Influence of Sulfur-Curing Conditions on the Dynamics and Crosslinking of Rubber Networks: A Time-Domain NMR Study

Exploring the Impact of Bio-Based Plasticizers on the Curing Behavior and Material Properties of a Simplified Tire-Tread Compound

The tire industry needs to become more sustainable to reduce pollution and fight climate change. Replacing fossil ingredients in a tire-tread compound with bio-based alternatives is an approach to create a more sustainable product. For instance, the plasticizer can be replaced, which is a petroleum-based ingredient used in relatively high amounts in the rubber. This approach was followed in the current study. Three plant-based plasticizers were selected as potential substitutes for treated distillate aromatic extract (TDAE) in a simplified tire-tread compound formulation, namely, sunflower oil, coconut oil, and cardanol. Additionally, squalane was used as a TDAE replacement to further investigate the possible interactions between plasticizers and other compound ingredients. Squalane (C30H62) is a fully saturated substance, containing six methyl groups but no additional chemical functional groups. Therefore, it was expected that squalane would result in limited interactions within the studied system. All alternatives to TDAE showed an increased cure rate and decreased scorch time, except squalane. This indicates that the three bio-based plasticizers might interact with the vulcanization system. For example, they could function as an additional coactivator of the curing system and/or shield the silica surface. A severe decrease in maximum torque and an increase in elongation at break were obtained for cardanol and sunflower oil. Both plasticizers also resulted in lower crosslink densities compared to the other compounds. A model study with the bio-plasticizers and sulfur verified that the unsaturation in the cardanol and sunflower oil reacted with the crosslinking agent. This leads to less sulfur available for the curing reaction, explaining the low maximum torque. The tan δ curves showed that all replacements resulted in a decrease in the glass transition temperature of the compound. Although all oil alternatives displayed promising results, none of them are suitable as a direct substitute for TDAE in a tire-tread compound due to its ability to interact additionally with other rubber ingredients and contribute in this form to the reinforcement of the compound.

Sulfur and Peroxide Vulcanization of the Blends Based on Styrene-Butadiene Rubber, Ethylene-Propylene-Diene Monomer Rubber and Their Combinations

Rubber blends based on styrene-butadiene rubber, ethylene-propylene-diene monomer rubber and a combination of both rubbers were cured with different sulfur and peroxide curing systems. In sulfur curing systems, two type of accelerators, namely tetramethylthiuram disulfide, N-cyclohexyl-2-benzothiazole sulfenamide, and combinations of both accelerators were used. In peroxide curing systems, dicumyl peroxide, and a combination of dicumyl peroxide with zinc diacrylate or zinc dimethacrylate, respectively, were applied. The work was aimed at investigating the effect of curing systems composition as well as the type of rubber or rubber combinations on the curing process, cross-link density and physical-mechanical properties of vulcanizates. The dynamic mechanical properties of the selected vulcanizates were examined too. The results revealed a correlation between the cross-link density and physical-mechanical properties. Similarly, there was a certain correlation between the cross-linking degree and glass transition temperature. The tensile strength of vulcanizates based on rubber combinations was higher when compared to that based on pure rubbers, which points out the fact that in rubber combinations, not only are the features of both elastomers combined, but improvement in the tensile characteristics can also be achieved. When compared to vulcanizates cured with dicumyl peroxide, materials cured with a sulfur system exhibited higher tensile strength. With the application of co-agents in peroxide vulcanization, the tensile strength overcame the tensile behavior of sulfur-cured vulcanizates.

Glass Transition Temperatures and Thermal Conductivities of Polybutadiene Crosslinked with Randomly Distributed Sulfur Chains Using Molecular Dynamic Simulation

The thermal conductivities and glass transition temperatures of polybutadiene crosslinked with randomly distributed sulfur chains having different lengths from mono-sulfur (S1) to octa-sulfur (S8) were investigated. The thermal conductivities of the related models as a function of the heat flux autocorrelation function, applying an equilibrium molecular dynamic (EMD) simulation and the Green-Kubo method, were studied for a wide range of temperatures. The influence of the length of sulfur chains, degree of crosslinking, and molar mass of the crosslinker on the glass transition temperature and final values of thermal conductivities were studied. First, the degree of crosslinking is considered constant for the eight simulation models, from mono-sulfur (S1) to octa-sulfur (S8), while the molar mass of the sulfur is increases. The results show that the thermal conductivities of the crosslinked structure decrease with increasing temperature for each model. Moreover, by increasing the lengths of the sulfur chains and the molar weight of the crosslinker, thermal conductivity increases at a constant temperature. The MD simulation demonstrates that the glass transition temperature and density of the crosslinked structure enhance as the length of the sulfur chains and molar mass of the sulfur increase. Second, the molar weight of sulfur is considered constant in these eight models; therefore, the degree of crosslinking decreases with the increase in the lengths of the sulfur chains. The results show that the thermal conductivities of the crosslinked structure decrease with the increase in the temperature for each model. Moreover, by increasing the lengths of sulfur chains and thus decreasing the degree of crosslinking, the trend in changes in thermal conductivities are almost the same for all of these models, so thermal conductivity is constant for a specific temperature. In addition, the glass transition temperature and density of the crosslinked structure decrease.

Preparation Optimization of CFRP and EPDM Composite by the Co-Curing Method

As the requirements of aerospace technology become more rigorous, the performance of solid rocket motor (SRM) cases needs to be further optimized. In the present study, a co-curing technique was used to fabricate carbon fiber reinforced polymer (CFRP)/ethylene-propylene-diene monomer (EPDM) composites whereby the properties of CFRP/EPDM composites were adjusted by varying the temperature, heating time and type of vulcanizing agent to obtain the optimum manufacturing process. The results of crosslink density (3.459 × 10^-4 mol/cm³) tested by nuclear magnetic resonance (NMR), a 90° peel strength test (2.342 N/mm), and an interlaminar shear test (ILSS = 82.08 MPa) demonstrated that the optimum mechanical properties of composites were obtained under the temperature 160 °C heated for 20 min with the curing agent DCP/S. The interfacial phase and bonding mechanism of composites were investigated by scanning electron microscopy (SEM). Thermogravimetric analysis (TGA) further indicated that EPDM/DCP/S had favorable thermal stability. This will provide valuable recommendations for the optimization of the SRM shell preparation process.

Peroxide-Based Crosslinking of Solid Silicone Rubber, Part I: Insights into the Influence of Dicumylperoxide Concentration on the Curing Kinetics and Thermodynamics Determined by a Rheological Approach

Predicting the curing behaviour of industrially employed elastomeric compounds under typical processing conditions in a reliable and scientifically driven way is important for rubber processing simulation routines, such as injection moulding. Herein, a rubber process analyser was employed to study the crosslinking kinetics of solid silicone rubber based on the concentration of dicumylperoxide. A model was proposed to describe the optimal cure time variation with peroxide concentration and temperature, based on the analysis of processing parameters applying kinetic and thermodynamic judgments. Additionally, the conversion rate was described with the aid of a phenomenological model, and the effect of dicumylperoxide concentration on the final crosslink state was investigated using kinetic and thermodynamic explanations. Optimal curing time was affected both by temperature and dicumylperoxide concentration. However, the effects were less pronounced for high temperatures (>170 ∘C) and high concentrations (>0.70 phr). A limit on the crosslink state was detected, meaning that the dicumylperoxide capacity to crosslink the silicone network is restricted by the curing mechanism. Curing restrictions were presumed to be primarily thermodynamic, based on the proton abstraction mechanism that drives the crosslinking reaction. In addition to providing more realistic crosslinking models for rubber injection moulding simulation routines, the results of this study may also explain the chemical behaviour of organic peroxides widely used for silicone crosslinking.

Networking Skills: The Effect of Graphene on the Crosslinking of Natural Rubber Nanocomposites with Sulfur and Peroxide Systems

Tailored crosslinking in elastomers is crucial for their technical applications. The incorporation of nanoparticles with high surface-to-volume ratios not only leads to the formation of physical networks and influences the ultimate performance of nanocomposites, but it also affects the chemical crosslinking reactions. The influence of few-layer graphene (FLG) on the crosslinking behavior of natural rubber is investigated. Four different curing systems, two sulfur-based with different accelerator-to-sulfur ratios, and two peroxide-based with different peroxide concentrations, are combined with different FLG contents. Using differential scanning calorimetry (DSC), vulcametry (MDR) and swelling measurements, the results show an accelerating effect of FLG on the kinetics of the sulfur-based curing systems, with an exothermic reaction peak in DSC shifted to lower temperatures and lower scorch and curing times in the MDR. While a higher accelerator-to-sulfur ratio in combination with FLG leads to reduced crosslinking densities, the peroxide crosslinkers are hardly affected by the presence of FLG. The good agreement of crosslink densities obtained from the swelling behavior confirms the suitability of vulcameter measurements for monitoring the complex vulcanization process of such nanocomposite systems in a simple and efficient way. The reinforcing effect of FLG shows the highest relative improvements in weakly crosslinked nanocomposites.