part 1   part 2    ;part 3

But why is hygroscopicity so important for insulation?

To begin with, let's find out what hygroscopicity is. By the most common definition, hygroscopicity is the ability of materials to absorb (absorb) moisture on their surface (convection water flows, water vapor, sweat) and transfer it to the environment or other materials. Simply put, the hygroscopicity of insulation is the ability of the material to absorb moisture in a vaporous state.

Thermal insulation materials both during storage and during operation must be protected from moisture. The ability of the material to moisten due to its hygroscopicity is called sorption. The wetter the air and the lower its temperature, the higher the sorption.

Now the role of insulation with zero hygroscopicity for clothing operated in special climatic regions with a "deep minus" becomes even more obvious and relevant. Two related physico-chemical processes – hygroscopicity and sorption – divide the insulation into effective and inefficient. And here's why.

The fact is that the heat exchange of hygroscopic and sorption fibers occurs much faster. In practice, this means that a person in clothes with insulation made of such fibers will give off their heat, and therefore freeze much faster.

So, moisture conducts heat – this is an axiom. But why has the principle of selecting exclusively non-hygroscopic insulation not yet become an axiom? A new generation of synthetic materials with zero hygroscopicity and sorption will protect a person from freezing, since heat exchange is actually converted into heat conservation. And this is already one of the main indicators of thermal protection, which is characterized by the ability of clothing as a whole to retain the heat released by the human body. We are talking about "dry heat", relatively speaking, the moisture from which is not removed into the insulation itself, but bypassing it, leaving the insulation dry!

Why is this happening? Everything is extremely simple: hollow 100% polyester and other chemical fibers of nonwovens do not absorb moisture (from the point of view of physics, such a fiber is an elementary non-absorbent surface). Their hygroscopicity index is less than 1%!

The amount of moisture absorbed depends on the nature of the fibers (synthetic, vegetable or animal origin). Thus, at a temperature of 20 °C and a relative humidity of 65%, the hygroscopicity of clothing made of cotton fabrics is 12-18%, linen 12, wool 17, silk 11, viscose 12, nylon 3, acetatendash; 7, triacetate – 4.5%. Merino wool can absorb and temporarily retain up to 30% of its weight. Just think about it – humidification of fiber insulation by 1% leads to a loss of thermal protection up to 10–30%!

One more detail: a significant decrease in strength when swelling in water (low strength in the wet state) is a significant disadvantage of hydrate cellulose (natural) fibers. As a result, the strength of the products decreases when washing or when used in a wet state. But synthetic hydrophobic fibers do not stretch when moistened. In addition, synthetic fibers that do not absorb moisture have high fatigue strength, relaxation processes occur faster. Hence the higher resistance to repeated deformations, the insulation holds its shape better.

The abrasion resistance of the same fibers is highly dependent on humidity. For example, for viscose fiber, the abrasion resistance in the wet state is 20 to 30 times lower than in the dry state. On the other hand, synthetic fibers (for example, high-tech insulation, this property is also enhanced by thermal bonding!) the resistance to abrasion in dry and wet conditions is the same. That is, even if such fibers are still moistened, it will happen with the preservation of abrasion properties, which is extremely important for the functional set of any insulated clothing with the use of such material.

Synthetic insulation materials with higher hygroscopicity (for example, popular foreign brands with microfibers) inevitably "provoke" the insulating layer to evaporation and condensation, as well as the adhesion of fibers and loss of volume, since moisture "binds" inside and between the fibers. Microfibres in the insulation retain moisture both due to their hygroscopicity and due to the huge number of additional micro-surfaces, which for convection moisture become nothing more than an additional barrier.

According to the expert of the Research Institute of Nonwovens G. K. Mukhamedzhanov, with the same surface density, the total thermal resistance of thermally bonded insulation and foreign microfiber insulation is almost the same, but the air permeability is much higher.

The basis of the wool fiber is keratin protein, which makes up 70% of its weight. The high content of active groups interacting with water (NH2, OH) in keratin causes high hygroscopicity of wool fiber. It has the highest moisture absorption among all fibers. Under the influence of water, the wool swells intensively. And after drying, it returns its original volume. This property of natural fibers is inherent in nature itself.

Synthetic fibers do not have this property. They can't swell. Moisture is retained only on the surface (shell) of the fiber. Non-woven thermally bonded materials made of 100% polyester fiber have the least hygroscopicity, sorption and capillary effect.

Hygroscopicity of the insulation is also the property of the material to change the moisture content not only depending on the humidity and ambient temperature, but also on the temperature and humidity of the body during physical exertion at subzero temperatures. This is an extremely important feature, because if the fibers contain a certain amount of moisture, then with an increase in air humidity or an increase in its temperature, the humidity of the fibers increases even more, and vice versa. If the fiber has such a property, then it is hygroscopic. And if it is hygroscopic, then it affects the heat-protective properties of the insulation as a whole. Hygroscopic fibers are conductors of cold to the human body and conductors of the most valuable heat into the atmosphere.

Hygroscopicity is also important for maintaining thermal equilibrium in the system "man"clothes". Under the influence of moisture, heat is quickly removed from a person into the environment. Accordingly, the heat-saving characteristics are reduced. This is due to the fact that the coefficient of thermal conductivity of water is ten times higher than air. 

To be continued

Author: Vladislav Ivanov,

& nbsp;         Director of Plant Development  Thermopol», (TM "Holofiber").