Mooney Viscosity Stability and Polymer Filler Interactions in Silica Filled Rubbers

Abstract

Abstract The change in Mooney viscosity ( ML 1+4 ) with aging was followed for silica filled compounds containing various silanes and polar additives. Several mechanisms for the aging stability are postulated and evaluated through experimentation. The type of silane or polar additive used can cause the ML 1+4 to increase or even decrease during aging. When bis(triethoxy silanes) are used in silica filled rubbers, the ML 1+4 growth during aging is caused by hydrolysis. Silica-silica bridging was found to be responsible for the ML 1+4 growth in rubber compounds containing a more thermally stable polysulfide or a sulfur-free bis(triethoxy silane). When the bis(triethoxy silane) is bis(3-triethoxysilylpropyl) tetrasulfide (TESPT), a fraction of TESPT is attached to the unsaturated rubber to give polymer-silica attachments. These attachments further enhance the hydrolytic ML 1+4 increase during aging. Chemical coating of the silica with a monofunctional silane or a physical coating with a trialkyl amine compound effectively stops the ML 1+4 increase upon aging. The prevention of ML 1+4 growth is so efficient that a reduction in the ML 1+4 can be realized by absorption of ambient moisture. The extent of ML 1+4 reduction caused by moisture depends on the degree of hydrophobation of the coated silicas. Hydrolytic stability was also studied with an amine or a sugar fatty acid ester that formed either strong or weak polar associations to the silica.