1, more preferably
between 15 and 0.5, and most preferably between 10 and 1.
[0034] In many crosslinker/catalyst formulations, the formulation
may contain 5-10 ppm platinum, which is preferably in the form of
platinum 1,3-diethylenyl-1,1,3,3-tetramethyldisiloxane complexes.
Platinum divinyltetramethylsiloxane complex is an exemplary noble
metal catalyst for forming the coating of the invention. The noble
metal catalyst is normally present in a concentration of 1 to 100
ppm, more preferably 5 to 10 ppm, calculated as the weight of the
noble metal catalyst. The noble metal catalyst can be formed
in-situ, or it can be formed prior to addition to a reaction
solution. Although the noble metal catalyst can be insoluble or
soluble in the reaction solution, it is generally preferred that
the noble metal catalyst be soluble in the reaction solution.
Nonlimiting examples of noble metals are nickel, copper, palladium,
silver, platinum, and gold. Other catalysts such as zinc and tin
chlorides, zinc acetates, zinc octoates, and peroxides can also be
used.
[0035] The vinyl terminated siloxane, crosslinker, and noble metal
catalyst may be dissolved in a solvent. The solvent may be a
hydrocarbon such as, for example, hexane or toluene. The selected
solvent should not react with or dissolve the substrate or the
crosslinked coating.
[0036] The coating formulation containing the vinyl terminated
siloxane, crosslinker, noble metal catalyst, and solvent, is
contacted with the substrate at a temperature between 15 and
30.degree. C. In a preferred embodiment, the contacting takes place
at approximately room temperature. The two part crosslinker systems
thus may include catalysts and silanol-terminated polymers with a
molecular weight of about 26,000 to 200,000 Daltons.
[0037] The substrate is soaked in the solution for about 15 seconds
to 5 minutes, more preferably 30 seconds to 3 minutes, and most
preferably 1 to 2 minutes. The coated substrate is then removed
from the coating solution and is air dried for 1 to 180 minutes,
then oven cured at a temperature of 100 to 150.degree. C., more
preferably 105 to 130.degree. C., and most preferably 110 to
120.degree. C. for 1 to 180 minutes, more preferably 5 to 120
minutes, and most preferably 10 to 60 minutes, to produce the
coated filter of the invention.
[0038] As a third alternative, crosslinking may be achieved by
impregnating the substrate with polydimethyl siloxane and then
exposed to ultraviolet (UV) radiation to cure the coating on the
substrate. Likewise, in a fourth embodiment, methyl silicone may be
impregnated into the substrate to form the coat, and the coat then
may be crosslinked by heat curing at a temperature above
100.degree. C.
[0039] Vinyl-terminated siloxanes or vinyl-terminated
fluorosiloxanes may be dissolved in a solvent selected from the
group consisting of fluorocarbons, hydrocarbons, and alcohols such
as, for example, isopropanol. Preferably, the solvent is not a
solvent of the substrate, and can be a hydrocarbon such as, for
example, hexane. The solution containing solvent and
vinyl-terminated siloxanes and crosslinkers is contacted with the
substrate at a temperature of about 15 to 30.degree. C. for about
30 seconds to 5 minutes. Preferably, the contacting takes place at
approximately room temperature for several seconds to 2
minutes.
[0040] Embodiments of the coating process can be used to coat
substrates including asymmetric or isotropic membranes, or other
media such as, for example, melt blown, woven, and non woven
material. Melt blown material may include polypropylene or ECTFE,
and are commercially available from U.S. Filter/Filtrate Division,
Timonium, MD.
[0041] The substrate can be treated with sufficient coating agent
so that the coated filter contains at least 0.1 wt % of the coat
comprising vinyldimethyl-terminated siloxane, more preferably
between 0.5 and 6 wt % coat material, and most preferably 1 to 3 wt
% coat material.
[0042] The hydrophobic, oleophobic filters of the invention,
employing any useful substrate, also can be bonded to a textile
fabric or other woven or nonwoven material to form a layered fabric
capable of excluding the passage of liquid while allowing passage
of vapors and gasses therethrough. Such a layered fabric can be
useful in a variety of applications, as will be appreciated by
those or ordinary skill in the art. Bonding a hydrophobic,
oleophobic filter to a fabric can be accomplished by conventional
adhesives, thermal bonding, and the like, and can also be achieved
by layering the filtration medium substrate together with the
fabric, and curing or otherwise crosslinking the coating
formulation thereafter. In this embodiment, the substrate may be
coated prior to layering, or the coating may be simultaneously
with, or after, the layering of substrate with fabric.
EXAMPLES
[0043] The following examples are provided to illustrate the
present invention. However, such examples are merely illustrative
and are not intended to limit the subject matter of the
application.
[0044] The first three examples demonstrate the modification of the
surfaces of substrates having three different pore sizes by
chemical crosslinking.
Example 1
Oleophobic Modification of Very Large Pore Microfiltration
Membranes by Chemical Crosslinking
[0045] A 1.8 .mu.m polysulfone membrane (BTS-X, sold by US Filter,
San Diego, Calif.) was treated with a hexane solution containing 1%
by weight vinyldimethyl terminated siloxane (available from United
Chemical Technologies (UCT), Bristol, PA), 0.1 wt % hydrosilicone
(also available from UCT), and 10 ppm platinum
divinyltetramethyldisiloxane catalyst (also available from UCT).
The membrane was soaked for a few seconds in the solution. The
membrane was then air dried for one minute and oven cured at
140.degree. C. for 10 minutes (Example 1a) or 15 minutes (Example
1b).
[0046] The weight percent of coating on the filter was determined
in all of the examples by weighing the filters before and after
treatment. The weight gain divided by the total weight of the
filter after treatment is the weight percent. A filter weighing 99
mg before treatment and 100 mg after treatment would have a coating
value of 1 wt %. The average coating value for filters treated
according to Example 1 is about 1 wt %.
[0047] The resulting samples were tested for water penetration
pressure and air flow. Both tests were performed with a 90 mm disk,
having an effective filtration area of about 45 cm.sup.2. The
results are shown in Table 1. The water penetration pressures of
the treated filters were 20% (Example 1a) and 40% (Example 1b)
higher than for the untreated substrate. The air flow rates for the
treated filters were equal (Example 1a) and 10% higher (Example 1b)
than for the untreated substrate.
Example 2
Oleophobic Modification of Large Pore Microfiltration Membranes by
Chemical Crosslinking
[0048] A 1.2 .mu.m polysulfone membrane (BTS-5H, US Filter, San
Diego, Calif.) was treated with a hexane solution containing
(Example 2a) 1% by weight vinyldimethyl terminated siloxane, 0.1 wt
% hydrosilicone, and 10 ppm platinum divinyltetramethyldisiloxane
catalyst or (Example 2b) 2% by weight vinyldimethyl terminated
siloxane, 0.2 wt % hydrosilicone, and 20 ppm platinum
divinyltetramethyldisiloxane. Each membrane was soaked for a few
seconds in the solution. The membranes were air dried for one
minute then oven cured at 140.degree. C. for 30 minutes.
[0049] The resulting samples were tested for water penetration
pressure and air flow. Both tests were performed with a 90 mm disk.
The results are shown in Table 1. The water penetration pressures
of the treated membranes were 57% (Example 2a) and 43% (Example 2b)
higher than the untreated membrane. The air flow rates for the
treated membranes were 80% (Example 2a) and 8% (Example 2b) higher
than for the untreated membrane.
[0050] The substrate of Example 2a was reacted with twice as much
siloxane, silane, and catalyst as the substrate of Example 2b. The
air flow of the filter of Example 2a was higher than the air flow
for the filter of Example 2b. The higher concentration of
components in the coating compound in the filter of Example 2b led
to a relatively lower air flow.
Example 3
Oleophobic Modification of a Smaller Pore Microfiltration Membrane
by Chemical Crosslinking
[0051] A 0.2 .mu.m hydrophobic polysulfone membrane (BTS-55H, US
Filter, San Diego, Calif.) was treated with a hexane solution
containing 1% by weight vinyldimethyl terminated siloxane, 0.1 wt %
hydrosilicone, and 10 ppm platinum divinyltetramethyldisiloxane
catalyst. The substrate was soaked in the solution for a few
seconds. The filter was air dried for one minute, then oven cured
at 140.degree. C. for 15 minutes.
[0052] The resulting filter was tested for water penetration
pressure and air flow. Both tests were performed with a 90 mm disk.
The results are shown in Table 1. The water penetration pressure of
the treated filter was 62% higher than for the untreated substrate.
The air flow rate for the treated filter was 13% lower than for the
untreated substrate.
[0053] The following example shows that the treatment can be
effectively performed using either hydrophobic or hydrophilic
substrates.
Example 4
Oleophobic Modification of Hydrophobic and Hydrophilic
Microfiltration Membranes by Chemical Crosslinking
[0054] Hydrophobic (Example 4a) and hydrophilic (Example 4b) 0.45
.mu.m polysulfone membranes (BTS-25H and BTS-25, US Filter, San
Diego, Calif.) were treated with a hexane solution containing 1% by
weight vinyldimethyl terminated siloxane, 0.1 wt % hydrosilicone,
and 10 ppm platinum divinyltetramethyldisiloxane catalyst. The
membranes were soaked in the solution for a few seconds, air dried
for one minute, then oven cured at 140.degree. C. for 15
minutes.
[0055] The resulting samples were tested for water penetration
pressure and air flow. Both tests were performed with a 90 mm disk.
The results are shown in Table 1. The water penetration pressure of
the treated filter was 9% higher (Example 4a) than for the
untreated substrate. The hydrophilic membrane of Example 4b had a
water penetration pressure of 0 psi before treatment and 30 psi
after treatment. The air flow rates for the treated filters were
both about 20% lower than for the corresponding untreated
substrates.
1TABLE 1 Air Flow and Water Penetration Results for Untreated
Substrates Compared with Filters Treated by Chemical Crosslinking
Water Water Example Air Flow Penetration Penetration Number Air
Flow* Change.sup..dagger. Pressure* Pressure Change.sup..dagger. 1
(Control) 55.7 -- 5 -- 1a 55.7 none 6 +20% 1b 61.2 +10% 7 +40% 2
(Control) 18.9 -- 7 -- 2a 34 +80% 11 +57% 2b 20.4 +8% 10 +43% 3
(Control) 9.2 -- 26 -- 3 8 -13% 42 +62% 4a (Control) 13.6 -- 20 --
4a 10.9 -20% 38 +90% 4b (Control) 13.6 -- 0 -- 4b 10.9 -20% 30
undefined (.infin.) *Air flow is expressed in units of
(ml/min/cm-H.sub.2O/cm.sup.2). Water penetration pressure is
expressed in psi. .sup..dagger.Changes in air flow and water
penetration are compared to control values for each example.
[0056] The examples in Table 1 show that substrates having a wide
range of pore sizes can be made oleophobic and hydrophobic by being
coated with a coat comprising polydimethylsiloxane. Further, both
hydrophobic and hydrophilic substrates can be treated to be
hydrophobic and oleophobic.
[0057] In the following example, the coat was made by heat
crosslinking rather than by chemical crosslinking.
Example 5
Oleophobic Modification of Large Pore Microfiltration Membranes by
Heat Crosslinking
[0058] A solution containing 50 wt % methyltrimethoxysilane
(available from UCT) and 50 wt % isopropylalcohol (IPA) was
prepared. Two moles of water per mole of methyltrimethoxysilane
were added to the methyltrimethoxysilane/IPA solution. The combined
solution was then heated at 60-70.degree. C. for several hours to
hydrolyze the methyltrimethoxysilane. The resulting solution was
then diluted with IPA to 2 wt % of the polymer.
[0059] The solution was used to coat a large pore hydrophobic
polysulfone microfiltration membrane with a pore size of 1.2 .mu.m
(BTS-5H). The substrate was soaked for a few seconds, air dried for
one minute, then heat crosslinked at 140.degree. C. for 12 minutes.
A 90 mm disk of the crosslinked filter was tested for water
penetration pressure and air flow. The results are shown in Table
2. The coated filter had a higher water penetration pressure than
the untreated substrate (15 psi versus 7 psi). The air flow rate
for the treated filter was 20% higher than the air flow rate for
the untreated substrate.
2TABLE 2 Air Flow and Water Penetration Results for an Untreated
Substrate Compared with a Filter Treated by Heat Crosslinking Water
Water Example Air Flow Penetration Penetration Number Air Flow*
Change.sup..dagger. Pressure* Pressure Change.sup..dagger. 5
(Control) 18.9 -- 7 -- 5 22.7 +20% 15 +114% *Air flow is expressed
in units of (ml/min/cm-H.sub.2O/cm.sup.2). Water penetration
pressure is expressed in psi. .sup..dagger.Changes in air flow rate
and water penetration are compared to control values for each
example.
[0060] In the following example, a polyvinylidene difluoride
membrane was treated by chemical crosslinking, demonstrating that a
variety of substrates can be treated according to the
invention.
Example 6
Oleophobic Modification of a Hydrophilic PVDF Microfiltration
Membrane by Chemical Crosslinking
[0061] A 1.0 .mu.m hydrophilic polyvinylidene difluoride (PVDF)
membrane (commercially available from U.S. Filter/Filtration
Division, San Diego, Calif.) was treated with a hexane solution
containing 1% by weight vinyldimethyl terminated siloxane, 0.1 wt %
hydrosilicone, and 10 ppm platinum divinyltetramethyldisiloxane
catalyst. The substrate was soaked for a few seconds in the
solution. The membrane was air dried for one minute then oven cured
at 140.degree. C. for 15 minutes.
[0062] The resulting filter was tested for water penetration
pressure and air flow. Both tests were performed with a 90 mm disk.
The results are shown in Table 3. The water penetration pressure of
the treated filter was 5 psi versus 0 psi for the untreated
substrate. The hydrophobicity of the treated filter was therefore
increased by the treatment. The air flow rate for the treated
filter was 28% lower than the air flow rate for the untreated
substrate.
3TABLE 3 Air Flow and Water Penetration Results for an Untreated
PVDF Membrane Compared with a PVDF Membrane Treated by Chemical
Crosslinking Water Water Example Air Flow Penetration Penetration
Number Air Flow* Change.sup..dagger. Pressure* Pressure
Change.sup..dagger. 6 (Control) 29.2 -- 0 -- 6 21.1 -28% 5
undefined (.infin.) *Air flow is expressed in units of
(ml/min/cm-H.sub.2O/cm.sup.2). Water penetration pressure is
expressed in psi. .sup..dagger.Changes in air flow rate and water
penetration are compared to control values for each example.
[0063] Treating the PVDF membrane to form a coat comprising
polydimethylsiloxane therefore made the hydrophilic PVDF
hydrophobic.
Example 7
Laminated Oleophobic Filters
[0064] Filters from Example 2 were laminated to a nonwoven
polyester substrate, Hollytex 3256, with a layer of polyethylene
(PE85) in between the filter and the polyester substrate. Hollytex
3256 is commercially available from Ahlstrom, Mount Holly Spring,
Pa.; PE85 is commercially available from Bostic, Inc., Dana Point,
Calif. Lamination setup temperature was about 138.degree. C. The
resulting laminated oleophobic filters displayed improved
mechanical strength as compared with the filter or the polyester
substrate alone, and were also tested for air flow rate and water
penetration pressure. The air flow was the same as in Example 2b,
and the water penetration pressure was further improved to be about
17 psi as compared to 10 psi from Example 2b.
Example 8
Determination of Oleophobic Nature of Modified Filters
[0065] The relative oleophobicity of modified filters and
unmodified substrates is determined by testing the filters and
substrates (collectively, filtration media) for their ability to be
wetted by a low surface-tension fluid. A drop of 2-methoxyethanol
is gently placed on the surface of the filtration medium using a
glass pipette, and the wetting time is recorded. If the medium is
not wetted by the 2-methoxyethanol within 30 seconds, the result is
recorded as "No Wetting". Filters of the invention are generally
resistant to wetting by 2-methoxyethanol, and are relatively more
oleophobic than untreated substrates.
Equivalents
[0066] The present invention has been described in connection with
specific embodiments thereof. It will be understood that it is
capable of further modification, and this application is intended
to cover any variations, uses, or adaptations of the invention
following, in general, the principles of the invention and
including such departures from the present disclosure as come
within known or customary practices in the art to which the
invention pertains and as may be applied to the essential features
hereinbefore set forth, and as fall within the scope of the
invention and any equivalents thereof.
* * * * *