Non-porous gas permeable membrane

A process for placing an ultra thin layer of a non-porous gas permeable polymer continuously over an entire filter surface area allows the fabrication of compact, high flux, fouling resistant gas filters. The process involves contacting a dilute coating solution of gas permeable polymer in a solvent with one side of a microporous substrate. The pore size of the substrate is chosen for its ability to effectively filter the gas permeable polymer from the coating solution. Solvent of the coating solution is made to flow through the microporous substrate which causes an ultra thin layer of polymer to build up on the side of substrate. When a desired thickness of polymer is built up, the solution and solvent is removed and residual solvent is evaporated, preferably by passing a gas at high rate over the surface of the polymer layer. This process can be used to coat flat sheet and hollow fiber substrates. The process is particularly useful for coating multiple hollow fibers assembled in modules which by virtue of the high surface area density of small diameter fibers and the efficient packing of many fibers in the modules, provide very high filter surface area in a small volume. The coated fiber modules thus can be used as high flow rate gas filters in applications such as filtering nuisance particules from recovered gas streams, collecting hazardous or valuable droplets suspended in a gas process stream for protection or salvage. Specific applications include filtering air for microelectronic circuit assembly facilities and filtering microbes from the atmosphere in biomedical "clean rooms". A preferred gas permeable polymer for use with this invention is an amorphous copolymer of perfluoro-2,2-dimethyl-1,3-dioxole. The superior hydrophobicity and oleophobicity of this material in the form of a non-porous, continuous layer provides a barrier for solid particles and liquid droplets from blocking flow through a gas filter. Furthermore, a gas filter made according to this invention can be easily cleaned and restored to near-original gas flux performance.

FIELD OF THE INVENTION 
This invention relates to a process for producing gas permeable membranes. 
More specifically, the invention pertains to non-porous gas permeable 
membranes for separating a gas from an aerosol, and especially to 
fluoropolymer membrane filters for removing ultra-fine solid particles or 
liquid droplets suspended in gas. 
BACKGROUND AND SUMMARY OF THE INVENTION 
Many modern industrial processes involve contacting liquids or solids with 
a gas which produces a suspension of liquid droplets and/or solid 
particles in the gas. Frequently, the suspended substance is a hazardous 
or expensive material or merely is nuisance contamination in a valuable 
gas stream. Filtration of droplets and particles from gas thus becomes 
important for many commercial applications such as recovery or containment 
of valuable or dangerous particulate materials; venting purified exhaust 
gas for disposal to the environment; and purifying a contaminated gas 
stream for use as a raw material in a later process step. 
Membrane technology increasingly is applied to the filtration of industrial 
gases. Because fluoropolymers have certain physical properties such as 
good hydrophobicity, inertness to a variety of chemical and biological 
materials, and good thermal stability, these materials, and especially 
microporous, expanded polytetrafluoroethylene ("E-PTFE"), are popular for 
use in porous membrane filters. One notable shortcoming is that oil "wets" 
E-PTFE. Wetting refers to the affinity of a liquid for the membrane 
material. E-PTFE can become so wet with oil that the oil clogs the pores 
of the membrane. This reduces and sometimes totally blocks the gas flow 
through the membrane. Oil is present in a large number of gas processing 
applications, including oil lubricated compression and automotive 
applications, for example. Hence, the oleophilic nature of E-PTFE 
significantly reduces the effectiveness of this material in membrane 
filtration. 
A gas permeable membrane with both high hydrophobicity and oleophobicity 
has been sought for improved gas filtration performance. U.S. Pat. No. 
5,554,414 of Moya et al. provides a process for producing a composite 
porous article having a porous polymeric substrate and a 
hydrophobic/oleophobic polymeric surface formed from a cross-linked 
ethylenically unsaturated monomer containing a fluoroalkyl group. The 
polymeric surface is formed by coating a porous membrane substrate with a 
solution of a polymerizable monomer, a cross-linking agent, and a 
polymerization initiator. The polymerizable monomer is polymerized and 
cross-linked onto the porous membrane substrate in a way that the entire 
surface of the porous membrane, including the inner surfaces of the porous 
membrane, is modified with a cross-linked polymer. However, the composite 
porous article retains substantially all of the original properties of the 
substrate, particularly porosity. 
U.S. Pat. No. 5,116,650 of Bowser describes the use of an amorphous 
copolymer of 10-40 mole percent tetrafluoroethylene ("TFE") and a 
complementary amount of perfluoro-2, 2-dimethyl-1, 3-dioxole ("PDD") for a 
gas filter. The amorphous copolymer is coated onto a gas permeable 
material which has passageways, or continuous pores, through the material. 
The amorphous copolymer coats at least a portion of the interior of the 
passageways but does not block them. 
The above-cited references describe completely porous membrane structures 
for gas filtration. Gas molecules can travel readily through such a 
structure via the passageways formed by the pores. As a result porous 
structures generally provide high gas flux, that is, gas transmission per 
unit of filter surface area. Hence, a moderately sized porous filter 
element usually can transfer gas at industrially acceptable rate. Although 
the pores are coated with enhanced oleophobic compositions to reduce the 
tendency of oil to adhere to the membrane, the open pores still provide 
the opportunity for oil to penetrate and eventually clog the membrane. 
Solid particles that may be suspended in a gas can also enter and occlude 
the pores. Additionally, penetrating liquid and solid contaminants can 
become embedded in the pores and can be difficult to clean out. Thus, the 
gas flow through a porous membrane gas filter can decrease over time in 
service. 
A practical, non-porous permeable membrane for a gas filter has not been 
available previously. Gas flux through a non-porous permeable membrane is 
directly proportional to permeability of the membrane composition and 
inversely proportional to membrane thickness. Most polymeric compositions 
have low gas permeability. Consequently, to provide a filter element of 
practical size surface area with industrially significant gas flux, a 
non-porous membrane of even moderately high permeability would need to be 
extremely thin. Heretofore a method for making a sufficiently thin 
non-porous gas permeable membrane for a gas filter has not been known in 
the art. 
A process for making a membrane structure comprising an ultra-thin, 
continuous layer of a non-porous, gas permeable polymer composition now 
has been discovered. The polymer composition has good permeability which 
provides high initial gas flux. Additionally, the non-porous structure of 
the continuous layer imparts superior resistance to oil and solid particle 
penetration and improved stability of gas flux. Furthermore, if the novel 
membrane structure becomes fouled, it can be cleaned easily to restore gas 
flux to near-original gas transmission rate. As a consequence of this 
invention, it is now possible to produce a very thin film of a non-porous, 
gas permeable polymer in a membrane structure adaptable for use as a gas 
filter and for other gas transfer operations. 
Accordingly, this invention provides a process for making a membrane 
structure comprising the steps of: 
(a) dissolving a gas permeable polymer in a solvent to obtain a coating 
solution; 
(b) selecting a microporous substrate of a pore size effective for 
filtering dissolved polymer from the coating solution, the substrate 
having a first side, and a second side; 
(c) contacting the first side of the microporous substrate with the coating 
solution; 
(d) making the solvent flow through the microporous substrate to the second 
side; 
(e) removing coating solution and solvent from the membrane structure; and 
(f) evaporating solvent from the membrane structure, thereby forming a 
continuous, non-porous layer of the gas permeable polymer on the first 
side. 
In another aspect, the present invention also provides a process for 
coating hollow fibers with an ultra-thin, continuous layer of a gas 
permeable polymer. 
Additionally, there is provided a process for making a gas filter 
comprising the steps of: 
(a) dissolving a gas permeable polymer in a solvent to obtain a coating 
solution; 
(b) providing a filter module including 
(1) an elongated casing having two ends, the casing defining a shell side 
cavity; 
(2) a first tube sheet at one end of the casing having a first tube sheet 
outboard face; 
(3) a second tube sheet at the other end of the casing having a second tube 
sheet outboard face; 
(4) a plurality of open ended, microporous hollow fibers extending in 
substantially parallel alignment within the casing from the first tube 
sheet outboard face to the second tube sheet outboard face, the hollow 
fibers collectively defining a tube side cavity; wherein the pore size of 
the hollow fibers is effective to filter dissolved polymer from the 
coating solution; and 
(5) at least one shell side port through the casing; 
(c) causing the coating solution to flow through one of the shell side 
cavity and the tube side cavity; 
(d) making the solvent flow from the coating solution through the 
microporous hollow fibers to the other of the shell side cavity and the 
tube side cavity; 
(e) draining coating solution and solvent from the module; and 
(f) evaporating solvent from the hollow fibers thereby forming a 
continuous, non-porous layer of the gas permeable polymer on one side of 
the hollow fibers. 
In another aspect, the present invention provides a method of separating a 
gas from an aerosol comprising permeating the gas through a filter surface 
area of a membrane structure including 
a microporous substrate; and 
a non-porous gas permeable layer on the substrate and continuous over the 
entire filter surface area of an amorphous copolymer of 
perfluoro-2,2-dimethyl-1,3-dioxole having a permeability to oxygen of at 
least 100 barrers at a temperature below the glass transition temperature 
of the amorphous copolymer. 
Still further there is provided a novel gas filter comprising a membrane 
structure having a filter surface area for permeating a gas to separate 
suspended droplets from the gas, the membrane structure comprising: 
a microporous substrate having a pore size of about 0.005-0.1 .mu.m; and 
a non-porous gas permeable layer on the substrate and continuous over the 
entire filter surface of an amorphous copolymer of 
perfluoro-2,2-dimethyl-1,3-dioxole having a permeability to oxygen of at 
least 100 barrers at a temperature below the glass transition temperature 
of the amorphous copolymer.

DETAILED DESCRIPTION 
In one aspect, the present invention involves a method of separating a gas 
from an aerosol. The term "aerosol" means a suspension of fine liquid 
droplets or solid particles, (hereinafter, collectively, "droplets") in a 
gas. The invention is suitable for filtering either liquid droplets, solid 
particles or both simultaneously. Thus the present invention can be 
utilized to make a more concentrated aerosol by removing a portion of the 
gas or to substantially completely remove the gas for collection of the 
droplets. The purified exhaust gas will be substantially free of droplets, 
hence the invention can be used to obtain a clean gas from an aerosol. 
The size of the droplets is determined by various factors. These can 
include system pressure and temperature, physical properties of the 
droplets such as composition and liquid viscosity, and the method by which 
the droplets are created, e.g., by condensation and by atomization. The 
droplets can be of uniform size or have a size distribution. Generally, 
the size of droplets suspended in the aerosol will lie in the range of 
about 0.01 .mu.m to about 1 mm. The concentration of droplets is not 
critical. It should be appreciated that liquid droplets normally will 
coalesce on contact. 
Separation is effected by filtering the gas through a gas filter which 
includes a gas permeable membrane structure. The nature of the gas in the 
aerosol is not particularly important as long as the gas can permeate the 
membrane structure. However, one can readily appreciate that the gas 
should not react with or otherwise adversely affect the materials of 
construction of the membrane structure or parts of the gas filter to which 
the gas is exposed. Representative gaseous components include elemental 
gases such as helium, hydrogen, neon, nitrogen, argon, oxygen, krypton and 
xenon; hydrocarbons such as methane, ethylene, ethane, acetylene, propane, 
propylene, cyclopropane, butane and butylene; halocarbons or 
halohydrocarbons such as dichlorodifluoromethane, methylene chloride, and 
methyl chloride; and miscellaneous industrial and environmental gases such 
as nitrous oxide, carbon dioxide, ozone, hydrogen sulfide, ammonia, sulfur 
dioxide, carbon monoxide, phosgene and any mixture of any of them. 
One element of the membrane structure is a non-porous film of a gas 
permeable substance. Preferably, the gas permeable substance is an 
amorphous copolymer of a certain perfluorinated dioxole monomer, namely 
perfluoro-2,2-dimethyl-1,3-dioxole ("PDD"). In some preferred embodiments, 
the copolymer is copolymerized PDD and at least one monomer selected from 
the group consisting of tetrafluoroethylene ("TFE"), perfluoromethyl vinyl 
ether, vinylidene fluoride and chlorotrifluoroethylene. In other preferred 
embodiments, the copolymer is a dipolymer of PDD and a complementary 
amount of TFE, especially such a polymer containing 50-95 mole % of PDD. 
Examples of dipolymers are described in further detail in U.S. Pat. Nos. 
4,754,009 of E. N. Squire, which issued on Jun. 28, 1988; and 4,530,569 of 
E. N. Squire, which issued on Jul. 23, 1985. Perfluorinated dioxole 
monomers are disclosed in U.S. Pat. No. 4,565,855 of B. C. Anderson, D. C. 
England and P. R. Resnick, which issued Jan. 21, 1986. The disclosures of 
all of these U.S. patents are hereby incorporated herein by reference. 
The amorphous copolymer can be characterized by its glass transition 
temperature ("T.sub.g "). The polymer property of glass transition 
temperature is well understood in the art. It is the temperature at which 
the copolymer changes from a brittle, vitreous or glassy state to a 
rubbery or plastic state. The glass transition temperature of the 
amorphous copolymer will depend on the composition of the specific 
copolymer of the membrane, especially the amount of TFE or other comonomer 
that may be present. Examples of T.sub.g are shown in FIG. 1 of the 
aforementioned U.S. Pat. No. 4,754,009 of E. N. Squire as ranging from 
about 260.degree. C. for dipolymers with 15% tetrafluoroethylene comonomer 
down to less than 100.degree. C. for the dipolymers containing at least 60 
mole % tetrafluoroethylene. It can be readily appreciated that 
perfluoro-2,2-dimethyl-1,3-dioxole copolymers according to this invention 
can be tailored to provide sufficiently high T.sub.g that a membrane of 
such composition can withstand exposure to steam temperatures. Hence, 
membranes of this invention can be made steam sterilizable and thereby 
suitable for various uses requiring sterile materials, especially those 
involving biological materials. Preferably, the glass transition 
temperature of the amorphous copolymer should be at least 115.degree. C. 
The amorphous copolymer is further characterized by substantial 
hydrophobicity and oleophobicity. This incompatibility of the PDD 
copolymer with both water and oil also makes the gas permeable membrane 
not more than negligibly soluble or swellable in a wide range of liquids. 
This characteristic assures the preservation of the structural integrity 
and dimensional stability of the membrane while in contact with many 
liquid compositions. 
The shape of the membrane structure of the present invention can be a flat 
sheet or other geometric configuration. A flat sheet can comprise one or 
more monolithic films of the non-porous, gas permeable substance. Gas flux 
through a permeable membrane is inversely proportional to the thickness 
and directly proportional to the gas transport area of the membrane. One 
of skill in the art will readily appreciate that to obtain a practically 
acceptable gas flux through a gas permeable film of reasonable surface 
area, a very thin film should be used. This is true even though the 
permeability of many commercially significant gases through the amorphous 
copolymer preferred for use in this invention is quite high. The preferred 
non-porous film thickness for desirable gas flux is about 0.01 to about 25 
.mu.m. 
Polymer film of less than about 12 .mu.m generally is non-self supporting. 
Thus, in a preferred embodiment, the gas permeable membrane structure of 
this invention comprises an amorphous copolymer present as a non-porous 
layer on a microporous substrate. The substrate maintains structural 
integrity of the non-porous layer in service. The structure of the 
substrate should be designed to have porosity so as not to impede the flow 
of the gaseous component. Representative porous substrates include a 
perforated sheet; a porous mesh fabric; a monolithic microporous polymer 
film; a microporous, hollow fiber and a combination of them. 
The non-porous layer is located adjacent or directly on the microporous 
substrate and may be manufactured by any of a variety of methods known to 
those skilled in the art, including coating techniques such as dipping, 
spraying, painting and screeding. Preferably, the non-porous layer will be 
applied by a solvent coating method, and more preferably, by a novel 
solvent coating method suitable for placing an ultra-thin, continuous, 
non-porous amorphous copolymer layer onto a microporous substrate, as will 
be explained in greater detail, below. In context of thickness of the 
non-porous layer, the term "ultra-thin" means about 0.01 to about 10 
.mu.m. 
The membrane structure can also have a tubular configuration. A hollow 
fiber is a particularly preferred form of substrate for use in the present 
invention. The term "hollow fiber" refers to high aspect ratio bodies with 
extremely small cross section dimensions. By "high aspect ratio" is meant 
the ratio of the fiber length to fiber cross section dimension. Although 
other hollow shapes are possible and are contemplated to fall within the 
breadth of the present invention, cylindrical hollow fibers are preferred. 
The fiber outer and inner diameter generally is about 0.1-1 mm and about 
0.05-0.8 mm, respectively. 
The separation process of this invention basically is carried out by 
placing the aerosol in contact with the gas permeable membrane component 
of a membrane structure in a gas filter and allowing the gas to permeate 
through the membrane Leaving the droplets in the aerosol. The term "filter 
surface area" means the effective area available for gas transport. 
Generally, the filter surface area is the gas transport area of the 
membrane measured normal to the direction of gas flow. For example, the 
filter surface area of a rectangular flat sheet membrane is the product of 
sheet length and width. Similarly, the filter surface area of a single, 
cylindrical hollow fiber is the product of the fiber length and the 
circumference of the cylinder. 
The preference for hollow fiber substrate derives from the ability to 
create a very large filter surface area in a small volume, and especially, 
in a volume of small overall cross sectional area. The filter surface area 
of a hollow fiber per unit of fiber volume increases inversely with the 
diameter of the fiber. Thus, surface area density of individual small 
diameter hollow fibers is very great. Additionally, a large number of 
fibers can be bundled substantially parallel to the axis of fiber 
elongation and manifolded. This effectively pools the filter surface area 
to the total of the bundled individual fiber filter surface areas. Due to 
the fiber geometry, a total effective filter surface area of a hollow 
fiber bundle can be many multiples of the overall cross sectional area of 
the gas filter unit. Hollow fiber substrate also is preferred because the 
surface area very effectively contacts the aerosol. That is, aerosol flow 
can be directed through bundled hollow fibers in the fiber axial direction 
in a way that the aerosol sweeps across all of the available gas filter 
area. In contrast, a gas filter based upon flat sheet filter elements can 
have poorly purged "dead spaces" of aerosol and/or filtered gas in 
stagnant contact with the elements which causes a reduced rate of transfer 
through the filter elements. Nevertheless, an ultra-thin, non-porous layer 
on a flat sheet substrate according to the present invention can provide a 
highly effective and useful gas filter. 
According to the present invention, the non-porous layer of amorphous 
copolymer is continuous over the entire filter surface area of the 
membrane. That is, the non-porous layer is coextensive with the substrate 
and uninterrupted, being substantially free of voids, perforations or 
other channels which could provide open passageways through the membrane 
for gaseous communication between the aerosol side and gas side of the gas 
filter other than by permeation. Preferably, the non-porous layer is on 
the aerosol side of the gas permeable membrane. It can be appreciated that 
the non-porous layer presents an uncompromised barrier to penetration of 
liquid droplets or solid particles into the micropores of the membrane. 
The separation process of the present invention thus provides a high flux, 
gas filter that resists clogging so that the high gas flux will remain 
stable for extended duration. Furthermore, the liquid droplets and solid 
particles cannot become embedded in the membrane structure because of the 
non-porous layer barrier. Therefore, if the filter should become fouled, 
it can easily be cleaned and restored to near-original gas flux 
performance. 
In contrast to the present invention, conventional gas filters for 
aerosols, such as those disclosed by Bowser and Mayo et al., mentioned 
above, rely on a microporous structure coated with hydrophobic and 
oleophobic material. The mechanism for gas filtration in such filters is 
understood to rely on the flow of gas through narrow, tortuous but open 
passageways of pores across the complete thickness of the structure. The 
coated material does not eliminate penetration of these substances into 
the passageways. The resistance to clogging and stability of gas flux 
obtained from gas filters according to the present invention are superior 
to the performance of conventional filters. 
Absence of passageways makes the gas filter of the present invention 
particularly well suited for processing gas in contact with biological 
fluids. The term "biological fluid" includes human and other animal 
natural body fluids such as blood, and other natural, synthetic or 
combined cell culture media. Such fluids typically contain cells and other 
microorganisms which tend to adhere to and grow on many substrate 
materials. While porous PDD copolymer membranes might resist adhesion and 
for a time maintain good gas flow in biological fluid systems, cells or 
microorganisms can grow in the pores to eventually block flow. However, 
the microorganisms cannot penetrate the non-porous layer of the novel gas 
filters. Moreover, if cell growth or other fouling occurs on the gas 
filter surface of the non-porous layer, the surface can be cleaned easily 
to restore performance as described above. 
PDD-containing, amorphous copolymer has a high gas permeation selectivity, 
especially with respect to oxygen and nitrogen. The O.sub.2 /N.sub.2 
selectivity is a function of the PDD content in PDD copolymer. The 
oxygen/nitrogen selectivity of 50-90 mol a PDD copolymer is at least about 
1.5:1. This high gas selectivity of PDD copolymer can be used conveniently 
to test whether the gas permeable element of the membrane structure is 
non-porous. For example, the membrane structure can be checked easily for 
absence of holes by sequentially exposing one side of the membrane 
structure to selected pure component gases at constant pressure. The 
selectivity can be determined by calculation of the ratio of individual 
gas fluxes. Approximately equal flux for each gas indicates that the 
non-porous layer may be defective (i.e., has perforations or holes). 
However, an appropriately high gas selectivity confirms that the gas 
permeable layer is non-porous. Because the non-porous, amorphous copolymer 
layer of this invention can be extremely thin, variability in processing 
technique provides the opportunity of making a defective gas permeable 
layer. Hence, it is helpful to assure the integrity of the non-porous 
layer by gas selectivity testing. 
The novel gas separation method will usually be operated at about ambient 
temperature, but may be performed at higher temperatures. However, the gas 
permeable membranes that include PDD copolymer should be used at a 
temperature below the glass transition temperature, and especially at 
least 30.degree. C. below the glass transition temperature of the 
amorphous copolymer used in the non-porous layer. As previously explained, 
PDD copolymers can have remarkably high T.sub.g. Hence, the amorphous 
copolymer membranes used in the method of the present invention are 
capable of being utilized at elevated temperatures, including in some 
embodiments at temperatures above 100.degree. C. The method of the present 
invention may be operated at relatively low temperatures, e.g., about 
10.degree. C. Of course, the gas filter should not be operated beyond the 
temperature service range of the substrate material. 
According to this invention, a new process has been discovered which 
enables placing an ultra-thin layer of polymer onto a microporous 
substrate. The novel process can be used with any suitably high molecular 
weight polymer which is capable of dissolving in a low viscosity, liquid 
solvent. Of course, the solvent should not dissolve, react with or 
otherwise degrade the substrate material. As mentioned, amorphous 
copolymers of PDD and TFE are preferred for making wetting-resistant gas 
filter membrane structures. Certain "perfluoro" solvents can be used to 
dissolve these PDD/TFE copolymers in carrying out the novel process. 
Suitable perfluoro solvents include liquid compounds of highly or 
exclusively fluorine substituted carbon compounds or fluorocarbons 
containing ether oxygen linkages. Representative perfluoro solvents 
suitable for use in the novel process include perfluoroalkanes, such as 
perfluorohexane, perfluoroheptane and perfluorooctane, available from 3M 
Company, Minneapolis, Minnesota under the tradenames PF5060, PF5070 and 
PF5080, respectively, and FC-75 Fluorinert brand Electronic Liquid, also 
from 3M. FC-75 is a solvent of perfluoro compounds primarily with 8 
carbons, believed to include 2-butyltetrahydrofuran. 
The novel process includes dissolving the high molecular weight polymer in 
a solvent to produce a dilute, coating solution. A suitable microporous 
substrate which can effectively filter the polymer from the coating 
solution is selected. The coating solution is brought in contact with a 
first side of the microporous substrate. The solvent is made to flow 
through the microporous substrate to the second side of the substrate 
opposite the first side. This flow continues while a layer of high 
molecular weight polymer builds up on the first side. When a preselected 
thickness layer of polymer has built up, the solution is removed from 
contact with the first side. The solvent residue inside the pores and in 
the wet polymer layer is evaporated. 
Solution concentration will depend upon process parameters such as 
composition of the dissolved polymer and the pore size of the microporous 
substrate. Preferably, the concentration of polymer in dilute solution 
should be less than about 1 wt %, more preferably less than about 0.5 wt 
%, and most preferably less than about 0.1 wt %. 
By "effectively filter" is meant that the substrate is a barrier to the 
dissolved polymer molecules but allows the solvent to pass through. Thus, 
the pore size of the substrate is related to the size of the dissolved 
polymer molecule. That is, the substrate pore size is small enough to 
filter the polymer from the solvent. 
A preferred technique for choosing a substrate pore size for use in the 
novel process takes into account the molecular weight of the gas permeable 
polymer and the desired gas flux of the product membrane structure. Once 
the gas permeable polymer is selected, the molecular size of the polymer 
in solution can be identified. For example, PDD copolymer typically has a 
molecular weight of about 600,000. A microporous filter having a molecular 
weight cut off ("MWCO") value of about 50,000, for example, would thus 
effectively filter a solution of this PDD copolymer. According to The 
Filter Spectrum, published by Osmonics, Inc., Minnetonka, Minn. the 
nominal size of an approximately 50,000 molecular weight polymer molecule 
on the Saccharide Type number scale is about 0.02-0.03 .mu.m. 
To verify that the substrate effectively filters the dissolved polymer, one 
can observe that solvent drawn through the substrate is substantially free 
of dissolved polymer. It is not necessary that the filtrate be absolutely 
free of dissolved polymer. Certain gas permeable polymers suitable for use 
in the present invention can have a broad molecular weight distribution 
characterized by a weight average molecular weight. The distribution thus 
will have certain high and low molecular weight fractions above and below 
the average molecular weight, respectively. The substrate may adequately 
filter most of the high molecular weight fractions and allow some portion 
of the low molecular weight fractions to pass through. Preferably, the 
filtrate is considered substantially free of dissolved polymer if the 
concentration of dissolved polymer in the filtrate is less than about 10 
percent of concentration of dissolved polymer in the coating solution. 
Verification that the filtrate is substantially free of dissolved polymer 
can be determined in a number of ways. For example, a sample of filtrate 
can be quantitatively analyzed by chemical analysis for polymer, or the 
solvent can be evaporated from a sample of filtrate to reveal the 
existence of polymer residue. A preferred method of verifying effective 
filtration involves measuring and comparing the liquid viscosity of the 
filtrate to the viscosity of pure solvent. The filtrate will be deemed 
acceptably dissolved polymer-free if the filtrate viscosity is about the 
same as the viscosity of the pure solvent. 
The substrate pore size should not be too small because an extremely small 
pore size microporous substrate can restrict the flow of gas. The lower 
limit of the substrate pore size preferably should be one that would 
provide at least about five times the gas flux of the desired non-porous 
polymer layer. For example, the oxygen flux of a 0.5 .mu.m thick layer of 
PDD/TFE amorphous copolymer with a T.sub.g of 240.degree. C. is about 
2,000 gas permeation units ("GPU"). A gas permeation unit is defined as 1 
cm.sup.3 /cm.sup.2 -sec-cm Hg.times.10.sup.-6. In this case, the gas flux 
through the bare substrate should be at least 10,000 GPU. Polysulfone 
hollow fibers of pore size equivalent to 50,000 MWCO are rated for gas 
flux of 240,000 GPU. Therefore, in this example the minimum thickness for 
a layer of a PDD/TFE copolymer with a T.sub.g of 240.degree. C. on 
polysulfone should be about 0.02 .mu.m, which is equivalent to about 
48,000 GPU. 
The term "suitably high molecular weight polymer" can now be better 
understood to mean that the molecular weight of the polymer is selected to 
provide a molecule large enough that it can be filtered from the polymer 
solution by the microporous substrate. The pore size of the microporous 
substrate should be small enough to filter the polymer from solution. The 
microporous substrate should be of a pore size corresponding to a MWCQ 
less than the molecular weight of the polymer. Preferably, the MWCO 
characteristic of the substrate should be at most about 50% of the polymer 
molecular weight, and more preferably, at most about 20% of the polymer 
molecular weight. 
Evaporation of residual solvent can be carried out, for example, by 
sweeping a clean, inert gas through the membrane structure under pressure, 
by drawing a vacuum on the membrane structure, or by both. Evaporation 
continues until the membrane structure is effectively dry of solvent. 
Effective dryness is achieved when the flux of a gas, such as O.sub.2 or 
N.sub.2, through the membrane remains constant at a given set of 
permeation conditions (e.g., pressure and temperature). The excess solvent 
can be evaporated in either direction across the thickness of the membrane 
structure. However, when sweeping is employed, preferably the sweep gas is 
supplied to the coated side of the structure. When vacuum is used, 
preferably the vacuum is applied to the second side to draw residual 
solvent from the first side to the second side. Although, the pressure of 
the inert gas or vacuum is not critical, excessive pressure differential 
across the structure should be avoided to prevent blowing or sucking a 
hole through the membrane. Generally, slight pressure or mild vacuum are 
sufficient to complete the evaporation step within a matter of hours. 
Preferably, evaporation is done at ambient temperature. It is possible to 
evaporate at elevated temperature, provided that the substrate, the 
polymer and the apparatus holding the membrane structure are not adversely 
affected. 
The flow rate of blowing gas over the non-porous layer for evaporating 
residual solvent is very important, especially for coating hollow fibers. 
The gas sweep should flow across the surface of the non-porous layer at a 
high rate effective to prevent the polymer from drying in non-uniform 
thickness on the substrate, for example in clumps. When coating inside 
surfaces of hollow fibers, the clumps can occlude the bore of the fibers. 
If a sufficiently high gas rate is used, the thickness of the non-porous 
layer will dry to a substantially uniform thickness over the whole 
substrate. One of ordinary skill in the art should be able to determine 
the minimum sweep gas flow rate necessary to produce uniform thickness 
coating without undue experimentation. 
The novel process can be carried out in a single cycle or in a series of 
cycles. In a single cycle process, the step of drawing solvent through the 
microporous substrate continues until the preselected thickness of polymer 
is formed. Alternatively, the sequence of drawing and evaporating steps 
can be repeated multiple times in series. Each time a partial amount of 
the total thickness of polymer to be coated is achieved. The cycle 
repetition can be continued until the desired coating thickness builds up 
on the substrate. 
The novel process can be applied with great advantage to coat a hollow 
fiber substrate. As mentioned, hollow fibers can have very high surface 
area density. Utilizing the present method, an entire filter surface area 
of a fiber can be coated with a continuous, ultra-thin layer of a 
non-porous gas permeable polymer. The novel process thus produces a very 
high surface area density tubular membrane structure because the gas 
permeable polymer layer is extremely thin. 
Gas filters often are designed for high capacity in a small volume. That 
is, it is sought to achieve maximum gas flow with minimum pressure drop 
across the filter while maintaining small overall filter cross section 
dimensions. The coating process of this invention further provides for the 
ability to fabricate a module containing a plurality of closely packed, 
coated hollow fibers for use as a gas filter. Hence, the present invention 
presents the additional advantage of providing for construction of a very 
high capacity, compact gas filter. 
Many shapes of the module are possible, however, a generally cylindrical 
configuration is preferred. A cylindrical gas filter is easy to make and 
can be fit into existing processes quite simply, often by connecting the 
module between flanges in a pipeline. The circular cross module between 
flanges in a pipeline. The circular cross section of a cylindrical gas 
filter additionally provides the ability to pack a large number of fibers 
per unit of overall filter cross sectional area. Hence, a very large 
number of fibers can be packed together to produce a relatively compact 
but extremely high surface area gas filter. For example, a coating on the 
outside of a 250 .mu.m outside diameter polypropylene hollow fiber yields 
a gas transport area per unit volume of 16.4 cm.sup.2 /cm.sup.3 with a 
fiber packing density of 40%. Packing density refers to the cross 
sectional area of all the fibers as percentage of the overall cross 
sectional area of the gas filter. For a cylindrical gas filter, the cross 
section is measured perpendicular to the cylindrical axis. In contrast, 
the typical area density for a flat sheet geometry membrane structure is 
only 1.1 cm.sup.2 /cm.sup.3 or one sixteenth of packed hollow fibers. 
Additionally, the amorphous copolymer used in the present invention also 
has very high permeability. For example, PDD/TFE copolymer membranes 
exhibit a permeability for oxygen of at least 100 barrers, especially at 
least 200 barrers and in particular at least 500 barrers. Consequently, 
the present method provides a gas filter with superior gas flux compared 
to conventional methods due to the combination of high gas permeability 
and ultra-thin layer of the non-porous membrane composition, and to the 
high filter surface area present in a compact space. 
A compact hollow fiber gas filter module suitable for use in the present 
invention is illustrated in FIG. 1. The filter module 10 has a generally 
elongated cylindrical casing 2 housing a plurality of hollow fibers 4. The 
fibers are held in place by tube sheets 8. The fibers extend through the 
tube sheets allowing open ends 5 to emerge on the outboard faces 9 of the 
tube sheets. The effective filter surface area of each fiber is defined by 
the fiber diameter and by the length 11 between tube sheets. 
FIG. 1 shows the fibers as being perfectly parallel. This is an ideal 
condition which need not, and usually, is not satisfied in practice. Owing 
to the extremely high length-to-diameter aspect ratio and the polymeric 
composition, each fiber is quite flexible. It is acceptable that the 
fibers are aligned substantially parallel, provided that space between 
neighboring fibers is effective to permit gas contact with a major 
fraction of the outer surface of all fibers. The interior of the casing, 
the outside of the fibers and the inboard surfaces of the tube sheets 
defines the shell side cavity 6. At least one port 7a, 7b through the 
casing is provided to allow flow into or out of the shell side cavity. 
Occasionally, the module is installed in a gas filter with covers (31, 32 
in FIG. 3) attached over the outboard faces of the tube sheets. The covers 
define inlet and outlet chambers which serve to conduct fluid into and out 
of the tubes. The space inside hollow fibers and within the inlet and 
outlet chambers, where applicable, is referred to as the tube side cavity. 
In the illustrated embodiment, FIG. 2, the interior gas filter surface 22 
of the each fiber 24 is coated with a layer 26 of gas permeable polymer. 
In a contemplated alternative embodiment not shown the gas permeable layer 
can be coated onto the exterior surface 28 of the fiber. FIG. 2 shows that 
the fiber is firmly embedded into the tube sheet which provides a fluid 
tight seal between the shell side cavity and the tube side cavity. 
Hollow fiber modules can be fabricated from fibers of various materials. 
Hollow fibers are available from Spectrum, Inc., Laguna Hills, Calif., and 
Hoechst Celanese Company, for example. A preferred method for mounting the 
fibers in tube sheets involves aligning a bundle of fibers and fixing the 
bundle together as a unit in a deep bed of thermoplastic or thermosetting 
cured polymer such as polyurethane. Another bed of cured polymer is used 
to secure the bundle at a distance (11 in FIG. 1) along the fibers from 
the first. A flat tube sheet outboard face can be made by cutting through 
one fixed bundle in a direction perpendicular to the axes of the fibers. 
At a convenient distance from the first outboard face, a cut through the 
other fixed bundle can be made to create the second outboard face. 
Finally, the tube bundle with tube sheets can be glued or otherwise sealed 
to the ends of an elongated casing to form the module. The method of 
making modules suitable for use in the present invention containing bare 
hollow fibers, i.e., fibers without a non-porous ultra-thin gas permeable 
layer, is known to those of skill in the art. Modules containing multiple 
uncoated hollow fibers are commercially available from such manufacturers 
as Spectrum, Inc. and Hoechst Celanese. 
Operation of the novel process of coating hollow fibers can be understood 
with reference to FIG. 3. An uncoated hollow fiber cylindrical module 30 
is equipped with tube side cavity covers 31 and 32. The covers have ports 
31a and 32a, for conducting fluid to and from the tube side cavity. In the 
illustrated embodiment, the module is placed in an upright orientation so 
that the longitudinal axes of the substantially parallel aligned hollow 
fibers 33 are vertical. The upright orientation of the fibers has been 
found to be preferred for achieving the desired result of a uniformly thin 
coating on the inside of the fibers. The bottom tube side port of the 
module is connected to a feed tank 34 by feed line 36. Pump 37 is used to 
pump polymer solution from the feed tank through the tube side cavity. 
Excess tube side fluid is returned to the feed tank from top tube side 
port 32a through discharge line 38 and throttling valve 40. Normally, 
upper shell side port 7b is closed with a line blank (not shown) or with a 
valve 35. Fluid from the shell side cavity also returns to the storage 
tank through overflow line 39. 
Polymer solution is recirculated by pump 37 from tank 34, through the tube 
side cavity of module 30 and back to the tank. Throttle valve 40 is 
adjusted to impose a slight pressure on the tube side cavity. This forces 
solvent to permeate the microporous hollow fibers which causes a layer of 
gas permeable polymer to build up on the interior of the fibers. When a 
preselected thickness of polymer layer has built up, recirculation is 
stopped and all fluid is drained from the tube and shell side cavities. 
Finally, a sweep of high flow gas is blown through the tube side to 
evaporate residual solvent. 
Initially a dilute polymer solution is prepared by dissolving gas permeable 
polymer in a suitable solvent. In one way of operating the process, the 
amount of polymer and solution is calculated in advance from the desired 
tube side coating thickness. That is, the total gas filter surface area is 
calculated from the dimensions of the hollow fibers and from the module 
manufacturer specifications. The amount of polymer needed to effect a 
selected thickness of coating can then be calculated. A solution 
containing at least the calculated amount of polymer is charged to a feed 
tank. The actual coating thickness can be determined empirically at 
conclusion of the process. 
The foregoing process description pertains to coating the inside surfaces 
of the hollow fibers with gas permeable polymer. Coating the outside 
surfaces of the fibers is contemplated as another embodiment of the 
present invention. The apparatus of FIG. 3 can be used to coat the 
external fiber surfaces by recirculating the polymer solution through the 
shell side cavity. This may be accomplished by pumping solution into lower 
shell side port 7a and out of upper shell side port 7b through valve 35. 
Similarly, top tube side cover port 32a is blanked or valved closed and 
permeate solvent is routed back to the storage tank through bottom tube 
side cover port 31a. Tubes 36, 38 and 39 and pump, 37, are re-connected to 
the ports as appropriate to achieve fluid recirculation. 
A hollow fiber module gas filter with a gas permeable polymer coating on 
the tube side of the fibers according to the present invention can be used 
as follows. An aerosol, for example can be caused to flow through the tube 
side cavity. A source of the aerosol is connected to the tube side inlet 
port and the aerosol is permitted to enter the inlet cover, pass through 
the coated interior of hollow fibers, discharge to the tube side outlet 
cover, and exhaust through aerosol outlet port to a collection reservoir. 
The tube side outlet port can be closed completely to dead-end the tube 
side cavity or restricted partially. It can be seen that the filtered gas 
component will migrate through the coating of amorphous copolymer to 
ultimately reach filtered gas outlet port on the shell side for 
collection. 
It can readily be appreciated that many variations in the modes of 
operation, number, shape and placement of module elements are suitable for 
use in the present invention. For example, the non-porous layer of 
amorphous copolymer can be placed on the exterior, shell side of the 
hollow fibers. In that case, preferably the aerosol would flow through the 
shell side of the module and the filtered gas would flow through the tube 
side. The drain port can be used to remove accumulated solid or 
precipitated liquid that is filtered from the aerosol over time. 
Preferred applications for the present invention include providing 
contaminant-free gas for clean room environments, such as in 
microelectronic equipment manufacturing facilities, automotive filtration 
and in biological material processing facilities. The novel gas filters 
can also be used to recover fine chemical contaminants in gases vented 
from chemical processes prior to emitting the gases to atmosphere. 
This invention is now illustrated by examples of certain representative 
embodiments thereof, wherein all parts, proportions and percentages are by 
weight unless otherwise indicated. Unless otherwise stated or the contrary 
is evident from context, all pressures referred to herein are relative to 
atmospheric pressure. Units of weight and measure not originally obtained 
in SI units have been converted to SI units. 
EXAMPLES 
Materials used in the examples, below, include the following: 
______________________________________ 
Polymer A 
Teflon .RTM. AF 2400 (E. I. du Pont de Nemours and Co., Wil- 
mington, Delaware), dipolymer of 85 mole % 
perfluoro-2,2-dimethyl-1,3-dioxole and 15 mole % 
tetrafluoroethylene, glass transition temperature 240.degree. C. 
Polymer B 
Teflon .RTM. AF 1600 (E. I. du Pont de Nemours and Co., Wil- 
mington, Delaware), dipolymer of 65 mole % 
perfluoro-2,2-dimethyl-1,3-dioxole and 35 mole % 
tetrafluoroethylene, glass transition temperature 160.degree. C. 
E-PTFE Expanded polytetrafluoroethylene 
O-1 SAE 10W-30 automotive motor oil from Quaker State 
O-2 Vacuum pump oil from Norton Petroleium Co., Newark, 
Delaware 
______________________________________ 
Example 1 and Comparative Examples 1-4 
A 0.025 wt % coating solution of Polymer B in FC-75 was prepared. 
Approximately 8.9 cm.times.20 cm of microporous E-PTFE rectangular sheet 
from W. L. Gore and Associates, Elkton, Md., designation Goretex.RTM. No. 
X19290-BAG 10F2, with nominal 0.05 .mu.m pore size was laid onto a clean 
glass plate with a narrow side in the 12 o'clock ("top") position. The 
E-PTFE sheet thickness was about 127 .mu.m thick. The sheet was adhered to 
the plate with pressure sensitive tape placed along the top edge. The 
sheet was placed in a transparent box purged with nitrogen gas to minimize 
contamination during coating. FC-75 was placed on the sheet to saturate 
the sheet. Excess solvent and possible air pockets were removed by drawing 
a rubber squeegee from top to bottom over the sheet while applying slight 
pressure. A bead of coating solution was laid on the sheet at the top edge 
and a 254 .mu.m deep casting bar was drawn down smoothly from the top of 
the sheet. The coating was allowed to dry for 1 hour at room temperature. 
Thereafter, the coated sheet was placed for 15 hours in a vacuum oven at 
50.degree. C. and purged with 10 cm.sup.3 /min. of nitrogen gas. 
A portion of the uncoated, E-PTFE sheet (Comparative Example 1) was placed 
in a membrane holder and the rates of oxygen and nitrogen through the 
sheet were measured separately. From the gas flux measurements shown in 
Table I, the O.sub.2 /N.sub.2 selectivity was calculated. Similarly, the 
gas flux and selectivity of the Polymer B-coated E-PTFE sheet (Comparative 
Example 2) was determined. Oils O-1 and O-2 were deposited separately on 
samples of the sheets Comp. Ex. 1 and 2 substantially in accord with the 
procedure described under "Visual Oil Wetting" of U.S. Pat. No. 5,116,650, 
incorporated herein by reference. Without blotting, observations of the 
wetting characteristics were made as described in Table II. Prior to 
contact with oil, the sheets were a uniformly light color. Wetting was 
visually observable as discoloration of the sheet to a contrasting gray 
color in the area of wetting. After 24 hours, the flat surface of the 
wetting test samples were tilted to a 45.degree. angle from horizontal. 
The tendency of the oil drop to roll down the inclined plane was viewed as 
indicating whether the oil had wet the sheet. A 0.01 wt % solution of 
Polymer A in FC-75 was prepared. The Polymer A solution was used to coat a 
fresh rectangular sheet of the E-PTFE (Comparative Example 3) as described 
for Comp. Ex. 2. 
The procedure for making the coated flat sheets was repeated with a 0.2 wt 
% solution of Polymer A used to coat a sheet of microporous polysulfone 
from Memtec of San Diego, Calif. The porosity of the polysulfone substrate 
had a MWCO rating of 100,000. The selectivity and oil wetting of uncoated 
polysulfone substrate (Comparative Example 4) and coated polysulfone 
(Example 1) were determined as above. 
Lower individual gas fluxes of both Comp. Examples 2 and 3 in relation to 
corresponding gas fluxes of Comp. Ex. 1 indicates that the coating on the 
E-PTFE somewhat reduced the pore size of the substrate. However, the fact 
that the selectivity for O.sub.2 and N.sub.2 remained close to unity 
verifies that the pores remained open after coating. Polymers A and B have 
a high O.sub.2 /N.sub.2 gas selectivity. Ex. 1 demonstrates the 
preferential permeability of oxygen over nitrogen through Polymer A by 
about two times and verifies that the polysulforne coating was non-porous 
and continuous over the whole substrate. 
Table II shows that the non-porous membrane structure of Ex. 1 was much 
more resistant to oil wetting than the porous coated E-PTFE structures. 
Coating E-PTFE did improve oil resistance relative to uncoated E-PTFE 
(Comp. Ex. 1). However, within only a few minutes after depositing oil on 
the substrate of Comp. Ex. 2, the drop began to the wet membrane structure 
as evidenced by a spreading gray spot. Comp. Ex. 3 was more resistant to 
oil wetting but after about 3.25 hours of contact, a significant area 
under the drop became wet. The non-porous coating of Polymer A (Ex. 1) 
dramatically improved oil wetting resistance in comparison to Comp. Ex. 4. 
No visual evidence of wetting was observed after 3.25 hours. Furthermore, 
the drops of oil on the non-porous coated polysulfone rolled freely down 
the inclined flat structure while the oil drops on the most resistant 
porous coated E-PTFE sample refused to move. After 168 hours, Ex. 1 still 
evidenced resistance to the oil drops. This behavior further indicates 
that the non-porous coated membrane structures are significantly more oil 
resistant than the conventional structures. 
TABLE I 
______________________________________ 
O.sub.2 flux 
N.sub.2 flux 
O.sub.2 /N.sub.2 
(GPU .times. 10.sup.-3) 
(GPU .times. 10.sup.-3) 
selectivity 
______________________________________ 
Comp. Ex. 1 
207 248 0.83 
Comp. Ex. 2 
185 211 0.87 
Comp. Ex. 3 
213 239 0.89 
Ex. 1 4.6 2.4 1.88 
Comp. Ex. 4 
1,653 1,797 0.92 
______________________________________ 
TABLE II 
__________________________________________________________________________ 
Observations of Oil Wetting 
Elap- 
Comp. Ex. 1 
Comp. Ex. 2 
Comp. Ex. 3 
Ex. 1 Comp. Ex. 4 
sed Uncoated 
Polymer B/ 
Polymer A/ 
Polymer A/ 
Uncoated 
Time 
E-PTFE E-PTFE E-PTFE Polysulfone 
Polysulfone 
__________________________________________________________________________ 
Exposure to oil O-1 
0 0.5 cm diam. 
0.5 cm diam. 
nearly -- 
mound shaped 
nearly spherical 
drop; sheet 
spherical 
drop; no 
gray under 
drop; no 
grayness 
drop grayness 
1-3 -- gray dots 
-- Mound shaped 
drop flat- 
min. appeared un- drop but no 
tened and 
der drop grayness 
was absorbed 
underneath 
into gray 
circle 
within 3 
minutes 
25 0.9 cm diam. 
drop still 
same as time 
ditto 
min. 
lower mound; 1 
nearly 0 
cm diam. gray 
spherical; 
circle extend- 
0.75 cm 
ing outward 
diam. gray 
from drop 
circle under 
drop 
90 lower mound; 
ditto nearly ditto 
min. 
1.15 cm diam. spherical 
gray circle drop; gray 
dots ap- 
peared under 
drop 
195 1.2 cm gray 
nearly nearly ditto 
min. 
circle spherical 
spherical 
drop; gray 
drop; 30% of 
circle diam. 
area under 
0.98 cm 
drop was 
gray 
24 -- -- drop did not 
drop freely 
-- 
hours move down 
rolled down 
45.degree. incline 
incline 
immediately 
Exposure to O-2 
1-3 same as for 
same as for 
same as for 
Mound shaped 
same as for 
min. 
O-1 O-1 O-1 drop but no 
O-1 
grayness 
underneath 
25 same as for 
same as for 
same as for 
ditto same as for 
min. 
O-1 O-1 O-1 O-1 
90 same as for 
same as for 
same as for 
ditto same as for 
min. 
O-1 O-1 O-1 O-1 
195 1.33 cm gray 
same as for 
same as for 
ditto same as for 
min. 
circle O-1 O-1 O-1 
24 -- -- drop did not 
drop freely 
-- 
hours move down 
rolled down 
45.degree. incline 
incline 
immediately 
__________________________________________________________________________ 
Example 2 
To 900 ml (1620 g) of FC-75 in a glass bottle was added 16.38 g of "Polymer 
A"). The bottle was capped and shaken by hand for about 10 minutes and 
then placed on a roll mill under a heat lamp overnight. A 1 wt % stock 
solution of Polymer A was thus produced. A 0.1 wt % coating solution was 
made by adding 810 ml of FC-75 to 90 ml of the 1 wt % solution in a clean 
glass bottle and shaking by hand for about 5 minutes. The dilution of 
stock solution was repeated to provide an ample supply of coating 
solution. 
A standard "Krosflo" hollow fiber module (Spectrum, Inc., Laguna Hills, 
Calif., part No. K25S 100 01N, with a 6.35 cm inner diameter polysulfone 
casing and about 5087 polysulfone hollow fibers of 460 .mu.m inner 
diameter.times.640 .mu.m outer diameter and pore size rated at 50,000 MWCO 
was modified by the manufacturer by removing the finger webs on the shell 
side ports. The overall length of the fibers was 22.86 cm with 19.05 cm 
effective length. The modified hollow fiber module was mounted in vertical 
orientation substantially as shown in FIG. 3. The upper shell side port 7b 
was blanked closed. Transparent covers were placed on the ends of the 
fiber module. The 3.8 cm nominal diameter top and bottom tube side ports 
31a and 32a and lower shell side port 7a were reduced with transparent 
barbed tubing adapters to receive nominal 79 mm inner diameter 
platinum-cured silicone rubber tubing. A "Masterflex L/S Quickload" 
peristaltic pump driven by a 6-600 rev./min. variable speed motor was 
placed about 15.25 cm below bottom tube side port 31a. Silicone rubber 
tubing was used to connect the module and pump with a 2L capacity, low 
density polyethylene carboy serving as feed tank in the configuration 
depicted in FIG. 3. 
Initially 1800 ml of 0.1 wt % coating solution was charged to the feed 
tank. The pump was started to establish flow from the feed tank to the 
bottom tube side port of the module. The bottom and top ends of the fibers 
within the module were visually monitored through the transparent covers. 
Flow rate was set such that the time elapsed from when the solution first 
flowed into all the fibers until solution overflowed from the top of the 
fibers was 15 seconds. After level of solution in the feed tank dropped 
due to filling the module, 400 ml more coating solution was added to the 
tank. At selected times after flow from the top of the tubes was 
established, flow from the lower shell side port was diverted and the time 
required to collect 25 ml of permeate was determined. Flow through 
discharge tube 38 was throttled by appropriate loosening or tightening of 
hose clamp 40 with a goal of maintaining collection time of 25 ml of 
permeate between 8 and 15 seconds. Actual measurement and collection times 
are shown in Table III. 
TABLE III 
______________________________________ 
Measurement Time 
25 ml Collection 
(min:seconds) Time (seconds) 
______________________________________ 
1:40 27 
2:45 23 
5:00 8 
7:30 10 
8:45 8 
______________________________________ 
Viscosity of the permeate was measured with a cross arm No. 191 viscometer 
from Technical Glass Products, Inc. (Dover, N.J.). Time for the standard 
amount of permeate to flow through the viscometer was 225.66 seconds. 
FC-75 flows through the viscometer in the range of 225-228 seconds. A 0.1 
wt % solution of Polymer A requires 325 seconds to flow through the same 
viscometer. These measurements confirm that the permeate is substantially 
free of dissolved polymer and that the microporous substrate effectively 
filtered the polymer from solution. 
The pump was stopped after 10 minutes of solution recirculation. Supply 
line tubing was clamped below the bottom tube side port with a hemostat 
and severed below the hemostat. The top tube side port tubing was also 
cut. The module was tilted to drain permeate from the shell side into the 
feed tank. Polymer solution from the tube side was drained into a clean 
beaker by unclamping the hemostat. The module was remounted in vertical 
orientation and a low pressure nitrogen gas supply was connected to the 
top tube side port. Nitrogen was purged through the tube side of the 
module at a rate of 30 L/min. for 5.5 hours. At several random times, the 
module was temporarily tilted to empty the shell side of any accumulated 
liquid. 
The thickness and gas selectivity of Polymer A on the fibers was determined 
as follows. The permeabilities of pure gases through Polymer A were 
determined from previously prepared tabulations. The tabulated data had 
been obtained from measurements of flow rate of pure gases through 
uniformly thick, monolithic membranes of Polymer A produced as describe in 
U.S. Pat. No. 5,051,114, which is incorporated herein by reference. One of 
the two top tube side cavity ports and one of the two shell side cavity 
ports were closed. A pure gas was admitted to open tube side cavity port 
at about 25.degree. C. and slightly positive pressure. The gas was 
permitted to permeate through the coated hollow fibers and was directed 
from the open shell side cavity port to a calibrated burette. The flow 
rate was measured by observation of the displacement of a soap bubble in 
the burette. The average thickness of the coating on the hollow fibers was 
calculated to be 0.1 .mu.m from the known permeability, the filter surface 
area of the module and the gas flow rate. The flow rate measurements were 
conducted for each of pure oxygen and nitrogen, separately. By dividing 
the oxygen flow rate by the nitrogen flow rate the O.sub.2 /N.sub.2 
selectivity was determined to be 1.90. This example demonstrates a method 
of coating the entire filter surface of hollow fibers with an ultra thin 
layer of an amorphous copolymer. 
Example 3 
A 0.025 wt % coating solution of Polymer A was made by adding 877.5 ml of 
FC-75 to 22.5 ml of the 1 wt % stock solution prepared in Example 2 in a 
clean glass bottle and shaking by hand for about 5 minutes. Dilution of 
the stock solution was repeated to provide an ample supply of coating 
solution. 
A new "Krosflo" hollow fiber module identical to the one used in Example 2 
was mounted vertically. The module 41 was connected to a solution 
circulation system shown schematically in FIG. 4. The same pump and feed 
tank as in Example 2 were used. The upper shell side port was blanked 
closed. The lower shell side port was connected via tubing 42 to a 1000 ml 
capacity KIMAX filter flask 43. The vapor space of the filter flask was 
vented through exhaust tube 44 to a vacuum source (not shown) consisting 
of a 36.8 cm long by 3.8 cm inner diameter two-piece vacuum trap submerged 
in ice and a Welch Duo-seal laboratory vacuum pump. Air was bled into the 
trap with an adjustable valve (not shown) to control the filter flask at a 
selected pressure. 
Initially 1800 ml of the coating solution was charged to the feed tank. 
Pressure of the filter flask was set to a vacuum of 7.5 cm Hg absolute. 
The solution circulation pump was started which caused flow of coating 
solution into the bottom of the module. Solution flow was set at a rate 
such that the elapsed time to fill the fibers was 15 seconds as observed 
by visual inspection. Permeate began to collect in the filter flask. An 
additional 700 ml of coating solution was charged to the feed tank after 
the coating solution inventory level dropped sufficiently to make room. 
Viscosity of the permeate was checked as in Example 2 and found to be the 
same as the viscosity of FC-75. One hundred five seconds after the fibers 
had filled, pressure was adjusted to control filter flask vacuum at 3.8 cm 
Hg absolute. Two hundred ten seconds after the fibers had filled, the feed 
tank had emptied and the solution circulation pump was stopped. The module 
was disconnected from the solution apparatus and drained of liquid. The 
fibers were dried with 30 L/min. nitrogen purged through the tube side for 
5.5 hours as in Example 2. By the measurement methods described in Example 
2, the average thickness and the O.sub.2 /N.sub.2 selectivity of the 
Polymer A layer on the hollow fibers were found to be 0.1 .mu.m and 1.84, 
respectively. 
Examples 4-6 
The procedure for coating the inside surface of hollow fibers as described 
in Ex. 2 was repeated with the following changes. A 2.54 cm diameter 
Spectrum hollow fiber module No. M15S26 O/N with about 361 fibers of 14.2 
cm effective length providing a total of 680 cm.sup.2 filter surface area 
was used. Oxygen and nitrogen fluxes and selectivity of the membrane 
structures were determined and are shown in Table IV. These examples 
demonstrate that a small overall cross section gas filter can be made 
according to the present invention with a continuous, ultra thin layer of 
gas permeable membrane to provide high gas flux. 
TABLE IV 
______________________________________ 
Coated Fiber Module Performance 
N.sub.2 flux (GPU) 
O.sub.2 flux (GPU) 
O.sub.2 /N.sub.2 Selectivity 
______________________________________ 
Ex. 4 2,060 3,976 1.93 
Ex. 5 2,657 4,862 1.83 
Ex. 6 2,200 4,136 1.88 
______________________________________ 
Example 7 and Comparative Example 5 
The cylindrical, coated hollow fiber module of Example 4, identified by 
reference number 55 was connected-to a testing apparatus shown in FIG. 5 
(Example 5). The overall cross sectional area of the module based on 
cylinder diameter Dhf was 5.1 cm.sup.2. The filter surface of this module 
was 680 cm.sup.2. 
A centrifugal pump 80 was equipped with an oiler 82 consisting of a wick 
immersed in a container of O-2 oil. The wick was connected to the suction 
side of the pump which caused oil droplets to suspend in pump discharge 
air 83. Air was taken in at the pump suction and blown through valve 51 to 
an ambient vent through line 54. A portion of the oil-bearing air was 
diverted through manual control valve 52 and into a flat sheet membrane 
holder 60. Excess oil-bearing air was vented from the membrane holder 
through a line containing a manual valve 62. Air that permeated through 
the membrane in the holder was exhausted through tube 64 which was capable 
of being connected to the bottom of a glass, graduated cylinder, not 
shown. Soap solution was introduced into the graduate cylinder to measure 
volumetric flow rate of the permeate using conventional, expanding bubble 
technology. Another portion of the pumped air was diverted through manual 
control valve 53 into the top of the tube sheet cavity of fiber module 55. 
Excess oil-bearing air passed through the tubes and discharged through a 
line containing a manual valve 56. The permeate air from both upper and 
lower shell side ports was exhausted through common vent 57 which also was 
adapted to connect to a soap bubble gas flow measuring cylinder. 
A sectional schematic view of the 47 mm diameter, Dfs, circular flat sheet 
membrane holder 60 from Millipore Corporation, Bedford, Mass. is shown in 
FIG. 6. Overall cross sectional area of the flat sheet membrane holder was 
17.3 cm.sup.2 and the total filter surface area of the membrane was 9.6 
cm.sup.2. The holder includes a top block 61 and a bottom block 63 which 
mate to enclose a gas permeable membrane 65 and a rigid, perforated backup 
plate 66. The bottom block is machined to define an internal channel 77. 
Gas to be filtered was fed through inlet opening 68. All ports were 
equipped with tubing connections, not shown. The incoming air traveled in 
the direction of the arrow toward the membrane. The air was diverted into 
a narrow space adjacent to the membrane and collected in an outlet channel 
72 that discharged through port 69. Permeate air discharged from space 71 
through port 70 in the direction shown by the arrow. The permeate air was 
separated from the incoming air by the membrane and an elastomeric O-ring 
67. 
A 47 mm diameter circular section of the coated membrane from Comp. Ex. 2 
was placed in the flat sheet membrane holder with coated side toward the 
incoming gas (Comparative Example 5). Pump 80 was started causing 
oil-bearing air to flow to vent position 54 through valve 51. Excess oily 
air discharge valves 56 and 62 were closed. Manual valves 52 and 53 were 
opened to admit oily air to the flat sheet membrane and to the hollow 
fiber module, respectively. The valves were adjusted to provide 2 L/min of 
gas through each discharge line 64 and 57. The pressure on gauges 58 and 
59 and corresponding flow measurements were recorded periodically. This 
experiment thus exposed the novel gas filter and the conventional, flat 
sheet membrane to the same concentration of oil in gas. Data from the 
experiment are tabulated in Table V. 
TABLE V 
______________________________________ 
Elapsed 
Comp. Ex. 5 Example 7 
Time Pressure Flow Pressure 
Flow 
(hours) 
(lbs.sub.f /in .sup.2) 
(L/min) (lbs.sub.f /in .sup.2) 
(L/min) 
Comment 
______________________________________ 
0 2.8 2.0 3.1 2.0 
0.75 2.8 2.0 3.1 2.0 
2.75 2.8 2.0 3.2 2.0 
17.25 2.8 2.0 3.2 1.76 
17.42 2.8 2.0 3.8 2.0 
19.00 2.8 2.0 4.0 2.0 
22.42 2.8 2.0 4.1 2.0 
25.00 2.8 2.0 4.4 2.0 
41.25 2.8 2.0 4.5 1.69 
41.50 2.8 2.0 5.5 2.0 
42.92 2.8 2.0 5.5 2.0 
49.00 2.8 2.0 5.9 2.0 
50.75 2.8 2.0 6.2 2.0 
67.75 2.8 1.88 6.8 1.67 
69.75 2.9 2.0 8.5 2.0 
88.75 2.9 1.88 8.7 1.5 
89.08 3.0 2.0 8.5 2.0 Purged through 
valves 56 and 
62 
115.25 
2.9 1.88 9.0 1.88 
115.42 
3.0 2.0 9.6 2.0 
117.25 
8.0 2.0 8.0 2.0 Purged through 
valves 56 and 
62 
118.00 
9.5 2.0 8.5 2.0 
121.00 
11.0 0.0066 9.5 1.76 
121.42 
10.0 0.0121 9.0 2.0 Purged through 
valves 56 and 
62 
138.75 
10.8 0.0078 9.5 1.26 
139.25 
10.8 0.0150 9.5 1.67 Purged through 
valves 56 and 
62 
142.25 
11.0 0.0029 9.5 1.30 
165.25 
11.0 0.0014 9.8 1.10 
165.75 
11.0 0.0009 9.8 1.56 Purged through 
valves 56 and 
62 
168.25 
11.0 0 9.8 1.38 
168.75 8.0 1.06 Purged through 
valve 56 
188.50 8.0 .96 
193.08 8.0 0.93 
212.00 8.0 0.78 
212.31 8.0 0.91 
213.75 8.0 0.86 
______________________________________ 
The above data reveal that the differential pressure across the flat sheet 
membrane remained steady for about 90 hours then rose rapidly. At 121 
hours, the flat sheet membrane was substantially completely clogged. Flow 
through the flat sheet membrane began to show signs of slowing down at 
about 68 hours. It dropped off to nearly zero at 121 hours. The pressure 
differential across the coated, hollow fiber module gradually increased, 
starting very soon after the start of the test. Initially, the hollow 
fiber module pressure differential was larger than that of the flat sheet 
as might be expected from a continuous, non-porous barrier to gas flow in 
comparison to a porous membrane structure. 
Starting at about 89 hours, air was blown out through the excess oily air 
valves 56 and 62 for five minutes. This was done to attempt to clean out 
any oil that might have accumulated in the gas filters. During these "blow 
outs" no liquid oil was observed to discharge from the flat sheet membrane 
holder while liquid oil flowed from the fiber module. These phenomena 
suggested that oil penetrated the flat sheet but was prevented from 
passing through the hollow fibers. The module had a transparent case. 
Visual inspection during the trial showed that no liquid oil penetrated 
the fibers and settled in the shell side cavity. However, when the flat 
sheet holder was opened at the end of the test, liquid oil was found in 
the membrane and on the top surface of the membrane. These observations 
confirmed the suggestion with regard to penetration of oil through the 
membrane structures. 
As a result of blowing air out of the excess oily air lines, the flows of 
both gas filters directly returned to the goal amounts of 2.0 L/min. This 
purging procedure was repeated five additional times. In four of the five 
instances, flow through the hollow fiber module increased significantly. 
The flat sheet membrane structure did not respond to any of the subsequent 
purges. The permeate air flow through the hollow fiber module remained 
above 50% of starting goal for over 168 hours. After 213 hours at 
conclusion of the test , the hollow fiber module delivered 46% of goal 
flow. The pressure drop across the hollow fiber module was lower than the 
peak pressure differential across the flat sheet membrane. These data 
demonstrate that the novel membrane structure with a continuous non-porous 
barrier of gas permeable membrane will resist clogging by oil 
significantly longer than will a porous membrane structure. Furthermore, 
the non-porous membrane structure can be cleaned repeatedly to boost gas 
flow. 
Example 8 and Comparative Example 6 
A 47 diameter sample of fresh porous membrane from Comp. Ex. 3 was 
installed in a flat sheet membrane holder (Comparative Example 6). Ports 
68 and 69 (FIG. 6) were closed and oil O-2 was poured into port 70 to fill 
the interior of the holder. Similarly, valve 56 (FIG. 5) of the hollow 
fiber module of Ex. 6 was closed and the tube side cavity of the module 
was filled with oil O-2. The gas filters were held liquid-full for one 
hour. The oil was drained from the gas filters which were installed in the 
testing apparatus of FIG. 5. Excess air valves 62 and 56 were opened and 
pump 80 was started. Air was swept through the gas filters for 10 minutes 
to remove residual oil. Valves 62 and 56 were closed and valves 51, 52 and 
53 were adjusted to control pressure on gauges 58 and 59 to 5 lbs.sub.f/ 
/in.sup.2. Data from this experiment is shown in Table VI. 
TABLE IV 
______________________________________ 
Elapsed 
Comp. Ex. 6 Example 8 
Time Pressure Flow Pressure 
Flow 
(hours) 
(lbs.sub.f /in .sup.2) 
(L/min) (lbs.sub.f /in .sup.2) 
(L/min) 
Comment 
______________________________________ 
0.17 5.0 0.39 5.0 1.20 
0.67 5.0 0.22 5.0 1.34 
1.67 5.0 0.104 5.0 1.46 
2.42 5.0 0.04 5.0 1.46 
24.67 5.0 0.00 5.0 1.17 
24.92 5.0 0.00 5.0 1.30 Purged through 
valves 56 and 62 
______________________________________ 
This experiment demonstrates that the novel membrane structure with a 
continuous, non-porous layer of oleophobic, gas permeable polymer provided 
superior oil wetting resistance compared to a microporous structure coated 
with the same polymer. The gas flow through the novel hollow fiber module 
was high and stable for more than 24 hours. The conventional membrane 
structure clogged after less than three hours. 
Example 9 
A 0.1 wt % stock solution was prepared by adding 1.164 g of Polymer B to 
900 ml (1620 g) of FC-75 in a glass bottle and shaking manually for about 
10 minutes. The solution was agitated by rolling the bottle on a roll mill 
under a heat lamp overnight. A 0.005 wt % coating solution was prepared by 
diluting 50 ml of the stock solution with 950 ml of FC-75 in a clean 
bottle and shaking for about 5 minutes. The dilution was repeated to 
produce 4L of coating solution. 
A "MiniKros" hollow fiber module (Spectrum, Inc. part No. S555 001 HF-2) 
with a polysulfone casing of 3.2 cm inner diameter containing about 1,153 
microporous polysulfone fibers of 460 .mu.m inner diameter and 640 .mu.m 
outer diameter was mounted in a coating apparatus as shown in FIG. 4, but 
horizontally. The porosity of the polysulfone fibers was rated at 50,000 
MWCO. The effective length of the polysulfone fibers was 20 cm which 
provided a total filter surface area of 3895 cm.sup.2. 
The same pump and feed tank as in Example 2 were used. The two shell side 
ports were manifolded using silicone rubber tubing connected in a "Y" 
configuration. The common tube was connected to a 1000 ml capacity KIMAX 
filter flask. The vapor space of the filter flask was vented through 
exhaust tube 44 to a vacuum source (not shown) consisting of a 36.8 cm 
long by 3.8 cm inner diameter two-piece vacuum trap submerged in ice and a 
Welch Duo-seal laboratory vacuum pump. Air was bled into the trap with an 
adjustable valve (not shown) to control the filter flask at a selected 
pressure. 
Initially 1800 ml of the coating solution was charged to the feed tank. 
Pressure of the filter flask was set to a vacuum of 30 cm Hg absolute. The 
solution circulation pump was started which caused flow of coating 
solution into the module. Permeate began to collect in the filter flask. 
An additional 1700 ml of coating solution was charged to the feed tank 
after the coating solution inventory level dropped sufficiently to make 
room. 
Three minutes after starting solution recirculation the filter flask was 
filled. A valve in the vacuum line (44, FIG. 4) was shut temporarily while 
an empty flask was installed. Then the valve was opened and filtration was 
resumed. The filter flask change operation was repeated at 7 minutes and 
again at 11 minutes from start-up. The feed tank was empty after 14 
minutes of operation and the solution circulation pump was stopped. The 
module was disconnected from the solution apparatus and drained of liquid. 
The fibers were dried with 30 L/min. nitrogen purged through the tube side 
for 5.5 hours as in Example 2. 
Viscosity of the permeate was checked subsequently as in Example 2 and 
found to be the same as the viscosity of FC-75, i.e., 225 seconds drop 
time through No. 191 viscometer. By the measurement methods described in 
Example 2, the average thickness and the O.sub.2 /N.sub.2 selectivity of 
the Polymer B layer on the hollow fibers were found to be 0.45 .mu.m and 
2.59, respectively. 
Example 10 
A hollow fiber module was coated as in Example 9, except for the following 
changes. A 0.5 wt % coating solution of Polymer A was used. The coating 
solution was prepared by adding 500 ml of FC-75 to 500 ml of stock 
solution prepared in Example 2 and shaking by hand for about 5 minutes. 
The hollow fiber module used was a "MiniKros Sampler" (Spectrum, Inc. 
Special Lot No. 032596-1) which had a 1.6 cm inner diameter polysulfone 
casing and 1000 microporous polypropylene fibers of 200 .mu.m inner 
diameter and 250 .mu.m outer diameter. The pore size of the polypropylene 
fibers was nominally 0.05 .mu.m. The feed tank was filled with 500 ml of 
coating solution and pressure of the filter flask was set to a vacuum of 5 
cm Hg absolute. Then the module was mounted vertically in the 
recirculation apparatus. The solution circulation pump was started which 
caused flow of coating solution into the bottom of the module at a rate 
that filled the tubes in 4 seconds. After recirculating coating solution 
for 9 minutes and 15 seconds, the pump was stopped. The module was 
disconnected from the recirculation apparatus and drained of liquid. The 
fibers were dried with 1 L/min of nitrogen for 5.5 hours purging through 
the tube side. O.sub.2 /N.sub.2 selectivity of the coated module was 
measured to be 1.81 and the thickness of the non-porous polymer layer on 
the interior of the fibers was determined to be 0.9 .mu.m. 
Example 11 
A hollow fiber module was coated on the outside of the fibers, i.e. the 
fiber shell side. The procedure was similar to that of Example 10 in that 
the coating solution was drawn through the hollow fibers under vacuum. A 
total of about 200 ml of 0.1 wt % Polymer B stock solution of made as in 
Example 9 was used for the coating solution. A "MiniKros Sampler" 
(Spectrum, Inc. Specal Lot No. 081696-2) which had 1190 microporous 
polypropylene fibers of 0.05 .mu.m pore size and effective length of 21.3 
cm was mounted horizontally in the recirculation apparatus. The coating 
solution was introduced into one of the shell side ports and returned to 
the feed tank via the other shell side port. The two tube side ports were 
manifolded with a "Y" connector and the common tubing was connected to the 
permeate collection flask of the vacuum system. Thus the FC-75 was drawn 
from solution on the shell side and through the microporous fibers into 
the bore of the fibers. The vacuum on the tube side was maintained at 500 
mm Hg absolute pressure. 
Circulation was stopped when the feed tank had emptied. The module was 
disconnected from the circulation apparatus and liquid was drained. 
Nitrogen was purged through the shell side for 20 minutes at 8 L/min. An 
additional 0.5 L/min. nitrogen drying gas sweep was blown through the 
shell side for 8 hours to continue drying. After coating, the O.sub.2 
/N.sub.2 selectivity of the module was determined to be 1.72 and the 
coating thickness was 0.3 .mu.m. 
Example 12 
A module containing about 99 microporous polyvinylidene fluoride ("PVDF") 
hollow fibers was coated with Polymer A substantially as in Example 2 and 
with the following changes. The module was a Spectrum, Inc. S555001HF-12 
"Krosflow" model with a 1.6 cm inner diameter. The fibers were 1 mm inner 
diameter and 1.2 mm outer diameter and the pore size rating of the fibers 
was 500,000 MWCO. Overall length of the fibers was 13.6 cm and the 
effective length was 12.5 cm. 
The module was mounted horizontally in the circulation apparatus with 
silicone rubber tubing from each of the shell side ports manifolded with a 
"Y" connector. Instead of returning permeate to the feed tank, the common 
discharge line from the Y connector was lead to a collection bottle. Two 
hundred ml of coating solution was charged to the feed tank which had a 1L 
capacity. The pump was operated at about 5 ml/sec flow rate. The return 
line from the tube side exit to the tank was restricted with a hose clamp 
to adjust return flow to about 3.5 ml/sec. Thus about 1.5 ml/sec permeate 
was collected. When the feed tank had emptied, the pump was stopped. 
Liquid was emptied from the module and low pressure nitrogen gas was 
purged through the tube side at a rate of 2 L/min for 4 hours. 
After coating, the O.sub.2 /N.sub.2 selectivity of the module was 
determined to be 1.87 and the coating thickness was 2.70 .mu.m. 
Although specific forms of the invention have been selected for 
illustration in the drawings and examples, and the preceding description 
is drawn in specific terms for the purpose of describing these forms of 
the invention, this description is not intended to limit the scope of the 
invention which is defined in the claims. 
Comparative Example 6 
An 11.5 cm.sup.2 filter surface area, 12 hollow fiber module was coated on 
the inside of the fibers with a layer of "Matrimid 5218" polyimide polymer 
from Ciba Geigy using the procedure described in Example 9. The hollow 
fibers were microporous polypropylene of porosity of about 0.05 .mu.m 
which corresponds to a MWCO of about 100,000. The polyimide had a 
molecular weight of 30,000 and inherent viscosity of 0.6 in n-methyl 
pyrrolidone. The polymer was dissolved at 1.5 wt % in clear dimethyl 
formamide to make 100 ml of orange colored, coating solution. The coating 
solution was drawn through the fibers under vacuum of about 380 mm Hg 
absolute pressure. It was observed that the permeate liquid was orange 
colored which indicated that polymer was not effectively filtered by the 
microporous hollow fibers. 
Each of nitrogen and oxygen had a flux through the uncoated module of about 
65,000 GPU. After coating, both nitrogen and oxygen fluxes were reduced to 
14,700. The O.sub.2 /N.sub.2 selectivity remained approximately equal to 
unity which indicated that a continuous non-porous layer of gas permeable 
polymer had not been formed on the polypropylene substrate. Non-porous 
polyimide has an O.sub.2 /N.sub.2 selectivity of greater than 5.0. This 
example demonstrates the importance of establishing a relationship between 
the molecular weight of the dissolved polymer and the pore size of the 
substrate such that the substrate effectively filters the polymer to 
create a continuous non-porous layer on the substrate according to the 
present invention. 
Examples 13-14 and Comparative Examples 7-8 
Four modules of same construction as those of Examples 4-6 were coated on 
the inside of the polysulfone hollow fibers with ultra-thin layers of 
Polymer A according to the method of this invention. Pertinent data are 
presented in table VII. The primary difference among these examples is 
that the solvent was evaporated from the comparative examples by drying in 
a vacuum oven overnight at 100.degree. C. while the operative examples 
were swept with nitrogen gas at the stated flow rates overnight. 
The average coating thickness was calculated from measured flow through the 
fibers and known permeability of the coating composition. The average 
inside diameter of the fibers was measured through hydraulic calculations 
based upon measured flow at measured pressure drop across the bank of 
tubes. The uncoated fibers had average inside diameter of 420 .mu.m. The 
data show that despite very thin coating of much less than 1 .mu.m in all 
cases, the average inside diameter of the comparative example module 
fibers was significantly less than the expected dimension of about 418 
.mu.m. The difference is believed attributed to non-uniform thickness of 
the coating on the inside of the fibers. When the solvent was removed by 
gas sweeping at high rate, as in Examples 13 and 14, the observed average 
fiber diameter was much closer to the expected dimension. These examples 
thus demonstrate that a high flow of sweep gas is important to achieving 
the desired coating geometry according to the present invention. 
TABLE VII 
______________________________________ 
Coated 
Coating Non-porous 
Fiber 
Solution O2/N2 Layer Inner 
Conc'n. Select- 
Thickness 
Diameter 
Example Method (wt %) ivity (.mu.m) (.mu.m) 
______________________________________ 
Comp. Ex 7 
Vacuum 0.20 1.8 0.16 372 
Oven 
Comp. Ex. 8 
Vacuum 0.30 1.27 0.05 363 
Oven 
Ex. 13 4 L/min. 0.15 1.57 0.12 410 
Nitrogen 
Sweep 
Ex. 14 2 L/min. 0.20 1.89 0.5 413 
Nitrogen 
Sweep 
______________________________________