Volatile organic compounds (VOCs) inhibition and energy consumption reduction mechanisms of using isocyanate additive in bitumen chemical modification

Tens of millions of tons of polymer-modified bitumen are annually used in the construction of asphalt pavement worldwide, which induces serious volatile organic compounds (VOCs) emission and energy consumption associated with high working temperatures. To achieve cleaner production of high-performance bitumen, this study investigated the feasibility of using liquid isocyanate-based additive in bitumen modification. In comparison with the conventional styrene–butadiene–styrene (SBS) modified bitumen, mechanical and performance-related properties of isocyanate-modified bitumen were examined for practical application. The VOCs inhibition and energy consumption reduction mechanisms were interpreted based on the experimental and quantum-chemical investigations. The result shows that bitumen modification occurs through isocyanate's reactions with bitumen components. Compared to the widely used SBS-modified bitumen, at least a 20 °C reduction in bitumen production temperature, a 60% reduction in VOCs emission, and a 5% reduction in energy consumption can be achieved by isocyanate modification during the bitumen preparation, heating, and mixing processes. During isocyanate modification, both resins and asphaltenes are preferential targets for the chemical reactions with isocyanate. In this process, chemical crosslinking can occur at relatively low temperatures, which can also help reduce the existence of single polycyclic aromatic hydrocarbon (PAH) molecules and increase the difficulty of VOCs molecules' escaping. The findings not only provide a further understanding of the environmental performance of isocyanate-modified bitumen but also offer an impetus for future research.