CAPSULE-BASED SELF-HEALING EPOXY SYSTEMS USING
DIELS-ALDER REACTIONS
G.R. Palmese1, P.A. Pratama1, A.M. Peterson1 and M. Sharifi1
1
Department of Chemical and Biological Engineering, Drexel University, Philadelphia, PA 19104, USA – e-mail: palmese@coe.drexel.edu
Keywords: epoxy, Diels-Alder, solvent, capsule, self-healing ABSTRACT
We present recent results on the use of Diels-Alder reversible reactions for the design of self-healing polymer systems. Previous work has shown that solutions of maleimide functional monomers can be used to heal cracks of furan functionalized epoxy networks. Two topics will be discussed. First, the results of an investigation regarding the influence of solvent selection and maleimide type as well as concentration on healing efficiency, and second, our progress encapsulating solutions containing maleimide-based healing agents to create capsule-based self-healing systems that rely on Diels-Alder chemistry.
The effects of maleimide healing agent structure, concentration, and solvent type on the effectiveness of solution-based healing agents for a furan-functionalized epoxy-amine thermoset were investigated. These factors were found to influence healing behavior by affecting two mechanisms: (1) Diels-Alder bonding across the crack surfaces and (2) mechanical interlocking promoted by solvent-induced swelling. Aliphatic and aromatic maleimides with varying functionality were studied. The solvents evaluated were toluene, dimethyl formamide (DMF), and phenyl acetate. The maleimide healing agent structure and concentration were found to affect the degree of Diels-Alder interfacial bodnding, while solvent selection was found to influence both healing mechanisms.
A urea-formaldehyde (UF) encapsulation technique was used to encapsulate phenyl acetate – maleimide solutions. The resulting capsules were found to be thermally stable up to 90°C. It was not possible to encapsulate DMF solutions, and solutions using toluene resulted in low capsule yield. Phenyl acetate – maleimide capsules were incorporated into a furan functional epoxy creating a material system that heals autonomously. Compact tension specimens prepared with 10 wt.% capsules showed 75% strength recovery 24 hours after first fracture at ambient temperature. This compares favorably with the direct injection of healing solution into the crack. The new capsule healing system based on Diels-Alder chemistry also provides the opportunity for repeated healing cycles.
