Manifold Role of DPG Inside a Silica-filled Tire Tread Compound

Diphenyl guanidine (DPG) is widely used as an essential ingredient in silica-filled tire tread compounds, mainly as a secondary accelerator for the sulfur vulcanization reaction. Beside this, it influences also the silanization reaction: On the one hand, it can be adsorbed at the silica surface which would lead to a reduction in the polarity of silica. This would hinder the silane to access the silica surface and would decrease the coupling efficiency. On the other hand, it can accelerate the silanization reaction by interacting directly with the silane or with ZnO. This would lead to a faster silanization reaction. Both reactions have an impact on the final fillerpolymer coupling via the silane. The time when DPG is added to the compound during the mixing process influences the ratio between both reactions. Adding DPG in an early mixing stage can result in a higher amount of chemically bound rubber due to boosting of the silanization and filler-polymer coupling on the one side, and on the other side, the amount of free DPG is reduced when it is introduced in an earlier mixing stage which should result in a delayed vulcanization and lower crosslink density. Additional to all these considerations, when DPG covers additionally the silica surface, it hinders the adsorption of the primary accelerator(s) which can then contribute more effectively to the vulcanization. This should result in a higher crosslinking density. The possibilities to control these manifold reactions are discussed in the present work.