Hydrogen peroxide bleaching of mixed recycled fiber

Mixtures of recycled furnishes containing different ratios of old newspapers (ONP), mixed office wastepapers (MOW) and old corrugated containers (OCC) were bleached using 2% hydrogen peroxide and varying amounts of sodium hydroxide. Optical and physical properties of the bleached mixtures were compared to samples where the three recycled fiber grades had been bleached separately followed by mixing the bleached samples at the same ratios. Brightness response of the premixed material in all cases was the same or better than the material prebleached followed by mixing. Also, strength properties of the premixed samples were superior if compared at the same caustic application level. The effect was most pronounced for material containing higher levels of ONP (high kappa number). Bleaching to the same final pH of approximately 9.5 resulted in essentially the same strength properties independent of the bleaching/mixing sequence, but total caustic application required for the premixed samples was lower. No justification for sorting and separate bleaching of mixed grades recycled fiber under the bleaching conditions chosen in this study could be determined.