Plastomers are frequently employed as bitumen modifiers because they are less expensive than elastomers and have increased stiffness, which makes them more resistant to permanent deformation at high temperatures, or rutting. Phase separation caused by low polymer compatibility with bitumen, which results in two different phases, i.e., the asphaltene-rich phase and polymer-rich phase, is one of the disadvantages of using plastomeric modification. However, the storage stability is frequently a concern for chemically inert and non-polar polymers, such as polyolefins, but it is less of an issue when these plastics are mixed with polar substituents. By researching the interaction between polymers and bitumen, the mechanism of phase separation can be better understood. The polymer is inflated by the bitumen's maltene component during polymer-bitumen modification, forming a thermodynamically unstable and kinetically stable system. Due to the gravitational field's influence on this thermodynamically unstable system, phase separation occurs, which causes heavier asphaltene micelles to settle towards the bottom of mixes during hot static storage. 
When high-density polyethylene (HDPE) was employed to change the bitumen binder, Pérez-Lepe et al. discovered that the modification improved high-temperature performance but lowered storage stability, indicating that this kind of modification is less useful for pavement applications. Phase separation is undesirable and restricts how these polymers can be used for paving roads. As a result, attempts have been made to reduce or eliminate phase separation and improve the compatibility of the polymer with bitumen. The nonpolar chains of the polymers are to blame for the reduced compatibility of plastomers. By limiting or eliminating the non-polar groups, the compatibility can be increased. The addition of polar groups, for example through free radical polymerization with butyl acrylate or vinyl acetate, tends to increase the compatibility of polymers with bitumen. The best way to improve the compatibility of plastomers with bitumen is to add polar functional groups and substituents to the primary nonpolar backbone by copolymerizing (e.g., EVA) or grafting (e.g., MA-g-PE). On the other hand, it has been claimed that polar groups are more abundant in recycled plastics than in fresh polymers because the heating procedure used to recover plastic ages the material. 
Recycled plastics have improved compatibility with bitumen because the polarity of plastics increases with age. According to reports, factors influencing the phase separation of bitumen modified with polymers include storage circumstances including time and temperature, the kind of bitumen binder, and the characteristics and concentration of the polymer. The two most widely used plastomers are polyethylene and polypropylene; other plastomers include poly(ethyl methacrylate), ethylene-vinyl acetate, ethylene-butyl acrylate, polystyrene, and polyvinyl chloride. Long chain hydrocarbon polyethylene, produced by polymerizing ethylene, is a thermodynamically unstable, crystalline, and reasonably priced plastomer. Depending on co-polymerization or branching, which alters its density and level of crystallinity, it can take the form of high-density polyethylene (HDPE), medium-density polyethylene (MDPE), low-density polyethylene (LDPE), very low-density polyethylene (VLDPE), ultra-high molecular weight polyethylene (UHMWPE), and linear low-density polyethylene (L-LDPE). The popular PE known as metallocene-catalyzed PE (m-PE), which is used to make LLDPE and HDPE, has also been applied to bitumen modification. All different types of PE can be seen in a variety of objects, including HDPE, which is used in toys, pipes, and various household items like milk and shampoo containers, LDPE, which is used in containers and trays, reusable bags, and agricultural films, and L-LDPE, which is used in geomembranes and food packaging films. 
The density of HDPE is between 943 and 961 kg/m3, MDPE is between 926 and 948 kg/m3, LDPE is between 890 and 953, and L-LDPE is between 910 and 940 kg/m3. The melting points of HDPE, MDPE, LDPE, and L-LDPE are, respectively, 129-149 °C, 126-129 °C, 108-120 °C, and 124-128 °C, whereas the softening point is between 95 and 127 °C. These materials may be added to bitumen to create polyethylene-modified bitumen since the melting temperature is typically lower than the production temperature needed to make hot bitumen mixtures (i.e., 160–170 °C). The mixing conditions and the physical, chemical, rheological, and mechanical characteristics of virgin and waste plastomers modified binders. By weight of the binder, polyethylene is frequently mixed with bitumen at various percentages (0.1-6% for HDPE, 1-5% for MDPE, 2-10% for LDPE, and 4-6% for L-LDPE), with mixing temperatures ranging from 160 to 185 °C for HDPE, 165 to 170 °C for MDPE, 165 to 185 °C for LDPE, and 150 to 170 °C for L-LDPE; the mixing time is 0.5 For HDPE, MDPE, LDPE, and L-LDPE, the mixing speed ranges from 2500 to 4000 rpm, 3000 to 4000 rpm, 3000-5000 rpm, and 4000 rpm, respectively. The reported particle size for polyethylene-modified bitumen is 0.1-5 mm. The size of the polymer added to the binder is also thought to be a significant characteristic for homogeneous mixing. If the waste polyethylene is obtained directly from post-consumer streams, studies advise washing, drying, and extruding the material before trimming or grinding it; however, if the material is delivered clean, it can be immediately trimmed or ground to various sizes. 
Compared to post-consumer polyethylene, post-industrial polyethylene is typically cleaner and of a higher level of consistency. The addition of polyethylene alters the attributes of modified bitumen mixes. The in-service characteristics of asphalt mixtures, such as resistance to high temperature rutting, high temperature behavior, fatigue life, flexural stiffness, thermo-mechanical resistance, water resistance, adhesion, and elasticity, may be enhanced by the use of polyethylene-based polymers. Because crystallites may crosslink extended polymer chains and form a gel network, bitumen that contains polyethylene tends to crystallize when it is cooled down and tends to improve the glass transition peak of modified blends and the stiffness at high temperatures until the crystals melt. Because bitumen is polar and aromatic, polyethylene is said to be immiscible with it and interacts with bitumen less because of its propensity to crystallize. Despite being insoluble in bitumen binders, polyethylene-based polymers can nevertheless flow and distribute across the matrix of the binder (i.e., mechanical blending as opposed to chemical blending), which enhances the characteristics of the modified blend. To address the difficulties of low miscibility and compatibility of polyethylene-based polymers with bitumen, many procedures and techniques can be applied; grafting and chlorination, for example, are frequently used to disperse the polymer particles into the bitumen. 
Bitumen Plastomer
Properties and performance of plastomers-modified bitumen blends are significantly influenced by the blending circumstances and composition of the polymer and binder. For instance, the melt flow index, a polymer characteristic that depicts the architecture of the molecular structure of polymers and influences the physical and rheological properties of plastomers-modified bitumen, is an indirect indicator of molecular weight where higher MFI corresponds to lower molecular weight. It is hypothesized that a lower melt flow index led to a higher softening point, complicated viscosity, and lower penetration values, and that a higher mixing temperature raises the melt flow index, leading to an increase in the polyethylene-modified bitumen's performance. On the other hand, low mixing temperatures impair blend performance overall by resulting in compatibility and dispersion instability, as well as by lowering the melt-flow index. According to reports, the molecular weight and molecular weight distribution of polyethylene or polyethylene-based modifiers play a significant role in determining the phase separation, low temperature characteristics, and hot storage stability of polymer-modified bitumen. Modifiers made of polyethylene that have a wide molecular weight distribution and a low molecular weight are thought to be ideal for bitumen modification. The performance of polyethylene-modified bitumen is significantly influenced by the polymer concentration since high polyethylene concentrations (i.e., 5–15%) produce phase separation, hence 5% (wt%) has been proposed as the upper limit for pavement applications. 
For polyethylene and polyethylene-based polymers, several research revealed that the ideal weight percentages should be between 4% and 6%. The test displays the fluorescent pictures of polyethylene-modified bitumen captured by a UV microscope at various polyethylene concentrations. It shows the dispersed spots of bitumen blended with 2% polyethylene; as the concentration of polyethylene is increased from 2% to 4%, these scattered spots transform into a filamentous structure. As the concentration of polyethylene-based modifiers rises, the bitumen's softening point falls but penetration rises. According to Table 3, the reported softening point ranges for LDPE, L-LDPE, and HDPE-modified binders are 44-68.5 °C, 50-67 °C, and 51-79 °C, respectively, while the penetration values for LDPE, L-LDPE, and HDPE are 23.5-79.1 (0.1 mm), 13-64.7 (0.1 mm), and 21-36 (0.1 mm), respectively. However, depending on the source and kind of binder, neat bitumen's typical softening point and penetration values are 42–65 °C and 59.1–98 (0.1 mm), respectively. Greater consistency of the blends is shown by the increase in softening point and decrease in penetration with the inclusion of polyethylene-based modifiers, presumably indicating increased resilience to long-term high-temperature deformations. When polyethylene-based modifiers are used with a 3% content, the penetration value is lowered by 7% to 42%, whereas when polyethylene-based modifiers with a 5% content, the penetration value is reduced by 22-63%. A 2–50% rise at 3% polyethylene content and a 14–91.5% increase at 5% polyethylene loading were seen in respect to the softening point. 
Another crucial property of bitumen is its viscosity, which can improve performance at high temperatures (i.e., flow resistance), but can also make the asphalt mix difficult to work with and difficult to construct. According to a report, conventional, unmodified bitumen has a lower viscosity than polyethylene-modified bitumen. Due to the higher polymer-dominant phase in high-content polyethylene-modified bitumen, viscosity also rises when the concentration of polyethylene-based modifiers is increased. Contrarily, adding polyethylene-based modifiers reduces ductility, suggesting that polyethylene-modified binders may behave brittlely at low temperatures. When polyethylene was added, it was found that the ductility decreased by up to 97% at 15 °C and up to 35% at 25 °C. According to the polyethylene-modified bitumen's rheological properties, a high modifier concentration leads to significant increases in the complex shear modulus and a decrease in phase angle values, which point to a shift toward more elastic responses and increased stiffness. In their MSCR investigation of LLDPE-modified bitumen, Nizamuddin et al. discovered that the addition of LLDPE greatly boosted the percentage recovery while significantly decreasing the Jnr value. However, compared to larger percentages of LLDPE, the effect was more pronounced at low percentages of LLDPE (3% and 6%). 
Trans-polyoctenamer was added by Beena and Bindu to LDPE, and the modified samples' percent recovery and Jnr values were analyzed. The study discovered that, at lower additive percentages, Jnr value increased and percent recovery decreased, whereas the opposite was seen at higher trans-polyoctenamer additive concentrations. Gama et al .'s study of the MSCR analysis of HDPE modified binder revealed that while the neat binder's percentage recovery was approximately 2.4% at 3.2 kPa and 15.7% at 0.1 kPa, adding HDPE increased these values to 91.5% at 3.2 kPa and 95.8% at 0.1 kPa, respectively. This improved modified binder was found to maintain good elastic properties at high traffic levels and temperature. According to several studies, polyethylene modified bitumen has poor storage stability because it separates during storage into two distinct phases (polymer-rich phase and bitumen-rich phase) even after a short time. However, some studies have discovered that bitumen modified with polyethylene can be regarded as stable at low to intermediate polyethylene content levels, such as 5% of LLDPE [66] and 3% of HDPE or LDPE, by weight of bitumen. In comparison to LDPE and HDPE, LLDPE has generally been found to be more successful in enhancing the binder characteristics and low temperature performance. Although the HDPE modification is susceptible to phase separation issues because HDPE disperses less readily into the bitumen phase than LDPE, it frequently produces bitumen that is stronger than LDPE. 
LDPE-modified bitumen was shown to have greater low-temperature fracture toughness than unmodified mixtures, indicating improved resistance to low-temperature fracture. Additionally, 2.5% of LDPE was added to increase Marshall stability, fatigue life, robust modulus, and moisture susceptibility. Because of its low cost, good dispersion characteristics, mechanical features, physical properties, and increased storage stability, metallocene catalysed polyethylene (m-PE) has been suggested as a bitumen modifier. It has been proposed that metallocene catalysis regulates the molecular structure and molar mass distribution, giving polymers a uniform distribution of short chains and a narrow molar mass distribution. As a result, the melt elasticity is decreased and bulk qualities like crystallinity and viscosity are tuned, which increases the dispersion. To validate the prospective use of these polymers as bitumen modifiers, Spadaro et al. blended two different grades of metallocene catalyzed LLDPE with bitumen. It was found that the inclusion of metallocene-catalyzed LLDPE improved the modified bitumen's thermal and mechanical properties. By increasing the polymer concentration, the glass transition temperature was dramatically lowered, pointing to benefits of low temperature flexibility. 
At greater polymer concentrations, the modified bitumen mixes displayed relatively low dynamic shear viscosity. Another study examined the rheological and thermal properties of LLDPE and HDPE that had undergone metallocene catalysis. Blends of metallocene-catalyzed LLDPE and metallocene-catalyzed HDPE appear to be miscible, as indicated by the linear fluctuation in zero shear viscosity, relaxation time, and frequency findings across the weight fraction range. González et alresearch .'s study examined the viscoelastic properties and storage stability of metallocene-catalyzed LLDPE/bitumen mixes. By adding 1-3% metallocene-catalyzed LLDPE to bitumen at 180 °C, 1800 rpm, and for six hours, the metallocene-catalyzed LLDPE/bitumen blends were created. By preventing phase separation at high storage temperatures, it was discovered that LLDPE that had been metallocene-catalyzed had superior storage stability.