Studying the Effects of Thermo-oxidative Aging on the Mechanical, Tribological and Chemical Properties of Styrene-butadiene Rubber

Styrene-Butadiene Rubber (SBR) is a form of rubber compound that is widely used in the tire industry. This is due to some of their unique characteristics such as high strength, high elasticity and resilience, high abrasion resistance, ability to absorb and dissipate shocks and vibrations, low plastic deformation, high deformation at low levels of stresses, and high product life. One of the most important and often overlooked causes of SBR degradation and eventual tire failure is 'rubber aging.' It can be defined as an alteration in the mechanical, chemical, physical, or morphological properties of elastomers under the influence of various environmental factors during processing, storing and use. Some of these environmental factors are humidity, ozone, oxygen, temperature, radiation (UV rays), etc. This study focuses on the effects of two of these factors acting in tandem, oxygen and temperature. In the past, studies have been conducted to observe the effects of rubber aging on the mechanical and wear properties of rubber. Studies have also been conducted to study the reactions taking place in rubber during aging and changes in its chemical structure. These studies use different modelling techniques and experiments to quantify the effects of aging.

In this study, a material aging model that can predict the hyperplastic response of styrene-butadiene rubber (SBR) was mathematically developed using an integrated testing and continuum damage model framework. Coupling between the mechanical changes of SBR to the change in the chemical properties, specifically crosslink density (CLD) was also investigated. SBR dogbone shaped samples were accelerated aged in an aging oven at various temperatures and aging periods. Subsequently, hyperelastic tests were conducted to obtain the high strain response taking the 'Mullin's effect' into consideration. These responses were calibrated to different hyperelastic material models and the Arruda-Boyce model was chosen, due to its stable behavior and optimal fit. An aging evolution function was developed based on the variation in the model coefficients. This damage model is able to predict the hyperelastic response of SBR as it ages. A user material subroutine (UMAT) was also implemented in Abaqus based on the obtained aging evolution function to predict the stress response of SBR for varied applications. Additionally, to couple the chemical variations with the hyperelastic response, the rubber structure and composition was probed using Fourier-transform infrared spectroscopy (FTIR). The degradation of additives and SBR polymer chains were analyzed microscopically to explain the impact on the macroscopic properties. This study helps to correlate the change in crosslink density to ameliorate mechanical properties, such as strain at break, modulus, and stiffness.

The effects of aging on the viscoelastic properties of SBR were also studied. Dynamic Mechanical Analysis (DMA) was used to characterize the viscoelastic response. Master curves of storage and loss modulus were generated using the time-temperature superposition principle (TTSP). The friction coefficient was estimated from the storage and loss modulus using a simplified form of the Persson equation [1]. CLD was also estimated from DMA data. Wear experiments were conducted on the Dynamic Friction Tester (DFT) for various aging conditions. The estimated friction coefficient was compared to the one from the experiments. Archard's law was used to correlate the frictional energy to the volume loss during wear experiments. Correlation between the wear and the viscoelastic properties of SBR is also studied. Finally, the lifetime of SBR for various aging temperatures is predicted using various models.

[1] M. Ciavarella, "A Simplified Version of Persson's Multiscale Theory for Rubber Friction Due to Viscoelastic Losses," J. Tribol., vol. 140, no. 1, 2018, doi: 10.1115/1.4036917.