Light scattering and pore surface in paper under dynamic compression

Pressure pulses of 8.2 × 107 dyn cm-2, each lasting for about 0.01 sec, were applied to paper, and the scattered light from it was measured simultaneously in order to study the response of pores in paper under dynamic conditions. During the compression phase the deformation in pore surface, i.e. the loss in fibre-air interface, is found to be linearly related to the square root of pressure, and the minimum pressure required to compress a paper to the non-porous state is estimated to be around 5 × 108 dyn cm-2, depending on the type of paper. The decompression curve, however, always follows a different path; thus, a cycle of compression and decompression shows large hysteresis. An approximate calculation is made of the energy loss per cycle, and this shows the associated temperature rise to be less than 1 degc. The delayed recovery from compression is large compared with the elastic recovery, but the permanent set is very small. Both the resistance of pores to compression and the energy loss per unit volume seem to be related to the paper density.