Thermomechanics of Elastomers Undergoing Scission and Crosslinking at High Temperatures

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Abstract
The elastomeric materials used in tires are frequently subjected to severe thermal, chemical, and mechanical stress conditions. These conditions produce significant changes in material properties that affect their service life. The prediction of service life has become an increasingly important part of the engineering design process, and there is a need for a robust life-prediction model. There are many physical factors that affect the durability of an elastomeric material, such as deformation, conversion of mechanical energy to heat arising from dissipative effects, heat transfer within the component, and changes in material properties because of changes in microstructure. The goal of this work is the development of a thermomechanics model that incorporates these factors. This study focuses on the effect of high temperatures on an elastomeric component. There are two sources of temperature increase, a hot environment and internal heating attributed to mechanical loading such as occurs during cyclic loading. Internal mechanical heating can lead to substantial temperatures occurring within the component. When the temperature of the material becomes sufficiently high, macromolecules undergo time-dependent scission, recoil, and may crosslink to form new networks with new reference configurations. This process can affect the stiffness of the material system, induce anisotropy, and lead to permanent set. A constitutive theory is presented that accounts for this temperature-dependent microstructural change on the mechanical response. It is based on experimental results and is motivated by the two-network theory of Tobolsky. The theory is applicable for large deformation and varying temperature histories. An example is presented that illustrates the implications of scission and re-crosslinking.

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