Assertion (A): Mercerised and unmercerised cotton should not be dyed together in one bath.
Reason (R): The two take up dye at different rates and to different depths.
Answer: C
Both are true and the reason gives the consequence: the result is a two tone effect within a single batch, which is one of the classic causes of a shade complaint from a customer. Segregating the two lots is the only real answer.
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Arrange the stages of a chain mercerising range in order.
1. wash off the caustic while the tension is maintained
2. impregnate the fabric with caustic liquor
3. neutralise with acid and rinse
4. hold it under tension on the stenter chain
Answer: D
Tension must be maintained through the wash boxes or the fabric shrinks as the caustic is removed and the lustre is lost. Neutralisation is last and removes the final traces of alkali. The wash boxes sit inside the tensioned section.
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The caustic must be washed off while the fabric is still under tension because
Answer: A
Correct answer: releasing the tension early loses the lustre
The reorganised structure is not fixed until the caustic has been washed out. Release the tension while the fabric is still swollen and it shrinks, the fibres lose the alignment and the round cross-section, and the lustre goes with them - irrecoverably. The stretched fabric is therefore carried through the wash boxes on the chain or between rollers and only released once the caustic is removed.
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State whether the following is true or false.
Mercerising is normally carried out before dyeing rather than after.
Answer: B
The improved dye uptake is one of the reasons for the process, so it makes no sense to mercerise afterwards. It also has to precede any finish that would be removed by the strong caustic. The sequence is fixed by both of these considerations.
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The barium activity number is used to measure
Answer: C
It compares the barium hydroxide absorbed by the sample with that absorbed by an unmercerised control. A value around a hundred and fifteen or above indicates a well mercerised cotton. The test is a routine mill control.
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Consider the following statements about the barium activity number.
1. It compares barium hydroxide uptake against an unmercerised control.
2. A value of about 115 or more indicates good mercerisation.
3. A value of 100 indicates complete mercerisation.
Answer: D
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: the test compares barium hydroxide uptake against an unmercerised sample of the same cloth, and 115 or above indicates a good result. Statement 3 has the scale wrong - 100 is the unmercerised control by definition, so a value of 100 means no mercerisation has taken place at all. Full mercerisation is usually taken as 150 or more.
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Three of the following are properties improved by mercerisation. Which is the odd one out?
Answer: A
Crease recovery is improved by a resin finish rather than by mercerisation, which if anything makes cotton crease a little more readily. The other three are the three classic benefits of the process. Crease resistance needs a resin finish instead.
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Caustic recovery from a mercerising range is worthwhile because
Answer: B
Multiple effect evaporation concentrates the weak liquor back to working strength, and the economics are usually favourable. It also removes a large alkaline load from the effluent. Multiple effect evaporators are used for the duty.
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Assertion (A): Caustic recovery plants are common on mercerising ranges.
Reason (R): The wash liquor is strong enough that evaporating it back to strength repays the energy.
Answer: C
Both are true and the reason gives the economics, and the environmental benefit of keeping that alkalinity out of the effluent is a second and increasingly important reason. The payback is usually a matter of a few years.
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A range uses 3000 litres per hour of 250 grams per litre caustic and recovers 70 per cent. How much fresh caustic is needed per hour?
Answer: D
The total caustic used is seven hundred and fifty kilograms per hour and thirty per cent of that must be made up. That is two hundred and twenty-five kilograms, and the recovery saves the rest. Recovery rates above seventy per cent are common.
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