A force-based characterization of the extrusion pulping process is necessary to be able to predict the effects of extrusion on fibres. To determine which forces are beneficial and which only cause energy consumption, the nature and the origin of the forces have to be known. This paper discusses the behaviour of hemp bast fibres under compression to better understand the underlying mechanisms involved in the application of energy to the fibres. Untreated, water soaked and NaOH-soaked hemp bast fibres were subjected to varying loads and loading rates and the resulting stress-density and relaxation curves were studied. The stress-density relationship of hemp bast fibres was described with the compressibility equation. The coefficient of the compressibility equation decreased with temperature and moisture content. Fibres became more flexible and fibre bending became more important in the mechanism of fibre compression. The higher flexibility resulted in a lower stress over the whole density range. The compressibility equation also applied to the stress-density curve during compression of soaked fibres at low densities. However, at higher density the stress increased faster, because of flow resistance caused by the dense fibre mat. The flow limitation was advanced at higher strain rates and enhanced at higher sodium hydroxide concentrations. Relaxation was described with a generalized Maxwell model. The extent of relaxation slightly decreased with increasing maximum stress, increased with strain rate and increased with moisture content. Soaked fibres showed a much higher relaxation, which, however, was mostly caused by the high maximum stress because of the flow resistance at higher densities. When regarding the solid stress instead of the total stress, relaxation appeared to be lower. Mechanical pulping processes should therefore be operated at such low strain rates, that the stress observed only depends on the density obtained and not on the flow resistance. During repeated compression both maximum stress and the extent of relaxation decreased with the number of repetitions and the total plastic strain increased until a point was reached at which no further changes were observed. However, the total plastic strain showed a higher dependence on the maximum stress during repeated compression than on the number of compressions. Repeat compression of wet fibres resulted in the formation of a compact fibre mat with a high flow resistance causing high stress peaks.
There are no comments yet. You can post the first one!
Post a comment
Please log in to use this service. Login as Wageningen University & Research user or guest user in upper right hand corner of this page.