8 Expanded PTFE Use in Fabrics and Apparel

O U T L I N E

8.1 Introduction 171 8.4.2 Outdoor Footwear 186

8.4.3 Testing Footwear 186
8.2 Breathable Expanded Polytetrafluoroethylene
8.4.4 Outdoor Gloves 188
Fabric Structure 172
8.5 Protective Apparel 188
8.3 Development History 178
8.6 Summary 190
8.4 Outdoor Apparel 182
8.4.1 Testing Apparel 184 References 190

8.1 Introduction Fabrics treated with silicone, fluorocarbon, and

other water repellants usually allow evaporation of
The use of expanded polytetrafluoroethylene perspiration but are only marginally waterproof.
(ePTFE) began to revolutionize the apparel industry, They allow water to leak through them under very
and generally fabric design. The discomfort of low pressures, and usually leak spontaneously when
clothing and footwear from overheating, such as rubbed or mechanically flexed. Rain garments must
when wearing winter coats and boots, has been an old withstand the impingement pressure of falling and
issue that has been resolved by the use of ePTFE. We wind blown rain and the pressures that are generated
begin with a few words about the issue of clothing in folds and creases in the garment.
“comfort” with respect to body temperature. Comfort Garments must be “breathable” to be comfortable.
is a complicated concept and has many components Passage of some air through the garment makes it
including weight, drapeability, and feel of a garment. more comfortable in addition to the transmitted water
Numerous applications beyond coats have been vapor from inside to outside. In the absence of water
developed for breathable and waterproof fabric ex- accumulation the undergarments do not become
amples of which include tents, gloves, and uniforms wet allowing the natural evaporative cooling effect to
as well. take place. Breathability and ability to transport
Protective garments for wearing in rain and other interior moisture vapor to the external environment
wet conditions must keep the person wearing the are used interchangeably in this discussion.
clothes dry by preventing the leakage of water into The transport of water through a layer can be
the garment and by allowing perspiration to evapo- achieved in a number of ways. Wicking is the most
rate from the wearer to the atmosphere [1]. In the common way when large quantities of moisture are to
past, and through a long history of rainwear devel- be transferred. Wicking materials are hydrophilic in
opment, truly waterproof materials have not allowed that a drop of water placed on the surface of these
the evaporation of perspiration. Consequently, an materials forms an advancing water contact angle of
active person wearing the clothes becomes soaked less than 90 degree so that they wet spontaneously.
with perspiration. “Breathable” materials that do They are also porous with pores that interconnect to
permit evaporation of perspiration have tended to wet make complete pathways through the wicking
through from the rain, and they are not truly water- material.
proof. Waterproof materials like vinyl do not allow Liquid water moves by capillary action from inte-
water in but do not allow evaporation of perspiration. rior surface to exterior surface where it evaporates.

Expanded PTFE Applications Handbook. http://dx.doi.org/10.1016/B978-1-4377-7855-7.00008-0

Copyright © 2017 Elsevier Inc. All rights reserved. 171
172 E XPANDED PTFE A PPLICATIONS H ANDBOOK

Although some wicking materials may resist pressure-

induced flow of liquid water through them due to the
tortuosity and length of flow path, they readily trans-
port water by capillary action from the exterior surface
to the interior surface and so are unsuitable as rain
material. The comfort attributed to cotton garments in
warm climates results from its ability to transport
water to the exterior surface where it can readily
evaporate and provide cooling. Another natural
wicking material is leather that owes its great comfort
to breathability via wicking.

Figure 8.2 Basic model of moisture vapor transmis-

sion and liquid and particulate repellence of a fabric [3].
8.2 Breathable Expanded
Polytetrafluoroethylene Fabric
Structure side. This model was implemented using multiple
layers of especially designed materials including
Expanded PTFE membranes have more than microporous membrane of ePTFE.
1.4 billion pores per square centimeter (Fig. 8.1). The Fig. 8.3 shows the implementation of the model
pores are 20,000 times smaller than a drop of water, shown in Fig. 8.2 in actual apparel like a winter
but 700 times larger than a molecule of water. So the jacket. Those coats are a popular example for the
pores in the membrane are too small to allow water in application of ePTFE membrane. The outer shell is
its liquid form to penetrate the membrane, but mois- usually made from a strong durable fabric such as
ture vapor (a gas) can easily escape [2]. nylon, which is made water repellent by topical
Fig. 8.2 shows a basic model of a breathable fabric treatment with a fluorocarbon or silicone compound.
laminate and how it works. Ideally, water, liquids, The water that is not repelled travels through the
and particulate matter are repelled by the external outer shell but is stopped by the ePTFE membrane
(exposed) surface of the fabric/garment. The fabric that is quite hydrophobic. Water drops, even the
thus may not absorb (like cotton) or wick (like smallest ones, are simply too large to enter the ePTFE
leather) the liquid water. Simultaneously, the fabric membrane pores. It holds back water penetration
allows uninhibited transmission of moisture vapor because of its very low surface energy and small
from the inside surface of the fabric to the ambient pores. Indeed a significant amount of pressure is
environment. Moisture transmission takes place as required to force water into the pores of ePTFE
long as the partial pressure of the water on the interior membranes. That pressure is far larger than the hy-
side of the fabric is higher than it is on its external drodynamic pressure generated by even the most
torrential downpours.
The water vapor generated by bodily perspiration
goes through the polyurethane (PU)/ePTFE layer. PU
is permeable to water vapor. In the vapor phase, water
molecules are sufficiently small to diffuse through
the pores of ePTFE membrane. The body thus re-
mains cool because vapor removal allows continued
evaporation of perspiration, which is the body’s main
cooling mechanism.
Fig. 8.4 shows the components of the fabric
including the ePTFE membrane. The outer layer of
the fabric is usually only resistant to abrasion and
tear. It can be made permanently water repellent by
Figure 8.1 An example of expanded polytetrafluoro- applying a silicone or fluorocarbon chemical to its
ethylene membrane at 40,000
magnification [2]. surface; also called durably water repellent. Those
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 173

Figure 8.3 A practical structure of water repellent and breathable fabric containing expanded
polytetrafluoroethylene.

chemicals usually endure many wash cycles but Investigation by W. L. Gore & Associates revealed
would need to be reapplied after a period of time. The PTFE is not oleophobic, thus allows entry of oily
next layer is usually a mesh intended to protect the materials into the pores. Gore chose to attach (coat)
layers beneath it. the ePTFE membrane to a thin layer of PU to render
Early in the history of ePTFE development the it oleophobic. An alternative and newer method to
membranes were used in outdoor coats. After a achieve oleophobicity of ePTFE involves covering
period of time the first GORE-TEX coats began to the walls of the pores within the membrane with an
leak water in and the breathability lessened! In- oleophobic coating without blocking the pores. This
vestigations showed contamination (caused by dirt, technology has been in use by eVent Corporation,
body oils, sweat, sunscreen, insect repellent, or which is related to BHA Corp, in fabrics for outdoor
similar foreign matter) was the root cause of the leak. coats.
Contamination had become the unexpected enemy of The ePTFE membrane (Fig. 8.4) is coated with a
the early ePTFE laminates that were designed with PU resin to keep out the bodily oils, surfactants, and
plain ePTFE membranes. The membranes worked other oily compounds from blocking its pores. PU
fineduntil they collected dirt and oils, which possess partially penetrates the near-surface pores of the
higher surface energies than PTFE. Gradually, water ePTFE, thus keeps the two layers together. One or
makes contact with dirty or oily ePTFE membranes, more fabric layers are placed under the PU layer as
which eventually allow water through and thus inner shell (or backer). Aside from the ePTFE/PU
leakage occurs. layer being ubiquitously present in breathable

Figure 8.4 Construction of breathable and moisture repellent layers in an expanded polytetrafluoroethylene
fabric.
174 E XPANDED PTFE A PPLICATIONS H ANDBOOK

Figure 8.5 Schematic of water vapor transmission through expanded polytetrafluoroethylene membrane.

waterproof fabrics, there are many fabric designs pores. The answer is no because the pores are orders
depending on the intended end use and its of magnitude larger than the water molecules; thus do
requirements. not hinder their movement. Fig. 8.5 is not drawn to
There is, however, a difference between mois- scale; in reality water molecules are much smaller
ture vapor transmission of ePTFE alone and the than they are depicted.
PU/ePTFE combination membrane. In the case of An example of the chemical structure of typical
the expanded microporous PTFE membrane a PU can be seen in Fig. 8.6. In this case PU is the
simple driving force removes water vapor from the product of the reaction of a polyol with diisocyanate
interior of the apparel (like a coat) to the outside that produces a fairly polar polymeric structure
environment as seen in Fig. 8.5. That driving force capable of absorbing water. Examples of polar
is the partial pressure of the water vapor that must functional groups include the following.
be higher in the fabric interior, next to the body,
than the exterior environment. The larger the dif- II
– O –, – C – and – N –.
ference in partial pressures the more breathable I
the membrane. The moisture vapor transmission H

rate (MVTR) is thus proportional to the difference

between the partial pressure of water on the two The PU used for coating ePTFE membranes is
sides of the membrane. For example, on a cold dry hydrophilic and thus absorbs some water (Fig. 8.7). It
winter day a breathable coat works very well then acts as a reservoir or “jumping off platform” for
because of the relatively large magnitude of the water molecules to enter the pores of the membrane.
driving force. Table 8.1 lists the most common The singular benefit of the PU coatings is it pro-
methods for measurement of breathability of films tects the membrane pores from oils and oily com-
and coatings. pounds. The earliest ePTFE fabric designs did not
One reasonable question is whether the hydro- include a PU coating. Oils and other oily compounds
phobicity of the microporous PTFE membrane af- produced by the body or applied to the body clogged
fects the diffusion of water molecules through the the pores of ePTFE membrane relatively quickly
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 175

Table 8.1 Common methods for measurement of breathability of films and coatings

Method Name Standard Code Explanation of Purpose

Sweating guarded hot ISO 11092 Measurement of thermal and water vapor resistance
plate test ISO 1999 under steady state conditions
ASTM F1868
Upright cup method ASTM E96 Standard test methods for water vapor transmission
of materials
Inverted cup method ASTM E96 Standard test methods for water vapor transmission
of materials
Desiccant inverted cup ASTM E96 Method M-05 Standard test methods for water vapor transmission
method of materials
Dynamic moisture ASTM F2298 Standard test methods for water Vapor diffusion
permeation test resistance and air flow resistance of clothing
materials using the dynamic moisture permeation cell
Moisture vapor ASTM D1653 Standard test methods for water vapor transmission
transmission cell of organic coating films
Dynamic moisture ASTM F2298 Standard test methods for water vapor diffusion
permeable cell resistance and air flow resistance of clothing
materials using the dynamic moisture permeation cell

Figure 8.6 Example of the

polymerization reaction of a
two-part polyurethane [4].

causing the fabric to stop being breathable. PU air through the fabric [6]. So, can ePTFE membrane
coatings were selected because of their high water be used without a PU coating (Figs. 8.4 and 8.7)
permeation rate and oleophobicity that prevents successfully? The answer is yes and accomplished by
clogging of the membrane. applying an oleophobic fluoropolymer coating to the
Silicone resin has also been reported as a substi- ePTFE membrane. An example of the technique is
tute material for PU as a protective layer for ePTFE. described here.
US Patent 5,362,553 [5] reported on the discovery It is coalesced on the surfaces of the nodes and
that ePTFE coated with a silicone resin exhibited fibrils to provide resistance to oil and contaminating
improved resistance to surfactant activity. The in- agents without completely blocking the pores in the
ventors reported no loss in MVTR of silicone ePTFE membrane. An example of the oleophobic
resinecoated ePTFE compared to the membrane by fluoropolymer is a perfluoroalkyl acrylic copolymer
itself. In contrast, they contended ePTFE/PU film had with fluorocarbon side chains. The fluorocarbon side
a lower MVTR than that of the ePTFE alone. chains extend in a direction away from the surface of
Commercially either PU or no coating on the back- the nodes and fibrils that the coalesced oleophobic
side of ePTFE remains the prevailing system for fluoropolymer coats. The oleophobic fluoropolymer
manufacturing breathable waterproof fabrics. coating is coalesced on surfaces of the nodes and
It is known that some degree of air permeability is fibrils to provide resistance to oil and contaminating
desirable to increase user comfort. A drawback cited agents without completely blocking the pores in the
for PU is its impact on reducing permeation (flow) of membrane.
176 E XPANDED PTFE A PPLICATIONS H ANDBOOK

Figure 8.7 Schematic of water vapor transmission through polyurethane-coated expanded polytetrafluoroethy-
lene membrane.

A dispersion of perfluoroalkyl acrylic copolymer the molecular attraction between the solid and the
with fluorocarbon side chains was diluted with a water- liquid. The free energy of the solid relative to a
miscible wetting agent (Fig. 8.8). The dispersion was liquid is often referred to as the surface energy gSL
diluted at a ratio of water-miscible wetting agent to of the solid relative to the liquid. The free energy of
dispersion in a range of about 1:5 to 20:1. The diluted liquid relative to air is normally called the surface
dispersion had surface tension and relative contact tension of the liquid gLA. The free energy of the
angle properties that enable the diluted dispersion to solid relative to air is normally referred to as the
wet the membrane and coat surfaces of the membrane. surface energy of the solid gSA. The YoungeDupre
That included diluting the dispersion in a material equation relates all the free energies to the contact
selected from the group including ethanol, isopropyl angle as q [8]:
alcohol (IPA), methanol, n-propanol, n-butanol, N-N-
dimethylformamide, methyl ethyl ketone, and water- gSA  gSL ¼ gLA $cosðqÞ (8.1)
soluble e- and p-series glycol ethers.
Fig. 8.9 shows scanning electron micrographs of The degree to which a challenge liquid may wet a
ePTFE membranes coated with a PU film and an challenged solid depends on the contact angle q. At a
oleophobic coating like the one described in Fig. 8.8. contact angle q of 0 degree, the liquid wets the solid
The interior side of the ePTFE membrane is contin- so completely that a thin liquid film is formed on the
uously covered with the PU film while its exterior side solid. When the contact angle q is between 0 degree
retains the basic node and fibril structure. In case of and 90 degree the liquid wets the solid. When the
the oleophobic coating the pore wall surfaces and the contact angle q is more than 90 degree the liquid does
node and fibril structure of the membrane is not wet the solid.
preserved. For example, consider two different liquids on a
Wetting mechanism of PTFE surface is described polytetrafluoroethylene (PTFE) solid surface that has
further because of the importance of the subject to a surface energy gSA of 19 dyn/cm. One liquid, such
the coating of the membrane pores. The free energy as IPA has a surface tension gLA of 22 dyn/cm (which
between a solid and a liquid is inversely related to is a higher value than the surface energy gSA value of
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 177

Figure 8.8 Schematic of coating perfluoroalkyl acrylic copolymer on the walls of expanded polytetrafluoroethy-
lene membrane pores.

Figure 8.9 Scanning electron micrographs of expanded polytetrafluoroethylene (ePTFE) membranes coated
with a polyurethane (PU) film and an oleophobic coating [7].
Images courtesy of Dr Philip Gibson.
178 E XPANDED PTFE A PPLICATIONS H ANDBOOK

the PTFE material and in theory cannot wet the PTFE of SCCO2 and coating materials also have
material) and a relative contact angle q of about viscosity <0.5 cP. The viscosity and surface tension
43 degree relative to PTFE. Therefore, IPA “wets” of the resultant solution are low compared to tradi-
PTFE very well. The gSL of IPA relative to PTFE can tional solvents so resistance to flow is reduced; thus
now be calculated by rearranging Eq. (8.1) to lending itself to entering even the smallest pores in
Eq. (8.2) to: the membrane. Most solvents’ viscosity is >0.5 cP
and surface tension >15 dyn/cm which make it
gSL  gSA ¼ gLA $cosðqÞ (8.2) difficult for them to enter small pores of ePTFE.

Consequently, it is difficult to coat all the surfaces of

gSL ¼ 19  22
cos 43 ¼ 3 dyn=cm ePTFE membrane with those liquids.
The polymer coatings in the described method
Another liquid such as deionized water has a form very small “particle-like” precipitates in the
surface tension of about 72 dyn/cm and a contact CO2 fluid. These particles are very small as compared
angle q of 112 degree relative to PTFE and, therefore, to conventional dispersed particles. As the polymer
does not wet PTFE or is held out. The calculated particles precipitate from the low surface tension
value for the surface energy gSL of water relative to fluid the polymer stays highly swollen and the ePTFE
PTFE would be 38.5 dyn/cm [6,9]. material of base membrane remains completely
Another aspect of contact angle q is important. If wetted with the fluid and the CO2-plasticized poly-
the contact angle q that a given liquid makes relative mer [10].
to a solid is less than 90 degree, the liquid can be
drawn into capillaries existing in even an apparently
solid material. The amount of capillary force drawing 8.3 Development History
the liquid into the capillary will depend on the size of
the capillary. A relatively smaller capillary exerts a The development of breathable fabrics took place
relatively greater force on the liquid to draw the at W. L. Gore & Associates in the 1970s. One of the
liquid into the capillary. If the contact angle q is early patents is US 4,194,041 that describes the
greater than 90 degree, there will be a force to drive construction of breathable and waterproof fabrics
the liquid out of the capillaries. using ePTFE membranes. There have been a large
The capillary force relates to the surface energy number of patents since the Gore disclosure many of
gSA of the solid material and to the surface tension which have innovated over the initial invention.
gLA of the liquid. The capillary force drawing Today, a number of fabric designs with water repel-
the liquid into the capillaries increases with the lence and breathability characteristics are available
increasing surface energy gSA of the solid. from various companies.
The capillary force drawing the liquid into the cap- ePTFE was used in waterproof garments and tents
illaries also increases with decreasing surface tension because it kept liquid water out while permitting the
gLA of the liquid. YoungeLaplace Eq. (8.3) governs evaporation of the perspiration and the transfer of
the equilibrium state of liquid entry into a capillary. moisture vapor through the layered fabric [1]. At
least two layers had to be combined for this appli-
2gLA cation: (1) an interior, continuous hydrophilic layer
rp ¼ b cosðqÞ (8.3) (eg, PU) that allowed water to diffuse through, pre-
DP
vented the transport of surface active agents and
rp ¼ pore diameter; gLA ¼ surface tension of solide contaminating substances such as those found in
liquid, dyn/cm (mN/m) (calculated from Young’s perspiration, and was substantially resistant to the
equation); DP ¼ pressure difference applied across pressure-induced flow of liquid water and (2) an
the membrane; ß ¼ capillary constant; q ¼ contact ePTFE layer that permitted the transmission of water
angle. vapor and provided thermal insulating properties
In another method the solvent used for coating even when exposed to rain.
material is carbon dioxide in supercritical phase. The Fabrics of these materials were permanently
surface tension of the supercritical carbon dioxide waterproof. They repelled all exterior water
(SCCO2) solution is less than 0.1 dyn/cm so it can yet allowed the evaporation of perspiration whenever
enter very small pores of base membrane. Mixtures the partial pressure of water vapor inside the garment
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 179

exceeded that outside. In practice, these garments The fabric exhibited elastomeric properties of
withstood nearly all climate conditions. The hydro- stretch to break of 275% in the machine direction,
philic film had an MVTR >2000 g/m2 day, permitted and 145% in the transverse direction, and a total
no detectable transmission of surface-active agents stretch recovery of at least 39% after being stretched
and no detectable flow of liquid water at hydrostatic to 75% extension for 100 cycles. The waterproof and
pressures up to 172 kN/m2. The hydrophobic layer breathable elastomeric ePTFE laminate bonded to a
had an MVTR >2000 g/(m2 day), and an advancing stretch fabric was proven durable and possessed
water contact angle >90 degree [1]. MVTR of over 2000 g/m2 day.
Donovan [11] reported development of a breath- A water vaporepermeable, waterproof, and highly
able fabric using ePTFE that was quite strong and elastic film of ePTFE was developed by impregnating/
flame resistant. The exterior layer was Nomex poly- coating it on both sides with a water vaporepermeable
aramide, followed by the ePTFE membrane and the PU elastomer. The membrane had elongation of
interior layer was Kevlar polyaramide. This fabric >40% in at least one direction. The membrane was
was waterproof, windproof, and permeable to water durable in repeated stretching to 80% of its ultimate
vapor, properties that are desirable in tents for severe elongation for over 200,000 cycles. The membrane is
service applications, such as continued rough usage useful for construction of clothing, tents, and other
and usage under severe weather conditions. Tensile end uses in which water vapor transmission and
strength of the fabric was 2.3 and 2 MPa in the ma- waterproofness are required [14].
chine (warp) and fill (cross) directions. US Patent 4,961,985 [15] described innovations
A laminate developed for medical and biological over previously reported developments in surgical
applications exhibited strong resistance to bacterial gowns, drapes, and similar fabrics that protect sur-
penetration. It consisted of a flexible inner layer of gically prepared areas of the skin from contamina-
ePTFE with an MVTR >1000 g/(m2 day) and a tion. Similarly, they also protect surgeons and nurses
contact angle >90 degree, combined with a against contamination through contact with unpre-
continuous outer hydrophilic layer such as PU pared or contaminated areas of patient’s skin. In
attached to the inner surface of ePTFE. This PU summary, surgical gowns must provide a sterile
layer had a minimum MVTR of 1000 g/(m2 day). It barrier to protect patients from contamination
contained a solid powder or a liquid additive such through contact with the surgeons and operating
as color pigments and antistatic agents. An addi- room staff, and vice versa.
tional textile layer was attached to the inner surface An ePTFE membrane with a minimum porosity of
of the ePTFE layer for strength and aesthetic rea- 65% was used that had a per unit area weight of
sons. This laminate had, in addition to being 1e10 g/m2. A layer of hydrophilic PU resin was
breathable and waterproof, strong resistance to applied to one side of ePTFE. The PU resin diffused
bacterial penetration in excess of 5000 min, water into the pores near the surface of the ePTFE (see
entry pressure above 138 kPa, and an MVTR Figs. 8.4 and 8.7). The PU layer had a per unit area
>2000 g/(m2 day). This type of laminate is partic- weight of 10e20 g/m2. The laminate of ePTFE/PU
ularly useful in both biological and health-care had a minimum MVTR of 15,000 g/m2/24 and had a
applications [12]. significant resistance to the passage of microor-
In addition to being waterproof and breathable, it ganism barrier. For instance, when challenged by a
is desirable for fabrics to have the distinguishing Virus Barrier Efficiency Test at 27.6 kPa no virus
characteristic of stretch [13]. Stretchability offers passed through the membrane. The laminate exhibi-
many advantages including comfort, fit, reduction in ted air permeability <6 cm3/min by the Gurley air
pucker, improved wrinkle resistance, required fewer permeability test (ASTM D726) [15].
sizes, alterations, and greater design flexibility. In its A 2005 patent application [16] provides another
broad concept, “stretch” might be defined as an type of breathable fabric with a capability to control
important comfort factor in textile products. To microorganisms like bacteria. In addition to infection
accomplish the goal of stretchability, ePTFE was control in medical applications, growth of microor-
coated with an elastomeric hydrophilic polymer ganisms such as molds and bacteria damages fabrics
with an MVTR exceeding 1000 g/m2 day. The or produces an unpleasant odor. Examples include
stretchability of the elastomer coating had to be at garments worn for an extended period of time
least 5% higher than its yield point. without being changed, or used under circumstances
180 E XPANDED PTFE A PPLICATIONS H ANDBOOK

where it will not dry for extended periods. Military resistance to water. Manufacturers typically describe
uniforms are sometimes worn for long periods of the waterproof breathability of fabrics using two
time in extreme environments. Control of microor- numbers. The first is in millimeters and is a measure
ganisms in such instances is important. of how waterproof a fabric is. In the case of a
Silver compounds such as silver acetate, silver 10,000 mm fabric, if you put a square tube with inner
nitrate, silver protein, and silver sulfadiazine dimensions of 2.54 cm
2.54 cm (100
100 ) over a
are known for antimicrobial effects. They generate piece of said fabric, you could fill it with water to a
silver ion treatment purported to place some bacteria height of 10,000 mm (32.8 feet) before water would
in an active but nonculturable state and eventual begin to leak through. The higher the number, the
death [17]. Silver is formulated into fibers such as X- more waterproof the fabric.
Static available from Noble Biomaterials. The silver- The second number is a measure of how breath-
laden fibers are incorporated in various medical able the fabric is, and is normally expressed in terms
devices such as advanced wound care treatments, of how many grams of water vapor can pass through a
dressings, medical socks, and orthopedic soft square meter of the fabric from the inside to the
goods. They are also incorporated into soft surfaces, outside in a 24-h period. In the case of a 20k
such as privacy curtains, scrubs, lab coats, patient (20,000 g) fabric, this would be 20,000 g. The larger
apparel, and bedding to prevent the growth and the number the more breathable the fabric would be.
cross-contamination of bacteria on the surface of US Patents 5,026,591 and 4,532,316 described the
fabrics [18]. formulation and preparation technique for PU coat-
Silver-containing fibers are included in multilayer ings [19,20]. The PU described is a 100% solids
breathable fabric structure of ePTFE membrane thus mixture of polyol and an isocyanate such as diiso-
providing protection against microbial growth, along cyanate. A coating lamination process was reported
with anti-odor, anti-static, heat and moisture transfer that produced a laminate of PU, ePTFE membrane,
attributes. The breathable fabric included a mem- and a fabric substrate. The coated ePTFE may be
brane containing a porous ePTFE scaffold material used in waterproof-breathable products, such as
with a void volume (>60%). A resin such as PU can garments, shoes, or gloves. The fabric acted as a
be applied to at least one surface of the scaffold protective layer in the construction of the apparel.
material. The fabric also included silver-containing One of the benefits of the laminate is the wide
substrate placed in contact with the ePTFE mem- variety of fabric substrates that can be processed into
brane. One method of securing the silver-containing a laminated product. This is true because the sub-
substrate to the ePTFE membrane was by an adhe- strate does not control the film-forming process, nor
sive system. does the substrate’s geometry, properties, or charac-
Table 8.2 lists waterproof rating of fabrics in teristics control the penetration of coating into the
which pairs of numbers are used to describe the substrate. The ePTFE membrane controls the amount

Table 8.2 Waterproof rating of fabrics using pairs of numbers

Waterproof Rating (mm) Resistance Provided What it can Withstand

0e5000 mm No resistance to some resistance to Light rain, dry snow, no pressure
moisture
6000e10,000 mm Rainproof and waterproof under light Light rain, average snow, light pressure
pressure
11,000e15,000 mm Rainproof and waterproof except under Moderate rain, average snow, light
high pressure pressure
16,000e20,000 mm Rainproof and waterproof under high Heavy rain, wet snow, some pressure
pressure
20,000 mmþ Rainproof and waterproof under very Heavy rain, wet snow, high pressure
high pressure
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 181

of PU entering and adhering to the fabric substrate, adhering adjacent layers of seam tape together
and further controls in a unique way the geometry strongly enough that upon unrolling the spool, the
and the continuity of the coating. The fabric may expanded porous PTFE layer is damaged. The
have any geometry such as thickness, texture, open- densified surface of the ePTFE layer prevents the
ness, etc. The substrate, therefore, is selected pre- entry of the thermoplastic adhesive through cold
dominantly as the needs of the end use dictate. creep into the ePTFE layer while the same tape is
The coated products of this technique have unique stored in roll form (Fig. 8.11).
characteristics. It was discovered that the combina- The ePTFE films prior to densification have den-
tion of PU and ePTFE membrane was attached to the sities between 0.3 and 0.5 g/cm3, thickness
fabric substrate in a unique way. The PU/ePTFE was 25e50 mm, and porosity 40%. The PU (25e200 mm
attached only at select points. This was in contrast to thick) must have a sufficiently low viscosity as a
what was normally seen in previous techniques (see liquid to flow into the pores of the ePTFE and when
Fig. 8.4) where the PU/ePTFE seemed to follow the cured or partially cured must melt well above the
contour of the fabric substrate. It did not thus have an melting point of the thermoplastic hot melt adhesive
overall regular thickness. The US Patents 5,026,591 layer to prevent delamination. Materials with melt
and 4,532,316 produced laminates with regular viscosities between 10 and 200 P, at 100 C, are
thickness (Fig. 8.10). required. The melting point of the cured or partially
The problem of blocking of seam tape contain- cured thermosetting adhesive is in 200 C and
ing an ePTFE layer during storage, especially in preferably does not melt, but decomposes. In the solid
warm months, used to be a big impediment for the form, it must be insoluble in water and unaffected by
breathable waterproof garment manufacturing. US dry cleaning solvents.
Patent 5,162,149 reported on a non-blocking The preferred thermoplastic hot melt adhesive is
waterproof seam tape for covering sewn seams PU. Its melting point should be >100e180 C and
in 1992. have a melt flow rate (as determined by ASTM 1238
The seam tape consisted of an ePTFE layer in under conditions KISS/15) of >20 g/min and <150 g/
which one surface had been densified, a cured, or min to ensure adequate flow of the thermoplastic hot
attached to a partially cured PU adhesive layer and a melt layer when applied to a seam. Fig. 8.12 illus-
thermoplastic hot melt adhesive layer. Compressive trates how the sealant tape is applied to a sewn seam
forces especially in warm environment induce the of ePTFE fabric.
thermoplastic hot melt adhesive to creep into the More recently, copolymers of tetrafluoroethylene
pores of the adjacent layer of ePTFE layer, thus, (TFE) of the fine powder type have been developed.

Figure 8.10 Construction of a breathable and moisture-repellent fabric using an expanded polytetrafluoroethy-
lene laminate with uniform thickness [19,20].

Figure 8.11 Schematic design of seam sealing tape for apparel [21].
182 E XPANDED PTFE A PPLICATIONS H ANDBOOK

Figure 8.12 Schematic design of seam sealing tape for apparel [21].

Comonomer concentrations of over 5% by weight 8.4 Outdoor Apparel

have been demonstrated yielding copolymers that are
expandable. Examples of comonomers included A few examples of commercial product are
chlorotrifluoroethylene, hexafluoropropylene, vinyl described in this section. Even though there are many
fluoride, vinylidene difluoride, hexafluoroisobutylene, suppliers the selected examples are limited to a few
and trifluoroethylene. The TFE copolymers produced major suppliers that provide descriptions of those
strong, useful, expanded membranes with a micro- products.
structure of nodes interconnected by fibrils. Up to and Fig. 8.13 shows a basic GORE-TEX coat (GORE-
beyond a 25:1 stretch ratio allowed formation of a TEX is a trademark of W. L. Gore & Associates,
uniform viable membrane. Expandability of TFE co- Inc.). The two-layer construction, designed for a
polymers is entirely unexpected, and contrary to past wide range of outdoor activities, uses a specific
reports that a TFE copolymer can not be expanded GORE-TEX membrane bonded to the outer material
[22e24]. and protected on the inside by a separate lining. The

Figure 8.13 Example of basic GORE-TEX coat.

Courtesy: W. L. Gore & Associates, www.GORE-TEX.com, January 2015.
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 183

separate lining ensures better wearing comfort and Figs. 8.14 and 8.15 show examples of two fabric
versatility. designs that eliminate the added oleophobic (pro-
The three-layer construction, designed for more tective) layer of traditional products thus allowing for
demanding activities, uses a specific GORE-TEX increased breathability while preserving its protec-
membrane sandwiched between the outer material tion against contamination. The design in Fig. 8.14 is
and backer material. Three-layer provides added likely based on the technology discussed in the
durability without additional weight or bulk. Section 8.2 (Fig. 8.8) in which the pores are coated

Figure 8.14 Design of the

eVent fabric without
polyurethane oleophobic layer.
Courtesy: www.eventfabrics.com/
products/#protective.

Figure 8.15 Example of GORE-

TEX Pro Coat.
Courtesy: W. L. Gore & Associates,
www.GORE-TEX.com, January
2015.
184 E XPANDED PTFE A PPLICATIONS H ANDBOOK

Figure 8.16 Example of GORE-TEX

Paclite Coat.
Courtesy: W. L. Gore & Associates, www.
GORE-TEX.com, January 2015.

using a supercritical carbon dioxide solution of an both the outer material and a specially developed
oleophobic polymer. The construction in Fig. 8.15 robust inner lining. Outer materials meet demanding
uses a 100% ePTFE based multilayer membrane performance criteria (denier 40) and the thin, low
construction. It has up to 28% increase in breath- denier Gore Micro Grid Backer technology which
ability over current ePTFE laminates. The interior enhances breathability, reduces weight, internal
comfort of the garment is increased, thus enabling the abrasion, and snag resistance allow high performance
wearer to feel less clammy and more comfortable of the three-layer products.
over a wide range of temperatures and conditions.
The new membrane technology performs better in
lower humidity conditions, is more comfortable in
warmer conditions caused by solar loading, and has 8.4.1 Testing Apparel
improved dry out times in conditions with frequent Fabrics containing ePTFE membrane must meet
workerest cycles. the requirements of the intended outdoor wear just
Fig. 8.16 is an example of GORE-TEX Paclite like any other materials would. The essential tests
apparel advertised to have the lightest, most packable include mechanical durability, resistance to cold flex
fabrics. The garments are durably waterproof, fatigue, waterproofness, and comfort. These tests are
windproof, and breathable and are built for activities described briefly.
when weight and space are critical, but protection is The Wyzenbeek abrasion test (ASTM D4157) is
still important. The outside fabric is constructed of used primarily in North America. Although designers
high-performance polyester or nylon, and the inside in North America are less familiar with the Martin-
uses a specific ePTFE membrane with a protective dale test (ASTM D4966), it is gaining recognition as
layer made from an oil repellent substance and car- a reliable test. The Martindale is considered by many
bon. So no separate lining is required which makes to be a more accurate measurement of “real life” use.
the shells lighter and smaller to pack away. Special The fabric is mounted flat and rubbed in a modified
tape technology is said to ensure the seams and are figure-eight motion with a piece of worsted wool as
completely sealed waterproof. the abradant. The number of cycles that the fabric can
GORE-TEX Pro products for mountain sport ac- withstand before showing an objectionable change in
tivities (Fig. 8.17) use a revolutionary 100% ePTFE- appearance is counted. The inspection interval is
based multilayer membrane system with a unique dependent on the end point of the fabric and is usu-
microstructure. The membrane is strongly bonded to ally every 1000 up to 5000 rubs, every 2000 between
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 185

Figure 8.17 Example of GORE-TEX Pro products for mountain sport activities.
Courtesy: W. L. Gore & Associates, www.GORE-TEX.com, January 2015.

5000 and 20,000, every 5000 between 20,000 and management inside a garment. Other factors include
40,000, and every 10,000 above 40,000 [25]. lightweight, garment fit, and softness.
In the cold flex text the goal is to determine the The body regulates heat in four primary ways:
resistance of the fabric to failure under stress at radiation, convection, conduction, and evaporation.
reduced temperatures. The fabrics are squashed and In hot and humid environments or during physical
stretched repeatedly in extreme low temperatures for activity, evaporative cooling (wet heat transfer) is the
many hours. The fabrics must survive this punishing primary heat loss method.
test and emerge still durably waterproof.
W. L. Gore tests every garment style for water-
proofness before production. The testing facility is
designed to simulate a variety of rain conditions.
Specially engineered rain nozzles are strategically
positioned in the chamber to subject the garment to
conditions that range from light drizzle to wind-
driven rain [26].
While comfort is important in everyday garments,
it is critical during physical activity because of the
possibility of heat stress. Clothing is one of the four
key factors contributing to heat stress (Fig. 8.18).
Comfort is the combination of the garment properties
and each individual’s perception or preference, with
the region and work environment having significant
influence on this comfort perception. However, the
issue of comfort can seem very confusing because
there are many variables involved. Two important
factors of comfort are heat and humidity Figure 8.18 Factors contributing to heat stress [27].
186 E XPANDED PTFE A PPLICATIONS H ANDBOOK

Managing sweat/moisture is one of the most GORE-TEX Performance Comfort footwear

important aspects of heat management. Moisture design is suitable for outdoor use in moderate
travels in both vapor and liquid forms away from the weather conditions. They are waterproof and
body. Poor moisture management can make a garment breathable and may be worn in a wide range of
feel clammy, clingy, sticky, and heavy. Fabrics that use outdoor activities and changing weather conditions.
ePTFE membranes solve this problem. When moisture The GORE-TEX membrane in Fig. 8.20 is a com-
is removed rapidly through a breathable fabric/ bination of ePTFE and other materials to render it
garment, less liquid perspiration is formed because of waterproof.
its evaporation away from the body. GORE-TEX Insulated Comfort footwear
(Fig. 8.21) are designed for outdoor use in rain, snow,
and cold conditions. They combine waterproofness
8.4.2 Outdoor Footwear and effective breathability with insulation for use in
Breathable waterproof footwear has been devel- cold weather conditions. Water and snow remain on
oped by a number of companies, using ePTFE tech- the outside while moisture generated by perspiration
nology. W. L. Gore has developed a variety of designs escapes from the inside. The footware’s insulated
for a broad range of wear such as everyday life and inner lining allows reliable protection from the cold
strenuous sports such as skiing, climbing, and thus making them suitable for a wide range of out-
hunting. A few examples of GORE footwear are doors activities (Fig. 8.21).
described. A number of patents provide detailed information
Shoes constructed using GORE-TEX Extended about the development of breathable and waterproof
Comfort technology may be worn indoor and outdoor footwear material and the construction of shoes and
in moderate and warmer conditions or during higher boots [29e32].
activity levels. The footwear is (Fig. 8.19) waterproof
combined with effective breathability, offering
enduring weather protection. Water stays on the 8.4.3 Testing Footwear
outside while perspiration can easily escape from the There are a number of footwear tests for each type
inside. Their noninsulated construction allows of shoe and boot. Breathable and waterproof foot-
outstanding climate comfort and heat release [28]. wear are subjected to specific tests: walking simu-
The GORE-TEX membrane in Fig. 8.19 is a com- lator, wicking test, leak test, and breathability.
bination of ePTFE with other materials to ensure the The walking simulator tests the waterproof per-
footwear is waterproof and does not leak. formance of the footwear (Fig. 8.22). Test shoes are

Figure 8.19 Shoes constructed using GORE-TEX Extended Comfort technology [28].
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 187

Figure 8.20 Shoes constructed using GORE-TEX Performance Comfort technology [28].

Figure 8.21 Shoes constructed using GORE-TEX Insulated Comfort technology [28].

placed on flexible foot forms equipped with moisture and laces to ensure that the whole shoe or boot meets
sensors that are subjected to 200,000 steps in a water the waterproof performance standards. Fig. 8.23
bath. If moisture enters the shoe, the testing stops and shows the setup for the wicking test.
the sensor indicates the source of the leak. Leak test is run in a centrifuge using boots filled
In addition to the ePTFE membrane, there are other with water spun at high speeds. The pressure gener-
components in footwear that ensure durable water- ated by centrifugal force enables water to go through
proofness. All materials must also be nonwicking to even the smallest of holes.
prevent water from being transported into the shoe or Breathability is tested to ensure all the shoe
boot. Those include the shoe’s leather, foam, stitching, components work together properly.
188 E XPANDED PTFE A PPLICATIONS H ANDBOOK

8.4.4 Outdoor Gloves

Gloves have been made with ePTFE membranes to
protect hands from wind and keep them cool and dry
during activity (Fig. 8.24). Less moisture is trapped
in the insulation, so it remains drier thus keeping
hands warmer. According to W. L. Gore the gloves
provide enduring weather protection and personal
comfort, balanced heat transfer, and optimum mois-
ture managementdeven in harsh conditions. Hands
stay warmer when it is cold, and drier when as the
wearer perspires.
Gloves designed for sports such as skiing and
Figure 8.22 Example of a walking simulator test as-
hunting have specialized grips to enhance dexterity.
sembly [28].

8.5 Protective Apparel

Attempts have been made to utilize the properties
of microporous PTFE membranes in developing
protective apparel. An important consideration is the
breathability characteristic of ePTFE membranes.
This is a useful property for a broad range of
uniforms for firefighters, policemen, military
personnel, and other personnel. Yet the ability of
water vapor to move through breathable fabrics im-
plies other vapors and gases also could. This implies
ePTFE fabrics are useful when protecting against
Figure 8.23 The setup for footwear wicking test [28]. liquid challenge as opposed to vapor challenge.

Figure 8.24 Example of a breathable water repellent glove.

Courtesy: W. L. Gore & Associates, www.GORE-TEX.com, January 2015.
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 189

First responders perform physically demanding (ASTM F903) with liquid splash protection.
activities that increase the risk of heat stress. Multi- Clothing of this type is designed to protect the
Threat suits have been developed that are light- wearer from liquid contact, but allows exposure to
weight, flexible, and give the wearer freedom of vapors. Permeation data is not appropriate for
movement, increased range of motion, improved deciding material performance for the level of pro-
peripheral visibility, and excellent dexterity. Wetting tection it provides [34].
the outer layer of this fabric reduces heat stress on the Penetration test procedures are specified in
wearer by promoting evaporative cooling, allowing National Fire Protection Association (NFPA)
the wearer to remain engaged longer. An example of 1992dStandard on Liquid Splash-Protective En-
this type of fabric, that is not breathable, is GORE sembles and Clothing for Hazardous Materials
Chempak Ultra Barrier Fabric and suits [33]. These Emergencies. These procedures are identical to
suits are certified to NFPA 1994, Class 2 and NFPA those in ASTM F903, Procedure C. The penetration
1992 (Table 8.3). test measures the resistance of protective clothing
Liquid splash protection is needed, but not vapor materials to penetration by liquids using a 1-h, one-
protection, a certified liquid splash protective sided liquid exposure to the normal outside material
ensemble that meets NFPA 1992 must be selected. surface. The test is conducted at atmospheric pres-
These protective uniforms are selected for their sure and room temperature. During the sixth minute,
capability to protect against a specific chemical the test is conducted at 13.8 kPa to simulate the
based on penetration data (ASTM F903). Penetra- pressure from a burst pipe. Liquid penetration is
tion is the bulk flow of a liquid chemical through the detected visually at the end of the test. Penetration
material, seams, or suit closures. NFPA 1992 asso- results are recorded as either “Pass” or “Fail”
ciates liquid-tight integrity and penetration data (Table 8.4).

Table 8.3 Certified Protection in CB Hot Zone Environments [33].

Multi-Threat
Requirement Typical Results
NFPA 1994, Class 2 ensemble overall function and integrity
Man in simulant test Systemic physiological 361 PPDFsys 2100 PPDFsys
(MIST) protective dosage factor
(PPDFsys)
Material performance
Burst strength 35 lbf 310 lbf
Seam break strength 15 lbf/2 in 190 lbf/2 in
Chemical permeation Max level
Chemical warfare agents
Mustard (HD) <4.0 mg/cm2 60 min >720 min
Soman (GD) <1.25 mg/cm 2
60 min >720 min
Toxic industrial chemicals
Dimethyl sulfate (DMS) <6 mg/cm2 60 min >480 min
Acrolein <6 mg/cm 2
60 min >480 min
Ammonia (NH3) <6 mg/cm 2
60 min >480 min
Chlorine (Cl2) <6 mg/cm 2
60 min >480 min
Acrylonitrile <6 mg/cm 2
60 min >480 min
190 E XPANDED PTFE A PPLICATIONS H ANDBOOK

Table 8.4 Examples of Chemical Penetration Data [34].

Green (gray in print version)dThese chemicals represent liquid splash hazards as defined by NFPA 1992 Standards. GORE
Chemical Splash Fabric passes the penetration test for chemicals printed in green.
Yellow (light gray in print version)dThese chemicals represent both potential vapor and liquid splash hazards3. GORE Chemical
Splash Fabric passes the penetration test for chemicals printed in yellow. Significant amounts of chemical vapor permeate this
material.
Red (dark gray in print version)dDo not usedGORE Chemical Splash Fabric fails the penetration test for chemicals printed in red.

8.6 Summary [3] www.gore.com, December 2015.

[4] Introduction to Polyurethane Technology, Mad-
This chapter describes applications of expanded ison Chemical Industries, December 2015.
membrane of PTFE in fabrics aimed at apparel. The www.madisonchemical.com.
great advantage of ePTFE as a porous material is its [5] J.A. Dillon, M.E. Dillon, U.S. Patent 5,362,553,
ability to block liquid water but allow water vapor Assigned to Tetratec Corp, November 8, 1994.
through. The membrane is mechanically strong and [6] R.J. Klare, D.E. Chubin, U.S. Patent 6,228,477,
possesses the basic properties of PTFE resin. ePTFE Assigned to BHA Technologies, Inc., May 8,
has found use in nearly every type of apparel ranging 2001.
from outdoor garments to protective gear to medical [7] REI Co-Op, http://www.rei.com/learn/expert-
uniforms. Technology development for modifying advice/rainwear-how-it-works.html, Image
the basic ePTFE membrane for manufacture of new courtesy of Dr. Philip Gibson, December 2015.
and improved apparel products has continued nearly [8] S. Ebnesajjad, C.F. Ebnesajjad, Surface Treat-
half a century after its discovery. ment of Material for Adhesive Bonding, second
ed., Elsevier, Oxford, UK, 2014.
[9] R.J. Klare, U.S. Patent 6,854,603, Assigned
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[1] R.W. Gore, U.S. Patent 4,194,041, Assigned to 2005.
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[2] W.L. Gore & Associates, www.GOREprotective Assigned to BHA Technologies, Inc., August 5,
fabrics.com, January 2016. 2008.
8: E XPANDED PTFE U SE IN FABRICS AND A PPAREL 191

[11] J.G. Donovan, U.S. Patent 4,302,496, Assigned [22] L.A. Ford, U.S. Patent 8,637,144, Assigned to
to Albany International Corp, November 24, W.L. Gore & Associates, January 28, 2014.
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[12] D.J. Gohlke, U.S. Patent 4,344,999, Assigned to W.L. Gore & Associates, May 26, 2015.
W. L. Gore Associates, August 17, 1982. [24] L.A. Ford, U.S. Patent 9,193,811, Assigned to
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[14] H. Nomi, U.S. Patent 4,692,369, Assigned to [26] W.L. Gore & Associates, www.GORE-TEX.
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Bowser, P.L. Brown, U.S. Patent 4,961,985, [27] J. DeNardis, FR Garment Comfort e Explaining
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[16] M.E. Carr, B. Parker, W.F. McNally, S. Chandra, [28] W.L. Gore & Associates, www.GORE-TEX.
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[17] W.K. Jung, H.C. Koo, K.W. Kim, S. Shin, W.L. Gore & Associates, July 15, 1986.
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[18] Noble Biomaterials, January 2016. http:// October 30, 2012.
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[19] R.L. Henn, U.S. Patent 4,532,316, Assigned to 8,607,476, Assigned to W.L. Gore & Associates,
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