|Therman Insulation - Q&A
by Aristides Repoulias
Department of Textiles
T.E.I. of Piraeus
|Let's begin with something already known, but still very fundamental: why do we feel cold and in which way our clothes prevent us from it?|
Our body produces heat in order to keep us warm. According to a principle of physics, heat moves always from a hot body to a colder one, and never inversely (if there is no intervention from an external factor e.g. an air-conditioner). So, when we are in a cold environment, heat moves from our body to the outside. As a result, we have a body temperature fall and we feel cold. This phenomenon appears more intensively when we are sweating, which means that heat is also spent to evaporate water. This problem one tries to solve with clothes, which aim to prevent heat movement to the outside, to a degree that we don't feel either cold or hot (this would happen if there was no exchange of heat).
What makes polyester so much preferavle to other fibers as for winter clothing?
We will better understand the properties of the fibers we are about to examine, if we learn two fundamental notions:
1. a) Regain of a fiber sample, which is equal to the quotient of the water mass to its dry mass.
2. b) The heat which is produced from a textile material during moisture absorption is called Heat of Absorption (or Differential Heat of Sorption). This is a positive characteristic of textiles, as they can make us feel warm. Unfortunately, when we have satiation of moisture in a textile material (it is absolutely soaked) then the heat of absorption decreases rapidly, until it becomes zero.
Polyester presents very low Regain. For relative humidity 60%, it has a Regain of only 0.5%. This means that it absorbs very little water from the air, and as a result it is very easy to dry. At the same time it doesn't absorb heat from our body to evaporate water, so we don't feel cold. Even if snow is stuck on this textile, or if it has absorbed a high amount of water, it is easy to remove both by simply shaking the clothe. This is due to its low absorption!
The negative characteristic of polyester - at least in comparison to wool - is that it has a high Heat of Absorption
It is very light, in relation to the thermal insulation it provides. The fact is also very important that, in order to create the velour or bulky form of Fleece or Pile, the use of synthetic fibers is recommended, because of their big length and their resistance to attraction. Polyester is the best material for this, according to all its properties.
Moreover, its ability to dye with bright colors is very positive. Apart from the "warmth" of these light colors (psychology), one can also be discernible in a "white" landscape (safety).
Finally, the water repellence refinement, which is nothing more than chemical refinement, although not permanent, it increase efficiently the water repellence, yet maintaining breathability.
I disagree! In out grandparents' years, and leter, woolen clothes used to constitute the best protection agaist cold. Why not today?
First of all, wool has a very high Heat of Absorption. To have an idea, we can imagine that when someone moves from a room with a temperature 18░C and relative humidity 45%, to the outside with a temperature 5░C and relative humidity 95%, a woolen blouse of 1 kg. produces heat of 100,000cal (100 kcal.), which equals the heat produced in an hour by human metabolism.
In relation to polyester, for a relative humidity 60% wool has a Heat of Absorption 13.5 kJoule/gr, polyester only 0.5 kJoule/gr. Moreover for a relative humidity 80% wool has a Heat of Absorption 13.5 kJoule/gr, polyester only 1 kJoule/gr. It seems wool provides more warmth than polyester, even when it is wet.
Apart from this, wool also has the unique property to seem dry, although in reality it contains water in a proportion 15-18% of its weight.
Its resistance to flexibility makes it differ from other fibers. Wool fibers can flex 20,000 times without breaking, although cotton fibers can stand only 3,000 times.
These were the reasons why wool fibers are used from old times to offer thermal insulation. This selection was made from experience and not scientific knowledge! So it would be wrong to say that we shouldn't use wool, as it has not only aesthetic but also unique properties.
However two great disadvantages tend to keep them off from mountaineering.
1. They take a long time to dry, because of the fiber construction and the enormous quantity of water it keeps when it becomes soaked. Furthermore, it increases vertically in weight.
2. They are considered to be heavy, even when dry, in relation to a polyester one. So we don't prefer to carry it into our backpack!
And what is going on with the other materials?
Cotton and silk have a low Heat of Absorption (for a relative humidity 80% they provide 10-13 kJoule/gr). The only advantages of cotton is the increase in resistance up to 30% when it is wet, and also good breathability. On the other hand it takes a long time to dry.
Nylon is rejected, as it doesn't breath enough to avoid soaking, although it has the lowest water absorption after polyester (4%). Finally silk is a very good thermal insulator, but it decreases its resistance to friction up to 30% when it is wet. Furthermore, it is very expensive.
I found a very cheap blouse made of polyester. What is the difference with a famous one as they are made of the same material?
At first the difference lies in the raw material used, as polyesters are quite numerous. This means that the cheap blouse may probably have a great Regain, or other characteristics we don't want.
Moreover an expensive mountaineering blouse has sustained special frefinement of water repellence, wind resistance, resistance to pilling etc. Although they are not permanent, they are important.
Of course quality of dying and sewing have a part in the configuration of the final price.
Finally the obvious ergonomy of a blouse (pockets and their position, buttons, cords, zippers or press fasteners, collar, reinforcements to elbows etc.) surely affects the price.
Apart from all those, the "brand name" is a quality guarantee and it always costs a lotů
Let's make clear some notions: fleece, pile, polyester.
Fleece: Fabrics with special knitting and brushing processes, which provide them with velour and warm feeling. Their first appearance was in the 70s.
Pile: Fabrics made from knitted fibers with a special refinement to look pilled, be warm and bulky.
Polyester: Organic compounds category. In our case they are used for fiber-processing.
1. Textiles, G. Galanopoulos.
2. Textiles Physics, D. Kaminara.
3. World Sports Activewear, Volume 3, No. 3, Autumn 1997.