Rubber compounds filled with recycled rubber: Rheo NMR combined technology reveals changes in sulfur diffusion and crosslinking density

Research Background and Objective: Ground Rubber (GR), as a sustainable filler in rubber compounds, has always been a focus of attention in the field of rubber recycling in terms of the impact of its filling amount on material engineering properties. A study published in the Journal of Applied Polymer Science systematically investigated the effect of GR filling amount on the properties of rubber compounds using a unique combined Rheo NMR device. This study not only focuses on the regulation of engineering performance by GR as a sustainable component, but also deeply analyzes the underlying mechanisms of sulfur diffusion and cross-linking density changes.

Experimental Design and Methods

In the study, the rubber matrix and homemade GR were prepared using styrene butadiene rubber (SBR) and natural rubber (NR), respectively.

. The combined Rheo NMR device was used to quantify two key effects: one is the rheological behavior of the lotion like blend system formed by GR particles in the rubber matrix, and the other is the further vulcanization of GR particles due to sulfur diffusion. Through this device, researchers can simultaneously obtain rheological information and crosslink density (CLD) data.

Key findings and data

The study revealed an important mechanism: free sulfur in the rubber matrix diffuses into the interior of GR particles, which directly changes the crosslinking density between the matrix and GR particles.

. The NMR results showed that for the NR system, the crosslinking density of GR particles increased by 100%; In the SBR system, the crosslinking density of GR particles decreased by about 13%. This difference indicates that the type of base rubber has a significant impact on the direction of sulfur diffusion and the evolution of crosslinking density.

In terms of rheological behavior, GR particles form a lotion like blend structure in the rubber matrix, and its rheological characteristics can be described by an appropriate constitutive lotion model. Among them, the deviation between the predicted results of the Palierne model and the experimental data is within 15%, indicating that the model can well describe the rheological behavior of GR filled rubber compounds.

Significance for the Polymer Additive Industry

This study has multiple implications for the formulation design and recycling process optimization in the rubber additive industry, especially in the tire and rubber fields.

. Firstly, in the design of the vulcanization system, the phenomenon of free sulfur diffusing from the matrix to the GR particles means that in formulations containing recycled rubber, the amounts of sulfur and accelerators need to be rebalanced. If designed according to traditional formulas, the effective sulfur content in the matrix may decrease due to migration towards GR particles, resulting in insufficient cross-linking density of the matrix; At the same time, excessive increase in cross-linking density within GR particles may cause them to become harder and brittle, affecting the dynamic mechanical properties of the final product. The data that the crosslinking density of GR particles increases by 100% for NR matrix is particularly noteworthy, indicating that the redistribution of crosslinking density distribution by sulfur diffusion should be fully considered in the NR/GR blend system. Secondly, in the synergistic application of reinforcing fillers with recycled rubber, when GR is used as a sustainable filler to replace some carbon black or white carbon black, its interface bonding and sulfur diffusion behavior with the matrix will directly affect the mixing and vulcanization process. The Palierne model can predict rheological behavior with a deviation of less than 15%, providing a quantifiable tool for formulation engineers to estimate processing performance under different GR filling amounts, thereby reducing trial and error costs. Thirdly, in terms of rubber aging and weather resistance, changes in crosslink density directly affect the thermal oxidative aging resistance and dynamic fatigue life of rubber compounds. The significant change in crosslinking density of GR particles may become one of the sources of aging performance fluctuations. Therefore, when selecting and optimizing the dosage of anti-aging agents, it is necessary to take into account the unevenness of crosslinking density caused by the introduction of GR. Finally, this study has direct value for the field of tire retreading and recycling. As a sustainable component, optimizing the filling amount and regulating sulfur diffusion of GR are key to achieving high-quality recycled rubber materials. The study reported for the first time the opposite trend of crosslinking density of GR particles in NR and SBR, a finding not previously covered by literature, providing new scientific basis for the refined design of rubber recycling processes.

Conclusion and Prospects

Overall, this study quantitatively revealed the sulfur diffusion driven crosslinking density evolution law in GR filled rubber compounds through the combined use of Rheo NMR technology, and verified the applicability of the Palierne model in describing its rheological behavior.

. For the polymer additive industry, this means that in tire and rubber formulations containing recycled rubber, the design of vulcanization systems, reinforcing fillers, and anti-aging agents needs to shift from a “single-phase” mindset to a “multi-phase sulfur distribution” mindset to achieve a balance between performance and sustainability.

Information source:Effects of Ground Rubber Loading on Rubber Compounds Studied by the Hyphenated Rheo‐NMR and the Palierne Model， Organized by Herun Chemical.