For bitumen alteration, both virgin and, more recently, recycled polymers have been used successfully. Evidently, due to the recycling initiatives, many nations are putting in place in response to mounting waste problems like disposal, environmental concerns, and health concerns regarding plastic wastes, research for using recycled plastics in the bitumen industry has significantly increased over the past ten years. The disposal of waste plastics is one of the most significant concerns that is becoming more and more of global concern. Other health concerns include circulatory, respiratory, and lymphatic system problems for transport with eventual deposition in the kidney, gut, and liver. The environmental effects include entrapment and destruction of wildlife habitats, risk of ingestion, and transportation of organisms to the ecosystem by plastic. There is currently a lot of focus on using waste-recycled plastic polymers in roadways in order to reduce the health and environmental difficulties caused by this waste while still enhancing bitumen performance. Because governments are investing substantially in recycling and green technologies, it should be highlighted that research on waste plastics in bitumen began about 20 years ago even though it did not receive much attention until lately. 
The tensile strength, water resistance, durability, and general service life of bitumen roads have been proven to noticeably enhance research investigations employing waste plastics. Additionally, recycling, using eco-friendly technologies to build road pavements, and using plastics in the bitumen business have all been suggested to reduce carbon emissions by as much as one-third. Although it depends on the methodology used (wet or dry), a more recent LCA (life cycle assessment) study indicated that recycled plastics in bitumen can be advantageous. The wet methodology was associated with larger environmental advantages. Plastomers, elastomers, and chemically functionalized thermoplastics are among the three types of polymers utilized as bitumen modifiers based on their chemical compositions and characteristics. Although it is true that polymer modifiers increase bitumen's resistance to heat susceptibility, each type of polymer has a unique impact on the material's characteristics. For instance, whereas elastomer polymers enhance the elastic characteristics (resistance to fatigue) of bitumen, reactive and plastomer polymers tend to increase stiffness and resistance to deformation due to load. Reactive polymers are said to increase the compatibility of the polymer with bitumen as well as reduce the need for additives in order to stabilize the polymer phase. The mechanical properties, temperature sensitivity, and storage stability of the modified binder are frequently improved by reactive polymers. Reactive and non-reactive polymers were compared in terms of their effects on the rheological characteristics of modified binders by Navarro et al. 
They discovered that reactive polymers, particularly at low frequency and intermediate temperature, produced an evolution in rheological properties (G' and G"). Additionally, the reactive polymer modified binder maintains homogeneity, providing superior storage stability as a result. Plastomers, one of the three types of polymers used to modify bitumen, are typically less expensive, have a high stiffness at high temperatures, and are hence resistant to permanent deformation. Common plastomers also have melting points that are lower than those required to create hot bitumen mixes. The following section elaborates on recycled plastic waste as a potential modification for bitumen because recent research has focused on recycled plastic trash as opposed to virgin plastic for use on roads. With no other options for disposal or recycling, plastic usage has been rising significantly globally over the past few decades, severely polluting the environment. Only 130,000 of the 586,300 tons of plastics produced in Victoria, Australia, in 2017 were recycled, while 7200 tons (1.2%) were burned for energy recovery and the remaining 449,100 tons (76.6%) were disposed of in landfills. According to projections, Victoria would produce 100,000 tons more plastic every four years. According to the US Environmental Protection Agency (EPA), 34.5 million tons of plastic were produced in the country in 2015. Despite a local citizen recycling rate of 75%, only 3.14 million tons of plastic were recycled, while 5.35 million tons of plastic were burned for energy recovery, and 26.01 million tons of plastic were dumped in landfills (75.4%). The disposal of polymer and plastic wastes through recycling is recommended as preferable to other options including composting, incineration, or landfilling. Mechanical recycling and chemical recycling are the two ways that polymers are recycled. Unaltered plastics can be recycled mechanically and used to create new items.
Collection, sorting, shredding, washing or decontaminating, extrusion, quenching, and pelletizing are just a few of the processes involved in mechanical recycling. As an alternative, waste plastic can be recycled chemically to create energy or a source of feedstock for fuels and chemicals. Pyrolysis and gasification are two techniques for recycling chemicals. Mechanical recycling, on the other hand, is the technique for recycling plastic that is most frequently used; the outputs typically include clean and recycled resins in pellet form. The collection of post-consumer and post-industrial plastic waste is the first step in the recycling process. To meet ISO criteria for environmental safety, manufacturing and production organizations frequently use recycling companies' collecting services as a method of disposal. Post-industrial plastic wastes are normally recycled more effectively since they are gathered from each individual company and typically come from the manufacturing of particular polymers that don't need to be further sorted. In order to set up collection programs for local residents to collect both general and recyclable household garbage, plastic recycling companies frequently get licenses from local authorities through kerbside collection. Recycling businesses can also get their hands on plastic garbage at drop-off locations for municipal waste collection that are overseen by councils and where residents can dispose of their own domestic plastic waste in lieu of curbside pickup. Prior to kerbside collection, post-consumer plastic garbage is separated in various countries based on the appropriate identification categories. Instead of post-consumer, post-industrial plastic streams account for the majority of the plastics that are recycled and reused globally. The used plastic that is shredded after it is collected. Before being cleaned or decontaminated, large pieces of plastic are frequently shredded into smaller chunks and flakes. A set of revolving blades with distinct grids for size gradation and an electric motor driving them are used in the shredding process. 
To make coarse, amorphous plastic flakes, materials are put into the shredder. High-end industrial shredders can be reserved for particular polymers because waste plastics come in a variety of shapes and are often categorized as rigid and flexible plastics. However, because of their flexible and film-like nature, flexible plastics call for certain shredders. When film polymers tear, pollutants are more likely to be carried by film pockets. Cleaning and disinfection are necessary before moving on to the next step in the recycling of plastic. Plastics are divided into various types after decontamination. Monitoring density differences has traditionally been a strategy used by large-scale producers to separate polyolefins from ordinary trash. Plastics with lower densities than water, such as LDPE, LLDPE, HDPE, and PP, are examples of common polyolefins. As a result, garbage can be dissolved in water in a big tank, and whatever material floats to the top can be collected. Through X-ray fluorescent (XRF), which allows for the identification of chlorine within the plastic [80], plastics like PVC are separated. Hyper Spectral Imaging Technology is provided by optical sorters using a number of Near Infra-Red (NIR) cameras. The NIR wavelengths emit particular molecular vibrations upon coming into touch with the plastics that reveal particular chemical compositions. However, optical sorters can only distinguish between a limited number of plastic kinds, such as PET, HDPE, PP, and PE. The excess is typically categorized as "mixed." Transferring electrons from one particle to another is a different way to sort polymers. With the electrostatic sorting technique, plastics are fed onto a conveyor belt that is tailored to the materials. The plastics acquire a positive or negative charge when they enter a high-tension field, and then, in accordance with the various charges on each individual plastic particle, they are electrostatically separated into pure sorted fractions. Waste plastics are extruded after being sorted. Extrusion is used to homogenize and transform recycled plastics into easy-to-work-with materials. 
Plastic is molded through a die mold and driven along a tubular pipe as part of the process. The plastics must be in the shapes of flakes, powder, or pellets and are introduced by a feeder. Depending on the needed output size, the diameter of the Archimedes screw might vary. Similar to that, depending on the desired product outcome, numerous die mold shapes can be produced and used. To ensure that polymers are heated to the ideal temperature and can be shaped appropriately, heating coils are put on the exterior of the tubular pipe. The process of extrusion comes before quenching, in which the plastics are cooled before being pelletized.
- Alteration
The structural characteristics of the pelletized polymers are ultimately determined by the cooling rate. The technique of "quenching," which hardens and cools plastics quickly, does not permit the alteration of molecular orientation because there is not enough time for the chains to move freely and form crystalline zones. The two basic techniques used in this process are quenching with water and quenching with gases. Gas quenching quickly cools the plastics without oxidation to produce a higher-quality product than water quenching, which requires placing the polymers in a cold water bath. The cost of gas quenching is higher than that of conventional water quenching. On the other hand, if the plastics are cooled slowly, crystallization starts to take place, which enables the plastics' molecular orientation to take on a more organized and defined form. Pelletizing, the last step in recycling, includes feeding hardened plastics via an in-feed at a constant line speed and cutting them into rough cylindrical pellets with a rotor and a bed knife. The rotational speed of the blades will affect the pellets' size. The extruder's form, however, will determine the pellets' shape. Following post-treatment procedures, plastic pellets may require more drying if they have had a water quench, or further cooling if they have undergone a slow cooling procedure, packaging, and storage.
In order to enhance practices of construction and maintenance and reduce harm to the environment, road pavement technicians and scientists are adopting new techniques and approaches in tandem with the rapidly expanding asphalt sector. Evidently, over the past ten years, there has been a considerable surge in studies into using recycled plastics in the asphalt sector. Studies on the subject of employing plastics on roads have discovered considerable benefits in the road's mechanical and physical properties, including, but not limited to, tensile strength, water resistance, durability, and overall life span. In a physical analysis experiment, Khurshid et al. examined the impact of adding recycled LDPE and HDPE for bitumen modification. Following the mixing process, it was discovered that LDPE had a limited solubility in bitumen at mixing temperatures and that HDPE was insoluble in bitumen even at 0.5% by weight, producing a non-homogeneous mix with noticeable particles. As a result, HDPE was dropped from the experiment, making LDPE the sole type of polymer used. The added LDPE enhances the overall stiffness and increases the penetration value, according to the results. In comparison to conventional bitumen, LDPE modified bitumen showed a 16% decrease in penetration value at 2% polymer content. Overall, the findings showed that adding LDPE increased the softening point, flash point, and fire point, enabling a higher degree of resistance to high temperatures. Shopping bag LDPE was utilized by Nouali et al. to improve binder properties.
The results showed an increase in penetration index value and a 15% improvement in softening point while decreasing temperature susceptibility. However, the storage stability was poor at high temperatures because of the bitumen phase's low compatibility with waste polymers. The research was also done in this study comparing recycled LDPE-modified bitumen to regular bitumen in an asphalt mix. According to the findings, the recycled LDPE-modified bitumen had increased stiffness modulus, complex modulus, and water resistance by 13%, 20%, and 11%, respectively. Overall, it was established that the recycled-LDPE modified bitumen had enough compaction and workability for bitumen applications. Another study modified polymers using recycled PE, PP, and PS. Among the three chosen polymers, it was discovered that PE aggregate samples had the best resistance to plastic deformation. The use of recycled PE enhanced the bitumen mixture's rigidity by 60%. The many types of plastomeric polymers (either virgin or recycled) from these categories that have been extensively used as a bitumen modifier in prior studies are covered in depth in the following section.