No abstract

The impregnation pattern of alkali in fresh Cottonwood is analysed. Isothermal process, medium alkaliconcentration, temperature below 100 o C and tangential direction are considered. The degree of deacetylation in the wood is taken as an indicator of the whole chemical action of the alkali. Profiles of alkali concentration, alkali content, liquid content and acetyl content in the wood are experimentally determined. The results show that wood behaves rather like a glassy polymeric solid placed in contact with a solvent. An advancing boundary zone is established which separates an intact inner part from the outer swollen zone. Alkali concentration and acetyl groups content profiles can approximately be described by a model used to analyse the reaction of solids: the shrinking core model. The chemical reaction in the front is the controlling mechanism. The swelling of the cell walls together with the occupation of the hollow cavities of the wood by the liquor make it possible for the wood liquid content to increase up to almost 3 g liquid g wood -1 .The procedure here adopted and the impregnation pattern proposed are valuable tools to analyse the effect of wood direction and process variables such as temperature, alkali concentration and initial wood moisture. They could also be very useful to build a model for alkaline impregnation of wood under moderate conditions.

The low temperature alkaline treatment of Cottonwood (Populus deltoides) was studied by applying a factorial experimental design. The effects of treatment time and alkali concentration on alkali consumption, as well as treatment yield, carboxyl groups and acetyl groups contents were investigated. Relationships between these contents and wood swelling measured by the water retention value (WRV) were also analyzed. In the whole alkali-concentration range under study, the deacetylation process was responsible for a great part of the alkali consumption. Fitted curves show that the swelling obtained is always increasing despite the fact that the carboxyl groups content attains its higher levels at intermediate times. The unique relationship found for the WRV of the wood and its acetyl groups content, indicates that the deacetylation, rather than the acid group content, is the process determining the wood swelling brought about by the alkaline treatment.