Study On The Nonlinear Viscoelastic Behaviors Of Highly Filled Rubber Compounds And Its Gel

Carbon black (CB) is the widely used reinforcing filler in rubber industry due to its low price and the ability to substantially improve the processing and mechanical properties of elastomeric material such as tire and conveyor belt. The filling volume fraction (φ) of CB in filled rubbers is generally larger than 20%, CB particles interact with rubber matrix through particle-particle, particle-rubber interaction to form complex hierarchical structures. Such filled rubbers often exhibit striking nonlinear mechanical response, of which y-dependent modulus decline, i.e. Payne effect, is the most concerned issues, for its practical significance in reducing rolling resistance of tires thus enhancing fuel efficiency, and theoretical value to understand reinforcing mechanism of highly filled rubbers.Carbon black filled natural rubber (CB/NR) is a paradigm of nanocomposite materials, and it is well acknowledged that CB gel (CBG) network embedded in the entanglement rubber matrix is crucial for the reinforcement and viscoelastic nonlinearity. Based on such a consideration, we firstly obtain bulk CBG by solvent extraction of highly filled natural rubber (NR) compounds in toluene and investigate the composition, microstructure, and linear rheological response of CBG samples. Temperature-modulated differential scanning calorimetry was used to examine the existence of glassy rubber layer. We found that the CBG obtained from different highly filled compounds has the same composition, microstructure, and viscoelastic behaviors. CBG contains 40 wt% inextractable rubber fractions, among which ～12 wt% is glassy.Large amplitude oscillatory shear (LAOS) response of CB/NR compound, CBG, and masticated NR were studied and compared. The results indicate that CBG behaves as a jammed system and could be categorized into the soft glasses, the related Payne effect is analogous to an unjamming process without frequency and temperature dependence, which mainly involving the oscillatory shear-induced fluctuation, migration, and rearrangement of CB aggregates. For the highly filled compounds, both the breakdown of the CBG network and the frequency-dependent chain disentanglements are responsible for the Payne effect; the rapid unjamming of CBG network couples with the slow disentanglement of the rubbery network, leading to the Payne effect of compounds slower than that of CBGs.Modulus recovery kinetics, thixotropic behavior, and nonlinear stress relaxation response were measured to furture investigate time-dependent behavior of Payne effect. The results indicate that CBG exhibit rapid modulus recovery character, which is mainly due to the bound rubber mediated quick healing of interparticle association. While in the compounds, the recovery is driven by a diffusion process of CB aggregates and dominated by the viscoelasticity of the matrix rubber, the matrix rubber delay the modulus recovery thus delaying structure rebuilding.