Moisture Relation in Textiles

Moisture Relation in Textiles:

Importance of moisture in Textile- TextileSphere

The most significant textile fiber properties are closely related to its behavior with the atmospheric conditions. Different fibers react differently with the atmospheric conditions.
Some fibers are hygroscopic, which means they absorb water from the atmosphere or loose water in dry atmosphere. If the rate of exchange of water vapour or moisture from the atmosphere to the material or from the material to the atmosphere becomes equal, a state of equilibrium is achieved which is called as ‘Hygroscopic Equilibrium.’  

Influence of humidity on fiber properties:
The amount of moisture is not constant, it keeps changing. This changes the amount of moisture in the material. Due this change the physical properties of the textile fiber like rigidity, tensile strength, elastic recovery, elastic resistance, dimensions etc.
Let us consider cotton to understand this phenomenon properly, cotton absorbs moisture ore rapidly when exposed to high dampness in the atmosphere and as a result of which weight and strength of the material increases and other properties will be changed. But in the case of man-made fibers like acetate and viscose, the strength reduces. And synthetic fibers like Terylene and nylon are not affected much as they are not fully hygroscopic in nature.
In practical point of view all textile fibers show increase in spinning capability and pliability and greater electrical influence with increase in amount of moisture in the fiber.

Influence on Processing:
Under ideal humidity conditions the following benefits are achieved during processing:
  • It reduces fly and dust and allows retention of moisture already present in the material
  • It reduces generation of static charges and increases pliability of the material
  • It also provides bodily comfort for the workers

Some important terms:
  • Humidity: Humidity is a term used to describe the amount of moisture present in the atmosphere. This can be noted in terms of Relative humidity or Absolute humidity.
  • Relative Humidity (RH):  It is defined as the ratio between the actual vapour pressure and the pressure at the same temperature, expressed in a percentage.
  • Absolute Humidity (AH): it is defined as the weight of water present in a unit volume of moist air. It is expressed in terms of grains/cubic foot or grams/ cubic meter.
  • Moisture Regain(R): It is defined as the weight if water in a material which is expressed as a percentage to the oven dry weight of the material.
  • Oven dry weight: he oven dry weight of the material is defined as the constant weight of the material obtained after drying the material at a temperature of 105 degrees, till all the moisture in the material is expelled.
  • Moisture content (M): It is defined as the weight of water in a material expressed as a percentage of the total weight of the material.
  • Standard atmosphere: It is defined as an atmosphere at prevailing barometric pressure with relative humidity of 65% and temperature of 20 degrees Celsius. In tropical and sub-tropical regions lie India, temperature is higher, around 27 degrees Celsius. Standard Regain: Standard regain is also referred as reference regain, it is a regain measured at the standard testing atmosphere.

Reasons for differences in fiber regain:
  • It is noted that the presence of water attractive and inert groups in the fiber molecules influence the regain of the fibers. If the fibers are kept in the atmosphere and if they contain water loving groups that is, water attractive groups naturally, they will absorb more water molecules from the atmosphere and posses a higher regain value. For example, all natural, animal and vegetables have –OH groups and have hydrogen bond which is strong in nature.
  • If the fibers have inert group, they do not attract water molecules from the atmosphere which results in lower moisture regain. For example, acetate.

Regain- Humidity Relations to Textile:

Relations between regain humidity relationship

  • If two samples are taken one completely wet and one completely of the same material are considered into a given atmosphere and a interval o time, the regain value for each were determined.
  • By plotting the regain against time for both the samples, two curves could be obtained.
  • The regain changes quickly at first and then slowly as the equilibrium condition are approached.
  • The two curves obtained would not meet as their equilibrium regain values differ.
  • The sample which was originally wet is noted to have higher regain value than that of the dry sample
  • This effect is termed as ‘HYSTERESIS’.

 Absorption and Desorption Curves:

  • The behavior of textile material can be studied from the curve obtained by plotting a graph against relative humidity and regain. By plotting this graph any property of a textile material can be studied.
  • The curves obtained are ‘A’ and ‘D’ represents absorption and desorption respectively. Both these curves showed sigmoid or S-shaped characteristics. The figure mentioned below shows the nature of both the curves.
  • The regain increases rapidly at low humidity also the curve D does not follow back curve A. 
  • For an instance, point ‘b’ the equilibrium regain at RH 65% of the ample when approached.
  • From wet side point ‘a’ is the equilibrium regain when approached from dry side this is called as hysteresis effect which was discussed earlier.

Factors affecting the fiber regain: 
  • Relative Humidity: Regain of the fibers increases at low humidity and is follows a linear portion and then rises at high humidity.
  • The previous history of the sample: The previous history of the sample that is, if the sample is wet or dry can affect the equilibrium. Therefore, in the study of regain values, physical and chemical history of the material must be taken into account.
  • Time: A material placed in a given atmosphere takes a certain time to reach the equilibrium condition. The time taken depend size and form of the material and external condition.
  • Temperature: The curves of regain against RH depend to a little extent on temperature expect at high temperature and humidity, the regain decreases as the temperature increases, that is it has inverse relation.
  • The regain also affects the properties of the material, the dimensions, mechanical properties of the material, thermal effects and the electrical properties of the materials are changed.

Measurement of Atmospheric conditions: 
The instruments used in the measurement o humidity are called as Hygrometers and the different methods of measurement of humidity are chemical method, Dew-point method and gravimetric method. But these methods are not widely used in testing labs but the following instruments are generally used:
  • Wet and Dry bulb hygrometer
  • Thermo hygrograph
  • Electrolytic hygrometer

The wet and the dry bulb hygrometer is widely used instrument and it functions on the following principle:
If the bulb of the thermometer is surrounded by a film of water and the air around it is not saturated then the water evaporates from the bulb into the air at a rate which is proportional to the difference between the actual humidity and 100% humidity. Since the evaporation is accompanied by cooling, the temperature indicated by the thermometer will be less than the room temperature.

The following figure shows the set up wet and dry bulb thermometer:

  • The wet and dry bulb hygrometer consists of a frame on which two identical thermometers are placed.
  • The bulb of one thermometer is covered with a muslin cloth which is dipped in the trough of distilled water. This is the wet bulb thermometer.
  • The bulb of other thermometer which is uncovered and from that thermometer the dry bulb temperature can be determined.
  • The wet bulb thermometer shows temperature less than the dry bulb and the temperature is termed as wet bulb temperature

  • Principle of Textile Testing by JE Booth

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  1. It's useful ,thanks!Here is a machine looks good ,Temperature and Humidity Chamber For the conditioning of samples prior to testing, also applied for Water Vapour Permeability Testing when equipped with related kits. I hope to get more details about it.

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  3. Excellent post!!!