The impact of wood quality factors on the manufacture of spruce thermomechanical pulp (TMP) was examined for three different fiber sources: wood from first-thinnings, sawmill chips, and old-growth forests from several forest stands in southern Finland. Average wood and fiber characteristics, i.e., basic density, fiber morphology, percentages of moisture, latewood, heartwood, juvenile wood, bark and extractives, and their variation were determined for each raw material group.
Subsequently, each group equalling about 1500 m3 roundwood was pulped separately in commercial scale for a 24 h period, at constant process conditions. The properties of resulting newsgrade thermomechanical pulps were then determined and any differences were related to selected wood and fiber properties.
TMP from sawmill chips had the highest proportion of long fibers, smallest proportion of fines, and generally had the coarsest and longest fibers. In this respect, TMP from first-thinnings was just the opposite, whereas TMP from old-growth wood fell in between the two above pulps. Moreover, a high proportion of dry heartwood in old-growth wood produced slightly elevated shives content. No statistically significant differences were found in pulp specific energy consumption between the three wood categories.
Pulp tear index was clearly best in TMP made from sawmill chips, and poorest in pulp from first-thinnings wood, which had generally inferior strength properties. No dramatic differences in any of the strength properties were found between TMP from sawmill chips and old-growth wood. Pulp optical properties, in terms of brightness and light scattering coefficient, were superior in TMP from first-thinnings. Unexpectedly, no noticeable differences in sheet density, bulk, air resistance or roughness, which could be explained with fiber morphological factors, were found between the pulps.
The most important wood quality factors in this study were fiber length, fiber cross-sectional dimensions and percentage juvenile wood. Differences in the quality of pulp manufactured from the three groups of spruce wood suggest they could be pulped separately in order to obtain specific product characteristics and to minimize the unnecessary variation in raw material quality, and hence, pulp quality.
Key words: Picea abies, wood properties, fiber properties, pulp quality, thermomechanical pulp.