With multiplicity of quantitative descriptions for fibre strength, the comparison of world cottons would be a hazardous task as it might lead to wrong conclusions. Particularly significant are the effects of the last two tenacity descriptions. While most institutions in India use the ICC tenacity, many foreign cottons, especially those from the US and Australia, are labelled with HVI tenacity values which are much higher. Such comparisons have led to the questionable conclusion that Indian cottons are weaker their foreign counterparts in each staple length category.

There is need for a comparative study of the fibre tenacity of cottons popular in the world market, by measurement on the same machine under identical conditions. Institutions such as CIRCOT should take up such a study and find out conclusively whether Indian cottons are really inferior in strength or not. This information will also help cotton breeders in their effort to evolve high quality cotton for the textile industry.

The world-renowned fibre-testing machine, HVI, manufactured originally by Zellweger Uster, US, is now being increasingly used by the Indian cotton trade and the textile industry. While about 150 imported units are in use in textile mills and other organisations, two Indian companies have also started manufacturing similar machines.

When HVI first appeared in Indian laboratories in the early 1980s, the industry and trade were quite happy, as the machine was much faster and more accurate than its predecessors — the fibrograph, the micronaire apparatus and the Stelometer. As a result, the number of HVI users rose rapidly to about 120 by 1995. These machines were initially calibrated with the ICC Standards of the United States Department of Agriculture (USDA). Everything went well till 1996-97. And then the ICC Standards were stopped and a new set of calibration cottons, HVI calibration, introduced. The fibre parameters obtainable with the use of the new calibration cottons are, however, different. As a result of this changeover, two issues have surfaced on the Indian scene.

The production and supply of ICC standards were stopped in 1996, and `HVI calibration cottons' introduced instead. When HVI calibration cottons are used, the machine would give the following parameters: i) upper half mean length (UHML); ii) uniformity index (per cent); iii) micronaire; and iv) HVI tenacity.

When the HVI machine is set (calibrated) with this range of calibration cottons, the system said to be working in the `HVI calibration mode' or the `HVI mode'. These parameters, with the exception of micronaire, are by definition different from corresponding measures given by the same machine when operated in the ICC mode. The differences in the two sets of measures are as follows.

Fibre length: The 2.5 per cent span length is the distance up to which 2.5 per cent of the fibres caught in the sample holding comb are found to extend. On the other hand, UHML is the average length by number of the longer half (50 per cent) of the fibres distributed by weight.

Though different by definition, the two length measures are nearly equal. Hence, there is no difficulty in switching over from the old set of parameters to the new. The trade and industry will readily accept the change. These two indices of fibre length for a few varieties of cotton are given in the Table, from which it will be apparent that the values are quite close to each other.

Fibre length variation: The length variability index in the ICC mode is the uniformity ratio, of span length at 50 per cent and 2.5 per cent density levels.

UR per cent = (L50 / L2.5) 100

The UR per cent for all varieties of cotton lies in the 40-54 range.

The uniformity index (UI) determined in the HVI mode is the ratio of the mean length and the UHML:

UI per cent = (Mean length/UHML) 100

As mean length is very close to, though smaller than, UHML, the ratio is much larger than the UR per cent. Generally, the UI per cent is in the 70-98 range, the theoretical limit being 100 when the fibres are all of constant length.

The two indices of variability measured in the ICC and HVI modes of operation for a few varieties of cotton are also included in the Table, from which it is apparent that: i) the values are widely different; and ii) there is poor correspondence between the two measures of fibre length variation. There is bound to be a good deal of confusion when the new measures are adopted.

Tenacity: The fibre tenacity in the ICC mode is available at the Stelo or the Pressley level. The latter is about 18 per cent higher.

These are adjusted to agree with the tenacity obtained with conventional mechanical machines known as the Stelometer and the Pressley strength tester — the latter is known to be about 25 per cent higher than the other because the Pressley machine works at 10 times the speed of the Stelometer and the test result is prone to give an overshoot error while recording the breaking load.

In the HVI mode, the tenacity given by the machine is much higher than the two measures obtained in the ICC mode as shown by data in the Table. The HVI tenacity is about 25-30 per cent higher than even the tenacity obtained in the ICC mode.

The correlation between the two is, however, quite high unlike in the case of the length variability indices — UR per cent and UI per cent.

The discussion assumes significance in two contexts — adoption of new quality parameters by the cotton trade and industry in India, and quality comparison of Indian and foreign cottons.

Adoption of the new HVI parameters

The new quality measures — such as UHML, UI and HVI tenacity in place of 2.5 per cent span length, and uniformity ratio and tenacity at Pressley/Stelo levels — will upset an individual's perception of the quality level of cottons. It took over 10 years for the Indian trade and industry to understand and adopt the 2.5 per cent span length in the replacement of the staple length and the effective length. In fact, even today there are vestiges in the Indian textile industry preferring to use the effective length for machine adjustments in the spinning lines.

At a time when the 2.5 per cent span length is just being understood, another change may appear difficult to reconcile with.

But then, is there a choice? Today, both the sets of parameters are visible in test reports in circulation in the country, causing confusion in the process. How long can one put up with this situation?

HVI machines are being increasingly used. Besides the Zellweger machines, two Indian manufacturers — Premier Polytronics Ltd and Statex Engineering (P) Ltd of Coimbatore — have supplied over 150 machines. The TMC is supporting installation of HVI machines in major market yards and ginneries.

The calibration cotton now available from USDA required for all these machines can give only the new set of parameters, namely, UHML, UI per cent and the so-called HVI tenacity. Of course, CIRCOT calibration cottons, now popular among textile mills in India, are marked with both old and new sets of parameters. But this can only be seen as a temporary arrangement suitable for a transitional period. Sooner or later the Indian trade and industry will have to accept the new quality indices. It is pointless to wait, as the rest of the world has either accepted the change or is on the way.

(The authors are advisor and consultant, respectively, with Technology Mission on Cotton.)

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