Thiols as a Potential Surface Modifier for a Lignin-based Renewable Functional Filler

The application of lignin-based renewable functional filler (RFF) together with bis(triethoxysilyl)propyl tetrasulfide (TESPT) in a SSBR / BR blend improves the reinforcing properties. It was found just recently [1] that the RFF-silane coupling occurs between a thiol group of the silane and the hydroxy group of the RFF. In case of TESPT, a thiol-like function is formed by the breakage of the sulfur bond, enabling a coupling towards RFF. The ethoxy groups of the silane are less reactive towards the RFF surface and are capable of self-condensation.
Consequently, in the current study, two different thiol containing molecules: 1,2-bis(2-mercaptoethoxy)ethane and a liquid polysulfide polymer with thiol end groups were investigated. Both modifying agents have the potential to act as surface modifiers for RFF as well as a coupling agent towards the polymer matrix. An ex-situ modification of the RFF was carried out. After that, the modified RFF was mixed into a SSBR compound. The impact of the modified RFF on the in-rubber properties was studied in comparison to carbon black N330 and unmodified RFF filled SSBR compounds. Both thiol-modified RFF show a pronounced improvement in the curing and mechanical properties in comparison to the unmodified ones. This confirms that thiol can effectively modify the surface of RFF, contributing to a better compatibility between the filler and the rubber. The tensile properties of both thiol-modified RFF are still lower than that of the reinforcing carbon black. This is most likely based on the lower surface area of RFF applied here as well as a lower amount of polymer-filler interactions. Overall, the coupling of thiol towards lignin based renewable functional filler seems to be the preferred reaction in comparison to the coupling towards the polymer chain.
[1] P. Sekar et al, ACS Sustainable Chem. Eng. 2023, 11, 48, 16882–16892