Thermal and Mechanical Properties of LDPE/Sisal Fiber Composites Compatibilized with Functionalized Paraffin Waxes L. P. Nhlapo, A. S. Luyt Department of Chemistry, University of the Free State (Qwaqwa Campus), Phuthaditjhaba 9866, South Africa Received 24 August 2010; accepted 26 May 2011 DOI 10.1002/app.35023 Published online 22 September 2011 in Wiley Online Library (wileyonlinelibrary.com). ABSTRACT: The effect of maleic anhydride-grafted hard paraffin wax (MA-g-wax) and oxidized hard paraffin wax (OxWax), as possible compatibilizers, on the morphology, thermal and mechanical properties of LDPE/sisal fiber composites were examined. The differential scanning calo- rimetry (DSC) results show that sisal alone did not change the crystallization behavior of LDPE, while the two waxes influenced the crystallization behavior of LDPE in differ- ent ways, whether mixed with LDPE alone or in the pres- ence of sisal. The thermal properties seem to be influenced by the fact that the waxes preferably crystallize around the short sisal fibers, and by the fact that the two waxes have different compatibilities with LDPE. The TGA results show an increase in the thermal stability of the blends in the presence of the two waxes, with LDPE/ OxWax showing a more significant improvement. The presence of wax, however, reduced the thermal stability of the LDPE/sisal/wax composites. The presence of OxWax and MA-g-wax similarly influenced the tensile properties of the composites. Both waxes similarly improved the modulus of the compatibilized composites, but in both cases the tensile strengths were worse, prob- ably because of a fairly weak interaction between LDPE and the respective waxes. V C 2011 Wiley Periodicals, Inc. J Appl Polym Sci 123: 3627–3634, 2012 Key words: composites; LDPE; sisal fiber; functionalized waxes; morphology; properties INTRODUCTION The incorporation of natural fibers into a polymer commonly leads to significant changes in the me- chanical properties of the composites. However, a problem encountered when trying to combine natu- ral fibers with thermoplastic materials like polyole- fins is one of incompatibility due to the hydrophilic nature of the natural fibers. Therefore, the use of a compatibilizer or coupling agent, which alleviates gross segregation and promotes adhesion, is neces- sary to reduce the interfacial tension between the hydrophobic polyolefins and the hydrophilic natural fibers. The choice of compatibilizers or coupling agents is critical for optimizing the dispersion and properties of the polyolefins. Besides, a good compa- tibilizer should provide stronger adhesion between the natural fibers and the polymer, and form entan- glements and/or segmental crystallization with the polymeric matrix. The polymeric compatibilizer is expected to be compatible with the matrix material, as morphology has a significant effect on the poly- mer properties. Different techniques have been used to prepare polyolefin/natural fiber composites. These techni- ques include solution mixing, roll milling, melt mix- ing, as well as injection and compression molding. 1–12 The methods differ in terms of their operating princi- ples and processing parameters, which may lead to fairly different properties of the prepared composite materials. Polyolefin/natural fiber composites were generally pretreated on the surface of the fiber or incorporated with surface modifiers to improve the interfacial adhesion between the hydrophilic natural fibers and the hydrophobic polyolefins. This can be achieved by using treatments such as silane coupling agents, compatibilizers, maleated polyolefins (maleic anhydride grafted polypropylene or maleic anhy- dride grafted polyethylene), as well as alkali and radiation treatments. 1,2,8,13 Many studies focused on the preparation and morphology of non-treated composites. Colom et al. 7 and Mengeloglu et al. 15 focused on non-treated HDPE/wood fiber composites. They found poor ad- hesion between the non-polar HDPE matrix and the polar wood fiber, and inadequate wetting of the non-treated fibers within the HDPE matrix. They related these to the presence of large numbers of voids between the HDPE matrix and the wood fiber, and fiber-pull-out producing holes with smooth walls in the polymer matrix. The SEM results, in all Correspondence to: A. S. Luyt (LuytAS@qwa.ufs.ac.za). Journal of Applied Polymer Science, Vol. 123, 3627–3634 (2012) V C 2011 Wiley Periodicals, Inc.