Physico-chemical modification of asphalt bitumens by reactive agents

Abstract

Bitumen, a by-product from crude oil distillation, has long been used in numerous engineering applications that range from the construction of road pavements to waterproof membranes for the roofing industry. On account of its properties (impermeability, adhesiveness, elasticity, ductility, etc.), bitumen is the most suitable material to be used as a binder of mineral aggregates for paving industry, and consequently, roads are mainly constructed using a composite mixture of bitumen (~ 5 wt.%) and mineral aggregates. Despite its small proportion, bitumen forms the continuous matrix and is the only deformable component in the pavements. Therefore, as bitumen is responsible for the viscoelastic behaviour characteristic of this material, it plays a relevant role in predicting roads performance. It is known that, after a limited life-time, the combined action of extreme temperatures along with a continuous increase in traffic loads may lead to well-known asphalts distresses. In this sense, a considerable research effort has been expended in modifying the properties of bituminous binders to improve the performance of road pavements. One of the major types of modifiers that have found wide acceptance in the paving industry is the polymer modified bitumen (PMB). A variety of polymers (SBS, SBR, EVA, SEBS, PE, etc.) have been developed to improve perceived limitations of the bitumen and mix to fatigue resistance, permanent deformation, thermal fracture and moisture sensitivity. Alternatively, the paving industry is also interested in development of modified bitumens by reactive agents such as polyphosphoric acid, sulfur, etc. These additives are capable of chemically interact with specific bitumen fractions and may help overcome that storage-related disadvantage. In order to get novel binders with enhanced Theological properties for road paving industry, we herein propose the use of different reactive agents: two non¬polymeric additives (thiourea dioxide and thiourea, abbreviated as �ThD� and �Th�, respectively) and a castor oil functionalized with isocyanate groups (isocyanate-terminated prepolymers), referred to as MDI-CO. The specific objectives of this work may be summarized in the following items: 1. To find new bitumen modification routes by reactive agents for enhancing the engineering properties of bituminous binders. 2. To understand how the bitumen modification via non-polymeric additives (thiourea dioxide and thiourea) occurs. 3. To develop NCO-terminated prepolymers obtained from biomass-derived polyol (castor oil) as a more sustainable and environmental bitumen modification procedure. 4. To study the effects that isocyanate-prepolymer characteristics and processing/curing conditions exert on the thermal and rheological properties of the resultant bituminous binders. From these results, ThD/Th binders present enhanced thermo-rheological properties in a wide range of in-service temperatures. In addition, it is worth mentioning that the bitumen modification by these non-polymeric modifiers (well-known in many other industrial applications as a reducing agent) represent an alternative to other classic acid bitumen modification with PPA (polyphospheric acid), with the additional benefit of anticorrosion properties of ThD and Th in opposition to storage tanks corrosion. On the other hand, the use of polyurethanes as modifying agents has been extensively developed by the paving industry. They are produced as a result of the reaction between hydroxyl and isocyanate groups, at ambient temperatures, without the use of catalyst. In this work, a vegetable oil-based polyol (castor oil) functionalized with polymeric MDI (4,4�, diphenylmethane diisocyanate) may become a promising alternative to those from petrochemical feedstock, such as PPG or PEG, in agreement with the concept of sustainable and environmental development. Thus, the degree of modification achieved by the MDI/castor oil binders is influenced by two defined pathways: a) Prepolymer formulation: ratio of isocyanate to hydroxyl groups (i.e. -NCO/-OH molar ratio). b) Processing/curing conditions, that is, the influence that different procedures exert on modified samples (long or short processing conditions, curing processing at room temperature, final addition of water, etc.).