JOURNAL OF COMPOSITE MATERIALS Article Modeling of water diffusion mechanism in polypropylene/date palm fiber composite materials Sonia Boukettaya 1,2 , Ahmad Alawar 3 , Fahad Almaskari 4 , Hachmi Ben Daly 1 , Ahmed Abdala 4 and Sami Chatti 5 Abstract The aim of this work is to model the water absorption mechanism of the polypropylene/date palm fibers composite materials after their exposure at different immersion conditions. For short immersion period, a model combining the Fick’s law and the time temperature stress principle has been proposed to describe the water absorption mechanism. However, it has been noted that, after a saturation time, the water diffusion mechanism leads to a physical degradation and a mass loss into the composite material, especially under high temperatures and long times of water immersion. Microscopic observations have revealed a decrease of the interfacial adhesion between the fibers and the matrix and fiber degradations. A general new model describing the evolution of the water uptake mechanism and the degradation phenomenon has been also proposed in the present work. A good agreement between the theoretical and experimental data has been obtained. Keywords Polypropylene, date palm fiber, composite, modeling, water diffusion, stress relaxation Introduction In the recent years, there has been a significant growth in using natural fibers as reinforcements in composite materials thanks to their multiple advantages (recyc- lability, low densities, remarkable price/quality ratio) and their competitively with the synthetic fibers. 1,2 However, the main disadvantage of natural fibers is the presence of hydrophilic group in the main con- stituents (cellulose and hemicellulose), which leads to a high level of water uptake. 2 Such problem could greatly affect the compatibility between natural fibers and the hydrophobic thermoplastic matrix and, conse- quently, causes a subsequent decrease of the mechan- ical properties of the composite material. 3,4 The water diffusion process has shown to be governed by three different mechanisms. 5,6 The first one involves the dif- fusion of water molecules inside the micro gaps between polymer chains. The second mechanism involves the capillary transport into the gaps and flaws at the interfaces between fibers and the matrix. This is a result of poor wetting and impregnation during the initial manufacturing stage. The third one involves transport of microcracks in the matrix arising from the swelling of fibers. 7 Based on these mechan- isms, water diffusion kinetic has been described using different models. Most of the authors historically used a classical Fick’s model to represent the diffusive behavior of natural fibers subjected to hydrothermal aging. 8–13 According to this theory, the amount of the absorbed moisture increases linearly with the square root of time and then gradually slows down until equilibrium moisture content is reached. 1 LMS-ENISo, Universite ´ de Sousse, Tunisia 2 Ecole Polytechnique de Sousse, Boulevard Khalifa Karoui, Tunisia 3 Polytechnic Abu Dhabi, UAE 4 Khalifa University of Science Technology and Research, Petroleum Institute, UAE 5 Institut Supe ´rieur des Sciences Applique ´es et de Technologie de Sousse, Rue ibn Khaldun, Tunisia Corresponding author: Sonia Boukettaya, LMS-ENISo, Universite ´ de Sousse, B.P. 264, 4023 Sousse Erriadh, Tunisia. Email: sonia.boukettaya@polytecsousse.tn Journal of Composite Materials 0(0) 1–9 ! The Author(s) 2018 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998317752228 journals.sagepub.com/home/jcm