Waste Plastics as Asphalt Modifier
The widespread use of plastics, while bringing convenience to daily life and fostering economic and social development, has resulted in a significant environmental challenge. Approximately 50% of plastic materials are single-use, contributing to packaging, agricultural films, and disposable consumer items. Another 20 to 25% is utilized in long-term infrastructures like pipes, cable coatings, and structural materials. This extensive plastic consumption has led to a critical issue in plastic waste management and eco-environmental protection.
Most countries employ various strategies for municipal solid waste (MSW), but unfortunately, a large portion of postconsumer plastic materials worldwide is treated as waste. Landfill and incineration, the traditional methods of plastic waste disposal, have detrimental environmental consequences. Landfills, receiving 27 million tons of plastic in the US alone in 2018, demand extensive land resources and pose long-term risks of chemical leaching into soils and waters. Incineration, while reducing volume and mass, leads to air pollution and the discharge of heavy metals.
Improper handling of plastic waste further results in a high content of plastics in MSW incineration-bottom ash (MSWI-BA), adding contaminants to water, soil, and oceans. Microplastics, originating from plastic fragmentation, present a serious concern for public health, being challenging to remove from waste filtration systems.
Recognizing the urgency of the plastic pollution problem, countries worldwide have introduced policies to ban disposable plastic products. By 2020, many countries had already implemented such measures. This shift in awareness and policy has led to a significant reduction in the consumption of plastic shopping bags, showcasing a positive impact on carbon dioxide reduction. Some plastic waste enterprises have adapted by establishing recycling systems, contributing to a notable increase in plastic waste recycling rates.
In China, for example, a waste plastic industry recycling system has proven effective, leading to a 30% recycling rate for plastic waste in 2019. As the dangers of improper plastic treatment gain international attention, the recycling and utilization of plastic waste emerge as crucial strategies to mitigate environmental impact and natural resource depletion. While challenges persist, there is a growing global effort to address and curb the plastic pollution crisis, making it one of the most pressing environmental issues of our time.
(1) Waste LDPE: LDPE, with its unique polyethylene chain, enhances asphalt properties by forming three-dimensional structures. Studies show its effectiveness in asphalt modification, with LDPE from recycled sources being widely used. Research indicates that 10% LDPE offers superior resistance against rutting compared to other additives.
(2) Waste HDPE: The high crystallinity of HDPE poses challenges in asphalt compatibility but results in higher stiffness, viscosity, and improved moisture resistance. Studies highlight its higher stiffness but note drawbacks in resilience and creep recovery compared to other modified asphalts.
(3) Waste PP: Recycled PP enhances high-temperature performance in asphalt but reduces ductility and fatigue cracking resistance. Suitable for high-temperature areas, waste PP-modified asphalt requires improved viscosity.
(4) Waste PVC: Waste PVC increases viscosity and stiffness, improving rutting resistance. However, caution is needed due to the potential release of hydrogen chloride during heating, leading to air pollution.
(5) Waste PET: PET, a highly recycled plastic waste, enhances high-temperature performance when used for dry modification. Studies show improved performance with 30% and 50% PET dosage.
(6) Waste PS: Waste PS-modified asphalt exhibits higher rigidity, especially in warmer climates, improving rutting resistance. However, concerns arise regarding its performance in colder areas and the release of harmful substances at high temperatures.
(7) Waste EVA: Waste EVA has good compatibility with asphalt, acting as a reinforcing bar and enhancing high-temperature stability, low-temperature cracking resistance, and viscosity. Suitable for various asphalt improvements, including low-temperature performance.
(8) Waste ABS: ABS, common in electronic devices, improves asphalt viscosity and blending temperatures, decreasing rutting susceptibility. While having better storage stability than other modifiers, waste ABS-modified asphalt shows performance comparable to unmodified asphalt.
(9) Waste PU: Both synthetic and thermoplastic PU enhance asphalt properties, particularly at high temperatures. Waste PU-modified asphalt exhibits good deformation resistance, aging resistance, fatigue resistance, and high-temperature storage performance.
In summary, waste plastics like LDPE, PP, EVA, ABS, and PU demonstrate good compatibility with asphalt, impacting high-temperature stability and viscosity positively. Each plastic type has specific advantages and considerations, emphasizing the need for further research based on dosage and asphalt composition.