Humidity responsive bi-layered composites

Among cellulose-based materials, which are known for their hygroscopic behaviour, cellulose acetate (53.3 % of acetylation) has been chosen for this investigation. Membranes of cellulose acetate have been prepared via solvent evaporation of cellulose acetate and ethyl lactate solutions. Membranes with thicknesses within the range of 66÷200 μm have been manufactured. Via gravimetric measurement of preliminary dried membranes, the moisture absorption at room temperature and different relative humidity (RH = 21÷53%) have been monitored. A moisture diffusion coefficient of 3.35×10-6 mm2⁄s and a relaxation factor of 0.026 s-1 have been determined. A dependency of moisture concentration at saturation on relative humidity has been observed. The induced expansion due to the moisture absorption has been determined by thermomechanical analysis. The hygroscopic expansion coefficient of cellulose acetate has been evaluated as 192.5 (mm3/g). The experimental measurements have been considered as input of a detailed finite element model that couples moisture diffusion and hygroscopic expansion. The model allows to predict the response to the changes of relative humidity of a bi-layered composite made up of cellulose acetate and a non-hygroscopic textile.