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Article

FTIR Characterization of Asphalt SARA Fractions in Response to Rubber Modification

by
Mohyeldin Ragab
1,
Eslam Deef-Allah
2,3 and
Magdy Abdelrahman
2,*
1
Pure Asphalt Company, 3455 W 31st Pl, Chicago, IL 60608, USA
2
Department of Civil, Architectural, and Environmental Engineering, Missouri University of Science & Technology, Rolla, MO 65409, USA
3
Construction Engineering and Utilities Department, Faculty of Engineering, Zagazig University, Zagazig 44519, Egypt
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(14), 8062; https://doi.org/10.3390/app15148062
Submission received: 27 June 2025 / Revised: 14 July 2025 / Accepted: 18 July 2025 / Published: 20 July 2025
(This article belongs to the Special Issue Infrastructure Resilience Analysis)
Browse Figures
Versions Notes

Abstract

Asphalt–rubber binders (A-RBs) have a long and deep history of use; however, little is known regarding the interrelated chemical behaviors and miscibility of rubber with the asphalt fractions [saturates, aromatics, resins, and asphaltenes (SARA)]. This study comprehensively attempted to address this knowledge deficiency by employing Fourier transform infrared spectroscopy (FTIR) to investigate the chemical evolution of A-RBs. A-RB interacted at 190 °C and 3000 min−1 for 8 h was deemed to have the optimal rheological performance. FTIR of the liquid fractions of A-RB 190–3000 showed a prominent chemical shift in the SARA fractions, with new peaks that showed rubber polybutadiene (PB) and polystyrene migration into asphaltenes. Meanwhile, decreases in peaks with C–H aromatic bending and S=O stretching for the A-RB 190–3000 saturates showed that the rubber absorbed low-molecular-weight maltenes during swelling. Peaks associated with C=C aromatic appeared in saturates and aromatics, respectively, emphasizing that unsaturated components migrated from the rubber into the asphalt. Thermal analysis showed that rubber dissolution for this sample reached 82%. While a PB peak existed in asphaltenes of A-RB 220–3000, its intensity was diminished by depolymerization, thus compromising the integrity of the migrated rubber structure and generating less rheological enhancement. This study concludes that FTIR characterization of SARA fractions offers valuable insights into the interactions between asphalt and rubber, and that regulated processing conditions are essential for enhancing binder performance.

1. Introduction

Asphalt has a complex chemistry that gives it unique physical and rheological properties [1,2]. It is a residue of crude oil distillation and is composed of many hydrocarbons, from highly polar, highly condensed aromatic systems to nonpolar, saturated hydrocarbons [3]. Mainly, asphalt is composed of two distinct classes of substances: asphaltenes and maltenes [4]. Asphaltenes are dispersed in maltenes, which are composed of nonpolar, saturated hydrocarbons, less polar to moderately polar aromatic components, and polar resins [5,6]. The presence and association of asphaltene with some other components cause asphalt to increase in viscosity. The intricacy of asphalt has resulted in an increasing interest in enhancing it using sustainable, waste-derived modifiers (e.g., scrap tires).
Scrap tire disposal in landfills causes severe problems, including environmental hazards and waste of valuable materials. Most scrap tires are repurposed for fuel, for construction applications, or as ground rubber [7]. Tire rubber is a complex crosslinked material composed of valuable components, including natural rubber (polyisoprene) with synthetic rubbers, styrene-butadiene, and butadiene rubber, that are crosslinked with sulfur and reinforced with carbon black. Valuable additives such as aromatics and antioxidants are also incorporated to increase flexibility, ease of processing, and resistance to aging or oxidation [8,9]. Ground tire, crumb rubber, was first introduced for asphalt surface treatments in the mid-1960s and later adopted in hot mix asphalt applications during the 1970s [10]. When the asphalt binder is mixed with rubber, a combination of swelling, partial dissolution, and release of rubber constituents into the asphalt matrix occurs [11]. The complex interactions in asphalt–rubber binders (A-RBs) can be monitored effectively using Fourier transform infrared spectroscopy (FTIR).
FTIR is a crucial tool for detecting chemical changes arising from additive inclusion into asphalt through changes in functional groups [12]. Carbonyl (C=O) at a 1700 cm−1 wavenumber and sulfoxide (S=O) at 1036 cm−1 were lessened for A-RBs compared to neat binders, indicating improved aging resistance [13]. The increase in aromatics (C=C) at 1600 cm−1 in A-RBs was more pronounced in unaged samples; however, both short- and long-term aging reduced this increase due to the breaking of C=C bonds during aging [14]. Furthermore, increasing rubber content led to higher C=C intensity due to the degradation of rubber chains, releasing additional unsaturated components into the binder’s matrix. Surface treatment of rubber with zinc stearate caused a higher intensity of the S=O peak at 1030 cm−1 [15], influencing the high-temperature performance of binders. Earlier studies chemically examined the liquid phase of A-RBs, without the undissolved rubber, and established evidence for rubber polymeric component release into the asphalt matrix. Small sharp peaks emerged in the A-RBs at 966 cm−1 for the C–H bending of trans-alkene in polybutadiene (PB), C–H bending of terminal-alkene in PB at 911 cm−1, and out-of-plane bending of the C–H in the monosubstituted aromatic ring in polystyrene (PS) [16,17]. All these studies identified rubber-derived compounds in A-RBs’ matrix; however, a better understanding of these modifications necessitates further investigation into their effects on asphalt saturates, aromatics, resins, and asphaltenes (SARA) fractions.
The composition of the SARA fractions of asphalt binder affects its performance and aging characteristics, as confirmed through both experimental studies and molecular dynamic simulations [18,19,20,21,22,23,24,25]. As the asphalt aged, the contents of resins and asphaltenes increased, while saturates and aromatics decreased [19,20,22]. This conversion turns aromatics into resins that can further be converted into asphaltenes [26,27]. In A-RBs, there was a positive correlation between asphaltene and resins content and adhesion, while saturates and aromatics had negative correlations with adhesion. The rubber particles’ size and proportion in A-RBs influenced SARA composition. Increasing the rubber percentage or decreasing the rubber particle size caused increases in the asphaltene and resins components and decreases in the saturate and aromatic components. This was due to the absorption of the asphalt’s light components by rubber particles [21]. Rubber inhibited the increase of carbonyl in asphaltenes, resins, and aromatics, promoting the aging resistance and low-temperature performance of A-RBs [28].
Previous studies [18,21,22,28] examined A-RBs’ entire matrix, along with SARA components, chemically and rheologically. Due to the rubber modifications, the distribution of SARA fractions was altered, influencing rheological performance. These alternations were promoted due to the rubber swelling through absorbing low-molecular-weight fractions (saturates and aromatics). However, these studies lacked in-depth analysis of the interaction between rubber and SARA components through FTIR spectral changes. Consequently, the primary objective of this study was to understand the chemical interactions between rubber and asphalt fractions. This was executed through (1) utilizing FTIR to detect functional group changes and spectral shifts in the SARA fractions of the liquid phase as a result of rubber modifications; (2) evaluating the effect of changing interaction conditions (shear speed and temperature) on these spectral changes; and (3) correlating thermal analysis of the rubber particles (dissolution extent and dissolved components) and rheological performance of the A-RBs (stiffness and elasticity evolution) with the FTIR spectral changes.

2. Materials and Methods

2.1. Materials

This study employed asphalt binder with a performance grade of 52–34 and one type of cryogenically processed crumb rubber, a mixed source of scrap tires. The SARA fractions composed 16.23%, 40.46%, 29.64%, and 13.67% of the asphalt, respectively, according to ASTM D4124-09 [29]. Rubber particles had sizes of 30–40, passing through a #30 sieve (0.60 mm) but being retained on a #40 sieve (0.42 mm). According to ASTM E1131-20 [30], the rubber particles were composed of 7% oil (OL), 28% natural rubber (NR), 21% synthetic rubber (SR), and 44% filler (FR).

2.2. Methods

The experimental program consisted of four stages, as illustrated in Figure 1. The first stage involved asphalt–rubber interactions. After this stage, the A-RBs were obtained at various interaction conditions. The A-RB matrix consisted of undissolved rubber and the liquid phase of asphalt (Figure 2). Partial dissolution of the rubber occurred during the interaction, and the remaining particles were still present as undissolved particles. The second stage was to extract the liquid phases, asphalt without undissolved rubber particles, from the A-RBs. Following that, the neat binder and the A-RBs were fractionated into asphalt SARA fractions. The neat binder and A-RB SARA fractions were evaluated chemically, and their FTIR spectra were compared. The third stage aimed to confirm the FTIR spectra comparison using a thermogravimetric analyzer (TGA). Undissolved rubber particles were extracted from the A-RBs and evaluated thermally using a TGA to understand the dissolved rubber percentage and its components released in the binder. Finally, the FTIR and TGA analyses will be aligned with the rheological analysis of the neat binder and A-RBs, revealing the effects of the interaction between rubber and asphalt on altering the performance of the overall modified binder. The following subsections elucidate the experimental program’s stages.

2.2.1. Asphalt–Rubber Interactions

Asphalt was mixed with rubber particles in a 3.785 L can that was inserted into a heating mantle connected to a temperature controller with a 304.8 mm probe. The selected rubber dosage was 10% by weight of the neat binder. High rubber dosages above 10% can impede its dissolution in the binder matrix due to the lack of aromatics, reducing aging resistance [31]. The mixing temperatures were 190 and 220 °C, and the mixing speeds were 600 and 3000 min−1, controlled via a high-shear mixer with a mixing time of 8 h. An interaction temperature of 190 °C, along with a speed of 3000 min−1 and a time of 8 h, was found to optimize the performance of the A-RB, forming internal network structures and enhancing its rheological performance [17,32]. The 8 h interaction time maximized the release of rubber’s polymeric constituents in the liquid phase of asphalt, in contrast to shorter interaction durations, thereby improving the elasticity and rheological properties of the A-RB [17]. The 220 °C temperature with the same interaction speed (3000 min−1), as well as the 600 min−1 speed at 190 °C, were selected as additional variables to understand their influence on the SARA FTIR spectra, in comparison to the SARA spectra of the optimal interaction (190–3000). The interaction matrix is presented in Table 1. All interactions were conducted under a nitrogen blanket to prevent oxidation, as proposed by an earlier study [32].

2.2.2. Extraction of Liquid Phases

The liquid phases, asphalt without undissolved rubber particles, of the A-RBs were extracted following each interaction. A-RBs were heated to 165 °C and then poured on to a #200 (75 μm) sieve mesh cloth that was fitted over the top of a metal can. This system was kept in the oven at 165 °C for 25 min [32].

2.2.3. Asphalt Fractionation

Following extraction of the liquid phases, the neat binder and A-RB SARA fractions were assessed. The test was conducted following ASTM D4124-09 [29] utilizing two duplicates, and the average results were evaluated. The method involves separation of the asphalt (asphaltene and maltene) fractions using n-heptane precipitation, followed by filtration. The maltene fraction was then subjected to a chromatographic separation process using a column packed with activated alumina and silica gel to separate its components (saturates, aromatics, and resins). A detailed method can be found in a previous work by Ragab [33]. The colloidal instability index (Ic) developed by Gaestel and introduced in Equation (1) reflects the stability of asphalt’s system: as the Ic increases, the asphalts’ systems are less stable [34,35].
The IC is represented by the following equation:
I C = S a t u r a t e s + A s p h a l t e n e s A r o m a t i c s + R e s i s n s

2.2.4. FTIR Analysis

SARA fractions of neat binder and A-RBs were assessed chemically using the Thermo Scientific Nicolet 8700 FTIR-transmission method (Thermo Fisher Scientific, Waltham, MA, USA). Each fraction was diluted into toluene with a concentration of 50 mg of asphalt fraction per mL of toluene [33,36,37,38]. Then, the diluted samples were placed on a potassium bromide disc and kept in air for 15 min before testing to ensure complete toluene evaporation [33]. The test was conducted using 32 scans, four resolutions, and wavenumbers from 4000 to 600 cm−1. The test was performed using two duplicates, and the average results were assessed. FTIR spectra peak assignments were made according to well-established literature values for functional groups [17]. Baseline correction and Gaussian peak fitting were performed using OMNIC software version 9.13.1224 (Thermo Fisher Scientific, Waltham, MA, USA) to separate overlapping bands and enhance the accuracy of the identification of functional groups. FTIR quantitative analysis aimed to calculate the changes in carbonyl (ICO), sulfoxide (ISO), aromatic (ICC), and aliphatic (ICH) indices using Equations (2), (3), (4), and (5), respectively [16,17,19,39].
Equations (2) and (3) represent the ICO and ISO, which are as follows:
I C O = a r e a   a r o u n d   1700   c m − 1 ∑ t h e   a r e a   a r o u n d   1460   a n d   1376   c m − 1
I S O = a r e a   a r o u n d   1030   c m − 1 ∑ t h e   a r e a   a r o u n d   1460   a n d   1376   c m − 1
The ICC and ICH are denoted by Equations (4) and (5), respectively, as follows:
I C C = a r e a   a r o u n d   1600   c m − 1 ∑ a r e a   a r o u n d   1700 ,   1600 ,   1460 ,   1376 ,   a n d   1030   c m − 1
I C H = ∑ t h e   a r e a   a r o u n d   1460   a n d   1376   c m − 1 ∑ a r e a   a r o u n d   1700 ,   1600 ,   1460 ,   1376 ,   a n d   1030   c m − 1

2.2.5. Extraction of Rubber

Undissolved rubber particles were extracted from A-RBs using the rubber dissolution test [17,32]. 5 ± 2 g of A-RBs were placed on a #200 (75 μm) sieve and washed with toluene until the filtrate became colorless, indicating that no asphalt remained with the rubber particles. The washed rubber was placed in an oven at 60 °C for 12 h to evaporate the remaining solvent. Details of calculating the dissolved rubber percentages were discussed in earlier studies [17,32].

2.2.6. TGA Analysis

The original and extracted rubber were characterized thermally using a TA instruments Q-500 TGA (TA Instruments, New Castle, DE, USA), following ASTM E1131-20 [30], to assess the compositional changes of rubber during the interaction with asphalt. The test was conducted with two duplicates, and the mean results were evaluated. Following previous studies [40,41], the test was conducted using a ramp heating method with a 20 °C heating rate from room temperature up to 600 °C. The rubber composition was identified based on the decomposition temperature range of each component [42]: OL decomposed at 300 °C, NR and SR decomposed from 300 to 500 °C, and FL was the remaining component at 500 °C. NR and SR components can be identified through the derivative of thermograph (DTG): NR is thermally degraded from 300 °C to the lowest point between DTG peaks, and SR decomposed from this point till 500 °C.

2.2.7. Rheological Analysis

A temperature sweep test was carried out on neat binder and A-RBs using a dynamic shear rheometer Bohlin Instruments CVO (Bohlin Instruments, Worcestershire, UK), from 10 to 70 °C with 6 °C increments, following ASTM D7175-15 [43]. This test aimed to assess the viscoelastic response of A-RBs by measuring the complex shear modulus (G*) and phase angle (δ) at 10 rad/s. Samples with 25 mm and 8 mm diameters were used for temperatures above and below 45 °C, respectively. The thicknesses of samples were 1 and 2 mm for neat binder and A-RBs above 45 °C, respectively, and 2 mm below 45 °C for both types. The test was performed using two duplicates, and the average results were assessed.

3. Results and Discussion

3.1. Analysis of Asphalt Fractions

The SARA fractions of neat binder and A-RBs are depicted in Figure 3. The coefficient of variation (COV) for saturates varied from 0.02 to 1.12%, for aromatics from 0.75 to 4.00%, for resins from 0.58 to 1.77%, and for asphaltenes from 3.21 to 5.51%. A-RBs had lower saturates plus aromatics and higher resins plus asphaltenes compared to the neat binder. Saturates showed the highest reduction, reaching the lowest value for A-RB 220–3000 (13.12%). A-RBs 190–600 and 190–3000 showed a decrease in saturates by 17.31 and 15.59%, respectively, compared to the neat binder. Aromatics decreased by 2.89 to 15.08% for A-RBs as compared to neat binders. This reflects the role of saturates and aromatics in rubber swelling during the interaction with asphalt. Resins showed minimal variations. They decreased slightly in A-RB 190–600 and then increased in A-RBs 190–3000 and 220–3000. However, asphaltene increased across A-RBs. Asphaltene content increased by 30.65 and 30.21% in A-RBs 190–600 and 190–3000, respectively, and by 56.25% in A-RB 220–3000 relative to the neat binder. The Ic value of the neat binder was 0.43 and increased for A-RBs 190–600 and 190–3000, reaching a value of 0.46. For A-RB 220–3000, the Ic was 0.53, indicating colloidal instability (Ic > 0.5) [35]. The COV values for Ic ranged from 2.24 to 4.35%.

3.2. FTIR Spectral Comparisons of Neat Binder and A-RB Fractions

Figure 4 depicts the FTIR spectra of saturate fractions extracted from neat binder and A-RBs. The mutual exchange between rubber and saturates is confirmed via the FTIR spectral changes. The rubber low-molecular-weight components, aliphatic hydrocarbon chains, were migrated into saturates. This was highlighted in A-RB 190–600, showing a broader and higher intensity of C–H stretching vibrations for aliphatics (sp3 hybrids) [19,39] at 2955, 2925, and 2850 cm−1 peaks compared to the neat saturates. For A-RB 190–3000, a peak that appeared at 1571 cm−1 (extending from 1553 to 1590 cm−1) was related to C=C stretching vibrations in aromatics [44]. This reflects the migration of rubber aromatic components into saturates during the 190–3000 interaction. For this interaction, the rubber in the A-RB released both aromatic and aliphatic components in the saturates due to partial dissolution and thermal degradation of the rubber. Peaks at 1753 and 1723 cm−1 related to C=O stretching vibrations of esters and ketones [45,46] were noted for the saturates of the neat binder. Peaks at 1660 and 1627 cm−1, corresponding to C=C stretching vibrations of aromatics [47,48], were also observed for the neat saturates. However, these peaks disappeared in the A-RBs’ saturates, reflecting the migration of unsaturated polar components, shifting saturates to a more aliphatic fraction.
Two peaks appeared in the neat saturates at 816 cm−1 (extending from 851 to 780 cm−1) and 1036 cm−1 (extending from 1063 to 1006 cm−1), with areas of 15.24 and 8.67%.cm−1, respectively. These peaks are related to C–H out-of-plane bending on aromatic rings and S=O stretching vibrations, respectively [16,17]. For A-RB 19–600, the area at 816 and 1036 cm−1 increased to 206.28 and 86.34%.cm−1, respectively. However, for A-RB 190–3000, these areas had the lowest values, reaching 6.73 and 3.78%.cm−1 at 816 and 1036 cm−1, respectively. For A-RB 220–3000, these peaks’ areas increased again at 220 °C, hitting values of 39.97 and 17.98%.cm−1 at 816 and 1036 cm−1, respectively. At 190 °C and 600 min−1, rubber particles released aromatic and sulfur-containing components into saturates; however, at 190 °C and 3000 min−1, the rubber absorbed these aromatic species from saturates and dissolved more, as explained later by the TGA analysis. Furthermore, at 220 °C, excessive rubber dissolution, detected by TGA, occurred, and rubber depolymerization and excess devulcanization caused the rubber particles to release these components back to the saturate fraction. This shows that the rubber in A-RB 190–3000 absorbed aromatic species at 816 cm−1 and released other aromatic species at 1571 cm−1, indicating mutual exchange.
Figure 5 shows the FTIR spectra of aromatic fractions extracted from neat binder and A-RBs. The aliphatic C–H stretching vibrations between 2960 and 2850 cm−1 for the A-RBs’ aromatics were less intense than those in the neat aromatics. This reflects the decrease in the aliphatic chains of hydrocarbons, indicating the role of these components in the swelling of rubber particles. Another part of these aliphatic components could be thermally volatilized during the high-temperature interaction. At 220 °C, the intensities of these aliphatic components increased again due to depolymerization and excessive devulcanization of the rubber polymeric components. The C=C stretching vibrations peak at 1560 cm−1 [49] emerged for the A-RB 190–600 aromatics and disappeared for the other two types of A-RB aromatics. This reveals the existence of rubber unsaturated components in the aromatics at 190 °C and 600 min−1; however, at 190 °C and 3000 min−1, a similar peak emerged in the saturates (Figure 4). This highlights that increasing the interaction speed from 600 to 3000 min−1 urged the unsaturated components to migrate into the saturate (non-polar) fraction.
Figure 6 shows the FTIR spectra of resins fractions extracted from neat binder and A-RBs. The aliphatic C–H stretching vibrations between 2955 and 2850 cm−1 intensified for A-RB resins compared to the neat resin, highlighting the migration of aliphatic components from rubber into resins. The broad O–H stretching peak at 3300 cm−1 was intense for the neat binder resin; however, for A-RB 190–3000, this peak was extremely broadened. This could be related to hydrogen bonding or chemical interaction that broadened the O–H peak at 3300 cm−1 and disappeared the C=O peak at 1700 cm−1 [50]. However, the O–H and C=O peaks for the A-RB 220–3000 increased again due to oxidation and degradation of rubber components (e.g., depolymerization and excessive devulcanization). Contrary to saturates and aromatics, the spectra of resins did not show C=C stretching vibrations in aromatics at 1560 or 1571 cm−1. Thus, the aromatics in rubber migrated into aromatics or saturates rather than resins.
Figure 7 shows the FTIR spectra of asphaltenes fractions extracted from neat binder and A-RBs. During the interaction between rubber and asphalt at 3000 min−1 and 190 °C, the rubber polymeric components were absorbed with the asphaltene as a polar- and aromatic-rich asphalt fraction [51]. These rubber components were detected at 971 cm−1 (extending from 983 to 959 cm−1) and 735 cm−1 (extending from 752 to 718 cm−1), which were related to the C–H out-of-plane bending of trans-alkene in PB and out-of-plane bending of the C–H in the monosubstituted aromatic ring in PS, respectively [52,53]. These peaks were not detected in the asphaltenes of the neat binder and the A-RB 190–600. Moreover, the PB peak had higher intensity at A-RB 190–3000 than 220–3000, with areas of 4.38 and 1.98%.cm−1, respectively. The PS peak at 735 cm−1 for A-RB 190–3000 had an area of 10.62%.cm−1; however, at 220 °C, this peak disappeared, which was related to the depolymerization that occurred at 220 °C. The C–H stretching vibrations at 2925 and 2850 cm−1 were more intense in A-RBs than in neat asphaltene, showing deeper transmittance values. These peaks were related to the vibrations of aliphatic hydrocarbon groups [54] that intensified from the low-molecular-weight rubber products.
Figure 8 illustrates the FTIR indices of SARA fractions: ICO in Figure 8a, ISO in Figure 8b, ICC in Figure 8c, and ICH in Figure 8d. In Figure 8a, the COV values for ICO ranged from 15.03 to 37.68%. The neat saturates had the highest ICO value (0.23), which concurred with the results presented in Figure 4: neat saturates showed carbonyl groups at 1723 and 1753 cm−1. For A-RBs saturates, the ICO decreased reaching the lowest values for A-RBs 190–600 and 190–3000, hitting values of zero. This is in agreement with the results in Figure 4: the carbonyl groups for the neat saturates disappeared for A-RBs saturates. This reflected the influence of rubber on decreasing the carbonyl components and thus enhancing aging resistance [13]. A further contributing factor was the redistribution of ICO into more polar fractions, especially the resins, with A-RB 190–600 demonstrating a value of 0.08. For A-RB 190–3000 resins, the ICO had a value near zero (0.0018), concurring with the qualitative results displayed in Figure 6. For this sample, the hydrogen bonding or chemical interaction caused the O–H peak at 3300 cm−1 to be broadened and the C=O peak at 1700 cm−1 peak to disappear. In Figure 8b, the COV values for ISO ranged from 0.84 to 52.92%. ISO was the highest in the neat saturates and aromatics, while the AR-B saturates and aromatics had the lowest values. Furthermore, the resin fraction of A-RB 190–3000 revealed the highest ISO (0.65), while its saturate and aromatic values had the lowest ISO values. These findings suggested the absorbance of S=O components from saturates and aromatics into the rubber or rearrangement into more polar fraction (e.g., resins).
Figure 8c shows the ICC with COV values ranging from 1.92 to 29.14%. From saturates to asphaltenes, a general trend revealed an increase in the unsaturated components for neat binder and A-RBs. However, the general trend was different for the aliphatic components in Figure 8d, highlighting a decrease in the ICH values as the fractions progressed from saturates to asphaltenes. These trends were expected, because asphaltenes are rich in aromatic rings with short aliphatic chains, and saturates mostly consist of nonpolar linear, cyclic, and branched saturated hydrocarbons (e.g., naphthene and paraffin) [55,56]. The ICC was the highest for A-RB 190–600 aromatics, with a value of 0.16, which corresponds with the qualitative analysis in Figure 5, where a C=C aromatic peak developed at 1560 cm−1. Nevertheless, for A-RB 190–3000 saturates, it had the highest ICC, which agrees with the qualitative analysis in Figure 4. A C=C aromatic peak at 1571 cm−1 appeared in Figure 4 for the A-RB 190–3000 saturates and was absent in the aromatics of the same sample shown in Figure 5. This reveals that the lower interaction speed (600 min−1) promoted the rubber unsaturated components to migrate into the aromatics, while the higher speed (3000 min−1) shifted their migrations into the saturates.
Figure 8d shows the ICH with COV values ranging from 0.46 to 10.77%. Most of the A-RBs fractions had higher ICH values than the neat fractions. The A-RB 190–600 saturates had the highest ICH in Figure 8d, reaching a value of 0.95, higher than the neat binder value by 32%. The intensity of the aliphatic peaks for this sample was the highest, as depicted in Figure 4, which supported migration of the rubber aliphatic hydrocarbon chains into saturates. For the A-RB 190–3000 saturates, the ICH was lower than the value of A-RB 190–600 saturates. However, in the aromatics, this trend was reversed, suggesting that changing the interaction speed from 600 to 3000 min−1 altered the rubber’s aliphatic migration into aromatics. At 3000 min−1, rubber particles absorbed more aliphatic components, swelled more, partially dissolved more—as confirmed by the dissolved rubber percentage—and released more aliphatics back into the aromatics fraction. A temperature of 220 °C caused an increase in the ICH for saturates and resins as compared to 190 °C. This increase in aliphatic components resulted from excessive rubber dissolution, as confirmed by the rubber dissolved percentage, causing depolymerization and excessive devulcanization.

3.3. Rheological Analysis of Asphalt Matrix

Figure 9 illustrates the variations of G* and δ with respect to temperatures for neat binder and A-RBs. The COV values for G* varied from 0.00 to 9.09%, whereas those for δ ranged from 0.00 to 1.89%. The G* values in Figure 9a for the A-RBs were higher than those of the neat binder, while the neat binder exhibited the highest δ values (Figure 9b), demonstrating that A-RBs had the highest stiffness and elasticity values. Among A-RBs, the sample that interacted at 190–3000 had the highest stiffness and elasticity values, revealing more effective rubber swelling and partial dissolution. The mechanism of interaction between rubber and asphalt caused network structure formation, as revealed in a previous study [32], enhancing the viscoelastic properties. Furthermore, the plateau region in the δ curve [57,58], as shown in Figure 9b, particularly for A-RB 190–3000, revealed the formation of network structures. The 600 min−1 interaction speed was not sufficient to release the rubber components in the asphalt matrix, as confirmed by the results discussed in Figure 7 and the rubber dissolution results revealed in the upcoming section, causing the lowest enhancements in the rheological properties compared to other A-RBs. Increasing the interaction temperature from 190 to 220 °C reduced the G* and increased the δ values due to excessive devulcanization and depolymerization of the rubber components. The δ value consistently high for the neat binder above 40 °C, indicated that the neat binder was behaving mostly in a viscous manner, with little elastic contribution. In contrast, the steadily increasing δ of the A-RBs indicated that temperature was causing softening and slow degradation of the rubber-reinforced elastic network within the binder matrix as a result of temperature [59]. Chemical changes in the SARA fractions were linked to rheological enhancements, including higher G* and lower δ. Asphalt–rubber interactions reduced the amount of saturates and aromatics while increasing the amount of resins and asphaltenes. Additionally, the polymeric components released from rubber into the asphalt liquid phase were observed for the sample interacted at 190 °C and 3000 min−1, providing higher G* and lower δ values.

3.4. Compositional Changes of Rubber

Thermal analyses of original and extracted rubbers are depicted in Figure 10, illustrating the percentage of dissolved rubber and compositional components. The COV value for OL was 16.67%, for NR ranged from 9.52 to 10.33%, for SR ranged from 3.33 to 5.56%, and for FR ranged from 1.02 to 3.33%. Analyzing these data confirms the migration of rubber components into the asphalt fractions. The dissolved percentage of rubber increased from 34.3% in A-RB 190–600 to 82.0% in A-RB 190–3000, while increasing the temperature to 220 °C caused excessive dissolution that reached 92.0%. This increase in rubber dissolution corresponds to the decrease in the asphalt light fractions, saturates, and aromatics, as discussed in Figure 3, reaching the lowest values in A-RB 220–3000. Rubber particles absorbed low-molecular-weight components from asphalt, swelled, partially dissolved, devulcanized, and released their components back into asphalt. The rubber-released components were detected by FTIR and TGA. OL components in rubber decreased from 7.0% in the original rubber, 1.4% in A-RB 190–600, 0.5% in A-RB 190–3000, and 0.1% in A-RB 220–3000, supporting the C=C aromatic peaks that emerged in A-RB 190–3000 at 1571 cm−1 and in A-RB 190–600 at 1560 cm−1. NR and SR decomposed and reached the lowest values in A-RB 220–3000. Migration of these polymeric components was confirmed in A-RB 190–3000 at 971 cm−1 for PB and at 735 cm−1 for PS, forming network structures [32] that enhanced the rheological performance. FR decomposed from 44.0% in the original rubber to values between 36.1 and 4.3% in the extracted rubbers, showing more disintegration of rubber components in asphalt binders.

4. Conclusions

This study explored the chemical interactions between asphalt SARA fractions and rubber particles through FTIR-based investigation, accompanied by SARA fractionation and thermal and rheological analyses to better understand the effect of these chemical interactions. How rubber swelling, partial dissolution, and exchange of components with asphalt SARA fractions influenced the performance of A-RBs was emphasized. The conclusions from this study are as follows:
  • Asphalt–Rubber Interactions Influenced the Distribution of SARA Fractions: In A-RB 190–3000, saturates and aromatics decreased by 9.38%, while asphaltenes and resins increased by 12.28%, indicating that rubber absorbed light fractions and promoted swelling.
  • Rubber Released Its Components Partially in SARA Fractions: Rubber partially dissolved and released C=C aromatics at 1560 and 1571 cm−1 into aromatics and saturates. Aliphatic C–H peaks in saturates were intensified, highlighting the migration of rubber aliphatics. Rubber PB and PS components emerged in asphaltenes at 971 and 735 cm−1, respectively, for A-RB 190–3000.
  • Partial Dissolution of Rubber Enhanced Rheological Performance: Optimal interactions (190–3000–8) enhanced the rheological performance (higher G* and lower δ). Higher temperature and lower speed led to lower rheological performance due to excessive depolymerized or incomplete swelling, respectively.
  • Thermal Analyses of Rubber Confirmed its Partial Dissolution: Rubber dissolution reached 82.0% under optimal conditions, with OL, NR, SR, and FR dissolution exceeding 77%. At 220 °C, dissolution exceeded 90%; however, excessive devulcanization and depolymerization diminished performance merits.

5. Future Work

  • Examine how different sources of asphalt and sizes of rubber impact chemical and rheological changes in A-RB components.
  • Identify the short- and long-term aging processes that influence the interaction between asphalt and rubber using SARA-based chemical analysis.
  • Characterize the thermal stability and degradation characteristics of the SARA fractions in A-RBs using TGA.
  • Investigate the microstructural development of SARA fractions in A-RBs using atomic force microscopy to validate morphology–performance relationships.
  • Perform life cycle cost analysis of A-RBs to evaluate the economic feasibility of high-temperature and long-duration mixing processes.
  • Apply rheological modeling to quantify the viscoelastic behavior of A-RBs.

Author Contributions

Conceptualization, M.R. and M.A.; methodology, M.R., E.D.-A., and M.A.; software, M.R., E.D.-A., and M.A.; validation, M.R., E.D.-A., and M.A.; formal analysis, M.R., E.D.-A., and M.A.; investigation, M.R., E.D.-A., and M.A.; resources, M.A.; data curation, M.R., E.D.-A., and M.A.; writing—original draft preparation, M.R., E.D.-A., and M.A.; writing—review and editing, M.R., E.D.-A., and M.A.; visualization, M.R. and M.A.; supervision, M.A.; project administration, M.A.; funding acquisition, M.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research was partially funded by the Missouri University of Science and Technology (Missouri S&T) and the Missouri Asphalt Pavement Association (MAPA) endowment fund. Experimental work and material testing were parts of the Ph.D. research of Mohyeldin Ragab that was supported by the National Science Foundation, Award No. 0846861.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

Author Mohyeldin Ragab was employed by the Pure Asphalt Company. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
A-RBAsphalt–Rubber Binder
DTGDerivative of Thermograph
FRFiller
FTIRFourier Transform Infrared Spectroscopy
NRNatural Rubber
OLOil
PBPolybutadiene
PSPolystyrene
SARASaturates, Aromatics, Resins, and Asphaltenes
SRSynthetic Rubber
TGAThermogravimetric Analyzer

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Figure 1. Experimental program.
Figure 1. Experimental program.
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Figure 2. A-RB matrix and its components.
Figure 2. A-RB matrix and its components.
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Figure 3. SARA fractions of neat binder and A-RBs.
Figure 3. SARA fractions of neat binder and A-RBs.
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Figure 4. FTIR spectra of saturates: (a) 4000–600 cm−1 and (b) 2000–600 cm−1. The purple box delineates the fingerprint region, magnified on the right to illustrate specific chemical alterations.
Figure 4. FTIR spectra of saturates: (a) 4000–600 cm−1 and (b) 2000–600 cm−1. The purple box delineates the fingerprint region, magnified on the right to illustrate specific chemical alterations.
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Figure 5. FTIR spectra of aromatics: (a) 4000–600 cm−1 and (b) 2000–600 cm−1. The purple box delineates the fingerprint region, magnified on the right to illustrate specific chemical alterations.
Figure 5. FTIR spectra of aromatics: (a) 4000–600 cm−1 and (b) 2000–600 cm−1. The purple box delineates the fingerprint region, magnified on the right to illustrate specific chemical alterations.
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Figure 6. FTIR spectra of resins: (a) 4000–600 cm−1 and (b) 2000–600 cm−1. The purple box delineates the fingerprint region, magnified on the right to illustrate specific chemical alterations.
Figure 6. FTIR spectra of resins: (a) 4000–600 cm−1 and (b) 2000–600 cm−1. The purple box delineates the fingerprint region, magnified on the right to illustrate specific chemical alterations.
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Figure 7. FTIR spectra of asphaltene: (a) 4000–600 cm−1 and (b) 2000–600 cm−1. The purple box delineates the fingerprint region, magnified on the right to illustrate specific chemical alterations.
Figure 7. FTIR spectra of asphaltene: (a) 4000–600 cm−1 and (b) 2000–600 cm−1. The purple box delineates the fingerprint region, magnified on the right to illustrate specific chemical alterations.
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Figure 8. FTIR indices: (a) ICO; (b) ISO; (c) ICC; and (d) ICH.
Figure 8. FTIR indices: (a) ICO; (b) ISO; (c) ICC; and (d) ICH.
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Figure 9. Temperature sweep results of neat binder and A-RBs: (a) G* and (b) δ.
Figure 9. Temperature sweep results of neat binder and A-RBs: (a) G* and (b) δ.
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Figure 10. Rubber compositional components in (a) original; (b) 190–600-extracted; (c) 190–3000-extracted; and (d) 220–3000-extracted rubbers.
Figure 10. Rubber compositional components in (a) original; (b) 190–600-extracted; (c) 190–3000-extracted; and (d) 220–3000-extracted rubbers.
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Table 1. Interaction matrix.
Table 1. Interaction matrix.
Rubber
Percentage a (%)		Mixing
Temperature (°C)		Mixing
Speed (min−1)		Mixing Time (h)Code
101906008A-RB 190–600
3000A-RB 190–3000
2203000A-RB 220–3000
a The percentage was calculated based on the weight of the neat binder.
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Ragab, M.; Deef-Allah, E.; Abdelrahman, M. FTIR Characterization of Asphalt SARA Fractions in Response to Rubber Modification. Appl. Sci. 2025, 15, 8062. https://doi.org/10.3390/app15148062

AMA Style

Ragab M, Deef-Allah E, Abdelrahman M. FTIR Characterization of Asphalt SARA Fractions in Response to Rubber Modification. Applied Sciences. 2025; 15(14):8062. https://doi.org/10.3390/app15148062

Chicago/Turabian Style

Ragab, Mohyeldin, Eslam Deef-Allah, and Magdy Abdelrahman. 2025. "FTIR Characterization of Asphalt SARA Fractions in Response to Rubber Modification" Applied Sciences 15, no. 14: 8062. https://doi.org/10.3390/app15148062

APA Style

Ragab, M., Deef-Allah, E., & Abdelrahman, M. (2025). FTIR Characterization of Asphalt SARA Fractions in Response to Rubber Modification. Applied Sciences, 15(14), 8062. https://doi.org/10.3390/app15148062

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