Graduate Thesis Or Dissertation
 

A comparative study of preservative treatment of selected woods from Ghana and the United States

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  • The response of three hardwoods from Ghana and a hardwood and three softwoods from the United States to liquid and gas-phase impregnation was investigated. Impregnation with liquids included a sink-float test in a water-glycerine solution and pressure treatment with a copper sulphate solution and creosote under identical treating conditions. Flow of gases through wood was measured with an air permeability test and by the movement of chloropicrin vapour, an agricultural fumigant, through the samples. The effect of physical and anatomical characteristics of each species on its treatment was investigated. Penetration and retention of treating solutions in wood samples immediately after the time of removal from the treating cylinder followed nearly identical trends for the copper sulphate and creosote. That is, a particular species of wood would have large penetration and retention of both preservative solutions compared to other species of wood, although the values for each preservative solution differed. The sapwood and heartwood of Sugar Maple (Acer saccharum Marsh) from the United States gave highest penetration and retention among the hardwoods. Among the woods from Ghana, highest retention and penetration were in the sapwood and heartwood of Subaha (Mitragyna stipulosa [De Candole] 0. Kuntze). Low penetration and retention were found in Dahoma (Piptadeniastrum africanum [Hook f.] Brenan), and very low retention and penetration were present in Kaku (Lophira alata [Banks] Gaertner). Of the softwoods, the heartwood of redwood (Sequoia sempevirens [D. Don] Endl.) retained the greatest amount of preservative and was also the best penetrated sample. Sapwood of Ponderosa pine (Pinus ponderosa Laws) was penetrated better with preservatives and retained more preservatives than sapwood and heartwood of Douglas-fir (Pseudotsup menziesii [Mirb] Franco) but the heartwood of Ponderosa pine was the least penetrated sample and retained the smallest amount of preservative. Retentions in samples treated with copper sulphate solution were higher than retentions in creosote treated samples. Loss of preservative solvent by evaporation or bleeding of preservative solution from treated samples 7 days after treatment was directly related to the amount of preservative retained in the sample immediately after withdrawal from the treating cylinder. Retention of preservative solutions in the heartwood of hardwood species was directly related to the distribution of vessels but indirectly related to the diameter of the vessel lumen. Gum was the most important inclusion which influenced penetration and retention in the hardwood species. Retention among the softwood species was directly related to the diameter of the tracheid lumen. The release rate of chloropicrin vapour from a vial attached to the treated specimen and the subsequent time taken by the vapour to diffuse through the length of the specimen were the two treating variables measured in vapour-phase treatment. These two variables were interdependent, and their values followed almost the same pattern as the air permeability measurements in hardwoods and softwoods. Longitudinal permeability (liquid and air flow) was the only physical property of the species which was related to their preservative treatment. The extent of the relationship between values for air permeability and values for retention, penetration and vapour release was tested by applying linear regression analyses. The results showed that air permeability correlated better with the release of chloropicrin vapour than it did with preservative retention or penetration. Air permeability was a better indicator of the treatability of softwoods from the United States than the hardwoods from Ghana.
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