Retarded sorption in wood : experimental study, analyses and modeling

Models of moisture flow processes in wood are normally based on a Fickian approach. In many instances, there are considerable discrepancies between modelling and experimental results. It is shown in this thesis that a major cause for these discrepancies is a so-called retarded sorption which is not accounted for in conventional theories.

This retarded sorption occurs in the wood cell wall. A special experimental set-up to isolate and measure this process has been developed. Extensive series of sorption response measurements with different sequences of relative humidities have been performed on very thin, well exposed wood samples. The results show that the sorption often has an intricate dependence on moisture history. Two small steps may not give the same sorption as the corresponding single larger step. The process is not linear. The part of the sorption that is retarded is especially large at high relative humidities. The retarded sorption increases with decreasing temperature. In special cases, retarded sorption for single cell walls with a time scale of more than a month has been observed.

Different models to reproduce the measured results have been tested. It is shown that models with one added internal node are not sufficient. A few internal nodes are needed. The conductances between the nodes must be decreasing inwards. Non-linear conductances, where the conductance increases with the difference in moisture state between the nodes, are required to reproduce the result for more irregular sequences of relative humidity. The best agreement was obtained with a non-linear model with five internal nodes, where the inner-most node accounted for hysteresis.

Tangential and radial swelling (and shrinkage) was measured in parallel for two series. The ratio between radial and tangential swelling was quite constant throughout the whole series with their many and varied changes of relative humidity. The variation in time of tangential swelling and moisture content followed each other quite well along lines with a constant slope, in particular for cyclic steps. A somewhat higher slope was obtained in all cases where the sorption contained a larger retarded part.