THE ROLE OF STILBENES IN BLEACHING AND COLOR STABILITY OF MECHANICAL PULPS .1. THE REACTION OF LIGNIN MODEL STILBENES WITH ALKALI AND OXYGEN

The reactions of lignin model hydroxystilbenes and TMP with alkaline h ydrogen peroxide and with alkali and oxygen have been studied to deter mine the susceptibility of stilbenes to oxidative degradation. Oxygen in the presence of alkali readily destroys lignin model stilbenes via cleavage of the olefinic double bond. The stilbene must contain a ring hydroxyl substituent for the reaction to occur. The reaction is faste st if the stilbenes have p,p'-dihydroxyl substituents. Stilbenes of th e type likely to be attached to the lignin in pulps have only one hydr oxyl group and are more stable. Hydrogen peroxide, by itself, is unrea ctive with stilbenes but the presence of oxygen as a peroxide decompos ition product should limit the accumulation of stilbenes during alkali ne peroxide bleaching of mechanical pulps. Alkaline peroxide bleaching of mechanical pulps under a pressurised atmosphere of oxygen gas lead s to a slight increase in brightness and brightness stability. Guaiacy l-substituted stilbenes are more stable to alkali and oxygen than are syringyl-substituted stilbenes. This may partially explain why it is e asier to bleach hardwood than softwood mechanical pulps.