Moisture
All
textiles are hygroscopic i.e. they absorb or release moisture depending on the
relative humidity of the surrounding air. If the atmosphere is drier than the
relative humidity of the textile, the textile will release moisture from the
air. If the air is too humid then the humidity of the textile will increase.
This moisture loss and gain occur at every stage from the initial processing of
the fiber to the final garment production, distribution, and use by the
consumer.
Moisture
is measured as “relative humidity”. This is defined as the amount of water in
an air sample compared to the maximum amount of water that can hold air at the
same specific temperature. It is expressed in the form of 0 to 100%.
Cold
air can retain less humidity than warm air so air humidity is “relative” to its
temperature. For example, a sample of air at 10°C may retain less humidity than the
same sample of air at 20°C.
Although both samples contain the same amount of water, the relative humidity
of the warm sample is lower because it may contain more moisture than the cold
air sample. This means that the humidity of the indoor air can be lower even in
humid weather when the cool moist air from outside enters a textile
manufacturing facility and its temperature rises. A 20°C temperature
increase can lead to a normal drop in the humidity of about 60% RH (relative
humidity). This means that the humid air entering a building at 80% RH will
become only 20% RH when heated. This can cause big problems in textile
production.
Moisture Regain
Moisture
regain is defined as the percentage of water present in a textile material of
oven-dry weight. The percentage of moisture in a textile material brought into
equilibrium with a standard atmosphere after partial drying, calculated as a percentage
of the moisture-free weight.
Say,
Oven
dry weight = D
Weight
of water = W = Original weight - oven-dry weight
Moisture
Regain = MR or R
Then,
R (%) = 100 W/ D
Moisture Content
The
moisture content is the amount of moisture in a material when expressed as a
percentage of the total weight is known as the percentage. It is defined as The weight of water in the material is expressed as a percentage of the total weight.
Say,
Oven
dry weight = D
Weight
of water = W = Original weight- oven-dry weight
Moisture
Content = MC or C
Then,
C (%) = 100 W/ (W + D)
Relation Between moisture regain and moisture content
Say,
Oven
dry weight = D
Weight
of water = W
Regain
= R
Moisture
content = C
For
moisture regain
R=
100W/D............ (1)
For
moisture content
C=
100W/ (D + W).......... (2)
From
equation (1)
D=
100W/R
From
equation (2)
C=
100W/ (100W/R + W)
C=
100/ (100/R + 1)
C=
100R/ (100 + R)
100R=
100C + RC
R=
100C/ (100-C)……… (3)
Equation
(3) explain the relationship between moisture regain and moisture content.
Factors affecting the moisture regain of textile fibers
Different
fibers absorb different amounts of moisture depending on their affection for
water. There are various factors that affect the return of moisture to textile
materials, such as;
1.
Relative humidity
2.
Time
3.
Temperature
4.
Previous history of the sample
Relative humidity
The
relative humidity of an air-water mixture is defined as the ratio of the partial pressure of water vapor in the mixture to the equilibrium vapor the pressure of water at the surface of pure water at a certain temperature:
Relative
humidity = partial pressure of water vapor/equilibrium vapor pressure of water
In other words, relative humidity is the ratio of how much water vapor is in the
air and how much water vapor it can hold at a potential temperature. This
varies with air temperature: cold air may contain less vapor, so water vapor
may condense when some air is cooled. Similarly, warming some foggy air can
cause fog to evaporate, as the air in the water droplets is able to retain
water vapor. So, changes in air temperature can change relative humidity, even
when absolute humidity remains constant.
Relative
humidity only considers invisible water vapor. Mist, cloud, fog, and water
aerosols do not count for measuring the relative humidity of the air, although
their presence gives an indication that a body of air may be near the dew
point.
The relative humidity is usually expressed as a percentage; A higher percentage
means the air-water mixture is more humid. At 100% relative humidity, the air
is saturated and at its dew point.
Time
It
takes a certain amount of time to reach a new equilibrium at a certain relative
humidity, which is then transferred to the atmosphere with different relative
humidity. The time depends on the physical appearance of the material and how
easily moisture can reach or escape from the individual fibers. For example,
the British standard for yarn count testing offers one hour for yarn in the
form of hanks to achieve compatibility, but three hours for yarn in the
package.
Temperature
Temperature is a measure of warmth or coolness expressed on any one of the different scales, including Fahrenheit and Celsius. Temperature indicates that heat energy will flow spontaneously from a hot body to a cold body. The effect of temperature on moisture regain is not important. A change of 10°C will bring about a 0.3 percent change in cotton moisture regain. This effect can be ignored. Now we can say that for practical purposes the temperature does not affect the moisture regain of a sample.
The previous history of the sample
The previous history of the sample may affect the regain of balance. In the case of
the hysteresis effect, the recovery value of the wet sample is higher than that
of the dry sample of equilibrium. Processing recovery can also change. Removal
of oil, wax and other impurities also changes the recovery. For example, formal
regain of scoured wool is 16% and oil-combed tops are 19%.
Different fibers moisture regain & content (%)
Different
fibers moisture regain & content (%) are below-
Textile Fiber Moisture Regain
(MR%) Moisture Content (MC%)
Cotton
8.5 7.34
Jute 13.75 12.1
Viscose 11.0 9.91
Silk 11.0 9.91
Wool 16.0 13.8
Acrylic 1.5 0
Flax 12.4 10.4
Hemp 12.4 10.4
Acetate 6.0 0
Polyester 0.4 0
Nylon 4.0 3.1
Azlon 10
Metalic 0
Triacetate 3.5
Rubber 0
Wool
yarn 18.25
Glass
0
Moisture level for different fiber
In
terms of performance, natural fibers are much more sensitive to moisture than
man-made ones. However man-made garments suffer more from static charge
build-up.
Cotton
and linen have to be processed at very high levels, about 70-80% RH because
they are very fragile. By wetting every process from raw material combing to
carding, twisting, spinning, and weaving, the manufacturer can ensure that the
product remains flexible and prevents breakage. The longer the fiber, the finer
it can cut.
Wool is similarly sensitive to dry air, although a little more forgiving, requiring a moisture level of about 65% RH. Man-made fibers below 45% RH require accurate, although low, moisture levels, they are prone to generating static electricity. Silk should be processed between 65-70% RH, although artificial silk spinning requires a high level of 85% RH.
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