Highly Bleachable Wood Fibers Containing Less Condensed Lignin from Acid Hydrotropic Fractionation (AHF)
Qingzhi Ma, Kolby Hirth, Huimin Zhai, J. Y. Zhu
Abstract
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 °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 13C–1H 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 °C removed 79% of residue lignin from a batch fractionated fiber sample with 7% β-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 β-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.