The effect of wood fibre dimensions on energy consumption and pulp properties in pressurized grinding of Norway spruce

The physical and chemical properties of the raw materials to be pulped are decisive for the overall pulp quality. A considerable variation of fibre dimensions exists within a species and even within an individual tree. Unlike chemical pulping where mainly chemical composition and fibre flexibility are altered, mechanical pulping results in severe changes in respect to fibre dimensions and shapes. The objective of this study was to investigate the performance and development of fibre populations with distinctive morphological features in pressurized grinding. Two wood assortments hum Norway spruce (Picea abies) representing a population of short and fine juvenile wood fibres from a young tree as well as a population of coarse and long mature fibres from an old tree were ground under well defined and controlled laboratory conditions to pressure groundwood pulps of various freeness levels. The pulp quality obtained was evaluated in terms of individual fibre properties, pulp bulk properties, McNett fraction properties and handsheet properties. There was a clear relation between tracheid morphology and specific energy consumption. The mature wood required between 15 and 33% more energy than juvenile wood to produce pulp of a similar freeness level. The mature wood pulps contained considerably more shives and had a higher long fibre content and less fines than those produced from juvenile wood. When compared at a certain freeness or energy level, the pulps from juvenile wood afforded paper with superior smoothness and opacity. Also, its tensile index was equal or higher than for mature wood pulps as compared at a certain freeness or sheet density, while the tear strength was clearly lower.