High Strain Compression and Sliding Friction of Wood Under Refining Conditions, 2007 International Mechanical Pulping Conference

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Compression and friction characteristics are needed in order to gain a better grasp of the forces acting during refining. To this end, both stress-strain relations and frictional behavior of wood were investigated under simulated chip refining conditions (hot saturated steam, high strain rate compression, and high sliding speed). Two new, custom-designed, experimental set-ups were used. The equipment used for compression tests was based on the split Hopkinson pressure bar (SHPB) technique and the friction tester is a pin-on-disc type of tester. Both pieces of equipment allow a testing environment of hot saturated steam. The wood-steel friction investigations indicate that when making measurements in the lower temperature region (100°C−130°C), surface properties such as lubrication have a great influence on the coefficients of friction. Traces of lubricating layers, comprising fatty acids, were found on friction-tested pine surfaces using a staining technique and light microscopy; in the higher-temperature region no traces of lubrication could be detected in this way. During the friction measurements, the specimens of the different wood species started to lose fibers (produce wear debris) at different characteristic temperatures, indicated by peaks in the coefficient of friction, approximately at 150°C for both Radiata pine and Scots pine while at 165°C for Norway spruce. Since this paper presents the results of initial trials using the SHPB equipment for testing in a steam atmosphere, the most important result is that the measurements made seem consistent with those made in earlier investigations at lower strain rates, which confirms the performance of the equipment. As well, combining the results of the experiments and comparing them with those of refining trials raises interesting possibilities. For example, the generally lower shives content of pine thermomechanical pulps (TMP) than of spruce TMP may partly be explained by differences in the frictional properties of the two kinds of wood.