Experimental investigation of the oxidative ageing mechanisms in bitumen
Introduction
Bituminous binders are composed of a variety of organic molecules consisting of about 85% carbon, 10% hydrogen, heteroatoms such as nitrogen (0–2%), oxygen (0–2%), sulfur (0–9%) and traces of metals such as vanadium, iron and nickel [1], [2]. Among the myriad of chemical functional groups of neat bitumen are the hydroxyl groups of phenols, imino groups of pyrrolic compounds, as well as carbonyl groups of ketones, carboxylic acids and 2-quinolones [3], [4]. The heteroatoms in bituminous organic molecules not only modulate the polarity but also constitute chemical functional groups that can react and change. In particular, a wide variety of sulfur-containing compounds occurs preferentially within bituminous binders, such as sulfides, disulfides, sulfoxides, ring compounds (thiophenes, benzothiophenes and dibenzothiophenes) and their alkyl derivatives [5].
Previous studies have demonstrated the significance of bitumen chemistry as it can assist in the unravelling of the oxidative ageing mechanisms in bitumen [6]. More specifically, ageing of bitumen is a process of autoxidation as the binder reacts with oxygen, generating new compounds that may continue to react with oxygen. To date, a number of studies support that during the ageing process, the active functionalities of bitumen molecules are decomposed through the oxidative dehydrogenation of polycyclic perhydroaromatics generating intermediate hydroperoxides [7], [8]. Through the years, different mechanisms have been proposed to describe this phenomenon ranging from an oxycyclic reaction mechanism [9] up to a dual sequential binder oxidation mechanism [4], [10]. According to the latter, two major oxidation routes may exist, namely the chemically distinct “fast-spurt” and “slow/long-term” routes [6], [10], [11], [12]. This idea has gained considerable support since a direct link with the asphalt production stages can be established [13].
Among other factors, it should be acknowledged that the temperature may vary between different asphalt mixture applications (hot, warm, cold) or stages of the service life. Thus, certain standardised ageing protocols exist in order to simulate the different stages. It has been reported previously that the temperature during the short-term ageing in production stage has a stronger effect on the intensity of oxygenated products than the temperature during the long-term ageing in service life [14], [15]. Moreover, the literature emphasises that an increase of 10 °C may double the reaction rate and a relatively high temperature may destroy certain microstructures of the polar species [6]. Care should be taken when reviewing these mechanisms by taking into account the corresponding thermal history varying per asphalt application. Nevertheless, the ageing mechanisms will remain somewhat similar independently of the thermal history and they may be affected only in terms of quantity of products and reaction rate.
The studies presented so far provide evidence that the ageing-produced ketones and carboxylic acids are of high polarity, generating strong associations, expressed through their Van der Waals forces. Subsequently, the polar compounds of bitumen may interact with each other [16]. Possible chemical changes could induce stronger interactions and change the bitumen microstructure which may have implications in the mechanical behaviour. Given this, there is a growing body of evidence that an increase in apparent molecular weight due to increased molecular interactions can reduce the mobility of molecules to flow which, in turn, will influence the bitumen rheology [17], [18], [19], [20]. Eventually, there is a widespread recognition that the severity of ageing can be tracked by capturing the change in certain functional groups [13], [18], [19], [21], [22].
It appears that experimental validation of the underlying mechanisms has been confined primarily to sulfoxide and carbonyl formation. Considerable research has been devoted to the determination of these functional groups via chemical analysis such as Fourier-Transform Infrared spectroscopy (FTIR) [14], [21], [22]. When it comes to the fractions of bitumen, previous studies were also limited to the explanation that a sequential reaction, of aromatic to resin and finally to asphaltene fraction, takes place [23], [24], [25]. No further evidence for the mechanisms behind these changes and the accompanied reasoning has been provided apart from microstructural changes observed with the use of microscopy [26], [27], [28], [29]. Challenges arise when specific products of ageing in bitumen are needed to be confirmed experimentally.
This study addresses a number of questions regarding the ageing mechanisms of bitumen. An important issue is the validation of the previously proposed oxidation schemes. This was achieved by utilising a number of spectroscopic techniques. In particular, support was provided by FTIR, EPR and TOF-SIMS spectroscopy. Links between the results of the three techniques under predefined oxidation time and temperature, finally, review certain hypotheses for the ageing mechanisms of bitumen.
Section snippets
Materials and ageing treatment
Three bituminous binders were used as specified in Table 1, namely, a hard binder A and two soft binders B and C. Binders A and B originate from a wax-free crude oil, and differ only in the degree of distillation processing. Binder C is a visbroken residue, containing natural wax (crystallisable compounds) and coming from a different crude oil.
To simulate the oxidative ageing of the three binders, a modified thin film oven test (M−TFOT) was used: a binder film of approximately 1 mm thick was
FTIR analyses
The results of FTIR analyses show a steep increase of the normalised sulfoxide intensity followed by a steady milder increase for all three binders (Fig. 4). Binders A and C were found to have almost completed the initial rapid increase at about 5 days, whereas binder B reached this transition point at about 2 days. It becomes apparent that at 8 days of controlled ageing the slow-rate oxidation reaction has been initiated for all the binders.
Given that sulfoxides are considered to be one of the
Conclusions
In this work, the oxidative ageing mechanisms of three binders were investigated by FTIR, EPR and TOF SIMS. Binders were aged with modified thin-film oven test at a temperature of 50 °C. Consistent with previous studies the results from FTIR support the existence of two rate-determining oxidation phases, a fast- and a slow-rate and a rapid sulfoxide formation during the fast. For the examined binders this transition point was found to be between 2 and 5 days. Synchronous EPR measurements
CRediT authorship contribution statement
Georgios Pipintakos: Methodology, Investigation, Validation, Formal analysis, Writing - original draft. H.Y. Vincent Ching: Software, Formal analysis, Investigation, Validation, Writing - review & editing. Hilde Soenen: Conceptualization, Supervision, Writing - review & editing. Peter Sjövall: Formal analysis, Investigation, Writing - review & editing. Uwe Mühlich: Supervision, Writing - review & editing. Sabine Van Doorslaer: Conceptualization, Writing - review & editing. Aikaterini Varveri:
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors gratefully acknowledge support from Nynas AB and the European Union for H.Y. Vincent Ching's H2020-MSCA-IF grant (grant number 792946. iSPY).
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