| The ultraviolet (UV) aging of bitumen has been attracting more and more attentions with the proposal of long-life bituminous pavement and the rapid development of pavement construction in the western areas of China where the UV irradiation is relatively strong. As a result, the research on the measures to prevent bitumen’s UV aging has become an increasing concern in the field of bituminous materials all over the world. In recent years, UV absorber that is widely applied in polymers has been used in bitumen to improve its photostability. However, the effects of the UV absorber on physical, rheological and aging properties of bitumen are very complicated due to the complexity of bitumen components and structures. Therefore, it is of great significance to investigate the effect of UV absorbers on performance of bitumen and to reveal the mechanism between them. This will provide theoretical guidance for the preparation of excellent durable bitumen with ultraviolet absorbers, thus enhancing the durability of pavement.In view of these, this paper selected three different kinds of UV absorbers and five bitumens from different sources as the main object, and investigated the influences of these UV absorbers on physical, rheological, thermal-oxidative and UV aging properties of the bitumens. The generic fractions, colloidal structures, functional structures as well as microstructures of different UV absorber modified bitumens were analyzed by thin layer chromatography with flame ionization detection (TLC-FID), Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM). The effects of the UV absorbers on chemical structures of each fraction were also studied after the separation of generic fractions through SARA methods. Based on these, the relationship of chemical components, structures and properties of UV absorber modified bitumens was established, and the mechanisms of action between UV absorber and bitumen were revealed. Besides, the effects of the UV absorbers on properties of different styrene-butadiene-styrene (SBS) modified bitumens were investigated for the choice of proper UV absorbers. The main conclusions are as follows: (1)The physical and rheological properties of bitumen are significantly influenced by the UV absorbers. The penetration of bitumen is slightly increased, whereas the softening point and viscosity are decreased with the addition of UV absorbers. In particularly, the UV absorbers enhance the ductility of bitumen obviously. The complex shear modulus reduces slightly while the phase angle increases when the UV absorbers are added. The UV absorbers can improve the ability of bitumen to resist cracking.(2) The aging evaluation reveals that the effects of UV absorbers on aging properties of different bitumen are complex. The thermal-oxidative and UV aging of bitumen are accelerated or restrained by the UV absorbers, which depends on the origin of bitumen and type of UV absorber. Bitumen from different origins shows specific selectivity for different UV absorbers. For SK-70bitumen, the aging resistance (including both thermal-oxidative and UV aging) can be improved by the three UV absorbers (Octabenzone, Bumetrizole and Tinuvin770); for TH-90bitumen, both Octabenzone and Tinuvin770can improve its aging resistance; for SL-70and PJ-90bitumens, only Bumetrizole can improve their aging resistance; but for QP-70bitumen, all the UV absorbers degrade its aging properties.(3) TLC-FID analysis shows that the generic fractions are also affected obviously by the UV absorbers. The content of aromatics increases largely while that of resins and asphaltenes decreases with the addition of UV absorbers. The content of saturates changes slightly when the UV absorbers are added. Therefore, the colloidal structure of the bitumen tends to be sol with the addition of UV absorbers, which may be the cause for changes of bitumen properties. After the thermal-oxidative and UV aging, there is no specific relation between the changes of generic fractions and aging properties for the UV absorber modified bitumens. However, a good correspondence between the changes of colloidal structure and aging properties can be found for the samples. The decrease of gelation index indicates an improved aging property by the UV absorbers whereas the increase of gelation index indicates a deteriorated aging resistance.(4) FTIR analysis shows that the addition of UV absorbers can increase the relative content of aromatic structure (1601cm’1), but has little influence on the branched aliphatic structure (1377cm"1). It can be found from different functional indexes that in the processes of bitumen aging, if reactions of chain scission, polycondensation and oxidation are inhibited by the UV absorbers, the aging resistance of the bitumen is improved, and vice versa.(5) The effects of UV absorbers on chemical structures of each bitumen fraction were investigated by the FTIR. The results indicate that the structures of the UV absorbers are not changed whereas the chemical structures of bitumen fractions are changed obviously after the mixing of UV absorbers and bitumen fractions. During the UV aging process, the UV absorbers can prevent the formation of carbonyl and sulfoxide in aromatics and thus restrain the UV aging of aromatics significantly. Besides, the UV aging of saturates and resins can be inhibited by the UV absorbers to some extent. But the influence of UV absorbers on the UV aging of asphaltenes is not abvious.(6) AFM analysis shows that the dispersed phase of asphaltenes and continuous phase of light fractions exist in the base bitumen, causing a rough surface. With the addition of UV absorbers, the surface of bitumen turns smooth and the phase becomes even, leading to a relatively homogeneous system. This can be ascribed to the changes of bitumen fractions affected by the UV absorbers. The asphaltenes are well dissolved by the light fractions and the bitumen tends to be sol. For the UV absorber/bitumen system with superior aging resistance, the two-phase trend of bitumen is not obvious after aging, indicating that the association of asphaltenes is restrained. With respect to the UV absorber/bitumen system with inferior aging resistance, the two-phase trend of bitumen is obvious after aging, indicating a severe association of asphaltenes in bitumen.(7) Based on the TLC-FID, FTIR and AFM results, we can conclude that:(a) The mechanism of improved aging resistance of bitumen by the UV absorbers is that:the UV absorbers dispersed in the bitumen can convert the photon energy of the UV rays into harmless heat energy by the intramolecular hydrogen bonds (Octabenzone and Bumetrizole), or trap the free radicals in bitumen by secondary amines (Tinuvin770). Consequently, the addition of UV absorbers can slow down the reactions of chain scission, polycondensation and oxidation of bitumen molecules during the aging processes, causing the formation of asphaltenes decreased and the gelation process restrained, and thus the aging resistance is improved; (b) The reason why some UV absorbers can deteriorate the aging properties of bitumen is that the structures of the UV absorbers are changed when they are reacted with the bitumen molecules, and thus the UV absorbers lose the ability to convert the absorbed photon energy into heat energy or to trap the free radicals. To make things worse, the broken molecules of UV absorbers may generate harmful radicals to further activate the free radical reactions in the binders. Consequently, the reactions of chain scission, polycondensation and oxidation of bitumen molecules are accelerated, the gelation process is promoted, and thus the aging properties are degraded.(8) The addition of UV absorbers can increase the penetration, ductility and softening point but decrease the viscosity of SBS modified bitumens. The influences of the UV absorbers on complex shear modulus of SBS modified bitumens are slight, but the phase angle is affected complicatedly by the UV absorbers. The aging evaluation of UV absorber/SB S modified bitumens reveals that the UV absorbers also show specific selectivity for different SBS modified bitumens. The Tinuvin770can improve both the thermal-oxidative and UV aging properties of the commercial SBS modified bitumen significantly. For linear SBS modified bitumen, Bumetrizole and Tinuvin770can only improve the UV aging resistance; with respect to the radial SBS modified bitumen, Octabenzone and Tinuvin770can inhibit both the thermal-oxidative and UV aging processes to some extent. |
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