Mechanical and electrical properties of alumina–natural rubber composites N. Tangboriboon 1 , S. Chaisakrenon 1 , A. Banchong 1 , R. Kunanuruksapong 2 and A. Sirivat* 2 The mechanical and electrical properties of natural rubber (NR)–alumina (Al 2 O 3 ) composites crosslinked with dicumyl peroxide (DCP) were investigated. Scanning electron micrographs indicate the interaction of the C–C microstructure and that alumina particles are moderately dispersed in the matrix. The X-ray diffraction patterns indicate that Al 2 O 3 is of the b phase polytype that possesses high ionic conductivity. The electrical conductivity of the composites with DCP is greater by nearly three orders of magnitude than that of NR–Al 2 O 3 composites without DCP and pure NR. The increase in the electrical conductivity and the mechanical property is caused by the strong C–C bond, the tunnelling phenomenon and the ionic polarisation of the alumina particles acting as the dispersed phase in the elastomer matrix. Keywords: Electromechanical properties, Crosslink agents, Tunnelling theory, Crosslink density Introduction The backbone of natural rubber (NR), i.e. polyisoprene, is related to the polyacetylene backbone through the saturation of every other double bond. Polyisoprene is a potential candidate for materials used in various devices: solar cells, light emitting diodes and field effect transistors. 1,2 Raw elastomers, e.g. NR, have poor pro- perties and need to be reinforced. Reinforcement provides improvement in properties, such as tear strength, abra- sion resistance, stiffness and hardness. The solid particles (reinforcing agent or filler) and curing agents, to a large extent, control the mechanical properties of the rubber matrix. 3 STR XL is one kind of NR grades which means Standard Thai Rubber type XL or dried NR in an extralight colour slab with a low dirt content of ,0?04 wt-% having the chemical structure as shown in Fig. 1 and used as astarting material in this work. Alumina is a linear dielectric material with ionic polarisation and potential uses as electrolytic capacitors, optical materials and photoluminescents. 4 The most common form of crystalline alumina is corundum, which has a rhombohedral Bravias lattice with a space group R- 3c (number 167 in the International Tables). Alumina also exists in other phases, namely g, c, h and d theta alumina. All the phases have a structure with a spinel-like Al–O network similar to that found in b-alumina polytypes. The structures of b- and b0-alumina are closely related polytypes; both have the same general stacking pattern of Al–O spinel-like blocks separated by Na–O planes. 4 The b- and b0-alumina are high temperature solid electrolytes exhibiting high ionic conductivity. 4 These materials have been used in energy storage, alkali–metal thermal to electric conversion cells and gas sensors. 5 b0- Alumina, which has higher ionic conductivity, is pre- ferred over that of b-alumina. In general, the properties of Al 2 O 3 are low thermal expansion, good thermal shock fracture resistance, low density, high creep resistance, good chemical and thermal stability, high melting point (1828¡10uC) and excellent toughness and strength. Previous studies 6,7 reported that alumina can reduce heat, humidity, light, ozone and gamma radiations and crack growth in NR. Al 2 O 3 is a potential candidate as the dispersed phase in the NR matrix. 8–10 Sulphur is the most widely used material as curing agent for rubbers. When it is used with an accelerator and an activator at elevated temperatures, thermally stable covalent bonds are formed between the elastomer chains at the carbon–carbon double bonds. Oh and Koenig 11 reported the preferred use of peroxide as crosslink agent. An important feature of peroxide as curing agent is the superior strength of the C–C bond compared to the S–S or C–S bond in sulphur curing. The stronger bond strengths result in superior heat and creep resistance at high temperatures compared to sulphur crosslinked systems. Bhowmick et al. 12 reported that C–C crosslinking is more resistant than C–S crosslinking and has better compression set proper- ties. The C–C crosslinking is induced by peroxides, which are basically divided into two types: peroxide with a carboxylic acid group (dibenzoyl peroxide) and peroxide without a carboxylic acid group [dicumyl peroxide (DCP)]. Peroxide without carboxylic groups has less sensitivity to acids and higher decomposition tem- peratures and is suitable for compression. The gen- eral advantages of peroxide based crosslinking are high temperature resistance, good elastic behaviour in 1 Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900, Thailand 2 The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand *Corresponding author, email anuvat.s@chula.ac.th 26 ß Institute of Materials, Minerals and Mining 2013 Published by Maney on behalf of the Institute Received 20 July 2011; accepted 22 August 2011 DOI 10.1179/1743289811Y.0000000047 Plastics, Rubber and Composites 2013 VOL 42 NO 1