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The LOVED coat and the HOT coat: ePTFE’s Simplicity and Sophistication

Posted on December 18th, 2015 by in New Materials & Applications

WLGoreandAssociates

Image Source: WL Gore and Associates

Winter is arriving in the Northeast US. It is time to put on warmer coats and clothes.  I have some favorite ones and a few jackets I do not like as much.  It is all about comfort, and to me, that means a good coat does not soak in rain and overheat me after a little physical activity.  You may have some of those jackets too.  A good coat like any other one, but there is more to it than just the cool brand names.

The Engineering Behind ‘Breathable and Waterproof’

Believe it or not there are all types of clothing available that repel water, let out the moisture our bodies produce by perspiration and let in the outside air (Figure 1).  The inner and outer shells of the jackets are made of nylon, polyester or other fibers.  Sandwiched between the two shells is a highly engineered laminate that performs those three functions. A laminate is a combination of different layers of materials attached to one another.  In breathable and waterproof fabrics this relatively thin laminate works quite cleverly based on simple principles of physics and chemistry.

Figure 1: An ideal model of breathable waterproof fabric

Figure 1: An ideal model of breathable waterproof fabric

The actual implementation of the model in Figure 1 in fabrics and apparel is more involved (Figure 2).  A winter or ski jacket is a good example of clothes made from this type of fabric. The outer shell is usually a strong durable fabric like nylon made durably water repellent (DWR) by treatment with a fluorocarbon or silicone compound. Water drops not repelled enter the outer shell but are stopped by a hydrophobic microporous membrane called ePTFE. Even the smallest water drops are simply too large to pass through the pores of the ePTFE membrane. The membrane blocks water penetration because of its very low surface energy (18 dynes/cm compared to 72 dynes/cm for water) in addition to the size of the pores.  Another example of low surface energy is a non-stick pan.  Water and oil bid up on the pan and do not spread or wet its surface.

The water vapor generated by perspiration goes through the protective coating layer and ePTFE layer.  While even the smallest liquid water drops are too large to enter the ePTFE pores, in vapor phase water molecules are sufficiently small to travel through the membrane pores unhindered.  Human body remains cool because vapor removal allows continued evaporation of perspiration, which is the body’s main cooling mechanism.

Figure 2: A schematic of the structure of water repellent and breathable fabric containing ePTFE

Figure 2: A schematic of the structure of water repellent and breathable fabric containing ePTFE

That driving force of vapor removal is the partial pressure of the water that must be higher in the fabric interior, next to the body, than it is in the exterior environment.  Another way of stating this point is: humidity inside the jacket must be higher than it is in the ambient. The rate of vapor transmission is proportional to the difference between the humidity on the two sides of the membrane. For example, on a cold dry winter a breathable coat works very well and feels quite comfortable because of the relatively large magnitude of the driving force.

ePTFE Membranes and How They Work

“ePTFE” is the abbreviation for expanded PTFE (polytetrafluoroethylene).  PTFE is better known as Teflon® (trademark of DuPont Co.) and is a fluorinated thermoplastic polymer that withstands extreme temperatures (-260 to 260oC) in contact with most corrosive chemicals.  It is hydrophobic and non-stick with applications in industrial, consumer, medical, automotive, aerospace and other market segments.  PTFE films can be expanded or stretched, monoaxially or biaxially o produce microporous (pore size > 0.2 µm) membranes as thin as 1-2 µm (Figure 3).  The best-known commercial example of the membrane is Gore Tex® developed by WL Gore and Associates in the 1970’s.

The asymmetric porous ePTFE membrane (Figure 3) consists of nodes linked by fine fibrils. These fibrils are wide and thin with maximum width of 0.1 µm (100 nm). The minimum width may be one or two molecular diameters or in the range of 0.5 or 1 nm. The nodes vary in size from about 400 µm to <1 µm, depending on the stretching conditions.

Figure 3: A Scanning electron micrograph of a biaxially expanded PTFE membrane (5,000X)

Figure 3: A Scanning electron micrograph of a biaxially expanded PTFE membrane (5,000X). Source: H. Banning, WL Gore, Improving Shielding Effectiveness of Flexible Coaxial Cables, Microwave Journal, March 24, 2008.

Oils produced by the body or applied to the body clog the ePTFE membrane quickly causing the fabric to stop being breathable.  To solve the problem the ePTFE membrane is coated with a polyurethane (PU) resin to keep out the bodily oils and other oily substances from blocking the pores (Figure 4).  PU partially penetrates the near-surface pores of the ePTFE thus keeping the two layers together.  One or more material layers are placed under the PU layer as protective ply and inner shell.

Figure 4: Design Structure of breathable and moisture repellent layers in an ePTFE fabric

Figure 4: Design Structure of breathable and moisture repellent layers in an ePTFE fabric

An issue with the polyurethane coating on ePTFE is somewhat slower moisture vapor transmission than ePTFE alone.  It also has to be saturated with water molecules from the perspiration before transmission of moisture into ePTFE begins.  More recently technology has eliminated the PU layer and replace it with a hydrophobic coating on the pore medium’s walls.  That coating functions as a barrier to the entrance of oily substances into the ePTFE pores. US patent 7,771,818 reported the use of supercritical carbon dioxide solvent to deposit a fluorinated urethane polymer on the surfaces of fibrils and nodes without blocking the pores.  The latest apparel offered by eVent contains the new technology, which are breathable with effective blocking of oils from the entering the ePTFE membrane. The ePTFE membranes have entered functional fabric designs for applications such as sportswear, military uniforms and protective clothing.

Figure 5. The structure of eVent water-repellent and breathable fabric using pore wall-coated ePTFE. Source: www.eventfabrics.com

Figure 5. The structure of eVent water-repellent and breathable fabric using pore wall-coated ePTFE. Source: www.eventfabrics.com

The possibility of coating the pores of expanded PTFE membranes without complete blocking offers truly tantalizing possibilities for the development of functional and smart membranes.  Most recent developments have demonstrated the feasibility of coating the pores with a material like fluorinated vinyl-based copolymers with sulfonyl functionality.  Possibility of coating the pores with an incompatible material using a tie layer has also been demonstrated.  An obvious application for those membranes is as proton exchange membrane material in fuel cells. [US Patents and 7,588,796, 7,635,062, 7,635,062] The tenability of ePTFE porous media has resulted in development of application technologies in medical devices, biological medium, filtration, electronics and numerous other end uses.

Close to half a century after the serendipitous discovery of expanded PTFE it continues to find new applications beyond comfortable and protective apparel [Source: Ebnesajjad, S., Introduction to Fluoropolymers, PDL, Elsevier, 2013].

As Leonardo da Vinci said, “Simplicity is the ultimate sophistication.”

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