Asphalt is an organic colloidal mixture of lightweight components, such as various small hydrocarbon compounds and some unsaturated aromatic hydrocarbons, and is one of the oldest engineering materials. Refined asphalt, most of which was used in road construction, was first produced in the United States in the early 20th century. However, over the last few decades, continually increasing traffic volumes, heavier vehicle loads, unexpected weather extremes, and slight variations in ground conditions have posed major challenges to the consistency of pavements. Indeed, these challenges are mainly due to the inadequate mechanical properties of asphalt [1
]. Considering that it is difficult for small-hydrocarbon compounds in asphalt to chemically react with other substances, and thus improve their mechanical properties, polymers usually modify asphalt to enhance its viscoelasticity. Numerous studies and methods have shown [1
] that the modification of asphalt using polymers effectively solves problems such as rutting, cracking, and fatigue. Polyethylene waste can produce modified asphalt at a low cost, mitigate environmental pollution, enable the upgrade of energy-efficient construction technologies, and develop green construction projects [6
]. In the spirit of environmental protection and lower costs, research on the use of waste polyethylene as modifiers to produce modified asphalt shows no sign of slowing down. PE is a kind of thermoplastic resin polymer that is commonly used for the production of films, tubes, wires, cables, and moldings. The residual pollution of used agricultural films has become one of the outstanding problems restricting the green development of agriculture in China. The capacity of agricultural waste films to be consumed must be improved due to the high impurity of agricultural waste film, low utilization value of recycled materials, and large-scale production. There is a need to continue to design and develop industrial products using agricultural waste film as a raw material [11
]. PE-modified asphalt has increased rigidity and enhanced resistance to permanent deformation at high temperatures [12
]. However, non-polar crystalline PE is less compatible with asphalt and is incorporated into the asphalt as a modifier. It has a poor low-temperature ductility and is prone to phase separation [14
]. Many researchers have explored the storage stability problems of PE-modified asphalt, but these efforts remain in the ready-to-use stage of the application process. According to previous studies, EVA has been extensively studied in asphalt blended with PE to improve the phase separation of PE in asphalt [16
EVA is a thermoplastic resin polymer modifier consisting of the copolymerization of ethylene and vinyl acetate, obtained by modifying the vinyl segment with the polar functional group vinyl acetate to increase the polarity of the vinyl segment and reduce its crystallinity, thus providing excellent flexibility and improving its compatibility with asphalt [18
]. EVA has a similar molecular structure to PE and, according to the principle of similar solubility, is very compatible with PE. Adding EVA can effectively improve the storage stability and low-temperature ductility of PE-modified asphalt [16
]. However, the addition of EVA alone is insufficient to completely solve the storage stability problem of PE-modified asphalt.
The optimal state of compatibility between polymer and asphalt occurs when the polymer is thoroughly dispersed in the asphalt in the most diminutive possible form, where it is instantaneously transformed from a discontinuous state to a continuous state, creating a rigid three-dimensional network structure. Good compatibility can considerably delay phase separation and thus improve storage stability, which significantly impacts the performance of the final modified asphalt [15
]. To enhance the compatibility of PE/EVA composites with asphalt, attempts were made to add DCP and KH-570 to induce crosslinking reactions between PE/EVA composites via heating to form a three-dimensional network structure in the asphalt phase [22
which has the advantages of high strength and thermal stability and thus is often used to improve the mechanical properties of polyolefins, can further enhance the deficiencies in the mechanical properties of PE/EVA-modified asphalt [24
]. The three-dimensional network structure formed by the crosslinking reaction of PE/EVA composites may encapsulate CaCO3
, achieving a uniform dispersion of CaCO3
in the asphalt. As a result, the toughness, stiffness, and heat resistance of the PE/EVA-modified asphalt are improved. At the same time, the polarity of the PE/EVA increases, which significantly improves the dispersion of the PE/EVA in the base asphalt and further improves the compatibility of the PE/EVA with the asphalt. The addition of excess PE and EVA can disrupt the state of colloidal equilibrium in the base asphalt due to the spatial organization and arrangement of the SARA fractions present in the base asphalt themselves [26
], so the choice of adding furfural extraction oil can make up for the loss of lighter components in the modified asphalt, allowing the colloid to return to equilibrium and avoiding the problem of an excess of PE/EVA that is not very compatible with the asphalt [28
In a comprehensive consideration of the high- and low-temperature performance of PE/EVA-modified asphalt, this study utilized asphalt modified with 4% PE and 4% EVA to investigate the effect of furfural extract oil, DCP, KH-570, and CaCO3 on the rheological properties, fatigue resistance, and storage stability of PE/EVA-modified asphalt. The aim was to completely solve the high-temperature storage stability of PE-modified asphalt while further improving the high-temperature deformation resistance and fatigue resistance of PE/EVA-modified asphalt, providing a theoretical basis for a non-dissociation preparation scheme for PE/EVA-modified asphalt. The effects of furfural-extracted oil, DCP, KH-570, and CaCO3 on the conventional physical properties of PE/EVA-modified asphalt were determined using orthogonal experiments and grey correlation analysis, and the rheological properties and fatigue resistance of PE/EVA-modified asphalt were investigated using dynamic shear rheology (DSR) tests. DSR tests include a frequency sweep at 25 °C, multiple stress creep and recovery (MSCR) at 64 °C, and a linear amplitude sweep (LAS). Furthermore, the phase distribution between the polymer modifier and asphalt was investigated using fluorescence microscopy (FM).
In this study, the effects of different ratios of furfural-extraction oil, DCP, KH570, and CaCO3 on the conventional physical properties of PE/EVA-modified asphalt, as well as the rheological properties at medium and high temperatures, were explored. In addition, fluorescence microscopy was used to assess morphology and microstructure. A research method for the preparation of PE/EVA-modified asphalt without segregation was obtained. The following conclusions can be drawn.
(1) Based on orthogonal experiments and grey correlation analysis, the factors that affected the softening point, flexibility, and penetration were furfural extract oil, CaCO3, DCP, and KH-570, in that order. However, KH-570 had a greater effect on the storage stability of samples than DCP. Furfural-extracted oil improved the ductility and needle penetration of PE/EVA-modified bitumen and reduced the softening point and softening point difference. The addition of CaCO3 increased the softening point and decreased the ductility and needle penetration, but the effect on the difference in softening point was not significant, depending on the degree of cross-linking of PE and EVA. It is worth noting that, when 1.4% furfural extraction oil, 0.03%DCP, 0.01%KH-570, and 0.05%CaCO3 were added, the difference in softening point was reduced by 12.3 °C, the ductility increased by 6.3 cm, the softening end increased by 2.2 °C, and the penetration decreased by 4.6 (0.1 mm) compared to when no filler was added.
(2) MSCR tests showed that the addition of DCP significantly improved the elastic recovery of PE/EVA-modified asphalt under repeated negative traffic loading at high temperatures. The addition of an appropriate amount of KH-570 further improved the viscoelasticity of the modified asphalt at high temperatures, but too large or too small a proportion of DCP to KH-570 had a negative impact on the viscoelasticity of the modified asphalt. More importantly, higher viscoelasticity is beneficial for enhancing the resistance of the asphalt mix to permanent deformation at high temperatures.
(3) The sweep tests showed that the shear deformation resistance of PE/EVA-modified asphalt was improved and the elasticity was enhanced at 0.01–0.03% of DCP and KH-570 in the case of vehicle speed increase at a medium temperature. The increase in furfural-extracted oil slowed down the degree of transition from viscosity to elasticity of the modified asphalt. LAS tests showed that KH-570 significantly improved the wear resistance and enhanced the fatigue life of the PE/EVA-modified asphalt. dCP significantly improved the elasticity of the PE/EVA-modified asphalt, leading to a reduction in fatigue life.
(4) The phase distribution of the polymers in the asphalt was examined using fluorescence microscopy. Under the condition of DCP as initiator, the addition of KH-570 can induce PE and EVA to cross-link, and its fine filamentous mesh structure can wrap CaCO3, promoting the uniform dispersal of CaCO3 in the asphalt phase and significantly improving the storage stability of PE/EVA-modified asphalt at high temperatures.