Short hold times in dynamic vapor sorption measurements mischaracterize the equilibrium moisture content of wood

Recently, the dynamic vapor sorption (DVS) technique has been used to measure sorption isotherms and develop moisture-mechanics models for wood and cellulosic materials. This method typically involves measuring the time-dependent mass response of a sample following step changes in relative humidity (RH), fitting a kinetic model to the data, and extrapolating the asymptotic mass. A series of steps covering the full RH range is used to generate the sorption isotherm. The majority of prior DVS data were taken with hold times of either 60 min or until the change in moisture content was < 0.002% per minute over a 10-min period. Here, DVS measurements on wood and isolated wood polymers are presented where the hold times at certain relative humidity steps were much longer, ranging between 24 and 50 h. The data clearly show that the criteria for hold time in previous DVS measurements result in significant errors in prediction of the asymptotic mass. Although the data at short times are consistent with previous measurements, the data exhibit slow sorption behavior with characteristic times on the order of 500-2000 min that cannot be identified with shorter hold times. The results suggest that new hold time criteria need to be developed for dynamic vapor sorption measurements in wood.