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Dynamic asymmetry in polymer modified bitumen: a viscoelastic model for phase separation

Polymer modified bitumen (PMB) has been widely used in many countries, but some challenges still exist. One of the common problems is the storage instability of the final PMB products, which is believed to have a close relationship with the phase separation process in PMB. When investigating phase separation in PMB, it is reasonable to consider PMB as a pseudo-binary blend: one polymer-rich phase and one bitumen-rich phase. Based on the theory of viscoelastic phase separation, the dynamic asymmetry between polymer and bitumen may have important effects on the phase separation process. Dynamic asymmetry means the different dynamics for the two components of a binary mixture (one slow component and one fast component). For blends, its physical origin is mainly the large difference in the glass transition temperature between the two components. Because of the influence of thermal history and the complex glass transition phenomena of bitumen and some polymer modifiers, it is not possible to claim important dynamic asymmetry effects in PMB just from the difference in glass transition temperature or characteristic rheological time. However, by analyzing related literature and image data, it is found that some features of viscoelastic phase separation are shown during the phase separation process in some PMBs, i.e. the network structure formation and downward trend in volume fraction of the slower phase. So it is reasonable to hypothesize that phase separation in these PMBs can be described by a viscoelastic model, although more work still needs to be done to confirm this. By assuming composition-dependent mobility, shear and bulk stress, the effects of dynamic asymmetry can be introduced into the energy relation under incompressible condition. Then the equations of motion would be obtained by analyzing the energy relation. In this, the stress-diffusion coupling plays a key role in the model.

Author(s):

Jiqing Zhu    
Division of Highway and Railway Engineering, Department of Transport Science, KTH Royal Institute of Technology
Sweden

Niki Kringos    
Division of Highway and Railway Engineering, Department of Transport Science, KTH Royal Institute of Technology
Sweden