Whiteness of a bleached fabric is measured by
Answer: A
Several indices exist, and a fabric can rank differently on two of them, so the index used should be stated. Absorbency is the test for scouring rather than for bleaching. Two indices can rank the same pair differently.
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Assertion (A): A whiteness value should be quoted together with the index used to compute it.
Reason (R): Different indices weight the blue end of the spectrum differently and give different rankings.
Answer: B
Both are true and the reason gives the pitfall: two fabrics can swap places between one index and another, which matters greatly when a shade is being matched against a customer standard. The index must accompany the number quoted.
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Fibre damage during bleaching is assessed by
Answer: C
Both tests detect oxycellulose, which forms when the bleach attacks the fibre rather than the colour. A tensile test confirms the damage but is less sensitive at low levels. Oxycellulose is the product of over-bleaching.
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State whether the following is true or false.
A fabric can be bleached to an excellent white and still be badly damaged.
Answer: D
Whiteness and fibre strength are independent, and pushing the bleach hard improves one at the expense of the other. This is why damage tests are run alongside whiteness measurements. A balance has to be struck between the two.
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All of the following are ways of raising the apparent whiteness of a fabric EXCEPT
Answer: A
Fabric weight has no bearing on how white the cloth looks. The other three are the three approaches actually used, and they are often combined in a single recipe. Brighteners work by fluorescence rather than by removal.
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Combined scouring and bleaching in one bath is attractive because it
Answer: B
Correct answer: saves a stage of water, energy and time
Peroxide bleaching runs in the same alkaline conditions as scouring, so the two can share a bath and a pass - one heating, one set of wash boxes and one lot of effluent instead of two. What is given up is control, since one set of conditions must serve both purposes, so the risk of fibre damage rises and the whiteness achieved is usually a little below what separate stages give.
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Consider the following statements about bleaching blends.
1. The cotton component needs the bleach and the polyester generally does not.
2. Conditions must avoid damaging either fibre.
3. Polyester requires the same peroxide treatment as cotton.
Answer: C
Correct answer: 1 and 2 only
Statements 1 and 2 describe the requirement correctly: only the cotton carries natural colouring matter needing removal, and the conditions must be gentle enough not to hydrolyse the polyester while still bleaching the cotton. Statement 3 is wrong - polyester needs no bleaching at all in the ordinary case, and where its whiteness must be improved that is done with an optical brightener rather than an oxidising bleach.
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