Influence of pore structure and water retaining ability of fibres on the strength of papers from unbleached kraft fibres

The influence of pore structure and the water retaining ability of fibres on different strength properties of papers from unbleached kraft fibres was investigated and the effects of pulp yield, counter-ion types, pH and homification were determined. NMR relaxation measurements of water were used to determine the pore structure of the fibres and WRV and FSP measurements were used to determine their water retaining ability. The average pore radius, as determined by NMR, was almost unaffected by changes in pulp yield whereas changes in counter-ion and pH had a significant effect on the average pore radius. The detected changes in NMR were suggested to be due to changes in the swelling forces both within the fibre wall and at the fibre surface. The WRV value decreased with decreasing yield and it was significantly affected by pH and counter-ion. Changes in WRV were explained to be largely associated with changes in the amount of water associated with the fibre surface. The FSP values decreased with decreasing yield just as the WRV's. Homification upon drying and reslushing significantly lowered the average pore radius, whereas the FSP only showed a minor decrease, suggesting that the surface area available to water was changed without drastically changing the overall fibre wall volume. The differences between FSP NMR and WRV can hence be traced back to what the methods are actually measuring. It was concluded that the different measuring methods contain unique information and that a combination of the methods is necessary to give as complete a picture as possible over the changes that occur in the fibre wall upon varying the condition for the fibres. The influence of pore size on sheet tensile properties was also investigated. It was found that fibres with larger pores produced an increased tensile index and tensile stiffness of the paper made from these fibres. It was suggested that fibres with larger pores allow for a larger molecular contact area between fibres, stronger fibre/fibre joints and consequently a higher strength of the formed sheets.