The theoretical basis for the use of microwave processing of bitumen for high-quality asphalt production.
One of the main technical policies in the field of organic binders, implemented in Russia and in the world, is to focus on improvement in road bitumen quality. The widespread method of bitumen modification today is the use of surfactants and polymeric additives (Rangwala, S.C., 2005; Sessions, J., 2007). The application of the above technology involves the following problems: additional power costs for the production of additives of surfactants and polymer modifiers, costs for their transportation to consumers, and environmental safety of these methods. An alternative is to use ultra-high frequency energy to control service properties of road bitumen.
The development of powerful microwave-field generators, electronic databases, and scientific study of microwave energy effects on various construction materials and complex polymer compositions have opened up broad prospects for industrial applications of microwave engineering and technology. Recently, researchers have not been interested in microwave heating because of complexity and high cost of equipment and lack of stable favourable effects of it is use.
At present some effective systems of RF energy effect on different dielectric materials (liquid and dry ones) have been developed (White, G.O., 1992; Volkov, V.V., 2009).
The study of physical effect methods, which was conducted in the field of building materials modification, demonstrated their high effectiveness and the prospects of their use (Akimov, A.E., 2010; Yadykina, V.V., 2010).
Bitumen of BND 40/60 60/90, 90/130 brand produced at Moscow refinery in compliance with GOST 22245-90 "Viscous oil road bitumen. Specifications" was used as test samples. The microwave field source was a "Samsung" microwave stove having 2.45 GHz frequency and maximum radiated power of 850 watts. Bitumen temperature before treatment was 100-120[degrees]C, and it was 140-160[degrees]C after treatment, the treatment time period was 0.5-4 min.
The changes in the composition of bitumen under influence of the microwave field was evaluated by means of infrared spectroscopy (Barbara, H. Stuart, 2004).
The specimens for spectral studies were films of organic binder applied onto the glass. After activation of bitumen in a super-high frequency field, asphalt was dissolved in chloroform, the spectrogram of which did not have absorption bands in the test area up to the concentration of 0.5 %.
The infrared spectra were recorded in the region of 7000-400 [cm.sup.-1], using the SFM 1202 instrument connected with a computer.
All organic compounds were characterized by absorption bands in the frequency region of 2926 -2957 [cm.sup.-1] which were are caused by stretching vibrations of C-H bonds in methylene group (C[H.sub.2]); the availability of aromatics indicates the presence of double bonds of C=C in the region of 1600 cm-1. The content of naphthenic hydrocarbons can be judged by a characteristic "trident" with frequencies in the region of 725-810-880 [cm.sup.-1]. To eliminate the influence of error thickness of the deposited film on the spectrograms, an internal standard was introduced. It was used for decoding and quantitative assessment of the spectrum.
To study the changes in the structure of bitumen, electron microscopy and the electron paramagnetic resonance method were used.
The essence of the electron spin resonance phenomenon is the resonance absorption of electromagnetic radiation of the unpaired electrons. The relative viscosity of the bitumen was determined, using a viscometer and a penetrometer according to the standard procedures.
The Main Part:
The previous studies showed some significant advantages of using the method of bitumen activation by the ultrahigh frequency field and a positive effect on physical and mechanical properties of asphalt concrete (Akimov, A.E., V.V. Yadykina, 2010; Gentsler, I.V., A.S. Karapetyan, 2001; Burminskiy, N.I., M. Barachova, Ye, 1999). In particular, there is an increase in asphalt adhesion with stone materials and an increase in cohesive strength of the binder.
The positive effect is connected with the changes in the composition and properties of bitumen after its microwave field activation. The study results have showed that the amount of asphaltenes in bitumen changes insignificantly, some accumulation of aromatic hydrocarbons occurs (Fig. 1), and it is evidenced by the increased stretching vibration band of benzene rings (1600 [cm.sup.-1]), as well as the increase in the amount of acid groups (absorption band at 1700 [cm.sup.-1]).
The determination of the binder acid number showed that the bitumen acid content was increased by 30-35 % under the influence of the microwave field. Obviously, the acid number increase is due to oxidation of hydroxyl end groups up to acid ones. These changes will have a favourable effect on bitumen bonding with mineral materials, and, therefore, will contribute to improvement in strength characteristics of asphalt concrete and its durability.
It has been established has been established by using the electron paramagnetic resonance (EPR) method that the number of paramagnetic centers (Fig. 2 a, b) increases as a result of microwave treatment of bitumen for 2 min, because the intensity of the ESR signals increases. This demonstrates that oxidation occurs according to free radical mechanism under the influence of the microwave field. The increase in the amount of free radicals will provide for a better interaction with the surface of stone materials, and may also contribute to the formation of intermolecular bonds. The data obtained match the results of Iliopolova's study (Iliopolov, S.K., I.V. Mardirosova, 2003), according to which the rise in the binder softening temperature increases the number of unpaired electrons. The reduction in the EPR spectra intensity after bitumen cooling (Fig. 2) is due to the recombination of free radicals in the formation of supramolecular associates, as evidenced by increased viscosity of bitumen.
The structure and mechanical properties of organic binders are known to be determined by the nature of interaction and the properties of macromolecular compounds within their composition.
In accordance with the theory of solutions of high molecular compounds (HMC), bitumen may be in various thermodynamic states, depending on external conditions. It passes sequentially through all the stages--from true solutions (at high process temperatures) to colloidal solutions of supramolecular structures (associates) of asphaltenes and resins, to plastic ones and then solids.
The ordered structures of varying degrees can be formed in solutions by means of the interaction of molecules of higher-molecular compounds. All structural formations are destroyed and reappear when concentration and temperature change, and they are significantly different from the colloidal particles.
The intermolecular interactions in solutions of the higher-molecular compounds may occur between adjacent molecules as well as between the individual links of the same molecules by means of hydrogen bonds. These links are capable of forming -OH, -NSO, -COOH groups and others. Their association in solution, viscosity increase, and change in other properties is conditioned by the presence of the hydrogen bonds. The bitumen structure transformation under the influence of the microwave field is confirmed by microscopic studies (Fig. 3). These pictures show that before microwave treatment (a) bitumen is a dispersed system in which asphaltenes are distributed among oils and form the contacts between them. After microwave activation of bitumen and its cooling, the homogeneous structured system is observed (b). It appears that when processing bitumen by microwave energy, the bonds between the molecules and the molecules inside are ruptured first to form active functional groups and free radicals. As cooling occurs, "the cross-linking" of macromolecules is observed due to the formation of hydrogen bonds and recombination of free radicals, causing the increase in bitumen viscosity. As a result, bitumen molecular structure becomes more uniform and fine.
Since the relative viscosity of the bitumen is determined at a sufficiently low temperature (250[degrees]C), the formation of associates, apparently, plays a significant role under these conditions, which is reflected on the bitumen penetration index after microwave treatment (Fig. 4). It has been established that microwave activation causes a significant increase in bitumen viscosity. The increase in viscosity was 35, 36 and 51 % for bitumen of grade 40/60, 60/ 90 and 90/130, respectively. It is necessary to specify the nature of bitumen viscosity variations: a sharp increase in the viscosity under two minutes' treatment is characteristic of all brands, then the slowing-down of the process is observed.
It is known that at high temperatures the influence of the chemical structure of component molecules comes to the foreground, which determines the degree of flexibility and rigidity of the components, and the forces between the molecules and structures play a less role.
As the temperature goes up, thermal motion increases, leading to the destruction of the associates, and the system viscosity is reduced, i.e. the spontaneous recovery phenomenon is observed. This is confirmed by measuring the relative viscosity of bitumen at the temperature of 1400[degrees]C determined by a viscometer (Table 1).
The results show that the microwave activation of bitumen causes a significant reduction in its viscosity at processing temperature (by 37% for bitumen of grade 60/90, and by 28% for bitumen of grade 90/130 under 2 4 minutes' treatment).
The viscosity reduction is obviously due to the destruction of associates, the breaking of intermolecular bonds and formation of oxygen-containing functional groups which stabilize the bitumen dispersion, and this is confirmed by the bitumen micrographs obtained previously before and after microwave treatment.
It has been established that in case of bitumen treatment by microwave field, firstly, the increase in the content of acid groups and free radicals occurs; secondly, the change in the structure and rheological properties of bitumen is observed. Microwave treatment causes the reduction in the bitumen viscosity at high processing temperatures, which contributes to better aggregate wetting. As the temperature decreases, in case of compacting the asphalt mix and continued operation, bitumen has a higher viscosity, and therefore a greater cohesive strength owing to its finer and more homogeneous structure, which will have a positive effect on physical and mechanical properties of asphalt concrete.
The present study results regarding the effect of the activation process by a microwave field on bitumen properties are the theoretical basis for controlling the properties of road bitumen, and, consequently, the performance of bitumen-mineral composites, which lays the foundation for future large-scale application of the proposed technology along with the development of new high-performance microwave systems.
Received 25 January 2014
Received in revised form 12 March 2014
Accepted 14 April 2014
Available online 5 May 2014
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Valentina Vasilievna Yadykina, Andrey Evgenevich Akimov, Anna Ivanovna Trautvain
Belgorod State Technological University, 308012, Belgorod, Kostyukova, 46, Russia
Corresponding Author: Valentina Vasilievna Yadykina, Belgorod State Technological University, Russia, 308012, Belgorod, Kostyukova, 46
Table 1: Modified bitumen viscosity at operating temperature after the microwave treatment. Treatment time, min Viscosity, sec. for bitumen grades 00 00,5 11 22 23 24 60/90 38 35 28 26 25 24 90/130 25 23 19 17 16 18
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|Author:||Yadykina, Valentina Vasilievna; Akimov, Andrey Evgenevich; Trautvain, Anna Ivanovna|
|Publication:||Advances in Natural and Applied Sciences|
|Date:||Apr 1, 2014|
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