Technical report Effect of kaolin–metal oxides core–shell pigments on the properties of styrene–butadiene rubber composites S.H. El-Sabbagh a , N.M. Ahmed a,⇑ , A.A. Ward b a Polymers and Pigments Department, National Research Centre, Dokki, Cairo, Egypt b Microwave Physics and Dielectrics Department, National Research Centre, Dokki, Cairo, Egypt article info Article history: Received 13 February 2012 Accepted 1 April 2012 Available online 19 April 2012 abstract This study investigated the effects of core–shell kaolin–metal oxide pigments on the rheological, physico- mechanical and dielectric properties of styrene–butadiene rubber composites. In this way, newly prepared core–shell pigments based on kaolin as the core representing 90% of the whole pigment was covered with different metal oxides (CaO, MgO and CaOMgO) comprising the shell which represents only 10% of the prepared pigments were incorporated with different concentrations in styrene–butadiene (SBR) rubber composites. Studying the different properties of pigmented and unpigmented SBR compos- ites were done. Scanning electron microscopy (SEM) was used to feature out the surface morphology. Addition of the new pigments increased the tensile strength and strain energy, while elastic modulus was decreased. This study revealed that there is a significant effect of the new prepared pigments on SBR properties and the optimum pigment loading was 40 phr for CaO/kaolin, while it was 2.5 phr for MgO/kaolin. The dielectric results also showed that, the values of e 0 (relative permittivity) and e 00 (dielec- tric loss) increased with increasing core shell content. Moreover, the samples containing MgO/kaolin and MgOCaO/kaolin showed promising dielectric properties with low relative permittivity and electrical insulating properties. The different measurements showed good agreement in their results. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Reinforcement has been one of the most important technologies needed for the utilization of synthetic rubbers. The use of fillers is of utmost importance to obtain the desired physical properties of rubber vulcanizates. Filled rubbers are a very important class of engineering materials [1]. Polymers are generally known to be good insulating materials due to their stable physical and chemical properties. However, both mechanical and electrical properties can be further improved or modified with the addition of inorganic fill- ers as demonstrated by the increase in the mechanical strength of the composite and changes in the electrical properties [2]. The properties of particulate filler polymers are determined by several factors, such as the component properties (matrix and filler), com- position and structure. In addition to the component properties, the mechanical characteristics of these materials are significantly influenced by the interfacial interactions, which depend on the size of the interface and the strength of the interaction [3]. The usual approach to synthetic amorphous elastomers like styrene–butadiene rubber (SBR) is to use reinforcing filler that would somehow dissipate energy during service. The reinforce- ment of SBR with filler not only reinforces it but also impart new mechanical and electrical properties. The modified properties of these composites are very complicated depending on a large num- ber of parameters such as size, surface area, structure and the dis- persion of filler particles [4]. Composites consisting of a polymeric matrix and clay as the reinforcing fillers are attracting interest in many industries because of their advantages over synthetic fillers. The mechanical, thermal and electrical properties of the polymers enormously increase, while gas permeability decreases, even with a small amount of clay addition [5,6]. In general, inorganic pigments are superior to organic pigments in heat and light stability, weathering, migration resistance and low price. Recently, a lot of interest in core–shell pigments has arisen due to their importance in various fields of science and tech- nology, such as biological labels, optical resonances, catalysis, mag- netic, ceramics, and pigments [7]. By building up such shells, various surface properties of dispersed matter can be altered in order to meet some required specifications. Some examples of sur- face properties that can be improved or modified are flowability, dispersibility, solubility, wetability (hydrophilic/hydrophobic properties), electrostatic, electric, magnetic, optical, color, particle shape/sphericity, sinterability, and solid phase reactivity [8,9]. On the other hand, mixed or core–shell pigments are new trend of pigments that contain more than one component chemically deposited on each other. The core is always a cheap extender 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.04.004 ⇑ Corresponding author. Tel.: +20 201003448034; fax: +20 23334455146. E-mail addresses: niviahmed@yahoo.com, nm.hussein@nrc.sci.eg (N.M. Ahmed). Materials and Design 40 (2012) 343–355 Contents lists available at SciVerse ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes