1. Introduction
Road networks are a crucial element of any nation’s economy. Their efficient functioning and sustainable development are essential prerequisites for a transition toward economic growth, ensuring the integrity and national security of a country and improving the standards and quality of people’s lives. This necessitates the creation of reliable and safe roads for both people and the environment. At the same time, the construction of new roads and maintenance of existing ones require unlimited access to resources, including bitumen. This requirement imposes economic and environmental constraints on the road construction industry. The need for affordable and high-quality resources while also respecting environmental standards requires an evaluation of the products used throughout their life cycle. An additional factor is the annual increase in the automotive and consumer markets, which complicates the situation in terms of environmental safety. Global reserves of rubber product waste collectively represent a significant volume, reaching around 80 million tons, with a constant annual increase of at least 10% in total reserves [
1,
2]. Waste from used tires falls under the IV hazard class and takes 12 to 140 years to decompose in the environment. Presently, there are several ways to recycle used tires (see
Figure 1) [
2].
Based on the provided data (
Figure 1), crumb rubber grinding appears to be the most attractive method of disposal to minimize the environmental damage. However, it is important to note that as the scale of this method’s implementation increases, significant reserves of unused ground crumb rubber (CR) are generated. This underscores the relevance of research into the potential for effectively utilizing crumb rubber, including as a modifying component in bitumen binders [
3].
The quality of the bitumen directly depends on the state of its structure. According to the “classical colloidal theory” of bitumen’s structure, proposed by Nellensteyn, in 1923 [
4], its structure consists of lyophobic asphaltenes surrounded by lyophilic resins, which form an adsorption–solvation shell that prevents the aggregation of asphaltenes. These colloidal formations are micelles distributed in an oily medium. The micelle core can be a particle composed of carbon surrounded by adsorbed asphaltenes. Later, Traxler [
5] developed this theory, establishing that the most polar, aromatic, and high-molecular-weight components are located in the micelle core, and as the distance from the core increases, the polarity and molecular weight of the components decrease. This theory, known as “colloidal theory”, is based on the Tyndall cone effect (i.e., light scattering) in bitumen solutions in benzene and the ability of their dispersed components to undergo Brownian motion. Depending on the concentration and liposomal properties of the asphaltenes, bitumen mycelia may interact with each other through thin layers of the dispersion medium (oil), either forming a coagulated structure or remaining stabilized as a solution. As a result, the bitumen binder acquires the properties of a gel, sol, or an intermediate colloidal structure of the sol-gel type [
6]. However, it should be noted that this theory does not explain the thermodynamic stability of bitumen and even suggests the heterogeneity and instability of the system.
The development of the colloidal theory of petroleum-based dispersive systems (“modern colloidal systems”) was furthered in the works of Syunyaev [
7]. He considered the dispersive phase of bitumen as a system composed of complex structural units (CSUs), represented by asphaltene–resin complexes consisting of asphaltenes, as well as solid and high-melting resins, which are distributed in a dispersive medium of maltenes, composed of oils and low-melting resins [
8]. According to this theory, the structure and properties of bitumen are determined not only by the chemical compositions of the components but also by the size of the associates, which influence the molecular, supramolecular, inductive (topological), and colloidal-dispersion levels—highlighting the importance of the entire complex of structural formation phenomena.
However, it is important to note that when adding additional components to bitumen, in this case, crumb rubber (CR) and plasticizers form more complicated structures of organized polydispersive materials. These represent a complex statistical ensemble of macro- and microsized components with varying physical and mechanical properties, granulometric composition, and geometric shapes, distributed within a multiphase medium and interacting with each other [
9].
The mechanism of interaction between the particles of the crumb rubber and the bitumen binder plays a major role in determining the quality and performance properties of the crumb-rubber-modified binder. To establish the influence of crumb rubber on the structures of complex systems, modern scientific and technical sources describing the interaction mechanism of crumb-rubber-modified binder components were studied and analyzed [
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20,
21,
22,
23,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39,
40].
It was found that in the process of interaction between CR and bitumen, the following two main mechanisms occur: swelling of crumb rubber particles [
10] and their physical or chemical destruction (devulcanization and depolymerization) [
11,
12]. Rubber swelling is a physical process of aromatic substance adsorption and partial diffusion. Typically, the swelling process is followed by destruction. The chemical destruction of rubber causes the cross-linking to break down. Most often this involves sulfur, less often resin or peroxide [
13,
14]. Reorientation of the molecules occurs forming new substances. In the literature, researchers use the term “rubber dissolution” to describe the state of rubber particles in bitumen during their interaction [
15,
16]. Rubber dissolution is measured by extracting rubber particles from the binder matrix; the portion of rubber particles that passes through a fine sieve (usually 75 µm) is considered the dissolved part [
11,
17]. This definition of the term “dissolution” is inaccurate and does not determine whether the chemical destruction of rubber has occurred. It should be noted that swelling and complete dissolution of rubber are at two opposite ends of the spectrum of interactions between bitumen and rubber, depending on the interaction conditions.
Uncrosslinked polymers easily swell because of the action of compatible solvents and then undergo partial dissolution [
18,
19]. The dissolution of an uncrosslinked polymer in a solvent involves the following two phenomena: swelling caused by solvent diffusion and chain disentanglement [
11,
20]. Polymers with a crosslinked structure, where bonds exist among chains or segments, can swell by absorbing solvents, but the dissolution process is minimal. For polymers with a crosslinked network structure, such as crumb rubber derived from ground used tires, a characteristic property is limited swelling (or partial dissolution) [
11,
16]. When rubber is mixed with bitumen at high temperatures, light fractions of the bitumen diffuse into the crumb rubber, causing it to expand in volume, which can be described as swelling [
20]. The change in the volume of the rubber particles and the formation of a gel layer adjacent to the rubber–bitumen interface reduces the distance between the RC particles and alters the ratio of components in the remaining bitumen, leading to increased stiffness of the composite material [
11,
21].
Polymer destruction is the process of breaking down the rubber network under harsh interaction conditions, such as excessively high mixing temperatures, high shear forces, and prolonged mixing times. Under these conditions the sewn polymer mesh of the crumb rubber is destroyed by high thermal energy and shear energy [
22]. The first process of devulcanization involves the breaking of disulfide bonds (SS) and carbon–sulfur bonds (C–S); that is, the cross-links breaks. This is why a sulfoxide odor can be detected during the preparation of crumb-rubber-modified binder at high temperatures. Next, depolymerization occurs, breaking the carbon–carbon (C–C) bonds in the main chain, thereby reducing the average molecular weight of the rubber [
11]. Chemical destruction of the polymer network in crumb rubber negatively affects the mechanical properties of bitumen binders [
23,
24] but positively influences improvements in crumb-rubber-modified binders’ storage stability [
25].
It is also important to note that crumb rubber derived from ground used tires contains a mixture of various components—plasticizers, carbon black, and inorganic fillers—which are released into the bitumen matrix during interactions under high temperatures and mixing [
26]. Their influence on the properties of the bitumen binder cannot be ignored—it has been noted that these components significantly affect the aging and rheological properties of rubber–bitumen binders [
27,
28].
Thus, according to modern technology, the mechanism of interaction between crumb rubber and bitumen can be described in the following three main stages [
12,
29]:
- (1)
The first phase: swelling, whereby RC particles begin to increase in volume, absorbing light fractions of the bitumen, and a gel-like layer forms adjacent to the bitumen oil fractions–RC interface;
- (2)
The second phase: The subsequent swelling and beginning of destruction occur, when the swelling of the crumb rubber particles continues. Chemical destruction of crumb rubber occurs due the breakdown of the vulcanized polymer mesh. As a result, the swollen crumb rubber particles break down into smaller pieces;
- (3)
The third phase: destruction and complete dissolution, whereby the already devulcanized RC particles undergo further polymer network breakdown until the RC particles are completely dissolved in the bitumen matrix, resulting in a homogeneous modified bitumen binder.
The degree of swelling and destruction of crumb rubber in bitumen plays a crucial role in the formation of the performance properties of crumb-rubber-modified binder [
30]. Therefore, controlling the degree of RC swelling and destruction by adjusting the conditions for producing crumb-rubber-modified binder has attracted the attention of many researchers seeking to obtain crumb-rubber-modified binder with improved performance properties.
To study the mechanism of RC particle swelling and devulcanization in bitumen binder, a method involving the stepwise extraction of swollen rubber was used [
31], allowing the CR–bitumen interaction zone to be divided into four layers for sequential investigation. Gel permeation chromatography results reveal that bitumen fractions with lower molecular weights were absorbed into the deeper layers of the swelling rubber. It was also noted that there was minimal absorption of bitumen fractions containing carbon–oxygen (C-O) bonds. In a modeling study [
21], dibutyl phthalate was selected to simulate the light components in bitumen, and a soaking test was conducted with rubber. A use test was conducted for the impregnation of rubber. The gas chromatography-mass spectrometry results led to the conclusion that chemical reactions occur in the “rubber–dibutyl phthalate” system, including light component absorption, rubber decomposition with chain breakage, and the formation of new compounds. The study [
21] also confirmed that changes in viscosity can reflect the physical–chemical properties of RC particles in bitumen. In particular, the viscosity first increased and then decreased after reaching a peak value when the crumb rubber content was more than 20 wt.% and the mixing time was more than 90 min. It was shown that both high temperatures and prolonged times are causes of CR polymer chain destruction, leading to a decrease in viscosity. In conclusion, it can be noted that the viscosity of crumb-rubber-modified binder gradually increases, reaching an equilibrium as the CR swells, and then decreases with an increase in the degree of degradation and the amount of degraded CR.
The studies and methods considered have both obvious advantages [
3,
32] and disadvantages for binders modified with crumb rubber [
33,
34]. To eliminate these disadvantages, several promising methods have been developed, generally involving CR activation, which consists in the surface destruction of crumb rubber due to shear effects [
10,
35,
36], exposure to ionizing radiation [
37], or treatment with a devulcanizer [
12,
38,
39,
40].
However, these methods still face the problem of dispersing the crumb rubber in the bitumen. Because the melting process results in significant increases in particle surfaces and/or changes in the surface wettability of the crumb rubber particles, this subsequently leads to the formation of aggregates composed of the unwetted particles, which has an intense elastic aftereffect, leading to cracking, especially during low-temperature operation, and, therefore, premature destruction of road surfaces [
41,
42]. It should also be noted that the lack of knowledge about crumb rubber compositions and their stability and compatibility with plasticizers, as well as the influence of various factors on the efficiency of the rubber devulcanization process and its subsequent connection with bitumen, leads to variability in research results and ambiguity in the recommended technological modes. Current problems include a lack of studies aimed at establishing a mechanism for the interaction between plasticizers and crumb rubber (macromolecular or supramolecular). Moreover, a lack of understanding of the interaction process between hydrocarbon plasticizers and devulcanized crumb rubber prevents the effective control of its dissolution process to obtain dispersed systems with specified property parameters and stability.
According to regulatory documents enforced in the Russian Federation, one of the most important indicators for crumb-rubber-modified binder is crumb rubber’s solubility in a bitumen volume of at least 99%; otherwise, such a modified binder cannot be used for road surface construction. Currently, there are no crumb-rubber-modified binders in our country that meet these requirements. Thus, this work is devoted to the study of crumb rubber compositions and their stability, as well as the determination of the compatibility between the crumb rubber and plasticizers and investigation of the mechanism of interaction between them (macromolecular or supramolecular). New data will allow for the formulation of a hypothesis for the formation of a sustainable and thermodynamically stable structure of crumb-rubber-modified binder. This will also contribute to the increased stability and durability of asphalt concrete while simultaneously reducing construction costs and improving the environmental aspect of used tire disposal.
4. Conclusions
It was found that the average mass ratio of the polymer component in the investigation crumb rubber was 93.3 ± 1.8%. The stability of the polymer components in the investigation crumb rubber taken from different batches and sizes was analyzed using pyrolytic gas chromatography-mass spectrometry. It was established that they were characterized by identical polymers, leading to the conclusion that the raw materials and final crumb rubbers products produced at the same plant were stable.
It was found that the ACR LST sample had the lowest glass transition temperature at 9.59 °C, while the nonactivated NCR LST and CR CRP 0.5 samples had values 34% and 100% higher, respectively. The temperature at which destruction occurred for all samples studied was within the range of 300 to 310 °C. So, the activated CR LST sample is a promising modifier in terms of the temperature range for property retention. It was found that both nonactivated and activated crumb rubbers from large-sized tires were based on butadiene rubber, while CR CRP 0.5 was based on butadiene–styrene rubber. Considering that butadiene rubber has high elasticity over a wide temperature range and in accordance with the existing knowledge about the best interaction of activated crumb rubber with bitumen, activated crumb rubber was selected for further research.
An analysis of the compatibility parameters allowed for the identification of differences among them when using different characteristics. Stability in the calculated values was observed only for chemical compatibility parameters. Therefore, chemical compatibility parameters were used as criteria to justify the selection of compatible components. They were based on the condition of achieving better compatibility by those plasticizers characterized has having a large content of maltene, which is the most compatible with polymers in crumb rubbers. Only purified waste frying oil meets two compatibility chemical criteria at once.
It was found that for the studied dispersed systems of “hydrocarbon plasticizer–crumb rubber” the supramolecular plasticization mechanism was insufficient to produce sustainable crumb-rubber-modified binder. It was found that the use of activated-crumb-rubber ARC LST did not ensure the achievement of a stable and sustainable structure in crumb-rubber-modified binder. All compositions of crumb-rubber-modified binder were heterogeneous and did not meet the requirements for solubility and delamination.
A scientific hypothesis was formulated arguing that the formation of a thermodynamically stable dispersed system of “bitumen–plasticizer–crumb rubber” should be based on the realization of a supramolecular mechanism (often applied in practice) and a molecular plasticization mechanism. This requires the partial (controlled) physical devulcanization of crumb rubber to reduce the energy barrier of the subsequent swelling, postswelling, secondary destruction, and solubility. The results of testing this hypothesis are expected to be presented in a second article.