Optical Brightening Questions and Answers

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Most brighteners for cellulose are anionic, and a cationic softener forms an insoluble complex with them that quenches the fluorescence - so a white that measured well before softening comes out visibly duller after it. Non-ionic or specially compatible softeners are used on brightened whites for exactly this reason.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: a deep shade absorbs the emitted blue along with everything else, so the brightener is wasted, while on a pastel it lifts the shade and makes it look cleaner and more brilliant. Statement 3 confuses two properties - a brightener adds no fastness to the dye and, having modest light fastness itself, is usually the first component of the finish to fail.

Answer: C

Correct answer: True

Brighteners are large conjugated molecules and the same conjugation that makes them fluoresce makes them vulnerable to photodegradation. A brightened white therefore yellows on prolonged exposure, sometimes to below the whiteness it started at, since the brightener is gone and the substrate’s own yellowing has continued. It is why brighteners are of limited use on outdoor textiles.

Answer: D

Domestic detergents carry brighteners precisely to replace what is lost, which is why washed cottons keep their appearance. Without them the original treatment washes off over a number of cycles. Detergent brighteners are chosen for cotton.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the pad route applies the solution by nip and then dries or cures, and the add-on is the product of the pick-up and the concentration, so an uneven pick-up across the width gives an uneven white. Statement 3 claims too much - padding is faster and suits continuous ranges, but exhaust application relies on the brightener’s own affinity and can give better levelness on goods that are difficult to pad evenly.

Answer: B

Unevenness shows up strikingly on a white and is much less forgiving than on a coloured fabric. The underlying cause is usually uneven application or uneven residual yellowness from the preparation. A white shows every variation without mercy.