Acacia mearnsii, Eucalyptus grandis and Pinus patula are the main commercial plantation species grown in the Eastern Highlands of Zimbabwe. They are a potential source of pulpwood for a proposed bleached kraft pulp and paper mill. The aim of this study was to investigate and evaluate the technical factors to be considered when planning pulpwood plantations.
An extensive literature survey of important pulpwood properties, with emphasis on measurable and potentially controllable properties, revealed that neither cellulose, lignin nor extractive content are universally reliable indicators of pulp yield. Wood density emerged as the most important pulpwood property affecting wood handling costs, pulp yield, pulp quality and digester productivity. Among the fibre properties, fibre length and fibre coarseness were found to have large influence on paper properties although this is more important where large differences occur such as those found between softwoods and hardwoods.
To provide material for assessing the pulpwood quality, the hardwood species were sampled at single sites from 8 year old trees, while P. patula was sampled at two sites (Erin and Stapleford) from two age classes, 14 and 25 years (ie. 6 stands). For investigation of wood properties within trees, 25 mm thick discs were taken at six height levels (0, 10; 30, 50, 70 and 90% up to 8 cm diameter over bark) from three sample trees from each of the six stands. The material was also used for pulping within tree of P. patula, by simulating utilisation of 7 year old thinnings, toplogs and slabwood. In order to determine the average pulpwood quality, six trees were sampled from each of the P. patula stands and twelve trees from each of the hardwood stands. A single log (15 cm long) was taken from the trees in rotation from the midpoint of the butt, middle and top third of the stem.
Wood density was measured by an x-ray densitometric technique along the north and south radii of all sample trees. Fibre length and coarseness were measured by the Kajaani FS200 fibre analyser. Investigation of fibre length was carried out on the north radii of a single tree from each of the four P. patula site-age classes, and two trees of each of the hardwoods. Fibre coarseness was only investigated on a single tree of each hardwood species. P. patula fibres caused blockages of the Kajaani capillary which frequently stopped the analysis and invalidated the measurement.
A technique was developed for preparing whole fibres for determining length distributions and coarseness. Radial ships, while clamped between perforated metal plates, were partially delignified by kraft pulping and bundles of whole fibres were extracted from the pulped still), thereby avoiding the problem posed by cut and broken fibres.
A special procedure was developed, using Millipore filters, to estimate oven-dry mass of the fibres in the suspension used in the Kajaani fibre analyser and through this the fibre coarseness.
Wood properties were measured and recorded for each growth ring for P. patula and for every 5 mm section from the pith for the hardwoods. Within-tree data for P. patula were analysed on the basis of the age of tree when the growth ring was formed (year-number). Analyses of variance were used ito examine differences in density, fibre length and fibre coarseness between trees, between the north and south radii, between six height levels and between year- numbers or distances from pith. Regression analysis was used to estimate mathematical equations that best described the pattern of the wood property within the tree. Predicted values calculated from the equations were presented as grey scale maps to illustrate visually the within-tree variation pattern of the properties.
P. patula showed an increase in wood density with tree age. The area-weighted mean densities of the 14 year old trees of P. patula were 0.476 and 0.492 g/cm3 respectively for samples from Erin and Stapleford. The respective values for the 25 year old trees were 0.557 and 0.544g/cm3. A. mearnsii and E. grandis had area-weighted mean density values of 0.811 and 0.583 g/cm3 respectively. For the three species, variation of density between trees, between heights and with year-number or distance from pith was statistically significant. For P. patula and A. mearnsii, density decreased with tree height, while density increased after an initial decrease from 0 to 10% height level in E. grandis. In both P. patula and A. mearnsii, the overall decrease between 0% and 90% height levels was about 20% for all trees.
At all height levels, density increased from pith to bark. For P. patula the rate of increase from pith bark was similar at the different heights, resulting in a cylindrically symmetric within-tree pattern, similar to that reported for P. radiata (Cown & McConchie 1980). A similar pattern was found in A. mearnsii, but the central core of low density extended from about 30% height level upwards. The pattern of wood density in E. grandis requires further work since the regression equation obtained had a low coefficient of determination and there were no obvious trends apparent. Equations from previous studies (e.g. Malan 1988) also failed to describe the density pattern.
The corresponding points along the two radii of both P. patula and A. mearnsii were found not to differ significantly. Although a significant difference was observed in E. grandis, the difference was small in practical terms.
Fibre length in P. patula increased with tree age. The ring area-weighted mean fibre length for the 14 year old trees were 3.48 mm and 3.07 mm respectively for the samples from Erin and Stapleford. Fibre lengths for the 25 year old trees were 3.55 and 3.84 mm respectively for Erin and Stapleford. The fibre length of 0.89 mm for E. grandis was significantly (at the 5% probability level) longer than the 0.76 mm recorded for A. mearnsii.
Fibre length varied significantly with tree height and with year-number or distance from pith. The general pattern in all three species was for fibre length to increase from a minimum at the butt, reaching a maximum at 10 or 30% height level before decreasing towards the apex.
Radial increase from pith to bark was more than 100% at most height levels. In P. patula, most of this increase occurred in the first 10 growth rings from the pith. Unlike wood density, the within tree pattern for fibre length was nearly symmetric conically. These patterns closely resemble those reported for P. radiata (Cown & McConchie 1980). Both hardwood species exhibited a cylindrical distribution of fibre length within-tree. This difference arises from the linear radial increase found in the hardwoods as opposed to the curvilinear increase found in P.patula.
E. grandis had coarser fibres, yielding an area-weighted fibre coarseness of 6.19 mg/100M compared to 4.23 mg/100m for A. mearnsit. The trend in the mean fibre coarseness is different in the two species. For A. mearnsu, fibre coarseness increased up the tree but in E. grandis there was a decrease with tree height. In E. grandis, fibre coarseness increased with distance from pith, while in A. mearnsu it remained relatively constant. The interesting point is that the radial trends are the reverse of those found for density. These results contradict the generally held view that density and fibre coarseness are correlated. More research is needed to clarify this because, while the relationship was statistically significant for E. grandis, that for A. mearnsii was not significant.
Basic density of the pulpwood samples was 414 kg/m3 for E. grandis, 610 kg/m3 for A. mearnsii, and averaged 370 and 420 kg/m3 for the 14 and 25 year-old P. patula.
To achieve a Kappa number of between 20 and 30, P. patula, required 18 to 19% active alkali to produce 44 to 45% screened pulp yield. For Kappa number 20, A. mearnsii required 12.5% active alkali to yield 53.0% screened pulp and E. grandis 10.5% active alkali for 54.2% screened pulp yield. The study demonstrated that the two hardwood species can successfully pulped together resulting in a marginally lower screened yield. A. mearnsii had the highest pulp yield per cubic metre of wood (323 kg) followed by E. grandis (224 kg) and P. patula (174 kg). All pulps could be bleached to a brightness of 87-89% ISO.
Lightly beaten pulp from the 25 year old P. patula trees had the highest tear index (18 - 21 mN.m2/g), and highly beaten pulps from the 14 year old P. patula trees had the highest tensile index (112 - 115 N.m/g) and burst index (about 9 kPa.m2/g). Bleaching reduced tear index but increased tensile index.
At 300 Csf, the tensile and tear indices for E. grandis pulp (127 N.m/g and 11.0 mN.m2/g) and for A. mearnsii pulp (103 N.m/g and 9.4 mN.m2/g) would be classified as very good and adequate for use in unbleached packaging papers. A. mearnsii pulp had higher opacity (70 vs 65%) which is crucial for printing papers. Bleaching reduced tensile index by about 14% in both species.
The pulping and papermaking results of A. mearnsii demonstrates that the conventional prejudice against the use of this species as pulpwood in Zimbabwe may be wrong. Its high density means that on a volume basis, there is more wood material that can be transported and higher pulpwood productivity will enable pulp production from less volume of wood for a given digester capacity. These all have beneficial economic consequences to the pulp producer. The species has the added advantage that it can be successfully co-pulped with the other major hardwood species.
A new sampling scheme was developed to provide wood formed when trees were 7 years old. This was useful in demonstrating that pulp yield and papermaking properties of the simulated 7 year old thinnings of P. patula were not influenced by tree age from which they were extracted. Bulk density decreased in the order of slabwood, toplog and 7 year old. Slabwood produced pulp with the highest tear index. This property was about the same for toplogs and 7 year old material.
The test results for the P. patula components provided an opportunity for correlation analysis of wood density and fibre length with pulp properties (freeness, bulk, tear index, tensile index, stretch, burst index and air permeance). All handsheet properties examined (except freeness and stretch from high tearing strength pulp) could be predicted in terms of wood density and fibre length to the extent of accounting for at least 69% of the variation.
A model was developed for estimating plantation areas required for the production of the proposed pulp grades. It involves three parameters and the associated coefficients of variation, namely pulp yield (Y), basic density of wood (BD) and mean annual volume increment (MA1).
Using the growth rates of similar species in South Africa and test results from this study, it is estimated that the 60 000 t/y mill could be supplied from about 20 000 ha of dedicated pulpwood plantations.
The results show that growing P. patula pulpwood on 25 year rotation as against 14 year rotation will provide high strength pulps for packaging papers
