Antioxidants and ageing reduction in bitumen

Ageing of bitumen is strongly related to proliferation of oxidative species in the chemistry of bitumen which mostly are irreversible reactions which alter engineering properties of bitumen permanently. Certainly, the previous statements seem to be far ahead of the pertinent questions which are:

• • What is bitumen?
  • Where does it come from? and
  • Why is ageing an important aspect to its functional properties and performance as a road binder?

Let us first answer the pertinent questions before we engage in the ageing chemistry of bitumen and the role antioxidants play in reduction of ageing. In this article, once we have answered these basic points, we seek to highlight bitumen ageing and the role of antioxidants on the performance of bitumen as a binder in asphalt mixtures.

The basics

Bitumen is a byproduct of fractional distillation in the crude oil refinery. Normally, it is the distillate component at the bottom of the barrel and hence it also consists of other constituents from the refining process. The source of the crude oil determines its chemistry in addition to refining technology which also plays a key role in the final product and the age state of bitumen. Resultant changes in bitumen colloidal chemistry affect its performance as a binder. Bitumen consists of saturates, aromatics, resins and asphaltene fractions and their chemical changes as a result of aging are time and temperature dependent. Saturates, aromatics and resins are collectively known as maltenes. Volatilization of maltenes as a result of high temperatures disturbs the colloidal stability and increases both micellization and flocculation of asphaltenes. These irreversible and permanent changes alter the binder engineering properties. 

The ageing process

Many schools of thought have been suggested with regards to the mechanism of bitumen ageing and we will discuss some of them. A dual oxidation mechanism involving ketones and sulfoxides from hydroperoxide precursor leading to viscosity building alcohols from reactive sulfides scavenging hydroperoxides before decomposition to form ketones. Another school of thought suggests that upon exposure to air, the molecular size of bitumen grows, polar interactions increase, and aromaticity propagates pi-pi and dispersive interactions. Some suggest that contrary to irreversible changes are thixotropic effects, which reflect reversible molecular arrangement over time based of the free volume theory. Exposure to temperature and time leads to different mechanisms of ageing which occur as a combination of different chemical processes taking place concurrently. 

Antioxidants

The role played by antioxidants is critical in slowing down the generation of oxidative species in bitumen. Antioxidants confer stiffness to the bitumen, with marked increase in stiffness at high temperatures and lower stiffness at lower temperatures. Chemically, a decrease in volatilization and oxygen species is observed with use of antioxidants. Examples of antioxidants are vast and  hence for the purposes of this discussion we will focus on hydrated lime and carbon black as conventional antioxidants and commercially available primary antioxidant Irganox 1010 and secondary antioxidant Irgafos 168. Hydrated lime is an active filler that reduces chemical ageing through interaction with acids in the bitumen. Its main constituent is calcium hydroxide but with a mass component of magnesium hydroxide which together they are referred to hydrated dolomitic lime. The two mechanisms of use of hydrated lime in bitumen, consists of dual functionalities as a chemical ageing deterrent and stiffening above room temperature. 

Carbon black shows effective alleviation in ageing although it contributes to deterioration of low temperature bitumen performance. Carbon black exists as a solid pearlized powder known to be heat and UV light stable.  Irganox 1010 is a primary antioxidant known to peptize asphaltenes, leading to improvement in the colloidal index with improved resistance to thermal and UV ageing. Chemically, Irganox 1010 is a sterically hindered phenolic antioxidant normally used in processing and in-service thermostability. Secondary antioxidant, Irgafos 168 participates in reactions with hydroperoxides during processing, hence preventing process-induced degradation and synergistically extending performance of primary antioxidants such as Irganox 1010. 

Examining the different interactions

The relationship between saturates, aromatics, resins and asphaltenes changes as a result of ageing and to counteract that antioxidants reduce the extent to which chemical changes affect the performance of bitumen. Antioxidants reduce the susceptibility of bitumen to oxidative ageing and hence improve fracture behaviour and increased resistance to fatigue cracking. The fatigue resistance of bitumen has been found to fundamentally determine and directly correlate to the fatigue resistance of asphalt mixtures. Service life of asphalt is extended through modification of bitumen with antioxidants. Antioxidants reduce bitumen hardening as result of decrease in molecular size of asphaltenes, resins and increase in free volume space for saturates and in return this enhances the fatigue performance of asphalt. The correlation between laboratory and in-service ageing mechanisms can often be misleading with the laboratory conditioning limited in the exposure of all the possible distresses possible to the bitumen. However, oxidation remains the major form of chemical change key to bitumen aging although more recently studies have started including more spectroscopic analysis in understanding other functional groups that cause changes in saturates, aromatics, resins and asphaltenes. 

Importance of understanding ageing

Bitumen ageing is a multi-step and time dependent phenomenon consisting of different chemical processes and mechanisms that are governed by the nature of exposure to temperature and environmental conditions. Each bitumen is unique based on its parent crude source and furthermore on the refining technology at the different refineries that use crude oil as a feedstock. Ageing highly depends on the chemical composition of bitumen which varies from binder to binder. Antioxidants reduce the susceptibility of bitumen to ageing and this depends on the saturates, aromatics, resins and asphaltene fractions in the bitumen which determine the chemistry of the respective anti-aging mechanisms conferred by the different antioxidants. The performance of bitumen in asphalt depends on its ability to render its engineering properties to the ultimate mix which consists of all the other mix design components such as void content, and type of aggregate. Therefore, bitumen with good resistance to ageing can perform better and increase the longevity of the road in addition to resistance to other distresses that are exposed to the road surface. In essence, ageing is one of the governing factors that influences the bitumen and asphalt mixture properties which is directly related to pavement performance. 

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REFERENCES 

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