A substantial amount of research has allowed the introduction of efficient techniques, for genetic improvement of trees from Eucalyptus species. The basic density of wood has been identified as one of the main hereditary characteristics, to be employed as an important selection parameter for propagation of new forests. An important part of this program is the evaluation of the effect of various chemical and anatomical properties, which may be associated with wood basic density, upon industrial aspects of the production of pulp and paper.
The influence of wood basic density on chemical and anatomical characteristics, on production parameters, and on paper properties was evaluated for twenty-five hybrid trees of Eucalyptus grandis. The trees were selected at random from the same plantation site, with basic density values varying from 418 to 666 kg/m³, at seven years of age.
Anatomically, it was observed that fibers presented smaller diameters and thicker walls as wood basic density increased. On the chemical side, denser trees showed a tendency to have more lignin and less pentosans, as compared to individuals of the same species and age, with lower density values. Fiber length and extractives content were not correlated to wood basic density, which suggests that these properties may be genetically controlled.
Pulping evaluations, performed under simulated mill conditions, indicated that pulp yield increased when basic density varied from 418 kg/m to approximately 470 kg/m³, but decreased from this point up to 666 kg/m³, accompanied by a steady increase in rejects content. Nevertheless, an estimate for digester yield showed that production capacity can be improved through utilization of denser woods. Hence, from a production viewpoint, other aspects, such as the observed increase in chemical consumption during pulping, as well as the higher viscosity and solids content of black liquor, may become more relevant when pulping
denser woods. The latter may translate into a limiting factor in production, if the recovery system is not dimensioned to handle extra loads.
Unbleached pulps of the same kappa number showed diminishing trends in viscosity and pentosan content, when wood density was increased in the range studied. The properties of paper were shown to be strongly correlated with variations in basic density. Sheet consolidation and fiber bonding decreased almost linearly with wood density, as indicated by lower apparent density and tensile strength values, which are mere consequences of lower fiber
flexibility in denser woods. The porosity of handsheets, which is also closely related to sheet structure, demonstrated an almost exponential increase with wood density.
Trends in tear resistance were shown to be dependent on the amount of beating. At low to medium beating levels the tear strength was lower for denser woods, and after extensive mechanical treatment no correlation was found. All apparent variations have led to the conclusion that selection of Eucalyptus grandis trees with wood densities beyond a certain level may result in undesirable combination of paper properties, for most end-uses.