An optical brightening agent works by
Answer: A
The fluorescence adds blue light that was not there before, so the fabric returns more visible light than it receives. It removes nothing and is quite different from a bleach. The effect is additive rather than subtractive.
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Assertion (A): A fabric treated with an optical brightener can reflect more visible light than falls on it.
Reason (R): Energy absorbed in the ultraviolet is re-emitted in the visible blue region.
Answer: B
Both are true and the reason resolves the apparent paradox: the total energy is not increased, but light is moved from a region the eye cannot see into one it can. Reflectance above a hundred per cent is the visible result.
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State whether the following is true or false.
An optical brightener removes the yellow colouring matter from a fabric.
Answer: C
It leaves the colouring matter exactly where it is and masks it with added blue light. Bleaching is the operation that actually destroys the coloured compounds. The two operations are complementary rather than alternative.
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The blue light emitted by a brightener compensates for
Answer: D
Correct answer: residual yellowness of the substrate
A bleached fabric still absorbs a little at the blue end and therefore looks faintly yellow. A brightener absorbs invisible ultraviolet and re-emits it as visible blue fluorescence, and that added blue cancels the residual yellow to the eye. The fabric reflects more visible light than falls on it in the visible range, which is why it appears whiter than white rather than merely clean.
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All of the following are true of a fluorescent brightener EXCEPT
Answer: A
Removing colour is what a bleach does, and the brightener leaves every coloured compound where it was. The other three are the three facts that govern how a brightener is chosen and applied in a mill, and each of them limits what the treatment can achieve.
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Consider the following statements about brightener dosage.
1. Whiteness rises with dose only up to a maximum.
2. Beyond that point the fabric takes on a green or blue cast.
3. More brightener always gives a whiter result.
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 describe the dose response correctly: whiteness rises to a maximum and then declines. Statement 3 denies that maximum. Beyond the optimum the added blue overshoots the yellow it was cancelling and the goods take a green or blue cast - a fault called greening, which no amount of further brightener will correct because more of it is the cause.
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Assertion (A): There is an optimum dose of optical brightener beyond which whiteness falls.
Reason (R): Excess fluorescence tints the fabric green or blue rather than adding to the white.
Answer: C
Both are true and the reason names the greening point, which is determined experimentally for each brightener and substrate rather than being a single figure that applies everywhere. Suppliers publish a curve for each product.
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Brighteners for cotton are chemically related to
Answer: D
They are applied in much the same way as direct dyes and show the same kind of substantivity for cellulose. Those for polyester behave instead like disperse dyes and are applied at high temperature. Affinity for the fibre is what decides the class.
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Match each fibre with the class of brightener normally used on it.
List I
A. Cotton
B. Polyester
C. Wool
D. Polyamide
List II
1. a type stable under the mild acidic conditions used
2. a substantive type resembling a direct dye
3. a type with affinity under weakly acidic conditions
4. a disperse type applied at high temperature
Answer: A
A brightener must have affinity for the fibre in the same way a dye does, so no single product serves every substrate. Blends often need two brighteners together. Each class matches a family of dyes. No single product covers every substrate.
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State whether the following is true or false.
A single optical brightener can be used on any fibre.
Answer: B
Affinity for the substrate is required, exactly as it is for a dye, so a cotton brightener has no substantivity for polyester. A blend usually needs a combination of two products. Substantivity is fibre specific in both cases.
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