Use of fluorescent whitening agents to inhibit light-induced colour reversion of unbleached mechanical pulps

Because of their high lignin content, mechanical pulps arm yellow with time and light exposure. This colour reversion can be attributed to thermal and photo-induced yellowing of the lignin. Our goal is to study new chemicals that act as inhibitors of the photo-oxidative process. Some diaminostilbene derivatives, a UV screen and some triazines have been tested on unbleached softwood mechanical pulps. Very thin sheets were used to carry out the pho tochemical study which was followed by UV-visible diffuse reflectance and f luorescence spectroscopy. Results have shown that many chemicals were found to be very effective in inhibiting colour I-eversion: up to 80% after 5 h and crp to 65% after 60 h of intense reversion (respectively 1 month and I year of simulated natural exposure). However; there is a loss of fluorescen ce of the chemicals with rime. In a series of analogue diaminostilbene deri vatives, the inhibition efficiency can be related to the sulphonation degre e.