ON THE IMPORTANCE OF HYDROQUlNONE/ p -QUINONE REDOX SYSTEM IN THE PHOTOYELLOWING OF MECHANICAL PULPS

In the area of photoyellowing of mechanical pulps, recently obtained experimental evidence has shown that hydroquinone/p-quinone redox couple is present in lignin-rich mechanical pulps. It was also noted that compared to a control pulp the concentration of pquinones was significantly higher in a photoyellowed pulp. Under ambient conditions, upon exposure to light, the existent pulp hydroquinones were converted to p-quinones. To further investigate the importance of hydroquinone/p-quinone redox couple in photoyellowing, studies of methylhydroquinone and methyl-p-quinone were carried out Based on presently reported as well as previously published research, it is shown that the hydroquinone/p-quinone redox system can successfully explain most of the observations related to photoyellowing. Consequently, the hydroquinone/p-quinone redox couple plays a highly significant role in pulp photoyellowing. INTRODUCTION Recent FT-Raman studies of untreated, bleached (single and multiple stages), and photoyellowed thermomechanical pulps (TMPs) provided information that unequivocally established that p-quinones structures were present in photoyellowed and untreated samples. In the case of bleached TMPs, pquinone concentration depended upon the extent of bleaching (higher brightness pulps showed lower amounts). Moreover, when the pulp-handsheet brightness data were plotted against the decline of the p-quinone Raman band intensity a good linear correlation was obtained. Another method, C NMR spectroscopy, has recently provided further evidence in support of p-quinones. It was reported that such structures were present in softwood MWLs. Therefore, detection of p-quinones by two independent non-destructive techniques proves beyond any doubt that these structures are integral part of native lignin’s molecular structure. In other investigations, oxygen quenching of laserinduced fluorescence (LIF) was used to show that hydroquinone structural were present in significant concentration in woods, mechanical pulps, and MWLs. Based, in part, on these findings a photoyellowing mechanism was proposed’. This mechanism in conjunction with the studies of the methylhydroquinone/methyl -p-quinone (MHQ/M-p-Q) redox couple successfully accounted for several aspects of the photoyellowing behavior of mechanical pulps. In this report, results of further experiments that were performed using the model system and mechanical pulps are described. These as well as previously reported results are used to evaluate the importance of the role of hydroquinone/p-quinone redox system in photoyellowing. EXPERIMENTAL Experimental aspects of the work described in this abstract are provided in reference 1 and in references