Materials and Structures (2018) 51:12 https://doi.org/10.1617/s11527-018-1141-z
ORIGINAL ARTICLE
Physical and chemical characterization of rejuvenated reclaimed asphalt pavement (RAP) binders using rheology testing and pyrolysis gas chromatography-mass spectrometry Mohamed Elkashef
. R. Christopher Williams . Eric W. Cochran
Received: 25 October 2017 / Accepted: 8 January 2018 Ó RILEM 2018
Abstract The use of rejuvenators to restore the properties of reclaimed asphalt pavement (RAP) binders is gaining widespread interest mainly due to its ability to lower the binders’ stiffness and enhance their low temperature resistance. The effect that several rejuvenators have on the rheological properties of the modified binders is well-characterized. However, the nature of the interaction between the rejuvenator and the base binder is still not clearly understood. In this research, a rejuvenator made from soybean oil is blended at 6% dosage with an extracted RAP binder. The durability of the modified binder is assessed using laboratory simulated short-term and long-term aging. The performance grade and rheological properties of the modified binder are determined at different stages of aging. Pyrolysis gas chromatography-mass spectrometry (Pyrolysis/GC–MS) is used to probe the chemical composition of the rejuvenator and the chemical stability of the rejuvenator with aging is investigated. The unaged and PAV-aged modified binder are studied using pyrolysis followed by GC– MS, to examine the nature of the chemical interaction between the rejuvenator and the binder, both before M. Elkashef (&) R. C. Williams Civil, Construction and Environmental Engineering Department, Iowa State University, Ames, IA, USA e-mail:
[email protected] E. W. Cochran Chemical and Biological Engineering Department, Iowa State University, Ames, IA, USA
and after aging. Pyrolysis is used to thermally desorb the rejuvenator before it is being cryotrapped using liquid nitrogen. The cryotrapped evolved gases are then analyzed using GC–MS. The rheological testing shows that the rejuvenated RAP binder has lower stiffness, higher phase angle and significantly improved fatigue resistance. The pyrolysis/GC–MS results reveal that the rejuvenator is undergoing changes during aging as it interacts with the base binder. Keywords Pyrolysis Gas chromatography-mass spectrometry (GC–MS) Reclaimed asphalt pavement (RAP) Rejuvenator Soybean oil Fatigue
1 Introduction Rejuvenators are used primarily to lower the stiffness and improve the low temperature properties of aged binders [1–3]. Short-term and long-term in-service aging cause oxidation of binders which leads to an increase in the amount of asphaltenes and a reduction in the lower molecular weight maltenes portion [4]. Using rejuvenators helps restore the balance between asphaltenes and maltenes by adding more maltenes and/or improving the dispersion of asphaltenes within the maltenes matrix [5]. When the rejuvenators are
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added to the aged binder, they diffuse into the aged binder and improve its properties [6]. Recently, the use of rejuvenators in high reclaimed asphalt pavement (RAP) content mixtures is becoming more common. The move towards higher contents of RAP in asphalt mixtures is driven by the need to reduce energy consumption, sustain the use of natural resources, reduce cost, and preserve the environment. RAP binders lose their fatigue and thermal cracking resistance with aging hence it is important for a good rejuvenator to be capable of enhancing the cracking performance of the RAP binder. The linear amplitude sweep test (LAS) was introduced as an accelerated fatigue test to characterize asphalt binders [7]. The LAS test is easy to perform and does not take excessive amount of time which was the case with the time sweep test that can take several hours to run and often leads to irreproducible results [8]. In a recent study, the LAS was used to evaluate the cycles to failure for RAP binders rejuvenated with six different recycling agents [9]. The bio-derived recycling agents were shown to provide better fatigue resistance as compared to petroleum based recycling agents. The fatigue performance of the RAP binders rejuvenated with the bio-derived recycling agents showed improved significantly to a level comparable to that of a virgin binder. The chemical composition and physical properties of rejuvenators differ greatly. Rejuvenators produced from distilled tall oil, petroleum aromatic extracts, and soybean oil were reported to successfully improve the properties of aged RAP binders [10, 11]. Bio-derived rejuvenators are introduced as safe alternatives to carcinogenic aromatic extracts [1]. Previous studies have shown that the effectiveness of a particular rejuvenator is dependent on the chemical composition, performance grade, and source of the base binder [12, 13]. It is thus believed that the chemical and physical compatibility between the rejuvenator and the base binder largely determines the effectiveness of the modification process. The durability of the rejuvenator is also a major concern since some rejuvenators have been noted to accelerate the aging process [14]. Fourier-transform infrared (FTIR) proved to be very effective in studying the aging behavior of rejuvenated RAP binders [12]. The evolution of the carbonyl and sulfoxide indices can be used as an indication of the rate of oxidation with aging. Thermal gravimetric analysis (TGA) was also used to examine
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the thermal stability of rejuvenated binders [15]. The change in mass loss with temperature provides a means to assess the thermal stability of the rejuvenators and the rejuvenated binders. Gas chromatography-mass spectrometry (GC–MS) is an invaluable tool to analyze volatile thermally stable compounds. Chromatography allows separation of the constituent compounds that make up the rejuvenator. The resulting chromatogram shows well-resolved peaks that indicate different constituents of the analyte. GC–MS was used to identify the chemical structure of bio-oils produced from waste cooking oil [16]. The analysis revealed the complex nature of bio-oils being composed of more than 40 different structures. The dominant structures were identified as long chain fatty acid methyl esters that has both hydrophilic and oleophilic characteristics. In another study, GC–MS was utilized to characterize the chemical structure of different asphalt modifiers produced from agroindustry waste sources [17]. The structure of the modifiers was shown to include oxygen-containing moieties such as hydroxyl and carboxyl groups. The hydrophilic nature of these groups explains the poor behavior of the modified asphalt under moisture conditions. The chemical composition of a biobinder produced hydrothermally from swine manure was studied using GC–MS [18]. The GC–MS results revealed that the molecular weight distribution of the biobinder ranged between 240 and 450 g/mol. The structure of the swine based biobinder showed significant unsaturated hydrocarbons and alcohols. In this research, a rejuvenator produced from soybean oil was used to rejuvenate a RAP binder. The rheological and fatigue properties of the control and rejuvenated RAP binders were studied using a dynamic shear rheometer (DSR) and a bending beam rheometer (BBR). Pyrolysis/GC–MS was used to characterize the composition of the rejuvenator before and after aging. Pyrolysis coupled with cryotrapping was used to thermally desorb the rejuvenator from the asphalt binder before it was analyzed using GC–MS to assess changes in the rejuvenator due to interaction with the binder.
2 Materials and methods The RAP binder was extracted from milled pavements in the State of Iowa, USA. The design traffic for the
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RAP mixture was 1 million ESAL. The extraction process followed ASTM D2172-Method A using toluene as a solvent. The toluene was then evaporated to recover the extracted RAP binder using a rotary evaporator as per ASTM D5404. Nitrogen gas was used to avoid unnecessary oxidation of the RAP binder during the recovery process. A rejuvenator produced from soybean oil was blended with the RAP binder at 6% by weight of the binder using a shear mixer at 160 °C and 2000 rpm for 45 min. The performance grades of the binders were determined using a dynamic shear rheometer (DSR) and a bending beam rheometer (BBR) to measure the critical high and low temperatures, respectively. Short-term aging was simulated using a rolling thin film oven (RTFO) as specified in ASTM D2872 whereas long-term aging was performed on the RTFOaged binders using a pressure aging vessel (PAV) as per ASTM D6521. The intermediate temperature was determined for PAV-aged binder using a DSR. Frequency sweeps were performed on RTFO and PAV-aged binders using a DSR at frequencies ranging from 0.1 to 15.9 Hz. The RTFO-aged binders were tested at a range of temperatures from 64 to 88 °C at 6 °C increments while the PAV-aged binders were tested from 10 to 34 °C at 6 °C increments. A 25-mm plate geometry was used for the RTFO-aged binders and an 8-mm plate geometry was used for the PAVaged binders. The frequency sweep data were used to construct complex shear modulus and phase angle master curves with the aid of the Christensen-Anderson-Marasteanu (CAM) model [19]. The reference temperatures for the master curves were 76 and 22 °C for the RTFO-aged and PAV-aged binders, respectively. The linear amplitude sweep test was conducted using the PAV-aged binders according to the AASHTO TP101-12 provisional standard. For the Pyrolysis/GC–MS analysis, a time-of-flight (TOF) accurate mass GC–MS instrument with Electron ionization (EI) operated in positive ion mode was used. The instrument is equipped with an Agilent 6890 GC. The GC column was baked out at 300 °C for 10 min prior to running the separation to ensure no contaminants are retained on the column from previous runs. The instrument was set to split injection mode and a split ratio of 100. Software called MassLynx was used to acquire and process the data. The samples were introduced using a Frontier EGA/ PY-3030D multi-shot pyrolyzer that is attached to the
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inlet of the GC instrument. Samples were accurately weighed in a sample cup that was directly introduced into the pyrolyzer. The samples were thermally desorbed in the pyrolyzer under a purge of helium gas. The temperature of the pyrolyzer was increased from 50 to 450 °C at a rate of 20 °C/min. The pyrolyzer interface temperature was set to a maximum of 320 °C with automatic control. The evolved gases were cryotrapped using liquid nitrogen to improve the resolution of the chromatographic separation. The GC column was increased in temperature from 40 to 360 °C in 18 min before it was allowed to stand at 360 °C for an additional 2 min.
3 Results and discussion 3.1 Performance grade The critical temperatures for both the control and rejuvenated RAP binders were determined as shown in Table 1. The continuous critical high temperature showed a considerable drop with rejuvenation. The same was true for the continuous critical low temperature which showed a significant drop however at a lesser magnitude than the critical high temperature. A similar drop was noted for the intermediate temperature. The overall performance grade of the RAP binder changed from a PG88-16 to a PG70-28 with the addition of the rejuvenator. The additional mass loss in the presence of the rejuvenator was negligible indicating the absence of highly volatile compounds in the rejuvenator. The parameter DTc was calculated using the BBR results, where DTc is the difference between the temperature at a stiffness, S, of 300 MPa and that at an m value of 0.300. This parameter serves as an indication of the relaxation ability of the binder, where a highly negative value means poor stress relaxation performance. As shown in Table 1, the control RAP binder shows a large negative DTc which is typical for excessively aged binders since aging causes the binder to lose its ability to relax following stress application. On the other hand, the rejuvenated RAP binder shows a much improved DTc denoting significant enhancement in the stress relaxation ability.
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Table 1 Properties of studied binders
Binder
Control RAP
Unaged (High temp.), °C
93.5
75.9
RTFO (High temp.), °C
89.5
72.4
PAV (Intermediate temp.), °C
25.2
14
PAV (Low temp.), °C
- 17.4
- 30.1
DTc, °C
- 8.3
- 1.9
Mass loss
0.4%
0.5%
Performance grade (PG)
PG88-16
PG70-28
3.2 Master curves
where G* and Gg are the complex shear and glassy modulus respectively, xc is the crossover frequency, and both w and m are curve-fit coefficients. The parameter w describes the rate at which the modulus reaches both the upper and lower asymptotes, and m is equal to log(2)/R, where R is the rheological index. R is equal to the difference between Gg and the modulus at xc. Figures 1 and 2 show the complex shear modulus and phase angle master curves for the RTFO-aged binders. The rejuvenated RAP binder clearly shows a reduced complex shear modulus and an increased phase angle at all frequencies. The PAV-aged binders
Phase angle, degrees
90
The data from the frequency sweep tests was processed using the concept of time–temperature superposition to develop master curves for both the control and rejuvenated RAP binders. Shift factors were calculated using the Williams–Landel–Ferry (WLF) [20] model and the data was fit using the CAM model according to the following equation [19]: h x m iw=m c jG j ¼ Gg 1 þ x
80 70 60
RAP Rejuvenated RAP
50 40 0.01
0.1
1.
10.
100.
Reduced Frequency, Hz
Fig. 2 Phase angle master curves for RTFO-aged binders at 76 °C
shown in Figs. 3 and 4 also reveal the same trend where the rejuvenated RAP binder has a lower complex shear modulus and a higher phase angle at all frequencies. The effect of the rejuvenator reverses the effect of aging which typically causes an increase in the stiffness of the binder accompanied with a reduction in the phase angle. The increased stiffness renders the aged binder more susceptible to low temperature cracking and the reduced phase angle make the binder less viscous hence loses its ability to undergo stress relaxation. The addition of the
1,00,000.
1,000. 100.
10,000.
10.
G*, KPa
G*, KPa
Rejuvenated RAP
1.
RAP Rejuvenated RAP
0.1
1,000.
RAP Rejuvenated RAP
100.
0.01 0.001 0.01
0.1
1.
10.
100.
1,000.
Reduced Frequency, Hz
Fig. 1 Complex shear modulus master curves for RTFO-aged binders at 76 °C
10. 0.001
0.01
0.1
1.
10.
100.
1,000.
Reduced Frequency, Hz
Fig. 3 Complex shear modulus master curves for PAV-aged binders at 22 °C
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RAP Rejuvenated RAP
60 50 40 30 20 10 0 0.001
0.01
0.1
1.
10.
100.
1,000.
Reduced Frequency, Hz
Fig. 4 Phase angle master curves for PAV-aged binders at 22 °C
rejuvenator improves the performance of the RAP binder due to its effect on both the stiffness and the phase angle. The effect of the rejuvenator was still sustained following long-term PAV aging which attests to the durability of the rejuvenator. 3.3 Linear amplitude sweep The linear amplitude sweep test was introduced to examine the fatigue properties of asphalt binders under conditions of extreme loading. The current Superpave specification assesses fatigue performance based on a single parameter (G*sind) that describes the material behavior under linear viscoelastic conditions. Such an approach which is based on linear viscoelastic properties is clearly limited and does not represent actual traffic loading conditions. The assumption that the fatigue behavior of binders can be fairly estimated by considering the linear part of the loading response curve does not seem to apply to all binders anymore. This is especially true for modified binders which are designed to sustain more fatigue loading and exhibit non-linear stress–strain behavior [21]. The linear amplitude sweep test is an accelerated test that characterizes fatigue damage resistance of binders by applying an increasing strain rate loading at a constant frequency of 10 Hz. A strain rate of 0.1% is initially applied for 10 s to obtain undamaged response. The strain rate is then allowed to increase from 1 to 30% in increments of 1%. The test is performed on PAV-aged binders using a DSR with an 8-mm plate geometry. A frequency sweep test precedes the linear amplitude sweep test at 0.1% strain rate over a range of frequencies from 0.2 to 30 Hz. The data from the frequency sweep test is used to obtain a material constant that is used in the calculation of the
damage growth. The data from the linear amplitude sweep test and frequency sweep test is analyzed using the viscoelastic continuum damage model (VECD). A relationship that relates the material integrity to the level of damage is derived. The material integrity, defined in terms of G*sind, is related to the damage intensity, D, as follows [21]: jG j sin d ¼ Co C1 ðDÞC2 where Co is the average value of |G*|sind at 0.1% strain rate, and C1 and C2 are curve-fit coefficients. The damage intensity and the curve-fit coefficients are determined as detailed in AASHTO TP101-12. The stress–strain diagrams for the control and modified RAP binders obtained at a test temperature of 16 °C are shown in Fig. 5. The control RAP binder shows high stiffness and exhibits a significant drop in stress beyond a shear strain of about 5%. The rejuvenated RAP binder can sustain higher strain rates and has a lower stiffness. The viscoelastic continuum damage curves at a temperature of 16 °C is shown in Fig. 6, where the damage intensity is plotted against a material integrity parameter, C. The parameter C is taken as the value of |G*|sind normalized to the undamaged initial |G*|sind at a strain rate of 0.1%. The control RAP binder deteriorates quickly with damage, whereas the rejuvenated RAP binder demonstrates improved material integrity at higher levels of damage. At this temperature, the rejuvenated RAP binder can sustain about twice the same level of damage as the control RAP binder. To compare between the performance of the binders at their corresponding intermediate temperatures, linear amplitude sweep tests were conducted at the intermediate temperatures of 25.2 and 14 °C, as given in Table 1, for the control and modified RAP 1,200
Effective Shear Stress, KPa
Phase angle, degrees
70
12
1,000
RAP Rejuvenated RAP
800 600 400 200 0
0
10
20
Effective Shear Strain, %
Fig. 5 Stress-strain diagram at 16 °C
30
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Materials and Structures (2018) 51:12 1
1.2 1.0
0.6
C
C
RAP Rejuvenated RAP
0.8
RAP Rejuvenated RAP
0.8 0.6
0.4
0.4 0.2
0.2 0.0
0
0
50
100
150
200
250
0
50
100
150
200
250
300
Damage intensity
Damage Intensity
Fig. 6 Viscoelastic continuum damage curves at 16 °C
Fig. 8 Viscoelastic continuum damage curves at the binders’ intermediate temperatures
binders respectively. Figure 7 shows the resulting stress–strain diagrams from the linear amplitude sweep tests. Both binders perform similarly within the linear portion of the curve, however they start to deviate at higher strain rates. The results in Table 1 suggest that the fatigue behavior of the control RAP at its intermediate temperature of 25.2 °C should be similar to that of the rejuvenated RAP at its intermediate temperature of 14 °C, however the linear amplitude sweep test results showed this to be true only at low strain rates while at higher strain rates the rejuvenated RAP performed notably better. The intermediate temperatures reported in Table 1 reflect the performance of the binders only within the linear viscoelastic range which explains why the two binders initially showed similar behavior at low strain rates before the rejuvenated RAP binder proved to have better performance at high strain rates. The VECD curves in Fig. 8 also shows that the rejuvenated RAP binder can sustain more damage at its respective intermediate temperature compared to the control RAP binder at its respective intermediate temperature.
3.4 Pyrolysis/gas chromatography mass spectrometry (Pyrolysis/GC–MS) A sample weighing 10 lg of the rejuvenator was placed in a sample cup and dropped inside the pyrolyzer. The sample was thermally desorbed under helium gas to prevent oxidation and the evolved gases were collected using a cryotrap. The cryotrapping technique works by flowing nitrogen gas onto a copper coil placed in a Dewar of liquid nitrogen. The condensed nitrogen liquid is then fed into a glass jet inside the GC oven. The evolved gases get trapped inside the glass jet ready to be analyzed by the GC– MS. The constituents of the rejuvenator are separated inside the GC column based on their affinity to the column’s stationary phase. Constituents with less affinity to the stationary phase elute first and thus have lower retention times while those with high affinity to the stationary phase elute last and have higher retention times [22]. The resulting total ion chromatogram for the rejuvenator is shown in Fig. 9. The chromatogram indicates a total of five distinct and well-resolved peaks representing different constituents within the
RAP Rejuvenated RAP
500
80
400 300 200 100 0
0
10
20
30
Effective Shear Strain, %
Total ion current, x103
Effective Shear Stress, KPa
600
60 40 20 0 15
16
17
18
19
Retention time, minutes
Fig. 7 Stress-strain diagrams at the binders’ intermediate temperatures
Fig. 9 Total ion chromatogram for the used rejuvenator
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60
Total ion current, x103
Total ion current, x103
80
40
20
60 40 20 0 15
16
17
18
19
20
Retention time, minutes
Total ion current, x103
Fig. 11 Total ion chromatogram for PAV-aged rejuvenator 80 60 40 20 0 15
16
17
18
19
20
Retention time, minutes
Fig. 12 Total ion chromatogram for unaged rejuvenated RAP
Total ion current, x103
40
20
0 15
16
17
18
19
20
Retention time, minutes
Fig. 13 Total ion chromatogram for RTFO-aged rejuvenated RAP
Total ion current, x103
rejuvenator formulation. The peaks eluted at different retention times of approximately 15.4, 16.2, 16.8, 17.3, and 17.4 min. The increasing baseline with time is due to column bleeding. This was observed in all the chromatograms obtained in this research. Column bleeding is a characteristic of GC due to the degradation of the column material at high temperatures [22]. A pure sample of the rejuvenator was subjected to RTFO-aging and PAV-aging following the same specifications used to age asphalt binders. The RTFO-aged and PAV-aged rejuvenator samples were analyzed using Pyrolysis/GC–MS to assess the chemical stability of the rejuvenator with aging. Figures 10 and 11 provides the total ion chromatogram for the RTFO-aged and PAV-aged rejuvenator samples, respectively. Both chromatograms showed the same peaks at the same retention times and with similar relative intensity as in the unaged rejuvenator sample indicating that the chemical composition of the rejuvenator was kept intact to a great extent following aging. A 200-lg sample of the unaged rejuvenated RAP binder was put in a sample cup. The sample was heated under a helium blanket to a temperature of 450 °C in the pyrolyzer to outgas the rejuvenator which is then cryo-focused using a cryotrap. At this temperature, some of the binder constituents are also volatilized and collected. The collected evolved gases are then analyzed using GC–MS and a total ion chromatogram is obtained as shown in Fig. 12. Samples of the RTFOaged and PAV-aged rejuvenated RAP binders are also analyzed in the same way and their total ion chromatograms are shown in Figs. 13 and 14, respectively. The total ion chromatogram of the unaged rejuvenated binder shows the rejuvenator’s peaks in
12
60
40
20
0 15
16
17
18
19
20
Retention time, minutes 0 15
16
17
18
19
20
Retention time, minutes
Fig. 10 Total rejuvenator
ion
chromatogram
for
the
RTFO-aged
Fig. 14 Total ion chromatogram for PAV-aged rejuvenated RAP
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addition to other smaller peaks attributed to the binder itself. The structure of the rejuvenator, as revealed by the chromatographic peaks, appears to change with aging when added to the binder which was not the case when the rejuvenator was aged in the absence of the binder. This change implies that the rejuvenator is interacting with the binder leading to chemical alteration in the rejuvenator’s structure. The most striking observation is that the two peaks at 17.3 and 17.4 min reduced in intensity with aging to a point where they entirely disappeared in the PAV-aged binder.
4 Summary and conclusions In this paper, a rejuvenator derived from soybean oil was added to an extracted reclaimed asphalt pavement (RAP) binder at 6% by weight of the binder. Dynamic shear rheometer (DSR) and bending beam rheometer (BBR) measurements were used to obtain the performance grade of the control RAP and rejuvenated RAP binders. The control RAP binder was found to be a PG88-16. The addition of the rejuvenator caused a drop in both the high and low temperature grades resulting in a rejuvenated RAP binder with a PG70-28. The mass loss of the rejuvenated binder did not differ much from that of the control binder indicating the absence of highly volatile components within the rejuvenator. The BBR results showed that the DTc parameter was significantly improved with the addition of the rejuvenator. The value of DTc changed from - 8.3 for the control RAP to - 1.9 for the rejuvenated RAP, indicating enhanced stress relaxation ability. The complex shear modulus master curves showed that the rejuvenated RAP binder had a lower modulus compared to the control RAP binder at all frequencies. This observation was true for both the RTFO-aged and PAV-aged binders indicating the sustained effect of the rejuvenator. The phase angle master curves revealed that the rejuvenation process led to an increase in the phase angle. Again, this effect was noted for both the RTFO-aged and PAV-aged binders. The linear amplitude sweeps conducted on the PAV-aged control and rejuvenated binders clearly indicated that the rejuvenator enhanced the fatigue performance of the binder. At the same temperature, the rejuvenated binder can sustain higher strain rates while maintaining its material integrity. The control
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RAP binder however degraded notably with increasing strain rates beyond 5%. At the respective intermediate temperatures for both the control and rejuvenated RAP binders, the behavior was similar within the linear range but the rejuvenated binder outperformed the control RAP binder at higher strain rates. The Pyrolysis/GC–MS analysis had two objectives, first to study the chemical stability of the rejuvenator, and second to determine whether the rejuvenator is interacting with the binder. To verify the chemical stability of the rejuvenator, GC–MS was conducted on unaged, RTFO-aged and PAV-aged samples of the rejuvenator. The total ion chromatogram at different stages of aging were identical. All chromatograms showed five peaks denoting five different constituents of the rejuvenator. Similar relative intensity and retention times of the peaks were noted across all three chromatograms indicating that no or minimal change in the chemical composition of the rejuvenator took place with aging. To determine if there exists an interaction between the rejuvenator and the binder, thermal desorption was used to outgas the rejuvenator and the evolved gases were collected using cyrotrapping. The evolved gases were analyzed using GC–MS and the resulting chromatogram showed some changes in the rejuvenators’ peaks. Some peaks were totally gone indicating that the rejuvenator was undergoing changes due to its interaction with the binder. This research constitutes a preliminary investigation into the extent and nature of the interaction between the rejuvenator and the binder. Further analytical work needs to be done in the future to verify the findings of this research. Compliance with ethical standards Conflict of interest The authors declare that they have no conflict of interest.
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