The role of liquid and solid polymers in bitumen emulsions: A microstructural and nano-mechanical investigation of homogeneity and performance

Ordinary bitumen emulsions often face significant challenges, especially when incorporated into cold asphalt mixtures, slurry seals, and microsurfacing. Their poor thermal storage stability and unsuitable viscosity limit their effectiveness. To address these issues, polymer-modified bitumen emulsions (PMBE)s have become a preferred alternative. This study assessed the homogeneity and stability of three PMBEs, using either liquid SBR latex and solid SBS/HDPE polymers (3% or 5% concentration). For solid polymers, the mixing temperature was reduced by 20 o C for energy efficiency, compensated by using small 0.3 mm particles to maintain production output. A comprehensive suite of laboratory techniques was employed to evaluate these PMBEs. The Sessile drop method was used for surface free energy (SFE) measurements, providing insights into their adhesion potential. Atomic force microscopy (AFM) revealed nanoscale phases, polymer distribution uniformity, nanoscale adhesion, and elastic modulus. Field emission scanning electron microscopy (FESEM) and X-ray energy dispersive spectroscopy (EDS) were used to analyze elemental composition and microstructural homogeneity. A new method was presented to analyze the homogeneity and stability of PMBE at microstructure. In this method, the maps obtained from the EDS were analyzed using image processing techniques with Python 3.13.5. The output of this analysis was a homogeneity coefficient at the microstructure scale, which was calculated for different PMBAs. SBR samples demonstrated the highest overall homogeneity, but a new method for SBS and HDPE using smaller particle sizes and lower mixing temperatures achieved comparable microstructural quality. SBR enhanced both the polar and non-polar SFE components, suggesting superior adhesion to aggregates, a finding supported by AFM analysis. Conversely, SBS proved more effective at increasing internal strength and nano-mechanical properties, with a 15% higher elastic modulus than the reference sample. Unlike the others, HDPE lowered the polar SFE component, making it a poor choice for adhesion but a strong performer for stiffness, with an elastic modulus over 70% greater than the reference sample. This makes HDPE ideal for dry climates where high stiffness is valued. A multi-criteria analysis, considering factors like adhesion and homogeneity, determined optimal concentrations to be 4.2% for SBR, 3.8% for SBS, and 3.4% for HDPE. − Presenting a new method based on EDS map analysis to evaluate the storage stability of PMBEs. − Modifying the mixing temperature of polymer-bitumen and the size of polymer particles to improve homogeneity. − Nanoscale analysis of bee structure to assess the homogeneity of PMBEs containing SBR latex, SBS and HDPE. − Optimizing the polymer content based on microstructural homogeneity, adhesion, and AFM elastic modulus.