Structural integrity and healing efficiency study of micro-capsule based composite materials via ¹H NMR relaxometry

Area of Science:

• Materials Science
• Composite Materials
• Non-Destructive Testing

Background:

• Microcapsules are crucial for self-healing in advanced aerospace composites.
• Assessing the structural stability and quantity of encapsulated agents is vital for performance.
• Existing methods may be destructive or lack in situ capabilities.

Purpose of the Study:

• To introduce a novel nuclear magnetic resonance (NMR) relaxometry approach for evaluating microcapsule structural stability.
• To correlate NMR signal intensity with the quantity of encapsulated self-healing agent.
• To assess microcapsule integrity under ambient and harsh environmental conditions, including thermal cycling.

Main Methods:

• Utilized 1H NMR spin-spin relaxation techniques on urea-formaldehyde (UF) microcapsules of varying diameters containing an epoxy healing agent.
• Performed NMR experiments during thermal cycling to simulate flight conditions.
• Employed a portable NMR spectrometer for in situ testing.

Main Results:

• Successfully correlated NMR signal intensity with the encapsulated self-healing agent mass, confirming it non-destructively.
• Demonstrated that decreasing capsule diameter increases the amount of healing agent but reduces shell integrity, leading to fragility.
• Identified 147 nm and 133 nm diameter microcapsules as having the most sensitive shells under thermal fatigue.

Conclusions:

• NMR relaxometry is a powerful, non-destructive tool for quantifying encapsulated agents and assessing microcapsule stability.
• Microcapsule size significantly influences structural integrity under thermal stress, critical for aerospace applications.
• The developed portable NMR spectrometer enables in situ testing, paving the way for quality control in microcapsule production.