Four Unlabelled Samples, One Reactive Dye Bath
At a job-work unit in Tiruppur, four unlabelled 20 cm × 20 cm samples arrive from a buyer who wants 50 kg of fabric dyed in a single reactive-dye shade at 60 °C. The only records are burn observations made by the supervisor. Sample A ignites quickly, burns with a yellow flame, smells of burning paper, leaves soft grey ash and continues to glow after the flame is removed. Sample B shrinks away from the flame, melts into a hard black bead, gives a sweet chemical odour and self-extinguishes. Sample C burns slowly, sputters, smells of burning hair and leaves a black bead that crushes between the fingers. Sample D melts and chars, leaves a hard black bead plus grey powdery ash, and gives a mixed odour. The unit dyes D with the same reactive recipe that works on A and obtains a pale, ring-like, patchy result. No fibre composition certificate is available, and the buyer's delivery date is in six days.
1. Identify the most probable fibre for Samples A, B and C, and explain the evidence for each. Then explain what Sample D is and why the reactive recipe produced a pale, ring-like result.
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A is most likely cotton: it burns rapidly like paper, leaves soft grey ash and continues to glow, which is typical of a cellulose fibre. B is most likely polyester: it shrinks from the flame, forms a hard black bead, gives a sweet chemical odour and self-extinguishes, all signs of a thermoplastic fibre. C is a protein fibre, wool or silk: the burning-hair smell and the black bead that crushes between the fingers are characteristic.
D is a polyester–cotton blend. The hard bead comes from the polyester melting, while the grey powdery ash comes from the cotton; the mixed odour supports two fibre types. A reactive dye forms covalent bonds with cellulose hydroxyl groups, so it dyes the cotton portion but has no comparable sites on polyester. The result is that only part of the fabric takes colour, giving a pale shade, and the two fibres absorb dye at different rates, producing the ring-like, patchy appearance. Adding more reactive dye will build colour only on the cotton and will not correct the polyester portion.
2. The unit has a laboratory that can do solubility tests but not infrared spectroscopy. Describe a confirmatory test plan to establish that D is a polyester/cotton blend and to estimate the blend ratio, and state one safety precaution.
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Weigh a dry 1 g specimen of D to 0.001 g. Treat it with 70% sulphuric acid at about 40 °C for 45–60 minutes with occasional stirring; cotton dissolves, while polyester is resistant under these conditions. Filter through a tared sintered crucible, wash the residue thoroughly with water until neutral, dry to constant weight and weigh. The residue is the polyester fraction; the loss in weight is the cotton fraction. Calculate polyester % = residue mass ÷ original mass × 100. Run a reagent blank and, if possible, a known 65/35 polyester–cotton control to check the method.
A burn test alone cannot quantify a blend because both components can leave char and a bead, and the odour is mixed. The solubility test gives a quantitative split. Safety: 70% sulphuric acid is corrosive, so wear gloves and goggles, add acid to water rather than water to acid, work in a fume cupboard, and neutralise the filtrate before disposal.
3. Recommend a dyeing route for Sample D and one care-label instruction for Sample B.
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D needs a two-stage route because two dye classes are required. First dye the polyester with a disperse dye at about 130 °C under pressure, pH 4.5–5, for 30–45 minutes, then reduction-clear with about 2 g/L sodium hydrosulphite and 2 g/L sodium hydroxide at 70 °C for 15 minutes to remove surface dye. After rinsing, dye the cotton with a reactive dye at 60 °C using controlled salt addition and soda ash fixation, then soap thoroughly. A one-bath two-stage route is possible only with compatible dyes and careful pH control; the two-bath route is safer for a 50 kg batch with a six-day deadline.
For B, which is polyester, the care label should specify a low iron temperature, washing at 40 °C or below, no chlorine bleach and low tumble-drying. Polyester is thermoplastic, so a hot iron can glaze, shine or permanently pleat the surface.