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Warm outer layers for cold days

"There's no such thing as bad weather - just the wrong clothing". Every year, sudden changes in the weather and icy temperatures make this truism topical once again. But what functions does clothing have to fulfil and how do modern materials accomplish that? Dr. Andreas Schmidt, Head of the "Function and Care" department at the Hohenstein Institute in Bönnigheim, studies and improves the wearing comfort of clothing and knows the answers to these questions:

The thermal insulation of garments is assessed using the thermal articulated manikin "Charlie".

® Hohenstein Institute

What are the main functions of clothing?

Clothing helps people to survive in the ambient conditions. That is to say, on the one hand, it has to keep us warm, and, on the other, it has to allow sweat to evaporate so that the body is kept sufficiently cool when necessary.

Why do we wear clothing instead of having fur?

The loss of our fur was a milestone in the history of our becoming human. Almost all mammals regulate their body temperature by respiration, but this greatly restricts the scope for heat dissipation. Early man, on the other hand, used his entire body for heat dissipation, making him superior to most other animals in terms of stamina and adaptability. However, the ability to sweat is only really effective if there is no fur to hinder air circulation. That is why in the course of evolution man largely lost his body hair.

Consequently, early man was only able to settle in colder regions of the world by inventing protective clothing. Nevertheless, even in climatic regions where protection for the body is not actually necessary, in the course of our cultural development typical types of clothing have still come to be worn for ethical and religious reasons.

Why does our body need to be protected from the cold?

Like all mammals, humans are warm-blooded, and their temperature (37°C) has to be kept constant, within a very narrow range, in their body core, i.e. in their head and trunk. Even a slight variation in the core temperature, an increase or decrease of just 2°C, can cause important functions within the body to fail.

The activity of the organs and muscles is constantly producing varying amounts of heat in the body, and this "output" can be expressed in watts. To maintain a constant temperature in the body core, the production and emission of heat by the body must be kept in balance. This requires complex regulatory mechanisms. For example, heat is dissipated from the body very effectively by the evaporation of sweat from the skin. In cold conditions, the body restricts the circulation of blood to the hands and feet and so reduces heat loss. The body can produce heat when it is cold by shivering. Because of the large surface area of the skin, humans can dissipate more body heat through the skin than they can by, for example, exhaling warm air.

Humans are able to adapt to different temperatures. This makes the surface of their bodies more tolerant of temperature changes. Variations can be tolerated the least in the trunk, where the vital organs are located. Our hands and feet, however, can put up with downward temperature changes of 10°C or more.

How does clothing keep us warm?

It's not the textile material that clothes are made of that keeps us warm but the air that is trapped by clothing: the purpose of clothing is to create a layer of air around the body which serves as an insulating layer between the body and the ambient conditions. Just like in a Thermos flask, the heat generated by the body is retained by the cushion of air in the clothing around the body. All fibres, regardless of whether they are woollen, silk or chemical fibres, conduct heat at least ten times better than air.

This means that what determines how warm a garment feels to us is its ability to trap air between the fibres and stop it being exchanged with the surrounding air. In nature, the fur on mammals and the feathers on birds work according to the same principle.

A garment not only has to offer good thermal insulation but also, depending on the area in which it is to be used, has to be windproof, so that the insulating cushion of air is not disturbed. Furthermore, the garment manufacture, i.e. the cut and workmanship, also plays an important role: for example, elasticated cuffs prevent too much exchange of air taking place as the body moves, thereby increasing the warming effect of the clothing. On the other hand, when the body is under strain openings for ventilation (which can be closed off), for example under the armpits, can transport excess thermal energy away from the body through the exchange of air with the surroundings.

And what happens when we start to sweat?

Physical activity increases heat production by the body. To make sure we don't overheat as a result, we start to sweat, even, for example, when skiing in freezing temperatures. Excess heat is removed from the body by the evaporation of sweat on the skin. However, for this to happen, the moisture has to be wicked away from the body. That is why some ski suits have ventilation slits under the armpits which the wearer can open if required. Modern membrane materials also allow the evaporated sweat to escape outside while still offering effective protection against the rain and wind.

If the sweat cannot be transported away from the body and dissipated into the surrounding air, it accumulates in the layers of clothing next to the skin. This is not only uncomfortable but, when the level of activity drops and so less heat is produced, it can even pose a risk to health. Since water is an excellent conductor of heat, wet clothing clinging to the body can cause people to lose a lot of heat. Together with the loss of energy through evaporation, this can have a serious chilling effect. We can observe the same effect in summer when the parts of the body that are covered by a wet swimsuit become uncomfortably cold.

In what ways is modern winter clothing different from that of 50 years ago?

Right up until the 1960s, clothes were made almost exclusively of natural materials like wool, cotton, linen, leather or fur. Admittedly, the first synthetic textile fibres had already been developed, with `Nylon´ invented by Dr. Wallace Hume Carothers in the USA in 1935 and `Perlon´ by Dr. Paul Schlack in Berlin in 1938. But chemical fibres really only made their big breakthrough when scientists learned how to deliberately influence specific desirable properties in them.

Now, by adjusting the fineness and therefore the stiffness of the fibres, and by using special processing techniques, the volume of air that is trapped by the textile, and consequently its thermal insulation, can be controlled and maximised. Jackets and sleeping bags filled with fleece materials made of hollow-core fibres with excellent lofting properties can provide a level of thermal insulation close to that of down filling. Since the hollow filaments are relatively stiff, they cannot be easily crushed and so they retain their warming cushion of air even under pressure.

When it comes to protection from wind and rain, membrane systems have become well-established since they were first introduced at the end of the 1970s. The membranes can be made of different high-tech materials: for example, the pores of porous polytetrafluorethylene are smaller than the smallest drops of water and so do not allow any rain to penetrate. But they are larger than a single molecule of water vapour, so they allow sweat in the form of vapour to evaporate to the outside. Membranes can also be made of special polyester or polyurethane which similarly do not let water droplets in but let sweat vapour out and are effective at keeping out the wind. A traditional oilskin jacket made of PVC- or polyurethane- (PU-)coated cotton fabric does offer good protection from a downpour or icy winds – but its breathability is practically nil, which is why after a short time the wearer becomes soaked in his own sweat and feels uncomfortably chilled.

In 1980, the Austrian ladies' team at the Winter Olympics in Lake Placid was equipped with the world's first ever two-layer underwear which had been developed in partnership with scientists at the Hohenstein Institute in Bönnigheim. Ever since, modern functional textiles have offered both professional and amateur sportsmen and -women clear advantages compared with traditional cotton underwear when it comes to heat and moisture management: the synthetic fibres of the "double-face material" lie next to the skin and conduct perspiration quickly and efficiently away from the body and into the outer cotton layer. In combination, the two materials are far more comfortable to wear than cotton underwear, because of the drier feeling on the skin.

Developments in this field are far from at an end. At the Hohenstein Institute, too, researchers are constantly testing new combinations and modifications of materials to see what the benefits are in terms of wearing comfort.

How can I judge how comfortable clothing will be to wear when I'm still in the shop?

Even for an expert, it's hard to assess the wearing comfort of a garment just by looking at it. The manufacturers' claims are often expressed in very flowery language, but can be hard to compare. So anyone wanting to know which ski suit offers good thermal insulation but won't leave you sweating buckets during the aprés-ski, or which sports underwear will best absorb perspiration without sticking uncomfortably to their skin, needs to rely on an objective, manufacturer-independent assessment. This is available in the form of the wearing comfort mark, awarded by the Hohenstein Institute on the basis of a whole series of measurements. The wearing comfort mark, generally shown on the product together with the Hohenstein quality label, ranges from 1 for "very good" to 6 for "unsatisfactory". It covers not only the thermophysiological properties of a textile material, such as its thermal insulation, breathability and moisture management, but also the skin sensory aspects of wearing comfort, that is to say, whether the textiles feel pleasantly soft and supple or, by contrast, are uncomfortably scratchy and prone to sticking to skin that is wet with perspiration. The Hohenstein scientists have developed objective methods of measuring all these textile properties and the results are used to calculate the wearing comfort mark.

Features that are optional on clothing for everyday use are already compulsory on cold protective clothing for professional use (e.g. in cold storage warehouses): for such clothing, manufacturers have to have the thermal insulation tested and declare the result on the garment. Then the user can determine, from a table that is included in the relevant standard, for how long the clothing can be worn for any given intensity of work or ambient conditions.

What is the ideal outfit for cold weather like?

There won't be an all-round outfit that you can wear in any temperature any time soon. However, the aim of clothing physiology research is to work out which clothing is suitable for which purpose and area of use, and provide the wearer with information accordingly. You can already see the results of this work in the case of sleeping bags: a standardised process is used to calculate the temperature range in which a product can be used without the user feeling uncomfortable or having any reason to fear damaging their health, and this is shown on the product. For bedding, too, a system developed by the Hohenstein Institute enables consumers to use a chart to identify the best bedding for them, depending on the ambient temperature and the body weight of the sleeper.

With clothing, unlike with sleeping bags or bedding, the level of activity and the associated varying rates of heat production by the body have to be taken into account. Here it is still useful to apply the "onionskin principle" in cold weather, i.e. to wear several layers of clothing on top of one another which can be removed as necessary. When choosing them, though, you should definitely remember the issues of heat and moisture transport described above and coordinate the different garments to achieve the best possible heat and moisture management. After all, with the right clothing, there's no such thing as bad weather.

Weitere Informationen:

Andrea Höra | Hohenstein Institute

Further reports about: Hohenstein activity clothing cotton evaporation heat materials moisture skin sleeping bags sweat temperature textile

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