Influence of Hydration State on Permeation Testing and Vapor

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Influence of Hydration State on Permeation Testing and Vapor Powered By Docstoc
					 Influence of Hydration State on Permeation Testing and
 Vapor Transport Properties of Protective Clothing Layers
                           Phillip W. Gibson, Ph.D, and Heidi L. Schreuder-Gibson, Ph.D.

           U.S. Army Natick Soldier Research, Development, and Engineering Center, Natick, MA, USA

                                                  Correspondence:
                                Phillip W. Gibson, email: phillip.gibson@us.army.mil


    NamePPhillip i Schreuder-Gibson U.S. Army Natick Soldier Research, Development, & Engineering Center,
                     Natick, Massachusetts, to: Phillip Gibson, email: Phillip.Gibson@us.army.mil
ABSTRACT                                                         for air-permeable sorptive layers are not designed to
Protective     clothing     systems    composed       of         probe the effect of different hydration levels of a
permselective polymer film laminates are an                      polymer film. Polymer films which have a mass
alternative to standard air-permeable garments based             transfer resistance that is very sensitive to water
on activated carbon. These polymer layers are                    content are greatly affected by the relative humidity
designed with high water vapor permeation rates and              of the testing conditions. In particular, testing with a
low permeation of chemical warfare agents. Polymer               dry gas sweep on one side of the membrane (0%
films that have a significant water vapor flux usually           relative humidity), can dry out the film so much on
also have an affinity for water, and will hydrate and            one side that the membrane becomes a significant
swell significantly at high humidity levels. The                 barrier to water transport. Testing with a dry gas
polymer film’s increase in water content has the                 sweep on one side of the sample is a common
potential to affect the transport rate of chemical               practice in permeation testing of films and
warfare agents in vapor and liquid form, and usually             membranes.       For films intended for clothing
also has a large effect on the intrinsic water vapor             applications, this test condition of 0% relative
permeability of the membrane.                                    humidity is not reasonable or realistic. Although
                                                                 testing with a dry gas sweep on one side of a polymer
INTRODUCTION                                                     membrane is convenient for experimental and
Protective clothing systems that include polymer                 theoretical reasons, it should be supplemented by
films as part of a multilayer laminate are now                   testing that produces a realistic state of hydration in
available as alternatives to standard air-permeable              the film. A polymer membrane swollen with water is
garments based on activated carbon. Many varieties               usually much more permeable to chemical vapors
of polymer membranes that include permselective or               than a dry polymer membrane. These realistic
reactive polymer components continue to be                       conditions apply to both water vapor permeability
developed. Most of these polymer films are designed              and to chemical vapor permeability.
to have high water vapor permeation rates and low
permeation of chemical warfare agents. A high water              EXPERIMENTAL
vapor transport rate is important for reducing the heat
stress potential of chemical and biological protective           Materials
clothing. Polymer films that have a significant water             A variety of materials were included to illustrate
vapor flux usually also have an affinity for water, and          differences in permeation behavior. The primary
will hydrate and swell significantly at high humidity            material used to show the influence of polymer
levels.     High humidity may be due to the                      hydration state on permeation behavior was a coated
environment, or due to high sweating rates of the                fabric. The fabric was a ripstop nylon, laminated
person wearing the protective clothing system.                   with a porous expanded polytetrafluoroethylene
                                                                 (ePTFE) film, and then coated with a blend of
Water uptake in the polymer film has the potential to            poly(vinyl alcohol – vinylamine) (PVA-Am) and
affect the transport rate of chemical warfare agents in          poly(ethyleneimine) (PEI). This PEI/ PVA-Am
vapor and liquid form, and usually also has a large              coated fabric allowed measurable permeation of the
effect on the intrinsic water vapor permeability of the          chemical agent simulant, even at low hydration levels
membrane. Test methods that have been developed                  (many of the other materials are good barriers at low

Journal of Engineered Fibers and Fabrics              11                          http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
hydration levels).    Several materials added for          isotherm. Figure 1 shows a schematic of the test
comparison purposes showed no chemical agent               system and detail of the test cell. More details on the
simulant permeation at any hydration level. These          test method are available in Reference 1.
materials include pure PVA-Am coated onto an
ePTFE substrate, Nafion, W.L. Gore’s Chempak
membrane laminate, and the standard military
chemical protective garment fabric with activated
carbon spheres. Other materials with low barrier
properties were also included, such as an untreated
ePTFE membrane, and two developmental materials
that had been intended as permselective films: a
sulfonated pentablock copolymer and a laminated
elastomeric polymer film (both compositions
proprietary).

Permeation Test Method                                                                    (a)
 The permeant was chloroethyl ethyl sulfide (CEES),
commonly called half-mustard, which is a simulant
for mustard agent. The test method used a stagnant
head space, with a specified amount of liquid applied
in the small stagnant volume above the sample, and a
gas sweep flow underneath the sample to carry any
permeant which diffused through the sample to a
detector. Drops of CEES were placed on a glass                                    (b)
fiber filter suspended above the test sample. This
                                                           FIGURE 1. Stagnant head space vapor diffusion cell for finite-
glass filter layer was used to wick and spread the         source liquid/vapor challenge with sweep flow. (a) system
droplets of the liquid evenly for the vapor diffusion      schematic; (b) cell detail.
test. A stainless steel screen supported the filter
paper, and prevented liquid from wicking into the test     Test Conditions: Temperature = 30°C, dry air flow
sample by separating the upper surface of the test         sweep flow rate = 100 cm³/min as set from mass flow
sample from the filter paper. This test method             controller (reference conditions of 0°C), mass flow
sometimes also includes a very thin microporous            rate of 0.002156 g/s, sample exposed area = 2.85
expanded polytetrafluorethylene (ePTFE) layer              cm². 3 L of CEES (3.2 mg) was applied to the
placed on the surface of the test sample to ensure that    porous glass fiber filter pad above the test sample.
no liquid comes in contact with the sample surface,        The liquid loading produces a challenge level of 11
and to help ensure uniform diffusion and vapor             g/m², which is close to the challenge level of 10 g/m²
exposure down to the sample. This particular series        used in several standard test methods.
of tests did not use the ePTFE layer. The top opening
of the diffusion cell was closed with a stopper, and       The mass flux of vapor across the sample in these
the test began with an air sweep of the underside of       types of tests is calculated by:
the sample to a flame ionization detector (FID) or a
photo-ionization detector (PID). The closed test cell      
                                                           m /A = QC / A                                            (1)
volume above the test sample has no ventilating air
flow, and is a small-volume stagnant air space that
becomes saturated with the test liquid and serves as a       
                                                            m = mass flux per unit area across the sample (g/s)
vapor source for the top surface of the test sample.       A = sample area (cm²)
The air sweep was controlled to a specified relative       Q = gas flow rate (cm³/s)
humidity -- most commonly 80%.                 Specific    C = concentration change (g/cm³) in sweep gas as it
experiments to measure the effect of polymer water         flows across the sample (same as measured
content on permeation behavior used relative               concentration exiting cell if the gas entering the cell
humidity levels of 0, 20, 50, 80, and 95%. Prior to        contains no vapor).
the application of the CEES liquid to the test cell, the
sample was allowed to equilibrate in the humidified        As shown in Figure 2, a simple mass balance check
sweep flow for a period of 1 to 10 hours, depending        was used to check the performance of the detectors.
on its anticipated water content as determined             Known amounts of liquid were injected into the test
previously from the measured water vapor sorption          cell without a sample present. All the liquid
Journal of Engineered Fibers and Fabrics              12                      http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
evaporated and was carried to the detector. Since the                                                        liquid remains in drop form (as on the ePTFE
amount of liquid was known, the area under the curve                                                         substrate), or wicked out into the glass fiber filter
was equivalent to the mass of the liquid. The plot of                                                        pad, which maximized surface area, and causes
volume or mass of the test liquid versus the area                                                            evaporation to take place much faster.
under the curve should be linear if the system is
performing properly. Each FID detector differs                                                               Water Vapor Sorption
slightly in their signal response, so each detector                                                          A Cahn 1000 microbalance was used to measure
must be calibrated separately.                                                                               vapor content in polymers and adsorbents as a
                                                                                                             function of concentration. A computer controls the
                                            Area Under Curve = Mass of Drops Evaporated
                                                                                                             vapor concentration flowing past the sample by
                                  500                                                                        combining a saturated flow with a dry gas flow to
                                                                                                             produce the desired concentration.         The data
   FID or PID Signal (No Units)




                                                                                      FID
                                  400                                                                        acquisition system records the sample weight and
                                                                                                             temperature as a function of time. The data may be
                                  300                                                                        used for kinetic studies (time for the sample to
                                                                                                             equilibrate) and the equilibrium weight content at
                                  200                                                                        various vapor concentrations (sorption isotherm).
                                                                                      5 drops                Any desired number of setpoints may be specified in
                                                                       3 drops                               terms of vapor concentration, time, and gas flow rate.
                                  100                2 drops
                                                                                                             For this set of measurements, the sorption balance
                                        1 drop                                        PID
                                                                                                             was used to measure the water vapor sorption
                                    0
                                        0    1         2           3        4    5          6       7   8    isotherm at a temperature of 30°C, using a gas flow
                                                                                                             rate of 1000 cm³/minute, for 15 setpoints over the
                                                                   Time (hours)
                                                                                                             range of 0% to 98% r.h., with the time at each
                                                                      (a)
                                                                                                             humidity condition ranging from 3 to 4 hours.
                                                           Mass of CEES (mg)

                                        0            10                20        30             40
                                  500
  Integrated Area Under Curve




                                  400


                                  300

                                                                                                                                                                                     (a)
                                  200
                                                                                                              Water Uptake (g of water / g of dry sample)




                                                                                                                                                            0.20
                                  100                                                                                                                                        Wool Fabric
                                                                                                                                                                             Nafion Film
                                                                                                                                                            0.15             Cotton Fabric
                                    0
                                        0        1             2            3         4         5       6

                                                       Number of Drops of CEES
                                                               (b)                                                                                          0.10

FIGURE 2. (a) Detector output for various amounts of liquid
CEES (8 mg drop size) injected into test cell (no sample); (b)                                                                                              0.05
Integrated area under detector signal curve. The response for both
a typical FID (red) and a PID (blue) are shown, but only FIDs were
used for the work reported here.
                                                                                                                                                              0
The known amounts of liquid may be used to convert                                                                                                                 0   0.2     0.4   0.6     0.8   1.0
the detector output to concentration units such as                                                                               Relative Humidity
g/cm³ or ppm. This calibration is repeated for each                                                                                          (b)
individual FID detector at every relative humidity test                                                      FIGURE 3. (a) Automated microbalance to generate water vapor
condition. The intent of the liquid application is to                                                        sorption isotherms; (b) Typical water vapor sorption isotherms.
produce a vapor challenge, but the concentration and
duration of the challenge is dependent on whether the

Journal of Engineered Fibers and Fabrics                                                                13                                                                   http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
Water Vapor Transport: Diffusion Test                                                                                                                         TABLE I
Conditions for Membranes and Films                                Standard Test Conditions – Water Vapor Diffusion
                                                                  Temperature = 30°C; Gas Flow Rate 2000 cm³/minute (cocurrent);
Figure 4 shows the test configuration used for                    Sample Area = 10 cm²
determining the “breathability” of polymer
                                                                  Setpoint                                                         R.H.               R.H.              Mean                    Humidity
membranes and laminates [2-3].                                    #                                                                on                 on                R.H.                    Gradient
                                                                                                                                   Top                Bottom

                                                                  1                                                                0.55               0.05              0.30                    0.50
                                                                  2                                                                0.65               0.15              0.40                    0.50
                                                                  3                                                                0.75               0.25              0.50                    0.50
                                                                  4                                                                0.85               0.35              0.60                    0.50
                                                                  5                                                                0.95               0.45              0.70                    0.50

                                                                  Typical Water Vapor Diffusion Results
                                                                  Figure 5 (a, b) shows typical “breathability” results
                                                                  for a wide variety of commercially available
                                                                  membrane-based clothing materials. These materials
                                                                  can serve as benchmarks for acceptable levels of
                                                                  water diffusion properties required for membranes
                                                                  incorporated into chemical and biological protective
                                                                  clothing.
FIGURE 4. Schematic of test cell used for water vapor diffusion                                                  1800
testing.                                                                                                         1700
                                                                                                                               Conduit
                                                                   ( Lower Resistance is More "Breathable" )     1600          from Mountain Hardwear
                                                                     Water Vapor Diffusion Resistance (s/m)

                                                                                                                 1500                                                           Schoeller WB-Formula
                                                                                                                               Sympatex Laminate
                                                                                                                 1400
Air at two different relative humidities flows over the                                                          1300          The North Face
                                                                                                                                                                                (Schoeller Textiles)

                                                                                                                               "Hydroseal"
two sides of the test sample, and humidity detectors                                                             1200          (Burlington "XALT")
                                                                                                                 1100
measure how much water vapor is transported                                                                      1000
                                                                                                                               Gore-Tex (Standard)
                                                                                                                                Membrain from
through the sample. Results may be shown in terms                                                                 900           Marmot
                                                                                                                  800          Entrant Dermizax
of flux (grams/square meter/day) or water vapor                                                                   700
                                                                                                                               (Toray Industries)
                                                                                                                                                                                                        Omni-Tech "Titanium"
                                                                                                                               Gore-Tex XCR                                                             Columbia Sportwear
diffusion resistance, which has the units of s/m. The                                                             600
                                                                                                                  500          Lowe Alpine
resistance units are preferred because results obtained                                                           400          Triplepoint Ceramic

                                                                                                                  300
at different environmental conditions can be easily                                                               200
                                                                                                                               Nextec from
                                                                                                                               Patagonia "Velocity"                                                    Entrant GII XT Laminate (Toray Industries)
                                                                                                                               Schoeller Dryskin Extreme
compared. The lower the diffusion resistance, the                                                                 100          (Schoeller Textiles)
                                                                                                                                                                                                       EVENT Laminate (BHA Technologies)
                                                                                                                                                                                                       Expanded PTFE Membrane
                                                                                                                    0
more water vapor permeates through the material.                                                                           0         0.1       0.2      0.3      0.4      0.5        0.6     0.7         0.8       0.9         1.0

Testing is done at a constant relative humidity (R.H.)                                                          Mean Relative Humidity (Average of Humidity on Both Sides of Sample) (1.0 = 100% r.h.)

gradient, but the mean relative humidity is increased                                                                                                               (a)
at each setpoint. Mean relative humidity is the                                                                7500
average of the incoming relative humidity on the two                                                           7000        Expanded PTFE Membrane
sides of the sample. Higher mean relative humidity                                                             6500
                                                                                                               6000
means that a hydrophilic membrane contains more                                                                             EVENT Laminate
                                                                                                                                                                                               Schoeller Dryskin Extreme
                                                                                                                                                                                               (Schoeller Textiles)
                                                                         Water Vapor Flux (g/m -day)




                                                                                                               5500         (BHA Technologies)
water than at a lower mean relative humidity. Most                                                             5000
                                                                   2




hydrophilic membranes have a lower resistance to                                                               4500        Entrant GII XT Laminate
                                                                                                                           (Toray Industries)
                                                                                                                                                                                                          Gore-Tex XCR

                                                                                                                                                                                                          Gore-Tex (Standard)
water vapor diffusion when they contain more water                                                             4000
                                                                                                                                                                                                          Dermizax
                                                                                                               3500                                                                                       (Toray Industries)
in their structure. Porous materials such as most                                                                          Nextec from                                                                    Membrain from
                                                                                                               3000        Patagonia "Velocity"                                                           Marmot
textiles, or microporous membranes, have a nearly                                                              2500                                                                                      Sympatex
                                                                                                                                                                                                         (Akzo-Nobel)
constant water vapor diffusion resistance under the                                                            2000
                                                                                                                           Lowe Alpine
                                                                                                                           Triple Point Ceramic                                                          Conduit from
                                                                                                                                                                                                         Mountain Hardware
same conditions because water vapor is diffusing                                                               1500
                                                                                                                                                                                                       Omni-Tech "Titanium"
                                                                                                               1000                                                                                    Columbia Sportwear
through the air in the pore spaces, rather than                                                                 500                                        Schoeller WB-Formula     The North Face "Hydroseal"
diffusing through the polymer film.                                                                                0
                                                                                                                                                           (Schoeller Textiles)     (Same as Burlington "XALT")

                                                                                                                       0           0.1        0.2     0.3        0.4      0.5       0.6      0.7         0.8       0.9         1.0

                                                                                                               Mean Relative Humidity (Average of Humidity on Both Sides of Sample) ( 1.0 = 100% r.h.)


                                                                                                                                                                    (b)

                                                                  FIGURE 5. (a) Water vapor diffusion resistance of several
                                                                  commercial membrane laminates; (b) water vapor flux
                                                                  measurements for several commercial membrane laminates.

Journal of Engineered Fibers and Fabrics                     14                                                                                 http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
The commercial waterproof/breathable fabric                                   (b) Tetratex ePTFE film
laminates in Figure 5 illustrate two types of
permeation behavior. Materials that depend on water
vapor to diffuse through the pore spaces of the
membrane and fabric show little dependence on
hydration level (essentially flat lines in Figure 5).
Materials that have a solid water-permeable polymer
layer somewhere in the laminate have water vapor
transport properties that are dependent on hydration
level (lines with significant slope in Figure 5).

Figure 6 shows typical examples of the membrane
element responsible for these differences in transport
behavior. Figure 6 (a,b) shows examples of porous
expanded polytetrafluoroethylene membranes. These
membranes are bonded between protective fabric                          (c) Gore-Tex membrane (coated side)
layers to provide a barrier to liquid water penetration,
while the extensive pore spaces in the membrane
allow high water vapor flux. Figure 6 (c, d) shows
an example of a continuous water vapor permeable
polymer layer coated onto an ePTFE supporting
layer. The polymer coating layer allows more water
vapor transport when it is at higher hydration levels.




                                                                       (d) Gore-Tex membrane (uncoated side)

                                                           FIGURE 6. Scanning electron microscope images of typical
                                                           membranes used in waterproof breathable fabric laminates.
                                                           Images are of the membrane layer only (bonded supporting and
                                                           protective textile layers not shown).

                                                           “Mean Relative Humidity”
                   (a) eVent ePTFE film
                                                           The term “mean relative humidity” refers to the mean
                                                           (or average) of the humidity on one side of the
                                                           sample, and the humidity on the other side of the
                                                           sample.      The mean relative humidity is an
                                                           approximation of the water content in the polymer
                                                           membrane that would be equivalent to the sorption
                                                           isotherm measured with the same relative humidity
                                                           on both sides of the sample.           For transport
                                                           measurements, a concentration gradient is required
                                                           for water or the chemical agent simulant to permeate
                                                           across the material. Under actual use conditions,
                                                           such as in a protective clothing system, the material
                                                           will have an equilibrium vapor content that is
                                                           determined under both a temperature and a
                                                           concentration gradient. These gradients are due to
                                                           the differences between the local vapor pressure and


Journal of Engineered Fibers and Fabrics              15                      http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
temperature at the human skin surface, and the vapor                                                                  constant water vapor diffusion resistance. This is due
pressure and temperature of the outside environment.                                                                  to the fact that water vapor is diffusing through the
 It is possible to measure the water content of                                                                       interconnected air-filled pore spaces in their
polymer films by using the same humidity on both                                                                      structure.
sides of the film, or alternatively with a humidity
gradient across the film. It is difficult to measure the                                                              In contrast, the polymer membranes and laminates all
“gradient isotherm” in the normal Cahn balance                                                                        show much higher water vapor transport rates when
setup, but such measurements may be performed by                                                                      they are well hydrated (higher mean relative
conditioning a polymer sample in the water vapor test                                                                 humidity) than when dry. At high mean relative
cell (Figure 4) in the following manner: after the                                                                    humidity, several of the polymer layers have better
sample has equilibrated at each condition of a                                                                        water vapor transport rates than the standard
particular humidity gradient, it can be quickly                                                                       chemical protective suit material. It is important to
removed, weighed on a laboratory balance, and                                                                         have knowledge of this behavior, since standard test
replaced in the permeation cell for the next set of                                                                   methods which only use a single humidity condition
humidity conditions.                                                                                                  to evaluate or rank materials may completely reverse
                                                                                                                      material rankings depending on whether the test
Figure 7 shows a comparison of the normal water                                                                       method produces low or high mean relative humidity
sorption isotherm and the “gradient isotherm” as                                                                      test conditions [4].
measured for a Nafion polymer film. There is good
agreement between the two measurements. This                                                                                                                                Elastomeric polymer membrane laminate
                                                                                                                                                                            PEI / PVA-Am coated on ePTFE laminated to nylon fabric
suggests that the use of the “mean relative humidity”                                                                                                                       W.L. Gore Chempack Laminate
                                                                                                                                                                            PVA-Am coated on ePTFE with mesh reinforcement
as an approximation of the polymer water content                                                                                                                            Gore-Tex XCR Laminate
                                                                                                                                                                            (Example of a Commercial "Breathable" Membrane Laminate)
defined by the normally-measured water vapor                                                                                                                                Standard Chemical Protective Suit
                                                                                                                                                                            (Woven Shell Fabric with Separate Carbon Bead Liner)
sorption isotherm is valid.                                                                                                                                                 Sulfonated pentablock copolymer
                                                                                                                                                                            Chemical Protective Suit Shell Fabric
                                                                                                                                                                            Nafion
                                                                                                                                                                            ePTFE
                                                0.20
 Water Content (g of water /g of dry polymer)




                                                                                                                       Water Vapor Diffusion Resistance (s/m)




                                                                                                                                                                3000
                                                                  Same Humidity on Both Sides of Sample                                                         2000
                                                                  Humidity Gradient Across Sample
                                                0.15              (Gradient Isotherm)
                                                                                                                                                                1000
                                                                                                                                   (Log Scale)




                                                                                                                                                                 500
                                                0.10


                                                                                                                                                                 200
                                                0.05
                                                                                                                                                                 100
                                                                                                                                                                       0    0.2           0.4           0.6            0.8             1.0

                                                  0                                                                                                                                  Mean Relative Humidity
                                                       0         0.2         0.4        0.6         0.8         1.0
                                                                                                                      FIGURE 8. Water vapor diffusion resistance of various polymer
                                                       Water Activity = Relative Humidity = Mean Relative Humidity
                                                                                                                      films, textiles, and laminates.

FIGURE 7. Water vapor sorption isotherms for Nafion determined                                                        CEES Permeation
by using the same humidity on both sides of the sample, and also
with a humidity gradient across the sample.                                                                           Permeation of half-mustard simulant (CEES) through
                                                                                                                      a variety of materials is shown in Figure 9. These
RESULTS                                                                                                               results are at the single humidity condition of 80%
                                                                                                                      r.h., 30°C, and liquid CEES loading of 11 g/m².
Water Vapor Transport
Figure 8 shows the water vapor transport properties                                                                   There was complete penetration of the porous ePTFE
of a variety of materials, as tested under a constant                                                                 film and the elastomeric polymer membrane
humidity gradient of 0.50 (50%), but with varying                                                                     laminate. Partial penetration was observed for the
mean relative humidity. The materials include                                                                         sulfonated pentablock copolymer film, and the PEI /
porous air-permeable layers, as well as several                                                                       PVA-AM coated fabric. No penetration was detected
polymer films and laminates. The porous layers                                                                        for the pure PVA-AM coated onto ePTFE, the W.L.
(ePTFE film, chemical protective suit shell fabric,                                                                   Gore Chempak laminate, the Nafion film, or the
and the combination of the chemical suit shell fabric                                                                 standard chemical protective suit material.
and the activated carbon bead liner) all show a
Journal of Engineered Fibers and Fabrics                                                                      16                                                           http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
                                                ePTFE 100% penetration                                                                                                            polymers has not been reported to date [5]. However,
                                                Elastomeric polymer membrane laminate 100% penetration
                                                Sulfonated pentablock copolymer 70% penetration
                                                PEI / PVA-Am coated on ePTFE laminated to nylon fabric 23% penetration
                                                                                                                                                                                  it has been reported that CEES will break down in the
                                                PVA-Am coated on ePTFE with mesh reinforcement No penetration
                                                W.L. Gore Chempak Laminate No penetration
                                                                                                                                                                                  presence of water on glass filters to a small amount
                                                Nafion No penetration
                                                Standard Chemical Protective Suit
                                                                                                                                                                                  of volatile product and a large amount may convert to
                                                (Woven Shell Fabric with Separate Carbon Bead Liner) No penetration                                                               a non volatile sulfonium product [1]. Materials that
                                                                                                            Time (hours)                                                          did not allow any permeation presumably are
                                      -6
                                         0             2                                      4                        6                       8                10                providing protection by different mechanisms: e.g.,
                             1.2x10
                                               100%                                                                                Time (hours)                                   the standard chemical protective material adsorbed
                                               Penetration
 CEES vapor flux (g/cm²-s)




                                               of Porous
                                                                                                -7
                                                                                           2.5x10
                                                                                                        0       2              4           6         8         10
                                                                                                                                                                                  the CEES onto the activated carbon, while the
                                               PTFE                                                                                                                               Nafion, W.L. Gore Chempak, and PVA-Am acted
                                                               CEES vapor flux (g/cm²-s)



                                                                                                -7
                                                                                           2.0x10
                                      -6                                                                       Sulfonated pentablock copolymer
                             0.8x10            Membrane
                                                                                                -7
                                                                                           1.5x10                                                                                 more as barrier films.
                                                                                                                            Elastomeric Membrane Laminate
                                                                                                -7
                                                                                           1.0x10
                                                                                                                                         PEI / PVA-Am

                             0.4x10
                                      -6                                                        -7
                                                                                           0.5x10
                                                                                                                                         coated on ePTFE
                                                                                                                                         laminated to nylon fabric                As a screening test, the stagnant head-space finite-
                                                                                                    0
                                                                                                        0           10000              20000         30000           40000
                                                                                                                                                                                  source permeation setup produces trends that suggest
                                                                                                                               Time (seconds)                                     a significant effect of hydration level on chemical
                                      0                                                                                                                                           agent simulant through protective clothing materials.
                                           0               10000                                               20000                           30000                     40000

                                                                                                    Time (seconds)
                                                                                                                                                                                  The PEI / PVA-Am coated fabric was the best
FIGURE 9. CEES permeation through materials at 80% relative
                                                                                                                                                                                  material for illustrating this effect, in that it had
humidity. Inset graph reduces y-axis range to better show                                                                                                                         measurable permeation levels at all hydration states.
permeation behavior of three materials of interest.                                                                                                                               The other materials shown in Figure 9 were either
                                                                                                                                                                                  very poor barriers, or complete barriers, and did not
These permeation results are generated with the                                                                                                                                   change their permeation properties significantly at
stagnant head-space finite-source screening test                                                                                                                                  different humidity levels.
described previously in the experimental section.
This test is not ideal for determining the true                                                                                                                                   Figure 10 shows that the CEES permeation behavior
transport properties of polymer membranes or                                                                                                                                      for the PVA-Am coated fabric was significantly
laminates. The permeation curves shown in Figure 9                                                                                                                                affected by different humidity levels. Testing at high
do not allow discrimination between different                                                                                                                                     humidities resulted in much higher CEES penetration
transport mechanisms. For example, since the liquid                                                                                                                               than at lower humidity conditions.
challenge is confined in the head space above the
sample, it would be expected that eventually all the                                                                                                                              The permeation curves for the PVA-Am coated fabric
permeant would diffuse through a non-chemisorptive                                                                                                                                are all very similar in duration. CEES breakthrough
/ non-reactive polymer layer. This is the case for the                                                                                                                            occurs very quickly, and there is little difference in
elastomeric permselective polymer laminate, but is                                                                                                                                the time during which penetration takes place. The
not observed for the sulfonated pentablock                                                                                                                                        level of hydration seems to have little effect on the
copolymer film, or for the PEI / PVA-Am coated                                                                                                                                    rate at which CEES penetrates, but does affect the
fabric. The results suggest that these two materials                                                                                                                              total amount which permeates the sample.
are adsorbing or reacting with the CEES in some                                                                                                                                                                                                   Time (hours)
way, thus immobilizing the penetrant in the polymer                                                                                                                                                                     -7
                                                                                                                                                                                                                           0      1             2           3         4         5
film. Other possibilities, such as leakage of the                                                                                                                                                              1.2x10
                                                                                                                                                                                                                                             3 L (0.00321 g) of CEES on glass filter
CEES vapor from the stagnant air volume, or                                                                                                                                                                                                  --calibration curve (complete penetration)

adsorption/reaction of the CEES with the materials in                                                                                                                                                                                                                                                Scale expanded
                                                                                                                                                                                   CEES vapor flux (g/cm²-s)




                                                                                                                                                                                                                                                                                                     to show full CEES
the head space, are less probable, since the                                                                                                                                                                            -7                                                                           calibration curve
                                                                                                                                                                                                               0.8x10                                                                            -6
                                                                                                                                                                                                                                                                                            1.5x10

elastomeric permselective membrane laminate (as
                                                                                                                                                                                                                                                                CEES vapor flux (g/cm²-s)




well as a wide variety of other polymer films tested                                                                                                                                                                                               95% r.h.
                                                                                                                                                                                                                                                                                                 -6
                                                                                                                                                                                                                                                                                            1.0x10




but not shown) allow all the CEES to permeate over                                                                                                                                                                                                                                               -6
                                                                                                                                                                                                                                                                                            0.5x10


                                                                                                                                                                                                                                                     80% r.h.
time spans of several hours.                                                                                                                                                                                   0.4x10
                                                                                                                                                                                                                        -7
                                                                                                                                                                                                                                                                                                     0
                                                                                                                                                                                                                                                                                                         0    5000       10000        15000   20000
                                                                                                                                                                                                                                                         50% r.h.                                                    Time (seconds)


                                                                                                                                                                                                                                                           20% r.h.
For the PEI / PVA-Am coated fabric, there is the                                                                                                                                                                                                               0% r.h.
additional possibility that the polymer layer is
reacting with the CEES, and the detector is not                                                                                                                                                                         0
                                                                                                                                                                                                                             0        5000           10000                                                   15000                            20000
measuring CEES vapor, but reaction products due to
                                                                                                                                                                                                                                                Time (seconds)
CEES decomposition. Although these polymers have
been shown to be reactive with nerve agent                                                                                                                                        FIGURE 10. Permeation of CEES at different humidity levels
                                                                                                                                                                                  through PEI / PVA-Am coated on ePTFE film laminated to nylon
surrogates such diisopropyl fluorophosphate (DFP),                                                                                                                                fabric.
similar chemical depletion of CEES within these
Journal of Engineered Fibers and Fabrics                                                                                                                                     17                                                  http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
Figure 11 shows a general correlation between                                                                     The dual flow apparatus produces a constant vapor
increasing water vapor permeability (flux), CEES                                                                  concentration on one side of the sample, and allows
penetration, and water content (uptake) as humidity                                                               time-dependent concentration measurements on the
increases.     Test and evaluation methods to                                                                     sweep side. This allows us to get both a steady-state
characterize the chemical resistance of protective                                                                permeation coefficient, and also some measure of the
clothing materials based on permselective and                                                                     diffusion coefficient, which we can extract from
reactive polymer layers need to consider test methods                                                             time-lag analysis of permeation following a
that produce a realistic state of hydration in the test                                                           concentration change in the vapor challenge.
samples.
                            80                                             4000                                   The material used for the dual-flow permeation was




                                                                                    Water Vapor Flux (g/m²-day)
                                                                           3000                                   the same material from the stagnant-head permeation
 Percent CEES Penetration

   Percent Water Uptake




                            60             Water Vapor Flux                2000                                   cell study: ripstop nylon, laminated with a porous
                                                                           1000
                                                                                                                  expanded polytetrafluoroethylene (ePTFE) film, and
                            40                                                                                    then coated with a blend of poly(vinyl alcohol –
                                                                           0
            or




                                                                                                                  vinylamine) (PVA-Am) and poly(ethyleneimine)
                            20             CEES Penetration
                                                                                                                  (PEI). As mentioned previously, the PEI/ PVA-Am
                                                                                                                  coated fabric allowed measurable permeation of the
                            0
                                                                                                                  chemical agent simulant, even at low hydration levels
                                           Water Uptake
                                                                                                                  (many of the other materials are good barriers at low
                                 0    20       40         60    80      100                                       hydration levels).
                             Relative Humidity or Mean Relative Humidity (%)
FIGURE 11. CEES penetration, water uptake, and water vapor                                                        The CEES vapor concentration was assumed constant
flux are generally correlated with relative humidity test conditions.                                             due to a nominal flow of 30 cc/min through the
                                                                                                                  CEES bubbler. Relative humidity of the air flows
The stagnant head-space finite-source method is a                                                                 through the top and bottom of the cell was adjusted
convenient screening test but is not ideal for directly                                                           by the relative flow rates of the dry and saturated air
measuring in a controlled way the effect of hydration                                                             streams. The top CEES flow of 30 cc/min was
level upon permeation behavior. Parallel testing                                                                  assumed to be part of the dry component of the air
using simulant vapor in a continuous stream (Figure                                                               flow, so the highest humidity that could be achieved
12) can provide steady-state permeation behavior of                                                               on the top flow was 70% r.h. (70 cc/min of water-
polymer films. This can correct some of the                                                                       saturated flow mixed with 30 cc/min of CEES-
deficiencies (particularly lack of mechanistic                                                                    saturated dry air). Lower humidities were achieved
knowledge) present in the stagnant head space test                                                                by adding in an extra dry flow from another flow
method described in the previous section. This test                                                               controller to the 30 cc/min of CEES flow, and
method is still being refined, but some preliminary                                                               decreasing the water-saturated flow to maintain a
measurements      that   confirm      hydration-level-                                                            total gas flow rate of 100 cc/min. The lower flow
dependent permeation behavior seen in the previous                                                                humidity was controlled by simple mixing of the dry
section are summarized below.                                                                                     and saturated flows; thus higher humidities could be
                                                                                                                  achieved in the bottom sweep flow (the total flow for
                                                                                                                  the bottom sweep was also 100 cc/min). This is
                                                                                                                  reflected in the final dual flow humidity test
                                                                                                                  condition where the humidity of the bottom sweep
                                                                                                                  flow was 95% r.h., but the humidity of the flow over
                                                                                                                  the top of the sample was 70% r.h. Otherwise, the
                                                                                                                  relative humidity on the top and bottom of the sample
                                                                                                                  were identical.

                                                                                                                  The dual flow test results in Figure 13 showed the
                                                                                                                  same material behavior that was evident in the
                                                                                                                  stagnant-head finite vapor source permeation test
                                                                                                                  method. The PEI/PVA-Am coated nylon fabric
                                                                                                                  becomes more permeable to CEES when it is
                                                                                                                  hydrated. There is some uncertainty in the sensor
FIGURE 12. Dual flow permeation test with CEES vapor source
                                                                                                                  calibration factors for these runs, so the sweep flow
for simultaneous CEES and water vapor permeation testing of                                                       concentrations are not converted to concentration
polymer membranes.                                                                                                units in Figure 13.
Journal of Engineered Fibers and Fabrics                                       18                                                  http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
                             200                                                                                                     Figure 15 shows a PVA-Am film, 30 m thick,
                                                                                                                       95/70
                                                                  3000
                                                                                                                                     tested with three different types of setpoints (A, B,
                                       FID Sensor for CEES Flow   2800
                                                                                                                                     and C listed below). The conditions with a dry gas
 FID Sensor for Sweep Flow




                             150                                  2600
                                                                                                                                     sweep (setpoints that produce low water content in
                                                                  2400
                                                                                                                                     the membrane), result in very little water transport
                                                                  2200                                                               through the film.
                             100
                                                                  2000
                                                                         0    20   40    60    80   100        70/70
                                                                              Relative Humidity                                           A. The constant gradient method (blue curve)
                              50                                                                                                     uses a constant humidity gradient of 0.50 (50%)
                                                                                                      50/50
                                       0/0                                   20/20                                                   difference across the film. This set of conditions is
                                                                                                                                     useful because the flux will also be constant if the
                               0                                                                                                     film does not change its properties according to
                                   0                                         20               40          60     80            100
                                                                                                                                     hydration state. In this case, the M12 film does not
                                                                                        Sweep Flow Humidity
                                                                                                                                     become very permeable until the mean relative
FIGURE 13. Permeation of CEES vapor through PEI/ PVA-Am                                                                              humidity reaches about 0.40.
coated fabric in dual flow apparatus. Inset graph is FID sensor
response for the CEES vapor carried in various humidity air flows                                                                         B. The most severe condition produced by
over the top surface of the sample, to show sensor response to air                                                                   holding one side at 0.0 and increasing the humidity
humidity levels (referenced to bottom humidity). Annotations                                                                         on the other side (green curve), shows that the film
refer to humidity on bottom flow / humidity in top flow.                                                                             does not become permeable until a mean humidity of
                                                                                                                                     0.45 is reached.
Dry Gas Sweep Influence on Testing Permeation                                                                                             C. The method of maximizing the gradient by
Properties of Polymer Films                                                                                                          holding one side at a high humidity, and gradually
Polymer films with a mass transfer resistance that                                                                                   increasing the sweep humidity (black curve), would
changes substantially with water content are very                                                                                    also be a reasonable way to test. However, the
sensitive to testing conditions. In particular, testing                                                                              gradient and the mean humidity are both increasing
with a dry gas sweep on one side of the membrane                                                                                     in this set of conditions, unlike the constant gradient
(0% r.h.), can dry out the film to such a degree on                                                                                  approach.
one side that the membrane becomes a significant
barrier to water transport. For films intended for                                                                                                                                 2000
                                                                                                                                                                                                                                                                   # Top Bottom
clothing applications, this test condition of 0% r.h. is                                                                                                                                #
                                                                                                                                                                                        1
                                                                                                                                                                                             Top Bottom
                                                                                                                                                                                             0.50     0.00
                                                                                                                                                                                                                  A.
                                                                                                                                                                                                               Constant                            B.              1  .50 .00
                                                                                                                                                                                                                                                                   2  .60 .00
not reasonable. Although testing with a dry gas                                                                                                                                         2
                                                                                                                                                                                        3
                                                                                                                                                                                             0.55
                                                                                                                                                                                             0.60
                                                                                                                                                                                                      0.05
                                                                                                                                                                                                      0.10
                                                                                                                                                                                                               Humidity
                                                                                                                                                                                                               Gradient
                                                                                                                                                                                                                                                 Bottom
                                                                                                                                                                                                                                                 Humidity          3  .70 .00
                                                                                                                                      Water Vapor Mass Transfer Resistance (s/m)




                                                                                                                                                                                                                                                                   4  .80 .00
sweep on one side of a polymer membrane is                                                                                                                                              4
                                                                                                                                                                                        5
                                                                                                                                                                                             0.65
                                                                                                                                                                                             0.70
                                                                                                                                                                                                      0.15
                                                                                                                                                                                                      0.20
                                                                                                                                                                                                               (50%)                             Held at Zero
                                                                                                                                                                                                                                                 and               5  .90 .00
                                                                                                                                                                                   1500
                                                                                                                                                                                                                                                                   6 1.00 .00
convenient for experimental and theoretical reasons,                                                                                                                                    6
                                                                                                                                                                                        7
                                                                                                                                                                                             0.75
                                                                                                                                                                                             0.80
                                                                                                                                                                                                      0.25
                                                                                                                                                                                                      0.30
                                                                                                                                                                                                                                                 Top Humidity
                                                                                                                                                                                                                                                 Increased         7  .95 .00
                                                                                                                                                                                        8    0.85     0.35                                                         8  .90 .00
it should be supplemented by testing that produces a                                                                                                                                    9    0.90     0.40
                                                                                                                                                                                                                                                 and then
                                                                                                                                                                                                                                                 Decreased         9  .80 .00
                                                                                                                                                                                        10 0.95       0.45                                                         10 .75 .00
realistic state of hydration in the film. These realistic                                                                                                                               11 1.00       0.50                                                         11 .70 .00
                                                                                                                                                                                   1000 Setpoints 1-3, no flux                                                     12 .65 .00
conditions apply to both water vapor permeability                                                                                                                                       (50/0, 55/5, 60/10)                                                        13 .60 .00
and to chemical vapor permeability. A polymer                                                                                                                                                 Water flux starts at
                                                                                                                                                                                                                                                     Setpoints 1-4, and 9-13, no flux
                                                                                                                                                                                                                                                     (50/0, 60/0, 70/0, 80/0)
membrane swollen with water is usually much more                                                                                                                                              Setpoint 4 (65/15)                                     (80/0, 75/0, 70/0, 65/0, 60/0)

permeable to chemical vapor than a dry polymer                                                                                                                                      500          C.
                                                                                                                                                                                              High Top     #     Top    Bottom
                                                                                                                                                                                                                                                     Water flux below detection limit
                                                                                                                                                                                                                                                     at mean r.h. < 0.40
                                                                                                                                                                                                           1     0.95    0.05
membrane.                                                                                                                                                                                     Humidity
                                                                                                                                                                                              and          2     0.95    0.15
                                                                                                                                                                                              Increasing   3     0.95    0.25
                                                                                                                                                                                              Bottom       4     0.95    0.35
                                                                                                                                                                                                           5     0.95    0.45
A convenient method for looking at the effect of a                                                                                                                                    0
                                                                                                                                                                                              Humidity
                                                                                                                                                                                          0         0.1        0.2      0.3      0.4   0.5     0.6        0.7      0.8       0.9        1.0
dry gas sweep is the “dual-flow” permeation cell
                                                                                                                                                                                                           Mean Relative Humidity (Average of Top and Bottom Humidity)
shown in Figure 14 (described previously in the
Experimental section).
                                                                                                                                     FIGURE 15. Water Vapor Mass Transfer Resistance for PVA-Am
                                                                                                                                     film is highly affected by test conditions that influence water
                                                                                                                                     content in the polymer layer. Water vapor transport is not
                                                                                                                                     significant until the mean relative humidity in the film exceeds
                                                                                                                                     40%. Test Conditions: Temperature = 30°C; Gas Flow Rate = 2000
                                                                                                                                     cm³/minute (Cocurrent); Sample Area = 5 cm².



FIGURE 14. Dual-flow permeation cell (vertical scale expanded).
The schematic is shown in the form of a computational fluid
dynamic simulation of gas flow and concentration contours during
the permeation experiment [6].

Journal of Engineered Fibers and Fabrics                                                                                        19                                                                             http://www.jeffjournal.org
Volume 4, Issue 4 – 2009
CONCLUSIONS                                                               [5]   Schreuder-Gibson, H., Gibson, P., Yeomans, W., Walker,
                                                                                J., “Development and Performance Assessment of
• Test and evaluation methods to characterize the                               Reactive Membranes for New Self-Detoxifying Materials
chemical resistance of permselective and reactive                               Systems for CB Protective Clothing,” Proceedings of the
polymer layers in protective clothing materials need                            2006 Scientific Conference on Chemical and Biological
to include conditions that produce a realistic state of                         Defense, Hunt Valley, MD, 13-15 November, 2006.
hydration in the test samples.                                            [6]   Gibson, P., "Multiphase Heat and Mass Transfer Through
                                                                                Hygroscopic Porous Media With Applications to Clothing
• Polymer membranes and laminates usually show                                  Materials," U.S. Army Natick Research, Development,
                                                                                and Engineering Center Technical Report, Natick/TR-
much higher water vapor transport rates when they                               97/005, 1996.
are well hydrated (higher mean relative humidity).
Standard test methods that only use a single humidity
condition to evaluate or rank materials may                             AUTHORS ADDRESSES
completely reverse material rankings depending on
whether the test method produces low or high mean                       Phillip W. Gibson, Ph.D.
relative humidity test conditions.                                      Heidi L. Schreuder-Gibson,Ph.D.
                                                                        Materials and Defense Sciences Division
• Test methods that have been developed for air-                        U.S. Army Natick Soldier Research, Development,
permeable sorptive layers are not designed to probe                     and Engineering Center
the effect of different hydration levels of a polymer                   Natick, MA 01760-5020
film. In particular, testing with a dry gas sweep on                    UNITED STATES
one side of the membrane (0% relative humidity), can                    ’S ADDRESS
dry out the film so much on one side that the
membrane becomes a significant barrier to water                         Phillip Gibson
transport. Although testing with a dry gas sweep on                     Molecular Sciences and Engineering Team
one side of a polymer membrane is convenient for                        Materials and Defense Sciences Division (MaDSci)
experimental and theoretical reasons, it should be                      U.S. Army Natick Soldier Research,
supplemented by testing that produces a realistic state                 Development, and Engineering Center
of hydration in the film.                                               Natick, MA 01760-5020
                                                                        USA
•    A polymer membrane swollen with water is
usually much more permeable to vapor than a dry
polymer membrane. This applies to both water vapor
permeability and to organic chemical vapor
permeability.

REFERENCES
  [1]   Schreuder-Gibson, H., Gibson, P., Owens, J., Gill, H.,
        Orlicki, J., Prugh, A., Thornberg, C., Engel, R., Walker, J.,
        Yeomans, W., Stote, R., Carney, D., Zukas, W., " Self-
        Detoxifying Materials for Chemically and Biologically
        Protective Clothing,” U.S. Army Natick Soldier Research,
        Development, and Engineering Center Technical Report,
        Natick/TR-08/008L, 2008.
  [2]   Gibson, P.W., "Water Vapor Transport and Gas Flow
        Properties of Textiles, Polymer Membranes, and Fabric
        Laminates," Journal of Coated Fabrics 28, 1999, 300-327.
  [3]   Gibson, P., "Effect of Temperature on Water Vapor
        Transport Through Polymer Membrane Laminates,"
        Journal of Polymer Testing 19 (6), 2000, 673-691.
  [4]   Lomax, G., “Breathable Polyurethane Membranes for
        Textile and Related Industries,” Journal of Materials
        Chemistry 17, 2007, 2775-2784.




Journal of Engineered Fibers and Fabrics                           20                      http://www.jeffjournal.org
Volume 4, Issue 4 – 2009