delamination in asphalt

Analytica Chimica Acta 444 (2001) 241–250

Direct and continuous methodological approach to study theageing of fossil organic material by infrared microspectrometry

imaging: application to polymer modified bitumen

Jérôme Lamontagne a,b,∗, Françoise Durrieu b,1, Jean-Pascal Planche c,2,Virginie Mouillet d,3, Jacky Kister a

a CNRS UMR 6171, GOAE, Case 561, Université d’Aix-Marseille III, 13397 Marseille Cedex 20, Franceb Laboratoire Central des Ponts et Chaussées, 58 Boulevard Lefebvre, 75732 Paris Cedex 15, France

c Centre de Recherche ELF-Solaize, B.P. 22, Chemin du canal, 69360 Solaize, Franced Laboratoire Régional des Ponts et Chaussées, CETE Méditerranée, B.P. 37000, 13791 Aix en Provence Cedex 3, France

Received 29 March 2001; received in revised form 26 June 2001; accepted 26 June 2001

Abstract

A new simulation method was developed for studying the ageing of polymer modified bitumens (PmBs). An ageing cell spe-cially was designed and fitted to a Fourier transform infrared (FTIR) microscope to continually and directly study the oxidationof PmB by FTIR microscopy imaging. The method was applied to a styrene–butadiene–styrene (SBS) modified bitumen. ThisPmB features a two-phase structure with polymer-rich areas along with polymer poor regions. Changes in the chemical prop-erties were observed during ageing. This structural and functional evolution was monitored in both phases simultaneously todetermine the kinetics of appearance and disappearance of chemical species. The method also allows the study of ageing by ki-netic mapping. Moreover, this methodology allows the analysis of PmB samples without destroying their internal equilibrium.This paper presents some of the possibilities of this new ageing cell. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: FTIR microscopy; Polymer modified bitumens; Ageing simulation; Microstructure; Process analysis

1. Introduction

Any fossil organic material evolves with time, dueto ageing. This is particularly the case for paving bitu-

∗ Corresponding author. Tel.: +33-4-91-28-83-16;fax: +33-4-91-28-82-34.E-mail addresses: [email protected] (J. Lamon-tagne), [email protected] (F. Durrieu), [email protected] (J.-P. Planche), [email protected](V. Mouillet).

1 Tel.: +33-1-40-43-51-64; fax: +33-1-40-43-65-14.2 Tel.: +33-4-78-02-64-78; fax: +33-4-78-02-60-90.3 Tel.: +33-4-42-24-78-27; fax: +33-4-42-24-78-18.

minous binders which are submitted to two main age-ing steps: firstly, the mixing process with aggregatesat high temperature and the laying process on the roadwhere their mechanical properties and chemical struc-tures evolve rather quickly, and secondly, the servicelife in the pavement where their evolution is slowerbut steady. Due to the increase of traffic, more andmore polymer modifiers are used to improve bitumenperformance. For a polymer to be effective in road ap-plications, it has to be compatible with the bitumenand to improve the binder contribution to the pavementresistance to the main distresses at pavement servicetemperatures, such as rutting, cracking from fatigue

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0003 -2670 (01 )01235 -1

242 J. Lamontagne et al. / Analytica Chimica Acta 444 (2001) 241–250

or low temperature, moisture damage, without makingthe polymer modified bitumen (PmB) too viscous atmixing temperatures. In other words, it must improvethe overall performance of the pavement [1–3], andmoreover, this improvement has to be durable withtime.

To analyse the durability of the asphalt mixture, itis necessary to study the evolution of the bituminousbinder used. Furthermore, ageing is one of the impor-tant issues being discussed to set up performance re-lated test methods and then specifications in Europe.

Ageing is already a very complex process in unmod-ified bitumens, and the degree of complexity increasesafter polymer modification [4]. Therefore, the possi-bility to understand and to predict how any changes incomposition or process can affect PmB evolution dur-ing ageing is of prime importance. The main cause forageing is oxidation. It has to be controlled or at leasttaken into account to prevent premature failures of thepavement which would cause industrial and econom-ical damages.

Depending on a number of parameters, PmBs gener-ally display polymer nodules dispersed in a continuousbitumen phase, or a bitumen phase dispersed withina continuous polymer phase, or even two interlockedcontinuous phases. Those parameters are mainly thebitumen composition, polymer nature and content andthe manufacturing method [5–7]. Most paving PmBsinvolve low polymer content. Their microstructure fea-tures polymer nodules swollen by bitumen fractions,dispersed in a continuous bitumen phase [8].

Global analytical techniques such as spectroscopydeveloped for neat bitumens are not sufficient enoughin the case of heterogeneous systems at a microscopicscale [9–12]. Microscopy techniques are indispens-able [13]. Current microscopy techniques can showPmB microstructure [14–16], but cannot inform onthe structural and chemical changes occurring witha polymer modification. Among other microspec-troscopy techniques, IR microscopy was chosen[17–19] because it allows at the same time the visual-isation of PmB microstructure and the chemical char-acterisation of the different observed micro-phasesseparately [20].

So, the knowledge of the PmB’s ageing chemicalmechanisms is essential to judge of their durability.Binder ageing is usually assessed through conven-tional standardised ageing tests such rolling thin film

oven test (RTFOT) and pressure ageing vessel (PAV).Those are supposed to simulate mixing for the RTFOT(75 min, 163◦C, air flow of 4 l/min) and field ageingfor the PAV (20 h, 100◦C, air pressure of 2.1 MPa).However, those ageing simulations give only punc-tual indications. One aim of this study was to developby Fourier transform infrared (FTIR) microscopy adirect continuous and rapid test method for studyingslow evolution phenomena like oxidation in each ofthe PmB micro-phases. To achieve this goal, an ageingcell was developed in our laboratory, to be speciallyadapted and designed for the FTIR microscope. So, itbecame possible to continuously study the oxidationof PmB by FTIR microscopy imaging. The presentpaper deals with the presentation and the characteri-sation of this ageing cell in order to demonstrate theapplicability of the method.

2. Experimental

2.1. Materials

Two PmBs were manufactured at 180◦C temper-ature, under moderate shear stirring, by adding 3%of linear styrene–butadiene–styrene (SBS) to a bitu-men, belonging to the 70/100 penetration grade in theFrench specification according the norm EN 12591.This bitumen can be considered as aromatic accord-ing to its characteristics, evaluated by FTIR and UVsynchronous fluorescence [21]. In a second step, thePmB was artificially aged through the PAV test [22].

2.2. Fourier transform infrared(FTIR) microscopy

The micro-FTIR analysis was performed on a Nico-let Protégé 460 spectrometer coupled with a NicoletNic-Plan IR microscope which was fitted with a MCTdetector cooled with liquid nitrogen. The microscopeis equipped with an CDD camera allowing to visu-alise the sample on the PC’s screen and to recordphotos. The standard analytical conditions were X 15IR objective, 40 �m diameter spot, 64 scans, spectralresolution of 8 cm−1. Spectra were rationed to thebackground collected on a clean area of the NaCl win-dow in the same analytical conditions. Assignmentsof the main IR bands were determined by reference

J. Lamontagne et al. / Analytica Chimica Acta 444 (2001) 241–250 243

to previous works [20]. Due to their high absorbance,PmBs can be analysed only in the transmission mode.The samples are prepared by leaving a hot droplet ofPmB flowing on a NaCl window (13 mm diameter,1 mm thickness), in order to obtain a film thickness ofabout 20 �m. Such a preparation permits to visualisethe sample heterogeneity and to quantify it by FTIRanalysis.

3. Procedure

3.1. Focal point analysis

In FTIR spectroscopy, the IR beam size is relativelywide and cannot accurately separate micro-phasesindividually. FTIR can only analyse the mean ofmicro-phases. On the other hand, by adjusting the IRbeam size (until 10 �m × 10 �m), FTIR microscopyallows to analyse point by point and separately themicro-phases (see Fig. 1). In the case of PmBs, FTIRmicroscopy allows to simultaneously visualise and

Fig. 1. Interest of the FTIR microscopy with regard to FTIR spectroscopy: (�) spectra of bitumen micro-phase; (×) spectra of polymermicro-phase.

Fig. 2. Chemical mapping of the PmB: video image (a), spectracollected on two different areas (b), chemical mapping (c).


Fig. 3. Visualisation by video image and by IR mapping structural and chemical structure evolution of the PmB after PAV.

characterise chemically the copolymer dispersion inthe bitumen, by identifying the chemical species ofbitumens that interact with the polymer to swell it [23].

So, this chemical analysis of the two micro-phases isrealised in situ, without modifying the internal equilib-rium, in a punctual way and using FTIR spectroscopycharacterisation techniques.

3.2. Analysis by mapping

The IR spectra of molecular compounds feature ab-sorption bands that can be associated with specific

Fig. 4. Principle scheme of the ageing cell.

functional groups. One can plot the variation of suchabsorption band intensities versus wave number onfew points of interest of the sample under study. Byusing a motorised x–y sample stage (Spectra-Tech ad-vantage motorised-mapping stage (AMS)) and appro-priate software techniques, it is possible to producefunctional group maps on a large area of the sam-ple (Fig. 2) [24]. In this way, the micro-morphologyof PmB can be characterised by recording IR map-ping of the two micro-phases distribution (polymerand bitumen). Moreover, the motorised stage underthe IR microscope allows to collect a large number of


spectra in a very short period of time, using a video im-age as guide (Fig. 2a). Once these spectra are collected(Fig. 2b), they can be correlated with their spatial lo-cation on the sample. FTIR micro-imaging can, thus,be used to complete other usual imaging techniquesand to correlate the microstructure (visible image) withthe chemical composition (spectroscopic characterisa-tion). IR mapping parameters are optimised to selecta beam size allowing a good compromise between thespatial resolution and signal/noise ratio of the spectra(about 40 �m × 40 �m).As already described, physi-cal blends of PmBs usually feature heterogeneous mi-crostructures including nodules. IR microscopy allowsto confirm the polymer nature of these nodules ac-cording to the polymer characteristic absorption bandsuch as 965 cm−1 for the butadiene, in the case of bi-tumen modified with SBS copolymer. The chemicalmapping (Fig. 2c) of SBS bitumen blend is based onan index defined as the ratio of the copolymer absorp-tion band at 965 cm−1 to the bitumen absorption bandat 1376 cm−1 [5].

3.3. Analysis of the conventional simulated ageing

So far, changes in PmB microstructure during age-ing has been studied discontinuously by characterisingthe sample by FTIR microscopy before and after con-ventional ageing in laboratory such as PAV test. Fig. 3shows chemical changes in the PmB before and afterageing. This method points out the evolution of thepolymer network (reduction of the interval betweenthe indexes [5]) generated by the ageing.

However, it is not possible to follow in continuousthe ageing phenomenon. This approach was developedby FTIR spectroscopy for neat paving grade bitumens[25–27]. The purpose of this work was to adapt thisapproach for FTIR microscopy in order to study PmBageing.

4. The heating ageing cell: design anddevelopment

In order to study the PmB’s structural evolutionduring ageing, an ageing simulation method directlyobservable and analysable by IR microscopy was de-veloped. An ageing cell was purposely designed witha number of requirements: horizontal mode, regulated

heating and adaptability to the motorised stage of themicroscope.

4.1. Scheme

The design and the development of the ageing cellhad to face a key problem that was the limited spacebetween the motorised stage and the objective of themicroscope. Consequently, the total thickness of theheating cell had to be limited to 1.8 cm. This cell isdisplayed in Fig. 4.

It consists of a metallic body heated by two heatingcartridges. These two cartridges are connected to aregulator (WATLOW 988) connected to a temperatureprobe fitted in the centre of the cell. A heated gasflow is flushed through the cell. The gas heating blockis connected to a second regulator WATLOW 988.Moreover, the system is equipped with a by-pass toselect the gas. Analysis can be performed either underoxidant gas (air) or under neutral gas (argon), allowingthe thermal effect to be dissociated from the oxidativeeffect during simulated ageing. Synthetic air (80% N2,20% O2) under 1 bar pressure was usually used.

A NaCl window (13 mm × 1 mm) supporting thesample is fitted at the bottom of the cell. A removableupper NaCl window (30 mm×2 mm) is fixed by somescrews during analysis conditions. The cell is lockedon a Teflon stand itself fitted in the motorised stage ofthe microscope. The use of a Teflon stand reduces theheat diffusion to the motorised stage, not to influencethe moving precision.

4.2. Validity of the cell

4.2.1. Temperature calibrationThe calibration of the cell was realised using

standard compounds of Köfler bench for which themelting points (Y) are well known. We recorded theiractual melting temperature (X) in the cell.

The testing are repeated three times for each com-pound in order to validate the results. The tempera-ture calibration curve is fitted with the equation Y =0.955X + 1.368.

4.2.2. Choice of the ageing temperatureA convenient oxidation test time is 2 h, because it

is sufficiently short without preventing from studyingearly initiation stages. It is also close to the duration


Fig. 5. Evolution of the PmB’s microstructure as a function of the temperature and the lighting of the sample. Picture dimensions as in Fig. 2.

of some of the industrial simulation test of ageing.The objective was, therefore, to determine the max-imal temperature making possible the study of themicrostructure evolution of the PmB during 2 h ofageing. Tests were, thus, performed at different tem-peratures (163–80◦C). The first observation in visiblemode show the microstructural evolution of the PmBduring the ageing at different temperatures in Fig. 5.

Fig. 6. Repeatability of rate of temperature increase. Temperature initially regulated at 25◦C.

The first test of oxidation of the PmB was performedwith the light continually on, allowing to observe thesample in visible mode at a temperature of 163◦C, asspecified in the conditions of the RTFOT normalisedtest. After 10 min, copolymer-rich areas are no longerobservable, as if the copolymer network had dis-appeared. A second test was performed at 163◦C,but switching the light on, only during video image


acquisition. On this second test, the copolymer net-work is observable during 50 min. So, a continuouslighting of the sample during the oxidation test at hightemperature artificially speeds up the degradation ofthe polymeric network by adding photo-oxidation tothermal ageing. This side effect undesirable as partof this study, was photo-oxidation because no tem-perature was associated to it as measured with thesounding line fitted in cell centre. Consequently, tominimise this effect, the light allowing visualisation ofthe sample was switched on only during video imagerecording. Other tests were performed then loweringthe temperature from 160 to 130◦C, by steps of 10◦C.In agreement with Petersen et al. [28] observationson the chemical oxidation kinetics of bitumen, theevolution of the PmB’s microstructure lasted for 2 hat 130◦C. The study was then conducted at 130◦Cduring the rest of the study.

4.2.3. Repeatability of the rate of temperatureincrease

A good repeatability of the rate of temperature in-crease is necessary because of the high reactivity ofsome chemical species during the first minutes ofheating (appearance or disappearance of some func-tions [27], such as the carbonyls).We designed the

Fig. 7. Kinetic of microstructural evolution of the PmB duringoxidation (130◦C, air). Areas analysed in continuous by IR: (×) forthe bitumen matrix and (�) for the copolymer-rich area. Picturedimensions as in Fig. 2.

cell with two regulators each fitted out with a mi-croprocessor allowing to program the temperature in-crease. Tests were performed with different rates oftemperature increase to reach 130◦C. The regulatorwas programmed to obtain a temperature increase of11◦C/min to 25–135◦C (instruction temperature cor-responding to an actual temperature of 130◦C). Thistime of 10 min is enough for the PmB sample to ho-mogeneously reach 130◦C. The repeatability of themethod is satisfactory (Fig. 6).

5. Results and discussion

From the sample already analysed (Fig. 3) the po-tential of this new ageing simulation cell is presented.

Fig. 9. Kinetic evolution of the polymer network of the PmB byIR mapping.


Fig. 8. Kinetic evolution of different species of the PmB: butadiene (a), carbonyl (b), aliphatic (c), aromatic (d); (�) for the bitumenmatrix and (�) for the copolymer-rich area.

5.1. Kinetic evolution by punctual analysis

At first, the evolution of this PmB microstructurewas studied for 2 h. Spectra and video images wererecorded every 15 min while the sample was oxidisedby the air. This kinetic evolution of this PmB mi-crostructure (Fig. 7) was observed in the visible mode,showing the copolymer nodule sizes to be stable for15–30 min, after which they appeared to swell.

The FTIR microscopy equipped with the ageing cellallows one to study continuously the indexes in twofocal points (Fig. 7).

These structural indexes [21] are calculated fromspectra band areas measured from valley to valley [9].These spectrometric indexes were set previously andallow to describe the state of the copolymer, oxy-genated functions, aliphatic and aromatic structures.

5.1.1. Copolymer sensitivityThe butadiene index is relative to the absorption

band at 965 cm−1. The FTIR microscopy analysis

confirms the degradation of the polymer network ob-served by visible microscopy (Fig. 8a). A fall of bu-tadiene rate occurs after 15–30 min. The drop of thisindex indicates with no doubt that the polybutadienepart of the copolymer undergoes significant degrada-tion during ageing. The bitumen is not the only con-stituent of the binder that chemically evolves. It wasalso observed by optical microscopy (Fig. 7) that thepolymeric network cohesion disappears after 60 minIR microscopy confirms this result. FTIR microscopywith this ageing cell is a new methodology which canshow that the copolymer is damaged by the oxida-tion of the unsaturated polybutadiene part resulting inchain scission, as already described in [29].

5.1.2. Formation of carbonyl compoundsThe carbonyl index (corresponding to the C==O

functions) is relative to the absorption band at1720 cm−1 [30,31]. The bitumen fraction trappedin the polymeric network produces less carbonylfunctions than the bitumen matrix (Fig. 8b) during


60 min, until such test time the copolymer networkhas not disappeared. The copolymer seems to protectthe bitumen fraction trapped in this network againstoxidation.

5.1.3. Sensitivity of aliphatic structuresAt the beginning, the aliphatic index is higher in the

copolymer enriched area than in the bitumen matrix(Fig. 8c). This is in agreement with [20]. The gapbetween the aliphatic index of the bitumen matrix andthat of polymer enriched areas lowers after 60 min.The copolymer seems to protect the trapped aliphaticin its network as long as this networks stays. This issimilar to the carbonyl formation.

5.1.4. Sensitivity of aromatic structuresAt the beginning, the aromatic index is more im-

portant in the bitumen matrix than in the copoly-mer enriched areas (Fig. 8d) as described in [20].During ageing, this index increase slightly in the bi-tumen matrix. In the copolymer-rich areas, the in-dex increases rapidly after 45 min. Transformation ofnaphthenic compounds into aromatic compounds isassumed to occur but cannot be confirmed at thispoint.

This new methodology of ageing simulation allowsat the same time to visualise in continuous the evo-lution of the microstructure of a PmB by optic mi-croscopy, to identify the chemicals structures respon-sible of this evolution by IR analysis and to study theirevolution during ageing.

By optical microscopy, we observed that thecopolymer nodules submitted to thermal oxidationhad an induction period. Then they swell until theydisappear in the bitumen matrix. From a structuralpoint of view, we showed that the copolymer seems toprotect the bitumen fractions trapped in its network.This protection results in the limitation of the forma-tion of carbonyl functions and aromatic structures andlimitation of the destruction of aliphatic structures.Moreover, we have demonstrated that the failure ofthe polymeric network observable in visible modecorresponds to the decrease of the butadiene index,that could be attributed to the scission of butadienechains. Ageing is a combined effect of the bitumenoxidation and the copolymer degradation. The bitu-men ages differently whether it is trapped or not by thecopolymer.

5.2. Kinetic evolution by infrared mapping

We tried to follow during the ageing the evolutionof the polymeric network of a PmB by IR mapping(Fig. 9). This was possible with the horizontal celldesigned and realised to be adapted to the IR micro-scope. Hence, several testing were performed in or-der to optimise the technical parameters, beam size(40 �m × 40 �m), analysis step (20 �m), number ofscans (16), resolution (8 cm−1), size of the area anal-ysed (130 �m×100 �m). Consequently, the ratio timeof mapping acquisition/time of kinetic was improved.Mapping and video images were recorded at regu-lar period of time (30 min) during air oxidation ofthe sample.The variation of the characteristic index ofthe polybutadiene repartition is 1.08 (maximum: 1.22,minimum: 0.16) at the origin and decreases down to0.17 (0.32−0.15) after 2 h of oxidation. So there is adegradation of the copolymer when exposed to heatin the presence of air. This new study allowed to visu-alise in visible mode the degradation of the polymericnetwork and to confirm it by IR mapping. This studyby continuous IR mapping will be particularly of in-terest in the case of more complex microstructures.

6. Conclusion

The present work reports a new methodology forsimulating the PmB’s ageing which occurs during bothmixing and laying processes in road construction andduring further field service conditions. A specially de-signed ageing cell was adapted to the FTIR micro-scope; it made possible the visualisation of the PmB’smicrostructural evaluations during continuous ageinginside the microscope. So very useful information wasprovided for the understanding of the oxidation phe-nomena through which undergoes ageing. Moreover,this work saved time in studies on PmBs in compar-ison with the usual FTIR analysis which addressesonly specified ageing times such as in the conditionsof the RTFOT or the PAV tests. Our study pointed outto what extent PmBs are affected by oxidation: oxi-dation level, oxidation rate and chemical functions in-volved. Such an accurate and continuous analysis ofthe formation or disappearance of chemical functionswould be helpful not only for neat bitumen or polymermodified bitumen, but also for other kinds of fossil


organic matter, e.g. heavy petroleum products, indus-trial oils, etc. This methodology may be extended tostudy other parameters such as environmental temper-ature and gas nature. It has been already suggested tobe apply it in the field of formulations for food andcosmetic industries which stress the effect of ageingin quality standards.

Acknowledgements

The authors thank the Laboratoire Central des Pontset Chaussées and Elf-Antar France for their financialsupport.

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