Highly Bleachable Wood Fibers Containing Less Condensed Lignin from Acid Hydrotropic Fractionation (AHF)

This study evaluated the bleachability of wood fibers from acid hydrotropic fractionation (AHF) of birch using p-toluenesulfonic acid (p-TsOH) at low temperatures below 90 degrees C. The wood fibers had varying amounts of lignin after AHF under different conditions using both batch and flow-through configurations. The extent of lignin reprecipitation in batch mode was evaluated by extended washing with p-TsOH. 2D C-13-H-1 nuclear magnetic resonance (NMR) spectroscopy indicated that residual lignin on fibers from batch reaction was significantly more condensed than that from flow-through under equivalent conditions. Furthermore, condensation substantially affected oxidative bleaching by reducing the reactivity of the residual lignin. Even a little variation in the extent of lignin condensation showed a marked effect on sample bleachability. Specifically, oxygen bleaching at 110 degrees C removed 79% of residue lignin from a batch fractionated fiber sample with 7% beta-O-4 aryl ether linkage and resulted in 55% ISO fiber brightness. The same oxygen bleaching removed 87% of residue lignin from a flow-through fractionated fiber sample with almost the same lignin content but with a higher beta-O-4 of 16%, or lower degree of lignin condensation, and higher brightness of 59% ISO. The 4% increase in brightness with the same loading of bleaching chemical translates to significant cost reduction. Furthermore, a brightness of 70% or more can be achieved with single stage peroxide bleaching for the flow-through AHF fibers.