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Isocyanate-based modification is lately gaining acceptance as a successful way to give added value to bitumen, a crude oil refining by-product. In order to study the influence of prepolymer type on the rheological properties of the resulting binders, six prepolymers synthesized from polypropylene-glycols (PPG) with varying molecular weight (between 440 and 2425) and different molar excess of a polymeric MDI (4,4′-diphenylmethane diisocyanate) were used. Two modification procedures, either involving or not water addition were followed. The modification achieved depends on both the selected polyol molecular weight and the excess in MDI (i.e., free isocyanate content), although not in a similar extent. Viscous flow and dynamic oscillatory shear tests, at 60 °C, demonstrated a much higher level of bitumen modification by using the prepolymer prepared with the polyol having a molecular weight of 940 and with a free isocyanate content of 17.4 wt.%, mainly after addition of water. On the other hand, bitumen nature greatly affects the final rheological properties of these bituminous products. In that sense, modification results much more effective when conducted on bitumen with a well-developed colloidal microstructure.

In this study, the suitability of a reactive polymer, synthesized by reaction of 4,4′-diphenylmethane diisocyanate (MDI) with a low molecular weight polyethylene-glycol (PEG), as a modifying agent for the manufacture of bitumen-based waterproof membranes, was evaluated. With that purpose, rheological and thermal analysis tests, and microstructural observations by AFM were carried out on different samples of modified bitumen having a MDI–PEG content ranging from 0 to 10 wt.%, cured at room temperature for a period of time within 0–30 days. The results obtained demonstrate that the addition of the reactive polymer proposed in this work to bitumen is very suitable at high in-service temperatures, because a noticeable increase in the values of viscosity, at 60 °C, of the resulting modified bitumen samples is observed on a time-scale of days. AFM observations, carried out at 50 °C, evidenced that the reactive polymer MDI–PEG leads to a new microstructure, displaying a higher level of stiffness. Therefore, this polymer should be seriously taken into consideration as a modifier of bituminous coatings for the waterproofing industry.

The influence that composition and processing variables exert on the linear viscoelastic properties of model synthetic binders has been studied in a wide range of temperature and frequency. Model synthetic binders were prepared by blending a non-modified colophony resin (40–65%), a process aromatic oil and a styrene–butadiene–styrene (SBS) triblock copolymer (5–15%). At high SBS content (11% and 15%) and gentle processing conditions (i.e. 150 °C and 60 rpm), a plateau region in G ′ is found in the mechanical spectrum. The microstructure of this binder is characterized by a continuous SBS-rich phase. On the contrary, a shoulder in G ′ is found at low polymer content. The resulting microstructure consists of a continuous resin-rich phase and a dispersed polymer-rich phase. Under severe processing conditions (180 °C and 1200 rpm) and low polymer concentration, the polymer influence is dampened and the glassy region appears at higher temperatures or lower frequencies. At high polymer concentration, a phase inversion can be induced by processing (i.e. 180 °C and 1200 rpm.). Both resin oxidation and SBS degradation may explain such microstructural changes.

Rheological measurements reveal that the viscoelastic properties of a 60/70 penetration grade bitumen are improved when either a virgin EVA or a recycled EVA copolymer of similar vinyl acetate content are mixed with it. Risk of cracking at low temperatures and rutting at high temperatures, are both reduced. Better viscoelastic features are obtained with the bitumen modified with recycled EVA, probably due to the presence of carbon black, which acts like a filler in this material. Stability tests performed combining oscillatory flow and microscopy results disclose that blends with the higher polymer proportion (3%) are susceptible of phase separation after 24 h of storage at 165 °C, but 1% blends are stable for at least 4 days. A general evaluation of the results indicates that the performance of this bitumen as a binder for road pavement, is particularly improved when 1% of recycled EVA or virgin EVA is added.