Testing of Cotton Fibre Maturity
A. Direct Methods
1. Degree of Wall Thickening
The extent of wall thickness development may be considered satisfactorily in terms of a ratio, the degree of wall thickening. This is defined as the ratio of the cross-sectional area of the wall to the area of the circle having the same perimeter as that of the fibres & is denoted by the Greek letter θ.
The ratio is unity if the fibre is completely solid & has a circular cross-section. The degree of wall thickening measures fibre maturity independently of the fibre perimeter expressing it on a numerical scale between zero & unity.
The direct determination of the degree of wall thickening from the measurement of the area & perimeter on fibre cross-sections is not suitable for use as a practical everyday test, as it is slow & laborious errors arising from cutting the sections. There are some indirect tests, which are much quicker to carry out.
2. Caustic Soda Swelling Test
Standard practice in the U.K.: A projection microscope with a traversing stage and magnification of 150x is used for the test. A thin tuft of fibers is drawn by means of tweezers and placed on a glass slide and covered by a coverslip. A drop of 18% NaOH solution is added to the fibers. The slide is placed on the stage of the microscope. Five slides each of 100 fibers are to be tested. Frequently a maturity test is carried out on samples previously subjected to comb sorter test. Five tufts are collected at intervals along with the diagram.
The cotton fibers swell differently for mature and immature fibers due to variation of cellulose content when treated with 18% (Sp. Gravity 1.28) NaOH solution.
In the examination, the fibres are classified into three main groups as follows:
- Normal Fibres (N): After the swelling. appear rod-like with no continuous lumen, no well-defined convolutions.
- Dead Fibres (D): After swelling, the wall thickness is one fifth or less of the maximum ribbon width. Dead fibres vary from float ribbons, with no secondary flat fibre wall, to highly convoluted forms with a greater wall development.
- Thin Walled (T): They are not well defined. Some have a fairly rod-like form, but are not classified as normal because they have thin but continuous lumen. At the other end of the range in this group the fibres approach the dead class but their wall thickness is greater than one fifth of the ribbon-width.
After identifying the fibres, Percentage normal and dead fibres are calculated.
Where,
- M – the percentage of mature fibers.
- H – percentage half mature fibers
- I- the percentage of immature fibers.
B. Indirect Methods
Indirect methods have been evolved mainly to reduce the time of testing and make it much simpler.
1. Polarized Light Tests
The fibres in the walls of the cotton fibre are anisotropic; they are doubly-refracting with two refractive indices n11 & n1. When plane-polarized light passes through fibre it suffers retardation and assumes thickness of the fibre wall & the orientation of the fibrils to the fibre axis.
A microscope, with a constant intensity source and polarizer and analyzer, fitted. The test is usually made with a magnification of 100 x.
A thin fringe of fibres is prepared and mounted on a microscope slide protected by a cover-glass, with the fibres substantially parallel to each other. The slide is placed on a traveling stage so that the fibres are parallel with the marked slow direction of the selenite plate. When the stage is advanced to bring the fibres into view beneath the objective they are seen to have different colours.
Very immature fibres appear violet or indigo, immature fibres are blue, mature fibres are green, and very mature fibres are yellow. The very immature fibres to have their colour almost extinguished in the 450 positions, changing to orange as the 900 positions are reached, approached.
The immature fibres change from blue to yellow in the 900 positions. The green mature fibres tend to become more yellow in the 900 positions. The yellow very mature fibres keep the colour the same on rotation through 900.
The colour changes which appear on rotation are used as a check-in classifying fibres subject to doubles.
Under the conditions of observations used in this test, the colour of a given depends markedly upon the absolute thickness of the wall rather than on the degree of thickening. In a range of cotton fibres of the varied perimeter, it would be expected that those giving the same maturity count by the polarized light method would not necessarily give the same estimates of the degree of wall thickening as estimated by the swelling method. The intrinsically fine cotton fibres of the small perimeter would tend to have the same absolute thickness as the intrinsically coarse cotton fibres, but to be relatively more mature as judged by the degree of wall thickening.
The classification by the polarized light technique is subjective. There is no finite & sharp distinction between the fibre classes. The colours of each class change gradually from one to another, because of the continuous form of distribution of wall thickness of different fibres in a sample. This leaves just as much scope for variation to occur in the judgment of differentials operatives as with the continuous change in the variation of the ratio of the wall thickness to ribbon width in the swelling test.
2. Differential Dyeing method
A 3-g sample is introduced into a boiling dye-bath consisting of Diphenyl Fast Red and Chlorantine Fast Green. After 15 minutes, 4% of (calculated on weight of fiber) of NaCl is added and, after a further 15 minutes, a further 4% of NaCl. When the sample has been in the bath for 45 minutes, it is taken out and rinsed three times in distilled water, after which it is centrifuged. The cotton is then rinsed in cold distilled water and carefully dried. The sample is now ground to powder in the mill, thoroughly mixed, and pressed into the form of a pad. The pads are then compared visually with pads prepared from standard American cottons of known maturity as measured by the standard ASTM maturity count. Mature samples appear predominantly red and immature samples predominantly green.
The test depends on two circumstances: (i) that, of the two dyes used, the red diffuses into, and also washes out of, the cellulose of the cell wall much more rapidly than the green; and (ii) that immature fibers have greater specific surface than the mature and so take up dye more rapidly. Thus, because of their greater specific surface, the immature fibers take up more green dye than the mature fibers do and, because of the slow diffusion rate of green dye, the difference between the two is not greatly affected by subsequent boiling wash. With rapidly diffusing red dye, on the other hand, a period of 45 minutes is long enough to cause both mature and immature to take up much the same amount of dye, but, in the 30-seconds boiling wash, the immature fibers lose much more of what they take up because of their greater specific surface.
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