Browsing by Author "Gonya, Nkosinamandla Artwel Sizwe"

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    An investigation into shrinkage and collapse behaviour of Eucalyptus grandis and Eucalyptus grandis x urophylla wood
    (Stellenbosch : Stellenbosch University, 2020-03) Gonya, Nkosinamandla Artwel Sizwe; Wessels, Brand; Stellenbosch University. Faculty of AgriSciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: Only 1 % of land is used for forestry in South Africa and the country might experience a shortage of structural softwood sawn timber in the near future. Despite this, South Africa actually produces more Eucalyptus logs than the local processing industry can process, hence end up exporting large volumes of Eucalyptus chips to foreign countries. Additionally, on average, the annual increment of South African E. grandis is almost twice that of South African Pinus species. Recently, the green processing of Eucalyptus lumber into engineered wood products has been investigated. However, the variation of dimensional changes during the drying process due to shrinkage and collapse make the efficient processing of Eucalyptus lumber a challenge. A better understanding of the shrinkage and collapse behaviour of Eucalyptus grandis and Eucalyptus grandis X urophylla might enable improved lumber separation and processing. The objectives of the study were to profile the variations of (1) collapse and (2) shrinkage (a) radially and (b) along the height of trees and to explore the relationships they have with basic physical properties of the tree. The hypothesis was that the non-uniform dimensional changes found in the end products of Eucalyptus grandis are due to these variations. Seventy trees of Eucalyptus grandis and Eucalyptus grandis X urophylla were collected from Tzaneen, Limpopo, South Africa. The trees were selected based on age, genetic improvement, as well as an additional splitting trial: 10x trees young (8 yrs) and genetically improved, 10x trees young (7 yrs) and genetically unimproved, 10x trees mature (13 yrs) and genetically improved, 10x trees mature (13 yrs) and genetically unimproved, 20x trees from a splitting trial (17 yrs) and an older trial (24 yrs). All trees were E. grandis except the mature groups which were E. grandis X urophylla. Disks that were later cut into wedges were removed from four heights (1.3 m, 6.4 m, 11.46 m, and 16.52m) from each tree. Two logs of 4.2 m were removed from two heights, processed into boards, and kiln dried. On the boards collapse was visually assessed and shrinkage measurements were taken before and after drying. On the wedge samples shrinkage, collapse, density, moisture content, permeability, extractives contents and heartwood/sapwood ratio were measured. All the samples and measurements were taken so that they give the radial variation between centre (pith), transition zone and outer wood (sapwood). An important finding in this study was that the properties measured on small wedge samples differed significantly from what was observed on sawn boards. This was true for both collapse and shrinkage - the properties on which this study focussed. On boards, collapse varied depending on group, log position and the radial position of the boards. The radial position of the boards had a highly significant effect on board collapse, with collapse generally decreasing from pith to bark. There were first order interactions between all the factors and board collapse behaviour was not consistent throughout groups and log positions. In terms of the causal factors, there were weak but significant correlations between collapse and extractives content, density, and permeability (although these results should be viewed with caution since it was obtained from the small wedge samples). A very interesting result is the weak correlation between radial and tangential collapse on small wedge samples showing the clear directional nature of collapse. On boards, shrinkage varied depending on group, log position and the radial position of the boards. Shrinkage in the thickness direction was higher in the centre boards and decreased towards the outer boards. This was due to the effect of collapse on the thickness direction as collapse is very size sensitive and no matter the direction (radial or tangential) the wider side experienced the most collapse. In the width direction, the shrinkage followed the normal pattern increasing from centre boards towards the outer boards. The width direction shrinkage had almost similar trends with the results found on wedges in both directions (radial and tangential). Twist, bow and cup also varied significantly radially and along the height. However, the magnitude of these was generally far better than the requirements of national standards. Bow had a similar trend as shrinkage in the width direction. The trends for twist and cup were not consistent. Some of the results obtained from small disk samples were inconclusive and contradicted what was observed in boards. It is recommended that future studies focus on obtaining a better understanding on the effect of specimen size. Although trends observed from the small samples were interesting, the practical significance is limited if the results cannot be related to observations in industrial size boards.