The crosslink density of elastomers is the essential property that determines many other but especially the mechanical product properties. Different raw materials, especially recycled and bio-based materials, influence the vulcanization and may change the crosslink density when they are used as a substitute to conventional raw materials. Aim of this study is to develop a procedure that allows the reliable determination of the crosslink density in highly filled EPDM compounds as basis for future investigations focussed on substitution with sustainable materials in this compound. Unfortunately, experiences with other rubber compounds like tire treads cannot be directly applied here because of the use of other filler types and amounts as well as the differences in the polymer with regard to active sites (available double bonds). Equilibrium swelling, Flory-stress-strain-measurements, freezing point depression and temperature-scanning-stress-relaxation are applied to a model EPDM compound with high filler and softener amount as typically used for sealings. For sensitivity investigation the amount of the crosslink agent sulfur was varied. Furthermore, the influence of different accelerators was investigated. All methods are able to determine the crosslink density but with different standard deviations due to measurement errors. Partially, they can be optimized for this use case. Based on the results a combination of Flory-stress-strain-measurements and freezing point depression was chosen to be used in the future.

Tire wear is a main contributor to microplastics. As we cannot fully avoid tire wear, otherwise we could not brake and stop, new solutions are needed to address this problem. Not only on roads tire wear is released to the environment, even more can be found at airports. The advantage there is that the Tire Wear Airstrip Particles are gathered while cleaning the pavement. This collection is an opportunity to recycle and add new value to it. Whereas rubber powder is a common way to recycle and reuse end-of-life-tires as raw material in rubber compounds, the question is if TWAP is reusable in the same or similar way. In this study TWAP and rubber powder from truck tire treads are analyzed and compared with regard to their morphology, particle size distribution and composition. The particle size distribution of TWAP is broader than rubber powder containing also much smaller particles. The mineral content of TWAP is about 60%. These minerals can be residues of the pavement, brake wear but also rubber ingredients. In comparison to rubber powder, the impurities of TWAP are expected to have an impact with regard to potential applications and should be better separated.

Ground tire rubber (GTR) is a product obtained by grinding worn tire treads before retreading them or via the cryogenic or ambient temperature milling of end-of-life tires (ELTs). The aim of this study is to evaluate if calcium carbonate can be substituted by GTR and, if so, to what extent. Different types of ground tire rubber are incorporated in an EPDM (ethylene–propylene–diene–rubber) model compound as partial or complete substitutes of calcium carbonate. The raw compounds and the vulcanizates are characterized to identify the limits. In general, it is apparent that increasing amounts of GTR and larger particles degrade the mechanical properties. The GTR also influences the vulcanization kinetics by reducing the scorch time up to 50% and vulcanization time up to nearly 80%. This is significant for production processes. The compounds with one-third substitution with the smaller-particle-size GTR show mostly similar or even better properties than the reference.

Pyrolysis is becoming increasingly important in the context of recycling and the volume of end‐of‐life tires worldwide. Sustainable carbon black (sCB), which is produced from pyrolysis oil instead of crude oil, and recovered carbon black (rCB), which is the remaining solid from pyrolysis, are promising secondary raw materials for rubber compounds as a substitute for industrial carbon black produced from fossil resources. This study investigates the possibility of substituting carbon black N550 partially or fully in an EPDM (Ethylene Propylene Diene Monomer) sealing compound. rCB contains impurities that affect the properties of the compound. Aging at higher temperatures, in the presence of oxygen is studied. The properties of the compounds are evaluated after heat treatment in air at different temperatures for up to 6 weeks. The results show that sCB is very close to N550 as a raw material and in terms of its in‐rubber properties. Due to the impurities, rCB alters the cross‐linking density and structure of the polymer‐sulfur network (shift to polysulfidic structure). Lower reinforcement is also observed, which is related to weaker polymer‐filler (decrease of I_3/1 for sCB by 3% and rCB by 43% related to vCB) and filler‐filler interactions. Aging effects are also more pronounced in the compounds containing rCB.

Every aircraft leaves tire wear particles on the ground of airports at take-off and landing. To maintain good grip properties, the pavement must be regularly cleaned up. The gathered Tire Wear Airstrip Particles (TWAP) normally get disposed by burning. As they are similar to rubber powder the question is if they might be used as a secondary raw material to keep them longer in the circular economy. In this study gathered TWAP of the runway of an airport in Germany are compared to ambient ground rubber powder from truck tire treads. Both are analyzed microscopically with regard to their morphology, particle size distribution and chemical composition by calcination with following elemental characterization of the ash by Energy-Dispersion-X-ray Analysis (EDS) in the Scanning Electron Microscope (SEM) and Thermal Gravimetric Analysis (TGA). The particle size distribution of TWAP is broader than rubber powder containing also much smaller particles. The mineral content is about 60% of the gathered TWAP. These minerals can be residues of the pavement or break wear and to a smaller amount from the rubber ingredients. Although TWAP on their own are similar to rubber powder, the impurities they contain may have an impact with regard to potential applications.