The effect of SBS morphology on field prformance and test results

The phase morphology of SBS polymer modified binders was studied using a fluorescence microscope and dynamic shear rheometer (DSR). The morphology at high temperature was determined by rapidly cooling small samples, and thus ‘freezing’ the morphology in microscope slides and in DSR samples. As SBS binders are cooled from their manufacturing temperature there is an increasing tendency for a single phase structure (if one is present) to become a two-phase structure with irregular globules of SBS distributed in an asphaltene-rich phase. The morphology of field samples of asphalts and sprayed seals was also studied and shown to be determined, in the case of asphalt, by the relatively slow cooling rate of the binder during transport and compaction. On the other hand, sprayed seal binders cool rapidly and their morphology reflects that present in the binder at spraying temperatures. Thus the same binder may have different properties when used in an asphalt and a seal. It is, therefore, desirable to ensure that the morphology in specification test samples is the same as that in the field application of a binder. This may mean modifying sample preparation procedures. The compatibility of the bitumen used to manufacture an SBS binder had an important effect on morphology, with incompatible bitumens forming globular particles but binders made with compatible bitumens retaining a homogeneous appearance. Morphology appeared to have little effect on viscosity but could have a major effect on elasticity. Rutting performance is unlikely to be influenced by morphology since it has been shown to be related to binder viscosity. Fatigue testing of asphalt beams with different morphology binders suggested that morphology might affect fatigue life.