Using waste products in the production of bituminous binders and mixtures has become common due to the economic and environmental benefits. In particular, the application of recycled waste plastics in asphalt modifiers and admixtures has attracted more and more attention. This review presents an analysis and comparison of the different forms of waste plastics used for asphalt modification, as well as methods for incorporating waste plastics into asphalt mixes. It focuses on the properties of waste plastics, asphalt binders and asphalt mixtures. Overall, adding plastic waste to asphalt mixes can significantly improve high temperature performance with potential economic and environmental benefits. The performance of modified asphalt is closely related to several factors such as the waste source, the amount of waste plastic, the blending conditions and the pre-treatment methods of the waste plastic.
In addition, this paper discusses the current challenges facing waste plastic modified asphalt, including the stability of blends, low temperature properties, modification mechanisms, and laboratory issues. The use of chemical methods such as additives and functionalization is considered an effective way to achieve better contact between waste plastics and adhesives and to improve the full utilization of waste plastics. While both methods offer alternative options for producing waste plastic-modified asphalt with stability and high performance, the optimal ratio of materials used in the mix and the microscopic mechanism of mixed modified asphalt is not well understood and needs to be further explored. The widespread use of plastic brings great convenience to daily life and promotes economic and social development. However, about 50% of plastic materials are single-use materials such as packaging, agricultural films and single-use consumer products. 20% to 25% of plastics are used in long-term infrastructure such as pipes, cable coatings and structural materials. At the same time, the consumption of plastic in large quantities leads to the pressure of plastic waste management and ecological protection.
Currently, many countries have different management strategies for total municipal solid waste (MSW) effluents. Unfortunately, most of the world's post-consumer plastic materials are considered wasteful. In the United States, China and other countries, plastic waste has long been disposed of only by landfilling and incineration. As a traditional plastic waste disposal method, landfills have great demands on land resources, which has also become a major issue of plastic waste disposal in many countries. Long-term exposure to chemicals leaking from plastics into soil and water is a serious environmental hazard. Furthermore, the most serious problem is that this leaching of chemicals can take at least decades or even centuries, as most plastics are not biodegradable. On the other hand, incineration effectively reduces the volume and mass of plastic waste, but air pollution is generated during the heat treatment process in incinerators, releasing toxic substances such as carbon monoxide and dioxins. Waste plastics also contain heavy metals such as cadmium (Cd) and lead (Pb), which are released from soot and residues generated during incineration. In addition, improper disposal of plastic waste can lead to high plastic content in MSW Incineration Bottom Ash (MSWI-BA). Disposal of MSWI-BA leads to an increase in pollutants polluting water, soil and oceans.
Microplastics from plastic debris and corrosion are an even more serious public health concern because they are small enough to pass through waste filtration systems and difficult to remove. As a result, "plastic pollution" has become one of the most pressing environmental issues in the modern world. The recycling and use of plastic waste is considered a golden management strategy to reduce the environmental impact and consumption of natural resources. As awareness continues to grow about the harms of improper handling of plastics, the issue has also gained international attention. Countries around the world have introduced policies to ban the use of plastics. From 2008 to 2016, the consumption of plastic shopping bags in supermarkets decreased by more than 2/3, and the cumulative reduction of plastic shopping bags was approximately 1.4 million tons, which is equivalent to a reduction of about 30 million tons of carbon dioxide. At the same time, some waste plastic enterprises quickly adjusted, installed and operated the waste plastics industry recycling system, and supported the government's strategy, which has achieved results in my countries. Asphalt is a thermoplastic material that exhibits viscosity under most pavement operating conditions and thus plays an important role in pavement performance. The use of modified asphalt instead of raw asphalt has long been suggested as an effective method to improve the durability and reliability of asphalt pavement to meet climate, traffic and other requirements. It has been known for some time that virgin polymers can improve asphalt properties, particularly high temperature stability. But virgin polymer materials are hard to find and unconstitutional to use as modifiers.
The high construction cost, along with an awareness of environmental management, encourages the use of waste plastics in asphalt modification. Various studies and research projects have been undertaken to find suitable applications for the use of waste plastics in asphalt production, discussing the properties, modification mechanisms and environmental issues of waste plastics-modified asphalt. In general, it is desirable to increase the use of waste plastics in bitumen, as long as performance is not adversely affected. Based on evidence from previous literature, the use of waste plastics as modifiers in asphalt production can give virgin polymers properties similar to asphalt, which can greatly reduce manufacturing costs and protect the environment from additional pollution. Plastics are synthetic materials mainly obtained from refining crude petroleum products. Generally speaking, the main sources of waste plastic in the environment are plastic containers, plastic packaging and other common plastic industrial products. Specifically, most single-use plastics, such as bottles, packaging, and single-use products, are made of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and polystyrene (PS), while Long-term use plastic products are made of polyethylene terephthalate (PET), polypropylene (PP), polyvinyl chloride (PVC), ethylene-vinyl acetate copolymer (EVA), etc. In addition, the researchers also looked at other waste plastic types such as polyurethane (PU) and acrylonitrile butadiene styrene (ABS).