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Testing of cotton for Fineness and Maturity - Instruments and Precautions

N.Balasubramanian

Retired Jt. Director (BTRA) and Consultant


Abstract> Different methods of testing cotton for fineness and maturity are reviewed and the precautions to be taken to get accurate results are discussed. Merits and limitations of the methods are indicated. Relationship between the values from different methods are discussed.
Fibre fineness and Maturity are important quality parameters of cotton. Fineness determines strength, evenness, spinning value of cotton. Maturity determines processing performance and yarn and fabric appearance. Immature fibres lead to sticking tendency, roller lapping, fibre breakages and nep generation. Dye uptake is poor and uneven. In this article an attempt is made to critically review various test methods for determine fineness and maturity, their relative merits and precautions to get accurate results.
Fibre fineness
Geometric Fineness
Cross section
Fibre cross section is prepared and viewed under microscope. Image is projected on a monitor and pen is used to trace the boundary. Initially polar perimeter is used to measure cross sectional area and curvimeter was used to determine perimeter`1. Hebert2 et al use a graphic digitizer to trace the outline of cross sections viewed under microscope. The data processed by a computer program is used to calculate cross sectional area, perimeter and major minor axes of equivalent ellipse. Merit - provides estimate of variability in cross sectional area, perimeter and circularity. Limitation – It is time consuming and laborious. Measurement of area and perimeter were later automated by the use of computer.
Image analysis system
Cross sectional measurements Xu3 et al developed a method for fully automated measurement of cross sectional shape by image processing technique. Area, roundness, ellipticity and cumulative angular function of cross sectional boundary are measured. Xu and Huang have4 developed special algorithms for cross sectional images which improve automation and accuracy of separating fibres contacting each other. Using a special software program with a macro, cross sectional area, perimeter, wall thickness area, lumen area and circularity have been measured. Another macro is used to get maturity parameters from this. Two softwares viz; FIAS and CSIRO are available to determine maturity and fineness from image analysis. Hequet5 et al has developed a set of reference cottons with preset maturity and fineness by using FIAS. However FIAS introduced immature fibre bias and tended to overestimate maturity by8- 9 % . FIAS was upgraded by Xu and Guv6 to overcome this problem. Higgerson7 et al found good agreement in the perimeter values by the two methods. But cell wall area and consequently maturity and fineness are 5 % lower with CSIRO software. FIAS software miscalculates lumen area in some cases.
Longitudinal Measurement
Image analysis for longitudinal views under microscope is also used to determine fineness and maturity. An adaptive threshold method helps to reduce errors from unfocussed fibres8. Look up table and double scanning algorithm improve efficiency of tracing and accuracy. Longitudinal data show a good correlation with fineness measurements by traditional methods. Boylston compared maturity measurement by longitudinal and cross section method. Maximum projection along the length of the fibre agree with cross sectional measurement while minimum projections do not. Cross sectional area provides true measure of maturity9.
Maturity
Circularity
Circularity θ, is given by the ratio of fibre wall thickness (shown as shaded area in Fig 1 ) by the area of the circle of same perimeter10.
Fig 1 :Circularity
Θ= (4 π A)/P2
Where A = cross sectional area of wall thickness P = Perimeter Θ = 0.577 when maturity factor is unity. Empirical relation between % mature and dead fibres and Θ is Θ = .00309(N-D) + 0.403 A value of N-D = 60 is taken as standard, Θs, standard circularity is given by Θs = 0.59
Maturity ratio can be estimated by the following equation
Maturity Ratio = Θ/Θs = (N-D)/200 + 0.7
Where N = % of fibres with Θ > .5 and D = % of fibres with Θ < .25 Degree of thickening
Fig 2 : Degree of thickening
Degree of thickening is given by (2 Π T)/P (Fig 2) Where T= wall thickness and P = Perimeter Xu4 et al found good correlation between degree of thickness and circularity which represent independent measures of maturity. Matic-Leigh and Cauthen found a curvilinear relationship between the two as shown in Fig 3 .
Fig 3 : Relation between circularity and Degree of Thickening
Fineness (H)
Fineness expressed as weight per unit length (H) is related to area of wall thickness (A) by H = ρ A Where ρ is density of cellulose in wall, which is 1.52 gms/cm3 Specific surface area (S)
HS which is standard hair weight per cm is given by H/M Specific surface area per gm S = P/H = 3.8 √HS /H = 3.8/√MH as Perimeter P = 3.8√HS
Maturity Factor
Matic-Leigh and Cauthen11 define maturity factor by (Fig 4) Fig 4 : Maturity Factor
Maturity Factor, Mf=100×(2t/D)
Where D = Effective circular diameter obtained by dividing perimeter P by π , t = Wall thickness of cross sectional area, L = Lumen diameter Mf = (100×2×π×t)/P
Wall thickness is given by ((D-L))/2
Wall thickness area is obtained by subtracting lumen area from cross sectional area11. Ratio of maximum to minimum fibre width along longitudinal direction is correlated with maturity12. Xu and Huang4 contend that from maximum, minimum, mean and Standard deviation of widths of each fibre maturity can be obtained by the equation
M= Wsd/Wmean Where Wmean = Mean width of fibre Wsd = SD of width Barker13 has developed a method for determining circularity of fibre cross section from longitudinal measurement. Fullness Fullness, which is considered a better estimate of maturity14, is given by P2= (A- Al )/(A max) = (4 π ( A-Al ))/P2 Where A = Overall area of cross section, Al = Area of lumen cross section Amax = Area of the circle with same perimeter P BR>Gravimetric method
Fibres are counted either as whole fibres or after being cut to a finite length (about 1.5 cm) under a microscope and weighed in a sensitive balance. About 500 to 1000 fibres have to be weighed to get accurate results Time consuming Maturity Caustic soda method (ASTM D1442- 06) Fibres irrigated in 18% caustic soda are examined under microscope. Mature fibres swell and assume a round cross section and immature fibres take a ribbon like shape. Based on the ratio of lumen width to wall thickness upon viewing under microscope of 400X magnification, fibres are classified as Mature, Half Mature and Immature based on the following criteria Mature (M) -(L/W < 1) Half Mature (HM) - ( 1 < L/W < 2) Immature (I) -( L/W > 2) where, W = wall thickness and L = Lumen width If M, IM and I are expressed as % then
Matrurity ratio Mr according to Shirley Institute is Mr = (M-I)/200 +0.7DR> Iyengar and Ahmad15 of C TRL uses a term Matrurity coefficient Mc to define maturity. This is given by Mc = (M + .6*H + .4 * I)
This method has the merit that it takes into account half mature fibres.
American MethodDR> Mature (N) – L/W < 2 Immature (D) – L/W => 2 This has the merit that it reduces subjectivity and is faster According to Raes and Verchrabge16 microscopic measurements on swollen fibres do not give a correct measure of biological maturity. Measurements made on unswollen fibres give the correct measure and should be used to calibrate instruments for measuring maturity like Arealometer and Shirley FMIT.DR> Polarised Microscope Method (ASTM D1442- 06)DR> Maturity is determined by interference colour of fibres under polarized microscope of 100 X with first order retardation plate. Immature – Blue, PurpleDR> Half mature - Green – Yellow with blue parts, Green, BlueDR> Mature - Orange or Yellow, GreenDR> Schwarz and Hotte17 have given a detailed account of how the test should be done and the precautions to be taken. Merits - Less subjective than caustic soda method as there is less confusion between mature and immature fibres Limitation - Measurement is likely to be influenced by total wall area. Lower maturity is obtained with cottons with lower maturityBR> Differential Dyeing
Fibres dyed in boiling dye bath containing .036g Diphenyl Fast Red 5 BL and .084g Chloarantine Fast green BLL in 120g of water. Red dye has specific affinity for thick walled mature fibres and green dye has affinity for thin walled immature fibres. As a result Mature fibre is dyed to redBR> Immature fibre is dyed to GreenV By visual comparison of the dyed samples against standards, maturity of the sample is estimated. Rebenfeld and Wu18 present a method for quantitative estimation of maturity to minimize subjective errors. Dyes from differentially dyed sample are extracted and analysed under spectrophotometer to determine Green/Red uptake. Green/Red uptake shows a highly significant correlation with maturity. Estimation of maturity by differential technique is based on the fact that immature fibres being less crystalline absorb more dye19. Merits – Quick method, useful for mill application Limitation – Not very accurate
Air Flow Methods
Airflow methods are based on resistance to flow of through a plug of fibres. The rate of airflow is proportional to the reciprocal of square of specific surface area of fibre. If M is maturity ratio, H is fibre weight per unit length then specific surface area S is given by
S = 3.8 ×1/(√M H)
Micronaire ASTM D1448-11)
Determines rate of flow of air through a known mass of fibres (3.24 grams ) 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, Mc which indicates rate of airflow is therefore proportional to 14.4/MH as shown earlier. Based on a study of 100 cottons Lord20 found the following relation between Micronaire and Fineness and maturity of cotton.
MH = 3.86 (Mc)2 + 18.16 Mc +13
H in turn is equal to M × HS where HS is intrinsic fineness or fibre weight. HS depends upon the perimeter of fibre crosssection.
Precaution :The cotton should be opened into a fluffy mass before being packed into the sample older. Trash and foreign matter should be removed by cleaning. Tests should be carried out in a laboratory where humidity and temperature are maintained at standard level. Calibration should be done with International cotton calibration standards with a low Micronaire and high Micronaire cotton. Micronaire is calibrated on the basis of testing of American upland cottons in terms of microgms/inch and therefore a correction may be needed while testing India and Egyptian cottons.
Maturity by variable volume
The volume of the sample holder is increased by .3927 cu in by use of an 0.5 inch spacer. Difference in Micronaire readings with and without spacer shows a significant correlation with fibre maturity. This is offered as rapid method of measuring maturity. Chapman and Staten21 found a correlation of .9 between maturity from Micronaire by spacer and causticaire. Sundaram and Iyengar22 who used a 3/8 inch spacer in place of 0.5 inch spacer, found a correlation of 0.8 between maturity by spacer and maturity by caustic soda swelling method. Seshan23 et al varied the porosity of plug by varying the weight of the sample instead of spacer. Difference in Micronaire value with 50 gr ad 40 gr sample showed a correlation of 0.86 with maturity coeffiecient by caustic soda method and the correlation is nearly same as spacer method. Balasubramanian24 showed that the difference in Micronaire value at 2 porosity levels (by spacer or varying the mass) is related to MH in the same way as Micronaire in the standard test. Correlation between Micronaire difference and maturity arises mainly because of this. Therefore, not much additional information about maturity than that provided by Micronaire at standard test, is obtained by tests at 2 porosity levels. Chapman’s work25 confirmed these findings. Correlation between micronaire and causticaire maturity is same or slightly better than that between difference in Micronaire with and without spacer and causticaire maturity.Chapman further found that the curvilinear correlation between Micronaire and causticaire maturity and fineness is slightly but consistently better than linear correlation.
Causticaire
After usual Micronaire test, sample is treated with 18 – 20 % caustic soda, with 11/2, % wetting agent, by volume. Caustic soda is removed by thorough washing and sample is dried to low moisture content and conditioned. Micronaire test is again carried out on fluffed sample using causticaire scale. Maturity index and fineness are calculated using a formula26. The following formula gives relation between causticaire fineness and maturity index.
C =1.185+0.00075 T 2-0.020 MI,
Where C = Causticare fineness and MI = Maturity index. The following formula gives relation between Causticaire Fineness and Micronaire value.
C = 2.20 M - 4.34 Where C= Causticaire fineness and M= Micronaire. Du Bois and Van De Riet27 found that all cottons with MI <76 lie on the left side of the line given by the above formula and all cottons with MI>76 lie on the right side of the line. The formula therefore needs correction. Further they found that comber waste has a lower maturity than comber sliver as determined by causticaire. This is because comber removes immature cluster of fibres. Instrument for automatic treatment of caustic soda, washing and drying is also available which reduces the time for testing28. A batch of samples of around 60 can be treated in about 50 min. A dryer has to be used to dry and recondition the samples which may take 1.5 hrs.
Lord29 found results from causticaire are slightly biased depending upon coarseness of fibre and has developed a correction to remove bias. Airflow through mercerized cotton depends primarily on H√M .Calibration of instrument on the basis of MH for unmercerised cotton and H√M for mercerised cotton enables separate estimation of M, H and Hs Micromat Fineness and Maturity Tester by SDL The instrument provides independent estimates of fineness and maturity of cotton from measurements of pressure drop through a plug of cotton at 2 levels of compression.
Arealometer
The instrument determines specific surface area, fineness and maturity of fibres. Resistance to air flow through a plug of fibres is measured at two pressure levels. The increase in resistance to air flow at low porosity (high compression) over that at high porosity is correlated to immaturity of cotton. It is proposed that immature fibres get flattened at high compression and therefore offer higher resistance to air flow30. While the instrument is useful for measuring of fineness and maturity on raw cotton, it does not give accurate results on mechanically processed material31. On the other hand, Balasubramanian32 et al found that Micronaire value by both conventional and HVI testing do not show any significant change from raw cotton to Lap, card sliver, comber sliver and finisher drawing sliver. Worley33 et al found the following relationship between Micronaire (Mic) and Arealometer
Mic = 17.146 -0.044268 Ar + 0.000032 Ar2 The instrument is not widely used like Micronaire.DR> Speedar
Speedar34 is a high speed airflow instrument to measure specific surface area. Instrument is much faster as there is no need to weigh the specimen. Using specimen between 5 to 10 gm, equipment automatically adjusts itself to weight of specimen. Merits - Much faster, not widely used.
Port Air
Port air is a portable instrument which uses 8 gm sample. It gives Micronaire value and Equivalent thickness (FT). Its main merit is that it can be used in the field. FT = 2000/A Where A = specific surface area
HVI (ASTM D5867-05)
HVI incorporates Micronaire or Micromat as one of the modules for determining fibre fineness. Testing procedure is same as in standard Micronaire except that that the sample need not be weighed to the exact level. Microcomputers are incorporated to test the specimen for a range of weights. For estimating maturity, Shirley HMT instrument or near infrared spectroscopy is used. Advanced Fibre Information System (AFIS)
AFIS employs scattered infra red light from a single fibre to determine fineness and maturity. An aeromechanical separator separates the fibres to individual level Shape of the fibre is viewed by sensor optically to determine fineness. Each fibre is viewed at different angles to determine the shape. Maturity ratio of fibre is given by the ratio of high degree (θ ≥ 0.5) and low degree (θ ≤ 0.25) wall thickening of fibres . Instrument also gives fibre fineness fraction which is % of fibres with fibre cross sectional area by number, less than 60µm2. Merits - Rapid, requires small sample about 500mg, no subjective errors, Limitation - requires calibration, costly
Near Infrared spectroscopy (NRI)
Maturity is determined by distinctly different absorbance spectra. It has 3 components viz; spectrophotometer, sample presentation system and chemometric software. Photons in the light that strike the fibre are scattered away from detector and do not fall on detector. Specific surface area of the fibre determines the intensity of light falling on the detector. NRI uses this principle for determining fineness and maturity of fibre35. A minimum of 5 gm of cotton fibre compressed in a cell is placed in quartz bottom cell and NRI spectrophotometer placed in it scans the sample. Software determines Miconaire, maturity ratio and fineness from the spectral analysis. As NRI is a secondary measurement, it should be calibrated with HVI or other classical instruments. Maturity and Fineness by bench type and portable NRI systems show good agreement but the former is more accurate. NRI measurements show good agreement with FMT in regard to maturity, fineness and Micronaire36. Optimum instrumental conditions for NRI is with the use of a glass-covered sampling port and increased instrumental gain, with high R 2 values, low residuals, and with < or = 12% outliers37.
Ramsey38 used NRI instrument with wavelengths 1.53, 1.97 and 2.32µm to determine fineness properties. High correlation (.89 and above) was found with Micronaire and Arealometer in regard to Micronaire value, causticaire maturity index, specific surface area and cross sectional area. Technical and instrumental time are less with NRI compared to other instruments except for Micronaire.

Merits Test is very rapid compared to FMT, Arealometer and Micronaire. It is easy to use, more accurate and non destructive.
Limitation
Equipment is costly and requires to be calibrated with calibration cottons
Fourier transform infrared spectroscopy39 can also be used to determine Micronaire and specific surface area of cotton with good accuracy. But it cannot be used for accurate measurement of maturity.
Thermogravimetric (TGA)
Weight loss in Thermogravimetric (TGA) analysis in the region 225 to 4250C is significantly related to maturity of cotton. Weight loss shows a significant negative correlation with HVI Micronaire, and maturity ratio by AFIS. This method can also used to determine maturity and Micronaire40.
Cottonscan
Cottonscan is an instrument by CSIRO for determining fibre fineness. Fineness is estimated from total length and weight of fibres cut to short lengths or snippets. Pre weighed fibre snippets are suspended in an aqueous medium and image analysis is used to measure total length.
Cottonscope
Cottonscope is an automated instrument for measuring fineness and maturity of cotton employing birefringence property of the fibre. The equipment is based on image analysis and polarized microscopy on small cut fibres (snippets) suspended in water medium. Mature fibres appear with a red hue and immature fibres appear translucent. From the total length and weight of snippets fineness is estimated. Standard test involves measurement on 2000 snippets in 25 - 35 sec. Merits - Rapid, accurate, requires small sample about 50 mg, free from subjective errors Limitation - Needs calibration, Costly, Maintenance of temperature and humidity at the level used in calibration is important to get accurate results of fineness.DR> Siromat
Siromat is an automated polarized microscope for measuring maturity. Use of digital colour cameras, Image analysis and powerful computer enables accurate measurement of interference colours transmitted by fibre, from which fibre maturity is estimated. The instrument is calibrated in terms of maturity ratio.
Meits - Rapid Limitation - Not accurate, shows poor correlation with manual results.BR> UAK-1+ by ITRU
The tester uses image analysis of longitudinal images of single fibres to determine fibre fineness and maturity through sophisticated algorithms. Other parameters displayed are circularity coefficient., fibre perimeter,area of wall, standard fibre fineness, causticare maturity %, degree of fibre flatness and micronaire value. Time taken for test is 20 - 40 sec.
Merits - Rapid Limitation - Expensive, accuracy not well established
Comparison of results from different instruments
Nair42 et al found close agreement in fineness values by HVI and gravimetric method. This is surprising as HVI fineness is influenced both by gravimetric fineness and maturity. AFIS and gravimetric fineness are close to each other in the medium range (150 – 160 mtex). AFIS gives higher linear density in finer cottons and lower linear density in coarser cottons. Agreement between maturity by AFIS and caustic soda swelling method is found to be poor. On the contrary, Xu4 et al found good correlation between Fineness by AFIS and perimeter by Image analysis as also between maturity by AFIS and circularity by image analysis. Kim43 et al concluded that AFIS does not give accurate measurement of maturity on developing immature fibres during growth. HVI, Cottonscope and Image analysis results correlate well for developing fibres. But for fully developed fibres, maturity by AFIS agrees with that of Image analysis, Maturity ratio by AFIS is greater than that from Cottonscope. Further, Cottonscope is more sensitive to maturity than AFIS. Fineness from Cottonscope is 20 % higher than AFIS. Rodgers44et al found Cottonscope to be more responsive to maturity of developing fibres during growth and is a useful aid in breeding program. Rodgers45 et al also found good correlation between measurements of fineness and maturity by Cottonscope and standard image analysis. Frydrych46 et al found good correlation between Cottonscan and AFIS but AFIS values are lower than that of Cottonscan. Siromat results show poor correlation with other methods of measuring maturity. Micronaire by UAK - 1 shows good correlation with digital microscope estimates. Micronaire by airflow methods have a lower correlation with digital microscope estimates46. Abidi and Hequet47 found good correlation between FTIR measurements and HVI Micronaire and specific surface area by AFIS. Long48 et al report that polarised light technique underestimates maturity with cottons having lower perimeter whereas crosssection method overestimates maturity for such cottons. However, Schwarz and Hotte35 have shown that wall thickness of mature low perimeter cotton is higher than that of immature and half mature high perimeter (coarse) cottons. So the criticism that polarised light technique cannot distinguish between mature low perimeter and immature large perimeter fibre is not justified. D. P. Thibodeaux and K. Rajasekaran49 conclude that image analysis method can be used as a reference technique for measuring maturity of cotton and for calibration of instruments just like ITMF robin test cottons. Image analysis can be used to predict Micronaire value. Hussain50 et al found that for a given variety of Indian cotton perimeter is nearly constant irrespective of maturity. So Micronaire value gives a good indication of maturity for a given variety. Webb and Burley26 found a better correlation between causticaire maturity and nep generation in processing, yarn appearance and strength than caustic soda maturity and Arealometer. Montalavo51 found that Micronaire is more sensitive to wall thickness than perimeter. This is because Micronaire varies as square of maturity and linearly with perimeter. However for high Micronaire cottons influence of perimeter is higher. Goynes52 found that differences in maturity of cotton grown at different locations is not shown by Image analysis and AFIS pro though all other methods show the difference clearly. When samples are hand combed to remove clumps of fibres a greater amount of fibre gets removed from immature sample than from mature sample. So sample preparation and the extent of combing to remove clumps of fibres affect maturity measurement.
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