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

Abstract

The characterization of the structural and dynamic properties of rubber networks is of fundamental importance in rubber science and technology to design materials with optimized mechanical properties. In this work, natural and isoprene rubber networks obtained by curing at three different temperatures (140, 150, and 170 °C) and three different sulfur contents (1, 2, and 3 phr) in the presence of a 3 phr accelerator were studied using a combination of low-field time-domain NMR (TD-NMR) techniques, including 1H multiple-quantum experiments for the measurement of residual dipolar couplings (Dres), the application of the Carr–Purcell–Meiboom–Gill pulse sequence for the measurement of the transverse magnetization decay and the extraction of 1H T2 relaxation times, and the use of field cycling NMR relaxometry for the determination of T1 relaxation times. The microscopic properties determined by TD-NMR experiments were discussed in comparison with the macroscopic properties obtained using equilibrium swelling, moving die rheometer, and calorimetric techniques. The obtained correlations between NMR observables, crosslink density values, maximum torque values, and glass transition temperatures provided insights into the effects of the vulcanization temperature and accelerator/sulfur ratio on the structure of the polymer networks, as well as on the effects of crosslinking on the segmental dynamics of elastomers. Dres and T2 were found to show linear correlations with the crosslink density determined by equilibrium swelling, while T1 depends on the local dynamics of polymer segments related to the glass transition, which is also affected by chemical modifications of the polymer chains occurring during vulcanization.

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

“…Industrially-applied elastomers, like the silicone rubber, are usually crosslinked by the use of specific chemicals (here denoted as crosslinkers), which along with temperature trigger the crosslinking reaction. Many are the chemical reactions that are able to join two adjacent macromolecules, being the most well-known ones the vulcanization (a mixed ionic-radical reaction at high temperature and applying sulphur, accelerators, and other additives) and the radical crosslinking (at high temperature usually employing organic peroxides) [ 5 , 6 , 7 ]. These two crosslinking methods are currently widely applied for producing rubber parts, employing different polymer processing techniques, such as compression and injection moulding.…”

Exploring the Effect of Resins of Different Origin on the Structure, Dynamics and Curing Characteristics of SBR Compounds

“…Upon curing, T g increases to a similar extent for all the investigated samples. A rise in T g is expected because of the formation of chemical crosslinks [ 41 , 43 ]. However, the decrease in M c −1 with the addition of resin suggests that the observed rise in T g is also associated with processes other than crosslinking, occurring during vulcanization in the presence of resin and leading to structural modifications of the polymer chains [ 43 ].…”

“…A rise in T g is expected because of the formation of chemical crosslinks [ 41 , 43 ]. However, the decrease in M c −1 with the addition of resin suggests that the observed rise in T g is also associated with processes other than crosslinking, occurring during vulcanization in the presence of resin and leading to structural modifications of the polymer chains [ 43 ]. Interestingly, these processes appear to be more relevant with SMD and Dertoline compared to Kristalex.…”

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

“…CFRP has been employed as the SRM case because of its low density, high strength, dimensional stability, and corrosion resistance [ 3 , 4 ]. EPDM rubber has been applied as the thermal barrier for SRM because of its density (0.85 g/cm 3 ), low thermal conductivity and high thermal stability [ 5 , 6 , 7 ]. However, when the rocket is launched, high-speed heat flow can generate extremely high temperatures (2000–4000 °C) and pressure (approximately 60 bar) [ 8 , 9 ], which can lead to the degradation and debonding of barrier [ 6 , 10 ].…”