Modification of xylan in alkaline treated bleached hardwood kraft pulps as classified by attenuated total-internal-reflection (ATR) FTIR spectroscopy

The glucuronoxylan composition of a pulp affects the bonding between cellulosic fibres, and thus correlates with such network properties as tensile strength. Here, we demonstrate the promise of attenuated total-internal-reflection (ATR) FTIR spectroscopy as a rapid means for classifying the xylan contained in commercial bleached kraft pulps. This study draws upon samples composed of bleached eucalyptus kraft pulps and combinations of eucalyptus with other commercial bleached kraft pulps. We subject these pulp samples to systematic extraction by sodium hydroxide solutions with concentrations ranging from 0.5% to 6% to build a standard sample library with varying xylan content, quantified by acid hydrolysis, HPLC carbohydrate separation and titration. This pulp chemistry of mild alkaline extraction removes up to two-thirds of the xylan. In the NaOH concentration regime of 0.5-4%, the infrared spectral variance reflects the decrease in hemicellulose concentration as well as the cellulose crystallinity. A residual xylan component remains resistant to base solutions of higher concentrations. Principal component analysis of infrared spectra distinguishes this residual xylan as structurally variant. Both partial least squares multivariate analysis and univariate analysis confined to a feature at 964 cm−1 in normalized second derivative IR spectra show a very good correlation with xylan content quantified by HPLC.