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Fundamentals of breathability
Testing a material's breathability by using a sweating guarded hotplate device helps in developing successful outdoor clothing.
Manufacturers of outdoor garments often express the overall comfort attributes of their products by making performance claims about 'breathability'. What they usually mean by this is that the material of the garment allows water vapour, generated as sweat during physical activity, to pass through the material keeping the wearer dry. A material which prevents the passage of water vapour therefore is often said to be 'non-breathable'.
One of the effects of a non-breathable material is that water vapour will build up inside the garment and ultimately condense on the inside surface and on the wearer's inner clothing, causing discomfort. By contrast, a 'breathable' garment is expected to limit the build-up of moisture and can help to maintain a higher level of comfort. The amount of water vapour generated by a human body depends on several factors such as the individual's physical characteristics, ambient temperature and humidity, and the level of physical activity.
Uncoated woven and knitted garments are often inherently breathable but this will change when a coating or membrane is incorporated. Coatings or membranes are usually used to improve properties such as resistance to wind and water penetration but can have a negative effect on breathability. Indeed, some of these composite materials are virtually non-breathable.
The problem faced by manufacturers of outdoor clothing is to incorporate a coating or membrane which enhances the wind or rain resistance, without significantly reducing the breathability of the material. The properties of the coating or membrane are therefore critical. Fortunately, there are now many types of membranes and coating materials available which allow the transmission of water vapour. They fall into two main groups, namely hydrophilic and microporous materials.
These work by absorbing water vapour molecules on one surface, allowing them to pass through the material before finally releasing them from the opposite surface of the material.
These contain huge numbers of tiny holes, or pores, which are designed to be big enough to allow passage of sweat in the form of water vapour molecules, but too small to allow the ingress of liquid water.
There is one further requirement which must be met before water vapour will transfer from one side of a material to the other. In technical terms there must be a 'humidity gradient' – a difference in the concentration of water vapour between the inside of the garment and the outside environment. If there is a higher level of water vapour inside the garment than outside and the garment materials are 'breathable', then water vapour will transfer from the inside of the garment to the outside.
Measurement of breathability
Many test methods exist to measure the transmission of water vapour through a material. All of them rely on creating a humidity gradient in the test system between the two sides of the material. There are two main types of test. The first type measures the loss or gain in weight over a period of time of a vessel or container sealed with the test material (for example, BS 7209, SATRA TMs 23, 47, and 172, IUP 15). The vessel will contain either water or a desiccant such as silica gel.
The second type of test measures the latent heat of vaporisation as liquid water evaporates from the surface of the test material (for example, EN 31092/ISO 11092 or ASTM F1868-02). It needs to be remembered, however, that some of these different methods do not correlate easily with each other. For this reason certain methods have come to be preferred. This is not to say that other methods are of no value. However, they may be limited to use in quality control within a manufacturing environment.
Sweating guarded hotplate
In Europe, EN 31092 (equivalent to ISO 11092) has become the method favoured for measuring the water vapour transmission of protective clothing and it uses a 'sweating guarded hotplate' device. Sometimes known as the 'skin model', EN 31092 measures the electrical power required to maintain a heated porous metal plate at a constant temperature. A water supply is fed to the underside of the plate and the test specimen is mounted on top of a reference permeable membrane placed on top of the plate. As water vapour evaporates from the plate and passes through the material, the plate cools owing to the loss of the latent heat of vaporisation of water.
The control system maintains the plate at a constant 35°C by means of electrical heating. The surrounding environment is also maintained at 35°C. These iso-thermal conditions ensure that any heat losses from the plate are due to evaporation of water and not to convective or other heat losses. The relative humidity of the surrounding environment is maintained at 40 per cent whilst the atmosphere immediately above the plate is near saturation (100 per cent), thus providing the moisture vapour gradient.
Results for this test are expressed as m2.Pa.W-1 where the Pa factor represents the water vapour gradient expressed as a difference in water vapour pressure, m2 is the area of the porous plate and W is the steady state power in watts required to keep the porous plate at the required temperature. The result of the bare plate test (with only the reference membrane in place) is subtracted from all results to obtain the final value.
Conversely, the water vapour transmission of protective gloves according to EN 420 is measured using the Nice-Mitton (IULTCS) test (equivalent to the SATRA TM172 test method). In this test, a sample of the test material is sealed across the top of a small bottle. The area of the test material across the bottle is recorded in cm2 for use when calculating the final result. Inside the bottle is a quantity of dried silica gel. This produces a dry atmosphere with a relative humidity (RH) close to 0 per cent. The mass of the bottle is recorded and then the bottle is placed in a rotating frame in front of a fan. This is to remove the layer of still air from the surface of the test specimens, ensuring that the water vapour permeability (wvp) recorded is that of the test specimen, not the still air layer.
The test takes place in a laboratory with a controlled temperature and humidity (23°C/50 per cent RH). In this case the gradient across the test sample is created by the dry silica gel inside the sealed bottle and the 50 per cent RH external atmosphere. After a set period of time (several hours) the bottle is re-weighed. The initial weight is subtracted from the final weight. The difference (in grams) represents the amount of water vapour which transferred through the material to be absorbed by the silica gel. The wvp is expressed as g.cm2.h-1.
These procedures are often used as part of an overall package of tests for the certification of personal protective equipment (PPE). Manufacturers of non-PPE products can also use these tests to provide information about the breathability of their products to the consumer.
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