Characterisation and effects of new anionic groups formed during chemithermomechanical pulping of spruce

The amount of total anionic groups (TAGs) in laboratory-scale chemithermomechanical pulps (CTMP) were assessed by Methylene Blue (MB) and quinoline sorption. The amounts of surface anionic groups (SAGs) were determined by MB sorption followed by X-ray photoelectron spectroscopy (XPS) and by polyelectrolyte (PE) titration. The content of uronic acids in the pulps was determined by acid methanolysis and gas chromatography (GC) and the methyl-esterification degree of pectins by alkaline hydrolysis. Pulps were produced by chemical pretreatments of sapwood chips of Norway spruce (Picea abies) prior to refining in a small-scale batch refiner. The chips were pretreated with alkaline, sulphite, alkaline sulphite and alkaline peroxide liquors. The TAGs in the pulps increased linearly with alkali and sulphite dosages. Sulphonic groups in lignin and new carboxyl groups formed in pectins accounted for a major part of the new anionic groups. New anionic groups were also created in the fibre material due to lignin oxidation in alkaline peroxide treatment. Pretreatment with sulphite resulted in a lower surface lignin content compared to pretreatment with alkali or alkaline sulphite. This may indicate more preferential fibre separation in the primary wall layers than in the middle lamella or be a result of redeposition of alkali-dissolved lignin onto fibre surfaces. The amount of SAGs, determined by XPS, was doubled for the chemically pretreated pulps compared to a reference TMP. This is probably a result of primary wall exposure and substantial pectin demethylation. Sulphonation did not result in more SAGs.