Qualities of Kraft and Thermomechanical Radiata Pine Papermaking Fibres

Abstract

This review examines the extent to which wood property variation in New Zealand’s radiata pine resource determines pulp quality. The qualities of radiata pine papermaking fibres are very dependent on their original position within a tree (growth rings from pith and/or height in tree), as well as the geographic altitude and latitude of sites on which the trees are grown. Two categories of radiata pine pulpwood are recognised in New Zealand: slabwood of high basic density from the outside of sawlogs; and corewood of relatively low basic density from the smaller logs (non-sawlogs) of the upper part of a tree and from whole-trees less than 20 years old.

The kraft pulp fibres from corewood are shorter and have thinner walls than corresponding fibres from slabwood, but the diameters of these two fibre populations are essentially identical. The handsheet properties (apparent density, and burst, tear, and tensile indices) are strongly correlated with, and can be predicted from, the wall thickness:diameter ratio of pulp fibres or the basic density of the wood sample pulped. These trends hold for whole trees of different age, for parts of trees, and for commercial pulpwood and slabwood material obtained from throughout New Zealand.

Mechanical pulps can be correlated with wood properties to a lesser extent than are kraft pulps. In thermomechanical (TMP) pulp production, slabwood consumes more energy to a given freeness and produces pulps of higher strength than corewood. Pulps from corewood, however, have excellent optical properties whereas those from slabwood are of slightly lower quality. These differences are partly explained by the very different qualities of slabwood and corewood fibres and fines. Slabwood TMP pulps are rich in fibrillar fines, which have a strong consolidating influence on the long and relatively stiff fibres of this furnish. Alternatively, corewood fines are of a more heterogeneous arid coarse quality (and have a lesser consolidating effect on fibres which are shorter and more collapsible than corresponding slabwood fibres) and therefore pack more tightly within handsheets. The handsheets of corewood pulps have excellent optical properties since the fibres and fines of this furnish give more air-to-fibre and air-to-fibre element interfaces than do those of corresponding slabwood fibres and fines.