POLYMER-MODIFIED ASPHALTS AS VISCOELASTIC EMULSIONS

Linear viscoelastic properties of polymer modified asphalts (PMAs) wer e studied at various temperatures and frequencies. The materials consi sted of blends of paving grade asphalt cements (ACs) and diblock poly (styrene-b-butadiene) (SB) or triblock poly(styrene-b-butadiene-b-styr ene) (SBS) copolymer up to 6 wt % concentrations, which yielded hetero geneous PMAs with an emulsionlike morphology: a polymer-rich phase dis persed within an asphalt phase. In addition, the 6% SE modified binder was studied before and after dynamic vulcanization (i.e., in situ cro sslinking of the polymer-rich inclusions to increase the PMA stability ). The rheological response of the blends was calculated using the Pal ierne emulsion model, knowing the mechanical properties of each phase, the volume fraction of dispersed phase and the capillary number of th e dispersed droplets. The interfacial tension then acted as an adjusta ble parameter and was estimated to be of order of 10(-5) N/m. As a con sequence of the colloidal nature of ACs (a solid phase-the so-called a sphaltenes-dispersed in a liquid phase, the maltenes), their mechanica l properties were highly improved after polymer modification for two r easons: (1) swelling of the polymer caused a decrease in the maltene c ontent of the matrix, leading to an increase in its asphaltene content when compared to the initial material composition and a subsequent in crease in modulus, (2) the presence of the dispersed phase modifies th e rheology of the materials as described by the Palierne emulsion mode l. Thus, a highly swollen polymer will create a very hard:matrix with a high volume fraction of soft inclusions, whereas a lightly swollen p olymer will generate a blend with a matrix almost similar to the origi nal AC with a low volume fraction of harder polymer particles. Therefo re, the swelling extent of the polymer should be controlled to optimiz e the properties of PMAs. (C) 1998 The Society of Rheology.