Effect of surface properties of fibres on some paper properties of mechanical and chemical pulp

Abstract

The overall goal of the thesis was to find correlations between the surface precipitates of pulps and selected strength properties of paper sheets. Special attention was paid to the surface lignin and extractives of fibres. The main surface-characterising techniques employed were electron spectroscopy for chemical analysis (ESCA), atomic force microscopy (AFM) and the polyelectrolyte titration method. In addition, a Wilhelmy balance for single fibres, time-of-flight secondary ion mass spectrometry (ToF-SIMS) and the fourier transform infrared (FTIR-PAS) method were applied.

The main object of the thesis was to characterise the surfaces of different mechanical pulps and the effect of different treatments, such as peroxide and dithionite bleaching, alkaline and ozone treatment and water washing, on these surfaces. Surface lignin was preferentially modified rather than decreased after sulphonation, peroxide bleaching and ozone treatment. All these treatments increased the hydrophilicity and Lewis base parameter of fibres, and introduced acidic groups into the pulp. Surface extractives were efficiently decreased after water washing and peroxide bleaching. Acidic groups were found to be important for the strength of the paper sheet. The more acidic groups were introduced, the more the fibres swelled, and thereby the flexibility and conformability of fibres appeared to increase. Extractives removal increased the hydrophilicity and basicity of fibres and increased the adhesion between fibres and water.

Adsorption experiments with model compounds of extractives (sodium abietate and sodium oleate), pitch and kraft lignin adsorbed on fully bleached kraft pulp were also carried out. Precipitated kraft lignin on the kraft fibres did not alone significantly affect the strength properties of the pulp sheets. By adsorbing first the cationic polymer before the anionic polyelectrolyte, such as kraft lignin and polyethene sulphonate, the strength properties increased markedly. Lowering of the pH of a pulp slurry taken after the oxygen delignification stage resulted in a clear precipitation of kraft lignin, which impaired the strength properties of the sheet. At a small coverage of model compounds of fatty and resin acids, the bonding between fibres was increased until a certain concentration was reached, after which the bonding decreased noticeably. Numerous samples were collected from different bleaching stages in highly closed softwood and hardwood kraft mills and compared with laboratory-washed samples. Surprisingly, no clear traces of any precipitates on the surfaces of these pulps were found.