Investigation on the Effect of a Compatibilizer on the Fatigue Behavior of PP/Coir Fiber Composites Sı´lvia H.P. Bettini, 1 Marcela C. Antunes, 2 Rodrigo Magnabosco 3 1 Department of Materials Engineering, Universidade Federal de Sa ˜ o Carlos, Rod. Washington Luiz, km 235, Sa ˜ o Carlos, Sa ˜ o Paulo, CEP 13565-905, Brazil 2 Departament of Metallurgical and Materials Engineering, Centro Universita ´ rio da FEI, Av. Humberto de Alencar Castelo Branco, 3972. Sa ˜ o Bernardo do Campo, Sa ˜ o Paulo, CEP 09850-90, Brazil 3 Department of Mechanical Engineering, Centro Universita ´ rio da FEI, Av. Humberto de Alencar Castelo Branco, 3972. Sa ˜ o Bernardo do Campo, Sa ˜ o Paulo, CEP 09850-90, Brazil The mechanical behavior of polypropylene (PP) and 30 wt% coir fiber reinforced PP composites, with and without compatibilizer, were assessed through mono- tonic (tensile and bending) and cyclic (fatigue) tests. Fatigue load controlled tests were conducted under tension loads at a frequency of 6 Hz. The fracture mechanism was accompanied by surface fracture anal- yses using both optical microscopy and scanning elec- tron microscopy. The compatibilizer used was the PP grafted with maleic anhydride. The compatibilized composites exhibited longer fatigue life times. It was also concluded that the presence of coir fibers changed the preferential fatigue mechanism, because the fracture mechanism in PP was mainly caused by heat generated by viscous effects during solicitation (thermal fatigue), whereas in the compatibilized and noncompatibilized PP/coir composites the predomi- nant fracture mechanism was mechanical fatigue. However, thermal fatigue was also observed in the composites, especially in the noncompatibilized ones. POLYM. ENG. SCI., 51:2184–2190, 2011. ª 2011 Society of Plastics Engineers INTRODUCTION The interest in using materials from renewable sources as reinforcement in thermoplastics has intensified the search for adequate processing conditions and improved adhesion in these composites [1–6]. These materials include natural fibers, such as coir fibers, which are widely available in tropical countries such as Brazil. However, polymer composites reinforced with natural fibers cannot be processed at temperatures exceeding 2008C due to thermal degradation of the fiber constituents (cellulose, hemicellulose, and lignin). This restraint limits the range of polymers that can be used as matrix for these compo- sites [7]. The use of coir fiber as reinforcement in polymers has been the object of several works. The most commonly used matrixes include polypropylene (PP) [8–11], low- density polyethylene [12], natural rubber [13, 14], and polyester resins [15, 16]. PP is used as matrix in coir fiber composites because of its relatively low melting temperature (1658C), which allows processing within the limits imposed by the fibers. However, PP is incompatible with these fibers, because of its nonpolar character against the polar character of the fiber constituents. Many investigations have proposed alternatives to improve adhesion between PP and natural fibers. To improve adhesion for adequate stress transfer compatibilizers or treatment with coupling agents are required. In addition, fiber surface may undergo chemical attack to confer a more irregular surface, which facilitates adhesion to the polymer matrix [4–8]. The most com- monly used compatibilizers are maleic anhydride grafted PP (PP-g-MA) and lignin. In addition to the incorporation of compatibilizer adequate mixing is required and fiber size should be controlled [9–11]. Investigation on monotonic properties, which include tensile, impact, and bending tests, are dealt with in the aforementioned studies, and it has been observed that incorporation of compatibilizers is essential for achieving higher tensile and flexural strength than those of the pure polymer; however, increase in stiffness, proportional to elastic modulus, is obtained even for the noncompatibi- lized composites. Despite the fact that many of these composites might be used in applications under cyclic loadings, no investigation assessing fatigue life has been encountered so far. Correspondence to: S.H.P. Bettini; e-mail: silvia.bettini@ufscar.br DOI 10.1002/pen.21990 Published online in Wiley Online Library (wileyonlinelibrary.com). V V C 2011 Society of Plastics Engineers POLYMER ENGINEERING AND SCIENCE—-2011