Novel organoclay/isocyanate-based modification of bitumen for the development of high-perfomance composites

Resumo

Bitumen, by-product resulting from the vacuum distillation of crude oíl, consists of a complex mixture of organic and inorganic compounds, comprising mainly aliphatic, aromatic and naphthenic hydrocarbons. Even though its composition greatly depends on the crude oíl source and the refining process, a basic characterisation can be performed based on its solubility in n-alkanes, resulting in maitenes, further subdivided into saturates, aromatics and resins; and asphaltenes, which constitute the most polar fraction of bltumen. This separation configures the so-called SARAs fractions of bitumen. Amongst bitumen properties, waterproofing, ductility, adhesiveness, and resistance to weathering and chemicals have favoured its development for a wide range of applications, mainly in the fields of pavement construction and roofing membrane manufacture. However, bitumen properties and mechanical behaviour are highly influenced by temperature and externa! agents, which may lead to severa! problems such as thermal cracking and rutting, and induce ageing processes, eventually provoking its premature failure. In an attempt to overcome such issues, bitumen has traditionally been modified by the use of different types of additives, commonly polymers like SBS, EVA, etc., which when added to bitumen and properly mixed, improve its mechanical properties. However, many different problems arise from their mutual thermodynamic incompatibility, which tend to provoke an eventual phase separation, with the subsequent loss of improvement in properties. Interestingly, chemical modification by means of reactive polymers has shown to be an effective way to noticeably enhance bitumen properties, and amongst reactive agents, polymeric MDI (a blend of diphenylmethane di-isocyanate and its higher homologues) has led to significant improvements in performance. Another type of material, layered silicates (clays), has centred the interest of both polymer and bitumen modifications, given the enhancement it may induce on their properties when interacting at the nanometre scale. Nanoclays are naturally occurring minerals, inherently inorganic, which might be nevertheless rendered organic if subjected to ion-exchange reactions with quaternary ammonium salts, hence resulting in increased compatibility with bitumen and polymers, allowing for their dispersion in clay layers to produce nanocomposites. Thus, targeting at high performance materials for civil engineering applications, the optimisation of the processing conditions that allow far the delamination of clay layers requires information on the materials' rheological properties. Accordingly, in this work, a further approach has been taken by preparing ternary bituminous composites consisting of an organically-modified montmorillonite clay, Closite 20A®, along with a reactive isocyanate-based oligomer, polymeric MDI. These two additives have been used to modify bitumen, both separately and jointly, resulting in bituminous systems that were studied in a twofold perspective. On the one hand, the influence exerted by different processing variables, such as shear and time, on the end properties. On the other hand, the effects of those variables when combined with the reactivity of isocyanate groups. Thus, investigation has focused on factors behind the formation of a nanoreinforced structure in bitumen/clay systems, so as to produce nanocomposites with improved thermo-mechanical performance; and to what extent the reactivity of isocyanate groups in ternary systems influenced such structure and performance. To proceed so, systems were subjected to a wide range of rheological tests, complemented by thermal, microstructural, and chemical studies. Consequently, a deeper insight has been taken into bitumen/clay nanocomposites, and to the best of our knowledge, a first approach into the characterisation of isocyanate modified bitumen/clay composites, and the influence of sorne majar controlling variables on their end properties.