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Electron Microscopy

It was hoped that electron microscopy would, from analysis of fracture surfaces reveal information, on the behavior of MDF over the range of humidities and the effect of the heat treatment. The following observations were made

• From samples taken from the seven different humidity values it was not possible to distinguish characteristics from the next hunidity higher or lower. Despite gradual distinctions not being obtainable, there was however clear differences of fracture surface topograohy in those samples that had been exposed to the higher humidities (eg and KCl )than those at the lower humidities (eg and LiCl)
• The heat treated samples were not distingishable from the control samples. Some wood fibres appeared to have been flattened by the pressure of the heat treatment, but not on a consistent enough scale for observations to be related to other measured results
• Samples were observed from tensile fracture and bending fracture, which has a tensile failure zone and compressive failure zone, above and below the neutral axis, respectively. Overall the differences in failure mechanism did not appear as significant on the microstrucural scale.
• For all samples at high humidities fibre pull-out (the wood fibres seperating from the resin matrix) was the primary failure mechanism. At low humiditied fibre shear appeared to be the primary failure mechanism. I suspect my terminology is lacking here
• At the extreme humidity exposures, the MDF and HD3 samples showed consistently different fracture surface topographies. At high humidities the degree of fibre pull-out in MDF was observably greater than in HD3. At low humidities MDF showed distortion of the fibre resin matrix in conjunction with fibre shearing. HD3 displayed little disruption of fibre-resin matrix, and the failure axis of the whole sample was adjacent to fibre failure axis.