Polymer modified asphalts as viscoelastic emulsions

Linear viscoelastic properties of polymer modified asphalts (PMAs) were studied at various temperatures and frequencies. The materials consisted of blends of paving grade asphalt cements (ACs) and diblock poly (styrene-b-butadiene) (SB) or triblock poly(styrene-b-butadiene-b-styrene) (SBS) copolymer up to 6 wt % concentrations, which yielded heterogeneous PMAs with an emulsionlike morphology: a polymer-rich phase dispersed within an asphalt phase. In addition, the 6% SE modified binder was studied before and after dynamic vulcanization (i.e., in situ crosslinking of the polymer-rich inclusions to increase the PMA stability). The rheological response of the blends was calculated using the Palierne emulsion model, knowing the mechanical properties of each phase, the volume fraction of dispersed phase and the capillary number of the dispersed droplets. The interfacial tension then acted as an adjustable parameter and was estimated to be of order of 10(-5) N/m. As a consequence of the colloidal nature of ACs (a solid phase-the so-called asphaltenes-dispersed in a liquid phase, the maltenes), their mechanical properties were highly improved after polymer modification for two reasons: (1) swelling of the polymer caused a decrease in the maltene content of the matrix, leading to an increase in its asphaltene content when compared to the initial material composition and a subsequent increase in modulus, (2) the presence of the dispersed phase modifies the rheology of the materials as described by the Palierne emulsion model. Thus, a highly swollen polymer will create a very hard:matrix with a high volume fraction of soft inclusions, whereas a lightly swollen polymer will generate a blend with a matrix almost similar to the original AC with a low volume fraction of harder polymer particles. Therefore, the swelling extent of the polymer should be controlled to optimize the properties of PMAs. (C) 1998 The Society of Rheology.