6 textiles and moisture

7/29/2019 6 Textiles and Moisture

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Natural fibres are hygroscopic and they are able to

absorb water molecules from a moist atmosphereand to give up water to a dry atmosphere;

The properties of many textile fibres such as mass,tensile strength, elongation, elastic recovery,electrical resistance, fibre dimensions and rigidity arestrongly affected by the amount of moisture in theair;

Moisture content can have a significant effect on themass of the material. Commercially, it is important toknow the amount of moisture contained in thematerials.


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Dimensional

Mechanical

Electrical

Thermal


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The behaviour of the fibre (swelling and deswelling) to changes in the relative

humidity of the environment is responsible for the dimensional stability

problems of certain fabrics


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Some fibres, such as wool and viscose, lose strength

when they absorb water and some, such as cotton, flax,

hemp and jute, increase in strength;

The extensibility, that is the extension at a given load,can increase for some fibres when wet;

Wool fibre (like most protein fibres) loses strength but

gains in elongation when wet compared with a drysample;

Besides tensile strength other properties such as fabric

tearing strength are affected by fibre strength.


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o The moisture content of fibres has animportant effect on their electricalproperties;

o The electrical resistance decreases andcapacitance increases with increasingmoisture content;

o Static electrical charges are more readilydissipated when the atmospheric relativehumidity is high.


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When moisture is absorbed by the fibre(esp wool), heat (heat of absorption orwetting, when fibre wets), is generated;

The thermal conductivity of the fibreincreases with moisture;

The application of both facts is importantin the design of clothing in coldenvironment.


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Therelative humidity is a ratio of the mass of watervapour in a given volume of air to mass of water vapourrequired to saturate this volume (100% humidity) at thesame temperature and is expressed as a percentage;

It can also be defined as the ratio of the actual vapourpressure to the saturated vapour pressure at the sametemperature;

The absolute humidity is defined as the weight of waterpresent in moist air (g/m3);

The amount of moisture contained by fibres that are inequilibrium with the atmosphere appears to depend onthe relative rather than the absolute humidity.


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The relative humidity affects the moisture content oftextile fibres and their properties;

It is thus necessary to specify the atmosphericconditions in which any testing is carried out;

A standard atmosphere for testing purposes is definedas a relative humidity of 65% and a temperature of2OoC;

In practice, certain tolerances in these values areallowed so that the testing atmosphere is RH 65% 2%,20 2oC. In tropical regions a temperature of 27 2oCmay be used;

Hygrometer is used to measure relative humidity.


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The amount of moisture in a fibre sample can beexpressed as eitherregain ormoisture content.Regain, popularly used in the textile industry, is theweight of water in a material expressed as a

percentage of the oven dry weight:

Regain,% = (10O x W) / D ;

where D is the dry weight and W is the weight ofabsorbed water

Moisture content is the weight of water expressedas a percentage of the total weight:

Moisture content, % = (100 x W) / ( D + W)


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Hysteresis: If two identical samples of fibre, one wet and

one dry, are placed in a standard atmosphere of 65% RH,the one that was originally wet is found to have a higher

regain than the one that was originally dry.

Equilibrium

Time

Regain, %

When samples are being conditioned, it is necessary for reproducibility of moisture

content for a sample to approach equilibrium in the std testing atm from the same

direction every time. The samples are dried in an oven at a low temperature (5OoC)

before conditioning in the std atmosphere

absorption

desorption

Regain, %

Relative humidity, %


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To measure the regain of a sample of textilematerial it is necessary to weigh the material, dry itand then weigh it again. The difference betweenthe masses is then the mass of water in the sample.

Regain, % = (mass of water x 100) / oven dry mass

Regain is based on the oven dry mass, which formost fibres is the constant mass obtained by drying

at a temperature of 105 2oC for most fibres forabout 1-2 hours. Constant mass is achieved bydrying and weighing repeatedly, at short intervals,until successive weighings differ by less than 0.05%.


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A conditioning oven is a large oven which contains the fibre

sample in a mesh container which is connected to a balance.This ensures that the weight of the sample can be monitoredwithout the need to remove it from the oven

A continual flow of air at the correct relative humidity is passedthrough the oven which is maintained at 105oC. The use of a

conditioning oven to dry a sample is the correct standardprocedure;


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A 'correct invoice weight' has to be determined when textile

materials, affected by RH, are bought and sold by weight;

A sample (~200 g), stored in air tight container, is taken from theconsignment and tests are made to give the correct invoiceweight;

The samples are weighed and then the oven dry weight isdetermined as described above.

The oven dry mass of the consignment, C = (M x D) / S ;

M is the mass of consignment at time of sampling, D is the oven dry mass

of sample, and S is the original mass of sample.

To the oven dry mass is added an official or standard regainallowance (as set out in BS 4784) for moisture depending on thenature and type of the material;

In the case of a blend the overall allowance is calculated fromthe fraction of each component in the blend multiplied by its

regain value.


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Testing laboratories require the atmosphere to be maintained at65 2% RH and 20 20oC in order to carry out accurate physicaltesting of textiles;

The temperature is controlled in the usual way with a heater/refrigeration and thermostat;

The relative humidity is controlled by a hygrometer whichoperates either a humidification or a drying plant;

The laboratory enclosure should be such (adequate insulation,air-tight doors and windows) that it helps to maintain theatmosphere within the tolerance bands;

When a fully conditioned testing laboratory is not available,testing should be carried out in a room in which ambientconditions are as uniform as possible throughout the year;

Small cabinets may be used to condition samples before testing,the actual tests being carried out in a normal atmosphere

straight after removal from the conditioned atmosphere.


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Tendency of fibers getting charged with static electricity changes according to

surface resistivity of them. Relative humidity directly affects surface resistivity.

6 textiles and moisture

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