HUMIDITY RESPONSIVE SMART TEXTILE: FROM NATURE TO APPLICATION

Plants can show an active response to the changes in the humidity level as they contain a great amount of cellulose. Pine cones, in particular, show reversible mechanical bending movement due to the changes in relative humidity. This behavior derives from the difference in dimensional changes due to moisture absorption between the inner and outer layers (bi-layered composite) of pine cone scales. A thin layer of a hygroscopic material, submitted to an increase of the environment relative humidity, expands linearly according to its Coefficient of Hygroscopic Expansion (CHE). If this hygroscopic thin layer is coupled with a layer of another material with a negligible CHE, the variation of relative humidity creates a bending deformation in the bi-layered composite. The scope of this work is to produce a bi-layered textile that can show a spontaneous response to the changes in the environment humidity level. Mimicking this instantaneous self-actuation requires a meticulous study of the hygroscopic behavior of the materials and a proper design of their coupling. Cellulose-based polymers, such as cellulose acetate, which are known for their sensitivity to the moisture absorption and desorption, are perfect candidates to mimic the autonomous response of plants to humidity changes. The stand-alone membrane of cellulose acetate (53.3% degree of acetylation), presented in this study, has already shown instantaneous response to the variation of environment humidity. Coupling this material as a membrane on textile as substrate can provide bi-layered composite with the desired actuation mechanism. A detailed model must be foreseen to predict the moisture absorption and the effect on the mechanical behavior of the cellulose acetate membrane and of the bi-layered composite. Although numerical finite element codes could be suitable tools for accurate modeling of the humidity response bi-layers textile-based composites, to the best of the author's knowledge, these codes do not allow direct simulation of the hygroscopic behavior. Such limitation can be overcome considering the similarities of governing equations of conduction heat transfer, by substituting the thermal conductivity and temperature with diffusion coefficient and equilibrium moisture concentration, respectively. This substitution is valid when the diffusivity is constant through the process of absorption. The moisture absorption process for a series of cellulose acetate membranes (65 – 200 μm of thickness) has been monitored at a constant temperature (23° C) and at different relative humidity (RH = 20,30 and 40%). Since cellulose acetate shows a sigmoidal diffusion behavior, the experimental measurements were fitted and extrapolated by an analytical model that can provide a constant diffusion coefficient. Extracted values from the analytical model for diffusion coefficient and equilibrium moisture concentration were used as input data for finite element analysis. Comparing the results of the simulation shows a very good agreement with the extrapolated and experimental data. Finally, a preliminary prediction of hygroscopic expansion of cellulose acetate membrane has been simulated by finite element analysis.