Surely many have heard this word more than once and may even have wondered how it is written and what it means. But not everyone is aware that knowledge of this physical property of materials can be useful in everyday life. That is why we will get to know him better.
Definition
Hygroscopicity is the property of any material to absorb and retain moisture from the air. The letter “g” in the first part of the word may confuse some, because we all know that complex terms related to water usually begin with the prefix “hydro”. But here we are talking a little about something else. Hygroscopicity takes into account the absorption by materials of only that water that is sprayed in the air in the form of steam, which means that the prefix is needed completely different. “Hygro” means the word refers to moisture. Everything is simple.
We've figured out the definition, and now it's time to find out what the word really means. The air around us has a certain humidity - this is even indicated in the weather forecast. Some fibers are able to absorb this water, often changing their properties. Thanks to hygroscopicity, clothes and shoes can get wet even without rain. In what cases it is good, and in which it is bad, we learn below.
What materials are hygroscopic?
This article will focus mainly on tissues. But not only they know how to absorb moisture from the air. The hygroscopicity index of a particular material is often necessary for builders, furniture makers, manufacturers of sophisticated equipment, and many others.
For example, we all know that wood has a porous structure, this increases its hygroscopic properties. Water, penetrating the structure of a tree, deforms it. That is why wood furniture is practically not installed in rooms with high humidity. Special impregnations can be used to reduce hygroscopicity.
No less important are the hygroscopic properties of the insulation used in construction. The air in the pores of the material retains heat in the room. But if the insulation gets wet, it will instantly lose its basic properties. Therefore, the materials used for these purposes should have minimal hygroscopicity. The ideal rate is 0%.
Hygienic properties of the fabric
All materials have different physical indicators, such as density, strength, etc. But for fabrics that subsequently need to turn into wardrobe items, other properties are important - hygienic. They determine how comfortable clothes from one or another material will be.
- Breathability . The name speaks for itself. Fabrics with a high air permeability index are able to “breathe”, and with a low one they protect from wind.
- Vapor permeability . The ability of a tissue to pass moisture in order to divert sweat and other fluids from the body.
- Water resistance . Protects the body from fluids. This property of the fabric is increased by various impregnations and polymer coatings.
- Dust extraction . This property allows the tissue to hold small particles on its surface. The more loose the material, the higher the dust removal rate.
- Electrification is the ability of a tissue to accumulate static electricity.
Do not forget about the heat-shielding properties of the fabric. This is the ability to maintain normal body temperature while it is cold outside. And let's talk about the last property in more detail.
Hygroscopicity of tissue
This indicator relates to the hygienic properties of textiles, which, in turn, determine the comfort of a particular material when worn. Moreover, the requirements for clothing largely depend on its purpose.
Hygroscopicity is the most important property of a sports uniform or summer clothes. The increased temperature of the air and body leads to profuse sweating, which, in turn, creates considerable discomfort for a person. It is the high hygroscopicity of the tissue that allows you to get rid of excess moisture. This property is an important indicator for manufacturers of everyday underwear.
What determines the ability of a fabric to absorb moisture from the environment? First of all, from the fibers of which it is made. In addition, the presence of protective coatings and impregnations is important.
Types and hygroscopicity of fibers
The materials from which the fabrics are made may have a different origin. There are natural fibers and synthetic ones. First, let's talk about the first. They are created by nature itself, although not without human involvement.
Wool sheared from various animals is most often used to produce warm clothing. It is she who is one of the leaders among natural fabrics in her ability to absorb moisture. The hygroscopicity of wool fibers is approximately 15-17%. But the rate of absorption of moisture is relatively low.
This indicator is significantly higher in many other tissues. For example, the hygroscopicity of cotton is only 8-9%, but it is able to absorb moisture much faster than wool. Another natural material is flax, obtained from bast fiber. Its ability to absorb moisture can range from 12 to 30%.
Artificial and synthetic fibers
The first type includes materials obtained from natural compounds. A striking example is viscose. It is created using natural cellulose. Viscose fibers are characterized by strength, heat resistance, as well as high hygroscopicity, equal to almost 40%.
Synthetic fibers are created from petroleum products and coal. These include polyamides. Nylon, nylon and anide are made from these fibers. The hygroscopicity of such materials is quite low, only 3-4%, but they retain tensile strength and are very durable. The polyester fibers from which the lavsan fabric is created have a high rate of heat and light resistance. But their hygroscopicity is minimal - only 0.4%.
Polyurethane fibers, which are the basis for lycra and spandex, also do not differ in their ability to absorb moisture from the environment. From the foregoing, we can conclude that the hygroscopicity of clothing made of synthetic materials is significantly lower than that of natural fabrics. But is it really a flaw?
Is hygroscopicity good or bad?
Everything in the world is relative. The same can be said about the topic that we raised. It cannot be said unequivocally that hygroscopicity is good. Yes, it makes it easier for people to survive the heat, and for athletes to perform exercises in more comfortable conditions. But excessive moisture can only hurt some tissues.
By the example of a heater, we have already found out that water reduces the heat-insulating properties of materials. In addition, some fabrics are deformed by moisture - we all know how to knit after stretching after washing. The same fate, only on a smaller scale, can befall some materials with very high relative humidity. Therefore, it is not always possible to say with certainty that tissue hygroscopicity is a plus. The question is the purpose of this or that material.
How to determine this indicator?
In the 80s of the XX century, GOST 3816-81 was created in the USSR. It contains a detailed description of methods for determining certain properties of textiles, including hygroscopicity. Here's how to do it.
Specialists take tissue samples of 5 x 20 cm in size and each is placed in a separate weighing cup. The main objective of the experiment is to find out how much water the material absorbs under certain conditions. To do this, the sample cup is placed in a desiccator, in which the air humidity is 97-99%. After 4 hours, the sample is weighed, and then at a temperature of 105-109 ° C the material is dried and its new weight is determined.
The hygroscopicity index (H) in percent is determined using the formula: H = (Mv - Ms) / Ms x 100, where for Mv and Ms, respectively, the mass of wet and dry tissue is taken.