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 peroxide-bleached stoneground wood pulp by two phenolic UV screens (methyl 3-hydroxy-4-benzoylphenoxy acetate (DHBE) and methyl 3-(2 ''-benzotriazinyl)-4'-hydroxy-5'-terbutylphenyl propanoate (HBTE)) and two polyethyleneoxythiols (2-(2'-methoxyethoxy) ethanethiol (MSH) and 2,2'-oxydiethanethiol (DSH)), individually or in combination (DHBE + MSH and HBTE + DSH), was investigated. A synergistic photostabilization effect was observed between the thiol and the UV absorber, but was not retained during irradiation due to thiol consumption probably by Michael-type addition on the quinones, generated by light. Moreover, during the irradiation, the HBTE derivative was found to be very stable on paper in contrast with the DHBE compound. This effect was related to the presence in HBTE of a bulky substituent (tert-butyl) adjacent to the phenolic group which locks the molecule in a favorable conformation for proton transfer in the ground and excited states.