Synthesis and Characterization of Melamine-Urea-Formaldehyde Microcapsules Containing ENB-Based Self-Healing Agents

Microcapsules for self-healing applications were produced with a melamine-urea-formaldehyde (MUF) polymer shell containing two different healing agent candidates, 5-ethylidene-2-norbornene (ENB) and ENB with 10 wt.-% of a norbornene-based crosslinking agent (CL), by in situ polymerization in an oil-in-water emulsion. Relatively neat outer surfaces with minor roughness were observed on the MUF microcapsules under optical and scanning electron microscopy. Shell thickness of the capsules ranged from 700 to 900 nm. Particle size analysis of the microcapsules showed narrow size distributions with a mean diameter of 113 μm for ENB-filled and 122 μm for ENB + CL-filled microcapsules at an agitation rate of 500 rpm. The microcapsules were found to be thermally stable up to 300 °C and exhibited a 10 to 15% weight loss when isothermally held at 150 °C for 2 h from thermogravimetric analysis. Overall, these MUF microcapsules exhibited superior properties compared to the urea-formaldehyde (UF) microcapsules used extensively for self-healing composites to date. In addition, the manufacturing process of MUF microcapsules is much simpler than those made from UF. Additional advantages of MUF microcapsules for self-healing composites are discussed.