CERAMIZABLE SILICONE RUBBER-BASED COMPOSITES D. M. Bielinski 1, 2 , R. Anyszka 1 , Z. Pedzich 3 , J. Dul 2 1 Technical University of Lodz, Faculty of Chemistry, Institute of Polymer and Dye Technology, 12/16 Stefanowskiego Str., 90-924 Lodz, Poland corresponding author; phone: +4842 631 32 14, fax: +4842 636 25 43, e-mail: dariusz.bielinski@p.lodz.pl 2 Institute for Engineering of Polymer Materials and Dyes, Elastomers and Rubber Technology Division, 30 Harcerska Str., 05-820 Piastow, Poland 3 AGH – University of Science and Technology, Faculty of Materials Science, Advanced Ceramics Department, 30 Mickiewicza Av., 30-059 Cracow, Poland Abstract The work reviews the state-of-the-art in the field of ceramizable silicone rubber-based composites used in cable industry. Phenomenon of ceramization is based on preventing volatiles of polymer thermal decomposition from evacuation by creation of ceramic layer in composite surface. Usually, the layer is composed of mineral filer particles, connected by fluxing agent. The ceramic barrier has to characterize itself by micro-porous structure: - protecting copper wire inside the cable from heat transfer leading to melting, and - exhibiting good mechanical strength, assuring integrity of electrical circuit. Subject literature provides information on the application of Ca- or Al-based mineral fillers, combined with fluxing systems, to fill silicone rubber - silica compounds. In this study various mineral fillers, together with boron oxide as a fluxing agent, have been tested. Acidic character of B 2 O 3 , inhibiting peroxide curing of silicone rubber, was compensated by admixing of MgO and its good dispersion in the composite matrix, was obtained by co-grinding of the minerals, enabling the decrease of their particle size. The best ceramic phase, created on fire, was found for composites filled with wollastonite or mica. They represent the lowest mass loss and the stable porosity of ceramic phase up to the highest temperatures tested. Keywords: composites, silicone rubber, mineral fillers, ceramization 1. INTRODUCTION Despite the significant progress in the area of engineering of materials, resources and systems of fire precautions and tools for firefighting, fires of flat buildings all the time threaten to the life of people and are the reason of substantial material losses [1]. This is why special security regulations have been imposed, referring especially to the objects of public utility, like shopping centers, sport halls, museums, cinemas and theatres, airports, underground and railway stations – in a word all places of large concentration of people, great material or cultural value. One of the obligatory regulations concerns the flame resistance of electrical cable insulation which should additionally assure integrity of electrical circuit under fire for not less than 90 minutes, making possible the safe evacuation of people and goods from buildings. Neither silicone rubber, nor chlorosulphonated polyethylene, polyvinyl chloride or crosslinked polythene, used so far by cable industry, are able to meet the present requirements, due to their limited thermal stability and low mechanical strength at high temperature under fire. Solution to the problem seems to be the application of ceramizable rubber composites. Electrical insulation made of silicone rubber filled with a proper selection of mineral fillers and fluxing system, can create the integral, effective heat barrier under fire, protecting copper wire from melting, additionally assuring durability of cable construction. 2. CERAMIZATION There are some reports on application of Ca-based (CaO, Ca(OH) 2 , calcite – CaCO 3 , wollastonite – CaSiO 3 ) or Al-based (Al 2 O 3 , Al(OH) 3 , boehmite – AlOOH, mica, montmoryllonite) minerals as fillers for silicone rubber [2, 3]. In a combination with an adequate selection of fluxing agents (glass frites, zinc or ferric oxides, zinc borate etc.), shifting the melting of ceramic phase to lower temperatures, it results in the ceramization process taking place simultaneously to degradation of a polymer matrix (starting already from 350 °C), limiting the escape of its volatile products [4, 5]. Finally, on surface of the composite a porous but integral ceramic layer is created, which does not break into fragments when the material is directly exposed to the action of fire (even > 1000 °C – close to the melting temperature of copper) – Fig. 1 [6].