Study On Viscoelastic Behavior Of Filled Rubber During Cure Processing

Vulcanization of rubber products is an important part of many manufacturing processes. The raw rubber is mixed with the filler and then is cured. The microstructure of rubber is changed from one characterized by long-chain network to one characterized by three-dimensional network during this process. Consequently, the rubber’s transition from viscoelastic fluid to viscoelastic solid takes place. Problems in coupling multiphysics, such as deformation and in chemical reactions and thermal fields, can present themselves during the curing process and significantly affect the mechanical response of filled rubber. The research of mechanical behavior of filled rubber during curing is of theoretical interest and application value. Starting from the microstructure of rubber, a novel tri-networks model is proposed to consider the interaction between filler and rubber. Based on the tri-networks model, the constitutive equation is deduced by taking the effect of multiphysics into consideration. Experiments have been performed to obtain the law of dynamic module varied with cure extent. The test data was used to verify the accuracy of the constitutive model and determine the parameters of the model. The constitutive model is employed in the structure analysis of a tire to study the effect of cure conditions on the mechanical behavior of tire. The data and findings of this research can be utilized as a guideline for improving rubber formulation and fine-tuning and optimizing curing conditions. The main contents of this paper are as follows:Firstly, the novel tri-networks model based on the crosslinking network model is proposed and the modification effect of entanglement & filler has been taken into account. By introducing the effective molecular chain density, the molecular chain length between interaction points and the microscopic elongation, the descript ion system of tri-networks model is established. The amplification of micro-stretch is taken into consideration. In addition, the barrier theory and the temporary network model are introduced to reveal the microscopic mechanism of how temperature and load influence the viscoelastic behavior of filled rubber. Meanwhile, the relations between micro-structure parameters and deformation & temperature are acquired. The tri-networks model can reflect the changes of rubber ’s microstructure and stressstrain relationship before and after curing.Based on the analysis above, we take into account temperature change history during curing process. By utilizing the multiplicative decomposition method, the overall deformation gradient is decomposed into three part, such a s temperature induced deformation gradient, equilibrium and non-equilibrium mechanical deformation gradient. According to the evolution law of crosslink density, entanglement density and similar entanglement density during curing, the viscoelastic constitutive relation of filled rubber considering the influence of temperature, load and cross linking reaction is built up. The model can be used to illustrate the two typical stress softening phenomena(Mullins effect and Payne effect), meanwhile the parameters’ variation can be used to describe the change of micro-structure.In order to acquire the relation between cure extent and temperature & time, the reaction kinetics of silica filled natural rubber is studied by the differential scanning calorimetry and rubber process analyzer. Isoconversional method and nonlinear model-fitting method are used to estimate the kinetic triplets avoiding compensation effect. Constant heating rate experiments and isothermal experiments are performed to acquire the non-isothermal cure data and isothermal cure data. The apparent activation energy vs. extent of cure is obtained by advanced integral isoconversional method under isothermal and non-isothermal cure condition respectively. Comparison between the isothermal and non-isothermal cure data reveals that the oscillatory shear have great effect on the later stage of cure reaction.Using the kinetic model of silica filled natural rubber, two kinds of dynamic test method during curing have been performed. Firstly, dynamic test is performed under different fixed strain amplitudes during curing process, the law of dynamic modulus varied with cure time under different test conditions are acquired. Secondly, the samples with different cure extent are prepared through rapid cooling method, and the strain sweep test are performed to acquire the evolution law of Payne effect during curing. Predictions by the proposed coupling model are in good agreement with the experimental results. The influences of temperature, deformation and crosslinking reaction on microstructure of filled rubber are obtained.Finally, using the experimental and simulation methods, tire simulation system is built and the state of cure at arbitrary positions of tire are calculated quickly and efficiently. Then the state of cure distributions are introduced as a predefined field in the structure analysis. The tri-networks model is employed in the loading analysis of tire to study the effect of inhomogeneous cure on the mechanical behavior of tire. It is observed that the contact pressure distribution and internal stress distributions are influenced by the non-uniform cure. The results show that consideration of nonuniform cure in the tire performance analysis is necessary.