Carmen Algra, of Tikipunga Primary School, asks :-
What's in washing powder to make clothes so white?
Stan Simpson, a chemist with the Wool Research Organisation of New Zealand, Lincoln, responded.
Perfectly white materials such as clothing, paint, paper or flowers appear so because they reflect light which has just the right balance of all the colours we can see when they are spread out as in a rainbow. The blue and the yellow parts of this sunlight spectrum are especially important when it comes to judging whether a cotton sheet for example is close to having a perfectly white appearance. If the sheet reflects too little blue light we see it as slightly yellow.
All undyed textiles tend to become more yellow or brownish in shade as they age. Detergents will clean off dirt and grease but have no effect on this gradual discolouration of the actual textile material. An old-fashioned remedy was to rinse fabrics with a little blue dye. The dye taken up certainly restored the balance of spectral colours needed for a nice white appearance. However, the dye also absorbs a lot of light and especially the yellow part so the fabric has a dull appearance.
Optical brighteners take advantage of another idea. They are chemically constructed to absorb some of the invisible ultraviolet part of the sunlight spectrum and then release most of this energy as a blue fluorescence. This extra blue again balances up the reflected radiation and the yellowish fabric now appears white. Since the optical brightener increases the retlected radiation, unlike the blue dye which reduces it with a dull effect, with this treatment the fabric is both whiter and brighter, and hence its name.
This may appear to be the perfect answer to overcoming the yellowing problems of textiles. However, optical brighteners eventually deteriorate with continued exposure to sunlight. After many repeated applications in the laundry the fabric may accumulate enough degraded brightener to lend it a slightly dull or grey tone.