THE COLOR REVERSION OF PAPERS MADE FROM HIGH-YIELD PULP - A PHOTOCHROMIC PROCESS

A modified fluorescence microscope has been used to sequentially irradiate paper with UV and blue light for known periods of time and to monitor the changes in fluorescence intensity during irradiation. It was confirmed that cellulose shows colour reversion but this effect is far more marked for paper made from high yield pulps. A similar effect is also seen for such papers which have been weathered, bleached with hydrogen peroxide and photobleached with thiourea-dioxide. The colour reversion process appears to involve both photodegradation and photochromic reactions. Chemical species contained in a hydroxypropylcellulose film, which may be responsible for the photochromic effect, were studied, i.e. a quinone and a phenylcoumarin. Some evidence to support the view that phenylcoumarins are photochromic is presented.