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Breathability

Introduction

Breathability is the ability of a fabric to allow moisture vapour to pass through it. It is an essential contributing property to thermal and Physiological Comfort http://www.hse.gov.uk/temperature/thermal/factors.htmin clothing, and is vital in filtration and medical textiles.

Waterproof-breathable fabrics are a highly competitive market, where Gore-tex? reigns supreme, though several new fabrics have recently emerged that aim to usurp its position. There are numerous laboratory testing methods for determining breathability, and none correlate perfectly with human trial testing. This leads to problems in determining which fabric is ‘most breathable’, as the properties of each fabric will vary according to the testing method used.

Breathability is crucial in many areas of textiles, but particularly in clothing, as an impermeable wet garment will lead to garments becoming saturated by sweat. Wet garments conduct heat away from the body 25 times faster than dry garments do. In addition, wet skin feels uncomfortable: it has been shown that Physiological Comfort http://www.hse.gov.uk/temperature/thermal/factors.htmis directly related to skin wettedness.

Market Sectors

Breathability is of relevance to the medical and hygiene market sectors, where bandages and dressings must be comfortable; to the interiors market sector, where chair coverings must be comfortable; to technical apparel, where garments must be comfortable even when worn in difficult or challenging circumstances; and to the sports and leisure market, where breathability is one of the most important factors in determining performance. The sports and leisure market is the major driving force behind the development of new breathable fabrics. For example, mountaineers strive to use the most breathable fabrics possible so sweat does not build up inside the fabrics, potentially causing dangerous cooling and loss of insulation.

Terms and Definitions

Breathability does not, as the term might imply, relate to an exchange of air. Instead it is the ability of a fabric to allow moisture vapour to pass through it. A fabric may carry this out in numerous different ways, and exchange of air is only one possible avenue. Moisture vapour permeability (MVP) and moisture vapour transmission (MVT) are perhaps better, and certainly more technical, terms to use than breathability. Neither ASTM nor the Textile Institute define breathability, but MVP and MVT are both defined in test standards.

According to EN 31092:1993, water-vapour permeability is “a characteristic of a textile material or composite depending on water-vapour resistance and temperature. Water-vapour permeability is expressed in grams per square metre hour pascal.” According to BS 3546:2001, water-vapour permeability is the “ability of a coated fabric to transmit water vapour above a specified level whilst maintaining a high degree of water penetration resistance”.
The water-vapour permeability index is the ratio of thermal and water-vapour resistances, in accordance with the definition in standard EN 31092:1993.

Water vapour resistance, Ret, can be thought of as the ‘opposite’ to breathability and is, according to BS 3546:2001 the “water-vapour pressure difference between the two faces of a material divided by the resultant evaporative heat flux per unit area in the direction of the gradient.”

Air permeability is intrinsically linked to breathability. Air permeability is “the velocity of an air flow passing perpendicularly through a test specimen under specified conditions of test area, pressure drop, and time” (according to BS EN ISO 9237: 1995). All air permeable fabrics are breathable to some extent, though not all breathable fabrics are air permeable.

Regulations, Legislation and Standards

There are few, if any, regulations surrounding breathability. However, there are a great deal of different testing standards on the subject. Some of the major standards are listed below:

• The sweating guarded hot plate test: BS EN 31092:1994 and  ISO 11092: 1993
• The specification for water vapour permeable garments: BS 7209:1990
• Evaporative dish method testing standard: British Standard 7209.
• Similar to evaporative dish method is the Canadian control dish method (CAN2.42-M77, 1977).
• Determination of air permeability: BS 5636 (now superseded by BS EN ISO 9237: 1995)

 

Basic Principles

Moisture vapour will move by diffusion from an area of high concentration (pressure) to one of lower concentration. Unimpeded, this happens readily. However, barriers such as fabrics reduce the rate at which this occurs. It is crucial to remember that vapour pressure drives moisture vapour: humidity is not the essential factor.

Modern waterproof-breathable fabrics can be divided into three different types, and each work in slightly different ways. It is not entirely accurate to state that fabrics can be both breathable and waterproof because the holes in their structures are large enough to let water vapour through, but too small to let water droplets through.

Fabrics with microporous coatings or membranes  are prevalent in many garments. PTFE (polytetrafluoroethylene) and PU (polyurethane) tend to be used to make these coatings and membranes. They act as a filter: their microporous structures contain billions of holes that link together in contorted pathways. They stop water penetrating them by maintaining a very low Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy. However, if the membrane or coating becomes contaminated then they can leak, as their Surface Energyhttp://en.wikipedia.org/wiki/Surface_energy increases. Microporous membranes are usually made by mechanical fibrillation, whereas microporous PU coatings are usually produced by wet coagulation.

eVent (BHA Industries) is a well-regarded microporous PFTE membrane. Its structure is protected from contamination by lining the pores with hydrophobic and oleophobic chemicals. By doing this, eVent remains air permeable, which maximises its ability to transmit water vapour.

Fabrics with continuous Hydrophilic? coatings or membranes contain no pores, making them impermeable to air. They are usually made of PU and PEO (polyethylene oxide). Moisture vapour transport in these structures occurs by molecular wicking: the water molecules are first adsorbed to the surface of the Hydrophilic? material, then they desorb and adsorb to the next molecule along. This process continues throughout the thickness of the Hydrophilic?.

The breathability of Hydrophilic? materials tends to be slightly lower in lab tests than that of PTFE membranes. However, their breathability is strongly affected by temperature and they are developed to operate best at temperatures just above freezing, whereas many lab tests are conducted at skin temperature.

Modern Gore-tex? is a bicomponent microporous and Hydrophilic? laminates: holes of the microporous PTFE membrane are partly-filled with Hydrophilic? polyurethane. This leads to excellent durability, though the structure is impermeable to air, limiting the maximum possible breathability. Water vapour is transmitted through the structure in a way analogous to that of the Hydrophilic? fabrics.

General Links for Basic principles:

Waterproof Breathable Active Sports Wear FabricsThis document expands upon the information given here, explaining moisture transport and how this is achieved in textiles.

Video outlines how the eVent fabric technology works. 

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Laboratory Tests for Breathability

There are many different laboratory-based tests for breathability. Some are simple methods (cup methods), whereas some are extremely complex.
Cup methods are performed under steady-state conditions and so poorly represent the changing environments of real-life use. One of the most commonly-used methods is the Upright Cup Method, where an aluminium cup is filled with distilled water and covered with the test fabric. After a predetermined period of time the dish weighed and the amount of water that has evaporated is calculated. The inverted cup method is identical to the upright cup method except the apparatus is inverted, which removes the air-gap, thus providing a different testing situation. The Evaporative Dish Method (British Standard 7209) is similar to the upright cup method except a series of samples are prepared and rotated on a turntable, which provides an air current. It shows a good correlation with field trials. The desiccant inverted cup method uses a desiccant such as potassium acetate to draw water vapour through the waterproof-breathable fabric. The technique measures the mass increase that occurs in the cup due to water vapour uptake.

Complex methods of testing breathability continue to be developed, though most do not go on to form international standards. Companies may develop new methods of testing to suit their own fabrics. The DMPC (Dynamic Moisture Permeation Cell) was developed at America’s Natick Army testing laboratory. The test controls both humidity and temperature between flows of gas. Thermal manikins can also be used to determine water vapour permeability. A manikin such as ‘Walter’ is made from water and a ‘skin’ composed of the sample fabric. This sort of testing is exceptionally expensive, but provides results that can be obtained through few other methods, as the manikin not only sweats, but may be able to move as well. Another recent development in testing breathability is the Human-Clothing-Environment (HCE) Simulator, which can be seen as a combination of the sweating manikin model and the, somewhat simpler, sweating skin model. The HCE Simulator separates a cold chamber from a ‘sweat’-distributing hot chamber using a sample fabric. A well-known industry method is the sweating guarded hotplate test, which can perform under dynamic and steady-state conditions. It forms both ISO 11092 and EN 31092. In the test, the sample is held over a pool of water on a heated plate and the amount of energy required to keep the plate at the same temperature, despite evaporating water, is measured. The test takes place in a controlled environmental chamber.


 

Challenges and Potential Innovation

Greater breathability is continually demanded by consumers, and they want their fabric’s breathability to be maintained despite long term use. Gore-tex?’s latest product, Active Shell,  is their most breathable fabric yet and has achieved this by being thinner, thus reducing the resistance to water vapour transmission. However, durability will almost inevitably suffer.

Polartec, a new competitor in the waterproof-breathable fabrics market, have used a new technique – electrospinning – to produce their Neoshell membrane. Making breathable membranes from electrospinning leaves great room for innovation as there are an exceptional number of parameters that can be changed in this technique to produce new materials.

Developing a laboratory test for breathability that correlates well with field testing is of continuous concern: if a cheap, simple and reliable technique could be developed that correlated perfectly with user trials then this would be a significant advancement in the field of breathability appraisal.

A great challenge for the sector is recyclability and reducing environmental impact. PTFE membranes are not Biodegradable? and numerous toxic chemicals are used in their energy-intensive manufacture. A Gore-tex? jacket cannot be recycled and does not degrade over time. Conversely, Sympatex, a product more prevalent in mainland Europe than in the UK or USA, is made using a polyester membrane. If this membrane is bonded to a polyester face fabric then the whole garment can be recycled.

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