Asphalt concrete (AC) coatings include two main components: bitumen and aggregates (crushed stone). Bitumen performs the function of a substance that binds particles of mineral material to form an asphalt concrete coating. Being nonpolar, bitumen has high waterproofing properties. It is known that high-resinous and low-paraffinic petroleum is the most suitable for the production of road and construction bitumen. However, due to the limited supply of this type of petroleum almost any oil residues are used. As a result, the quality of bitumen is very unsure at the moment and that leads to poor quality of bituminous materials resulting in poor quality asphalt concrete pavements.
Poor adhesion between the bitumen binder and aggregate may lead to stripping (delamination of the binder from the aggregate surface) in the presence of water which leads to pavement failure such as raveling, potholes formation, and rutting . The situation is aggravated by the continuous increase in capacity and traffic volumes, including heavy truck axle loads, super singles, and high tire pressure that leads to a significant increase of dynamic loads on the pavement. Therefore, to obtain high quality pavements, key factor is in ensuring high adhesion between bitumen and the mineral component of pavement. For this reason adhesion additives are used; most frequently these are cationic surfactants. Classical cationic surfactants are diphilic structures in which nonpolar part is a long chain hydrophobic alkyl and the polar part (so-called “head”) is a positively charged ammonium center, or nitrogen atom capable of being protonated in acidic mediums. This fundamental property, diphilicity, is the reason why surfactants are lined up at the interface “bitumen-mineral material”. Herewith, the positively charged (hydrophilic) groups are fixed on polar surface of the mineral material and the hydrophobic hydrocarbon chains are fixed in the bitumen.
Nowadays imidazolines and their derivatives (Ia–Iд) as well as bisimidazolines (IIa, IIб) that exhibit the ability to increase the bitumen adhesion to mineral material are most widely used as additives for bitumen; the mechanism of their action is shown schematically on Figure 2. Adhesion additive “Amidan”, which presents the technical product, containing a mixture of imidazoline (Ia–Iд) and bisimidazoline (IIa, IIб) derivatives, can be taken as example.
Composite modifiers such as sulfur, rubber (polybutadiene, natural rubber, butyl rubber, chloroprene, etc.), organic-manganese compounds, thermoplastic polymers (polyethylene, polypropylene, polystyrene, ethylene-vinyl acetate (EVA)), thermoplastic rubbers (polyurethane, olefin copolymers, and block copolymers of styrene-butadiene-styrene (SBS)) are also used in order to improve physical-chemical properties of BB as well as the physical-mechanical properties of AC.
The effectiveness and quality of additives are largely related to their thermal stability, and this is due to the following reasons (most relevant for Russia):
1) Road bitumen is produced in bulk by liquid-phase oxidation of vacuum residue, the optimal process temperature is 250°C, and it is more viable to inject the additive to the oxidized bitumen directly at its outlet from the oxidation column, to obtain bitumen with improved adhesion properties.
2) The hot-mix asphalt concrete is the most used in road construction, and the compounding of bitumen with mineral material is carried out at relatively high temperatures (160–170°C).
It should be emphasized that a disadvantage of modern adhesive modifiers for bitumen is that they demonstrate a low thermal resistance (up to 120–175°C), while for some systems there is a rapid loss of adhesion at 150°C. On the other hand their use often leads to deterioration of physical-mechanical properties of the corresponding AC.
2. Materials and Methods
According to the concept of nonclassic cationic surfactants, they have hydrophobic hydrocarbon chains which are fragmented (broken) by polar heteroatomic groups. This structure ensures their physical and chemical properties and adsorption behavior, different from the classic surfactants. Based on this concept high-performance chemicals were obtained that were useful for the production, treatment, and transportation of petroleum as well as for the production of corrosion inhibitors, emulsifiers, demulsifiers, the biologically active substances, and so on. We used the same concept to develop desired nitrogen-containing compounds.
2.1. Modifiers Synthesis
In accordance with the concept of nonclassical surfactants, we had assumed that it is necessary to increase polarity of the molecule of constructed imidazolines by increasing the number of polar imidazoline centers from one to two in order to create compounds with needed properties, as well as to introduce nonclassical spacer into the molecule, wherein the hydrophobicity of an alkyl chain is broken (fragmented) by one or more alkanoylamide fragments.
On one hand, this structure of the spacer provides an increase in the molecule polarity that is responsible for interaction between bitumen and hydrophilic surface of the aggregates, because extra polar amide groups appear along with two imidazoline cycles. On the other hand, the use of alkanoylamide fragments allows introducing additional hydrophobic long chain alkyl groups which increase the possibility of adhesive to fix in the bitumen media. This enhances the adhesive properties of the designed compounds. As a result, we obtained N-acylated bisimidazolines with (ethylene-N-alkanoylamide) spacers, namely, 1-(21-alkylimidazolinyl-11)-2-[(22-alkylimidazolinyl-12)-poly(ethylene-N-alkanoylamide)]ethanes, containing two imidazoline cycles, connected by spacers.
The authors assumed that this construction of the additive would lead to increasing of bitumen adhesion to the mineral component of pavement. Furthermore, the conversion of the secondary amino groups into alkanoylamides should also increase significantly the thermal stability of designed compounds due to the fact that tertiary amides are more stable than the starting amine-compounds.
The desired spacers were obtained with the average yield (the ratio of the mass of the obtained product to the mass of the starting material) of up to 90%, and are viscous and pasty brown liquids, the physicochemical characteristics.
The starting amino-compound and the resulting compounds were analyzed by the conventional method of potentiometric titration of a mixture of primary, secondary, and tertiary amines with the definition of the total basicity BT , the basicity of secondary and tertiary amines B2,3, and the basicity of tertiary amines B3. Then the joint basicity of tertiary amines and amides B3+amide was determined by potentiometric titration of the sample weight of test substance in acetic anhydride (as solvent) with the use of 0.1 N solution of HClO4 in glacial acetic acid. The basicity of amides Bamide was determined by the difference between the basicity B3+amide and the basicity B3.
Absorption bands with the following wave numbers were observed in the IR spectra of the obtained compounds (Figure 3(a)): υC=N = 1600–1630 cm−1, indicating the presence of imidazoline cycles; υC=О = 1740–1745 cm−1, υ–N(CO)R = 1660 cm−1, and υCN = 1260–1300 cm−1 can be related to alkanoylamide structures; δ(CH2)x = 720 cm−1 – corresponds to pendular oscillation of alkylene structure in a chain. Wide absorption bands at 1600 cm−1 in FTIR spectra of the original bitumen (Figure 3(b)) stand for the υCH bond in aromatic ring; sharp absorption bands with high intensity at 1440 and 1460 cm−1 correspond to stretch vibration of –CH2– structure fragment in long chains, confirming high content of paraffin waxes; absorption band at 1375 cm−1 is related to methyl groups.
2.2. Bitumen Modification and Performance Properties Analysis
Investigation of the effect of the synthesized compounds on the adhesion between bitumen and the mineral material was carried out by the use of example of oxidized bitumen of 60/90 penetration grade (BND 60/90 (the mark of bitumen in Russia means “petroleum bitumen for roads”)) that was produced in TAIF-NK PSC, with the following group chemical composition, wt.%: 22.62 of saturates (including 10.2 % of paraffin waxes), 15.14 of aromatics; 40.26 of resins; 21.98 of asphaltenes.
Bitumen adhesion is the strength of its adhesion to the skeletal materials of the organo-mineral composite. An indicator of the adhesion value is the ability of bitumen to remain on the surface of the stone material under the influence of an aggressive environment simulated by boiling in water for a certain period of time.
The bitumen modification process was carried out as follows:
(1) The BND was heated in a metal vessel to the temperature of 80°C with the use of stirring mechanism, kept under thermostatic control for 20 min to obtain homogeneous diluted substance with homogeneous distribution of components;
(2) The BND was gradually heated to the temperature of 120–130°C, at which it was kept for 20 min;
(3) Obtained PFM substances were injected into the heated bitumen and stirred for 20–30 min to ensure homogenous dispersion of the PFM in bitumen media.
The adhesion between bitumen and aggregates (mineral material) testing was conducted in accordance with GOST (GOST, Russian state standard) 12801-98, the essence of which is to determine the ability of the viscous bitumen that is precoated on the surface of mineral material to persist on it while being exposed by water. The quality of the adhesion assessment was rated according to Table 2.