1084 Research Article Received: 6 November 2009 Revised: 1 December 2009 Accepted: 15 December 2009 Published online in Wiley Interscience: 7 April 2010 (www.interscience.wiley.com) DOI 10.1002/pi.2832 Effect of various alumina nano-fillers on the thermal and mechanical behaviour of low-density polyethylene – Al 2 O 3 composites Giulio Malucelli, * Paola Palmero, Silvia Ronchetti, Alessandro Delmastro and Laura Montanaro Abstract Two different alumina powders were dispersed in low-density polyethylene (LDPE) to evaluate if any role can be ascribed to the crystalline phase, size and morphology of the alumina filler. In particular a submicrometric α-alumina and a nanocrystalline transition (γ /δ) alumina were added to the polymer at 5 wt% concentration, using a Brabender mixing unit. Both the neat inorganic fillers showed a good dispersibility in the polyolefin. The thermal and mechanical properties of the composites obtained were evaluated. As expected, a significant increase of the stiffness and abrasion resistance of LDPE was achieved in both cases. Furthermore, the composites showed a higher thermo-oxidative stability with respect to neat LDPE. Independent of their crystalline phase, size and morphology, both fillers gave a similar enhancement of composite features. c  2010 Society of Chemical Industry Keywords: LDPE composites; alumina powders; mechanical properties; thermal stability INTRODUCTION Polymer composites and, more recently, nanocomposites have attracted much research interest, since such materials exhibit im- proved thermal and thermomechanical properties with respect to the neat polymers. Such (nano)composites are widely used for various applications, such as in the aeronautical and aerospace en- gineering, automobile, biomedical, coating and microelectronics industries. 1–8 As far as particulate composites are concerned, several papers refer to ZnO, TiO 2 , SiO 2 and Al 2 O 3 micro- and nanoparticles embedded in thermoplastic and thermoset matrices. Most work in this field is devoted to the development of epoxy resins reinforced with alumina, silica or zirconia nano-fillers. The addition of such nanoparticles to polymers leads to a significant improvement in mechanical properties like stiffness, wear resistance, tensile and shear strength, and fracture toughness; in addition, a certain dependence on the nano-filler amount, size and morphology has been reported. 9–13 As far as thermoplastic matrices are concerned, addition of fillers has also been found to improve some mechanical properties, such as Young’s modulus, 14 fracture strength, 15 and friction and wear behaviour, 16,17 as well as thermal stability, 18 flame retardancy, 19 chemical resistance, barrier properties 20 and conductivity. 21 Among thermoplastic polymers, low-density polyethylene (LDPE) is of technological interest since it is an easily processed non-polar semi-crystalline polymer, which can be used as a representative of the polyolefin family. In addition, the production of nanocomposites based on LDPE could improve the mechanical properties to levels equivalent to those displayed by high-density polyethylene (HDPE), but with a low degree of crystallinity, which would make the modified LDPE more attractive for processes such as injection moulding, due to the expected reduction in cooling time and in mould shrinkage. The various applications of LDPE range from biomedical devices to use in the packaging industry. LDPE crystalline fraction can affect its mechanical properties, since stiffness and strength normally increase with increasing polymer crystallinity. The presence of fillers or additives can interfere with the polyolefin molecules fitting into the crystal structure and the overall mechanical response may decrease. However, if the loss in crystallinity is compensated by an enhanced interaction of the polymer matrix with the reinforcement filler, the yield strength and Young’s modulus are maintained or even increased. Thus, the matrix–filler interface plays a decisive role in obtaining the ultimate mechanical properties. Some papers have already reported changes and/or improve- ments of the physicomechanical properties of LDPE by the incorporation of inorganic nano-fillers. For example, the addi- tion of ceramic nanoparticles, such as alumina, silica and zirconia, has a positive effect on the photo-stabilization of the matrix. 22 In fact, under UV irradiation for various times (in the range 0 – 600 h), minor oxidation was found in the nanocomposites with respect to the pure matrix, even if the same degradation mechanism was recognized in all the materials. A dramatic increase in the elastic modulus and tensile strength and an anomalous elonga- tion at break of LDPE was found after the addition of nanosilica particles. 18 In addition, the glass transition temperature of sil- ica–LDPE nanocomposites was higher than that of the neat ∗ Correspondence to: Giulio Malucelli, Dipartimento di Scienza dei Materiali ed Ingegeneria Chimica and Local INSTM Unit, Politechnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy. E-mail: giulio.malucelli@polito.it Dipartimento di Scienza dei Materiali ed Ingegneria Chimica and Local INSTM Unit, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy Polym Int 2010; 59: 1084–1089 www.soci.org c  2010 Society of Chemical Industry