紡織品檢驗(Textile-Testing)授課課件2

1、Dimensions of Fibres & Yarns Basic Concepts, Units & Unit Systems Special Quantities Measurement MethodsTextile TestingMA Wilding1ZUT-Testing: Dimensions of Fibres & YarnsDimensions of Fibres & YarnsTeSuggested Reading Material JE Booth Principles of Textile Testing” (1983)Chapters 5 & 6 BP Saville“

2、Physical Testing of Textiles” (1999)Chapter 32ZUT-Testing: Dimensions of Fibres & YarnsSuggested Reading Material JE Most Natural Fibres Length varies from about 1 10 cm(“staple”) Most Man-made/Synthetic FibresCan be any length: 0 cm “infinity”(“continuous filament”)Fibre Dimensions - LengthFibre le

3、ngth is an extremely important parameterIt has far-reaching influences on yarn & fabric processing & product performanceCan divide fibres approximately into two groups: 3ZUT-Testing: Dimensions of Fibres & Yarns Most Natural FibresFibre DimeFibre Length - MeasurementContinuous filamentLength measure

4、ment not generally an issueStaple fibresA given batch of raw fibre (eg cotton or wool) will contain many different fibre lengthsMost methods are therefore statistical in natureThey are usually extremely difficult & time-consuming to carry out; instruments include:Various “comb” sorters (differ for c

5、otton & wool)Various photoelectric sorters (“Shirley” & “Fibrograph”)“Sledge” sorterSee, for example, Booth, Chapter 54ZUT-Testing: Dimensions of Fibres & YarnsFibre Length - MeasurementCont Fibres typically 20 microns (0.02mm) across Cross-section very difficult to determineeg woolFibre Dimensions

6、“Thickness”This is far from straightforward 5ZUT-Testing: Dimensions of Fibres & Yarns Fibres typically 20 micronYarns are often difficult, tooCross-section ill-defined, possibly “hairy” etcFibre Dimensions “Thickness”6ZUT-Testing: Dimensions of Fibres & YarnsYarns are often difficult, tooThere are

7、two types of system Usually specified in terms ofLinear Density (LD) or fineness Directeg tex systemValue is proportional to LD increases with thickness Indirecteg Cotton CountValue is inversely proportional to LD decreases with thicknessFibre Dimensions “Thickness”7ZUT-Testing: Dimensions of Fibres

8、 & YarnsThere are two types of system Fineness as a Measure of Cross-SectionMass = V x density () = ALLinear density = Mass/Length (eg in tex)= AL/L = AFor a given bulk density (), linear density is proportional to fibre cross-sectional area8ZUT-Testing: Dimensions of Fibres & YarnsFineness as a Mea

9、sure of CrossMass = V x density () = ALLinear density = Mass/Length (eg in tex)= AL/L = AIt works for any regular shape eg triangularFineness as a Measure of Cross-SectionFor a given bulk density (), linear density is proportional to fibre cross-sectional area9ZUT-Testing: Dimensions of Fibres & Yar

10、nsMass = V x density () = ALIt1. Direct tex = weight (g) of 1km decitex = weight (g) of 10 km millitex = weight (g) of 1000 km kilotex = weight (kg) of 1 kmFineness UnitsExample: 10 cm fibre weighing 0.02 mg= 0.00002/0.0001 g per km= 0.2 tex = 2 dtex = 200 mtex10ZUT-Testing: Dimensions of Fibres & Y

11、arns1. DirectFineness UnitsExample2. Indirect Count eg Cotton Count = No. of 840-yard hanks for 1 lb weightFineness UnitsExample:Suppose a standard-length hank of yarn weighs 1/30 lb Therefore need 30 hanks for 1 lbTherefore yarn = 30s Cotton Count11ZUT-Testing: Dimensions of Fibres & Yarns2. Indire

12、ctFineness UnitsExamp Stiffness & handle (ie drape etc) Torsional rigidity (ie how hard to twist)- square power dependence on finenessFineness - Technological importance Light reflection- fine fibres soft sheen- coarse fibres harsh glitter Absorption of liquids & fibre cohesion- related to surface a

13、rea Yarn uniformity- finer fibres give more even yarnFibre fineness impacts on a wide range of other properties - here are just some examples 12ZUT-Testing: Dimensions of Fibres & Yarns Stiffness & handle (ie drape Knowledge of effective thickness is usually essential for results to be meaningful(pa

14、rticularly when comparing different fibre types)Example:Suppose a given cotton fibre can withstand a greater load (tension) than a given nylon fibreIs cotton inherently stronger than nylon per se?We cant immediately tell, because thick fibres are stronger than thin fibres of the same typeTherefore,

15、in most cases, results must be normalised Fineness - Importance in testing13ZUT-Testing: Dimensions of Fibres & Yarns Knowledge of effective thickn There is a wide range of methods available These include gravimetric and non-gravimetric methods Some are direct; some are indirect Some are extremely c

16、omplicated & 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)Fineness Methods of determination14ZU

17、T-Testing: Dimensions of Fibres & Yarns There is a wide range of methTypically, 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 accurateSuppose the piece of yarn weighs W gr

18、amsIts 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 balanceThe count equivalent can be calculated using the appropriate conversion formula; for example, for English Cotton Count (NE):NE = 590

19、.5/texFineness Methods - YarnsSimplest: weigh a known length on a balance15ZUT-Testing: Dimensions of Fibres & YarnsTypically, 100 metres (ie 0.1 Fineness Methods - FibresRange of methods availableGenerally complicatedAppropriate choice depends on factors such as:What physical form the fibre is in-

20、Bale? Sliver? Yarn? Fabric?Is a single-fibre value required?Or some form of average for a bulk of fibres?16ZUT-Testing: Dimensions of Fibres & YarnsFineness Methods - FibresRangeFineness Methods - Fibres May estimate fibre fineness from overall yarn tex Need the number of filaments in the cross-sect

21、ion May be provided by yarn-producer If not, filaments need to be counted Microscope may be needed tedious and slowFor uniform synthetic fibres in continuous-filament yarns 17ZUT-Testing: Dimensions of Fibres & YarnsFineness Methods - Fibres May Fineness Methods - Fibres1. By microscopy Assumes fine

22、ness 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 calculatedFor single fibresSingle fibres are usually too small to b

23、e weighed reliably on a balance.Two examples of alternative methods:18ZUT-Testing: Dimensions of Fibres & YarnsFineness Methods - Fibres1. ByFineness Methods - Fibres2. Using a “Vibrascope”Principle of a vibrating stretched string- as in a musical instrumentFrequency (“pitch”) depends on tension, le

24、ngth and linear densityFibre hung between knife-edgesSmall known weight attached to lower endMade to vibrate at fixed frequency using electrostatic platesLength adjusted until get resonanceFineness (usually in dtex) read off dialRelatively straightforward and moderately quickMay not be highly accura

25、teFor single fibres19ZUT-Testing: Dimensions of Fibres & YarnsFineness Methods - Fibres2. UsFineness Methods - FibresIllustration of the Vibrascope principle20ZUT-Testing: Dimensions of Fibres & YarnsFineness Methods - FibresIllusFineness Methods - FibresAir-flow methods usually bestQuickGive an ave

26、rage valueFor bulk fibres (eg cotton or wool staple)Frompressure meterFlow meter & suction pumpChamber packed with wad of fibres of standard weightAirWeighted insertwith perforated baseAirTypical airflow system(schematic)21ZUT-Testing: Dimensions of Fibres & YarnsFineness Methods - FibresAir-fS is c

27、alled the “specific surface” of the fibreHence Fineness Methods Air-flowConsider an idealised, cylindrical fibre The finer the fibre the greater its specific surfaceLdP22ZUT-Testing: Dimensions of Fibres & YarnsS is called the “specific surf25 fibres of diameter d5 fibres of diameter 2dOccupy the sa

28、me total volume asFineness Methods Air-flowCompare two equal-volume batches of fibres with different diameters, and hence fineness but have twice the surface area Air-flow is restricted by drag over the fibre surface The finer fibres have twice the resistance to airflowLdL2d23ZUT-Testing: Dimensions

29、 of Fibres & Yarns25 fibres of diameter d5 fibreResistance to airflow as a measure of fineness2dFine fibres high resistanceCoarse fibres lower resistanceFineness Methods Air-flowd24ZUT-Testing: Dimensions of Fibres & YarnsResistance to airflow as a meaIn practice may measure either the pressure for

30、a given air-flow or the air-flow for a given pressureFineness (in tex) is:proportional to fibre cross-sectional areaproportional to fibre diameter squaredSpecific surface is:inversely proportional to diameterinversely proportional to square root of texPressure for given air-flow is:proportional to s

31、pecific surface inversely proportional to square root of texIdeally, for a mass of uniform cylindrical fibres Fineness Methods Air-flow25ZUT-Testing: Dimensions of Fibres & YarnsIn practice may measure either Where the fibres are not uniform and/or not cylindrical the results of air-flow measurement

32、s 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 meanFineness Methods Air-flowFor further details and practical systems for measuring fibre fineness see, for example, Booth, Chapter 526ZUT-Testing: Dimensio

33、ns of Fibres & Yarns Where the fibres are not unifMaturity (of Cotton Fibres)Maturity is a dimensional characteristic of natural cellulose fibres especially cottonIt indicates how well-developed the fibres are at harvestIt is extremely important in terms of down-stream processing and yarn/fabric qua

34、lityMaturity and fineness are interrelated, although not in a simple way27ZUT-Testing: Dimensions of Fibres & YarnsMaturity (of Cotton Fibres)MaMaturity (of Cotton Fibres)Structure and growth of cotton fibres - in briefLongitudinal ViewCross-sectional View“Convolution”28ZUT-Testing: Dimensions of Fi

35、bres & YarnsMaturity (of Cotton Fibres)StrMaturity (of Cotton Fibres)Structure and growth of cotton fibres - in briefCotton fibres begin (after the flower dies) as thin-walled hollow cylindersThey first lengthen without changing in diameter; takes around 20 daysThey then mature; takes about another

36、30 daysIn maturation, cellulose is deposited on the inside of the cylinder the secondary wallThe hole down the centre (the lumen) becomes progressively smallerThe outer diameter of the fibre remains virtually unchanged29ZUT-Testing: Dimensions of Fibres & YarnsMaturity (of Cotton Fibres)StrMaturity

37、(of Cotton Fibres)Structure and growth of cotton fibres - in briefA Cotton fibre grows first in length then in wall-thicknesslumenMaturity is related to the cell wall thickness in comparison to the lumen diameterSeed surfaceEmerging fibre30ZUT-Testing: Dimensions of Fibres & YarnsMaturity (of Cotton

38、 Fibres)StrMaturity (of Cotton Fibres)Degree of thickening ()AAo = A/AoLumenA = area occupied by celluloseCelluloseIdealised cross-section of a cotton fibreAo = total area (including lumen)31ZUT-Testing: Dimensions of Fibres & YarnsMaturity (of Cotton Fibres)DegThe fibres collapse when they dry out

39、on harvesting to give a “kidney-bean” shapeMaturity (of Cotton Fibres)Cross-sectional area changes, but perimeter remains approximately constantOn the plantPPDry32ZUT-Testing: Dimensions of Fibres & YarnsThe fibres collapse when they Technological importanceMaturity largely determines whether a batc

40、h of cotton can be spun into a good yarn - or indeed into any yarn Mature/Over-mature Immature “Dead”Some maturity VariationsCause “neps” clumps of matted fibresMaturity (of Cotton Fibres)33ZUT-Testing: Dimensions of Fibres & YarnsTechnological importanceMaturiThe fibre perimeter is related to both

41、fineness and maturity, and is involved in airflow methods for measuring these properties see Booth, Chapter 5Maturity (of Cotton Fibres)P1P2P1Air flow restricted through fine fibres because large specific surfaceAir flows more easily through coarse fibres because smaller specific surface34ZUT-Testing: Dimensions of Fibres & YarnsThe fibre perimeter is relatedMaturity (of Cotton Fibres)Maturity Count and Maturity RatioSuppose a large sample of cotton fibres is selected at random and treated with caustic soda. The mature fibres will swell back to