Textile Fibres: Classification, Properties and Identification
What the exam tests
The fibres domain checks whether you can place any fibre in the correct family, quote standard property values and choose a defensible identification route. Most marks sit on classification and on burning and solubility tests, with a short reasoning or calculation item on fibre behaviour in processing.
Classification
- Natural: vegetable (cotton, jute, linen), animal (wool, silk), mineral (asbestos).
- Man-made: regenerated (viscose, acetate), synthetic (polyester, nylon 6 and 6,6, acrylic, polypropylene), inorganic (glass, carbon).
- Regenerated fibres come from natural polymers; synthetic fibres from polymerised chemicals. Never merge the two groups.
Standard property values worth memorising
- Cotton (cellulose): moisture regain 8.5%; tenacity 3–4.5 g/den dry, roughly 20% lower when wet; damaged by acid, tolerant of alkali - the basis of scouring and mercerising.
- Wool (keratin): regain about 16%, tenacity only 1–1.7 g/den, high extension, felts on washing, degraded by alkali.
- Silk (fibroin): regain 11%, tenacity 3–4.5 g/den.
- Polyester: regain 0.4%, tenacity 5–7 g/den, thermoplastic, oleophilic.
Identification
Burning test: cotton smells of burnt paper and leaves grey ash; wool and silk smell of burnt hair and leave a crushable black bead; polyester melts, smokes and forms a hard bead. Confirm with microscopy (convoluted ribbon in cotton, scales in wool, triangular silk, striations in viscose) and solubility (cotton in 70% sulphuric acid, nylon in formic acid, polyester in m-cresol).
Worked example
A sample burns with a burnt-hair smell and leaves a crushable bead; it dissolves in sodium hypochlorite. Identify it. The smell indicates a sulphur-containing protein, so the fibre is animal; hypochlorite attacks protein, so the answer is wool rather than silk. Silk is confirmed by its triangular cross-section and higher lustre.
Traps
Calling viscose synthetic; saying cotton is stronger wet; treating the burning test alone as conclusive; forgetting that polyester is thermoplastic and heat-sensitive.