Louis A.E.M. Reuvekamp

Mechanistic aspects of the role of coupling agents in silica-rubber composites

Compared to carbon black, the use of silica as reinforcing filler for rubber results in lower hysteretic losses, for tyre applications leading to lower rolling resistance and consequently fuel savings. The compatibility of hydrophilic silica with a hydrophobic rubber polymer matrix is generally poor. Adding bi-functional coupling agents to the compounds, commonly bis(triethoxysilylpropyl) tetrasulphide (TESPT), enhances filler-matrix compatibility. The degree of hydrophobation of silica during rubber mixing then depends on many mutually interacting factors. Irreproducible conditions during mixing and vulcanisation are major causes of irreproducibility of silica-reinforced rubber compounds. Depending on the chemical composition of the coupling agent, the ultimate temperature obtained during the mixing process turns out to be the main factor governing the reactions of the coupling agent: the formation of a proper bond between the silica and the coupling agent, while avoiding a premature reaction with the rubber polymers, leading to premature scorch during mixing. Further, the mechanistic aspects of the reaction of various coupling agents, variants on TESPT, are covered, with silica as well as with the rubber. Of great importance are: the carbon and sulphur chain-lengths within the coupling agents, whether corrections are applied in the compound with elemental sulphur relative to the sulphur contained in the reference TESPT, and the moments the correcting amounts of sulphur are added to the compounds: during the first mixing stage, or together with the curing ingredients later-on in the process. The tensile properties of the vulcanised compounds are most prominently influenced. This is indicative of the dual role of sulphur: on the one hand as direct curative, on the other hand as part of the coupling agent becoming attached to the rubber polymers.

Effects of time and temperature on the reaction of TESPT silane coupling agent during mixing with silica filler and tire rubber

Abstract The addition of a coupling agent to silica-rubber compounds enhances the filler-matrix compatibility. Under certain mixing conditions the surface of the filler may be only partly activated...

The Influence of Silane Sulfur and Carbon Rank on Processing of a Silica Reinforced Tire Tread Compound

The effect of the sulfur rank (4-0) and of the carbon rank (2-10) of equivalents of bis(triethoxysilylpropyl)tetrasulphide TESPT as coupling agents for silica-reinforced tire tread compounds, is the subject of this study. The coupling agents are added in quantities equimolar to TESPT. Sulfur correction for lower sulfur ranks than TESPT is performed either in the final mixing step or in the first mixing step in an internal mixer. Without sulfur correction the silanes studied show a marked difference in processing as well as in the final properties of the rubber. When sulfur correction is made in the final mix together with the addition of vulcanization ingredients. all sulfur-containing silanes behave more like TESPT. The disulphide (TESPD) shows final properties similar to those of TESPT; the mixing behavior shows improved scorch safety. This is lost when sulfur correction is applied in the first mix. Sulfur-free silanes do not react on sulfur correction during processing and show only a slight improvement in mechanical properties. A silane without sulfur, having a carbon rank of 10 (DTES) shows the best processing, although final mechanical properties are inferior to TESPT.

Effect of zinc oxide on the reaction of TESPT silane coupling agent with silica and rubber

Abstract The addition of a coupling agent to silica-rubber compounds enhances filler-matrix compatibility. The coupling agent under study, TESPT, can also act as a sulfur donor, which leads to premature scorch in the mixer. The effect of the presence of zinc oxide in the internal mixer at higher dump temperatures was demonstrated by dynamic mechanical testing. A lower tendency to scorch was seen when zinc oxide was omitted during the internal mixing stage and added only later together with the curing additives on a two roll mill. Zinc oxide primarily interferes with the reaction between the coupling agent and the silica surface, as shown by the reaction rate constants obtained with rheological experiments. Further confirmation was obtained by HPLC-experiments: the reaction rate constants of TESPT with silica for the samples with zinc oxide present, were lower at higher temperatures in comparison with samples without zinc oxide. XPS data confirmed that zinc oxide can indeed react with the silica surface. W...

Influence of Network Structure on Glass Transition Temperature of Elastomers

It is generally believed that only intermolecular, elastically-effective crosslinks influence elastomer properties. The role of the intramolecular modifications of the polymer chains is marginalized. The aim of our study was the characterization of the structural parameters of cured elastomers, and determination of their influence on the behavior of the polymer network. For this purpose, styrene-butadiene rubbers (SBR), cured with various curatives, such as DCP, TMTD, TBzTD, Vulcuren®, DPG/S8, CBS/S8, MBTS/S8 and ZDT/S8, were investigated. In every series of samples a broad range of crosslink density was obtained, in addition to diverse crosslink structures, as determined by equilibrium swelling and thiol-amine analysis. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the glass transition process, and positron annihilation lifetime spectroscopy (PALS) to investigate the size of the free volumes. For all samples, the values of the glass transition temperature (Tg) increased with a rise in crosslink density. At the same time, the free volume size proportionally decreased. The changes in Tg and free volume size show significant differences between the series crosslinked with various curatives. These variations are explained on the basis of the curatives’ structure effect. Furthermore, basic structure-property relationships are provided. They enable the prediction of the effect of curatives on the structural parameters of the network, and some of the resulting properties. It is proved that the applied techniques—DSC, DMA, and PALS—can serve to provide information about the modifications to the polymer chains. Moreover, on the basis of the obtained results and considering the diversified curatives available nowadays, the usability of “part per hundred rubber” (phr) unit is questioned.

EFFECT OF POLYMER CHAIN MODIFICATIONS ON ELASTOMER PROPERTIES

ABSTRACT Considerable attention is paid to the influence of crosslink density and crosslink structures on the behavior of polymer chains and properties of elastomers. However, a very important parameter seems to be underestimated: the modifications to the polymer chains by curatives, formed by sulfur and fragments of accelerators. We draw attention to this important contribution to performance of spatial networks. The emulsion styrene–butadiene rubber samples, cured with tetramethylthiuram disulfide and sulfur (TMTD/S8) and zinc dialkyl dithiophosphate with sulfur (ZDT/S8), were studied. They were characterized in detail in terms of crosslink density and crosslink structures. Microscale techniques were used to obtain information about the behavior of the polymer chains: positron annihilation lifetime spectroscopy (PALS) to study the free volume structure and differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) to monitor the glass transition process. Properties such as static mech...

Upscaling of a Batch De-vulcanization Process for Ground Car Tire Rubber to a Continuous Process in a Twin Screw Extruder

As a means to decrease the amount of waste tires and to re-use tire rubber for new tires, devulcanization of ground passenger car tires is a promising process. Being an established process for NR and EPDM, earlier work has shown that for ground passenger car tire rubber with a relatively high amount of SBR, a devulcanization process can be formulated, as well. This was proven for a laboratory-scale batch process in an internal mixer, using diphenyl disulfide as the devulcanization aid and powder-sized material. In this paper, the devulcanization process for passenger car tire rubber is upscaled from 15 g per batch and transformed into a continuous process in a co-rotating twin screw extruder with a capacity of 2 kg/h. As SBR is rather sensitive to devulcanization process conditions, such as thermal and mechanical energy input, the screw design was based on a low shear concept. A granulate with particle sizes from 1–3.5 mm was chosen for purity, as well as economic reasons. The devulcanization process conditions were fine-tuned in terms of: devulcanization conditions (time/temperature profile, concentration of devulcanization aid), extruder parameters (screw configuration, screw speed, fill factor) and ancillary equipment (pre-treatment, extrudate handling). The influence of these parameters on the devulcanization efficiency and the quality of the final product will be discussed. The ratio of random to crosslink scission as determined by a Horikx plot was taken for the evaluation of the process and material. A best practice for continuous devulcanization will be given.

Designing of Cradle-to-cradle Loops for Elastomer Products

ABSTRACT Reuse of devulcanisates in virgin rubber compounds is the shortest loop in cradle-to-cradle cycles for elastomers. Due to the enormous complexity of these loops, the technology still stands in its infancy. Two elastomer types, EPDM (Ethylene–Propylene-Diene Terpolymer) and SBR (Styrene–Butadiene Rubber), are compared as to their ease of devulcanisation with an optimized devulcanisation aid. EPDM is relatively easy to devulcanise into a quality which allows for reuse of large amounts till 50% in virgin compounds for applications like roofing sheets and others. This is already being done on industrial scale. SBR is more difficult to devulcanise due to re-crosslinking under the influence of oxygen. Furthermore, tyre scrap is a mix of compounds and elastomers, which are impossible to segregate before devulcanisation. A lot more research and development is needed to bring this to a success.