Low temperature fracture properties of polymer-modified asphalts relationships with the morphology

A methodology for studying the relationships between fracture behavior and morphology of polymer-modified asphalts used as binders was developed by using the linear elastic fracture mechanics (LEFM) method and confocal laser scanning and environmental and cryo-scanning electron microscopies. Different types of polymers were used as modifiers: (i) copolymers from ethylene and methyl acrylate (EMA), butyl acrylate (EBA) or, vinyl acetate (EVA); (ii) diblock or star-shape triblock styrene-butadiene copolymers (SB or SBS^*). The 4 to 6 wt. % blends display an heterogeneous structure with a polymer-rich dispersed phase based on the initial polymer swollen by the aromatic fractions of the asphalt. The fracture toughness of the blends is higher than for the neat asphalt even if KIc of blends remains low compared to usual polymer blends due to the brittleness of the asphalt matrix. The fracture behavior which is strongly dependent on the nature of the polymer is discussed from the toughening mechanisms given for the filled polymers and the polymer blends. The EBA, SB, and SBS-based blends compared to the EMA and EVA-based ones display a higher KIc due to the elastomeric behavior of the polymer phase leading to a more efficient energy dissipation during crack propagation. The sample prepared with 4% crosslinked SB (Styrelf) and the corresponding physical blend (non-crosslinked) display the better fracture properties.