Metallocene Catalyzed Polyolefins For Improving Thermal And Mechanical Properties Of Bitumen Blends

Bitumen is a complex viscoelastic material used as a binder in applications ranging from road paving to waterproof membranes for roofing [1]. In such applications, bitumen is often blended with a polymer in order to improve its elasticity. The resulting polymer modified bitumens (PMBs) also generally show improvements in thermal and fatigue cracking resistance, and temperature susceptibility [2, 3]. A wide range of polymers have been used in PMBs, including thermoplastic elastomers, thermoplastics and thermosets [1], but among the most successful are styrene-butadiene-styrene (SBS) block copolymers, in which the polystyrene blocks provide cohesive strength and the rubbery polybutadiene blocks provide elasticity [3]. Because SBS is relatively expensive, the possibility of mixing bitumen with other polymers remains of interest [2-4] and metallocene-catalyzed linear low density polyethylenes (m-LLDPE) have recently been discussed as a possible alternative, combining low cost and elasticity with improved storage stability compared with conventional polyolefins [5, 6]. In the present study, PE/octene and PE/butene based m-LLDPE copolymers with different melt flow indices, glass transition temperatures, melting temperatures and degrees of crystallinity have been used to modify a roofing grade gel bitumen. The morphology, the thermal behaviour and the viscoelastic properties of the PMBs have been studied in the composition range 5 to 50 wt% m-LLDPE. In the case of the butenebased copolymer, fluorescence microscopy indicated a continuous m-LLDPE-rich phase matrix containing discrete asphaltene-rich domains at all polymer contents, although at 5 wt% of the octene-based copolymer; the continuous phase remained asphaltene-rich, and there was some evidence for phase inversion, the discrete m-LLDPE-rich domains containing additional asphaltene-rich domains. It follows that the melt rheological response of the PMBs was dominated by the m-LLDPE, but the excellent flow characteristics of this latter resulted in relatively modest increases in shear viscosity compared to those obtained with equivalent concentrations of SBS. Significant swelling by the maltene-rich components of bitumen led to a decrease of the melting point of both types of m-LLDPE, although the degree of crystallinity relative to the polymer content increased somewhat at low m-LLDPE concentrations. Time temperature superposition was used to provide an indication of the dynamic viscoelastic response in shear over a wide range of frequencies. The pure bitumen exhibited liquid-like behaviour over the entire temperature and frequency range investigated, the loss modulus, G’’, generally being significantly higher than the storage modulus, G’. However, whereas blending with the relatively crystalline octene-based m- LLDPE resulted in more elastic behaviour (G’ greater than G’’) the less crystalline butene-based m-LLDPE showed a crossover in G’ and G’’ for all the concentrations investigated. The crossover frequency was found to shift towards lower frequencies as the m-LLDPE content increased, suggesting it to be possible to fine tune the viscoelastic response, depending on the required end-properties. In view of the relative stability of the PMB microstructures referred to above, these results confirm the promise of m-LLDPE as additives to gel bitumen, and suggest there to be considerable flexibility in the range of m-LLDPE contents that may be envisaged, without compromising processing requirements. The consequences for applicative properties will be discussed.