Mitigating Stripping in Asphalt Mixtures: Pretreatment of Aggregate by Thermoplastic Polyethylene Powder Coating

Abstract

Thermoplastic polyethylene (PE) powder coating is proposed as a new technique to pretreat the aggregate of asphalt mixtures and improve the corresponding moisture resistance. In this paper, ethanol solution is investigated as a fluidized bed to disperse the PE powder. The surface tension of the solution, indicating the wettability of the liquid phase, decreases with increasing ethanol concentration and temperature. The PE powder could be stably dispersed into the solution when 15% ethanol solution heated to 40 to 50°C is used to reduce the surface tension of liquid to less than 40 mN/m. The suspension carrying the PE particles could be uniformly sprayed onto a hot aggregate surface (180–190°C) where the liquid phase evaporates fast while the PE powder melts and stays. The powder coating technique could prevent the aggregate from lumping and achieve a high-quality and efficient coating for aggregate. The PE monolayer model considering a hexagonal close packing of PE on aggregate surface is tentatively proposed to determine the theoretical amount of PE required to perfectly coat the aggregate. The predicted PE content was reasonably lower than that determined by the experimental results considering the ideal monolayer coating was hard to achieve. However, the modeling results could be used to determine the lower bound of the required PE content. The optimum PE content significantly increased with increasing particle size of PE and specific surface area of aggregate. The use of high-density polyethylene (HDPE) powder with a smaller particle size was more effective in the coating process. In addition, excessive PE could not further enhance the moisture resistance of asphalt mixtures efficiently. This study can contribute to both the recycling of waste plastic and the construction of durable asphalt pavements.

Waste Plastic in Asphalt Mixtures via the Dry Method: A Bibliometric Analysis

“…(d) Other methods: Xiao et al [22,69] sprayed powdered LDPE, using a fluidized bed of water and ethanol, over pre-heated aggregates at 180ºC. The chosen pre-heating temperature was way higher than the WP's melting point to ensure that the LDPE would melt and coat the aggregates.…”

“…Another crucial property is the particle size and shape of WPs. Plastic can be added to HMA in many shapes with varied sizes, such as powder (0.035-0.25 mm) [21,22], pellets (5-10 mm) [23], fibers (6.5-30 mm) [24][25][26], flakes (0.63-10 mm) [27][28][29][30], or granules (0.3-0.6 mm) [31,32]. WPs can also go through some processes for size reduction, such as shredding, grinding, crushing, and cutting [10,[33][34][35].…”