Textiles and moisture

Textiles and moisture

Introduction

The properties of textile fibers are strongly affected by the atmospheric moisture content. Moisture and humidity influenced on the results of testing of textile materials.

Many fibers, particularly the natural fibers are hygroscopic in nature and they will absorb moisture if kept in a humid atmosphere.

Conversely, they will tend to lose moisture in a dry atmosphere (a process known as desorption.)

Hydrophobic fibers (e.g. polyester) are those which do not absorb moisture if kept in a humid atmosphere.

Significance of moisture absorption

The importance of moisture relations cannot be overstated. Almost all the physical properties of fibers, yarns and fabrics are influenced by the presence of moisture.

The presence or absence of moisture can effect dimensions, mechanical strength, elastic recovery, rigidity and so on.

Mechanisms of moisture absorption

Moisture absorption in textile fibers occurs through the interaction of water molecules with polar groups within the structure (e.g. OH, -COOH).

In ordered (crystalline) parts of the structure, these polar groups are not attached but in disordered (amorphous) regions there can be very free groups which can act as hooks with which the water molecules can link through hydrogen bonding.

Thus the absorption of moisture takes place in the amorphous regions only.

Atmospheric conditions and humidity

The amount of water take up by a textile fiber can be expressed in terms of its ‘regain’. The regain depends on the atmosphere surrounding the material, i.e. humidity and temperature. Therefore it is vital to specify the atmospheric conditions when carry out testing.

Humidity

The amount of moisture in air is called as humidity”. There are two types of humidity

1. Absolute humidity

2. Relative humidity

1. Absolute Humidity

“The actual amount of water vapour present in a specific volume of air is expressed in terms of absolute humidity (AH).”

AH = Mass of water present per unit volume of air

The basic unit of absolute humidity is ‘Kgm-3

2. Relative Humidity

The 'relative humidity' (RH) of an atmosphere has two alternative (though almost equivalent) definitions:

a) The ratio of the AH of the air to that of air saturated with water vapour at the same temperature & pressure;

b) The ratio of the actual vapour pressure (VP) to the saturated vapour pressure (SVP) of the air at the same temperature, expressed as a percentage

r.h = vp/svp x100%

Atmosphere

“The relative humidity, temperature and pressure of the surroundings are termed as atmosphere.”

1) Saturated atmosphere

2) Standard atmosphere

3) Testing atmosphere

Atmosphere

1. Saturated atmosphere: “The atmosphere saturated with moisture

is called as saturated atmosphere.”2. Standard atmosphere: If the atmosphere has the following

specifications then it is called as standard atmosphere;

RH = 65% Temperature = 20 0C (at atmospheric

pressure)

Atmosphere

3. Testing atmosphere: It is an atmosphere in which we performs the tests

of the textile products.

If the atmosphere has the following specifications then it is called as standard ‘testing atmosphere’;

RH = 65 + 2 %

Temperature = 20 + 2 0C (27 + 2 0C in tropics and sub-tropics)

REGAIN & MOISTURE CONTENT

The amount of the moisture in a sample of material may be expressed in terms of regain or moisture content.

Moisture Regain The 'Moisture Regain' of a textile fiber is defined as:

The equilibrium weight of water contained by a specimen expressed as a percentage of its oven-dry weight. That is,

R=100(Ws-Wd)/Wd %


Moisture content:

The ‘Moisture Content’ of a textile fiber is defined as:

The equilibrium weight of water contained by a specimen expressed as a percentage of its total weight (i.e. including the water). That is,

M=100(Ws-Wd)/Ws %


Regain is more commonly used than moisture content.

The regain of any given fiber type gives a very good indication of how sensitive that fibers towards moisture.

Its actual value will clearly depends on the humidity of the surroundings.


Following diagram compares a number of typical fibers under standard testing conditions.

Regain-Humidity Relations of Textiles

If a piece of fabric or other textiles materials is placed in a room in which the humidity is constant, then it will eventually come to equilibrium by absorbing or desorbing moisture as necessary until it reaches a state of constant regain.

If the air humidity is in the normal range (around 65%rh) then wet fabric would become drier, whereas a dry fabric would gradually become wetter.

The rate of wetting or drying is rapid at first, becoming slower as equilibrium is approached.


Hysteresis:

If we plot regain VS time for wet and dry samples of the same material, both tend to be mirror image of one another as indicated in the following diagram.


Hysteresis: However the two curves do not come together at

equilibrium, the equilibrium regain value of the two curve is different.

specifically, the initially wet sample will end up somewhat wetter than the initially dry sample, no matter how long the two samples remain in the conditioned atmosphere.

This is an example of Hysteresis, and is very important where samples have to be conditioned for testing or other purposes.

It implies that the approach to equilibrium should always be from the same direction (i.e. either from wet to dry, or else from dry to wet).

Regain VS Relative Humidity Curve

Mostly the plots of Regain VS Relative Humidity (RH) are S-shaped (often described as ‘sigmoidal’). Because of Hysteresis, any given material will produce two different curves; the absorption curve and the desorption curve (desorption curve is always higher).

Curve A is the absorption curve, that is, the regain-r.h. percentage relation as a material takes up moisture.

Curve D is the desorption curve. For instance, point a is the equilibrium condition at 65 %

relative humidity when approached from the wet side, and point a is the equilibrium regain when approached from the dry side. this is the hysteresis effect.



Absorption curves of various materials

The following should be noted with regard to absorption curves;

Factors Affecting the Regain of Textile Materials

There are different factors that effect the regain of the textile materials, such as;

1. Relative Humidity

2. Time

3. Temperature

4. The Previous History of The Sample


1. Relative Humidity:

Relative humidity is the most important factor that affects the regain of textile materials. If the relative humidity in the air is more then there will be more moisture regain or more rate of conditioning of textile materials and vice versa.

2. Time:

A material placed in a given atmosphere takes a certain time to reach equilibrium. The rate of conditioning depends on several factors, such as;

a) The size and form of sample

b) The type of material

c) External conditions


3. Temperature: The effect of temperature on regain is not

important. A change of 10 0C will give a change in regain of cotton of about 0.3 percent. This effect can be ignored.

4. The previous history of the sample: The previous history of the sample can affect the

equilibrium regain of the sample. The hysteresis is a good example. Processing can also change the regain. When oils, waxes and other impurities are removed then regain may change.

METHODS OF MEASUREMENT OF RELATIVE HUMIDITY

The instruments used to determine the humidity are known as ‘hygrometers’ or ‘psychrometers’. Three main types of hygrometers are:

1) The wet-and-dry bulb hygrometer

2) Hair hygrometer

3) Electrolytic hygrometer

(1) The Wet-and-Dry Bulb Hygrometer

In this type of hygrometer there are two thermometers one of which is surrounded by a wet sleeve of muslin. The temperature difference between wet and dry thermometer bulbs is determined by the evaporation rate, which in turn can be related to RH through look-up tables.


EXAMPLE: Dry bulb reading =20 OC, Wet bulb reading = 14 OC Difference = 6 OC RH percent from the table = 51%


Its advantages include the simplicity and the fact that it needs no calibration.

Its disadvantages are that it requires a constant supply of distilled water and the fact that it is an indirect method, since tables are needed.

Its various types are;a) Wet-and-dry bulb hygrometer, wall mounted b) Wet-and-dry bulb hygrometer, sling typec) Assmann type hygrometerd) Recording type hygrometer

(2) Hair Hygrometer

Human hair has the property of lengthening or shortening as the humidity of the air increases or decreases. By connecting a band of hairs to a suitable lever system, the relative humidity can be indicated directly and, if needed, recorded on a chart. It is a fact that, it does not give great accuracy.

The main advantages of this type of hygrometer are the direct reading and elimination of distilled water.

Drawbacks (or disadvantages) are also present such as it requires frequent calibration and it have slow response to change in atmospheric conditions.

(3) Electrolytic Hygrometer

Electrolytic hygrometer operates by measuring the electrical current flowing in a skein of very fine fibers impregnated with salt solution (e.g. Lithium Chloride) which have the property of very rapidly attaining equilibrium with the surrounding atmosphere.

The heart of this type of instrument is an element consisting of a plastic frame carrying platinum-clad electrodes. Skein of very fine fibers impregnated with a chemical is wound across these electrodes.

(3) Electrolytic Hygrometer

Three advantages of electrolytic hygrometer are: They can have very fast response They take direct reading They require only low air currents They have no particular disadvantage. Some other types of hygrometer are;

1) Electrical Hygrometer

2) DuPont Hygrometer

EFFECTS OF REGAIN ON FIBER PROPERTIES

There are basically four effects of regain on fiber properties, such as;

1. Dimensions

2. Mechanical properties

3. Electrical properties

4. Thermal effects

(1) Dimensions of The Fiber

Moisture causes swelling of textile fibers. Because the fiber structure is essentially aligned along the fiber axis, the effect of the water molecules is to push the polymer chains apart laterally, so that the fiber diameter increases by a far greater fraction than does the length. In woven fabrics, swelling of the fibers very often causes a tightening up of the structure, resulting in an overall shrinkage.



Shrinkage due to swelling is not the only problem: a phenomenon known as 'hygral expansion' may also be encountered. This effect is partly due to the reduction in stiffness which accompanies the absorption of water, so that the fibers become more easily stretched.

(2) Mechanical Properties

Generally, the presence of water molecules in the fiber structure reduces the strength of the bonds holding the polymer chains together, thus reducing fiber strength. Exceptions to this include the vegetable fibers, such as cotton, in which water actually increases the strength.

(2) Mechanical Properties

In general, the shapes of fiber stress-strain curves are changed. The yield point may be dramatically lowered.

Other effects include changes in: Crease recovery Extensibility Flexibility ‘Setting ability’ by finishing process

(3) Electrical Properties

Most textile fibers when dry have very high electrical resistance, but this can fall by factors of hundreds of thousands at elevated regains.

Dielectric characteristics are also influenced by humidity, as is susceptibility to electrostatic problems.

Changes in dielectric properties can be a source of error in measuring the uniformity of slivers, rovings, etc, when capacitance-type instruments are used.

Where strict control of regain is required (e.g. in warp sizing) dielectric effects can be used in the design of the control system

(4) Thermal Effects

Just like electrical resistance, the thermal resistance (or insulating ability) of fiber is reduced on increasing humidity. Thus, a garment made form hygroscopic fibers gives better protection against cold in dry atmosphere than a damp one (humid atm.). It is not really the fibers themselves which provide real thermal effects, but the air that is trapped within the fabric surface (Air has very thermal conductivity).

(4) Thermal Effects

On the other hand, some fibers actually liberate heat when they absorb water. For example, in the case of wool this can be quite considerable. This is reason that why woolen sweaters are so popular in cold and damp climates.

The Control of the Testing Room Atmosphere (Environmental Control)

To measure the relative humidity of atmosphere is one thing, and to control it is a quite different matter. Controlling the atmosphere conditions of a laboratory is essential when standard conditions of testing are specified or when textile materials have to be stored for a long time. More or less complicated systems are in use for simultaneous control of temperature and relative humidity (which are not independent of one another). Such control systems are known as ‘Hygrosets’ or ‘Humidostates’.

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Textiles and moisture