Origin and surface distribution of anionic groups in different papermaking fibres

The origin and character of anionic groups (AGs) in a broad selection of papermaking fibres comprising chemical, mechanical and chemimechanical pulps was investigated using methylene blue sorption (MB) and determination of uronic and hexenuronic acid contents by different chromatographic methods. The amount of AGs on the fibre surfaces was estimated by labeling with Mg2+ and MB, and subsequent analysis using techniques with different depth resolution, i.e., energy dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS or ESCA). Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was also used to study the surface distribution of AGs. A significant part of AGs in chemical pulps was found being introduced during pulping and bleaching, independently of the wood used as raw material. In case of mechanical and chemimechanical pulps, a larger portion of AGs were from uronic acids already present in wood. Effects of different pulping and bleaching processes on the formation, depth profile and surface distribution of AGs were assessed. The content of AGs for different pulps at depths of 1000 and 10 nm were in the range of 60-97 and 1-3% of the total AG amounts, respectively.