Characterization of fiber carboxylic acid development during one-stage oxygen delignification

This study examines the changes in fiber carboxylate content during a series of one-stage oxygen delignification experiments using a southern United States pine kraft pulp. The carboxylic acid content of the oxygen-delignified pulps before and after holocellulose pulping was determined to establish the distribution of acid groups in lignin and pulp carbohydrates. The carboxylic acid content in residual lignin were also examined and compared with the holocellulose results in an attempt to decouple the responses from residual lignin and carbohydrate. It was found that oxygen-delignified kraft pulps exhibited an initial 4%-13% increase in total fiber carboxylic acid content in the first 10-30 min, followed by a steady state or slight decrease. In contrast, the holocellulose of oxygen-delignified pulps exhibited an initial 6%-8% decrease in carboxylic acid content in the first 10 min, followed by a small increase and then a slow decline. The residual lignin isolated from these pulps showed an initial 37% carboxylic acid content rise in the first 10 min, followed by a rapid drop and then a further slow increase at 100 degrees C with 2.5% NaOH and 800 kPa of O-2. The characteristic kappa numbers for the maximum carboxylic acid content were observed in the range of 19-24 for the oxygen-delignified fibers and the corresponding holocellulose, and similar to 25 for the residual lignin of oxygen-delignified pulps under experimental conditions. Overall, acid groups in the residual lignin significantly affect the total fiber carboxylic acid content in the initial phase, whereas acid groups in the carbohydrate fraction control the total fiber carboxylic acid behavior in the remaining phases. The optimal conditions for obtaining a pulp with high carboxylic acid content, under the conditions studied, were observed to be 100 degrees C with 2.5% NaOH and 800 kPa of oxygen.