Degradation during mixing in silica-reinforced natural rubber compounds
Ammarin Kraibut is a PhD student in the Department of Elastomer Technology and Engineering. (Co)Promotors are dr. W.K. Dierkes of the Faculty of Engineering Technology and dr. S. Saiwari and dr. K. Sahakaro from the Prince of Songkla University.
Degradation in silica/silane-filled Natural Rubber (NR) compounds during mixing arises from multiple factors. Mixing under thermo-mechanical and thermo-oxidative conditions induces molecular chain modifications such as chain scission, chain recombination, and network imperfections as well as changes in filler-rubber coupling within the compounds, ultimately degrading material properties. Apart from the influence of mixing conditions, this thesis examines the underlying causes of these degradation phenomena in depth. The effects of processing stabilizers, silica-silane filler systems, and blending NR with Butadiene Rubber (BR) were thoroughly investigated.
The findings reveal that commonly used rubber antioxidants, in this context hydroquinoline and phenylene diamines, i.e., TMQ and 6PPD, can act as mixing / processing stabilizers for silica-filled NR compounds. A tetrasulfide silane providing polysulfidic polymer-filler coupling bonds is prone to releasing sulfur during mixing. Blocked mercapto silanes exhibit a strong plasticizing effect at high mixing temperatures due to release of the blocking hydrocarbon chain, which coincides with rubber degradation. Blending NR with BR demonstrates a higher degree of long-chain branching and chain recombination along with the higher thermo-oxidative stability of BR, which helps mitigate the adverse effects of rubber degradation observed in silica-filled single NR compounds.
