Moisture absorption measurement and modelling of a cellulose acetate

Abstract

With a view toward the application of highly hygroscopic polymers as a humidity responsive self-actuator, the evaluation of the real time moisture concentration in the material becomes a priority. In this paper, the moisture diffusion process in a cellulose acetate (53.3% of acetylation) has been studied. Membranes of cellulose acetate (thickness within the range 66–200 µm) have been prepared, and the moisture absorption at room temperature and at a different relative humidity (RH within the range 21–53%) has been monitored. An analytical model has been used to describe the observed non-Fickian sigmoidal behavior of moisture diffusion. A relaxation factor (β) of about 0.026 s−1 and a moisture diffusion coefficient (D) of 3.35 × 10–6 mm2/s have been determined. At constant room temperature, the moisture concentration at saturation (Csat) has shown a linear relation with relative humidity. The identified values β, D and Csat of the analytical model have been used as input for the finite element simulation of the non-Fickian diffusion. The reliability of the finite element simulations has been confirmed with a second set of experiments.

Characterization and modeling the hygroscopic behavior of cellulose acetate membranes

“…Cellulose acetate (CA) powder (53.3% acetylation) was kindly provided by Mazzucchelli 1849 S.p.A. As it was described in a previous study (Khoshtinat et al 2021), a 20% w/w solution of Cellulose Acetate (CA) in ethyl lactate (purity C 98%, purchased from Sigma-Aldrich) was first prepared (Fig. 1a) for the production of cellulose acetate membranes.…”

“…Drying of CA film was carried out in several steps (Fig. 1b) to optimize the whole evaporation of the solvent (Khoshtinat et al 2021). Membranes with a thickness ranging from 66 ± 1.5 to 70 ± 4.5 lm were obtained (measured by micrometer).…”

“…The concentration at saturation (C sat ) has been determined during the absorption process via gravimetric measurements. Most of these measurements are reported in (Khoshtinat et al 2021), where data of CA membrane mass variation in time, monitored at 25 ± 1 °C and at different values of relative humidity (RH in the range 21 7 53%), are detailed. In the present work new tests have been performed at higher relative humidity values, up to 76%.…”

“…For weight measurement, an AS310.R2-RADWAG balance with a limit of readability of 0.1 mg was used. As reported previously (Khoshtinat et al 2021) the ''variable surface concentration'' model was adopted to describe the sigmoidal moisture absorption of the cellulose acetate membrane and to predict the concentration at saturation (C sat ) for different values of relative humidity. Figure 2 summarizes the obtained C sat values (at least 3 measurements) for different levels of Relative humidity (RH) by the best fitting of the variable surface concentration model interpolating the experimental data.…”

“…The physical and mechanical properties defined for the cellulose acetate in dry condition were: density 1.3 9 10 -4 (g /mm 3 ) (Wypych 2016), Young's modulus 1.1 GPa and Poisson's ratio 0.39 (Wypych 2016). A relaxation factor of b = 0.026 s -1 and a diffusion coefficient of D = 3.35 9 10 -06 mm 2 /s have been used (Khoshtinat et al 2021). The coefficient of hygroscopic expansion (a) has been defined as a function of relative humidity (RH) according to Eq.…”