Photoyellowing of lignin-rich paper: interaction of excited states with selected additives

Abstract

Specific chromophores in bleached thermomechanical pulps (BTMPs) were targeted by fluorescence quenching with cyclohexadiene. The results suggest that fluorophores in BTMPs are mainly conjugated aromatic hydrocarbons (ArH). The observed effect of cyclohexadiene on the photochemical yellowing of BTMPs supports recent work which ascribes a role in lignin photochemistry to ArH chromophores based on stilbene and biphenyl. The relative efficiency of triplet vs. singlet photochemistry was examined by applying a heavy atom agent, lead acetate, to the pulps. It is postulated that α-carbonyl triplets and ArH singlets are important sources of colour reversion. Based on a reconsideration of possible signlet state quenchers, several additives were tested as inhibitors; (EtO) 3P and Bu 3P were found to inhibit paper yellowing. Results of a tentative mechanistic study point to the reactions of the inhibitors with complex radical intermediates, rather than excited state quenching, as the key process in the observed inhibition.