Sustainable Bitumen Modification Using Bio-Based Adhesion Promoters

Abstract

The growing emphasis on sustainable road construction has stimulated interest in environmentally friendly bitumen modifiers. This study presents the development of biodegradable adhesion promoters synthesized via the amidation of renewable raw materials (rapeseed oil and higher fatty acids) with polyethylene polyamine. The main objective was to improve bitumen–aggregate adhesion while maintaining the essential physico-mechanical and rheological properties of the bitumen. The synthesized bio-based adhesion promoters were incorporated into penetration-grade bitumen at a dosage of 0.4 wt.%. Physico-mechanical testing confirmed that their inclusion does not significantly affect the fundamental properties of the bitumen, while substantially enhancing adhesion to both glass and mineral aggregates. Rheological analysis showed that the rapeseed oil-based adhesion promoter had minimal influence on viscoelastic behavior. In contrast, the fatty acid-based promoter increased the rutting resistance parameter (|G*|/sinδ) and decreased the phase angle (δ), indicating improved resistance to permanent deformation. FTIR spectroscopy further revealed that the fatty acid-based adhesion promoter significantly reduced the formation of carbonyl groups during short-term aging, suggesting a retardation in oxidative aging and potential rejuvenating effects. In conclusion, the proposed bio-based adhesion promoters, derived from renewable sources and fully biodegradable, represent a promising solution for enhancing bitumen performance and supporting the durability and sustainability of asphalt pavements.

1. Introduction

The development of the modern road construction industry demands not only the enhancement in technical characteristics of road materials, but also adherence to the principles of environmental safety and sustainability. One of the critical components determining the durability and performance of asphalt concrete pavements is bitumen—an organic binder that ensures the cohesion of mineral aggregates within the mixture. However, conventional road bitumen exhibits several limitations, including poor adhesion to mineral materials, low resistance to water, and high sensitivity to climatic factors. These drawbacks ultimately contribute to the premature deterioration of road surfaces [1,2].

Therefore, a comprehensive understanding of the mechanisms governing the interaction between bitumen and aggregates is essential for improving pavement durability and for the optimal selection of materials in asphalt concrete formulations [3,4].

To enhance the stability of the bitumen–aggregate interface, it is necessary to improve the adhesive properties of bitumen through the incorporation of various additives. Modified bitumen demonstrates superior wetting capabilities on aggregate surfaces, thereby ensuring stronger adhesion between the components of the asphalt concrete mixture [5,6,7,8]. Such modification can significantly extend the service life of pavements and improve their resistance to damage caused by moisture and mechanical stress [9,10]. Moisture infiltration into road surfaces can lead to a loss of interfacial adhesion between the bitumen and aggregates. The moisture resistance of asphalt concrete mixtures is influenced by several factors, including the mineralogical composition of the aggregate, surface texture, the chemical composition of the bitumen, and the overall compatibility between the bitumen and the aggregate [11,12,13].

Since strong adhesion between bitumen and mineral aggregates is one of the key factors influencing the durability of asphalt concrete pavements, a wide variety of adhesive additives based on different organic compounds has been developed worldwide. These include primary fatty amines, fatty amides and imidazolines, diamines, fatty acids, and their combinations with amines [14,15]. Among these, amine-based adhesive additives are particularly noteworthy. They often consist of long hydrocarbon chains and amino groups, where the hydrocarbon moieties interact with the bitumen, while the amino groups interact with the aggregate. This dual interaction improves the wetting of the aggregate by the bitumen, thereby enhancing adhesion [16,17].

One of the most widely used methods for improving the adhesion between bitumen and mineral aggregates in asphalt concrete mixtures is the use of surface-active agents (surfactants) as adhesive additives [18]. Surfactants are classified as cationic, anionic, or nonionic, depending on the type and presence of charged groups in their molecular structure. In road construction technologies—particularly in bitumen modification and adhesion enhancement—cationic surfactants are preferred due to their ability to interact effectively with negatively charged surfaces of mineral aggregates, such as acidic rocks (e.g., granite). These interactions result in the formation of a stable adsorption layer that promotes strong adhesion to the hydrophobic bitumen. Common cationic surfactants used in this context include fatty amines such as imidazolines, amidoamines, and diamines [19,20].

Surfactant-based additives act at the interface between the polar mineral surface and the hydrophobic bitumen [21,22,23,24]. Chemically, surfactants used in road construction may include fatty acid esters, their non-metallic salts, amines, and amides [15].

Among various adhesion additives used in asphalt concrete technologies, naphthenates—metal salts of organic acids obtained as by-products during the alkaline purification of petroleum fractions—are very effective ones. Of particular interest is iron naphthenate, which has demonstrated strong potential as an anti-stripping additive [25].

The use of adhesion additives is especially important in warm-mix asphalt (WMA) technologies, where maintaining proper material properties at reduced processing temperatures is critical. The incorporation of amine-based additives ensures adequate adhesion between bitumen and aggregates even at lower mixing and compaction temperatures, thereby enhancing the overall performance of the pavement [26,27,28].

Another class of promising adhesion promoters (APs) is based on organosilanes, typically represented by the general formula R₁Si(OR)₃. These compounds combine a hydrocarbon chain—providing compatibility with bitumen—and a polar silane group that interacts with the inorganic surface of mineral aggregates. Even in very small quantities, organosilanes can significantly enhance bitumen-aggregate adhesion [29].

Table 1. Overview of bio-based additives for improving bitumen–aggregate adhesion.

Bio-Additive Type	Bio-Material Source	Dosage (% wt. Bitumen)	Bitumen– Aggregate	Key Findings (Adhesion)	Advantages	Disadvantages	References
Cationic amidoamine (surfactant)	Waste rapeseed oil (condensed with diethanolamine)	~0.6–0.9 (optimum ≈ 0.9)	Penetration-grade bitumen (≈70/100); acidic (quartz) stone	Bitumen–stone wetting angle increased from 19.8° to 80.3° (much higher hydrophobicity), preventing stripping; rutting depth reduced (1.7 mm vs. 2.77 mm).	Improves binder hydrophobicity and adhesion; enhances rutting resistance; uses waste oil; no organic solvent needed.	No major disadvantages reported; long-term field performance still to be verified.	[30]
Plant-based oil extender (PBO)	Renewable plant oil (forest/pulp industry)	5–30 (typically ~5–15)	Base bitumens (penetration ~80/100 and 170/200); hard crystalline granite (Sweden)	Extended binders showed higher aggregate adhesion (better coating in rolling-bottle tests and higher ITS retention), and improved long-term durability. Bitumen remained fully miscible with oil.	Carbon-neutral/petroleum-offset potential; fully miscible; improved durability and adhesion.	At high additive content, binder softening occurs (must optimize dosage); some workers noted a different odor in field trials.	[31]
Curcumin-based additive (turmeric)	Turmeric powder (curcuma) and purified curcumin	~1 (tested 1–3; 1 found optimal)	Penetration 50/70 and 170/210 bitumens; acidic porphyry aggregate (porfido)	At 1% loading, refined curcumin (HPGC or RGC) nearly doubled adhesion (full coating in boiling test) vs. only ~35–40% coverage for raw turmeric. Improved bitumen–stone affinity was observed.	Natural, multi-functional (adhesion promoter + antioxidant); effective at low dosage with purified curcumin; enhanced binder–aggregate bonding.	Raw turmeric powder is less effective (requires higher dosage); cost/processing of purified curcumin may be high.	[32,33]
Vegetable fluxing additives	Sunflower oil esters (Oleoflux); vegetable resin (Green Seal)	0.5–5	50/70 bitumen; typical aggregates (field mixes)	Adding 0.5–5% flux lowered bitumen viscosity (enabling warm-mix paving) and promoted binder–aggregate coating. Moisture resistance was maintained or slightly improved due to better wetting.	Allows warm-mix production (lower temp, energy savings) and better coating; environmentally friendly (natural oils).	Slight reduction in stripping resistance observed in WMA mixes; excessive dosage (esp. Green Seal) can over-soften binder.	[34]

summarizes recent studies on bio-based adhesion additives for road bitumen, including eco-friendly surfactants and fluxing agents derived from vegetable oils, tall-oil derivatives, and natural products.

Recent studies consistently show that bio-based surfactants and oils can significantly improve bitumen–aggregate adhesion. For example, a rapeseed oil-derived amidoamine increased the stone wetting angle from ~20° to 80°, and sunflower oil esters (Oleoflux) were reported to “promote the binder’s adhesion to the aggregate”. These additives typically create a more hydrophobic bitumen film and better coating on aggregates. Common advantages include reduced production temperatures (warm-mix feasibility) and enhanced moisture resistance (or at least no loss of adhesion). They also leverage renewable waste materials, potentially lowering the carbon footprint of asphalt. However, care is needed to balance performance: excessive dosage can over-soften the binder or alter aging behavior. Some studies noted issues like altered odor or modest decreases in stripping resistance (especially in warm-mix scenarios). Overall, moderate dosages (typically 1–5 wt.%) of well-characterized bio-additives are recommended. In practice, formulations should be tailored to the base bitumen and aggregate type; for instance, formulations fully miscible with the binder [31] performed best.

Based on the literature, bio-based adhesion promoters are best used as small-dose anti-strip agents. They are typically blended into the bitumen (or bitumen emulsion) at fractions around 0.3–1.0 wt.% by weight. Such additives are most valuable when aggregates are acidic or prone to stripping, and they can be combined with standard modifiers (e.g., polymers or warm-mix additives) without incompatibility. The research suggests formulating them with chain lengths (18–22 °C) that match common fatty acids (oleic, linoleic, etc.) for strong adsorption. Field use should follow the proven recipes, e.g., the novel rapeseed additive was applied undiluted in the mix, while commercial products are used according to manufacturer guidance. In every case, performance should be verified (e.g., via boiling or surface energy tests) to ensure adhesion goals are met.

In summary, a growing body of work finds that carefully engineered bio-based surfactants can effectively replace conventional anti-stripping agents. They provide clear adhesion and rutting benefits with sustainability advantages. Key observations are that modified vegetable oils (amidoamines, waxes, etc.) significantly improve aggregate bonding, reduce moisture damage, and can even rejuvenate aged binders. Their benefits include lower mix temperature, improved low-temperature flexibility, and reduced environmental impact. Major limitations involve moisture sensitivity (if overdosed) and sometimes reduced high-temperature stiffness, so formulations must be optimized. Overall, bio-adhesion additives from fatty acids and oils show strong promise for asphalt use, but should be applied at recommended dosages and chemically activated forms to ensure consistent effectiveness.

The technical approach of this study involves the synthesis of cationic bio-based adhesion promoters from fatty acid feedstocks, their incorporation into paving-grade bitumen, and the evaluation of physical, mechanical, and rheological properties of the modified binders before and after aging.

2. Materials and Methods

The principal scheme of the research is presented in Figure 1.

2.1. Materials

For the synthesis of AP, two types of renewable raw materials were used: rapeseed oil (RO) and higher fatty acids (FAs) obtained from the hydrolysis of plant oils.

For the amidation of RO and FA, polyethylene polyamine (PEPA)—POLY7, manufactured by Tosoh Corporation (Tokyo, Japan), was used. The amine number of PEPA was 1250 mg KOH/g.

As a raw material for the modification, bitumen of grade 70/100 according to DSTU EN 12591:2017 [35], selected at PJSC Ukrtatnafta (Kremenchuk, Ukraine), was used. Physical and mechanical properties of the bitumen are given in

Table 2. Physical and mechanical properties of virgin bitumen 70/100.

Index		Units of Measurement	Value	Methods	Requirements According to [35]
Penetration at	5 °C	0.1 mm	5	DSTU EN 1426:2018 [36]	-
	15 °C		17		-
	25 °C		70		70 … 100
	35 °C		199		-
Softening point (SP)		°C	48.5	DSTU EN 1427:2018 [37]	43 … 51
Fraass breaking point (FBP)		°C	−17.5	DSTU EN 12593:2018 [38]	≤−10
Ductility at 25 °C (D25)		cm	99.8	DSTU 8825:2019 [39]	-
Penetration index calculated by SP		-	−0.78	DSTU EN 12591:2017 [35]	−1.5 … +0.7
Plasticity interval (PI = SP − FBP)		°C	66.0	-
Temperature at which the penetration is 800 × 0.1 mm (T800)		°C	45.5	-
ΔT = SP − T800		°C	3.0	-	-
Penetration index calculated by T800		-	−1.63	-
Adhesion to glass at 85 °C in water for 25 min		%	38.8	DSTU 9169:2021 [40]	-
Temperature at which the penetration is 1.25 mm		°C	−6.0
Resistance to hardening at 163 °C (RTFOT)				DSTU B EN 12607-1:2015 [41]
Retained penetration at 25 °C		%	75.7		≥46
Increase in softening point		°C	4.4		≤9
Resistance to hardening at 163 °C (TFOT)				DSTU EN 12607-2:2019 [42]
Retained penetration at 25 °C		%	67.1		-
Increase in softening point		°C	4.0		-
Acid value (AV)		${\displaystyle \frac{\mathrm{m}\mathrm{g} \mathrm{K}\mathrm{O}\mathrm{H}}{\mathrm{g} \mathrm{b}\mathrm{i}\mathrm{t}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{n}}}$	0.60	ASTM D664 [43]	-

.

2.2. Methods

2.2.1. Obtaining Bio-Based Adhesion Promoters

Amide-type APs were prepared as follows: 200 g of RO or FA and the required amount of PEPA were poured into a reaction flask equipped with a stirrer, thermometer, and dispenser. The mixture was heated to 140 °C and kept at this temperature and constant stirring (350 rpm) for 4 h.

2.2.2. Modification of Bitumen

Bitumen modification was carried out with an AP content of 0.4 wt.%. A dosage of 0.4 wt.% was selected as representative of the typical industrial range (0.3–0.6 wt.%). The effect of dosage will be presented in future studies. Mixing was carried out at a speed of 1000 rpm for 30 min, with the temperature maintained at 150 ± 2 °C throughout the process.

2.2.3. Properties of Bitumen

Aging was simulated using the Rolling Thin-Film Oven Test (RTFOT) according to DSTU B EN 12607-1:2015 [41] and the Thin-Film Oven Test (TFOT) according to DSTU EN 12607-2:2019 [42].

The rolling bottle test was conducted in accordance with DSTU EN 12697-11:2018 [44]. For the research, the 5–10 mm fraction of quartzite and granite of Ukrainian origin was used.

The determination of the physical and mechanical properties of the virgin and modified bitumens, both before and after short-term aging using the RTFOT and TFOT methods, was carried out according to the methods presented in Table 2.

2.2.4. Dynamic Shear Rheology

The dynamic shear rheometer (DSR) test was used to analyze the viscoelastic properties of the virgin and modified bitumens, both before and after short-term aging (RTFOT and TFOT), according to ASTM D7175-23 [45], by determining the complex shear modulus (G*) and phase angle (δ). Depending on the specific purpose, this test can be conducted at different temperatures. This study assessed the rutting parameter (|G*|/sin δ) for bitumen at temperatures of 46, 52, 58, 64, and 70 °C. These temperatures were selected based on the performance grade (PG58) of the virgin bitumen to represent its high-temperature service range and to evaluate potential improvements in rutting resistance after modification.

The effect of aging on the properties of the control and biochar-modified bitumen was evaluated through RAI at 58 °C based on the SuperPave™ methodology, rutting parameter:

$$\mathrm{R}\mathrm{A}\mathrm{I}={\displaystyle \frac{{\mathrm{G}}^{\ast }/\mathrm{s}\mathrm{i}\mathrm{n}\mathsf{\delta }\mathrm{i}\mathrm{n} \mathrm{s}\mathrm{h}\mathrm{o}\mathrm{r}-\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m} \mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e}}{{\mathrm{G}}^{\ast }/\mathrm{s}\mathrm{i}\mathrm{n}\mathsf{\delta }\mathrm{i}\mathrm{n} \mathrm{u}\mathrm{n}\mathrm{a}\mathrm{g}\mathrm{e}\mathrm{d} \mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{e}}}$$

(1)

Lower RAI values indicate a low susceptibility towards aging and hence better aging resistance [46,47].

2.2.5. XRF Analysis

To determine the elemental composition of the aggregate, an X-ray fluorescence analyzer, ElvaX Light SDD, with the technical characteristics described in [48], was used.

2.2.6. SEM/EDS Analysis

The surface morphology of analyzed samples was tested with the use of the electron microscope Apreo 2 C LoVac, equipped with the UltraDry EDS detector (model No. ANAX-100PM-B) with the active area up to 100 mm² (ThermoFisher SCIENTIFIC, Vlastimila Pecha 1282/12, 62700 Brno, Czech Republic).

2.2.7. FTIR Analysis

The FTIR (Fourier-transform infrared spectroscopy, PerkinElmer, Llantrisant, UK) spectra samples were recorded on a Spectrum Two spectrometer using a diamond U-ATR (The Universal Attenuated Total Reflectance Accessory, PerkinElmer, Llantrisant, UK) single-reflection accessory. The spectra (16 scans per spectrum) of the samples were collected in the mid-infrared wavenumber range of 4000 to 400 cm⁻¹, with a spectral resolution of 4 cm⁻¹.

The effect of AP on the aging intensity of road bitumen was assessed using the FTIR method. As highlighted in the literature [49,50], chemical changes induced by aging of bitumen primarily manifest in the range from 1800 to 900 cm⁻¹. The main oxidation products of bitumen are ketones and sulfoxides, and oxidation is reflected by changes in the carbonyl (1700–1750 cm⁻¹) and sulfoxide (1030–1360 cm⁻¹) peaks. The quantitative effect of AP on the aging of bitumen was determined by changes in the carbonyl (Equation (2)) and sulfoxide indices (Equation (3)), as well as the chemical aging index (Equation (4)).

$$\mathrm{I}\mathrm{C}\mathrm{O}={\displaystyle \frac{{\mathrm{A}}_{1700}}{{\mathrm{A}}_{1460}+{\mathrm{A}}_{1375}}},$$

(2)

where A₁₇₀₀, A₁₄₆₀, A₁₃₇₅ are, accordingly, the area around the peaks 1700 cm⁻¹, 1460 cm⁻¹, 1375 cm⁻¹.

$$\mathrm{I}\mathrm{S}\mathrm{O}={\displaystyle \frac{{\mathrm{A}}_{1030}}{{\mathrm{A}}_{1460}+{\mathrm{A}}_{1375}}},$$

(3)

where A₁₀₃₀, A₁₄₆₀, A₁₃₇₅ are, accordingly, the area around the peaks 1030 cm⁻¹, 1460 cm⁻¹, 1375 cm⁻¹.

CAI = ICO + ISO

(4)

3. Results and Discussion

3.1. Synthesis Bio-Based Adhesion Promoters

The synthesis conditions of bio-based APs are given in

Table 3. Synthesis conditions of bio-based adhesion promoters.

Parameter	Units of Measurement	Value
		RO-AP	FA-AP
Raw materials of vegetable origin	-	Rapeseed oil	Fatty acids
Reaction mixture	wt.%
Raw materials		80
PEPA		20
Temperature	°C	140
Duration	h	4.0

. The two proposed APs differ only in the type of raw materials of plant origin.

The characteristics of the starting reagents and the resulting products, as well as the reaction equation for the amidation of RO and FA with polyamines (PEPA), are given below (Scheme 1 and Scheme 2).

It was found that during the amidation process of RO, the acid value (AV) remains low and does not change significantly, confirming the proposed reaction chemistry (Reaction Equation (1)). Under the conditions described in Table 3, the reaction product (RO-AP) consists of amides as well as mono- and diglycerides of FA. In contrast, amidation of fatty acids results in a more than twofold decrease in AV—from 124.8 to 57.4 mg KOH/g—supporting the proposed chemistry outlined in Reaction Equation (2). The resulting product (FA-AP) contains amides and residual unreacted FA. Therefore, RO-AP and FA-AP differ not in their “active” component—amides—but in the presence of neutral compounds (mono- and diglycerides) in RO-AP, and residual acidity in FA-AP.

The FTIR spectra of the raw materials (RO, FA, and PEPA) and the synthesized products (RO-AP and FA-AP) are shown in Figure 2, with spectral interpretation provided in

Table 4. Interpretation of the obtained FTIR spectra.

Peak (cm−1)	Vibration	References
Secondary amide
3260	N–H stretching	[51]
1640	C=O stretching In-plane N–H band (two bands)
1540
1284	C–N stretching
691	Out-of-plane N–H band
Tertiary amide
1640	C=O stretching (one band)	[51]
Ester groups in RO
1745	C=O stretching	[52]
Fatty acid
1708	C=O stretching	[53]

. FTIR spectroscopy confirmed the presence of amide groups in the synthesized RO-AP and FA-AP products, as evidenced by the green-marked spectra. The intensity of characteristic absorption bands at 1640 cm⁻¹ (C=O stretching of amides) and 1540 cm⁻¹ (N–H bending vibrations) indicates the successful incorporation of predominantly secondary amide groups. The absence or reduced intensity of the 1540 cm⁻¹ band would otherwise suggest the presence of tertiary amides.

Additionally, the FTIR spectra of the initial reagents and final products confirm incomplete amidation of both RO and FA. This is evidenced by the presence of absorption bands corresponding to ester groups (1745 cm⁻¹) and carboxylic acid groups (1708 cm⁻¹), respectively, in the product spectra.

3.2. Modification of Bitumen with Bio-Based Adhesion Promoters

3.2.1. Physical and Mechanical Properties

The physical and mechanical properties of bitumens modified with RO-AP and FA-AP are presented in

Table 5. Physical and mechanical properties of the tested bitumen.

Index		Units of Measurement	Virgin	RO-AP	FA-AP
Penetration at	5 °C	0.1 mm	5	4	6
	15 °C		17	23	21
	25 °C		70	65	69
	35 °C		199	196	194
Softening point (SP)		°C	48.5	49.5	48.9
Fraass breaking point (FBP)		°C	−17.5	−16	−17
Ductility at 25 °C (D25)		cm	99.8	101.4	99.2
Penetration index calculated by SP		-	−0.78	−0.70	−0.71
Plasticity interval (PI = SP − FBP)		°C	66.0	65.5	65.9
Temperature at which the penetration is 800 × 0.1 mm (T800)		°C	45.5	45.0	46.5
ΔT = SP − T800		°C	3.0	4.5	2.4
Penetration index calculated by T800		-	−1.63	−1.96	−1.38
Adhesion to glass at 85 °C in water for 25 min		%	38.8	97.4	95.8
Temperature at which the penetration is 1.25 mm		°C	−6.0	−5.5	−8.5

. It can be observed that the addition of the proposed modifiers—synthesized from renewable raw materials (RO and FA)—at a concentration of 0.4 wt.% to oxidized road bitumen 70/100 does not alter the primary physical and mechanical characteristics. The only significant effect is an increase in the adhesion of the bitumen to glass, indicating that these modifiers function as typical APs.

3.2.2. Short-Term Aging Properties

Thermal stability is a critical requirement for APs. Therefore, bitumens modified with RO-AP and FA-AP were subjected to aging using the RTFOT and TFOT methods. The physical and mechanical properties of the aged bitumens are provided in

Table 6. Physical and mechanical properties of the tested bitumen after aging by the RTFOT and TFOT methods.

Index		Units of Measurement	Virgin		RO-AP		FA-AP
			RTFOT	TFOT	RTFOT	TFOT	RTFOT	TFOT
Penetration at	5 °C	0.1 mm	5	5	4	5	5	6
	15 °C		18	17	18	16	21	20
	25 °C		53	47	46	50	45	52
	35 °C		124	110	119	112	118	109
Retained penetration at 25 °C		%	75.7	67.1	70.8	76.9	65.2	75.4
Softening point (SP)		°C	52.9	52.5	53.0	52.7	53.1	52.4
Increase in softening point		°C	4.4	4.0	3.5	3.2	4.2	3.5
Fraass breaking point (FBP)		°C	−17.0	−16.0	−16.5	−17.0	−17.0	−18.0
Ductility at 25 °C (D25)		cm	55.5	–	49.1	–	49.2	–
Penetration index calculated by SP		-	−0.37	−0.74	−0.67	−0.55	−0.70	−0.53
Plasticity interval (PI = SP − FBP)		°C	69.9	68.5	69.5	69.7	70.1	70.4
Temperature at which the penetration is 800 × 0.1 mm (T800)		°C	51.0	53.5	51.0	52.5	52.5	54.5
ΔT = SP − T800		°C	1.9	−1.0	2.0	0.2	0.6	−2.1
Penetration index calculated by T800		-	−0.82	−0.51	−1.15	−0.60	−0.84	−0.05
Adhesion to glass at 85 °C in water for 25 min		%	44.0	41.4	59.6	40.9	49.1	39.3
Temperature at which the penetration is 1.25 mm		°C	−9.0	−9.0	−7.0	−9.0	−10.0	−12.5

.

A comparison of Table 4 and Table 5 reveals that heating the bitumens modified with the proposed APs (i.e., RO-AP and FA-AP) leads to a notable reduction in adhesion to glass at 85 °C in water. This suggests insufficient thermal stability of the synthesized APs. We believe this instability may be attributed to the relatively low content (20 wt.%) of the active component—PEPA—used in the synthesis of the APs (see Table 3). During amidation of RO with PEPA (resulting in RO-AP), a mixture of amides and mono-/diglycerides of fatty acids is formed, indicating incomplete amidation (see Reaction Equation (1)). In the case of FA amidation (leading to FA-AP), amides are also formed, but the product additionally contains unreacted free FA (see Reaction Equation (2)). Notably, the amide and free FA contents in FA-AP are approximately equal, as indicated by a halving of the acid value from 124.8 to 57.4 mg KOH/g. The effect of varying the proportions of renewable raw materials (RO and FA) used in AP synthesis on performance characteristics will be discussed in future publications.

3.2.3. Adhesion Properties

To gain a deeper understanding of the adhesion properties of bitumen to glass and two types of mineral aggregates (quartzite and granite), additional studies were conducted. The results are presented in Figure 3 and Figure 4.

In Ukraine, a widely used method for evaluating bitumen adhesion to glass is applied [40]. The results showed that virgin oxidized bitumen 70/100 exhibited the poorest adhesion both before and after RTFOT aging, while the best performance was observed for oxidized bitumen 70/100 modified with 0.4 wt.% of RO-AP. According to the Ukrainian standard [54], the required adhesion to the glass surface at 85 °C in water (test duration—25 min) must meet the following criteria:

(1)

Before aging—more than 75%;

(2)

After RTFOT—more than 60%;

(3)

After TFOT—more than 65%.

Based on these criteria, the first requirement is satisfied by bitumens modified with RO-AP (97.4%) and FA-AP (95.8%); the second requirement is partially met by RO-AP (59.6%); the third requirement is not met by any of the tested samples (see Table 4 and Table 5, Figure 3). It is important to highlight a significant difference in adhesion to the glass surface at 85 °C in water after TFOT aging compared to RTFOT. After the TFOT method, this indicator is significantly lower.

The traditional European method for assessing the adhesion of bitumen to mineral aggregates—the rolling bottle test—was also applied (Figure 4). Two types of aggregate, quartzite and granite, were used. Although many studies report that bitumen generally adheres better to granite than to quartzite [55], our results showed a slightly different trend: virgin bitumen exhibited higher coverage on quartzite (60%; Figure 4).

A comparative analysis of the surfaces of quartzite and granite was conducted using X-ray fluorescence (XRF,

Table 7. XRF analysis of aggregate.

Element	Wt.%
	Granite	Quartzite
Si	35.4238 ± 0.0619	44.5820 ± 0.0302
Al	7.4730 ± 0.0769	2.0397 ± 0.0522
K	3.0169 ± 0.0201	0.0280 ± 0.0058
Ca	1.8497 ± 0.0161	0.0782 ± 0.0047
Fe	1.8429 ± 0.0099	0.2209 ± 0.0036
Mg	0.3147 ± 0.1129	-
Ti	0.2333 ± 0.0060	0.0725 ± 0.0037
S	0.0452 ± 0.0087	0.0293 ± 0.0066
Sr	0.0376 ± 0.0004	0.0112 ± 0.0003
Mn	0.0314 ± 0.0039	0.0080 ± 0.0027
Zn	0.0235 ± 0.0084	0.0287 ± 0.0059

) and scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS,

Table 8. SEM/EDS of the surface of aggregates.

Element	Granite		Quartzite
	At.%	Wt.%	At.%	Wt.%
C	7.1 ± 0.1	4.5 ± 0.0	1.2 ± 0.0	0.7 ± 0.0
O	65.0 ± 0.3	54.7 ± 0.3	67.0 ± 0.3	54.0 ± 0.3
Na	2.4 ± 0.0	2.9 ± 0.0	-	-
Mg	0.4 ± 0.0	0.5 ± 0.0	-	-
Al	3.7 ± 0.0	5.2 ± 0.0	1.8 ± 0.0	2.5 ± 0.0
Si	20.4 ± 0.1	30.1 ± 0.1	29.8 ± 0.1	42.2 ± 0.1
K	0.4 ± 0.0	0.9 ± 0.0	-	-
Ca	0.4 ± 0.0	0.8 ± 0.0	-	-
Fe	0.2 ± 0.1	0.4 ± 0.1	0.2 ± 0.1	0.6 ± 0.2

; Figure 5 and Figure 6).

The analysis confirmed that quartzite contains a significantly higher proportion of silicon (44.58 wt.% according to XRF, 42.2 wt.% according to SEM/EDS; see Table 6 and Table 7) compared to granite (35.42 wt.% and 30.1 wt.%, respectively), which is attributed to the dominance of silicon dioxide (SiO₂)-based minerals. The variation in Si content measured by XRF and SEM/EDS is relatively minor. Granite, in contrast, exhibits higher levels of Al, K, Ca, and Fe, reflecting its more complex mineralogical composition, which includes aluminosilicates such as feldspars and micas. These compositional differences influence the surface characteristics of the aggregates—particularly surface polarity and their interaction with bitumen and adhesion promoters.

It is also noteworthy that the oxygen content in both aggregates is nearly identical, averaging around 54 wt.% (Table 8).

The higher carbon content observed in granite suggests a greater presence of carbonates, which may further influence the adhesion behavior within the bitumen–aggregate system.

Figure 5 and Figure 6 show the phase maps of granite (a) and quartzite (b) surfaces obtained via scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS). Several distinct phases are identified in granite, with two predominant ones: P1 (46.1 area%) and P5 (53.7 area%). These phases contain appreciable amounts of C and Al, indicating the presence of carbonates and aluminosilicates, respectively. Notably, P5 exhibits higher concentrations of both C and Al than P1.

For quartzite, the majority of the surface is composed of a homogeneous SiO₂-based phase (P1—80.5 area%), confirming its uniform mineralogical composition typical of quartz-rich rocks. The high combined content of silicon and oxygen (exceeding 80%) accounts for the hydrophilic nature of the quartzite surface, which is a critical factor in evaluating bitumen adhesion.

The superior adhesion of virgin bitumen to quartzite is likely due to this mineralogical homogeneity of the surface, in contrast to granite, which exhibits a more heterogeneous surface structure with numerous inclusions of aluminosilicate and carbonate phases. However, since granite is the predominant aggregate used in road construction in Ukraine, improving adhesion between bitumen and granite is essential.

Currently, there is no unified regulatory standard within the European Union specifically governing the requirements for bitumen modified with adhesion promoters (APs). For instance, in Germany, according to [56], the degree of bitumen coverage after 6 h in the rolling bottle test must be at least 80%. As shown in Figure 4, all of the modified bitumens tested prior to aging meet this requirement when either quartzite or granite is used as the aggregate.

As shown in Figure 4, all the modified bitumen samples before aging demonstrate a coating degree above 80%, regardless of the aggregate type (quartzite or granite), indicating strong initial adhesion due to the incorporation of cationic bio-based adhesion promoters.

After aging (RTFOT and TFOT), a decrease in coating degree is observed for all the samples, which is typical due to oxidative hardening of bitumen. However, the reduction in coating performance is less pronounced when quartzite is used compared to granite. This suggests a more stable bitumen–aggregate interface with quartzite during oxidative aging. This difference may be attributed to the surface chemistry of the aggregates. Quartzite, being richer in silica and more acidic, may interact more strongly with the polar functional groups (e.g., amide and amine moieties) present in the synthesized APs. These interactions likely form more durable physico-chemical bonds, which are better retained after aging. In contrast, granite, with its more heterogeneous mineral composition, may form weaker or less uniform interactions with the same APs, leading to greater degradation in adhesion over time.

These findings highlight not only the effectiveness of bio-based APs under initial conditions, but also their differential performance depending on the aggregate type during both short-term aging, which is critical for long-term pavement durability.

3.2.4. Rheological Properties

The |G*|/sin(δ) and phase angle δ of the tested bitumen before and after aging (RTFOT and TFOT) are shown in Figure 7.

The results of the study demonstrated that the addition of RO-AP has virtually no effect on the rheological properties of bitumen 70/100, whereas the addition of FA-AP leads to an increase in |G*|/sin(δ) and a reduction in the phase angle δ (Figure 7a). The increase in |G*|/sin(δ) upon the addition of FA-AP suggests that chemical transformations occur in the FA-AP/bitumen system during modification. The observed decrease in the phase angle δ after the addition of RO-AP indicates an increase in the viscous component of the system rather than the elastic one. Based on the performance grading, both the virgin and RO-AP-modified bitumen correspond to the PG58 grade, while the FA-AP-modified bitumen corresponds to PG64, indicating improved resistance to rutting.

Simulation of short-term aging processes using the RTFOT (Figure 7b) and TFOT (Figure 7c) methods revealed the following trend in aging susceptibility, based on rheological properties: virgin bitumen > RO-AP > FA-AP. This trend is also supported by the calculated RAI values after RTFOT and TFOT aging (Figure 8).

Upon addition of 0.4 wt.% RO-AP, the RAI decreases only slightly. However, the same amount of FA-AP leads to a more than twofold reduction in RAI—from 2.04 to 0.96 after RTFOT and from 2.53 to 1.21 after TFOT (Figure 8). These findings suggest that the additive synthesized via amidation of FA with polyethylene polyamines, using an excess of FA, can effectively serve as a bitumen rejuvenator.

3.2.5. Chemical Properties

In terms of the bitumen aging process, Fourier-transform infrared spectroscopy (FTIR) is very helpful in analyzing the changed chemical structures of bitumen constituents occurring as a result of their oxidation and aromatization reactions. The aging of bitumen is monitored using the stretching vibration bands of the polar C=O as well as S=O bonds at wavelengths of about 1700 cm⁻¹ and 1030 cm⁻¹, respectively [57] (Figure 9).

The structural changes in the components of bitumen were quantitatively analyzed by means of the ICO, ISO, and CAI determined using (2), (3), and (4). The findings are presented in Figure 10.

For both the virgin and RO-AP-modified bitumen, the short-term aging simulations led to an increase in CAI (Figure 10a,b). This indicates that the AP derived from rapeseed oil (RO-AP) does not inhibit the oxidative reactions responsible for bitumen aging and, therefore, cannot function as a bitumen rejuvenator. In contrast, Figure 10c clearly demonstrates that the use of FA-AP inhibits the formation of both S=O and C=O groups, indicating a suppression of oxidation processes. This observation is further supported by the acid value measurements conducted before and after short-term aging (Figure 11).

The addition of the AP obtained from fatty acids (FA-AP) significantly reduces the formation of carboxylic groups during short-term aging via the RTFOT method. Moreover, this product is characterized by a considerably higher acid value (57.4 mg KOH/g) compared to RO-AP (4.2 mg KOH/g). The underlying mechanism and chemical interactions responsible for the rejuvenating effect of FA-AP on bitumen remain insufficiently understood and warrant further investigation to elucidate the molecular-level processes involved in its interaction with the products of bitumen aging.

4. Conclusions

Based on the results of this study, the following conclusions can be drawn:

(1)

Adhesion additives synthesized from rapeseed oil and higher fatty acids using polyethylene polyamine are effective in enhancing the adhesive properties of road bitumen. The incorporation of these bio-based additives at a concentration of 0.4 wt.% significantly improves the adhesion of bitumen to both glass and mineral aggregates without notably altering the physical and mechanical properties of the bitumen.

(2)

The additive derived from higher fatty acids improves the rheological performance of bitumen by increasing the |G*|/sin(δ) parameter and decreasing the phase angle (δ), indicating enhanced rutting resistance at elevated temperatures.

(3)

Infrared spectroscopy and acid value measurements confirmed that the fatty acid-based additive inhibits oxidative aging of bitumen by reducing the formation of S=O and C=O groups. This makes it a promising agent not only for improving adhesion but also for enhancing the aging resistance of bitumen.

(4)

Thermal stability testing revealed that the effectiveness of the adhesive additives decreases significantly after the simulated short-term aging. This suggests the need for further optimization of the synthesis process, particularly with regard to the ratio between the amine and acid components.

(5)

The findings confirm the potential of biodegradable adhesion additives—especially those based on higher fatty acids—as environmentally friendly modifiers that can improve bitumen performance and contribute to the durability of asphalt pavements.