Sorption Properties of Steam Treated Wood and Plant Fibres

Hygrothermal treatment of wood and plant fibres was carried out to improve the dimensional stability of the fibres and products made from these. Fibres of Norway spruce, beech, wheat and hemp were included. The efficiency of the treatment was assessed by studying the moisture sorption properties. Chemical analyses were employed to possibly explain the observed changes of sorption characteristics. The fibres were steam treated at temperatures between 140 oC and 190 oC. The duration of treatment was from 5 to 60 minutes. Conditioning was done in a high precision climate chamber allowing weighing of the fibre samples to take place without removing these from the climate chamber. A mild treatment resulted in a reduction of sorption at all levels of relative humidity (RH), although relatively most pronounced at low levels of RH. At progressively longer durations of treatment or progressively higher temperatures the behaviour was different for low/medium RH and high RH. For low/medium range RH (<ca. 85 %) the reduction of sorption continued until a lower limit was reached corresponding to approximately 30 % reduction of moisture uptake. The reduction of sorption developed at a faster rate the lower the RH. This behaviour corresponds to the reduction of the number of primary sorption sites on e.g. hemicellulose. For high range RH (> ca. 85 %) two additional mechanisms may have been active. One is the capillary sorption in micropores created as a result of the thermal degradation of cell wall matter. A second mechanism may be the gradual filling of such micropores by lignin made to flow by further steam treatment. As a result of these counteractive mechanisms, sorption at high RH in steam treated fibres was seen to first grow, then drop and ultimately settle at a level corresponding to pure sorption at primary sorption sites. The annual plant fibres proved less susceptible than wood fibres to chemical breakdown from steam treatment. The component most susceptible to chemical breakdown was hemicellulose. Beech, wheat and hemp showed only a modest decrease of cellulose content, even at high temperatures, whereas spruce, surprisingly, exhibited a marked breakdown of cellulose. The latter may be correlated with the structure of softwood lignin.