An experimental and finite element simulation study of moisture absorption in an ordered PVAF-reinforced thermoplastic starch composite film

A two-dimensional (2D) model was established by the finite element method to study the transient hygroscopic process and moisture-induced stress to better understand the moisture absorption microscopic process of ordered polyvinyl alcohol fiber (PVAF)-reinforced thermoplastic starch (TPS) composite films and the corresponding moisture-induced stress. The Mises stress contour of moisture distribution at five time points (36, 68, 100, 176, and 348 h) under four relative humidities (R _H =57%, 75%, 84%, 98%) is demonstrated. The variation in moisture content along the two paths of width (AB) and thickness (CD) in the 2D model and the moisture-induced stress were also studied in detail. The results show that when the time t is 348 h, the saturated moisture contents along CD are 5.27, 17.22, 30.58, and 59.14%, and along AB are 5.27, 17.19, 30.57, and 59.07%. In addition, the change in moisture content, regardless of whether the path was AB or CD, mainly occurred before 176 h. The simulation results of moisture absorption are in good agreement with the experimental results. The ultimate moisture-induced stress is the largest in the fiber surroundings, which are 0.398, 1.30, 2.31, and 4.48 MPa at four RHs, respectively, and the internal moisture stress of 4.48 MPa was within the tensile strength (2–6 MPa) of PVAF/TPS (PVAF <0.89 vt%), which resulted in composite film failure. This paper provides a new way to understand the moisture absorption mechanism and explore possible methods for improving the water-resistance of TPS films.