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Coatings for personal protection.

Coatings not only protect buildings and other infrastructure, they play an important role in some unexpected aspects of daily life. For example, when applied to fabrics used for the manufacture of outerwear and clothing designed to be worn in harsh environmental conditions, waterproof and breathable textile coatings also help keep people warm and dry. The most famous of these coatings is Gore-Tee (from W.L. Gore & Associates), based on expanded polytetra-fluoroethylene (ePTFE), which was developed over 40 years ago. Newer systems based on polyurethanes that incorporate ceramic particles and foaming agents, as well as others that involve control of surface charges and the hydrophobicity/hydrophilicity of the coatings, have been introduced more recently.

An effective waterproof fabric provides much more than water repellency. Although it must repel water (cause water to "bead" on the fabric surface), the fabric also must resist water penetration and provide breathability, i.e., allow moisture vapor transmission. Membranes or coatings on the underside and durable water-repellent (DWR) finishes on the exterior of fabrics are used to achieve this performance. Membranes, such as ePTFE, as well as polyurethane and polyester films, form laminates when bonded to the undersides of fabrics. DWRs are designed to cause water beading without inhibiting breathability.

The industry benchmark for water repellency is retention of 80% repellency after 20 home launderings, according to Adam Clark, vice president of coatings manufacturer ASF Group. When water beads, it can easily roll off the fabric surface, which prevents any penetration into the fabric and also allows water vapor to escape. "Commercial coatings that provide durable water repellency typically last for a long time--with today's technologies, even up to 100 home launderings--but their performance is definitely enhanced with proper care, including using the correct washing, rinsing, drying, and ironing conditions," he says. In fact, water repellency is restored through proper washing and drying, and thus garments that have not been washed for an extended period of time exhibit reduced performance.

Water resistance refers to the ability to resist water penetration into the fabric, and is measured using two different tests, both of which apply water to the fabric under pressure. In the low hydraulic pressure test (JIS L 1092 A/ISO 811), or water column test, water is applied at increasing pressures up to 2,000 mm (-3 psi) at a rate of 600 mm/min, and simulates the pressure applied by a six-foot-high column of water. In the high hydraulic pressure test (JIS I 1092 B), water resistance is measured at pressures ranging from 2,000 mm up to 30,000 mm at a rate of 10,000 mm/min. Each test is completed when the fabric begins to leak.

Water repellency and resistance are not sufficient for an effective waterproof garment if sweat remains trapped against the body, leaving the wearer clammy. Breathability is just as important, and refers to the ability to transport water vapor (from perspiration) through the coating into the atmosphere. Various tests are used to demonstrate breathability for given applications. "Because the conditions under which garments are used vary so widely, it has been found that there is no one global test method that can predict the breathability of different garments under different end-use conditions," Clark explains.

There are three generally accepted tests--the upright CLIP test (JIS L 1099 Al). the inverted cup test (JIS L 1099 B1), and the sweating hot plate test (ISO 110-2). The upright cup test is used to test microporous hydrophobic coatings and laminates and measures water or perspiration vapor transmission. The inverted cup test is appropriate for monolithic hydrophilic films or PTFE films and is designed to measure the amount of water or perspiration that is absorbed, because hydrophilic nonporous films "breathe" by absorbing water or perspiration. The sweating hot plate test, also known as the "skin model," simulates the heat and mass transfer that occur next to human skin and measures both thermal and water vapor resistance in order to determine heat loss through moisture evaporation.

Finally, for a waterproof coating on a garment composed of separate pieces to be truly effective, the fabric must be seam-sealed using high performance tape and state-of-the-art seam-sealing techniques. For example, to achieve good adhesion with microporous waterproof fabrics, the glue must penetrate and saturate the fabric.

The membranes used in laminates are microporous/ mesoporous materials with pores that are small enough to prevent typical droplets of water but large enough for water vapor molecules to pass through. PTFE. in addition, has a very low surface energy and can only be wetted by liquids with a similar low surface energy, which excludes water. The water prefers to form spherical droplets and thus slides off. The performance of such membranes can be affected if the surface energy is modified due to the presence of contaminants, such as dirt, sunscreen, insect repellent, body oils, etc. To address this issue, ePTFE is often covered with a protective film of some type, such as a polyurethane (PU), to form a bicomponent laminate. These PU films are typically modified with hydrophilic functional groups or combined with a hydrophilic polymer, such as polyethylene oxide (PEO), so that they can adsorb sweat molecules, which then diffuse through the film and ultimately escape through the micropores in the ePTFE. One downside of the use of bicomponent films is the slower rate of water removal from within the garment, which can result in a sensation of dampness for the wearer. Alternatively, the PTFE can be fabricated such that the individual ePTFE filaments are coated with an oleophobic coating without affecting the pores. Such an approach allows for more rapid moisture vapor transport.

Microporous PU films can also be used directly in laminates themselves, as can polyester films. While ePTFE remains the most widely used film for waterproof and breathable fabrics, the technology for the production of PU films is rapidly advancing. PU films are attractive because they do not become as stiff as ePTFE at lower temperatures and they tend to be stretchier than ePTFE.

Polyester films have also been explored. They can be lower in cost than the ePTFE films. In one example, Sympatex offers a pore-free film with hydrophobic polyester and hydrophilic polyether components that transmits water vapor via an adsorption-diffusion-desorption process similar to that seen in PU films. Although the performance of polyester films tends to be slightly poorer than that of PU films, they have the advantage of being recyclable.

PU coatings, rather than bonded films, can also be used to produce waterproof, breathable fabrics. Microporous coatings behave similarly to the microporous films found in laminates. The pores can be formed by adding either a foaming agent or solid particles to the formulation. When released, the foaming agent expands within the coating to create the microporous network. As the coating dries, the solid particles lead to the formation of tiny cracks and fissures that ultimately result in a micro-porous network.

Coatings are the least expensive option for waterproof and breathable garments and tend to provide lighter garments that can be packed into a smaller space. They tend, however, to be less durable than laminates, and thus are generally not appropriate for use by outdoor athletes involved in extensive, high energy, and high impact activities. They are ideal for travelers and those involved in casual sporting activities.

ASF Group offers a range of waterproof. breathable polyurethane coatings under the trade name Eclipse Microporous Coating. -Our most recent generation of microporous coatings has a larger number of much smaller pores, which provides increased breathability and waterproof performance. In addition, it is a thinner coating, which results in a softer, more comfortable feel," Clark says. Eclipse Ceramic contains ceramic particles that produce extremely fine pores. The company has also developed a microporous, bicomponent system that incorporates both hydrophobic components to transport vapor and hydrophilic components to transport condensed perspiration.

In addition to coatings, AFS Group offers membranes for the production of laminates. For example, its hydrophilic laminates consist of stretchy monolithic barrier membranes with no micropores. The PU resin is modified with hydrophilic amino acids that result in a charged surface on the membrane, which enables the absorption and transport of moisture and moisture vapor. "The advantage of a monolithic barrier is that the membrane has no holes that can be clogged with dirt, detergent, perspiration, or other contaminants," Clark notes. Another option is a moisture-permeable and waterproof fabric based on microfiber technology.

Currently one of the main drivers of technology development in the waterproof and breathable textile market, and thus the coatings and membranes/films used to produce them, is the increasing consumer demand for improved environmental performance. "Customers' awareness of environmental issues is growing, and as it does, consumers are demanding apparel and other products that are more eco-friendly," observes Clark. Manufacturers are responding by seeking more environmentally friendly raw materials and improving their manufacturing processes.

"ASF Group has made a lot of progress in increasing the efficiency of our production processes, with reductions in waste and emissions. We are also exploring the use of more natural ingredients wherever possible," Clark says. He adds that the group's polyurethanes are chemical products, but the company is looking at numerous ways to make the environmental footprint of its products and processes as small as possible. One option that the company has investigated is the use of biobased polyols, but currently they carry a premium that is higher than what most consumer brand manufacturers are willing to accept. "We will continue to investigate all options for enhancing their performance and increasing the eco-friendliness of our waterproof/ breathable textile coatings and films/ laminates, and do so at a reasonable cost," Clark concludes.

"Coatings are the least expensive option for waterproof and breathable garments and tend to provide lighter garments that can be packed into a smaller space."
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Publication:JCT CoatingsTech
Date:Oct 1, 2014
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