Fibre properties and testing

Textile Technology (B.E./B.Tech) · Fibres · free study notes for exams, placements and interviews

In short

Fibre properties are the measurable physical, mechanical and chemical characteristics - length, fineness, strength, maturity, crimp, moisture regain, density, thermal and chemical behaviour - that decide what yarn and fabric a fibre can make. Fibre testing is the standardised measurement of these properties using instruments such as the HVI, AFIS, fibrograph, Baer sorter and tensile tester, under conditioned atmosphere (27 ± 2 °C, 65 ± 2% RH for Indian tropical standard). The subject matters because roughly 70-80% of yarn quality variation traces back to fibre quality and blending, and because cotton in India is bought and sold on HVI parameters. It is examined in university Fibre Science and Textile Testing papers, in GATE TF (Textile Engineering and Fibre Science) Section on fibre properties and testing, and probed in interviews at mills (Arvind, Vardhman, Trident, Welspun, Raymond, KPR), at SIMA/ATIRA/BTRA/SITRA laboratories and at Cotton Corporation of India buying points.

Explained

Length and its distribution

Staple length: the length of a representative "staple" of fibres held in the hand - the commercial trade term for cotton (e.g. Shankar-6 at about 29-30 mm). Man-made staple fibre is cut length and is exact.
Mean length (ML): arithmetic mean of all fibre lengths in the sample. Upper half mean length (UHML): mean length of the longer half by weight - what HVI reports and what the trade quotes.
Uniformity index (UI) = (ML/UHML) × 100, in percent; uniformity ratio = (50% span length / 2.5% span length) × 100.
Span length (fibrograph): the distance spanned by a stated percentage of fibres caught at a random comb. 2.5% span length approximates effective/staple length; 50% span length relates to mean length.
Short fibre content (SFC): percentage by weight of fibres below 12.7 mm (1/2 inch). High SFC causes neps, ends-down, hairiness and waste; AFIS measures it fibre-by-fibre, HVI estimates it.
Effective length: upper quartile of the length distribution numerically, obtained from a Baer sorter diagram after neglecting the shortest fibres.
Instruments: comb sorter (Baer sorter, Suter-Webb array - gravimetric, gives a full length array), digital fibrograph (span lengths), HVI (UHML, UI, SFI), AFIS (length by number and by weight, SFC(n), SFC(w)).
Why length matters: it fixes the spinning system (short-staple ring spinning, worsted, etc.), the achievable count, drafting roller settings and twist.

Fineness, maturity and trash

Fineness of fibres is expressed as linear density, not diameter, because cross-sections are irregular.
- Tex = grams per 1,000 m; decitex (dtex) = g/10,000 m; denier = g/9,000 m; millitex = mg/km.
- Cotton also uses micronaire - an airflow reading on a 3.24 g plug; it is a combined fineness-maturity index (µg/inch).
Typical micronaire bands: below 3.0 very fine/immature, 3.0-3.9 fine, 3.9-4.9 medium (ideal for most Indian ring-spun yarn), 4.9-5.9 coarse, above 5.9 very coarse.
Maturity: degree of secondary wall (cellulose) deposition. Maturity ratio MR and percentage mature fibres are found by the sodium hydroxide swelling test (18% NaOH) and classifying fibres as normal, half-mature and immature (dead) under a microscope, or by the caustic soda/polarised light and Causticaire methods.
Immature fibres cause neps, white specks after dyeing (they absorb less dye), low strength and processing waste.
Trash and non-lint content: Shirley Analyser (mechanical separation), MDTA, HVI trash module (optical % area and particle count). Non-lint in Indian hand-picked cotton is typically 2-6%.
Fineness instruments: airflow (micronaire, Arealometer), gravimetric cut-and-weigh (most accurate reference method), Vibroscope and vibrating-reed for single filaments, optical microscope/image analysis, AFIS (fineness and maturity by electro-optical sensing).

Tensile and extension behaviour

Tenacity (specific stress): breaking load per unit linear density, in cN/tex, gf/tex or gf/den. Fibres are compared on tenacity, not stress in MPa, because density differs.
Breaking elongation (%): extension at break. Work of rupture: area under the load-elongation curve (energy to break, in mJ or cN·cm); work factor = work of rupture / (breaking load × breaking extension), 0.5 for a straight line.
Initial modulus: slope at the origin - stiffness; high in polyester and flax, low in wool and spandex.
Yield point: the knee where the load-extension curve bends; beyond it, deformation becomes largely non-recoverable.
Elastic recovery and elastic resilience: fraction of imposed strain/work recovered on unloading - very high in wool and elastane, low in viscose when wet.
Bundle vs single fibre: HVI measures bundle tenacity (g/tex) with a Stelometer-type principle at 3.2 mm gauge; Pressley index was the older bundle measure. Single-fibre strength is measured on Instron/tensile testers per the relevant method, and is always higher per fibre than bundle tenacity because bundle breaks involve stress non-uniformity.
Effect of moisture: cotton, flax and jute gain strength when wet (about 5-15% up); viscose and wool lose strength when wet; polyester, nylon and acrylic are almost unaffected.
Effect of gauge length and rate of loading: longer test length gives lower apparent strength (weak-link effect); faster extension gives higher apparent strength.
Fatigue, abrasion and flex: repeated small loads and flexing cause failure below the single-break strength - tested by flex abrasion, Martindale (fabric level) and single-fibre fatigue testers.

Moisture, thermal, optical and chemical properties

Moisture regain R = (mass of water / oven-dry mass) × 100; moisture content M = (mass of water / total mass) × 100. Commercial invoicing uses agreed/standard regain, so these must never be interchanged.
Standard regains (approximate, at 65% RH, 20-27 °C): cotton 8.5%, viscose 11-13%, wool 14-16%, silk 11%, jute 12-14%, nylon 6 4.0-4.5%, polyester 0.4%, acrylic 1.5-2%, polypropylene ~0.
Hysteresis: regain reached by absorption is lower than by desorption at the same RH - always condition samples from the dry side. Heat of wetting raises temperature as fibres absorb moisture.
Conditioning and testing atmosphere: Indian/tropical standard 27 ± 2 °C and 65 ± 2% RH; ISO/temperate standard 20 ± 2 °C, 65 ± 4% RH. Samples are conditioned to equilibrium (commonly 24 h, or until successive weighings differ by less than 0.25%).
Density (g/cm³): cotton 1.54, viscose 1.52, wool 1.31, silk 1.34, nylon 1.14, polyester 1.38, acrylic 1.17, polypropylene 0.91 (floats on water).
Thermal: polyester Tg ≈ 70-80 °C, melts about 255-260 °C; nylon 6 melts about 215-220 °C, nylon 6,6 about 255-260 °C; polypropylene melts about 165 °C; cotton does not melt but chars/decomposes above about 150-250 °C. Heat setting of polyester is done at 180-200 °C.
LOI (limiting oxygen index): below ~21% means it burns readily in air - cotton ~18%, polyester ~21%, wool ~25%, modacrylic ~30%, aramid ~29-30%, PVC ~37%.
Chemical identification route: burning test → solubility series (acetone, DMF, 70% H₂SO₄, cold 20% HCl, m-cresol, NaOH) → microscopy (longitudinal and cross-section) → staining (Shirley Developer/Neocarmine) → melting point → FTIR for confirmation.
Optical and other: birefringence indicates molecular orientation (polyester ~0.19; viscose ~0.02-0.04); lustre, crimp and specific surface govern handle, cover and bulk.

Crimp, friction, bulk and dimensional behaviour

Crimp (%) = [(straightened length − crimped length)/crimped length] × 100; also expressed as crimps per cm. Natural in wool (fine merino may have 10-14 crimps/cm), mechanically or chemically imparted in man-made staple.
Crimp gives cohesion (essential for drafting and carding), bulk, loft, thermal insulation and extensibility.
Fibre friction: measured by capstan/twist methods; wool's directional friction effect (DFE, scale effect) is the cause of felting and of shrinkage in wool knitwear.
Flexural rigidity ∝ E × (linear density)²/density² - stiffness grows steeply with coarseness, hence coarse fibres prickle.
Swelling: cotton swells 20-30% in diameter in water (viscose 35-66%), causing fabric shrinkage and the need for compressive shrinkage/sanforizing.
Fibre quality index/spinning consistency index: HVI-derived composite indices (SCI from length, strength, micronaire, colour) used in India to rank bales for a given count.

Sampling, statistics and standards

Sampling: zoning technique (pinch small tufts from many places in a bale, discard half repeatedly) for cotton; random draw by mass for staple. Testing is destructive and sample-dependent, so sampling error dominates.
Statistics reported: mean, standard deviation σ, CV% = (σ/mean) × 100, and standard error of mean = σ/√n; number of tests n is fixed from the required confidence (commonly 95%) and tolerable error.
Confidence interval = mean ± t × σ/√n; textile data is highly variable, so CV% and n must always be quoted with a mean.
Calibration: HVI is calibrated with USDA/ICC calibration cottons; balances and tensile testers need periodic calibration traceable through NABL-accredited laboratories.
Standards bodies and practice in India: BIS Textile Department standards (TXD committees) cover methods for fibre length, fineness, maturity, moisture regain, tensile properties and conditioning atmosphere; ASTM D (D1447 length, D1445 bundle strength, D1448 micronaire, D2495 moisture) and ISO methods are equally used by export mills. Quote the current IS edition from the BIS catalogue before citing a number, since several older IS textile method standards have been revised or withdrawn.
Trade and regulation: cotton is traded on HVI certificates; the Cotton Corporation of India and the Minimum Support Price mechanism use staple length and micronaire bands for grading; Legal Metrology (Packaged Commodities) Rules govern declared net mass, for which moisture regain matters; GST on textile fibres/yarn/fabric sits in the 5%/18% structure in force since 22 September 2025 - confirm the current rate for the specific HSN before quoting.
Quality systems: Uster Statistics benchmark yarn results; mills run fibre testing laboratories under ISO 9001, with calibration and test reports forming part of OEKO-TEX and buyer audits.

Key points

UHML - mean length of the longer half of fibres by weight; the HVI length the cotton trade quotes.
Uniformity index - ML/UHML × 100; below about 80% signals excessive short fibre.
Short fibre content - % by weight under 12.7 mm; the main source of neps, hairiness and ends-down.
Span length - length spanned by a given % of fibres at a random comb; 2.5% SL ≈ effective length.
Micronaire - airflow fineness-maturity index in µg/inch; 3.9-4.9 is the preferred band.
Tex/denier - tex = g/km, denier = g/9 km; denier = 9 × tex; lower number means finer.
Maturity ratio - secondary wall development; immature fibres give white specks on dyeing.
Tenacity - breaking load per unit linear density in cN/tex; the only fair strength comparison between fibres.
Work of rupture - area under the load-elongation curve; toughness, not strength.
Initial modulus - stiffness at the origin; high in polyester, low in wool and elastane.
Regain vs content - regain divides by dry mass, content by total mass; regain is always the larger number.
Tropical standard atmosphere - 27 ± 2 °C, 65 ± 2% RH for testing in India.
Hysteresis - condition from the dry side, since desorption gives higher regain than absorption.
Wet strength - cotton and jute gain, viscose and wool lose, synthetics unchanged.
Crimp - gives cohesion, bulk and extensibility; wool's DFE causes felting.
Weak-link effect - longer gauge length gives lower measured strength.
CV% - σ/mean × 100; textile results are meaningless without CV% and the number of tests.
HVI vs AFIS - HVI is fast bundle/ optical testing for trading; AFIS is single-fibre testing for process diagnosis (neps, SFC, maturity).

Formulas and examples

Linear density: tex = mass(g) × 1000 / length(m); denier = 9 × tex; dtex = 10 × tex; Nm (metric count) = 1000/tex
Tenacity = breaking load / linear density [cN/tex]; 1 gf/den = 8.83 cN/tex; 1 gf/tex = 0.981 cN/tex
Breaking length (km) = tenacity in gf/tex ÷ 1000 × 1000 → numerically equal to tenacity in gf/tex (km)
Moisture regain R% = (W − D)/D × 100; Moisture content M% = (W − D)/W × 100
Conversions: M = R/(1 + R/100) ; R = M/(1 − M/100) [R, M in %]
Correct (conditioned) mass = oven-dry mass × (1 + standard regain/100)
Crimp% = (L_straight − L_crimped)/L_crimped × 100
Uniformity index = (Mean length / UHML) × 100 ; Uniformity ratio = (50% SL / 2.5% SL) × 100
Maturity ratio MR = [(N − D)/200] + 0.7, where N = % normal fibres, D = % dead fibres (NaOH swelling test)
Work of rupture = ∫ F ds ≈ work factor × breaking load × breaking extension
Fibre diameter from linear density: d(µm) = 2 × √[tex × 10⁶/(π × ρ)] with ρ in kg/m³ (circular cross-section)
CV% = (σ/mean) × 100 ; Standard error = σ/√n ; n = (t × CV%/allowable error%)²

Example 1 - linear density and tenacity conversion. A bundle of 500 polyester fibres, each 60 mm long, weighs 12.45 mg. A single fibre breaks at 9.6 gf.

Total length = 500 × 0.060 = 30 m; mass = 12.45 mg = 0.01245 g.
Mass per fibre = 12.45/500 = 0.0249 mg = 2.49 × 10⁻⁵ g over 0.060 m, so mass per 1,000 m = 2.49 × 10⁻⁵ × (1000/0.060) = 0.415 g → single-fibre linear density = 0.415 tex = 4.15 dtex = 3.74 denier.
Cross-check from the bundle: tex = 0.01245 × 1000/30 = 0.415 tex, which is already the per-fibre value, because the 30 m is the combined length of all 500 fibres (mass per unit length of one fibre = total mass / total length).
Tenacity = 9.6 gf/0.415 tex = 23.1 gf/tex = 23.1 × 0.981 = 22.7 cN/tex = 22.7/8.83 = 23.1/9 ≈ 2.57 gf/den.

Example 2 - moisture regain, content and invoice mass. A cotton lot weighs 102.5 kg as received; an oven-dry test on a 50.00 g sample gives 46.30 g dry.

Regain R = (50.00 − 46.30)/46.30 × 100 = 3.70/46.30 × 100 = 7.99%.
Content M = 3.70/50.00 × 100 = 7.40%; check with M = R/(1 + R/100) = 7.99/1.0799 = 7.40% ✓.
Oven-dry mass of lot = 102.5/1.0799 = 94.92 kg.
Correct (commercial) mass at 8.5% standard regain = 94.92 × 1.085 = 102.99 kg - so the lot is about 0.49 kg "light" against standard regain.

Example 3 - HVI report interpretation. A bale reports UHML 29.5 mm, ML 24.3 mm, micronaire 4.3, bundle strength 29.5 g/tex, SFI 8.5, trash 2.8%.

Uniformity index = 24.3/29.5 × 100 = 82.4% - good (80-82% average, above 83% high).
Micronaire 4.3 lies in the premium 3.9-4.9 band - no discount, low nep risk.
Tenacity 29.5 g/tex = 29.5 × 0.981 = 28.9 cN/tex - "strong" by HVI bands (29-30 g/tex).
Suitability: 29.5 mm UHML with 82% UI comfortably supports combed 40s Ne ring-spun yarn.

Example 4 - maturity ratio. In a NaOH swelling test on 100 fibres: 68 normal, 22 half-mature, 10 dead.

MR = [(68 − 10)/200] + 0.7 = 58/200 + 0.7 = 0.29 + 0.70 = 0.99 - a mature cotton (MR ≈ 1.0 is normal; below 0.8 is immature).
Percentage maturity coefficient = (68 + 0.6 × 22 + 0.4 × 10)/100 × 100 = (68 + 13.2 + 4)/100 × 100 = 85.2.

Example 5 - number of tests needed. Single-fibre strength on a viscose lot gives mean 18.0 cN/tex with σ = 4.5 cN/tex. How many tests for ±3% error at 95% confidence (t ≈ 2)?

CV% = 4.5/18.0 × 100 = 25%.
n = (2 × 25/3)² = (16.67)² = 278 tests - which is why bundle tests (HVI) are preferred for routine trading.

Property	Cotton	Viscose	Wool	Silk	Polyester	Nylon 6	Acrylic
Tenacity, dry (cN/tex)	26-45	20-26	10-14	35-50	40-55	40-60	20-30
Wet vs dry strength	Higher	Much lower	Lower	Lower	Same	Slightly lower	Same
Breaking elongation (%)	6-10	15-25	25-40	15-25	15-30	20-40	20-35
Standard regain (%)	8.5	11-13	14-16	11	0.4	4.0-4.5	1.5-2
Density (g/cm³)	1.54	1.52	1.31	1.34	1.38	1.14	1.17
Melting/decomposition	Chars	Chars	Chars	Chars	255-260 °C	215-220 °C	Softens 220-255 °C

Instrument	Measures	Principle
Baer/comb sorter, Suter-Webb	Full length array, effective length	Manual combing, gravimetric groups
Digital fibrograph	2.5% and 50% span lengths	Photoelectric scan of a random comb beard
HVI	UHML, UI, SFI, strength, Mic, colour, trash, SCI	Beard scan + pneumatic + optical, 1-2 min/bale
AFIS	Length by number/weight, SFC, neps, maturity, fineness, trash	Individualised fibres through an electro-optical sensor
Shirley Analyser	Trash/non-lint %	Mechanical opening and separation
Micronaire/airflow	Fineness-maturity	Pressure drop across a fixed-mass plug
Vibroscope	Single-filament linear density	Natural frequency of a tensioned fibre
Stelometer / Pressley	Bundle tenacity	Flat bundle break at a fixed gauge

Where it is used

Spinning mills (Vardhman, Trident, KPR, Nahar, Arvind): bale-wise HVI data drives bale management/mixing so that micronaire, length and strength stay uniform mixing-to-mixing - the single biggest lever on yarn CV% and ends-down.
Cotton trading and ginning: CCI procurement, MSP grading and export contracts settle on HVI staple length, micronaire, strength and trash; disputes go to accredited labs.
Man-made fibre plants (Reliance, Grasim/Birla Cellulose, Indo Rama): dtex, tenacity, elongation, crimp, shrinkage in boiling water and dye uptake are in-process QC parameters on every tow/staple lot.
Textile testing laboratories - SITRA (Coimbatore), ATIRA (Ahmedabad), BTRA (Mumbai), NITRA (Ghaziabad), TEXTILES COMMITTEE labs, NABL-accredited private labs: third-party fibre certification, buyer-mandated testing, calibration.
Technical textiles and composites: tenacity, modulus, LOI and thermal data decide geotextile, airbag, ballistic, FR-garment and FRP-reinforcement selection.
Job roles: QA/QC executive, lab in-charge, mixing/blending engineer, process improvement engineer, buying-house quality merchandiser, R&D at fibre producers.

Interview questions

Q: Why is fibre fineness expressed as linear density rather than diameter? A: Fibre cross-sections are irregular and non-circular (cotton is bean-shaped and collapsed), so diameter is not measurable reproducibly; mass per unit length is.
Q: What exactly does micronaire measure? A: The air permeability of a fixed-mass fibre plug, which responds to specific surface - it is a combined fineness-and-maturity index, so a low mic can mean a fine fibre or an immature one.
Q: Difference between moisture regain and moisture content? A: Regain divides water mass by oven-dry mass, content by total mass; regain is always numerically higher, and commercial mass is computed from regain.
Q: Why does cotton get stronger when wet while viscose gets weaker? A: Water in cotton relieves internal stresses and improves load sharing between fibrils in a highly crystalline structure; viscose has low crystallinity and high amorphous content, so water plasticises and separates the chains.
Q: Why is bundle strength lower than single-fibre strength? A: In a bundle all fibres do not share load equally and weak fibres break first, so the mean breaking stress of the bundle is lower than the mean of individual fibres.
Q: What is the weak-link effect? A: Longer test lengths contain more potential flaws, so measured strength falls as gauge length rises - test length must always be reported.
Q: What causes white specks in dyed cotton fabric? A: Immature fibres with thin secondary walls, which absorb far less dye; the cure is better maturity in the mixing, not harder dyeing.
Q: Why is the testing atmosphere in India 27 °C instead of 20 °C? A: 65% RH at 27 ± 2 °C is the tropical standard atmosphere, practical for Indian climate; ISO temperate standard is 20 °C, and results are not directly comparable for hygroscopic fibres.
Q: HVI or AFIS - which would you use and why? A: HVI for fast bale-by-bale trading and bale management; AFIS when diagnosing neps, short fibre or maturity problems because it measures individual fibres.
Q: What is the practical effect of high short fibre content? A: More fly, neps and waste, higher yarn hairiness and imperfections, higher ends-down in spinning, and poorer fabric appearance.
Q: Why does crimp matter in staple fibre? A: It provides inter-fibre cohesion for drafting, plus bulk, loft, insulation and extensibility in the final fabric.
Q: How would you identify an unknown fibre in the lab? A: Burning behaviour first, then a solubility series, microscopy of longitudinal and cross-section views, melting point for thermoplastics, and FTIR for confirmation.

Common mistakes

Interchanging moisture regain and moisture content, or applying standard regain to as-received mass instead of oven-dry mass.
Reading micronaire as pure fineness and ignoring that it is a fineness-maturity composite.
Comparing fibre strength in MPa across fibres of different density instead of using cN/tex.
Quoting a mean strength or length without CV% and the number of tests, or without the gauge length and test atmosphere.
Mixing denier and tex (denier = 9 × tex) or forgetting that a lower tex means a finer fibre.
Conditioning samples from the wet side and ignoring hysteresis, which inflates regain and depresses strength readings.
Treating 2.5% span length, UHML, staple length and effective length as the same number - they are related but distinct.