Home > Ann Special > Story

Post-MFA challenges in cotton fibre quality assessment

S Sreenivasan and R P Nachane, CIRCOT

S Sreenivasan

Indian cotton as an industrial raw material for a flourishing textile industry is at the cross-roads today. On the one hand it has to relentlessly face the onslaught from foreign cottons imported into the country that are contaminant-free, clean and of better quality and on the other the increasing use of polyester in industrial and apparel sectors is slowly eroding the very base of this eco-friendly natural fibre. In the post-MFA period with the dismantling of the quota system and increasing competition, the challenge to cotton is perceived to be much more than ever before. With increasing availability of manmade fibres at competitive prices, be it polyester or regenerated cellulosic fibres in different forms, the writing is well on the wall. It is high time that all concerned in the cotton chain take note of this and act quickly.

Technology Mission on Cotton, intended to strengthen the raw material base in the country, is a step in the right direction, that needs coordination and concerted efforts right from developmental to extension, processing and marketing agencies. The position of cotton in the fibre basket is no different in the international scenario as well. Cotton share in the world textile market is expected to slide from the current 42 per cent to 40 per cent by 2005 and to 38 per cent by 2010. According to International Cotton Advisory Committee (ICAC), during the last decade of the previous century, a seven per cent market share was lost by cotton. Out of this, lack of price competitivenes had contributed to the decline by 1-2 per cent only. On the other hand, quality deficiencies and lack of diversity in end uses had significantly eroded the cotton market. Another study by ICAC goes to show that the consumer is ready to substitute cotton with other fibres. Out of one dollar spent on textiles by a consumer, on an average only 23 per cent of it goes to cotton while the rest is on the other fibres. Another trend that is discernible is that in areas where cotton used to be “all in all” 10 years back, currently only 40 per cent component remains cotton, the rest 60 per cent coming from manmade fibres and filaments. This situation is noted even in functional textiles where absorption characteristics are of prime concern. Fashion dictates are changing consumer appeal against the comfort requirement. All the above clearly bring out the fact that unless the quality of both raw material and end product improves and alternate uses for cotton are found encroaching upon newer areas, it may be difficult to hold on to cotton share in the world market.

Cotton is grown to be spun. If this objective is to be realised notwithstanding the erosion of base by manmade fibres, the raw material has to fully satisfy the spinner’s needs. The conventional and the most versatile ring spinning machine produces on an average about 20-30 metres of yarn per minute. With the development of open-end spinning systems, the production rate dramatically climbed upto 100-120 metres/min. The latest innovation viz, airjet spinning/vortex spinning, that remains a novelty for Indian industry, produces yarns at a phenomenal speed in excess of 350 metres/min. Apart from this, today we have mechanical bale handling systems to keep contamination at bay and also to enhance productivity while processing. To match increased delivery rates, the cotton fibres are cleaned, pre-processed, parallelised and drafted at very high speeds making it unavoidable for the fibres to be subjected to high pulls and pushes during preliminary mechanical processing. This makes huge demands on the fibre. The modern systems are highly influenced by the cleanliness of the fibres. As a result, the priority for conventional fibre attributes viz, fibre length, length uniformity, micronaire, strength and maturity has changed and apart from this knowledge of additional parameters that affect the yarn quality and appearance such as short fibre content, elongation, fibre neps, seed coat fragments in lint, trash content, colour, fibre friction, cleanability, dyeability has become crucial. Some of these parameters are difficult to measure while assessment of some others is tedious. There is an urgent need for technologists to devise methods of measurement for these “not frequently measured attributes” in a quick, accurate and reproducible manner to help the spinner in his arduous task.

Short Fibre Content: Presence of high amounts of short fibres affects the yarn quality, particularly, its hairiness, uniformity and imperfections. In modern day processing, it is often stated that excessive mechanical handling of cotton during cleaning prior to and after ginning increases the short fibre content. Apart from genetic and agricultural factors, shorter maturation periods and low maturity and harsher ginning also contribute to short fibres. Short fibre content is measured as a percentage by weight of those fibres having length less than 12.7 mm in terms of the total mass of the sample. This method is currently employed in HVI and AFIS. In the conventional array/Baer Sorter method, it is also measured as percentage by number of fibres having length less than half the effective length with reference to total number of fibres. The method currently adopted in HVI and AFIS does not give the true picture when cottons in the entire range right from the shortest (Bengal Desi, coarse short arboreums) to the longest (Suvin, Egyptian Giza) are examined. In this scale, Indian cottons in the short to medium staple category (below 24 mm) are shown to possess short fibres as high as 2/3 to 1/2 of the total weight of the sample. This is particularly unacceptable as rotor yarns could be spun from “so called short fibres”. A recent study at CIRCOT suggests that weight of fibres having length less than one third of 2.5 per cent span length (by HVI) with respect to the total weight of the sample can be safely considered as the short fibre content in cotton irrespective of its staple grade. It would suffice here to say that it is not only highly necessary to assess correctly the short fibres in cotton but also control it to the extent possible. This is particularly crucial because to produce good quality uniform yarns, normally, the short fibres are combed off and this process adds to the cost of manufacture.

Fibre Extension: It is often noted that more than fibre strength it is the fibre breaking extension and its distribution in an ensemble that predominantly decides the yarn strength and its extensibility. Therefore accurate and precise assessment of fibre extension is crucial for a spinner. Although conventional bundle testing instruments like Stelometer and Pressley could measure this parameter to a certain extent of accuracy and reproducibility, they cannot be used for the reasons that they employ parallelised bundle of fibres and the slow nature of testing. The High Volume Instrument although is able to measure fibre extension employing a beard, its reproducibility and precision are yet to be proved. It is essential that a method be devised for quick and accurate estimation of fibre extensibility in order that the raw material is assessed as comprehensively as possible.

Trash Content: Estimation of trash content is very crucial to decide the grade and thereby the commercial value of cotton lint. Conventional gravimetric method of separating the trash from the lint and estimating the same as percentage of total uncleaned lint is although very precise, it involves time and hard work. The number of samples that could be estimated is also limited.

HVI employs imaging and analysis of trash particles in a fibre bed to estimate trash content. Earlier studies at CIRCOT had clearly indicated the lack of correlation between the trash count/area estimated by HVI with the gravimetric measure. The method of sample presentation before light, identification of trash, its size and density, its colour as compared to background are all crucial factors that affect the HVI measurement of trash. Here again there is an urgent need to develop an instrumental method of trash analysis that is quick and accurate so that the drudgery involved in the conventional method could be avoided. Since this parameter has direct bearing on the commercial value of cotton, the method has to be beyond scrutiny.

Colour: Colour is a crucial parameter that affects the appearance. HVI measures colour in terms of reflectence (Rd) and yellowness (+b). The colour grades developed for American cottons and incorporated in the machine are not straight away applicable to Indian cottons. The basic colour parameters seem to depend on the agroclimatic conditions and whiteness/lustre of cotton apart from its genetic build-up. Although the basic measurements by HVI for colour are beyond doubt, the colour grade for Indian cottons can be computed only after developing suitable colour scale. No serious attempts so far have been made to develop such a scale for Indian cottons. It is high time that this exercise is taken up in right earnest.

Fibre friction: It is a known fact that fibre to fibre friction and that between fibre to metal play a crucial role in the processing and formation of yarn. Inter-fibre friction is also found to affect the yarn quality and its performance in chemical processing. A reliable, quick and accurate estimation of fibre friction has so far not been possible. Although voluminous data have been generated by several workers in the past, the method of measurement and the interpretation of results obtained have been far from satisfactory. Even the factors that influence the measurement and the value of inter-fibre friction have not been clearly identified.

Rigorous efforts by CIRCOT and several others have clearly indicated the role of micronaire value (intrinsic fineness and maturity put together) in deciding the frictional properties of cotton fibres. Although CIRCOT has devised a method for measuring fibre friction by employing a pad of fibres, this needs to be quickened. Certain intriguing results obtained on yarns spun by otherwise two similar cotton fibres are often ascribed to frictional properties of fibres and those between fibre and metal parts. An accurate and faster estimation is the need of the hour.