Quality of cotton accounts for 50- 70 % of yarn and fabric quality. Longer and finer fibres result in higher yarn strength and enable spinning of a finer count. At the same time, finer fibres are more prone to breakages and nep generation and call for gentler treatment. Maturity of cotton influnces nep generation in processing and consequently determines the appearance of yarn and fabric. So while assessing fineness, it should be done independent of maturity and this has posed the gratest difficulty in developing suitable testing methods. Stronger the fibre stronger will be the yarn. The same holds good for elongation. Presence of trash, seed coats and other foreign matter in large amounts in the cotton increases waste losses and adversely affects appearance of yarn and grey fabric. Seed coats in the fabric get flattened during calendering and lead to ugly blotches that downgrade the fabric. Test methods used for measuring cotton have undergone several improvements over the years. The emphasis has been more on reducing the testing time and High Volume Testing instruments, popularly known as HVI have been developed. Though rapid, these instuments suffer from some limitations. Conventional and HVI instruments used for testing cottons are breifly reviewed here. Limitations of HVI are outlined.
Length
Balls Sorter-Determines frequency length distribution of fibres from which mean,CV are etimated. This is a classical method, time consuming and is not commonly used
Baer Sorter Determines cumulative fibre length distribution. Effective length, Mean length and % short fibres are important parameters determined. Merits-1.Effective length is close to Grader's staple length 2.Provides accurate estimate of short fibre content Limitations. 1. Time consuming(2hrs per sample 2. Calls for considerable operator skill in sampling and preparing the diagram
Fibrograph - Fibres are randomly clamped at any point along their length on a comb, the beard thus obtained optically scanned from which 2.5%, 50% span length, uniformity ratio and short fibre% are determined. Merits 1. Simulates beard formed by fibres held by back or front roller nip of a drafting system 2. Very rapid(abou 15 min per sample 3. Does not depend much upon operator skill
Effective Length by Baer Sorter = 4 + 2.5% Span length in mm
Fibre fineness
Gravimetric method - Fibres are counted either as whole fibres or after being cut to a finite length and weighed in a sensitive balance. Time consuming
Air Flow Methods
Micronaire Determines rate of flow of air through a known mass of fibres packed in a cylinder of known dimensions under constant pressure. This is expressed in mirograms/inch. Micronaire value is dependent upon not only liner density but also on maturity of fibres. Micronaire M c = K/ MH where M = Maturity Ratio H = Fibre weight/unit length and K=constant
Micromat Fineness and Maturity Tester by SDL- Provides independent estimates of fineness and maturity of cotton
Arealometer-Determines specific surface area, fineness and maturity of fibres
Maturity Coefficient
Caustic soda method - Fibres irrigated in 18% caustic soda are examined under microscope. Based on the ratio of lumen width to wall thickness, fibres are classified as Mature, Half Mature and Immature
Mature (M) - L/W <1
Half Mature (HM) - 1 < L/W < 2
Immature (I) - L/W > 2
M c = (M + .6*H + .4 * I)/100
American Method
Mature (N) - L/W < 2
Immature (D) - L/W >or= 2
Maturity Ratio M r = (N -D)/200 + 0.7
Polarised Microscope Method
Immature - Blue or purple
Mature - Yellow or Green
Differntial Dyeing
Fibres dyed in boiling dyebath containing .036g Diphenyl Fast Red 5BL and .084g Chloarantine Fast green in 120g of water
Mature - dyed to red
Immature Dyed to Green
Bundle Strength
Pressley - Operates on constant rate of traverse. Rate of loading increases with traverse and there is risk of overshootin and overestimation of strength
Pressley Index(PI) = Breaking strength at "0" gauge length in pounds/weight in mg
Pressley Ratio(PR) = Breaking strength at 3mm gauge length in lbs/weight in mgs
Tenacity at"0" gauge length, g/tex = 5.36 * PI
Tenacity at 3mm gauge length,g/tex = 6.8 * PR
Tensile strength in 1000psi = 10.81 * PI
Breaking tenacity, g/tex = Breaking strength in 1000psi *0.496
Stelometer- operates on constant rate of loading. Generally preferred
Trash content
Shirley Analyser is the standard equipment used for measrement of trash content. The equipment determines
primary tash consisting of coarse type of trash including seed coats
Finer trash
Micro dust larger than 150 microns
Microdust between 50 and 150 microns. About 200 gms of cotton are used and the test takes about 1hr
Denkendorf Mirodust and Trash Analyser enables quick estimation of trash from 25- 30 gms in 12-15min
HVI Testing
Conventional testing involves considerable time and results are not available in time for cotton selection and mixing preparation. High volume Instument(HVI) have speeded up testing substantially and testing of a cotton is completed in about 2-4 minutes. Bale wise testing of cootons is possible and bales with substandard characteristics can be weeded out.HVI determines Upper half mean length or 2.5 and 50% span lengths, uniformity index or ratio, Micronaire and bundle strength. Optional attachments for estimating trash level by optical scanning and colour of cotton and moisture level are available. In Micronaire test HVI requires an approximate weighment of cotton. As an option Macromat which measures both fineness and maturity can be incorporated in HVI. HVI, though rapid, has certain limitations in terms of accuracy of results. These include in case of
Fibre Length
Fibre breakages occur during comb preparation on HVI as this is done at high speed. Such breakages are more with harsh, entangled cottons. This results in lower estimates of fibre length. Breakages are also higher with card sliver compared to comber sliver.Over estimation of fibre fractionation is therefore found in HVI.
Bundle Strength
HVI works on constant rate of extension while stelometer works on constant rate of loading. Further, time for break is much lower in HVI to speed up testing.
A randomly clamped beard is used is used in stelometer and fibres are combed to remove short fibres. So all fibres are gripped by both jaws. In HVI, fibre bundle is not combed to remove short fibres before clamping. All fibres are therefore not clamped by both jaws.
The mass of broken specimen is weighed in a balance in stelometer while in HVI indirect estimate of mass (by optical means) is obtained. The state of crimp in the fibre affects the estimated mass.
As a result, significant differences are found in bundle strength values by HVI and conventional testing. HVI does not give true results for material at different stages of spinning.
Uster-AFIS
Fibre samles are opened into individual fibres by an opening roller and passed on to different modules for measuring
fibre Length
Fibre Fineness
Maturity
Nep content
Nep measurement is the most useful part of this instument. A computer distinguishes individual fibres from neps. Seed coat neps are separately assessed. A good correlation is claimed between neps measured with AFIS and neps in yarn by Uster imperfection tester. A second sensor measures trash and dust.
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