Polyphosphoric acid (PPA) in road asphalts modification
Source: | Author:Hanna JAROSZEK | Published time: 2023-12-15 | 315 Views | Share:

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.