Test MethodStudies of conventional sulfur vulcanization of SBR rubber: Analysing the reaction products from thermal degradation of the accelerator by means of MCC-IMS technique
Introduction
Studies on the mobility of ions in an electric field, together with the theory of radiation and ionization, have contributed to the creation of the first commercial analytical instrument in this field - plasma chromatograph (PC) [1]. Over its approximately 50 years of existence, ion mobility spectrometer (IMS), which is a highly developed apparatus, has gained a lot of interest. IMS instruments can be successfully applied in different fields of industry. Starting from military and defense applications, they are used as detectors for warfare agents, explosives or drugs [2,3]. They are also employed in various biological and medical applications [4,5]. More unconventional applications of IMS are described in review publications; for example in the food industry, IMS is applied to test food quality [6,7]. This paper describes another novel and unconventional application of MCC-IMS technique in the field of polymer materials and technology.
From a scientific and an industrial point of view, the behavior of polymer materials can be described by many theories as well as by using various analytical techniques to determine their processing parameters. From time to time and in line with current trends and progress in material engineering, new materials and processes have to be studied, requiring new approach to investigations e.g. vulcanization of rubber, being the subject of this study. The problem becomes much more complicated when rubber mixes are concerned. A variety of newly developed additives such as fillers, antioxidants, accelerators, activators, retardants, curing packages of different sulfur to accelerator (S/A) ratios etc., require continuous testing and optimization/evaluation of vulcanization processes. Despite such the significant development, some reaction routes either still remain unresolved or ambiguous, leaving room for controversy or inclination to questions on possible mechanisms taking place at each particular stage of rubber vulcanization.
One of such examples is sulfenamide accelerated vulcanization of natural rubber and the creation of specific reaction products involved in the crosslinking process. The main product, 2-bisbenzothiazole-2-2′-disulfide (MBTS) is created from different mercaptobenzthiazoles and mercaptobenzothiazol sulfeamides like mercaptobenzothiazole (MBT), N-cyclohexylbenzthiazolsulfemanide (CBS), N-tert-Butylbenzothiazole sulfenamide or 2-(morpholinothio)-benzothiazole (MBS) at the first step of the vulcanization [8,9].
Discussion on the formation of an active intermediate is still ongoing with postulations of it either being radical or ionic in nature. Son et al. [10] demonstrate ionic character of the formation of the active intermediate but, as an alternative to this reaction, also don't exclude the mechanism based on radical intermediates. Despite demonstration of an active intermediate with an ionic nature by Son et al. [10], the possibility of a mechanism with radical intermediates has not been completely ruled-out. The latter statement seems to be confirmed by significant enhancement of the reaction rate in polar solvents. Another evidence on the existence of radicals comes from the works of Fairbrother et al. [11], Trivette et al. [12] and Shelton and McDonel [13]. Discussion on the nature of reaction is not so important in this case, since the rate equations for the depletion of e. g. MBS and MBT and the formation of MBTS are the same for the both mechanisms [14].
To understand the idea of the formation of different by-products during the vulcanization process, various techniques to detect each of the species are employed. One of these methods is Model Compound Vulcanization (MCV), where samples based on squalene were used as a model substance for natural rubber [[15], [16], [17], [18], [19]]. As a matter of fact, industrial applications usually require more than one polymer matrix to be used, thereby making the topic more complicated. Rubber compounds used in the tire industry consist mainly of NR, SBR and BR rubbers mixed in various proportions, thus influencing the system morphology. Going beyond the model compound vulcanization analysis and trying to adapt another one, MCC-IMS technique in combination with developed sample introduction systems are introduced as an effective analytical method to monitor vulcanization processes. The technique makes it possible to analyse volatile organic compounds (VOCs), which are directly correlated with rubber processing and formation of co-products during vulcanization process. To minimize the number of possible VOCs being created, rubber samples based on styrene-butadiene rubber (SBR) cured with conventional sulfur system (CV) with varying N-tert-butyl-2-benzothiazyl sulfenamide (TBBS) to sulfur ratios, were studied. All the details about the composition of these rubber compounds will be described in the next section.
The idea behind the research described in this paper came from the simple assumption that – if it is possible to “smell” some specific species (VOCs) released during the vulcanization process, then the “marker” compounds being released and their intensity/concentration should be related to crosslink density and structure of the vulcanizates, and also eventually to their engineering properties. In order to minimize the effect of other additives and factors which can affect structure and properties of the vulcanizates, it was decided that the only variable in the study should be the quantity of accelerator. The amount of accelerator influences the crosslink density and crosslink structure, subsequently influencing the mechanical properties of the material such as its tensile strength or elongation at break. The accelerator used in this study was TBBS. Sulfenamides degrade at 210–220 ᵒC but in the presence of sulfur and zinc oxide, decomposition occurs nearly at vulcanization temperature. Decomposition leads to the formation of reactive but less stable polysulfidic complexes [20]. Knowledge on the chemical structure of TBBS provides the possibility to identify a marker from its decomposition products. Moreover, direct decomposition products like tert-butylamine or benzothiazole are very volatile chemical substances [21]. Benzothiazole is one of the most visible in MCC-IMS spectra, well separated from other signals and it plays crucial role in the crosslinking process of rubber as an part of MBT or MBTS molecule. Therefore, it was decided that the concentration of benzothiazole should be monitored and compared to selected properties of SBR vulcanizates cured with various CV systems.
Section snippets
Materials
To perform these measurements, six model rubber compounds were prepared. The materials studied were based on s-SBR rubber (25% styrene; 40% vinyl; TDAE oil extended 27.3%) and other commercial grade additives, commonly used by the tire industry: zinc oxide, stearic acid, TBBS and sulfur. Benzothiazole (96%) was used as a calibration standard, meanwhile toluene and diethyl ether were used for equilibrium swelling measurements. Piperidine (99%), 2-propanethiol (98%) and 1-dodecanethiol (98%)
Calibration of the MCC-IMS instruments using benzothiazole
Calibration of the MCC-IMS instrument started with two basic steps. The first step applies to operations on the calibration line to prepare the calibration source and to set the flows, whereas the second step focuses on operation of the MCC-IMS instrument. The first step was aimed at creating a self-made permeation vial with benzothiazole (96%, Sigma Aldrich). The vial was stored in an incubator at 30 °C to stabilize and then weighted over a period of time (Δt). The loss of weight (Δm),
Conclusions
The main idea of the presented study was to develop a methodology for testing the evolution of selected VOCs emission during the vulcanization of rubber samples by means of a MCC-IMS instrument and to compare the data with the results of conventional mechanical properties of the vulcanizates. Based on the experiments conducted, it could be confirmed that the calibration/dilution system hyphenated with a MCC-IMS instrument works properly for meeting the objectives of this research work. The idea
Declaration of competing interest
None
Acknowledgements
Dominik Pietrzak immensely appreciates Goodyear S.A (Colmar-Berg, Luxembourg) for the financial support in the form of a research grant and cooperation.
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