Expanded PTFE Use in Fabrics and Apparel

8 Expanded PTFE Use in Fabrics and Apparel

O U T L I N E 8.1 Introduction

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8.2 Breathable Expanded Polytetrafluoroethylene Fabric Structure 172 8.3 Development History

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8.4 Outdoor Apparel 8.4.1 Testing Apparel

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8.1 Introduction The use of expanded polytetrafluoroethylene (ePTFE) began to revolutionize the apparel industry, and generally fabric design. The discomfort of clothing and footwear from overheating, such as when wearing winter coats and boots, has been an old issue that has been resolved by the use of ePTFE. We begin with a few words about the issue of clothing “comfort” with respect to body temperature. Comfort is a complicated concept and has many components including weight, drapeability, and feel of a garment. Numerous applications beyond coats have been developed for breathable and waterproof fabric examples of which include tents, gloves, and uniforms as well. Protective garments for wearing in rain and other wet conditions must keep the person wearing the clothes dry by preventing the leakage of water into the garment and by allowing perspiration to evaporate from the wearer to the atmosphere [1]. In the past, and through a long history of rainwear development, truly waterproof materials have not allowed the evaporation of perspiration. Consequently, an active person wearing the clothes becomes soaked with perspiration. “Breathable” materials that do permit evaporation of perspiration have tended to wet through from the rain, and they are not truly waterproof. Waterproof materials like vinyl do not allow water in but do not allow evaporation of perspiration.

8.4.2 Outdoor Footwear 8.4.3 Testing Footwear 8.4.4 Outdoor Gloves

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8.5 Protective Apparel

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8.6 Summary

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References

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Fabrics treated with silicone, fluorocarbon, and other water repellants usually allow evaporation of perspiration but are only marginally waterproof. They allow water to leak through them under very low pressures, and usually leak spontaneously when rubbed or mechanically flexed. Rain garments must withstand the impingement pressure of falling and wind blown rain and the pressures that are generated in folds and creases in the garment. Garments must be “breathable” to be comfortable. Passage of some air through the garment makes it more comfortable in addition to the transmitted water vapor from inside to outside. In the absence of water accumulation the undergarments do not become wet allowing the natural evaporative cooling effect to take place. Breathability and ability to transport interior moisture vapor to the external environment are used interchangeably in this discussion. The transport of water through a layer can be achieved in a number of ways. Wicking is the most common way when large quantities of moisture are to be transferred. Wicking materials are hydrophilic in that a drop of water placed on the surface of these materials forms an advancing water contact angle of less than 90 degree so that they wet spontaneously. They are also porous with pores that interconnect to make complete pathways through the wicking material. Liquid water moves by capillary action from interior 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.

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Although some wicking materials may resist pressureinduced flow of liquid water through them due to the tortuosity and length of flow path, they readily transport 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.

8.2 Breathable Expanded Polytetrafluoroethylene Fabric Structure Expanded PTFE membranes have more than 1.4 billion pores per square centimeter (Fig. 8.1). The pores are 20,000 times smaller than a drop of water, but 700 times larger than a molecule of water. So the pores in the membrane are too small to allow water in its liquid form to penetrate the membrane, but moisture vapor (a gas) can easily escape [2]. Fig. 8.2 shows a basic model of a breathable fabric laminate and how it works. Ideally, water, liquids, and particulate matter are repelled by the external (exposed) surface of the fabric/garment. The fabric thus may not absorb (like cotton) or wick (like leather) the liquid water. Simultaneously, the fabric allows uninhibited transmission of moisture vapor from the inside surface of the fabric to the ambient environment. Moisture transmission takes place as long as the partial pressure of the water on the interior side of the fabric is higher than it is on its external

Figure 8.1 An example of expanded polytetrafluoroethylene membrane at 40,000
magnification [2].

Figure 8.2 Basic model of moisture vapor transmission and liquid and particulate repellence of a fabric [3].

side. This model was implemented using multiple layers of especially designed materials including microporous membrane of ePTFE. Fig. 8.3 shows the implementation of the model shown in Fig. 8.2 in actual apparel like a winter jacket. Those coats are a popular example for the application of ePTFE membrane. The outer shell is usually made from a strong durable fabric such as nylon, which is made water repellent by topical treatment with a fluorocarbon or silicone compound. The water that is not repelled travels through the outer shell but is stopped by the ePTFE membrane that is quite hydrophobic. Water drops, even the smallest ones, are simply too large to enter the ePTFE membrane pores. It holds back water penetration because of its very low surface energy and small pores. Indeed a significant amount of pressure is required to force water into the pores of ePTFE membranes. That pressure is far larger than the hydrodynamic 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 remains 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 applying a silicone or fluorocarbon chemical to its surface; also called durably water repellent. Those

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Figure 8.3 A practical polytetrafluoroethylene.

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repellent

chemicals usually endure many wash cycles but would need to be reapplied after a period of time. The next layer is usually a mesh intended to protect the layers beneath it. Early in the history of ePTFE development the membranes were used in outdoor coats. After a period of time the first GORE-TEX coats began to leak water in and the breathability lessened! Investigations showed contamination (caused by dirt, body oils, sweat, sunscreen, insect repellent, or similar foreign matter) was the root cause of the leak. Contamination had become the unexpected enemy of the early ePTFE laminates that were designed with plain ePTFE membranes. The membranes worked fineduntil they collected dirt and oils, which possess higher surface energies than PTFE. Gradually, water makes contact with dirty or oily ePTFE membranes, which eventually allow water through and thus leakage occurs.

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Investigation by W. L. Gore & Associates revealed PTFE is not oleophobic, thus allows entry of oily materials into the pores. Gore chose to attach (coat) the ePTFE membrane to a thin layer of PU to render it oleophobic. An alternative and newer method to achieve oleophobicity of ePTFE involves covering the walls of the pores within the membrane with an oleophobic coating without blocking the pores. This technology has been in use by eVent Corporation, which is related to BHA Corp, in fabrics for outdoor coats. The ePTFE membrane (Fig. 8.4) is coated with a PU resin to keep out the bodily oils, surfactants, and other oily compounds from blocking its pores. PU partially penetrates the near-surface pores of the ePTFE, thus keeps the two layers together. One or more fabric layers are placed under the PU layer as inner shell (or backer). Aside from the ePTFE/PU layer being ubiquitously present in breathable

Figure 8.4 Construction of breathable and moisture repellent layers in an expanded polytetrafluoroethylene fabric.

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

waterproof fabrics, there are many fabric designs depending on the intended end use and its requirements. There is, however, a difference between moisture vapor transmission of ePTFE alone and the PU/ePTFE combination membrane. In the case of the expanded microporous PTFE membrane a simple driving force removes water vapor from the interior of the apparel (like a coat) to the outside environment as seen in Fig. 8.5. That driving force is the partial pressure of the water vapor that must be higher in the fabric interior, next to the body, than the exterior environment. The larger the difference in partial pressures the more breathable the membrane. The moisture vapor transmission rate (MVTR) is thus proportional to the difference between the partial pressure of water on the two sides of the membrane. For example, on a cold dry winter day a breathable coat works very well because of the relatively large magnitude of the driving force. Table 8.1 lists the most common methods for measurement of breathability of films and coatings. One reasonable question is whether the hydrophobicity of the microporous PTFE membrane affects the diffusion of water molecules through the

pores. The answer is no because the pores are orders of magnitude larger than the water molecules; thus do not hinder their movement. Fig. 8.5 is not drawn to scale; in reality water molecules are much smaller than they are depicted. An example of the chemical structure of typical PU can be seen in Fig. 8.6. In this case PU is the product of the reaction of a polyol with diisocyanate that produces a fairly polar polymeric structure capable of absorbing water. Examples of polar functional groups include the following. II – O –, – C – and – N –. I H

The PU used for coating ePTFE membranes is hydrophilic and thus absorbs some water (Fig. 8.7). It then acts as a reservoir or “jumping off platform” for water molecules to enter the pores of the membrane. The singular benefit of the PU coatings is it protects the membrane pores from oils and oily compounds. The earliest ePTFE fabric designs did not include a PU coating. Oils and other oily compounds produced by the body or applied to the body clogged the pores of ePTFE membrane relatively quickly

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Table 8.1 Common methods for measurement of breathability of films and coatings Method Name Sweating guarded hot plate test

Standard Code

Explanation of Purpose

ISO 11092

Measurement of thermal and water vapor resistance under steady state conditions

ISO 1999 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

ASTM E96 Method M-05

Standard test methods for water vapor transmission of materials

Dynamic moisture permeation test

ASTM F2298

Standard test methods for water Vapor diffusion resistance and air flow resistance of clothing materials using the dynamic moisture permeation cell

Moisture vapor transmission cell

ASTM D1653

Standard test methods for water vapor transmission of organic coating films

Dynamic moisture permeable cell

ASTM F2298

Standard test methods for water vapor diffusion resistance and air flow resistance of clothing materials using the dynamic moisture permeation cell

Desiccant inverted cup method

Figure 8.6 Example of the polymerization reaction of a two-part polyurethane [4].

causing the fabric to stop being breathable. PU coatings were selected because of their high water permeation rate and oleophobicity that prevents clogging of the membrane. Silicone resin has also been reported as a substitute material for PU as a protective layer for ePTFE. US Patent 5,362,553 [5] reported on the discovery that ePTFE coated with a silicone resin exhibited improved resistance to surfactant activity. The inventors reported no loss in MVTR of silicone resinecoated ePTFE compared to the membrane by itself. In contrast, they contended ePTFE/PU film had a lower MVTR than that of the ePTFE alone. Commercially either PU or no coating on the backside of ePTFE remains the prevailing system for manufacturing breathable waterproof fabrics. It is known that some degree of air permeability is desirable to increase user comfort. A drawback cited for PU is its impact on reducing permeation (flow) of

air through the fabric [6]. So, can ePTFE membrane be used without a PU coating (Figs. 8.4 and 8.7) successfully? The answer is yes and accomplished by applying an oleophobic fluoropolymer coating to the ePTFE membrane. An example of the technique is described here. It is coalesced on the surfaces of the nodes and fibrils to provide resistance to oil and contaminating agents without completely blocking the pores in the ePTFE membrane. An example of the oleophobic fluoropolymer is a perfluoroalkyl acrylic copolymer with fluorocarbon side chains. The fluorocarbon side chains extend in a direction away from the surface of the nodes and fibrils that the coalesced oleophobic fluoropolymer coats. The oleophobic fluoropolymer coating is coalesced on surfaces of the nodes and fibrils to provide resistance to oil and contaminating agents without completely blocking the pores in the membrane.

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

A dispersion of perfluoroalkyl acrylic copolymer with fluorocarbon side chains was diluted with a watermiscible wetting agent (Fig. 8.8). The dispersion was diluted at a ratio of water-miscible wetting agent to dispersion in a range of about 1:5 to 20:1. The diluted dispersion had surface tension and relative contact angle properties that enable the diluted dispersion to wet the membrane and coat surfaces of the membrane. That included diluting the dispersion in a material selected from the group including ethanol, isopropyl alcohol (IPA), methanol, n-propanol, n-butanol, N-Ndimethylformamide, methyl ethyl ketone, and watersoluble e- and p-series glycol ethers. Fig. 8.9 shows scanning electron micrographs of ePTFE membranes coated with a PU film and an oleophobic coating like the one described in Fig. 8.8. The interior side of the ePTFE membrane is continuously covered with the PU film while its exterior side retains the basic node and fibril structure. In case of the oleophobic coating the pore wall surfaces and the node and fibril structure of the membrane is preserved. Wetting mechanism of PTFE surface is described further because of the importance of the subject to the coating of the membrane pores. The free energy between a solid and a liquid is inversely related to

the molecular attraction between the solid and the liquid. The free energy of the solid relative to a liquid is often referred to as the surface energy gSL of the solid relative to the liquid. The free energy of liquid relative to air is normally called the surface tension of the liquid gLA. The free energy of the solid relative to air is normally referred to as the surface energy of the solid gSA. The YoungeDupre equation relates all the free energies to the contact angle as q [8]: gSA  gSL ¼ gLA $cosðqÞ

(8.1)

The degree to which a challenge liquid may wet a challenged solid depends on the contact angle q. At a contact angle q of 0 degree, the liquid wets the solid so completely that a thin liquid film is formed on the solid. When the contact angle q is between 0 degree and 90 degree the liquid wets the solid. When the contact angle q is more than 90 degree the liquid does not wet the solid. For example, consider two different liquids on a polytetrafluoroethylene (PTFE) solid surface that has a surface energy gSA of 19 dyn/cm. One liquid, such as IPA has a surface tension gLA of 22 dyn/cm (which is a higher value than the surface energy gSA value of

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Figure 8.8 Schematic of coating perfluoroalkyl acrylic copolymer on the walls of expanded polytetrafluoroethylene 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.

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the PTFE material and in theory cannot wet the PTFE material) and a relative contact angle q of about 43 degree relative to PTFE. Therefore, IPA “wets” PTFE very well. The gSL of IPA relative to PTFE can now be calculated by rearranging Eq. (8.1) to Eq. (8.2) to: gSL  gSA ¼ gLA $cosðqÞ 

gSL ¼ 19  22
cos 43




(8.2)

¼ 3 dyn=cm

Another liquid such as deionized water has a surface tension of about 72 dyn/cm and a contact angle q of 112 degree relative to PTFE and, therefore, does not wet PTFE or is held out. The calculated value for the surface energy gSL of water relative to PTFE would be 38.5 dyn/cm [6,9]. Another aspect of contact angle q is important. If the contact angle q that a given liquid makes relative 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 the liquid into the capillary will depend on the size of the capillary. A relatively smaller capillary exerts a relatively greater force on the liquid to draw the liquid into the capillary. If the contact angle q is greater than 90 degree, there will be a force to drive the liquid out of the capillaries. The capillary force relates to the surface energy gSA of the solid material and to the surface tension gLA of the liquid. The capillary force drawing the liquid into the capillaries increases with the increasing surface energy gSA of the solid. The capillary force drawing the liquid into the capillaries also increases with decreasing surface tension gLA of the liquid. YoungeLaplace Eq. (8.3) governs the equilibrium state of liquid entry into a capillary. rp ¼

2gLA b cosðqÞ DP

(8.3)

rp ¼ pore diameter; gLA ¼ surface tension of solide liquid, dyn/cm (mN/m) (calculated from Young’s equation); DP ¼ pressure difference applied across the membrane; ß ¼ capillary constant; q ¼ contact angle. In another method the solvent used for coating material is carbon dioxide in supercritical phase. The surface tension of the supercritical carbon dioxide (SCCO2) solution is less than 0.1 dyn/cm so it can enter very small pores of base membrane. Mixtures

of SCCO2 and coating materials also have viscosity <0.5 cP. The viscosity and surface tension of the resultant solution are low compared to traditional solvents so resistance to flow is reduced; thus lending itself to entering even the smallest pores in the membrane. Most solvents’ viscosity is >0.5 cP and surface tension >15 dyn/cm which make it difficult for them to enter small pores of ePTFE. Consequently, it is difficult to coat all the surfaces of ePTFE membrane with those liquids. The polymer coatings in the described method form very small “particle-like” precipitates in the CO2 fluid. These particles are very small as compared to conventional dispersed particles. As the polymer particles precipitate from the low surface tension fluid the polymer stays highly swollen and the ePTFE material of base membrane remains completely wetted with the fluid and the CO2-plasticized polymer [10].

8.3 Development History The development of breathable fabrics took place at W. L. Gore & Associates in the 1970s. One of the early patents is US 4,194,041 that describes the construction of breathable and waterproof fabrics using ePTFE membranes. There have been a large number of patents since the Gore disclosure many of which have innovated over the initial invention. Today, a number of fabric designs with water repellence and breathability characteristics are available from various companies. ePTFE was used in waterproof garments and tents because it kept liquid water out while permitting the evaporation of the perspiration and the transfer of moisture vapor through the layered fabric [1]. At least two layers had to be combined for this application: (1) an interior, continuous hydrophilic layer (eg, PU) that allowed water to diffuse through, prevented the transport of surface active agents and contaminating substances such as those found in perspiration, and was substantially resistant to the pressure-induced flow of liquid water and (2) an ePTFE layer that permitted the transmission of water vapor and provided thermal insulating properties even when exposed to rain. Fabrics of these materials were permanently waterproof. They repelled all exterior water yet allowed the evaporation of perspiration whenever the partial pressure of water vapor inside the garment

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exceeded that outside. In practice, these garments withstood nearly all climate conditions. The hydrophilic film had an MVTR >2000 g/m2 day, permitted no detectable transmission of surface-active agents and no detectable flow of liquid water at hydrostatic pressures up to 172 kN/m2. The hydrophobic layer had an MVTR >2000 g/(m2 day), and an advancing water contact angle >90 degree [1]. Donovan [11] reported development of a breathable fabric using ePTFE that was quite strong and flame resistant. The exterior layer was Nomex polyaramide, followed by the ePTFE membrane and the interior layer was Kevlar polyaramide. This fabric was waterproof, windproof, and permeable to water vapor, properties that are desirable in tents for severe service applications, such as continued rough usage and usage under severe weather conditions. Tensile strength of the fabric was 2.3 and 2 MPa in the machine (warp) and fill (cross) directions. A laminate developed for medical and biological applications exhibited strong resistance to bacterial penetration. It consisted of a flexible inner layer of ePTFE with an MVTR >1000 g/(m2 day) and a contact angle >90 degree, combined with a continuous outer hydrophilic layer such as PU attached to the inner surface of ePTFE. This PU layer had a minimum MVTR of 1000 g/(m2 day). It contained a solid powder or a liquid additive such as color pigments and antistatic agents. An additional textile layer was attached to the inner surface of the ePTFE layer for strength and aesthetic reasons. This laminate had, in addition to being breathable and waterproof, strong resistance to bacterial penetration in excess of 5000 min, water entry pressure above 138 kPa, and an MVTR >2000 g/(m2 day). This type of laminate is particularly useful in both biological and health-care applications [12]. In addition to being waterproof and breathable, it is desirable for fabrics to have the distinguishing characteristic of stretch [13]. Stretchability offers many advantages including comfort, fit, reduction in pucker, improved wrinkle resistance, required fewer sizes, alterations, and greater design flexibility. In its broad concept, “stretch” might be defined as an important comfort factor in textile products. To accomplish the goal of stretchability, ePTFE was coated with an elastomeric hydrophilic polymer with an MVTR exceeding 1000 g/m2 day. The stretchability of the elastomer coating had to be at least 5% higher than its yield point.

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The fabric exhibited elastomeric properties of stretch to break of 275% in the machine direction, and 145% in the transverse direction, and a total stretch recovery of at least 39% after being stretched to 75% extension for 100 cycles. The waterproof and breathable elastomeric ePTFE laminate bonded to a stretch fabric was proven durable and possessed MVTR of over 2000 g/m2 day. A water vaporepermeable, waterproof, and highly elastic film of ePTFE was developed by impregnating/ coating it on both sides with a water vaporepermeable PU elastomer. The membrane had elongation of >40% in at least one direction. The membrane was durable in repeated stretching to 80% of its ultimate elongation for over 200,000 cycles. The membrane is useful for construction of clothing, tents, and other end uses in which water vapor transmission and waterproofness are required [14]. US Patent 4,961,985 [15] described innovations over previously reported developments in surgical gowns, drapes, and similar fabrics that protect surgically prepared areas of the skin from contamination. Similarly, they also protect surgeons and nurses against contamination through contact with unprepared or contaminated areas of patient’s skin. In summary, surgical gowns must provide a sterile barrier to protect patients from contamination through contact with the surgeons and operating room staff, and vice versa. An ePTFE membrane with a minimum porosity of 65% was used that had a per unit area weight of 1e10 g/m2. A layer of hydrophilic PU resin was applied to one side of ePTFE. The PU resin diffused into the pores near the surface of the ePTFE (see Figs. 8.4 and 8.7). The PU layer had a per unit area weight of 10e20 g/m2. The laminate of ePTFE/PU had a minimum MVTR of 15,000 g/m2/24 and had a significant resistance to the passage of microorganism barrier. For instance, when challenged by a Virus Barrier Efficiency Test at 27.6 kPa no virus passed through the membrane. The laminate exhibited air permeability <6 cm3/min by the Gurley air permeability test (ASTM D726) [15]. A 2005 patent application [16] provides another type of breathable fabric with a capability to control microorganisms like bacteria. In addition to infection control in medical applications, growth of microorganisms such as molds and bacteria damages fabrics or produces an unpleasant odor. Examples include garments worn for an extended period of time without being changed, or used under circumstances

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where it will not dry for extended periods. Military uniforms are sometimes worn for long periods of time in extreme environments. Control of microorganisms in such instances is important. Silver compounds such as silver acetate, silver nitrate, silver protein, and silver sulfadiazine are known for antimicrobial effects. They generate silver ion treatment purported to place some bacteria in an active but nonculturable state and eventual death [17]. Silver is formulated into fibers such as XStatic available from Noble Biomaterials. The silverladen fibers are incorporated in various medical devices such as advanced wound care treatments, dressings, medical socks, and orthopedic soft goods. They are also incorporated into soft surfaces, such as privacy curtains, scrubs, lab coats, patient apparel, and bedding to prevent the growth and cross-contamination of bacteria on the surface of fabrics [18]. Silver-containing fibers are included in multilayer breathable fabric structure of ePTFE membrane thus providing protection against microbial growth, along with anti-odor, anti-static, heat and moisture transfer attributes. The breathable fabric included a membrane containing a porous ePTFE scaffold material with a void volume (>60%). A resin such as PU can be applied to at least one surface of the scaffold material. The fabric also included silver-containing substrate placed in contact with the ePTFE membrane. One method of securing the silver-containing substrate to the ePTFE membrane was by an adhesive system. Table 8.2 lists waterproof rating of fabrics in which pairs of numbers are used to describe the

resistance to water. Manufacturers typically describe the waterproof breathability of fabrics using two numbers. The first is in millimeters and is a measure of how waterproof a fabric is. In the case of a 10,000 mm fabric, if you put a square tube with inner dimensions of 2.54 cm
2.54 cm (100
100 ) over a piece of said fabric, you could fill it with water to a height of 10,000 mm (32.8 feet) before water would begin to leak through. The higher the number, the more waterproof the fabric. The second number is a measure of how breathable the fabric is, and is normally expressed in terms of how many grams of water vapor can pass through a square meter of the fabric from the inside to the outside in a 24-h period. In the case of a 20k (20,000 g) fabric, this would be 20,000 g. The larger the number the more breathable the fabric would be. US Patents 5,026,591 and 4,532,316 described the formulation and preparation technique for PU coatings [19,20]. The PU described is a 100% solids mixture of polyol and an isocyanate such as diisocyanate. A coating lamination process was reported that produced a laminate of PU, ePTFE membrane, and a fabric substrate. The coated ePTFE may be used in waterproof-breathable products, such as garments, shoes, or gloves. The fabric acted as a protective layer in the construction of the apparel. One of the benefits of the laminate is the wide variety of fabric substrates that can be processed into a laminated product. This is true because the substrate does not control the film-forming process, nor does the substrate’s geometry, properties, or characteristics control the penetration of coating into 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 moisture

Light rain, dry snow, no pressure

6000e10,000 mm

Rainproof and waterproof under light pressure

Light rain, average snow, light pressure

11,000e15,000 mm

Rainproof and waterproof except under high pressure

Moderate rain, average snow, light pressure

16,000e20,000 mm

Rainproof and waterproof under high pressure

Heavy rain, wet snow, some pressure

20,000 mmþ

Rainproof and waterproof under very high pressure

Heavy rain, wet snow, high pressure

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of PU entering and adhering to the fabric substrate, and further controls in a unique way the geometry and the continuity of the coating. The fabric may have any geometry such as thickness, texture, openness, etc. The substrate, therefore, is selected predominantly as the needs of the end use dictate. The coated products of this technique have unique characteristics. It was discovered that the combination of PU and ePTFE membrane was attached to the fabric substrate in a unique way. The PU/ePTFE was attached only at select points. This was in contrast to what was normally seen in previous techniques (see Fig. 8.4) where the PU/ePTFE seemed to follow the contour of the fabric substrate. It did not thus have an overall regular thickness. The US Patents 5,026,591 and 4,532,316 produced laminates with regular thickness (Fig. 8.10). The problem of blocking of seam tape containing an ePTFE layer during storage, especially in warm months, used to be a big impediment for the breathable waterproof garment manufacturing. US Patent 5,162,149 reported on a non-blocking waterproof seam tape for covering sewn seams in 1992. The seam tape consisted of an ePTFE layer in which one surface had been densified, a cured, or attached to a partially cured PU adhesive layer and a thermoplastic hot melt adhesive layer. Compressive forces especially in warm environment induce the thermoplastic hot melt adhesive to creep into the pores of the adjacent layer of ePTFE layer, thus,

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adhering adjacent layers of seam tape together strongly enough that upon unrolling the spool, the expanded porous PTFE layer is damaged. The densified surface of the ePTFE layer prevents the entry of the thermoplastic adhesive through cold creep into the ePTFE layer while the same tape is stored in roll form (Fig. 8.11). The ePTFE films prior to densification have densities between 0.3 and 0.5 g/cm3, thickness 25e50 mm, and porosity 40%. The PU (25e200 mm thick) must have a sufficiently low viscosity as a liquid to flow into the pores of the ePTFE and when cured or partially cured must melt well above the melting point of the thermoplastic hot melt adhesive layer to prevent delamination. Materials with melt viscosities between 10 and 200 P, at 100 C, are required. The melting point of the cured or partially cured thermosetting adhesive is in 200 C and preferably does not melt, but decomposes. In the solid form, it must be insoluble in water and unaffected by dry cleaning solvents. The preferred thermoplastic hot melt adhesive is PU. Its melting point should be >100e180 C and have a melt flow rate (as determined by ASTM 1238 under conditions KISS/15) of >20 g/min and <150 g/ min to ensure adequate flow of the thermoplastic hot melt layer when applied to a seam. Fig. 8.12 illustrates how the sealant tape is applied to a sewn seam of ePTFE fabric. More recently, copolymers of tetrafluoroethylene (TFE) of the fine powder type have been developed.

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

Figure 8.11 Schematic design of seam sealing tape for apparel [21].

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

Comonomer concentrations of over 5% by weight have been demonstrated yielding copolymers that are expandable. Examples of comonomers included chlorotrifluoroethylene, hexafluoropropylene, vinyl fluoride, vinylidene difluoride, hexafluoroisobutylene, and trifluoroethylene. The TFE copolymers produced strong, useful, expanded membranes with a microstructure of nodes interconnected by fibrils. Up to and beyond a 25:1 stretch ratio allowed formation of a uniform viable membrane. Expandability of TFE copolymers is entirely unexpected, and contrary to past reports that a TFE copolymer can not be expanded [22e24].

8.4 Outdoor Apparel A few examples of commercial product are described in this section. Even though there are many suppliers the selected examples are limited to a few major suppliers that provide descriptions of those products. Fig. 8.13 shows a basic GORE-TEX coat (GORETEX is a trademark of W. L. Gore & Associates, Inc.). The two-layer construction, designed for a wide range of outdoor activities, uses a specific GORE-TEX membrane bonded to the outer material 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.

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separate lining ensures better wearing comfort and versatility. The three-layer construction, designed for more demanding activities, uses a specific GORE-TEX membrane sandwiched between the outer material and backer material. Three-layer provides added durability without additional weight or bulk.

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Figs. 8.14 and 8.15 show examples of two fabric designs that eliminate the added oleophobic (protective) layer of traditional products thus allowing for increased breathability while preserving its protection against contamination. The design in Fig. 8.14 is likely based on the technology discussed in the 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 GORETEX Pro Coat. Courtesy: W. L. Gore & Associates, www.GORE-TEX.com, January 2015.

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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 oleophobic polymer. The construction in Fig. 8.15 uses a 100% ePTFE based multilayer membrane construction. It has up to 28% increase in breathability over current ePTFE laminates. The interior comfort of the garment is increased, thus enabling the wearer to feel less clammy and more comfortable 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 improved dry out times in conditions with frequent workerest cycles. Fig. 8.16 is an example of GORE-TEX Paclite apparel advertised to have the lightest, most packable fabrics. The garments are durably waterproof, windproof, and breathable and are built for activities when weight and space are critical, but protection is still important. The outside fabric is constructed of high-performance polyester or nylon, and the inside uses a specific ePTFE membrane with a protective layer made from an oil repellent substance and carbon. So no separate lining is required which makes the shells lighter and smaller to pack away. Special tape technology is said to ensure the seams and are completely sealed waterproof. GORE-TEX Pro products for mountain sport activities (Fig. 8.17) use a revolutionary 100% ePTFEbased multilayer membrane system with a unique microstructure. The membrane is strongly bonded to

both the outer material and a specially developed robust inner lining. Outer materials meet demanding performance criteria (denier 40) and the thin, low denier Gore Micro Grid Backer technology which enhances breathability, reduces weight, internal abrasion, and snag resistance allow high performance of the three-layer products.

8.4.1 Testing Apparel Fabrics containing ePTFE membrane must meet the requirements of the intended outdoor wear just like any other materials would. The essential tests include mechanical durability, resistance to cold flex fatigue, waterproofness, and comfort. These tests are described briefly. The Wyzenbeek abrasion test (ASTM D4157) is used primarily in North America. Although designers in North America are less familiar with the Martindale test (ASTM D4966), it is gaining recognition as a reliable test. The Martindale is considered by many to be a more accurate measurement of “real life” use. The fabric is mounted flat and rubbed in a modified figure-eight motion with a piece of worsted wool as the abradant. The number of cycles that the fabric can withstand before showing an objectionable change in appearance is counted. The inspection interval is dependent on the end point of the fabric and is usually every 1000 up to 5000 rubs, every 2000 between

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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 40,000, and every 10,000 above 40,000 [25]. In the cold flex text the goal is to determine the resistance of the fabric to failure under stress at reduced temperatures. The fabrics are squashed and stretched repeatedly in extreme low temperatures for many hours. The fabrics must survive this punishing test and emerge still durably waterproof. W. L. Gore tests every garment style for waterproofness 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 winddriven 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

management inside a garment. Other factors include lightweight, garment fit, and softness. The body regulates heat in four primary ways: radiation, convection, conduction, and evaporation. In hot and humid environments or during physical activity, evaporative cooling (wet heat transfer) is the primary heat loss method.

Figure 8.18 Factors contributing to heat stress [27].

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Managing sweat/moisture is one of the most important aspects of heat management. Moisture travels in both vapor and liquid forms away from the body. Poor moisture management can make a garment feel clammy, clingy, sticky, and heavy. Fabrics that use ePTFE membranes solve this problem. When moisture is removed rapidly through a breathable fabric/ garment, less liquid perspiration is formed because of its evaporation away from the body.

8.4.2 Outdoor Footwear Breathable waterproof footwear has been developed by a number of companies, using ePTFE technology. W. L. Gore has developed a variety of designs for a broad range of wear such as everyday life and strenuous sports such as skiing, climbing, and hunting. A few examples of GORE footwear are described. Shoes constructed using GORE-TEX Extended Comfort technology may be worn indoor and outdoor in moderate and warmer conditions or during higher activity levels. The footwear is (Fig. 8.19) waterproof combined with effective breathability, offering enduring weather protection. Water stays on the outside while perspiration can easily escape from the inside. Their noninsulated construction allows outstanding climate comfort and heat release [28]. The GORE-TEX membrane in Fig. 8.19 is a combination of ePTFE with other materials to ensure the footwear is waterproof and does not leak.

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GORE-TEX Performance Comfort footwear design is suitable for outdoor use in moderate weather conditions. They are waterproof and breathable and may be worn in a wide range of outdoor activities and changing weather conditions. The GORE-TEX membrane in Fig. 8.20 is a combination of ePTFE and other materials to render it waterproof. GORE-TEX Insulated Comfort footwear (Fig. 8.21) are designed for outdoor use in rain, snow, and cold conditions. They combine waterproofness and effective breathability with insulation for use in cold weather conditions. Water and snow remain on the outside while moisture generated by perspiration escapes from the inside. The footware’s insulated inner lining allows reliable protection from the cold thus making them suitable for a wide range of outdoors activities (Fig. 8.21). A number of patents provide detailed information about the development of breathable and waterproof footwear material and the construction of shoes and boots [29e32].

8.4.3 Testing Footwear There are a number of footwear tests for each type of shoe and boot. Breathable and waterproof footwear are subjected to specific tests: walking simulator, wicking test, leak test, and breathability. The walking simulator tests the waterproof performance of the footwear (Fig. 8.22). Test shoes are

Figure 8.19 Shoes constructed using GORE-TEX Extended Comfort technology [28].

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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 sensors that are subjected to 200,000 steps in a water bath. If moisture enters the shoe, the testing stops and the sensor indicates the source of the leak. In addition to the ePTFE membrane, there are other components in footwear that ensure durable waterproofness. All materials must also be nonwicking to prevent water from being transported into the shoe or boot. Those include the shoe’s leather, foam, stitching,

and laces to ensure that the whole shoe or boot meets the waterproof performance standards. Fig. 8.23 shows the setup for the wicking test. Leak test is run in a centrifuge using boots filled with water spun at high speeds. The pressure generated by centrifugal force enables water to go through even the smallest of holes. Breathability is tested to ensure all the shoe components work together properly.

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8.4.4 Outdoor Gloves

Figure 8.22 Example of a walking simulator test assembly [28].

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 moisture 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 hunting have specialized grips to enhance dexterity.

8.5 Protective Apparel

Figure 8.23 The setup for footwear wicking test [28].

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 implies other vapors and gases also could. This implies ePTFE fabrics are useful when protecting against 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.

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First responders perform physically demanding activities that increase the risk of heat stress. MultiThreat suits have been developed that are lightweight, flexible, and give the wearer freedom of movement, increased range of motion, improved peripheral visibility, and excellent dexterity. Wetting the outer layer of this fabric reduces heat stress on the wearer by promoting evaporative cooling, allowing the wearer to remain engaged longer. An example of this type of fabric, that is not breathable, is GORE Chempak Ultra Barrier Fabric and suits [33]. These suits are certified to NFPA 1994, Class 2 and NFPA 1992 (Table 8.3). Liquid splash protection is needed, but not vapor protection, a certified liquid splash protective ensemble that meets NFPA 1992 must be selected. These protective uniforms are selected for their capability to protect against a specific chemical based on penetration data (ASTM F903). Penetration is the bulk flow of a liquid chemical through the material, seams, or suit closures. NFPA 1992 associates liquid-tight integrity and penetration data

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(ASTM F903) with liquid splash protection. Clothing of this type is designed to protect the wearer from liquid contact, but allows exposure to vapors. Permeation data is not appropriate for deciding material performance for the level of protection it provides [34]. Penetration test procedures are specified in National Fire Protection Association (NFPA) 1992dStandard on Liquid Splash-Protective Ensembles and Clothing for Hazardous Materials Emergencies. These procedures are identical to those in ASTM F903, Procedure C. The penetration test measures the resistance of protective clothing materials to penetration by liquids using a 1-h, onesided liquid exposure to the normal outside material surface. The test is conducted at atmospheric pressure and room temperature. During the sixth minute, the test is conducted at 13.8 kPa to simulate the pressure from a burst pipe. Liquid penetration is detected visually at the end of the test. Penetration results are recorded as either “Pass” or “Fail” (Table 8.4).

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

Multi-Threat Typical Results

NFPA 1994, Class 2 ensemble overall function and integrity Man in simulant test (MIST)

Systemic physiological protective dosage factor (PPDFsys)

361 PPDFsys

2100 PPDFsys

35 lbf

310 lbf

15 lbf/2 in

190 lbf/2 in

Material performance Burst strength Seam break strength Chemical permeation

Max level

Chemical warfare agents Mustard (HD)

<4.0 mg/cm2

60 min

>720 min

Soman (GD)

<1.25 mg/cm

60 min

>720 min

2

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

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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 This chapter describes applications of expanded membrane of PTFE in fabrics aimed at apparel. The great advantage of ePTFE as a porous material is its ability to block liquid water but allow water vapor through. The membrane is mechanically strong and possesses the basic properties of PTFE resin. ePTFE has found use in nearly every type of apparel ranging from outdoor garments to protective gear to medical uniforms. Technology development for modifying the basic ePTFE membrane for manufacture of new and improved apparel products has continued nearly half a century after its discovery.

References [1] R.W. Gore, U.S. Patent 4,194,041, Assigned to W.L. Gore Associates, March 18, 1980. [2] W.L. Gore & Associates, www.GOREprotective fabrics.com, January 2016.

[3] www.gore.com, December 2015. [4] Introduction to Polyurethane Technology, Madison Chemical Industries, December 2015. www.madisonchemical.com. [5] J.A. Dillon, M.E. Dillon, U.S. Patent 5,362,553, Assigned to Tetratec Corp, November 8, 1994. [6] R.J. Klare, D.E. Chubin, U.S. Patent 6,228,477, Assigned to BHA Technologies, Inc., May 8, 2001. [7] REI Co-Op, http://www.rei.com/learn/expertadvice/rainwear-how-it-works.html, Image courtesy of Dr. Philip Gibson, December 2015. [8] S. Ebnesajjad, C.F. Ebnesajjad, Surface Treatment of Material for Adhesive Bonding, second ed., Elsevier, Oxford, UK, 2014. [9] R.J. Klare, U.S. Patent 6,854,603, Assigned to BHA Technologies, Inc., February 15, 2005. [10] J. DeYoung, R.J. Klare, U.S. 7,407,703, Assigned to BHA Technologies, Inc., August 5, 2008.

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[11] J.G. Donovan, U.S. Patent 4,302,496, Assigned to Albany International Corp, November 24, 1981. [12] D.J. Gohlke, U.S. Patent 4,344,999, Assigned to W. L. Gore Associates, August 17, 1982. [13] D. Worden, F.T. Wilson, L.J. Grubb, U.S. Patent 4,443,511, Assigned to W.L. Gore & Associates, April 17, 1984. [14] H. Nomi, U.S. Patent 4,692,369, Assigned to Japan GORE-TEX®, September 8, 1987. [15] R.L. Henn, D.J. Sakhpara, C.E. Bailey, J.J. Bowser, P.L. Brown, U.S. Patent 4,961,985, Assigned to W.L. Gore & Associates, October 9, 1990. [16] M.E. Carr, B. Parker, W.F. McNally, S. Chandra, V. Naik, J.M. Furey, U.S. 2005/0196603, Merchant & Gold PC, September 8, 2005. [17] W.K. Jung, H.C. Koo, K.W. Kim, S. Shin, S.H. Kim, Y.H. Park, Antibacterial activity and mechanism of action of the silver ion in Staphylococcus aureus and Escherichia coli, App Environ. Microbiol. 74 (April 2008) 7. [18] Noble Biomaterials, January 2016. http:// noblebiomaterials.com. [19] R.L. Henn, U.S. Patent 4,532,316, Assigned to W.L. Gore & Associates, July 30, 1985. [20] R.L. Henn, C.H. Morell, E.J. Daniel, U.S. Patent 5,026,591, Assigned to W.L. Gore & Associates, June 25, 1991. [21] J. Reaney, U.S. Patent 5,162,149, Assigned to W.L. Gore & Associates, November 10, 1992.

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[22] L.A. Ford, U.S. Patent 8,637,144, Assigned to W.L. Gore & Associates, January 28, 2014. [23] L.A. Ford, U.S. Patent 9,040,646, Assigned to W.L. Gore & Associates, May 26, 2015. [24] L.A. Ford, U.S. Patent 9,193,811, Assigned to W.L. Gore & Associates, November 24, 2015. [25] Place Textiles, January 2016. http://placetextiles. com/abrasion-testing. [26] W.L. Gore & Associates, www.GORE-TEX. com/en-us/experience/quality/testingouterwear, January 2016. [27] J. DeNardis, FR Garment Comfort e Explaining the Mystery, DuPont Protection Technologies, 2014. [28] W.L. Gore & Associates, www.GORE-TEX. com/products/footwear/, January 2016. [29] G. Sacre, U.S. Patent 4,599,810, Assigned to W.L. Gore & Associates, July 15, 1986. [30] R.J. Wiener, U.S. Patent 6,935,053, Assigned to W.L. Gore & Associates, August 30, 2005. [31] A.W. Jessiman R.J. Wiener, U.S. Patent 8,296,970, Assigned to W.L. Gore & Associates, October 30, 2012. [32] A.W. Jessiman, R.J. Wiener, U.S. Patent 8,607,476, Assigned to W.L. Gore & Associates, December 17, 2013. [33] Chempak® Fabric, Multi-threat Suit, Published W. L. Gore & Associates, www.GORE.com, 2009. [34] Chemical Splash Fabric-Technical Data and Application Guide, Published by W. L. Gore & Associates, www.GORE.com, 2009.