Vulcanization kinetics of nano-silica filled styrene butadiene rubber Seyed Mostaffa Hosseini, Mehdi Razzaghi-Kashani * Polymer Engineering Department, Faculty of Chemical Engineering, Tarbiat Modares University, P. O. Box: 14115-114, Tehran 1411713116, I.R. Iran article info Article history: Received 5 August 2014 Received in revised form 29 September 2014 Accepted 30 September 2014 Available online 15 October 2014 Keywords: Rubber Vulcanization kinetics Nano-silica abstract It was shown that the physical filler-polymer and fillerefiller interactions, apart from the filler surface chemistry, has a substantial role in controlling the vulcanization kinetics of styrene butadiene rubber filled with nano-silica in a sulfur vulcanization system. Kinetic studies by the oscillating disc rheometer, differential scanning calorimeter, and swelling tests revealed that the vulcanization rate goes through a maximum as loading of silica increases, but conversion in crosslinking continuously decreases as the amount of silica increases. The effect of silica loadings on the vulcanization reactions was linked to the immobilization of rubber chains around particles as well as in a polymer-mediated filler network, which were differentiated by the nonlinear viscoelastic behavior of rubber vulcanizates. By surface modification of nano-silica, the accelerating/decelerating effects of nano-silica on the vulcanization reactions were altered corresponding to the non-linear viscoelastic behavior of the vulcanizates. Therefore, a mecha- nism was proposed which correlates vulcanization kinetics of rubber to the dynamics of chains influ- enced by the reinforcing fillers. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction Elastomers need to be reinforced with significant quantities (about 25 vol.%) of reinforcing fillers such as carbon black and/or silica to achieve the mechanical properties needed for high per- formance applications such as tires and other engineering prod- ucts. Silanized silica has gained wide attention in the Green-Tire technology because it provides lower rolling resistance at equal wear resistance and even better wet grip for the tire tread vulca- nizates than carbon blacks [1]. Specific mechanical and dynamic properties are expected from such complex vulcanizates, and the chemical crosslink network of rubber has indispensable contribu- tions in these properties. Contributions of the rubber network to the elastic modulus of rubber vulcanizates depend on the crosslink density as well as the nature of rubber matrix [2]. There is an op- timum degree of crosslinking in the rubber vulcanizates for prac- tical applications, which must be high enough to prevent viscous flow but low enough to avoid brittle failure in the vulcanizates [3]. Characteristics of the chemical crosslinks in the rubber vulcanizates are mainly defined by the mechanism and kinetics of the chemical reactions in the vulcanization process [4]. Kinetics of vulcanization for unfilled rubbers had been inten- sively investigated and well established for decades [5]. It is also well known that the presence of reinforcing fillers can influence vulcanization kinetics and crosslink density of rubber vulcanizates [4,6e8]. However, impacts of fillers on the vulcanization kinetics of rubber have mostly been studied by focusing on the filler surface chemistry. For instance, it has been concluded that the accelerating effects of carbon black is due to a large variety of oxygen containing functional groups, such as carboxyl, hydroxyl, phenol, lactones, quinones, ketones, aldehydes, and hydroperoxides [9,10]. Whereas, silica surface is occupied by acidic hydroxyl, siloxane and silanol groups, which adsorb basic accelerators by the surface hydrogen bonds, deactivate them, and slow down the rate and degree of vulcanization [11e 14]. Therefore, modification of silica surface by coupling agents, such as silanes, has been considered in order to reduce silanol groups of silica and their adverse effects on the vulcanization reactions. Ansarifar et al. [11] argued that introduc- tion and progressive increase in the silica loading (10e50 phr) significantly prolong the vulcanization time by decreasing the vulcanization rate. Choi [12] showed that in the compounds filled with both silane-modified silica and carbon black, vulcanization time extends by increase in the silica content. Mujtaba et al. [15] by double quantum nuclear magnetic resonance (NMR) measure- ments, showed that the average crosslink density in the silica-filled SBR vulcanizates decreases systematically with increasing content of nano-silica particles at high loadings (about 45% for filler * Corresponding author. E-mail address: Mehdi.razzaghi@modares.ac.ir (M. Razzaghi-Kashani). Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer http://dx.doi.org/10.1016/j.polymer.2014.09.073 0032-3861/© 2014 Elsevier Ltd. All rights reserved. Polymer 55 (2014) 6426e6434