# Preparation and Properties of Waterborne Polyurethane and SBS Composite-Modified Emulsified Asphalt
## 1 Introduction  
The increase in road construction activities correlates with a proportional rise in greenhouse gas emissions, contributing significantly to global warming. To mitigate greenhouse gas emissions, various industries are implementing diverse measures; for example, the road construction industry is striving to control its carbon footprint. One effective measure is the utilization of cold mix asphalt (CMA) in road construction to reduce greenhouse gas emissions. CMA is produced by mixing emulsified asphalt with unheated aggregates, maintaining a manufacturing temperature range of 0-40 \({}^{\circ}\)C. This process eliminates the need for heating and significantly reduces energy consumption [1]. Additionally, the production of CMA requires minimal investment in equipment, facilitating widespread adoption. CMA is particularly suitable for use in remote areas, both for initial construction with 100% original mix and for recycling asphalt pavement using reclaimed asphalt pavement (RAP). The primary advantages of CMA include high cost-effectiveness, environmental friendliness, low emissions, and ease of production. However, the use of water in CMA preparation and the absence of heating can affect the interaction between the binder and the aggregate, leading to decreased adhesion and various pavement issues. Repeated vehicle loading and environmental factors such as water, temperature, and ultraviolet radiation can cause common pavement failures, including permanent deformation (rutting) at high temperatures, cracking at low temperatures, and fatigue cracking [2; 3; 4]. Due to its inferior stability and durability compared to hot mix asphalt (HMA), CMA is mainly used for low-grade roads. Researchers are currently exploring various methods to enhance the road performance of CMA. Emulsified asphalt, a water-based material, relies on the performance of its residue after water evaporation [5]. To enhance the mechanical properties of emulsified asphalt, extensive research has been conducted both domestically and internationally. For instance, adding cement has been explored to improve the performance of emulsified asphalt [6]. However, the interaction mechanism between emulsified asphalt and cement is complex, with factors such as pH and concentration influencing cement hydration, potentially causing instability or cracking of emulsified asphalt [7; 8; 9]. Polyurethane (PU) has emerged as an asphalt modifier due to its unique molecular structure. [PERSON] et al. studied the preparation and properties of PU-modified emulsified asphalt and found that an appropriate PU content significantly improved penetration and ductility compared to those of SBS-modified emulsified asphalt, enhancing both high- and low-temperature ductility [10]. However, the production of commonly used PU prepolymers and PU elastomers poses significant environmental and safety risks. Waterborne polymer emulsions with low environmental impact and self-crosslinking capabilities, such as waterborne epoxy resin (WER), waterborne polyurethane (WPU) [10; 11; 12], and acrylate emulsions [13; 14], represent another class of materials for improving emulsified asphalt performance. Due to the limitations of single modifiers in meeting the traffic and environmental demands of existing asphalt pavements, composite modification technology has gained attention in the modification of emulsified asphalt. Water-based additives are water-soluble polymer systems stabilized in water through surfactants or by connecting hydrophilic polar groups, resulting in self-emulsification [15; 16]. As water evaporates, polymer droplets coalesce, initiating self-crosslinking reactions. This forms an interconnected polymer network, rendering the water-based additive amorphous and highly crosslinked [17], with strong bonding and high strength [18]. These properties make them suitable for use as coating materials [19], composite material adhesives [20], and fiber adhesives [15]. [PERSON] et al.