Introduction
Asphalt binders are commonly used in the pavement construction. To meet the raising requirements for durability of the road surface, alternative methods of improving binder have been developed. The wide range of viscoelasticity is essential to achieve long-lasting road surface, as it ensures consistency of asphalt’s rheological state in extreme service temperatures. Modifiers applied for improving asphalt’s viscoelasticity are [1]: elastomers, plastomers, synthetic resins, crumb rubber, metal-organic compounds, sulfur, natural asphalts and paraffins. Also polyphosphoric acid can be used for this purpose – pros and cons of its application are subject of this paper.
Bituminous binders Bituminous binders are materials of organic origin whose binding and hardening is caused by the temperature-related change of adhesion and cohesion of their molecules [1]. Those thermoplastic materials are divided into tars and asphalts due to their origin. Tars are produced by destructive distillation under pyrolysis from the organic materials (coal, lignite, wood, peat). Because of their toxicity and low quality they are not used in construction industry. Asphalt, also known as bitumen, is the mixture of hydrocarbons which are naturally occurring or obtained from crude oil distillation. The composition of asphalt is very complex and depends both on the origin and on the method of crude oil processing. Asphalts, especially modified, are also thixotropic – they flow like a liquid when a sideways force is applied. The properties of asphalt are function of temperature and duration of load. Bitumens are used as binders in asphalt concrete, a composite material used in the construction of road layers. It also contains mineral aggregate, filler and additives. All compounds are glued together by asphalt, so despite its low content (ca. 5%) its effect on properties of concrete and, therefore, on the pavement performance is critical. The physical and rheological properties of asphalt are dependent on its composition, chemical structure and colloidal structure.
Colloidal structure of asphalt Due to the complexity and a large number of molecules with different chemical structure, the structure of asphalt is divided into fractions of similar properties. There are several methods for extracting those asphalt group components [1÷3]. Bitumen is often characterized by its chromatographic fractions, the maltenes and the asphaltenes, soluble and insoluble in n-heptane, respectively. The maltenes can be fractionated further into saturates, aromatics and resins [2]. The asphaltenes consist mainly of highly condensed polycyclic aromatic hydrocarbons of average molecular weight 2-15 kDa [2]. Asphalt is commonly modeled as a colloid, with maltenes as the continuous phase and micelles of asphaltenes stabilized by associated resins as the dispersed phase [4]. There is no evident border between dispersed and continuous phase. Asphalt’s composition and its colloidal structure determines its physical and rheological properties. The penetration increases with increasing share of saturates and decreases with increase of asphaltenes and resins; changes in softening point and ductility are opposite. High content of asphaltenes increases viscosity, whereas resins contribute to the adhesion of asphalt to aggregate [3].
Asphalts classification In the USA until 1970s asphalts were classified due to their penetration at 25ºC, (ASTM D946), during 1980÷90s due to their viscosity at 60ºC (ASTM Standard D3381). From 1990s Superpave Performance Grade (PG) classification is introduced. It is based on relation between the binder’s properties and the conditions under which it is used. Asphalts are reported as PG X-Y, where X is the average seven-day maximum pavement temperature, Y is minimum pavement design temperature [1]. Introduction of this classification system has forced changes in the design of asphalt binders. To meet stiffness requirements in regions with extreme climatic conditions is not possible without asphalt binder modification. In Poland, as in many other European countries, the base of asphalt classification is their penetration at 25ºC. Road bitumens are classified according to PN-EN 12591. Polymer modified bitumen (PMB) are classified according to PN-EN 14023:2011. Polymer modified bitumens are reported as PMB X/Y-Z, where X/Y-penetration range at 25ºC, Z- lowest softening point.
Modification of asphalts, polymer modificators Modified bitumen is asphalt whose chemical or rheological properties were modified by addition of chemical compound [3]. First polymer modified bitumens were produced in first half of the XX century; in Poland – in 1990s. Earliest modifiers were natural rubber latex, sulfur and rubber; firstly synthetic modifiers were polychloroprene (CR) and styrene-butadiene copolymer (SBS), which is most commonly used nowadays. Increased attention in asphalt modifiers can be attributed to the following factors [6]: • increasing traffic volume, loads and tire pressures, causing increased pavement rutting and cracking • environmental and economic issues – possibility of recycling waste and industrial byproducts (tires, roofing shingles, glass, ash) with achieving benefit in pavement properties • public agency willingness to fund higher-cost asphalt additives for longer service life of pavement and its less maintenance. • Bitumen is modified to enhance the functional properties of asphalt concrete and the lifetime of pavement. Most important issue is to extend the range of viscoelasticity. Binder and polymer have to be miscible to form homogeneous mixture, without destroying colloidal structure of asphalt. Compatibility affects the long-term storage behavior as it prevents polymer phase separation. Miscibility depends on structure and properties both asphalt’s and polymer’s. Asphalt can be modified with elastomeric or plastomeric polymers. The first results in improved elastic recovery after removing applied stress, broaden range of viscoelasticity, reduced risk of rutting and reduced prone to thermal and fatigue cracking. Plastomers make bitumen stiffer but more prone to permanent deformation; low-temperature properties of such modified asphalt are not better than base asphalt [3]. The most popular polymer modifiers suitable for mixing with asphalt are: atactic polypropylene (APP), polychloroprene (CR), ethylene-vinyl-acetate (EVA), polybutylene (PB), styrene-butadienestyrene (SBS), synthetic styrene-butadiene-rubber (SBR), natural rubber latex, ethylene-propylene-diene terpolymer (EPDM), ethylenemethyl acrylate and ethylene-butyl acrylate (EMA, EBA) and styreneisoprene-styrene (SIS).
and a change in asphalt’s morphology is observed [10, 19]. Explanation of this phenomenon is based on colloidal structure of asphalt. According to recently proposed mechanism [10], asphalt’s weak bases (pyridines and amphoteric quinolones) form ionic pairs with strongly acidic PPA, thus, PPA displace weak acids such as phenols in their interaction with those bases. The loss of the hydrogen bonds and the release of an alkylated phenol from a larger aromatic structures results in compounds with lower molecular mass. Those lower molecular weight components enrich maltens fraction. PPA is also a catalyst in indole bridge formation. Formed large, covalently linked and therefore, stiffer molecules remain in fraction of asphaltenes. As a result, a decrease in average molecular weight of asphalt components and change in its morphology is observed, due to disintegration of asphaltenes resulting in smaller domains and amplification of the natural segregation of maltenes and asphaltenes. Low molecular weight compounds migrate to maltenes and decrease its Tg, and higher molecular weight covalently linked compounds raise the Tg of the asphaltenes [10]. Reactions of PPA with bitumen is strongly dependent on its chemical composition, which is related to its type and origin; thus properties of modified asphalt can slightly vary [19]. Main heteroatom in asphalt is sulfur, however, both aliphatic and aromatic sulfide groups, were proven to be inert when heated with PPA at 150°C for 1 hour [19]. Other studies [10] showed that reactivity of PPA with asphalt increases with increasing nitrogen and oxygen content. Given that indoles are found in higher concentrations than other functional groups (on second place there are pyridines), its content is expected to play a crucial role in the reaction of binders with PPA.
