Study of Road Bitumen Operational Properties Modified with Phenol–Cresol–Formaldehyde Resin

Abstract

Using a relatively inexpensive method, phenol–cresol–formaldehyde resin (PhCR-F) was produced utilizing the byproducts of coal coking. It is shown that petroleum road bitumens, to which 1.0 wt.% PhCR-F is added, in terms of basic physical and mechanical parameters, comply with the requirements of the regulatory document for bitumens modified with adhesive additives. Research on the operational properties of these modified bitumens as a binding material for asphalt concrete is described. It has been proven that modified bitumen can store stable properties during its application (resistance to aging). The interaction of bitumens modified by PhCR-F with the surfaces of mineral materials, which occurs during the creation of asphalt concrete coatings, was studied. It was shown that adding 1.0 wt.% PhCR-F to road bitumen significantly improves the adhesion of the binder to the mineral material and increases the hydrophobicity of such a coating. The production of effective bitumen modifiers from non-target coking products of coal will not only make it possible to use new resources in road construction but will also increase the depth of decarbonization of the coking industry.

1. Introduction

Asphalt concrete represents the predominant pavement material utilized in highway construction worldwide. Its widespread adoption is primarily attributed to its ability to meet critical performance criteria, including ride comfort, high-speed suitability, transportation cost-efficiency, and, most notably, traffic safety. When properly applied and maintained through timely rehabilitation interventions, asphalt concrete pavements are capable of sustaining long-term service lives extending over several decades. The primary binding agent in these pavements is road-grade petroleum bitumen, which plays a crucial role in ensuring structural cohesion and durability under varying environmental and load conditions.

The integrity of asphalt concrete can be compromised, leading to various surface defects, because of environmental influences on bitumen, such as ultraviolet radiation, moisture, oxygen in the air, and the catalytic effects of mineral materials [1,2,3]. Primarily, these factors weaken the bond between the binder and the mineral particles. A strong adhesion between bitumen and aggregate is crucial for achieving high strength and water resistance in asphalt concrete surfaces. Research has consistently shown that the mechanical and performance characteristics of asphalt concrete are closely linked to the strength of the interphase bond between bitumen and mineral particles [4,5].

The integrity of the adhesive bonds within asphalt concrete must remain stable under challenging traffic loads and environmental conditions. Of particular concern is the material’s resistance to moisture intrusion, which can occur through multiple mechanisms: surface water infiltration, capillary rise of groundwater from the underlying base layers, and moisture condensation within the pavement structure. Water ingress significantly undermines the adhesive bond between the bitumen binder and aggregate, ultimately leading to irreversible adhesive failure [6,7,8]. Such failure is characterized by complete detachment of the bitumen film from the aggregate surface, often leaving no residual binder, indicating total loss of adhesion [9,10,11]. This deterioration mechanism accelerates the onset of surface distresses, such as cracking, pothole formation, and material disintegration, thereby reducing pavement lifespan and performance [12].

To avoid an irreversible adhesion failure, it is essential for bitumen to exhibit both strong active and passive adhesion to the mineral material.

Asphalt concrete is generally recognized as a waterproof material; however, water can infiltrate its pores through various mechanisms: seepage from surface water, capillary action that draws water from the road base into the asphalt concrete, and water vapor that enters the pores and condenses because of air moisture. Additionally, the pumping action of vehicle wheels can expedite water entry into these pores. The pumping action of vehicle wheels refers to the repeated loading and unloading of the pavement surface as vehicles move across it. This dynamic pressure acts like a mechanical pump, forcing air out of surface-connected pores and drawing water into them. As a result, water penetration into the pavement structure is accelerated, which can weaken the material, reduce durability, and lead to premature damage, such as stripping or cracking.

Consequently, it is essential to achieve both passive and active adhesion of the bituminous binder to mineral materials. This approach facilitates the development of a category of materials known as surface-active substances. Active adhesion refers to the capacity of bitumen to bond with stone materials by forming and maintaining a robust chemical connection, even in the presence of water. In contrast, passive adhesion relies on a dependable bond that is only established with the surface of dry stone materials [6,13,14].

Researchers’ primary objective is to create techniques that enhance both the passive and active adhesion of asphalt concrete components. A key approach to improving the interaction between bitumen and mineral materials involves the modification of the binder using various additives.

Adhesives are classified into two categories based on the type of adhesion: Active Adhesion Promoters (AAPs) and Passive Adhesion Promoters (PAPs). AAPs are specifically designed to enhance the adhesion of bitumen to stone surfaces, even in the presence of residual moisture on the aggregate. In contrast, polyfunctional adhesion promoters (PAPs), commonly referred to as anti-stripping additives (ASAs), are designed to enhance the adhesive bond between the bituminous binder and aggregate, thereby mitigating delamination under moisture exposure [6]. Given that achieving a durable binder–aggregate interface is a critical requirement in asphalt concrete performance, active adhesive additives are among the most widely used solutions in pavement technology [15]. These additives typically function as surface-active agents or emulsifiers, which reduce the surface tension of the bitumen. Through the formation of molecular bridges between the binder and mineral surface, they reinforce both the physical and chemical interactions within the composite matrix [9].

Surfactants, categorized as cationic, anionic, or nonionic, are frequently used as additives in adhesives. Notably, cationic surfactants possess molecules that have one or more positively charged functional groups, and positively charged ions are formed in dispersed systems in their presence, including amines and their derivatives (mono-, di-, polyamines), amidoamines, etc. [6]. The group of anionic surfactants includes phosphate compounds, such as polyphosphoric acid ether (PPA); compounds based on organosilane-based adhesion promoters (OSAPs) R₁Si(OR)₃ and salts of higher carboxylic acids [15,16,17]. The class of nonionic surfactants is represented by oxygen-containing polymers, such as, for example, epoxy compounds [18].

In this study, we explore the potential for developing cost-effective bitumen modifiers and adhesive additives based on liquid byproducts of hard coal coking processes. The proposed approach emphasizes the use of relatively inexpensive raw materials and straightforward synthesis techniques to ensure economic and practical viability [19,20,21,22,23,24,25,26,27,28,29,30,31].

According to [32], the coke and chemical enterprises of Ukraine annually produce about 3360.0 thousand tons of coke, 144.0 thousand tons of coal tar, and 34.0 thousand tons of crude benzene. The resources of the phenolic fraction in the processing of 100% coal tar are about 3.4 thousand tons, from which 1.1 thousand tons of “crude” phenols can be obtained. According to [33], the use of oxidized bitumen in Ukraine is about 350 thousand tons per year. Using phenol–cresol–formaldehyde resin (PhCR-F) as an adhesive additive to road bitumen materials is most effective at an amount of 1% by weight [19,20,21,22]. Based on the above-mentioned production volumes of the phenolic fraction, the content of phenol and its derivatives, and the potential yield of phenol–cresol–formaldehyde resins in the amount of 1.0 thousand tons per year, it can be argued that PhCR-F can be used to produce up to 100 thousand tons per year of road bitumen, which will practically meet the needs of Ukraine for a binder with good adhesive properties.

Our existing studies [19,20,21,22] have yielded several fruitful results on modifying bitumen binders with phenolic resins synthesized from the phenolic fraction obtained during coal coking. It was shown that phenol–cresol–formaldehyde resin is a relatively high-quality adhesive additive to petroleum bitumens. The effectiveness of the proposed modifiers was compared with phenol–formaldehyde resins synthesized by “traditional” methods. However, these studies did not sufficiently describe the interaction mechanism of bitumen–PhCR-F–mineral fillers. The economic aspects of the use of phenol–cresol–formaldehyde resin were also not discussed.

This paper presents a study of the new operational properties of bitumen, namely, its behavior during storage and transportation, its behavior at low temperatures, the impact of technological aging, and the thermo-viscous properties of bitumen binders. Special attention was paid to studying bitumen adhesion to surfaces with mineral materials. Static tests (peeling in boiling water) and dynamic tests (bottle rolling tests) were used to determine the ability of the bitumen film to hold onto the surface of the mineral material previously coated with it. To confirm the empirical methods for assessing the adhesion of modified bitumen, we calculated the surface free energy of bitumen binders. Based on the studies mentioned above, the latest concepts of the interaction mechanism of the phenol–cresol–formaldehyde resin with road petroleum bitumen and crushed stone were formed. In addition to assessing the overall economic feasibility of modifying bitumen with the PhCR-F, the cost of the components for synthesizing PhCR-F was calculated, and the cost of producing bitumen modified with a commercial modifier and PhCR-F was compared.

This manuscript provides a description of the raw materials and reagents, the research methods and processing results, the results of the experiments, as mentioned earlier, and their discussion, the economic calculations of the feasibility of using PhCR-F as modifiers of petroleum bitumens, and conclusions drawn, which, among other things, cover the novelty of this work and its relevance.

2. Materials and Methods

2.1. Materials

In this study, oxidized pavement-grade bitumen 70/100, produced by the Public Joint-Stock Company “Ukrtatnafta” (Ukraine), was employed as the base binder. This type of bitumen is widely used in road construction across Ukraine because of its favorable performance characteristics. The physicochemical properties of the base bitumen utilized in the investigation are summarized in

Table 1. Characteristics of base bitumen 70/100.

Property	Value
Penetration at 25 °C (dmm), measurement error ± 2	70
Softening point (°C), measurement error ± 1	46
Ductility at 25 °C (cm), measurement error ± 4	63
Adhesion to glass (%), measurement error ± 5	33
Adhesion to crushed stone (mark), measurement error ± 0.5	3.0
Fraass breaking point (°C), measurement error ± 1	−18
Plasticity range (°C), measurement error ± 1	64
Resistance to hardening at 163 °C (RTFOT method):
Change in mass (%), measurement error ± 2	0.030
Softening point after RTFOT (°C), measurement error ± 1	52
Penetration at 25 °C after RTFOT (dmm), measurement error ± 2	55
Increase in softening point (°C), measurement error ± 1	6
Retained penetration (%), measurement error ± 2	79

.

For the modification of bitumen, a phenol–cresol–formaldehyde resin (PhCR-F) was employed. The synthesis procedure for this resin was previously described in detail in [19,20,22]. The relevant physical properties of PhCR-F are provided in

Table 2. Characteristics of PhCR-F.

Property	Value
Appearance (−)	Brown powder
Softening point (°C), measurement error ± 1	110
Free phenol (%), measurement error ± 1	8–12
Solubility in ketones (−)	Soluble
Resistance to bases (−)	Decomposes

.

To compare the performance properties of modified bitumens, we used the Wetfix BE adhesive additive manufactured by Nouryon Surface Chemistry AB (Sweden)—a surfactant.

Granite was acquired from the Mokriansk granite quarry in Ukraine to assess its adhesion properties to aggregates. Its characteristics are represented in Figure 1a and

Table 3. The mineral composition of crushed stone, as analyzed using the powder X-ray diffraction technique.

Compound Name	Chemical Formula	Content (wt.%)
Quartz	Si6O6	33.6
Periclase	Mg4O4	9.5
Potassium iron silicate	FeSi2KO8	15.2
Cristobalite	Si4O8	6.2
Aluminum oxide carbide	AlC8O5; Al2C23O10	35.6

.

Based on the results shown in Figure 1a and Table 3, the crushed stone aggregate used in this study is predominantly composed of silicon oxides (39.80 wt.%). The X-ray powder diffraction (XRD) analysis of the mineralogical composition confirms the acidic nature of the filler material.

2.2. Experimental Procedure

The experimental framework, including the main stages of this research, is illustrated in Figure 2. Detailed descriptions of each procedural step and associated methodologies are provided in Section 2.2 and Section 2.3.

Bitumen samples modified with phenol–cresol–formaldehyde resin (PhCR-F) were prepared via melt blending using a low-shear mechanical mixer (Daihan Scientific Co., Ltd., Wonju-si, Republic of Korea) equipped with a six-pitched-blade turbine impeller. A 650 g portion of the base bitumen was placed in a metal container and heated to a target temperature of 190 ± 2 °C. Upon reaching the desired temperature, the PhCR-F modifier was incrementally introduced into the molten bitumen at concentrations of 1.0 wt.% and 2.5 wt.%. The mixture was continuously stirred at a constant rotational speed of 1000 min⁻¹ for 60 min to ensure homogeneous dispersion of the additive within the binder matrix.

To characterize the hydrodynamic regime of mixing, the Reynolds number (Re) for the stirred system was calculated using Equation (1), as described in [34]. In stirred vessel operations, laminar flow typically occurs at Re < 10, while fully turbulent flow is established at Re > 10,000. The intermediate range of 10 < Re < 10,000 corresponds to transitional flow, where turbulence is often localized around the impeller region and may revert to laminar flow in the bulk fluid [35].

$$Re={\displaystyle \frac{N\cdot D^2\cdot \rho }{\mu }}={\displaystyle \frac{16.667\times {0.05}^2\times 906.51}{0.365}}=103.5$$

(1)

where N is the rotation speed, s⁻¹; D is the impeller diameter, m; ρ is the melted bitumen binder density at 190 °C (calculated from geometry of vessel), kg/m³; and μ is the fluid viscosity at 190 °C, Pa·s (see Figure 10).

2.3. Methods of Analysis

2.3.1. Analysis of Physical Properties of Bitumen Binders

The base bitumen and PhCR-F-modified bitumen samples were evaluated using standard European testing protocols to assess their physical and performance characteristics. The following test methods were employed: penetration at 25 °C [36], softening point [37], ductility and elastic recovery at 25 °C [38], Fraass breaking point [39], resistance to hardening via the rolling thin film oven test (RTFOT) at 163 °C [40], storage stability via the “tube test” at 180 °C [41], rolling bottle test for moisture susceptibility [42], and dynamic viscosity within the temperature range of 135–170 °C [43].

Adhesion to inert surfaces was characterized using two methods: adhesion to glass was assessed in accordance with the [44], while adhesion to mineral aggregates (crushed stone) was evaluated following the [45].

The plasticity range of the binders was determined as the difference between the softening point (SP) and the Fraass breaking point (FBP), as expressed in Equation (2).

$$Plasticity range=SP-FBP$$

(2)

The impact of aging on the characteristics of the bitumen binders was demonstrated through retained penetration. The calculation of retained penetration was performed using Equation (3):

$$Retained penetration={\displaystyle \frac{Penetration after RTFOT aging}{Penetration before RTFOT aging}}\cdot 100$$

(3)

2.3.2. Analysis of Adhesive Properties of Bitumen Binders at Low Temperatures

Given that the adhesive properties of bituminous binders can vary significantly with decreasing temperature, a simplified and reproducible methodology was developed to evaluate the temperature-dependent behavior of bitumen adhesion. This approach involves determining the adhesive strength to glass, based on DSTU 9169:2021, following a series of controlled freeze–thaw cycles, thereby enabling characterization of binder performance under cyclic thermal stresses. Freezing was performed at −15 °C, and thawing was performed at +25 °C for 12 h. To ensure the most significant effect of low temperatures on the adhesive properties of the tested samples, adhesion to glass was performed immediately after removing the samples from the refrigeration unit. The research sequence was as follows:

• Preparation of 8 samples;
• Determination of the adhesive properties of the initial samples;
• Determination of the adhesive properties after one freeze–thaw cycle;
• Determination of the adhesive properties after four freeze–thaw cycles;
• Determination of the adhesive properties after four freeze–thaw cycles and four days of aging at −15 °C.

The calculation of the value of low-temperature adhesion was performed using Equation (4):

$$A_{-15}={\displaystyle \frac{1}{2}}\left({\displaystyle \frac{A_1+A_2+A_3}{3}}+A_4\right),$$

(4)

where A_-15—low-temperature adhesion, %; A₁—adhesion to glass without freezing and thawing, %; A₂—adhesion index to glass after one freeze–thaw cycle, %; A₃—adhesion to glass after four freeze–thaw cycles, %; and A₄—adhesion to glass after four freeze–thaw cycles and four days of exposure to low temperatures.

2.3.3. Spectral Analysis of Crushed Stone

The analysis of crushed granite stone was conducted through X-ray diffraction (XRD) using the X-ray powder diffraction technique with the AERIS Research desktop powder X-ray diffractometer (Malvern Panalytical B.V., Almelo, The Netherlands). The X-ray tube employed a copper anode (Cu K-α = 1.5406 Å) with generator settings of 15 mA and 40 kV. The diffraction pattern was recorded over a range of 5–100° 2θ, with a step size of 0.022° 2θ. The initial processing of the experimental diffraction data for phase identification was performed using the HighScore Plus software version 5.1.

2.3.4. Application of Scanning Electron Microscopy in the Analysis of Bitumen Binders

The microstructure of the bitumen binders was analyzed through scanning electron microscopy (SEM). Images were obtained using a Hitachi S-4700 FE-SEM (Hitachi, Ltd., Tokyo, Japan) cold field emission high-resolution scanning electron microscope. The images of the bitumen binders were taken using the following settings: beam accelerating voltage—25 kV, emission current—11,500 nA (11.5 µA), working distance—16.3 mm (16,300 µm), and 50× magnification.

2.3.5. Surface Free Energy of Bitumen Binders

The sessile drop method was applied for contact angle (CA) measurements to calculate the bitumen binders’ surface free energy (SFE). Bitumen binder samples were applied to glass plates measuring 76.0 × 26.0 × 1.0 mm. A droplet of liquid, approximately 0.005 mL (5 µL) in volume, was placed on the surface of the bitumen binder film, which was created using a syringe at room temperature (23 ± 1 °C). Different needles were used for both liquids, such as distilled water and ethylene glycol, $n_D^{20}=1.4318$. To obtain a statistically significant contact angle value, 6 drops (for both liquids) were measured on each surface.

The samples were illuminated by a strong light source and recorded with a digital camera, Canon EOS 1100D, equipped with a macro lens (Canon Inc., Tokyo, Japan). The recordings of CA were taken immediately after the deposition of a drop on a studied surface to minimize the effect of liquid penetration and any chemical interaction on the contact angle.

Drop images (see Figure 8a–f) were analyzed for CA measurement by open-access software ImageJ version 1.45r with plugins, such as the Low Bond Axisymmetric Drop Shape Analysis (LB-ADSA) technique [46] and Half-Angle technique [47,48].

To calculate the SFE of the base bitumen and PhCR-F-modified bitumen samples, the dispersion force component and polarity component (hydrogen bonding and dipole–dipole interactions) were used in Equation (5) (using a modified Young–Good–Girifalco–Fowkes equation), which was adopted by Owens and Wendt [15,49]:

$$1+\cos (\Theta )=2\cdot \sqrt{{\gamma }_S^d}\cdot \left({\displaystyle \frac{\sqrt{{\gamma }_L^d}}{{\gamma }_{LV}}}\right)+2\cdot \sqrt{{\gamma }_S^h}\cdot \left({\displaystyle \frac{\sqrt{{\gamma }_L^h}}{{\gamma }_{LV}}}\right)$$

(5)

where θ denotes the contact angle formed at the interface; ${\gamma }_S^d$ and ${\gamma }_L^d$ are the surface free energy related to the dispersion force component for both the solid and liquid phases, respectively; ${\gamma }_{LV}$ is the surface free energy relevant for liquid–vapor; and ${\gamma }_S^h$ and ${\gamma }_L^h$ signify the contribution to the SFE that is due to hydrogen bonding and dipole–dipole interactions, termed the polarity component, for the solid and liquid phases.

The condensed representation of commonly recognized components and terms, which incorporates the values of these components (dispersion and polar) for the free energy of the solid surface for both test liquids, leads to Equation (6) in the following form (i = 1; 2, where water is represented by index 1 and ethylene glycol by index 2):

$$0.5\cdot {\gamma }_{LV-i}\cdot \left(1+\cos ({\Theta }_i)\right)=\sqrt{{\gamma }_S^d}\cdot \sqrt{{\gamma }_{LV-i}^d}+\sqrt{{\gamma }_S^h}\cdot \sqrt{{\gamma }_{LV-i}^h}$$

(6)

where i = 1, 2; index 1 represents water and index 2 denotes ethylene glycol.

In order to create a system of two equations with two unknowns, the appropriate designations X, A, B, and C are utilized, given that the contributions of the dispersion and polar aspects to the free energy of the solid surface are not known:

$$X_1=\sqrt{{\gamma }_S^d}\hspace{1em}{X}_2=\sqrt{{\gamma }_S^h}\hspace{1em}{A}_1=\sqrt{{\gamma }_{LV-i}^d}\hspace{1em}{B}_i=\sqrt{{\gamma }_{LV-i}^h}\hspace{1em}{C}_i=0.5\cdot {\gamma }_{LV-i}\cdot (1+\cos ({\Theta }_i)$$

The surface tension components of the solid surface are calculated through Equations (7) and (8):

$${\gamma }_S^d={\left(X_1\right)}^2={\left({\displaystyle \frac{B_1\cdot C_2-B_2\cdot C_1}{A_2\cdot B_1-A_1\cdot B_2}}\right)}^2$$

(7)

$${\gamma }_S^h={\left(X_2\right)}^2={\left({\displaystyle \frac{C_1-A_1\cdot X_1}{B_1}}\right)}^2$$

(8)

The values of the components (dispersive and polar) for the surface tension of both test liquids were taken from the work [50] and are represented in

Table 4. Values of the components (dispersive and polar) for the surface tension of both test liquids.

Liquid	${\mathit{\gamma }}_{\mathit{L}\mathit{V}-\mathit{i}}^{\mathit{d}}$, mJ/m2	${\mathit{\gamma }}_{\mathit{L}\mathit{V}-\mathit{i}}^{\mathit{h}}$, mJ/m2	${\mathit{\gamma }}_{\mathit{L}\mathit{V}-\mathit{i}}^{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$, mJ/m2
Water	21.80	51.00	72.80
Ethylene glycol	29.00	19.00	48.00

.

3. Results and Discussion

3.1. Definition of the Main Characteristics for Bitumen Modified with PhCR-F

To assess the feasibility of employing phenol–cresol–formaldehyde resin (PhCR-F) as a bitumen modifier, the primary performance characteristics of the base bitumen were compared with those of the bitumen modified with 1.0 wt.% and 2.5 wt.% PhCR-F. The selected concentration range reflects conventional practices: adhesion promoters—typically surface-active agents—are generally used at levels not exceeding 1.0 wt.% [6,12,15], while multifunctional performance-enhancing additives, such as natural zeolite, paraffin wax, synthetic and natural rubbers, crumb rubber, and styrene–butadiene block copolymers, are commonly introduced at concentrations approaching 3.0 wt.% [2,3,12,15,17]. PhCR-F-modified binders were prepared at 190 ± 2 °C under continuous stirring for 60 min. The key physicochemical properties of the base and modified binders are summarized in

Table 5. Performance characteristics of bitumen modified with PhCR-F.

Property	Base Bitumen	Comparison of the Characteristics of Bitumens with the Requirements of Regulatory Documents for Bitumens Modified with Adhesive Additives		Comparison of the Characteristics of Bitumens with the Requirements of Regulatory Documents for Bitumens Modified with Polymer Additives
		Base Bitumen + 1.0 wt.% PhCR-F	Requirements According to SOU 45.2-00018112-067:2011 1 [51]	Base Bitumen + 2.5 wt.% PhCR-F	Requirements According to DSTU 9116:2021 2 [52]
Penetration at 25 °C (dmm), measurement error ± 2	70	68	61–90	60	71–100
Softening point (°C), measurement error ± 1	46	48	47–53	49	≥55
Ductility at 25 °C (cm), measurement error ± 4	63	58	≥55	52	≥8
Fraass breaking point (°C), measurement error ± 1	−18	−18	≤−13	−18	≤−18
Elastic recovery at 25 °C (%) 3, measurement error ± 2	–	–	Not applicable	–	≥55
Adhesion to crushed stone (mark), measurement error ± 0.5	3.0	5.0	≥5.0	5.0	≥4.5
Adhesion to glass (%), measurement error ± 5	33	87	≥75	94	≥75
Low-temperature adhesion A–15, (%), measurement error ± 2	24	82	Not applicable	89	Not applicable
Plasticity range (°C), measurement error ± 1	64	66		67
Homogeneity (–)	–	Homogeneous	Not applicable	Homogeneous	Polymer clots and particles should not be observed in BMP

Note: ¹ SOU—Standard Organization of Ukraine; ² DSTU—National Standard of Ukraine; ³ Bitumen (PhCR-F modified bitumen) does not have elastic recovery.

.

As shown in Table 5, the incorporation of 1.0 wt.% PhCR-F results in an increase in the softening point from 46 °C to 48 °C, while a 2.5 wt.% addition raises the softening point to 49 °C. Concurrently, penetration values decrease from 70 dmm (base bitumen) to 68 dmm and 60 dmm for the 1.0 wt.% and 2.5 wt.% modified samples, respectively, indicating increased binder stiffness. Importantly, these modifications do not adversely affect the binder’s performance at low temperatures, as evidenced by the unchanged Fraass breaking point.

However, it is noteworthy that the addition of PhCR-F does not impart elastic recovery properties to the bitumen, suggesting limited modification of the binder’s elastic phase behavior. A particularly promising outcome is the significant improvement in adhesion properties: the PhCR-F-modified binders exhibited markedly enhanced adhesion to both glass and crushed stone surfaces compared to the unmodified bitumen. This suggests a potential dual functionality of PhCR-F as both a structural and adhesive performance enhancer. It is worth pointing out that for all samples of the bitumen binders, a decrease in the adhesive properties is observed after exposure to low temperatures. The same data indicate the feasibility of using the proposed methodology for determining the low-temperature adhesive properties of modified bitumen since they allow for predicting the adhesion of the binder to mineral surfaces when weather conditions change, including at low temperatures. A more detailed study of the interaction of the PhCR-F-modified bitumens with the surfaces of mineral materials, which occurs during the creation of asphalt concrete coatings, is considered in Section 3.3.

From the above data, it can be concluded that the bitumen with the composition of 2.5 wt.% PhCR-F does not meet the requirements of regulatory documents for bitumens modified with polymer additives (DSTU 9116:2021). Thus, it can be argued that the studied resin should not be used as a complex bitumen modifier that improves its plastic properties. However, bitumen modified with 1.0 wt.% PhCR-F meets the requirements of regulatory documents for bitumens modified with adhesive additives. The obtained PhCR-F resin can be used as an adhesive additive for bituminous binders in small quantities (1.0 wt.%). The addition of 2.5 wt.% PhCR-F to bitumen in the following studies was used only to determine the changing nature of the amount of modifier on performance properties.

It is also evident from the above results that bitumens modified with a small amount of PhCR-F resin meet the requirements of SOU. However, the mentioned standards do not fully characterize the behavior of such modified bitumens. Therefore, we further studied several indicators that allowed us to see the behavior of modified bitumen during storage, transportation, and preparation of bitumen–mineral mixtures.

To evaluate resistance to oxidative hardening, the modified binders were subjected to short-term aging using the rolling thin film oven test (RTFOT), conducted at 163 ± 0.5 °C, in accordance with the procedure outlined in Section 2.3.1. The results for the unmodified and modified bitumen samples are summarized in

Table 6. Effect of temperature on fatigue parameters for an RTFOT-aged bitumen modified with PhCR-F.

Property	Base Bitumen		Base Bitumen + PhCR-F
			1.0 wt.%		2.5 wt.%
	Unaged	RTFOT	Unaged	RTFOT	Unaged	RTFOT
Softening point (°C), measurement error ± 1	46	52	48	54	49	56
Increase in softening point (°C), measurement error ± 1	–	6	–	6	–	7
Penetration at 25 °C (dmm), measurement error ± 2	70	55	68	49	60	38
Retained penetration (%), measurement error ± 1	–	78	–	72	–	63
Change in mass (%), measurement error ± 0.05	–	0.030	–	0.085	–	0.114

. The aging performance was assessed in accordance with the EN 14023 standard [53], which specifies acceptable criteria for bituminous binder aging during production and handling.

As shown in Table 6, RTFOT aging leads to an expected decrease in penetration and a corresponding increase in the softening point across all samples. Among the tested binders, the bitumen modified with 1.0 wt.% PhCR-F demonstrated the most favorable aging profile, with a high retained penetration value (72%), a minimal mass loss (0.085%), and a moderate increase in the softening point (6 °C). In comparison, the softening point increase after aging was also 6 °C for the base bitumen, whereas it reached 7 °C for the sample modified with 2.5 wt.% PhCR-F, suggesting a slightly greater susceptibility to stiffening at higher additive concentrations.

3.2. Storage Stability and Scanning Electron Microscopy (SEM) Studies of Bitumen Modified with PhCR-F

From a practical point of view, one of the main limitations in the application of PMBs is the lack of storage stability, especially when they are stored for an extended period at high temperatures. Such storage or transportation conditions of modified bitumens can lead to the separation (delamination) of the bituminous binder and modifier because of their difference in density. The tendency to phase separation between the bitumen and the modifier will lead to changes in the physical properties of the system, which, in turn, can become critically important during the practical application of such modified bitumens.

To evaluate changes in the bitumen-modified PhCR-F behavior, its indicators were determined after long-term storage (72 h) at high temperature (180 ± 2 °C) according to the method given in Section 2.3.1. The results of this study are shown in

Table 7. High-temperature storage stability test results measuring phase separation of bitumen modified with PhCR-F.

Property	Base Bitumen		Base Bitumen + 1.0 wt.% PhCR-F
	Top	Bottom	Top	Bottom
Softening point (°C), measurement error ± 1	46.1	46.0	48.4	48.2
Softening point change (°C)	0.1		0.2
Penetration at 25 °C (dmm), measurement error ± 2	70	71	68	67
Penetration change (%)	1.4		1.5

.

Storage stability was evaluated using the tube segregation test, with results summarized in Table 7. The softening point values measured at the top and bottom sections of the tube indicate near uniformity, confirming the homogeneity of the bituminous binders. The bitumen containing 1.0 wt.% PhCR-F exhibited a slight increase in softening point variation (0.2 °C) compared to the base bitumen (0.1 °C), suggesting a minor reduction in storage stability, although both values fall within acceptable limits for homogeneous systems.

Previous studies [54,55,56] employing scanning electron microscopy (SEM) for morphological analysis of polymer-modified bitumens (PMBs) have demonstrated that the polymer-rich dispersed phase appears as bright regions (white spots), whereas the asphaltene-rich matrix remains dark. As observed in Figure 3b,c, no visible polymer-rich phase (white inclusions) is present in the bitumen samples modified with various concentrations of PhCR-F resin. This absence of phase separation suggests that the PhCR-F resin is uniformly dispersed within the bituminous matrix. Such homogeneity implies that PhCR-F does not form a distinct polymer phase but rather integrates at the molecular or colloidal level, potentially contributing to internal structural modification of the binder and enhancing its physical performance characteristics.

The above-mentioned results of storage stability and SEM confirm the previously advanced theory [22] that when PhCR-F resin is added to bitumen, at least partial chemical modification of the bituminous binder occurs. In our opinion, PhCR-F interacts with bitumen since the “raw” phenols, which are the raw material for the modifier, contain a large amount of phenol (33.551 wt.%) [21,22], the fragments of which are more reactive in the resin than in the bitumen. In addition, fragments of cresols can also interact with bitumen, as shown in Figure 4.

3.3. Adhesive Properties and Surface Free Energy (SFE) of Bitumen Modified with PhCR-F

From the results of the mineral composition of crushed stone (see Section 2.1), which was determined by the powder X-ray diffraction method, it can be seen that the used aggregate (crushed stone) consists of Si oxides (39.80 wt.%); that is, it shows an acidic character. It is known that in order to use the so-called “acidic” fillers for the formation of asphalt concrete, it is necessary to use bituminous binders with high adhesive properties, which will provide a complete, irreversible, and water-resistant bond in relation to the fillers (crushed stone) [5].

For this study, an investigation of the adhesive properties of PhCR-F-modified bitumen binders to crushed stone and glass (detachment in boiling water and the rolling bottle test (RBT)) was performed. The results are shown in Figure 5, Figure 6, Figure 7, Figure 8 and Figure 9.

As can be observed, the incorporation of PhCR-F into the bitumen binder significantly improves the adhesive properties on the surface of granite aggregate and glass (see Figure 5 and Figure 6). In this study, the index of adhesion to the surface of glass increased 2.5 times (for bitumen modified with 1.0 wt.% PhCR-F) compared to base bitumen binder (adhesion 33%) and binder modified with the addition of 2.5 wt.% PhCR-F (adhesion 94%). The index of adhesion to the surface of the granite increased from mark 3 to mark 5 (both when adding 1.0 and 2.5 wt.% PhCR-F to the bitumen composition).

Generally, adhesion to glass increases with the higher addition of a modifier. The high improvement in adhesion to both glass and granite aggregate with a small amount of PhCR-F in bitumen binder constitutes a very promising technological value for considering it as a potential adhesive modifier in pavement road technologies.

The results obtained from the adhesion test, which involved detachment in boiling water, align with the findings from the rolling bottle test (refer to Figure 7).

It can be observed that significant changes in the coverage of the bitumen–aggregate system were detected after 6 h (see Figure 7). So, for bitumen modified with 1.0 wt.% PhCR-F, the degree of bitumen coverage is 99%, and for base bitumen, it is 85%. Increasing the duration of rolling to 72 h leads to a significant decrease in the degree of bitumen coverage for the base bitumen (37%). The results of the same test method after 72 h for the bitumen modified with 1.0 wt.% PhCR-F show that the degree of bitumen coverage is 68%. This indicates that the bitumen containing PhCR-F resin is less prone to exfoliation, i.e., it has a stronger bond between the binder and aggregate (granite) compared to the base bitumen. Among the studied dosages of the PhCR-F modifier, we prefer its content in bitumen in the amount of 1.0 wt.% since at a modifier content of 1.0 and 2.5% wt.%, such modified bitumen exhibits similar properties.

The above static tests (detachment in boiling water) and dynamic test (rolling bottle test) make it possible to determine the ability of a bitumen film to hold onto the surface of a mineral material previously covered with it. Visual assessment methods of the percentage amount of bitumen that remains covered with a film on the aggregate are recommended for the analysis of samples that are examined in one laboratory. However, such methods are not easy to compare with other laboratories’ results (except the application of hyperspectral imaging and digital picture analysis) [57,58,59,60].

The surface free energy of bitumen binders is an important characteristic for the prediction of the hydrophobic properties of bitumen films, which is evidenced by a significant number of publications, as well as attempts to generalize and standardize these studies, as is performed, for example, in the report [61]. It is known that the adhesion of a bituminous binder to mineral material can also be determined based on the calculation of surface free energy (SFE) [50,62,63] and its polar and dispersive components [62,64] by measuring the contact angle (CA) of liquids with different hydrophilicity on bitumen binder films under ambient conditions (23 ± 1 °C).

The determination of the CA and the calculation of SFE were carried out according to the method given in Section 2.3.4.

The results are presented in

Table 8. Contact angles and surface free energies of bitumen binder films.

Bitumen Film Samples	Contact Angle (°) *		Surface Energy Components (mJ/m2)		Total Surface Energy (mJ/m2)
	Water	Ethylene Glycol	${\mathit{\gamma }}_{\mathit{S}}^{\mathit{d}}$	${\mathit{\gamma }}_{\mathit{S}}^{\mathit{h}}$	${\mathit{\gamma }}_{\mathit{S}}^{\mathit{t}\mathit{o}\mathit{t}\mathit{a}\mathit{l}}$
Base bitumen	84.82	65.34	14.90	9.21	24.11
Base bitumen + 1.0 wt.% PhCR-F	93.20	75.51	12.68	6.18	18.85
Base bitumen + 2.5 wt.% PhCR-F	85.88	73.25	7.85	13.19	21.04

Note: $\gamma$—surface energy (i.e., surface tension); subscript indices s pertain to the surface energy of solids; superscript index d means the dispersion force component of SFE for a solid; h means the polarity component (hydrogen bonding and dipole–dipole interactions) of SFE for a solid. * Standard deviations for contact angle measurements are in the range of 0.2–0.3°.

and Figure 8a–f, Figure 9 and Figure 10.

Figure 8. Photographs depicting sessile droplets (5 µL) in contact with a solid surface: (a) water droplet on the base bitumen; (b) ethylene glycol droplet on the base bitumen; (c) water droplet on the base bitumen containing 1.0 wt.% PhCR-F; (d) ethylene glycol droplet on the base bitumen containing 1.0 wt.% PhCR-F; (e) water droplet on the base bitumen with 2.5 wt.% PhCR-F; and (f) ethylene glycol droplet on the base bitumen with 2.5 wt.% PhCR-F.

Figure 8. Photographs depicting sessile droplets (5 µL) in contact with a solid surface: (a) water droplet on the base bitumen; (b) ethylene glycol droplet on the base bitumen; (c) water droplet on the base bitumen containing 1.0 wt.% PhCR-F; (d) ethylene glycol droplet on the base bitumen containing 1.0 wt.% PhCR-F; (e) water droplet on the base bitumen with 2.5 wt.% PhCR-F; and (f) ethylene glycol droplet on the base bitumen with 2.5 wt.% PhCR-F.

Figure 8a–f shows the CAs between the liquids (water and ethylene glycol) and bitumen binder films. CA values higher or lower than 90° show hydrophobicity or hydrophilicity of the samples, respectively. Figure 8a–f shows the increase in the CA of the PhCR-F-modified bitumen binders; such bitumens have a more hydrophobic nature, and, therefore, fewer water molecules are retained on the bitumen surface. This indicates that the bitumen modified with PhCR-F is less moisture-resistant than the base bitumen.

From the data presented in Table 8, it can be concluded that introducing PhCR-F into the bitumen composition reduces the total SFE of the modified bituminous binders. At the same time, the minimum value of the total SFE for the bitumen modified with 1.0 wt.% PhCR-F draws attention. It can also be seen that the dispersion force component of SFE for the bitumen binders was higher than the polarity component (hydrogen bonding and dipole–dipole interactions) of SFE, but exclusively, the bitumen modified with 2.5 wt.% PhCR-F, where the dispersion component is less than the polar one.

In order to achieve the minimum hydrophilicity of the bitumen surface, it is desirable that the total SFE and its polar component be as small as possible; therefore, bitumen modified with 1.0 wt.% PhCR-F will probably be the most optimal option. The doubling of the polar component (from 6.18 to 13.19 mJ/m²) and, together with it, the value of the total SFE (from 18.85 to 21.04 mJ/m²) when the PhCR-F content increases from 1.0 to 2.5 wt.% can be explained by the appearance of a larger number of polar groups on the bitumen surface, in particular, –OH groups from PhCR-F, which could not fully connect with the bitumen components in the volume. A similar phenomenon regarding the decrease in the total SFE is observed in the formation of cross-linked structures of phenol–formaldehyde resins when part of the –OH groups disappear from the surface (see Figure 9) [65].

Figure 9. A conceptual model representing the surface enhancement achieved by nonpolar components that contain a polar reactive functional group.

Figure 9. A conceptual model representing the surface enhancement achieved by nonpolar components that contain a polar reactive functional group.

It is also interesting to consider the relative contribution of the dispersive and polar components to the value of the total SFE for different contents of PhCR-F, which is shown in Figure 10. The maximum contribution of the dispersive component is observed (67%) for the bitumen modified with 1.0 wt.% PhCR-F in comparison with the two other investigated samples, where this contribution is 5 and 30% smaller, respectively, for base bitumen and the bitumen modified with 2.5 wt.% PhCR-F. At the same time, the minimum contribution of the polar component (33%) was obtained for the bitumen modified with 1.0 wt.% PhCR-F. The maximum contribution of the polar component (63%) agrees well with the results shown in Figure 5, Figure 6 and Figure 7, where the adhesion parameters of the bitumen modified with PhCR-F to polar hard surfaces (glass, crushed stone) increase with an increase in the content of PhCR-F in the bitumen from 1.0 to 2.5 wt.%.

Therefore, according to the results of the study on the total SFE, its components, and the assessment of the hydrophobicity of the bitumen surface, it can be stated that in this case, the bitumen modified with 1.0 wt.% PhCR-F is optimal. Since the surface of the aggregates is polar, the larger contribution of the polar component to the total SFE will contribute to the adhesion strength of the bituminous binder to the surface of the aggregates (crushed stone). However, here, it is also necessary to take into account the dispersion component, which obviously provides a significant contribution to the strength of the bitumen film on the aggregate. This is confirmed by data from the literature [66,67,68], where it is claimed that the strength of bitumen–aggregate adhesion mainly occurs because of the dispersion force component and covalent bonds.

Figure 10. The effect of adding PhCR-F on the contribution of surface free energy components in bitumen binder films.

Figure 10. The effect of adding PhCR-F on the contribution of surface free energy components in bitumen binder films.

3.4. Viscosity Test of Bitumen Modified with PhCR-F

For the base bitumen and the bitumen modified with PhCR-F, the influence of temperature on their dynamic viscosities was studied. The test results are shown in Figure 11.

For the production of asphalt concrete, bituminous binder is typically heated to temperatures between 140 and 160 °C [69]. Consequently, the viscosity characteristics of bitumen at these temperatures are significant. Figure 11 illustrates the temperature dependence of dynamic viscosity for the base and modified binders. Above 142 °C, the dynamic viscosity of the bitumen containing 1.0 wt.% PhCR-F is notably lower than that of the unmodified binder. This reduction in viscosity facilitates improved aggregate coating and promotes the formation of a continuous, homogeneous bitumen film during asphalt mix production. As a result, the modified binder is expected to exhibit superior active adhesion to mineral aggregates [5]. Moreover, the decreased viscosity at mixing and compaction temperatures has practical advantages, including reduced energy consumption and diminished oxidative aging of the binder during prolonged heating in bitumen boilers and asphalt plant operations [70].

The applicability of PhCR-F-modified bitumen for asphalt concrete production was further substantiated in our earlier work [21]. Taken together, the findings presented herein confirm that bitumen modified with low concentrations of PhCR-F exhibits advantageous rheological and adhesive characteristics at typical production temperatures, indicating its strong potential as a technologically viable additive for modern pavement applications.

3.5. Comparison of the Effectiveness of PhCR-F with Industrial Adhesive Modifier Wetfix BE

The next stage of this research was to obtain and compare the operational properties of PMBs containing the obtained PhCR-F, an industrial adhesive additive of the Wetfix BE brand (surfactant). It is worth noting that the conditions for modifying bitumen with the above industrial modifiers are close to the optimal conditions for modifying bitumen with the obtained PhCR-F resin (temperature 190 ± 2 °C and duration of 60 min) [21,29]. Therefore, the polymer-modified bitumen was obtained under the same conditions for comparison purposes.

The results of this research are presented in

Table 9. Comparison of the main performance characteristics of bitumens modified with PhCR-F and industrial adhesive additive Wetfix BE.

Composition of Polymer Modified Bitumen, wt.%			Penetration at 25 °C (dmm), Measurement Error ± 2	Softening Point (°C), Measurement Error ±1	Ductility at 25 °C (cm), Measurement Error ± 4	Adhesion to Glass (%), Measurement Error ± 5	Adhesion to Crushed Stone (Mark), Measurement Error ± 0.5
Bitumen 70/100	Wetfix BE	PhCR-F
100.0	–	–	70	46	63	33	3
99.5	0.5	–	87	46	>100	92	5
99.0	–	1.0	68	48	58	87	5
Requirements according to SOU 45.2-00018112-067:2011 1 [51]	–	–	61–90	47–53	≥55	≥75	5

Note: ¹ SOU—Standard Organization of Ukraine.

.

Based on the data in

Table 10. Calculation of the cost of 1 ton of PhCR-F.

Property	Quantity, Ton	Cost per Ton, USD	Cost, USD
1. Reagents: 1	–	–	1208
“Raw” (technical) phenols	1.05	875	919
Formalin	0.54	518	280
Catalyst (HCl)	0.03	314	9
2. Production costs (25% of point 1)	–	–	302
3. Profit (20% of points 1–2)	–	–	302
4. Cost without VAT 2 (points 1–3)	–	–	1812
VAT—20%	–	–	362
Total cost of 1 ton of PhCR-F	–	–	2174

Note: ¹ According to the data given in [19,20,21,22,23], the amount of a wide phenolic fraction for the synthesis of 1 ton of PhCR-F was calculated. ² VAT—value added tax.

, it can be concluded that the introduction of both the industrial adhesive additive (Wetfix BE) and the resulting PhCR-F into the bitumen composition significantly improves the adhesion of bitumen to both the glass surface and the crushed stone surface. It is worth noting that the introduction of both the industrial adhesive additive (Wetfix BE) and the resulting PhCR-F resin (in an amount of 1 wt.% by weight) allows for obtaining bitumen–polymer compositions that meet the requirements of regulatory documents [53].

3.6. Assessment of the Economic Feasibility of the PhCR-F Bitumen Modification Process

In order to assess the overall economic feasibility of the process of modifying bitumen with PhCR-F, the cost of the components of the modified bitumen was calculated. To determine the cost of PhCR-F, the cost of the raw materials, substances, materials, and production costs required to produce 1 ton of PhCR-F were calculated.

The prices of the industrial raw materials, reagents, and products used in the calculations were either commercially sensitive or currently unavailable, as new bitumen components were obtained by the authors during the course of this research. Table 10 and

Table 11. Calculation of the cost of polymer-modified bitumen.

Component	Amount, wt.%	Quantity, ton	Cost per Ton, USD	Total Cost
Modified bitumen with PhCR-F
Bitumen	99.0	0.990	518	513
PhCR-F	1.0	0.010	2174	22
Total	100.0	1.000	–	535
Modified bitumen with Wetfix BE
Bitumen	99.5	0.995	518	515
Wetfix BE	0.5	0.005	5428	28
Total	100.0	1.000	–	543

present the approximate costs of the investigated raw materials, products, and reagents based on data from prior collaboration with Ukrainian oil refineries and small-scale processing facilities.

All the values given below were based on prices for 2025. The results of calculating the approximate price of PhCR-F are given in Table 10.

Based on the data in Table 10, it can be seen that the commercial price of PhCR-F resin is USD 2174/t.

The market price of the bitumen 70/100 brand is USD 518/t. For comparison, the calculation of the bitumens modified with the industrial adhesive additive Wetfix BE brand (surfactant) was carried out. According to the data from online resources, the cost of Wetfix BE is USD 5428/t. We calculated the cost of raw material components of modified bitumen per 1 ton and summarized it in Table 11.

Based on the data in Table 11, it can be stated that the cost of the initial components of PMB using PhCR-F, which are necessary to obtain modified bitumen, is about USD 535/ton, which is 2.5% less than the cost of bitumen modified with the industrial adhesion additive Wetfix BE (USD 543/ton).

Thus, it was established that the use of PhCR-F as a modifier of petroleum bitumen is justified, since the obtained modified bitumen meets the requirements of regulatory documents, has excellent adhesive properties, and is cheaper compared to industrial analogs. With the productivity of a modified bitumen production plant of 300,000 tons/year, the economic effect can reach USD 2.5–7 million.

4. Conclusions and Recommendations

A novel modifier of road petroleum bitumens, i.e., phenol–cresol–formaldehyde resin (PhCR-F), was obtained from the non-basic liquid products of coal coking under optimal conditions using a relatively inexpensive method. Petroleum road bitumens with 1.0 wt.% PhCR-F added comply with the requirements of the Ukrainian regulatory document [51] for bitumens modified with adhesive additives in terms of the main technological indicators. Adding 1.0 wt.% PhCR-F to road petroleum bitumen increases adhesion to glass from 33% to 87% and mineral material from 3 to 5 points. The adhesion of bitumen to mineral material at low temperatures also improves. The aging resistance of bitumens with 1.0 wt.% PhCR-F added remains practically unchanged.

At the temperatures of preparing bitumen–mineral material mixtures during highway construction (140–160 °C), the dynamic viscosity of the bitumen that contains 1.0 wt.% PhCR-F is lower than that of the base bitumen. The bitumen containing 1.0 wt.% PhCR-F has a lower total surface energy than the original unmodified bitumen (18.85 mJ/m² and 24.11 mJ/m², respectively). During the preparation of asphalt concrete mixtures, this leads to the formation of a more uniform and unbroken bitumen film and better mixing with aggregate particles (crushed stone). In turn, the hydrophobicity of asphalt and asphalt concretes based on bitumen modified with the studied resin will be better (compared to the original bitumen).

It was established that using PhCR-F as a modifier of petroleum bitumen is justified since the obtained modified bitumen meets the requirements of regulatory documents and has excellent adhesive properties. Also, the PMB obtained based on PhCR-F is USD 8/ton cheaper than the analog obtained using an industrial modifier. With the productivity of a modified bitumen production plant of 300,000 tons/year, the economic effect can reach USD 2.5–7 million. Finally, it should be noted that the use of part of the phenolic fraction for the production of an effective bitumen modifier will allow for expanding the raw material base for the production of products for the road industry and using part of the coking products in non-fuel technologies (reducing the carbon footprint of the coal industry).