Chemical Modification and Performance Evaluation of Eucommia ulmoides Gum as a Natural and Sustainable Energy Resource and Its Application in Road Engineering

Eucommia ulmoides gum (EUG), a sustainable plant-derived natural polymer, was functionalized via three distinct routes, including vulcanization, epoxidation, and hydroxylation to yield vulcanized (VEUG), epoxidized (EEUG), and hydroxylated EUG (HEUG), respectively. We systematically characterized the effects of modification route and degree on the chemical structure, crystallization behavior, thermal stability, hydrophilicity, and mechanical properties of functionalized EUG and further evaluated the high/low-temperature performance, microstructure, and mechanical properties of the corresponding modified asphalt binders (VEMA, EEMA, HEMA) as a function of modifier type and loading. For VEUG, C-S cross-linking networks formed during vulcanization suppress EUG crystallization, enabling a rigid-plastic to elastic transition, while high-temperature cleavage of C-S bonds reduces its initial thermal stability. For EEUG, epoxidation breaks C=C double bonds and introduces epoxy groups to strengthen intermolecular interactions; subsequent ring-opening grafting of hydroxyl groups onto EEUG yields HEUG, which forms additional cross-links via dynamic hydrogen bonds. Increasing modification degree for both EEUG and HEUG reduces their number- and weight-average molecular weights with narrower distribution, diminishes crystallinity, enhances thermal stability and hydrophilicity, and drives a rigid-plastic to elastic transition, characterized by decreased strength (0.65 MPa < σHEUG < σEEUG < 10.18 MPa) and markedly improved ductility (143.6% < εEEUG < 262.0%, 679.9% < εHEUG < 1360.3%). In asphalt binders, VEUG’s cross-linked network endows VEMA with refined more abundant bee-like microstructures, drastically boosting high- and low-temperature performance: relative to pristine EUG-modified asphalt (EUGMA), VEMA’s DMT modulus decreases by 94%, and adhesion increases by 87%. EEMA forms covalent bonds with polar asphalt components via epoxy groups, while HEMA constructs a hydrogen-bonded cross-linked network; both effectively inhibit asphaltene aggregation. With increasing modifier loading, EEMA and HEMA exhibit increased modulus, reduced adhesion, and gradually improved high- and low-temperature performance, except for the non-significant high-temperature enhancement of HEMA at higher loadings.