Influence of bitumen colloidal nature on the design of isocyanate-based bituminous products with enhanced rheological properties

Virginia Carrera, Pedro Partal, Moise´s Garcı´a-Morales, Crı´spulo Gallegos,† and Antonio Pa´ez
Departamento Ingenierı´a Quı´mica, Facultad de Ciencias Experimentales, Campus de ‘El Carmen’, UniVersidad de HuelVa, 21071 HuelVa, Spain, and REPSOL YPF, Technology Centre, N-V Road, km 18, 28931 Mostoles, Spain
This work deals with the influence that bitumen colloidal nature exerts on the rheological properties of bitumen samples modified by isocyanate-based reactive polymers. Shear rheology tests, modulated differential scanning calorimetry (MDSC), chemical characterization by TLC-FID, and AFM microstructural analysis were carried out on four different 150/200 penetration neat bitumen samples and the corresponding MDI-PPG (a low molecular weight polypropylene glycol functionalized with a polymeric 4,4′-diphenylmethane diisocyanate) modified binders. The results obtained demonstrate that the bitumen modification degree depends on bitumen reactivity and microstructure. Thus, the highest modification capability is obtained with neat bitumen samples that exhibit both a well-developed three-dimensional network and a high chemical reactivity with the isocyanate groups. The results obtained may be used to improve the performance of these materials, according to their final application.
1. Introduction
Bitumen is a colloidal dispersion of asphaltenes into an oily matrix constituted by saturates, aromatics, and resins (which make up the maltene fraction). This composition is commonly known as SARA fractions [1]. The chemical composition of bitumen depends primarily on its crude source and processing.
Bitumen physicochemical behavior depends on the relative concentration of its different fractions. Thus, a variation in its composition strongly affects its mechanical properties and chemical reactivity [2,3]. Bitumen has some properties, such as impermeability, adhesiveness, elasticity, and ductility, which make it the most suitable material to be used as a binder of mineral aggregates for paving applications [4]. However, the performance of bitumen for road applications has been questioned, due to the fact that is brittle and hard in cold environments and soft in hot environments [5], yielding different road distresses. Bitumen modification with polymers may help to overcome such road distresses, i.e., rutting at high temperature, fatigue cracking, and thermal cracking [6,7]. Three main categories of polymers are generally considered for bitumen modification: thermoplastic elastomers, plastomers, and reactive polymers [8,9]. The first two classes of polymers usually present a very low compatibility with bitumen. The addition of reactive polymers, containing functional groups supposedly able to chemically interact with certain bitumen compounds, may yield some advantages in the resulting binder [2,3,8,10]. In this sense, an MDI-PPG (polypropylene glycol functionalized with 4,4′-diphenylmethane diisocyanate) prepolymer has been used in this research. Bitumen modification by this prepolymer is expected to take place by reaction of the isocyanate groups of the polymer with polar groups (-OH; >NH) in asphaltenes and resin molecules. Previous studies pointed out that bitumen modification with MDI-functionalized prepolymers is a complex process [11]. Thus, bitumen modification takes place by reaction of isocyanate groups with some bitumen to chemical reactions with air moisture. Thus, water reacts with the remaining isocyanate groups, yielding an unstable carbamic acid which decomposes into an amine and carbon dioxide.