Rheological Studies On Silica Filled Solution Polymerized Styrene Butadiene Rubber

Nowadays,great improvements are claimed for the"Green Tire"which isprepared by using commercial rubber filled with silica (SiO₂) with respect to lowerrolling resistance,better traction on water and snow,and lower heat build-upcompared with conventional tires filled with carbon black (CB).However thedispersion of SiO₂ is worse than that of CB due to existence of hydrophilic silanolgroups in SiO₂ surface.Silane coupling agents are usually used to improve thefiller-rubber interaction and to prevent SiO₂ agglomeration.Addition of colloidalparticles affects the rheological behaviors of the rubbery matrix.In this thesis,solution-polymerized styrene butadiene rubber (SSBR)/SiO₂compounds have been investigated.Silane coupling agent 3-hexanoylthio-1-propyltriethoxysilane (HXT) was synthesized and used in SSBR/SiO₂ compounds.Dynamic rheological measurement reveals that incorporation of HXT into SSBR/SiO₂compounds inhibit SiO2 agglomeration and improve dispersion of the particles in theSSBR matrix.The characteristic strain (γc) for Payne effect of the compoundscontaining HXT appears higher than that of those containing bis-(triethoxysilylp-ropyl)-disulfide (TESPD).The effect of different silane coupling agent (TESPT,TESPD and NXT) on thetheological properties for uncured SSBR/SiO₂ compounds mixed at two differenttemperatures (120℃and 150℃) were investigated.It is found that the dispersion stateof SiO₂ particles in the matrix is related to compounding temperature and themolecular structure of silane coupling agent.For 50 phr SiO₂ filled rubber,NXTcontent above 4.2 phr hardly influences theological behaviors because the reactionbetween SiO₂ and NXT almost approachs saturated.On the other hand,the effect of filler volume fraction (φ) on the nonlinearviscoelastic properties for uncured SSBR/SiO₂ compounds was examined.Resultsshow that the curves of dynamic storage modulus (G'),loss tangent (tanδ) anddamping function (h(γ)) versus strain amplitude (γ) for the filled rubber can besuperposed on those for the unfilled SSBR respectively,suggesting that the primary mechanism for the Payne effect is mainly involved in the nature of the entanglementnetwork in rubbery matrix.Moreover,the modulus recovery degree of filled rubber inthe thixotropy experiment becomes worse with increasingφ,indicating that thebreakdown and reformation process of filler network plays an important role in thefilled rubber besides the contribution of macromolecular chain.The effect of strain mode (steady shear,oscillatory shear and steady extension) onrheological behaviors for SSBR/SiO₂ compounds was studied.γand strain rate (γ)dependences of steady and complex viscosities (ηandη*),stress (Ï„) and modulus (G)for filled rubber are qualitatively similar to those for unfilled SSBR.Yielding-likebehavior is gradually weakened with increasingφin steady shear,indicating that themobility of polymer chain is restricted by addition of filler.On the other hand,Cox-Merz rule can be applied to examineηandη^* of SiO₂/SSBR compounds.Goodagreement between steady extensional viscosity (η_E) in the low strain (ε) amplitudeand the linear viscoelastic envelope prediction is observed.An overshoot in the tensilestress growth appears and the maximum valueη_m presents extension thinning with anasymptotic expression close toη_m～ε^-k.The corresponding characteristic strain value(ε_c) decreases with increasingφ,revealing the strain amplification effect of SiO₂particles.The effect of silane coupling agent and filler loading on the mechanical propertiesfor vulcanized SSBR/SiO₂ compounds was investigated.Compared with TESPT andTESPD,tensile strength of vulcanized rubber containing NXT becomes higher.Incase of NXT content above 4.2 phr,tensile strength remains constant and elongationat break only decreases slightly with increasing NXT content.Both tensile strengthand elongation at break increase with increasingφ,and the vulcanites perform best atφ=0.12.It is suggested that in the temperatures from -80℃to -50℃(around Tg ofSSBR),dynamicωsweep curves at different temperatures obey time-temperaturesuperposition (TTS) principle,and WLF equation can be used to fit the shift factor (α_Ï„)as a function of temperature.