ATTEMPT TO PHOTOSTABILIZE PAPER MADE FROM HIGH-YIELD PULP BY APPLICATION OF UV SCREENS IN CONJUNCTION WITH THIOLS

The stabilization against photoinduced brightness reversion of peroxid e-bleached stoneground wood pulp by two phenolic UV screens (methyl 3- hydroxy-4-benzoylphenoxy acetate (DHBE) and methyl 3-(2 ''-benzotriazi nyl)-4'-hydroxy-5'-terbutylphenyl propanoate (HBTE)) and two polyethyl eneoxythiols (2-(2'-methoxyethoxy) ethanethiol (MSH) and 2,2'-oxydieth anethiol (DSH)), individually or in combination (DHBE + MSH and HBTE DSH), was investigated. A synergistic photostabilization effect was o bserved between the thiol and the UV absorber, but was not retained du ring irradiation due to thiol consumption probably by Michael-type add ition on the quinones, generated by light. Moreover, during the irradi ation, the HBTE derivative was found to be very stable on paper in con trast with the DHBE compound. This effect was related to the presence in HBTE of a bulky substituent (tert-butyl) adjacent to the phenolic g roup which locks the molecule in a favorable conformation for proton t ransfer in the ground and excited states.