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Dimensions of Fibres & Yarns Basic Concepts, Units & Unit Systems Special Quantities

Textile Testing MA Wilding. Dimensions of Fibres & Yarns Basic Concepts, Units & Unit Systems Special Quantities Measurement Methods. Suggested Reading Material. JE Booth "Principles of Textile Testing” (1983) Chapters 5 & 6 BP Saville “Physical Testing of Textiles” (1999)

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Dimensions of Fibres & Yarns Basic Concepts, Units & Unit Systems Special Quantities

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  1. Textile Testing MA Wilding Dimensions of Fibres & Yarns • Basic Concepts, Units & Unit Systems • Special Quantities • Measurement Methods ZUT-Testing: Dimensions of Fibres & Yarns

  2. Suggested Reading Material • JE Booth • "Principles of Textile Testing” (1983) • Chapters 5 & 6 • BP Saville • “Physical Testing of Textiles” (1999) • Chapter 3 ZUT-Testing: Dimensions of Fibres & Yarns

  3. Fibre Dimensions- Length • Fibre length is an extremely important parameter • It has far-reaching influences on yarn & fabric processing & product performance • Can divide fibres approximately into two groups: Most Natural Fibres Length varies from about 1  10 cm (“staple”) Most Man-made/Synthetic Fibres Can be any length: 0 cm  “infinity” (“continuous filament”) ZUT-Testing: Dimensions of Fibres & Yarns

  4. Fibre Length- Measurement • Continuous filament • Length measurement not generally an issue • Staple fibres • A given batch of raw fibre (eg cotton or wool) will contain many different fibre lengths • Most methods are therefore statistical in nature • They are usually extremely difficult & time-consuming to carry out; instruments include: • Various “comb” sorters (differ for cotton & wool) • Various photoelectric sorters (“Shirley” & “Fibrograph”) • “Sledge” sorter See, for example, Booth, Chapter 5 ZUT-Testing: Dimensions of Fibres & Yarns

  5. Fibre Dimensions– “Thickness” This is far from straightforward … • Fibres typically ~ 20 microns (0.02mm) across • Cross-section very difficult to determine eg wool ZUT-Testing: Dimensions of Fibres & Yarns

  6. Fibre Dimensions– “Thickness” Yarns are often difficult, too Cross-section ill-defined, possibly “hairy” etc ZUT-Testing: Dimensions of Fibres & Yarns

  7. Fibre Dimensions– “Thickness” Usually specified in terms of Linear Density (LD) or ‘fineness’ There are two types of system … • Direct • eg ‘tex’ system • Value is proportional to LD • increaseswith thickness • Indirect • eg Cotton Count • Value is inversely proportional to LD • decreaseswith thickness ZUT-Testing: Dimensions of Fibres & Yarns

  8. Fineness as a Measure of Cross-Section For a given bulk density (), linear density is proportional to fibre cross-sectional area Mass = V x density () = AL Linear density = Mass/Length(eg in tex) = AL/L = A ZUT-Testing: Dimensions of Fibres & Yarns

  9. Fineness as a Measure of Cross-Section For a given bulk density (), linear density is proportional to fibre cross-sectional area It works for any regular shape – eg triangular Mass = V x density () = AL Linear density = Mass/Length(eg in tex) = AL/L = A ZUT-Testing: Dimensions of Fibres & Yarns

  10. Fineness Units 1. Direct • tex =weight (g) of 1km • decitex = weight (g) of 10 km • millitex =weight (g) of 1000 km • kilotex =weight (kg) of 1 km • Example: • 10 cm fibre weighing 0.02 mg • = 0.00002/0.0001 g per km • = 0.2 tex = 2 dtex = 200 mtex ZUT-Testing: Dimensions of Fibres & Yarns

  11. Fineness Units • 2. Indirect • Count • eg Cotton Count • = No. of 840-yard "hanks" for 1 lb weight Example: Suppose a standard-length hank of yarn weighs 1/30 lb Therefore need 30 hanks for 1 lb Therefore yarn = 30's Cotton Count ZUT-Testing: Dimensions of Fibres & Yarns

  12. Fineness - Technological importance Fibre fineness impacts on a wide range of other properties - here are just some examples … • Stiffness & handle (ie drape etc) • Torsional rigidity (ie how hard to twist) - square power dependence on fineness • Light reflection- fine fibres soft sheen - coarse fibres harsh glitter • Absorption of liquids &fibre cohesion- related to surface area • Yarn uniformity- finer fibres give more even yarn ZUT-Testing: Dimensions of Fibres & Yarns

  13. Fineness - Importance in testing Knowledge of effective thickness is usually essential for results to be meaningful (particularly when comparing different fibre types) Example: • Suppose a given cotton fibre can withstand a greater load (tension) than a given nylon fibre • Is cotton inherently stronger than nylon per se? • We can’t immediately tell, because thick fibres are stronger than thin fibres of the same type Therefore, in most cases, results must be‘normalised’ ZUT-Testing: Dimensions of Fibres & Yarns

  14. Fineness – Methods of determination • There is a wide range of methods available • These include gravimetric and non-gravimetric methods • Some are direct; some are indirect • Some are extremely complicated & time-consuming • Some are straightforward and quick • The most appropriate choice depends on many factors • An important one: are we testing fibre or yarn? • Several precautions may need to be taken – such as preconditioning sample in the lab (discussed later) ZUT-Testing: Dimensions of Fibres & Yarns

  15. Fineness Methods - Yarns Simplest: weigh a known length on a balance • Typically, 100 metres (ie 0.1 km) of yarn is wound off using a “wrap wheel” - Usually motorised, has a diameter of 1 metre, and a revolution counter to make length determination easy and accurate • Suppose the piece of yarn weighs W grams • Its linear density must therefore be W/0.1 tex (=10W tex) • Shorter lengths (eg a few centimetres) may be measured using a metre rule and a sensitive electronic balance • The count equivalent can be calculated using the appropriate conversion formula; for example, for English Cotton Count (NE): NE = 590.5/tex ZUT-Testing: Dimensions of Fibres & Yarns

  16. Fineness Methods - Fibres • Range of methods available • Generally complicated • Appropriate choice depends on factors such as: What physical form the fibre is in - Bale? Sliver? Yarn? Fabric? Is a single-fibre value required? Or some form of average for a bulk of fibres? ZUT-Testing: Dimensions of Fibres & Yarns

  17. Fineness Methods - Fibres For uniform synthetic fibres in continuous-filament yarns … • May estimate fibre fineness from overall yarn tex • Need the number of filaments in the cross-section • May be provided by yarn-producer • If not, filaments need to be counted • Microscope may be needed – tedious and slow ZUT-Testing: Dimensions of Fibres & Yarns

  18. Fineness Methods - Fibres For single fibres Single fibres are usually too small to be weighed reliably on a balance. Two examples of alternative methods: • 1. By microscopy • Assumes fineness proportional to cross-sectional area • Need to know the bulk density • may get approximate value from literature if know fibre type • Image of fibre cross-section projected & measured • Fibre area calculated from magnification • Fineness calculated ZUT-Testing: Dimensions of Fibres & Yarns

  19. Fineness Methods - Fibres For single fibres • 2. Using a “Vibrascope” • Principle of a vibrating stretched string • - as in a musical instrument • Frequency (“pitch”) depends on tension, length and linear density • Fibre hung between knife-edges • Small known weight attached to lower end • Made to vibrate at fixed frequency using electrostatic plates • Length adjusted until get resonance • Fineness (usually in dtex) read off dial • Relatively straightforward and moderately quick • May not be highly accurate ZUT-Testing: Dimensions of Fibres & Yarns

  20. Fineness Methods - Fibres Illustration of the Vibrascope principle ZUT-Testing: Dimensions of Fibres & Yarns

  21. Fineness Methods - Fibres For bulk fibres (eg cotton or wool staple) • Air-flow methods usually best • Quick • Give an average value Typical airflow system (schematic) Air Weighted insert with perforated base Air Flow meter & suction pump From pressure meter Chamber packed with wad of fibres of standard weight ZUT-Testing: Dimensions of Fibres & Yarns

  22. L P d Fineness Methods – Air-flow Consider an idealised, cylindrical fibre … S is called the “specific surface” of the fibre Hence … The finer the fibre the greater its specific surface ZUT-Testing: Dimensions of Fibres & Yarns

  23. L d L 2d Fineness Methods – Air-flow Compare two equal-volume batches of fibres with different diameters, and hence fineness 25 fibres of diameter d Occupy the same total volume as 5 fibres of diameter 2d … but have twice thesurface area • Air-flow is restricted by drag over the fibre surface • The finer fibres have twice the resistance to airflow ZUT-Testing: Dimensions of Fibres & Yarns

  24. d 2d Fineness Methods – Air-flow Resistance to airflow as a measure of fineness Fine fibres high resistance Coarse fibres lower resistance ZUT-Testing: Dimensions of Fibres & Yarns

  25. Fineness Methods – Air-flow Ideally, for a mass of uniform cylindrical fibres … Fineness (in tex) is: proportional to fibre cross-sectional area proportional to fibre diameter squared Specific surface is: inversely proportional to diameter inversely proportional to square root of tex Pressurefor given air-flow is: proportional to specific surface  inversely proportional to square root of tex In practice may measure either the pressure for a given air-flow or the air-flow for a given pressure ZUT-Testing: Dimensions of Fibres & Yarns

  26. Fineness Methods – Air-flow • Where the fibres are not uniform and/or not cylindrical the results of air-flow measurements must be treated with a degree of caution • The overall result will be some form of average for the batch • This may not be the simple arithmetic mean For further details and practical systems for measuring fibre fineness see, for example, Booth, Chapter 5 ZUT-Testing: Dimensions of Fibres & Yarns

  27. Maturity (of Cotton Fibres) • ‘Maturity’ is a dimensional characteristic of natural cellulose fibres – especially cotton • It indicates how well-developed the fibres are at harvest • It is extremely important in terms of down-stream processing and yarn/fabric quality • Maturity and fineness are interrelated, although not in a simple way ZUT-Testing: Dimensions of Fibres & Yarns

  28. Longitudinal View Cross-sectional View Maturity (of Cotton Fibres) Structure and growth of cotton fibres - in brief “Convolution” ZUT-Testing: Dimensions of Fibres & Yarns

  29. Maturity (of Cotton Fibres) Structure and growth of cotton fibres - in brief • Cotton fibres begin (after the flower dies) as thin-walled hollow cylinders • They first lengthen without changing in diameter; takes around 20 days • They then mature; takes about another 30 days • In maturation, cellulose is deposited on the inside of the cylinder – the ‘secondary wall’ • The hole down the centre (the ‘lumen’) becomes progressively smaller • The outer diameter of the fibre remains virtually unchanged ZUT-Testing: Dimensions of Fibres & Yarns

  30. lumen Maturity (of Cotton Fibres) Structure and growth of cotton fibres - in brief A Cotton fibre grows first in length … Emerging fibre Seed surface … then in wall-thickness Maturity … is related to the cell wall thickness in comparison to the lumen diameter ZUT-Testing: Dimensions of Fibres & Yarns

  31. Maturity (of Cotton Fibres) Degree of thickening () Cellulose Ao Lumen A Idealised cross-section of a cotton fibre A = area occupied by cellulose Ao = total area (including lumen)  = A/Ao ZUT-Testing: Dimensions of Fibres & Yarns

  32. Maturity (of Cotton Fibres) The fibres collapse when they dry out on harvesting to give a “kidney-bean” shape P P Dry On the plant Cross-sectional area changes, but perimeter remains approximately constant ZUT-Testing: Dimensions of Fibres & Yarns

  33. Mature/Over-mature • Immature • “Dead” Maturity (of Cotton Fibres) Technological importance Maturity largely determines whether a batch of cotton can be spun into a good yarn - or indeed intoanyyarn Some maturity Variations Cause “neps” – clumps of matted fibres ZUT-Testing: Dimensions of Fibres & Yarns

  34. Maturity (of Cotton Fibres) The fibre perimeter is related to both fineness and maturity, and is involved in airflow methods for measuring these properties – see Booth, Chapter 5 P1 Air flow restricted through fine fibres because large specific surface P2>P1 Air flows more easily through coarse fibres because smaller specific surface ZUT-Testing: Dimensions of Fibres & Yarns

  35. Maturity (of Cotton Fibres) Maturity Count and Maturity Ratio Suppose a large sample of cotton fibres is selected at random and treated with caustic soda. The mature fibres will swell back to cylinders, and appear rod-like. The immature ones will not swell but will appear ribbon-like Now count under a microscope: • The total number of fibres (T) • The number of mature (ie “normal”) fibres (n) • The number of “dead” fibres (d) • - ie those with wall thickness less than 0.2 lumen • The number of immature (but not dead) fibres (m) is then given by: • m = T-n-d ZUT-Testing: Dimensions of Fibres & Yarns

  36. Maturity (of Cotton Fibres) Maturity Count and Maturity Ratio Total number of fibres = T Number of mature fibres = n Number of dead fibres = d Number of immature fibres = m = T-n-d Now let N = 100 x n/T = % “normal” fibres and D = 100 x d/T = % dead fibres Two quantities defined: • Maturity Count = 100 x m/T = % immature • Maturity Ratio (M) = 0.7 + (N-D)/200 ZUT-Testing: Dimensions of Fibres & Yarns

  37. Maturity (of Cotton Fibres) Maturity Count and Maturity Ratio • M = 0.7 + (N-D)/200 • Gives M ~ 1 for a high-grade Egyptian cotton • M can be greater than 1 • M less than ~ 0.8 is not good • M less than 0.7 is very rare ZUT-Testing: Dimensions of Fibres & Yarns

  38. Maturity (of Cotton Fibres) Summary of measurement methods • Direct Method (counting fibres) • tediously slow! • Indirect Methods • polarised light microscopy (?) • differential dyeing (slow) • air-flow (best) ZUT-Testing: Dimensions of Fibres & Yarns

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