Studies on fiber strength and its effect on paper properties

The objective of this work was to study factors affecting the strength of pulp fibers used in papermaking and how their strength affects the properties of the fiber network, i.e. the paper. All the structural levels contributing to fiber strength starting from the organization of cellulose chains in microfibrils through to visible defects and fiber deformations were considered. Due to its wide availability, zerospan strength was used as an indication of fiber strength. Industrially made pulp fibers are mechanically damaged and thus weakened in the fiber line. Fiber strength may also degrade chemically. Chemical degradation may be homogeneous or heterogeneous, and the effects of these on fiber and paper properties differ. In this work, acid vapor-induced degradation was found to cause more heterogeneous degradation of fibers compared to ageing treatment at elevated temperature and humidity. In both treatments degradation occurs via the same mechanism – acid hydrolysis – but the difference is attributed to initial fast reaction of the acid vapors at fiber disorder sites. Z-directional fiber strength is less sensitive to fiber degradation than axial fiber strength. Spruce pulp fibers require the viscosity to drop below 400 ml/g before any difference can be detected in the Scott-Bond values. For birch pulp fibers the same happens at a viscosity of 700 ml/g. Neither treatment was observed to affect inter-fiber bonding. Zero-span strength measurements were shown to follow a normal distribution. This was expected based on the Central Limit Theorem and was also predicted by modeling. The variation in zero-span strength was found to increase as functions of decreasing sheet thickness and increasing span length. The former was predicted by the model. The reason for the latter is that the properties of the fiber network start to show in the measurement. Contrary to some earlier studies, it is suggested here that fiber curl itself maybe does not affect the zero-span measurement results. One reason for the often observed increase in zero-span strength during beating could be the favorable organization of fibrils. The Jentzen effect was not observed in the experiments probably because the test pulps likely had very few misaligned fibrils and on average a low fibrillar angle.