The Morphology, Chemistry and Pulping Characteristics of Reaction Wood

Abstract

In this paper, ‘reaction wood’ has been described and reference made to its wide occurrence in forest trees and the reasons for such occurrence. It has been emphasised that there may be all gradations from mild to severe reaction wood formation depending on the nature and intensity of the stimuli responsible. The macroscopic and microscopic features of the reaction wood of softwoods (compression wood) and that of hardwoods (tension wood) have been recorded. Particular attention has been paid to the variations in cell wall organization found in the two types of reaction wood and comparisons have been made with that of normal wood. Ways in which the cell wall organization might influence properties of both wood and pulp have been discussed. The chemical composition of reaction wood differs from that of comparable normal wood and these differences are particularly marked where the reaction wood is most severe. Compression wood is higher in lignin content and lower in cellulose content than normal wood of the same tree; the reverse is the case with tension wood and, in addition, the pentosan content is much lower. By staining techniques and ultra-violet microscopy, it has been shown that the cell wall of compression wood fibres is highly lignified and that of tension wood fibres is virtually unlignified. The lower pentosan content of tension wood has been correlated with the very poor papermaking qualities of chemical pulps prepared from it. On the other hand, the higher lignin content of compression wood does not apparently interfere with either the preparation or the properties of the chemical pulps prepared from it, although such pulps contain considerable quantities of lignin (in the cell wall). The association of cell wall deformations with the development of reaction wood has been referred to and it has been pointed out that such deformations are a source of weakness in pulps prepared by acid pulping methods. It was observed that tension wood produces a mechanical pulp much superior to that obtained from normal wood; on the other hand, compression wood gives a very poor mechanical pulp. Numerous investigations carried out have established that, although the presence of certain amounts of reaction wood adversely affects pulp strength properties, the decrease in strength, from a practical aspect, at least in the case of alkaline pulps, is not too great for most purposes. Finally, it has been stressed that, for the best integration of industries, those logs without reaction wood should be reserved for peeling and sawing, because in the converted timber the presence of reaction wood is likely to be extremely troublesome. This means, of course, that those logs in which reaction wood is present must go to the pulp mill, but here they can be converted into useful pulp by alkaline processes, although the strength of such pulp may be reduced somewhat, depending on the severity of the reaction wood present.