Dynamic mechanical loss properties of wood

This study is part of an extensive series of investigations of the friction and wear mechanisms which occur during the grinding of wood to produce papermaking pulp. A brief summary of these mechanisms is given. It is shown that dynamic mechanical losses in wood represent the major part of the friction losses which occur during wood grinding. Elastic shear modulus and dynamic mechanical loss, expressed as tan δ, of Eastern black sprure (Picea mariana) have been measured in an atmosphere of water vapour through the temperature range 20 to 140°C, and the pressure range 1 to 200 bar. A well-defined second-order transition is exhibited at about 85°C at the natural torsional frequency of about 1 Hz. This transition, which is due to softening of the amorphous lignin component of wood, shows a characteristic maximum in the tan δ-temperature relationship. The effect of hydrostatic pressure on elastic shear modulus and the transition temperature is, as expected, very small.