## 4 Performance of Waste-Plastic-Modified Bitumen Binder and Mixture
### Characteristics of Plastic-Modified Bitumen Binder
#### 4.1.2 Enhancing Compatibility and Reducing Ageing  
The compatibility of plastic modifiers with bitumen is a crucial factor influencing the mechanical performance, storage stability, and durability of plastic-modified asphalts. Recent discoveries indicate that the photo-oxidative ageing of PE improves its compatibility with bitumen. PE oxidized for roughly 8 days demonstrated the maximum binding energy with bitumen due to the development of polar carbonyl groups. This oxidation facilitated molecular coupling and enhanced resistance to diffusion, hence enhancing stiffness and rutting resistance [41]. Compatibility issues emerge from differences in polarity, molecular weight, and crystallinity between plastic and bitumen. HDPE and PP, having high crystallinity, frequently demonstrate phase separation when stored at elevated temperatures, whereas LDPE and EVA offer superior compatibility because of their reduced crystallinity and branching [11; 28; 33]. The compatibility of polymers with bitumen is determined by numerous fundamental factors. Differences in polarity affect miscibility; non-polar polyolefins such as polyethylene and polypropylene demonstrate minimal chemical interaction with bitumen, leading to phase separation unless compatibilizers, such as maleic anhydride grafted polymers, are employed [28]. Higher molecular weight and crystallinity (as in HDPE) diminish chain mobility and diffusion into bitumen, whereas polymers with reduced crystallinity, such as LDPE and EVA, with branched structures enhance entanglement and dispersion [11; 28]. Microstructural analyses using Scanning Electron Microscopy (SEM) have revealed that incompatible blends typically display distinct polymer domains with clear phase boundaries, whereas compatible systems exhibit homogenous morphology characterized by integrated polymer networks that improve elasticity and thermal stability [33]. FTIR analyses further indicate that oxidized PE introduces polar carbonyl groups, enhancing chemical affinity and decreasing diffusion coefficients within the binder [41]. The findings have been validated through molecular dynamics simulations, which demonstrate enhanced interfacial adhesion and decreased separation energies associated with the presence of polar functional groups on polymer chains [41]. Consequently, strategies for enhancing compatibility encompass oxidation, chemical grafting, or the incorporation of reactive additives to optimize dispersion, storage stability, and long-term performance.  
Aminolysis-based PET additives such as Bis(2-aminoethyl) terephthalalamide (BAET) and Bis(2-hydroxyethyl) terephthalalamide (BHETA) enhanced binder adhesion and stripping resistance [42]. BAET, due to its superior terminal group structure, delivered more pronounced improvements compared to BHETA [43]. Hybrid systems combining Styrene Butadiene Styrene (SBS), recycled LDPE/PP, and sulphur (as a compatibilizer) showed up to 330% improvement in elastic recovery after UV exposure and reduced oxidation indicators (carbonyl/sulfoxide peaks) in FTIR analysis [44; 45].  
Overall, achieving optimal compatibility necessitates the meticulous selection of plastic type, pre-treatment procedure, and optional use of compatibilizers to ensure uniform dispersion and long-term performance in asphalt pavements.