FIBRE TESTING - 3
AIR-FLOW METHOD(MICRONAIRE INSTRUMENT):
The resistance offered to the flow of air through a plug of fibres is dpendent upon the specific surface area of the fibres. Fineness tester have been evolved on this principle for determininG fineness of cotton. The specific surface area which determines the flow of air through a cotton plug, is dependent not only upon the linear density of the fibres in the sample but also upon their maturity. Hence the micronaire readings have to be treated with caution particularly when testing samples varying widely in maturity.
In the micronaire instrument, a weighed quantity of 3.24 gms of well opened cotton sample is compressed into a cylindrical container of fixed dimensions. Compressed air is forced through the sample, at a definite pressure and the volume-rate of flow of air is measured by a rotometer type flowmeter. The sample for Micronaire test should be well opened cleaned and thoroughly mixed( by hand fluffing and opening method). Out of the various air-flow instruments, the Micronaire is robust in construction, easy to operate and presents little difficulty as regards its maintenance.
Fibre maturity is another important characteristic of cotton
and is an index of the extent of
development of the fibres. As is the case with other fibre properties, the maturity of cotton fibres varies not only between fibres of different samples but also between fibres of the same seed. The causes for the differences observed in maturity, is due to variations in the degree of the secondary thickening or deposition of cellulose in a fibre.
A cotton fibre consists of a cuticle, a primary layer and
secondary layers of cellulose surrounding the lumen or central
canal. In the case of mature fibres, the secondary thickening is
very high, and in some cases, the lumen is not visible. In the
case of immature fibres, due to some physiological causes, the
secondary deposition of cellulose has not taken sufficiently and
in extreme cases the secondary thickening is practically absent,
leaving a wide lumen throughout the fibre. Hence to a cotton
breeder, the presence of excessive immature
fibres in a sample would indicate some defect in the plant growth. To a technologist, the presence of excessive percentage of immature fibres in a sample is undesirable as this causes excessive waste losses in processing lowering of the yarn appearance grade due to formation of neps, uneven dyeing, etc.
An immature fibre will show a lower weight per unit length than a mature fibre of the same cotton, as the former will have less deposition of cellulose inside the fibre. This analogy can be extended in some cases to fibres belonging to different samples of cotton also. Hence it is essential to measure the maturity of a cotton sample in addition to determining its fineness, to check whether the observed fineness is an inherent characteristic or is a result of the maturity.
DIFFERENT METHODS OF TESTING MATURITY:
The fibres after being swollen with 18% caustic soda are examined under the microscope with suitable magnification. The fibres are classified into different maturity groups depending upon the relative dimensions of wall-thickness and lumen. However the procedures followed in different countries for sampling and classification differ in certain respects. The swollen fibres are classed into three groups as follows
- Normal : rod like fibres with no convolution and no continuous lumen are classed as "normal"
- Dead : convoluted fibres with wall thickness one-fifth or less of the maximum ribbon width are classed as "Dead"
- Thin-walled: The intermediate ones are classed as "thin-walled"
A combined index known as maturity ratio is used to express the results.
Maturity ratio = ((Normal - Dead)/200) + 0.70
N - %ge of Normal fibres
D - %ge of Dead fibres
Around 100 fibres from Baer sorter combs are spread across the glass slide(maturity slide) and the overlapping fibres are again separated with the help of a teasing needle. The free ends of the fibres are then held in the clamp on the second strip of the maturity slide which is adjustable to keep the fibres stretched to the desired extent. The fibres are then irrigated with 18% caustic soda solution and covered with a suitable slip. The slide is then placed on the microscope and examined. Fibres are classed into the following three categories
- Mature : (Lumen width "L")/(wall thickness"W") is less than 1
- Half mature : (Lumen width "L")/(wall thickness "W") is less than 2 and more than 1
- Immature : (Lumen width "L")/(wall thickness "W") is more than 2
About four to eight slides are prepared from each sample and examined. The results are presented as percentage of mature, half-mature and immature fibres in a sample. The results are also expressed in terms of "Maturity Coefficient"
Maturity Coefficient = (M + 0.6H + 0.4 I)/100 Where,
M is percentage of Mature fibres
H is percentage of Half mature fibres
I is percentage of Immature fibres
If maturity coefficient is
- less than 0.7, it is called as immature cotton
- between 0.7 to 0.9, it is called as medium mature cotton
- above 0.9, it is called as mature cotton
AIR FLOW METHOD FOR MEASURING MATURITY:
There are other techniques for measuring maturity using
Micronaire instrument. As the fineness value determined by the
Micronaire is dependent both on the intrinsic fineness(perimeter
of the fibre) and the maturity, it may be assumed that if the
intrinsic fineness is constant then the Micronaire value is a
measure of the maturity
Mature and immature fibers differ in their behaviour towards various dyes. Certain dyes are preferentially taken up by the mature fibres while some dyes are preferentially absorbed by the immature fibres. Based on this observation, a differential dyeing technique was developed in the United States of America for estimating the maturity of cotton. In this technique, the sample is dyed in a bath containing a mixture of two dyes, namely Diphenyl Fast Red 5 BL and Chlorantine Fast Green BLL. The mature fibres take up the red dye preferentially, while the thin walled immature fibres take up the green dye. An estimate of the average of the sample can be visually assessed by the amount of red and green fibres.
The different measures available for reporting fibre strength are
- breaking strength
- tensile strength and
- tenacity or intrinsic strength
Coarse cottons generally give higher values for fibre strength than finer ones. In order, to compare strength of two cottons differing in fineness, it is necessary to eliminate the effect of the difference in cross-sectional area by dividing the observed fibre strength by the fibre weight per unit length. The value so obtained is known as "INTRINSIC STRENGTH or TENACITY". Tenacity is found to be better related to spinning than the breaking strength.
The strength characteristics can be determined either on
individual fibres or on bundle of fibres.
SINGLE FIBRE STRENGTH:
The tenacity of fibre is dependent upon the following factors
chain length of molecules in the fibre orientation of
molecules size of the crystallites distribution of the
crystallites gauge length used the rate of loading type of
instrument used and atmospheric conditions
The mean single fibre strength determined is expressed in units of "grams/tex". As it is seen the the unit for tenacity has the dimension of length only, and hence this property is also expressed as the "BREAKING LENGTH", which can be considered as the length of the specimen equivalent in weight to the breaking load. Since tex is the mass in grams of one kilometer of the specimen, the tenacity values expressed in grams/tex will correspond to the breaking length in kilometers.
BUNDLE FIBRE STRENGTH:
In practice, fibres are not used individually but in groups, such as in yarns or fabrics. Thus, bundles or groups of fibres come into play during the tensile break of yarns or fabrics. Further,the correlation between spinning performance and bundle strength is atleast as high as that between spinning performance and intrinsic strength determined by testing individual fibres. The testing of bundles of fibres takes less time and involves less strain than testing individual fibres. In view of these considerations, determination of breaking strength of fibre bundles has assumed greater importance than single fibre strength tests.
Go to Top of Page