In this article, we tried to discuss bitumen modifiers such as a polymer. Bituminous pavements collapse as a result of high traffic volumes and weather-related factors like humidity, temperature, and UV radiation. A lot of work has been done in recent years to improve the bitumen's material qualities, ultimately improving field performance and lengthening the service life of the pavement. The most common and among the best methods is regarded to be polymer modification. The most widely used polymers for bitumen modification are elastomers, chemically functionalized thermoplastics, and plastomers. Despite the fact that in polymer science, the term "plastomer" refers to a polymer with the simultaneous behavior of an elastomer and plastics (thermoplastics), this paper uses the term "plastomer" to refer to a thermoplastic polymer as it is more frequently found in civil and pavement engineering. Although some of them are more prone to phase separation and the ensuing storage instability, plastomers have a number of benefits and are frequently recognized to increase high-temperature stiffness. 
Many road authorities are considering the usage of recycled plastics in roadways today as a result of the recent drive for recycling. As a result, rather than originating from chemical businesses, some of the readily available plastomers—in pellet, flakes, or powder form—come from materials recycling operations. With an emphasis on recycled plastics, this review article discusses the specifics of using plastomers as bitumen modifiers and how they may be utilized to improve bitumen performance and road longevity. This paper also discusses chemical additives for enhancing the compatibility of plastomers with bitumen. Plastomers are discovered to have excellent rigidity at high temperatures, either individually or in combination with two or three polymers. For bitumen modification, a variety of polymers including HDPE, LDPE, LLDPE, MDPE, PP, PS, PET, EMA, and EVA have been used with success. However, as the study in-depth discusses, each of them has advantages and disadvantages of its own. New light has been shed on the use of virgin and recycled plastomers for bitumen modification as a low-cost and possibly environmentally advantageous solution for roads and pavements with the recent push toward the usage of recycled materials in roads. 
By-product of the petroleum industry, bitumen (or asphalt binder) is created by distilling crude oil. It has beneficial qualities like extended durability, good adhesion, and water resistance, which made it possible to use it as a road construction material. In order to create the asphalt mixture, bitumen is combined with aggregates in the construction of roads. The effectiveness of the bitumen binder has a big impact on how long asphalt mixtures last overall and on their performance characteristics. The majority of asphalt pavement failures are directly related to asphalt binder failure, which can occur due to thermal cracking at low temperatures, rutting at high temperatures that causes bitumen to soften and lose elasticity, or fatigue cracking at intermediate temperatures brought on by cyclic loads and aging of the pavement. Many efforts are made to prevent failures because bitumen pavement maintenance and repair are undesirable for socioeconomic and economic reasons. Numerous studies examined how bitumen may be altered to produce pavements with increased durability and superior quality. Polymer modification of bitumen is regarded as one of the most suited and widely used approaches among all researched modification techniques. To create polymer-modified bitumen, the polymer is either mixed into the bitumen through a chemical reaction (wet process) or mechanical mixing (dry process). 
For optimum chemical and physical interaction between the constituents, the polymer and bitumen are immediately blended at high temperatures for a predetermined amount of time in the wet process. The wet technique, for example, infuses plastic in the form of flakes, pellets, or powder into hot bitumen when the polymer is refined from waste plastics. Plastics chosen for the wet technique often require melting temperatures below the stated range, with typical mixing temperatures of 160–170 °C. In general, bitumen that has been modified with polymers using a wet method can be made more elastic, adherent, cohesive, and stiff, which increases durability, fatigue life, and rut resistance. The polymer is introduced directly into the aggregates at the start of the mixing process in the dry process since the bitumen and polymer are not premixed. However, several studies have used low-temperature melting point plastics with the dry method as a pre-coating of the hot aggregate before adding the bitumen. Recycled plastic is incorporated into the bitumen mix as a substitute for the aggregate when using waste plastics in the dry method, and it is suggested that its melting temperature is above the bitumen mixing temperature. 
The latter is occasionally referred to as the "mixed" method. The dry process minimizes the number of aggregates that must be utilized in the entire mixture, allowing for a higher percentage of plastics to be employed. According to studies, using this procedure enhances the road pavement mix's stiffness, fatigue life, and Marshall stability qualities. However, a number of drawbacks have also been noted for both wet and dry approaches, which are further discussed in the following sections. The finished blend's general qualities and the cost of the operation as a whole are greatly influenced by the blending/mixing process, whether it be chemical or mechanical. For instance, the increased blending temperature may cause the resulting blend to undergo primary aging, which can lead to polymer degradation and the oxidation of bitumen's maltene components (i.e., the low molecular weight compounds). The compatibility of the two phases is also impacted by the bitumen and polymer's different polarities and molecular weights. Additionally, polymer-modified bitumen blends are susceptible to phase separation when being stored at high temperatures and are not always thermodynamically stable. While polymer-modified bitumen has many advantages, there are also some drawbacks and difficulties that must be taken into account, such as the high cost of virgin polymers, resistance to aging, sensitivity to high temperatures for some types of polymer-modified bitumen (such as wax-modified bitumen), low elasticity, and poor storage stability. Numerous techniques and technologies, including sulfur vulcanization, saturation, the addition of antioxidants, functionalization, the use of reactive polymers, and the exploitation of hydrophobic clay minerals, have been proposed to address these drawbacks. 
Polymer Modifier
The polymer modifier and modification of bitumen has been the subject of several review articles; some of them concentrated on the impact of mixing conditions and the resulting modified bitumen properties, while others concentrated on the applications of elastomers—the most widely used polymers in the road sector—and reactive polymers. To the best of the authors' knowledge, however, there is very little in-depth information available on the comparison and use of waste (recycled plastics) and virgin plastomers for road applications. Therefore, the major goal of this work is to present a thorough review of virgin plastomer modification of bitumen for pavement applications and waste (i.e., resulting from recycling operations). The following section provides a quick overview of bitumen's history and physio-chemical properties. Additionally, evaluations are made on polymer incorporation techniques with bitumen. It is explored how plastomer-modified bitumen differs in terms of its chemical, thermal, rheological, structural, and mechanical properties. There is also an explanation of the procedures that result in the production of recycled plastics. Finally, plastomer-modified bitumen is discussed critically, and conclusions are drawn along with suggestions for future research projects. Although some of the significant literature studies from before 2010 are also cited in this study, a systematic literature review methodology was used to systematically review and collect a large amount of the literature reported in the most recent decade (2010–2020). Three research databases, including Scopus, Web of Science, and Google Scholar, were used to conduct the literature review. 
Bitumen, bitumen modification, polymer modified bitumen, PMB, virgin and recycled polymers used for bitumen modification, PMB performance, hybrid polymer, and chemical modifiers are some of the most pertinent keywords used to search the literature. A well-known engineering substance, bitumen is produced by fractionally distilling crude petroleum oil. Although bitumen's chemical makeup is diverse and complex, it is typically broken down into four general fractions: asphaltenes (80% carbon, 15% hydrogen), resins, saturates, and aromatic hydrocarbons (80% carbon, 15% hydrogen). Depending on the country of origin and refining method, bitumen typically contains 11.9–15.8% saturates, 39.6–53.1% aromatics, 22.8–34.8% resins, and 10.3–12.1% asphaltenes. Table 1 lists the parameters of conventional paving bitumen that have been recorded in various studies, albeit these numbers may vary globally depending on the bitumen supplier. The test also includes a list of the SARA (Saturate, Aromatic, Resin, and Asphaltene) composition and other fundamental attributes of conventional bitumen. Asphaltenes, a portion of bitumen that is black in color and insoluble in n-heptane, and maltenes, a mixture of resins, aromatics, and saturated compounds that are soluble in n-heptane, can be generally categorized as SARA components of bitumen. Bitumen has been used for a variety of purposes due to its characteristics, including as an adhesive, preservative, sealant, waterproofing agent, and a building material for roads and airports. According to reports, 85% of bitumen use is used to create pavements of various types; yet, in its unaltered standard form, bitumen still struggles to function properly in some conditions because of its heat susceptibility. 
This recommends using methods to improve bitumen performance, like polymer modification. The market for bitumen with synthetic polymer modifications has increased since they significantly improve a variety of bitumen qualities. About 75% of elastomers, 15% of plastomers, and 10% of shredded auto tire rubber and other materials are the most popular polymers used to modify bitumen. Over the last few decades, the idea of combining two or more materials—with properties that are entirely different from those of the parent materials—to create a new product for paving applications has been put into practice. The chemical, electrical, mechanical, rheological, and other qualities of the product are thought to be determined by the phase behavior that results, measured in terms of homogeneity. Entropy and heat of mixing are factors that affect the homogeneity and miscibility of mixtures. By including compatibilizers, cross-linking agents, and managing the phase morphology during the blending process, bitumen polymers' miscibility or homogeneity can be improved. The following qualities are advised for substances used to modify bitumen: not degrading at the temperature at which bitumen mixtures are produced; maintaining good chemical compatibility with bitumen; increasing deformation resistance and reducing thermal susceptibility; and being physically and chemically stable by not changing their characteristics during transportation, storage, processing, and other operations. The overall performance of the bitumen mixture, stiffness at high temperatures, moisture resistance, increased fatigue life, and resistance to cracking at low temperatures are all improved by polymer additives because they improve bitumen's mechanical properties, bitumen-aggregate adhesion, and temperature susceptibility. 
However, the specific characteristics of the polymer and bitumen, the amount and kind of polymer added to the bitumen, as well as the blending procedure, all have a significant impact on the final qualities of polymer-modified bitumen. The features of polymer-modified bitumen are closely related to the reactivity and chemical structure of particular polymers, which also affect their compatibility with bitumen. Different characteristics of the polymer and bitumen, as well as the bitumen itself, affects how well the two are compatible. Phase separation may happen when polymers and bitumen are combined because of the high molecular weight of the polymers and the insufficient maltene fractions for solvation. Phase separation results in the development of heterogeneous mixtures, which have poor storage stability and bitumen and polymer compatibility. These characteristics include, among others, molecular weight, density, solubility, and polarity. The storage stability of bitumen modified with polymers is a reflection of the compatibility between polymers and bitumen, with higher storage stability and simpler handling at the bitumen plant being produced by higher polymer-bitumen compatibility. Although polymers can enhance bitumen performance, mixing bitumen and polymers still presents some difficulties because of higher processing temperatures and specialized facilities, which raises plant costs and causes phase separation because of poor polymer-bitumen compatibility and, occasionally, high polymer content. A bitumen modifier is advised to maintain the following properties: I being highly soluble in bituminous mixtures to produce a viscous mixture that maintains homogeneity during storage; being highly resistant to water, thermal stresses, and ultraviolet radiation; not releasing harmful substances into the environment, and being widely accessible.