Horizons in Coupling of Sulfur-Bearing Silanes to Hydrothermally Treated Lignin toward Sustainable Development

The development of hydrothermally treated (HTT) lignin as a promising reinforcing filler for elastomers is challenged by the use of different sulfur-bearing silane coupling agents. Though the application of certain sulfur silane-modified HTT lignin, in particular bis­(3-triethoxysilylpropyl)­tetrasulfide, improves the in-rubber properties relative to the unmodified and other sulfur silane-modified ones, it results in the formation of distinctive spongelike filler textures within the rubber matrix, as observed by atomic force microscopy. It is of prime importance to understand the reason behind this formation of emerging filler cluster units and the less reinforcing efficiency of other sulfur silane-modified ones. This demanded the elucidation of the coupling reaction of hydrothermally treated lignin and sulfur silane modifiers as it can widen the application window for hydrothermally treated lignin in rubber technology and facilitate the use of these renewable materials. To gain insight into this phenomenon, HTT lignin and their model compounds were subjected to modifications using coupling agents bearing the specific silane functionalities, alkoxy and thiol. By detailed nuclear magnetic resonance investigations, it is shown that the underlying principle of coupling occurs between the thiols of silane and lignin. This systematic study also exemplifies that the ethoxy and/or the silanol groups of silane are unreactive toward the lignin surface and are only capable of self-condensation. In summary, a different coupling phenomenon is observed for hydrothermally treated lignin and sulfur silane, explaining both the cluster formation and the obtained reinforcing properties.