Marshall Properties and Thermal Conductivity of Asphalt Mixtures Modified with Polyurea-Shelled n-Nonadecane Phase-Change Nanocapsules

The rise in the surface temperatures of flexible pavements, driven by climate change and urbanization, increases the need for materials with a modified thermal response while retaining acceptable mixture properties. In this study, a previously developed n-nonadecane capsule system with a polyurea (PUA) shell was incorporated into bitumen at 1, 3, and 5% by weight of the binder, and the resulting asphalt mixtures were evaluated at the capsule/binder and mixture scales. The capsule system was characterized by SEM and FTIR, and its thermal behavior by DSC and TGA; the effective thermal conductivity of compacted mixtures was measured between 20 and 40 °C with a KD2 Pro thermal analyzer using a KS-1 transient needle probe, and Marshall volumetric and mechanical responses were determined on three replicate specimens per group. DSC showed melting at 34.5 °C for pure n-nonadecane and at ~33.8 °C for the capsule, with total endothermic enthalpies over the 20–40 °C interval of 212.2 and 92.8 J/g, corresponding to an enthalpy-based encapsulation ratio of ~44%. TGA showed no appreciable thermal decomposition up to the asphalt mixing temperature, but this measurement does not establish mechanical capsule survival during mixing and compaction. The measured effective thermal conductivity decreased with both temperature and capsule content, from 1.76 W/m·K for the neat mixture to 1.31 W/m·K for PCM-5 at 40 °C. Marshall stability was higher than the control at all tested dosages and reached 14.01 kN at PCM-3 versus 10.42 kN for the control; however, the volumetric response also shifted with capsule addition, and the modified mixtures were not independently optimized at each dosage. Accordingly, the present results are interpreted as formulation responses under a common 6.0% binder-content condition rather than as proof of an optimized pavement design or field-scale thermal regulation.