Use of tube flow fractionation in wood powder characterisation

Abstract

The size and shape of powdered wood particles are important properties when considering their use in wood burning or the production of biofuels, biocomposites and biochemicals. Different measurement techniques can be used to measure particle size, but unfortunately these do not provide information about the shape of the wood particles. In this study, a fast and reliable tube flow fractionation method (analysing time 3min; particle size range 1 μm-2000 μm; analysed particles per sample 30,000–200,000) is used to separate water-diluted wood powders which varies by size and shape. 30 different milled wood powder samples are analysed (average particle size from 20 μm to 300 μm and average aspect ratio from 3 to 10). The major benefit in tube flow fractionation method is that the concealment of the finest particles in CCD imaging is avoided by applying the imaging unit after the fractionation of wood particles. Wood powder can easily be separated into different fractions by leading the water-diluted sample flow into different beakers for certain time periods. The transmittance signal after tube flow fractionating yields information about the particle size, and optical imaging provides information about the morphology of the particles. It was found in this study that rotor impact milling to a finer size range produces small but elongated particles. Additionally there were only small differences in the particle size distributions between jet milled and oscillatory ball milled samples, but it was noticed that oscillatory ball milled powders have a significantly lower aspect ratio than jet milled samples.