1. Fibre length

The machines in a spinning mill with their settings are primarily sensitive to the fibre length, which is therefore recognized as the most important fibre property.

Measurement of fibre length is generally based on the concept of span length which represents the distance fibres extend into the drafting zones of machines. The measure most commonly used for the machine settings is 2.5% Span Length (S.L), which represents the distance 2.5% of the fibres extend into to drafting zones. Max Preysch(1) reports that in addition to 2.5% S.L.

The Uster Fibro graph 730 measures following fibre properties:

• 50% Span Length (S.L).
• Uniformity Ratio (UR).
• Short Fibre Index.

Whereas, Uniformity Ratio is 50% S.L., expressed as percentage of 2.5% S.L.

 W. Klein(2) reports that fibre length, i.e. 2.5% S.L. and U.R. influences:

• Spinning limit or spinning potential.
• Yarn strength, evenness and fullness.
• Handle and hairiness of yarn.
• Productivity.

On the basis of 2.5% S.L. Pakistani cotton are classified as following:

2. Fibre fineness

Fibre fineness is as important property as fibre length in the determination of spinning potential of a given variety of cotton. Fibre fineness is measured in micrograms per inch by testing the sample of cotton on Micronaire, the Fibre Fineness Tester.

Fineness can also be measured by testing sample of cotton on Uster HVI – Spectrum which has been available since 1999 as reported by Joseph M. Yankey(2).

Yarn characteristics influenced by fibre fineness in addition to those influenced by fibre length (2.5% S.L. & U.R.) are drape, luster and handle of textile products. On the basis of fibre fineness gms / inch cottons are classified as follows:

• Upto 3.1 very fine.
• 3.1 to 3.9 fine.
• 4.0 to 4.9 medium.
• 5.0 to 5.9 slightly coarse.
• Above 6 coarse.

3. Fibre strength

Fibre strength is also one of the most important fibre characteristics, however, full contribution of fibre strength cannot be realized in yarn strength. Gregory(3) has clarified that maximum strength in the twisted strand is a compromise reached between the increasing cohesion of fibres as twist increases and a decrease in the effective contribution to the axial loading of the strand due to the obliquity of fibres.

Since the fibres are inclined to the axis of the twisted strand, only component of the fibre stress resolved in the direction of the axis of the strand effectively balances the applied load and full contribution of the fibre strength is not realized.

This effect of obliquity is small at first, increasing more rapidly as the twist builds up. Consequently, all spinners use twist factor for spinning a particular count of yarn very carefully with the object of achieving maximum yarn strength. A twist factor which fulfils this object is known as Optimum Twist Factor.

Fibre strength by testing a sample of cotton on Pressley Fibre strength Tester. Cottons are classified on the basis of fibre strength as follows:

Pressley Strength in 103 LBS / Sq. inch.

• 93 and above               =          Excellent.
• 87 to 92                       =          Very strong.
• 81 to 86                       =          Strong.
• 75 to 80                       =          Medium.
• 70 to 74                       =          Fair.
• Below 70                      =          weak.

4. Fibre maturity

Maturity of cotton fibre is determined by the degree thickening of the call walls resulting from deposition of cellulose layers successively inside the walls during the second half of the growth period. It is equal to the area of the cell walls expressed as ratio of area of the circle of same perimeter.

For the determination of Maturity Ratio (M) of cotton, a sample of separated and parallel cotton fibres is placed on a microscopic glass slide, irrigated with 18% NaoH, covered with another glass slide and placed under the microscope. Image of the sample is projected on the screen. Normal (N) and Dead (D) fibres are counted. Maturity Ratio (M) is calculated as follows:

Value of M above 85 % is considered as good and from 75% to 85% as average.

Another measure of Maturity is Maturity Coefficient which is calculated as follows:

Maturity coefficient = Mature (M) + 0.6 H (Half Mature) + 0.4 I (Immature)
                                                                        100      

• 0.85 and above is good.

• 0.75 is average.

• 0.65 is poor.

Immature cotton fibres create neppiness, high proportion of short fibres leading to increase in ends-down at the spinning stage, loss of yarn strength and appearance of white specks in dyed fabrics.

4. Highest Standard Warp Count (H.S.W.C)

As already mentioned H.S.W.C. which can be spun from a given variety of cotton is directly proportional to its spinning potential and is determined by its fibre properties. Three most important fibre properties are length, fineness and strength. According to Max Preysch(1) yarn strength can be predicted on the basis of relative effect of these fibre characteristics as follows:

Pervez Ali Ismaili (4) has listed 27 cotton varieties which are currently under production in Pakistan. These varieties have been classified on the basis of 2.5% S.L. into groups and H.S.W.C. spinnable has been indicated for each group respectively as follows:

Acknowledgment

Assistance received from Danish Arian, Shoaib Akber and Adnan Akram final year students of TIP is gratefully acknowledged.

References

1. Max Preysch, “New developments of Digital Fibro graphs and their applications in cotton spinning mills”, Spinlab AG, Zurich (Switzerland), October, 1973.
2. Joseph M. Yankey, “Lecture delivered at Uster Symposium 2003: From fibre to fabric” Pakistan Textile Journal, February, 2003.
3. Gregory, J. “Cotton yarn Structure,” Journal of Textile Institute, 41 T (1950)
4. Pervez Ali Ismaili, “Quality Survey of Pakistan Cotton, Summary of Cotton Fibre Tests Results,” Crop Season 2007 - 2008. Pakistan Central Cotton Committee.