Impact of aging conditions on mechanical properties of thermoplastic polyurethane A. Boubakri a, * , N. Haddar a , K. Elleuch a , Y. Bienvenu b a Unité de Recherche de Chimie Industrielle et Matériaux, URCIM – ENIS, B.P. 1173-3038, Sfax, Tunisia b Centre des Materiaux P. M. Fourt, Ecole des Mines de Paris, Evry Cedex, France article info Article history: Received 26 January 2010 Accepted 12 April 2010 Available online 24 April 2010 Keywords: A. Thermoplastic polyurethane E. Degradation mechanisms E. Aging-factors abstract In this study, impact of environmental aging conditions on the mechanical properties of thermoplastic polyurethane (TPU) was investigated. Especially, effect of temperature on water diffusion has been stud- ied. Water-sorption experiments, tensile test and dynamic mechanical thermal analysis (DMTA) were performed after immersion in distilled water at different temperatures (25, 70 and 90 °C). The sorption process was analyzed by gravimetric measurements at different temperatures. Also, diffusion coefficients of solvent molecules in the TPU samples were identified. Therefore the activation energy and the mixing enthalpy were deduced. The aging impact on some mechanical properties of this material has been inves- tigated after various aging cycles. Degradation of mechanical properties was observed. In fact, elastic modulus and stress at 200% of strain were decreased. It was also shown that such degradation largely depends on both aging temperature and aging immersion duration. The storage modulus (E 0 ) was also affected by the hygrothermal (HT) environment. The modification of mechanical properties seems to be well correlated to structural observations obtained from scanning electron microscopy (SEM) photo- graphs. Finally, through thermal aging experiments, it was deduced that the combination of temperature with water seems to be a major factor of TPU degradation. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction In recent years, polymers are being increasingly used in many structural engineering applications. The advantages of these mate- rials consist of low-cost manufacturing processes witch has led to new applications for polymer materials in industrial sectors out- side. Studies on the durability of these materials are given more and more interest especially when they are used in aggressive environments such as temperature, moisture, ice, sunlight, radia- tion, aggressive chemicals, mechanical stresses and combinations of these factors. Different mechanisms may occur simultaneously depending on the degree of exposure conditions and which may lead to decrease of polymer lifetime. The mechanical and chemical effects are also a main reason behind the degradation of the poly- mer [1–4]. Thermoplastic polyurethane (TPU) represents a large family of these polymers which is used in many applications, including coatings, adhesives, foams, sneaker soles, tires, inflatable structures, conveyor belts, artificial joint, protective coverings, bio- materials, and spandex clothing [5–7]. It is a common knowledge that, during daily services of the TPU, the material may be sub- jected to different environmental conditions that affect its mechanical properties. As reported in the literature [3,8–11], several physico–chemical and mechanical changes take place in the material during aging. On the other hand, the degradation process is enormously influenced by several factors such as the chemical properties of the polymer material, the aging tempera- ture, the aging time and the immersion solution [1,12–17]. Mois- ture in any form is unfriendly for polymer and often causes swelling and degradation [2,18,19]. Such degradation, resulting from moisture absorption, is a big concern in most polymer appli- cations subject to normal atmospheric moisture, which can range from precipitation to mild humidity. Complete immersion in water represents the most severe environment, while humid air gener- ally results in lower maximum moisture content [20]. Generally, temperature accelerates diffusion rates of moisture and aging process. Fickian and non-Fickian diffusion behaviour have been reported [2,20–22]. Bao et al. [23] have investigated the effect of moisture absorption and HT aging in a bismaleimide resin. In particular they have found that absorbed water acts as a plasticizer and enhances the relaxation process of the matrix. Plasticization also results in a reduction in the glass transition temperature (T g ) and modulus which shifts the elastomeric range to lower temper- atures and decreases the strength of the polymer [2,19,20]. Mohd Ishak et al. [24] have studied the effect of HT aging on the tensile properties of short-glass–fiber-reinforced poly(butylene tere- phthalate) composites. They have found that the increase of immersion temperature induces a dramatic reduction in tensile strength, tensile modulus and elongation at break of the studied materials. Hollande and Laurent [12] have shown that water 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.04.023 * Corresponding author. E-mail address: boubakri_abid@yahoo.fr (A. Boubakri). Materials and Design 31 (2010) 4194–4201 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes