Surface characterisation of mechanical pulps by polyelectrolyte adsorption

The charge properties of Norwegian spruce (Picea abies) pressure groundwood (PGW), thermomechanical (TMP) and chemithermomechanical (CTMP) pulps were studied. Surface and total charge were determined by the polyelectrolyte adsorption method, detecting the end point of polyelectrolyte titrations with a dye indicator or a particle charge detector (PCD). PCD generally yielded slightly lower charges than the dye indicator. Conductometric and potentiometric titrations gave additional information on the origin of the charge in CTMP pulp. Surface carboxyl groups in PGW (in calcium form) were also determined by electron spectroscopy for chemical analysis (ESCA). The charge distributions in unbleached PGW and TMP were quite similar. Among the unbleached pulps, sulphonate group-rich CTMP had the highest charge of all. The effect of dithionite bleaching on the charge of PGW was insignificant. Alkaline treatment doubled the amount of free carboxyl groups. Peroxide bleaching introduced some additional carboxyls. About 35% of the charged groups were accessible to a high Mw polyelectrolyte (PDADMAC Mw 1•105-3•105). Ozone treatment modified the surface lignin and increased the surface charge; up to 70% of the acidic groups in PGW pulp were located in the surface layer. Methylglucuronic (MeGluA) and galacturonic acids (GalA) were analysed by high-performance liquid chromatography (HPLC) after acid hydrolysis together with sulphonic acid, and were found to account for 30-90% of the charge of the pulps. Extractives apparently covered some charged groups attached to other wood components, especially at a high extractives content. The results showed that details of the mechanical pulping process, in CTMP manufacture in particular, may have a strong effect on the fibre charge.