Bio-oil obtained by vacuum pyrolysis of softwood bark residues is a multi-phase viscous colloidal system containing an extractive rich upper layer. By centrifugation, the bio-oil can be separated into an upper layer and a bottom layer. The upper layer represents 16.3 wt. % of the whole oil, while the bottom layer represents 83.7 wt. %. Physicochemical characterization, chemical analysis and thermogravimetric analysis of the upper layer, bottom layer and whole oil were performed. Colloidal properties of the fresh and thermally treated upper layer, bottom layer and whole oil were studied by microscopy, DSC and rheometer. A microscopic analysis of the whole oil reveals the presence of a multi-phase system made of solid particles, structured materials and droplets which form a complex colloidal system. The upper layer represents the dispersed phase rich in extractives including structures of waxy materials and lignin derivatives. The bottom layer seems to be the matrix of the whole oil. The existence of microstructures was proven by DSC and rheological tests. The microstructures (e.g . waxy materials) in the bio-oil matrix are in part responsible for the bio-oil high viscosity and non-Newtonian flow behavior observed at low temperature (less than 50°C). A study of colloidal properties and a thermal stability test indicated that the bio-oil should be heated in the temperature range of 50–60°C in order to maintain its good fluidity and storage properties without bringing significant changes in its chemical composition.
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