Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Record number 309220
Title Compressibility of hemp bast fibres
Author(s) Westenbroek, A.P.H.; Roekel, G.J. van; Jong, E. de; Weickert, G.; Westerterp, R.K.
Source Nordic pulp & paper research journal 14 (1999)4. - ISSN 0283-2631 - p. 336 - 344.
Department(s) Agrotechnological Research Institute
Publication type Refereed Article in a scientific journal
Publication year 1999
Abstract 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.
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