Confocal Raman imaging, FTIR spectroscopy and kinetic modelling of the zinc oxide/stearic acid reaction in a vulcanizing rubber
Graphical abstract
Introduction
Vulcanization (i.e. the cross-linking reaction by sulphur) is one of the oldest and best developed processes in the rubber industry. Since its discovery by Goodyear in 1839, it has been continuously improved by the introduction of new ingredients (accelerators, activators, retarders), in order to improve mechanical properties. Nowadays, in the tyre industry, where the process represents the core technology, a vulcanizing rubber formulation is a very complex system comprising tens of components. Most of these formulations have been developed and optimized by skilful and lengthy techniques based on trial-and-error but, owing to the complexity of the chemistry involved and the multiplicity of ingredients, the mechanism of vulcanization has not been fully elucidated yet. It is generally recognized that the vulcanizing rubber is a heterogeneous system which, in turn, gives rise to a heterogeneous vulcanizate at the end of the process. How to precisely measure the degree of heterogeneity and to control it for a rational product design, remains an open issue [1]. Another relevant problem encountered in rubber processing is represented by undesired side reactions, whose occurrence increases with the number and reactivity of the formulation components. For instance, zinc oxide is still the best known activator for sulphur vulcanization.
Zinc oxide (ZnO) is in many vulcanisation systems a precursor to zinc-derived accelerators; it reacts with most accelerators to form the highly active zinc salt [1], [2] Complex formation of the zinc ion with different accelerators is critical to get efficient curing. In addition, there is also evidence that the inclusion of ZnO in a compound reduces heat-build-up and improves abrasion resistance. Due to its high thermal conductivity, ZnO acts as a ‘heat sink’, which accepts frictional energy dissipating local heat concentrations without a large increase in internal temperature. It has also been found that ZnO improves heat resistance of the vulcanisates and their resistance to the action of dynamic loading [2], [3], [4], [5], [6]. Moreover, several fatty acids are employed in rubber formulations as plasticizers, but for a few of them (for instance stearic acid, StA) it has been postulated that, beside acting as an internal lubricant [7], they play a further role as co-activators [4]. By reacting with stearic acid, zinc oxide forms water and the hydrocarbon-soluble zinc stearate (ZnSt), which has been claimed to be the actual activator species [7]. In fact, when the zinc coordinates the carboxylate ligands of stearic acid, the solubility and reactivity is strongly enhanced. It has been reported [8], [8](a), [8](b), [8](c), [8](d), [8](e), [8](f), [8](g), [8](h) that, in sulphur vulcanization of rubbers, the crosslink density increases with increasing zinc stearate concentration, while the effect of stearic acid consists in increasing the initial cure and in lowering the level of polysulfidic cross-links, thus enabling a more efficient use of sulphur [9], [10]. It has also been suggested that both these effects are caused by stearic acid allowing a better dispersion of ZnO. In another study [11], it was observed that, if the vulcanizing medium contains zinc stearate but no stearic acid, the end-properties of the final products worsened.
From the above arguments it follows that a quantitative characterization of the zinc oxide/stearic acid reaction, not only in terms of overall conversion but also with respect to the heterogeneity of the system, is of paramount importance towards a deeper understanding of the vulcanization mechanism. Previous literature reports have shown that, depending on the mixing conditions, the two reactants can already form zinc stearate during the compounding of the rubber. The calorimetric (DSC) results of Kruger and McGill [12] indicated the formation of zinc stearate and its subsequent reaction with sulphur, but the phase separation of the system components was not investigated.
In the present contribution we examine the interaction between zinc oxide and stearic acid in a medium (uncrosslinked styrene–butadiene rubber, SBR) suitable to simulate the actual vulcanizing system. The dispersion of the components before and after the reaction is investigated by confocal Raman microspectroscopy. This technique offers unique advantages in the characterization of heterogeneous polymer systems: it provides good contrast due to the specificity of the spectrum, molecular-level information and the possibility to implement quantitative or semi-quantitative analysis. The limiting spatial resolution (0.8–1.0 μm in the focal plane) is adequate for the systems under scrutiny. Furthermore, the state of aggregation of stearic acid in the rubber matrix and its reaction with zinc oxide are investigated by FTIR spectroscopy, which provides information on the H-bonding interaction of the stearic acid dimer and kinetic data at different temperatures. These are used to test the reliability of a semi-quantitative kinetic model developed on the basis of the spectroscopic evidences and based on a simple heterogeneous reaction scheme.
Section snippets
Materials
Styrene–butadiene rubber (SBR) had a styrene/butadiene molar ratio of 20/80 and a trans/vynil/cis C
C ratio of 24/56/20. Zinc oxide with a purity >99% was supplied by Bridgestone Technical Center Europe (TCE), Pomezia, Italy. The particle size distribution of ZnO, as determined by static light scattering (SLS), is reported in Fig. 1. The number average equivalent diameter () was 1.775 μm. Reagent grade stearic acid and zinc stearate were supplied by Aldrich Co., Milan (Italy) and were used
Raman spectroscopy
In Fig. 2A–D are reported the Raman spectra of the system components.
Zinc oxide displays two sharp peaks at 437 and 98 cm−1 (E2 modes) [13] which occur in an interference-free region and provide the required spectroscopic contrast. Stearic acid also displays intense inelastic scattering and a series of peaks in a region where the rubber matrix does not interfere (see inset of Fig. 2C). The intense and sharp components at 1296, 1130 and 1062 cm−1 have been assigned, respectively, to the CH2
Conclusions
The interaction between zinc oxide and stearic acid in a medium suitable to simulate a vulcanizing system has been investigated experimentally using vibrational spectroscopy techniques and interpreted theoretically by means of a kinetic model developed on the basis of a simple heterogeneous reaction scheme.
Confocal Raman microspectroscopy revealed that, at ambient temperature, both components are phase-separated in the form of microcrystals. When the reaction temperature is reached (80 °C and
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