Textile Testing MA Wilding Dimensions of Fibres Yarns

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

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 2

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 3

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 & timeconsuming 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 4

Fibre Dimensions – “Thickness” This is far from straightforward … • Fibres typically ~ 20 microns (0. 02 mm) across • Cross-section very difficult to determine eg wool ZUT-Testing: Dimensions of Fibres & Yarns 5

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

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 increases with thickness • Indirect eg Cotton Count Value is inversely proportional to LD decreases with thickness ZUT-Testing: Dimensions of Fibres & Yarns 7

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 8

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 9

Fineness Units 1. Direct · tex = weight (g) of 1 km · 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 10

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 11

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 12

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 13

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 14

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 (=10 W 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 15

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 16

Fineness Methods - Fibres For uniform synthetic fibres in continuousfilament 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 17

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 18

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 19

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

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 From pressure meter Flow meter & suction pump Chamber packed with wad of fibres of standard weight ZUT-Testing: Dimensions of Fibres & Yarns 21

Fineness Methods – Air-flow Consider an idealised, cylindrical fibre … L P d 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 22

Fineness Methods – Air-flow Compare two equal-volume batches of fibres with different diameters, and hence fineness L 25 fibres of diameter d d Occupy the same total volume as 5 fibres of diameter 2 d … but have twice the surface area L 2 d • 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 23

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 d 2 d 24

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 Pressure for 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 25

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 26

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 27

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

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 29

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

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

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

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

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 P 1 Air flow restricted through fine fibres because large specific surface P 2>P 1 Air flows more easily through coarse fibres because smaller specific surface ZUT-Testing: Dimensions of Fibres & Yarns 34

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: ZUT-Testing: Dimensions of Fibres & Yarns m = T-n-d 35

Maturity (of Cotton Fibres) Maturity Count and Maturity Ratio Total number of fibres Number of mature fibres Number of dead fibres =T =n =d Number of immature fibres = m = T-n-d Now let and N = 100 x n/T = % “normal” fibres 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 36

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 37

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 38