Migration processes in ground rubber containing compounds

Recycling waste is an essential part of Circular Economy. In the case of rubber products, the use of ground rubber is one possible recycling method to partially reuse the waste. For this, parts of the waste rubber of end-of-life tires are ground into small particles. This material can then be incorporated into new compounds to reduce the use of virgin material and thus the carbon footprint. However, the addition of Ground Rubber Particles (GRP) causes a deterioration in mechanical properties compared to compounds without recycled material.
This behavior can partly be explained by the migration of non-vulcanized sulfur from the surrounding matrix into the GRP. As a result of this diffusion, the crosslink density of the GRP is further increased. At the same time, it causes a poor crosslinking in the matrix and thus the reduction of mechanical properties.
This study investigates the conditions under which sulfur can migrate: Does only elemental sulfur diffuse? What is the influence of the zinc-accelerator complex formed during crosslinking? Which role does temperature play: does migration already occur at low temperatures (e.g. storage of compounds) or only at high temperatures (e.g. mixing and vulcanization)?
The answers to these questions can support the further use of GRP. Furthermore, the findings are also applicable e.g. for retreading or the migration of compound components between different layers during the tire manufacturing process.