4(%3)3 - Ecole nationale supérieure de chimie de Montpellier

the lower stiffness and fracture toughness of PDMS compared to matrix polymer. In a furthermodification to above work, PDMS was employed as the matrix polymer while containing two differentsets of microcapsules 25 . One set of microcapsules contained a PDMS copolymer with active sites forcrosslinking while the other kind contained a high molecular weight vinyl functionalized PDMS along withplatinum catalyst (initiator capsule). Up to 75% healing efficiency was achieved with 10 wt% resincapsules and 5 wt% catalyst capsules.Encapsulation of epoxy resin in microcapsules as healing agent is also one of the alternativesinvestigated 26,69-71 . However to accomplish healing, a latent hardener has to be incorporated into thematrix. Upon rupture of the microcapsule, the epoxy would seep into the crack and polymerizes due tocontact with embedded hardener thus closing the crack. This approach shares the same advantage asthe PDMS/catalyst system that the healing material generated during the healing process is identical asthe matrix material itself chemically. This result in the deviation of tear path in the healed sample fromthat of the virgin tear path, resulting in greater than 100 % healing efficiencies. The epoxy/hardenersystem, first reported in 2007 71 employed an imidazole-metal complex as a latent hardener. Thesolubility of CuBr 2 (2-MeIm) 4 hardener complex in epoxy ensured a homogeneous distribution all over thematrix and thus capable of curing epoxy at any point in the matrix. The reported diameter range of themicrocapsules was 30 – 70 µm with encapsulated epoxy resin. The incorporation of the hardener and theepoxy microcapsules did not impact the mechanical properties of the matrix as such though fracturetoughness was found to be increased slightly. Using 10 wt% epoxy microcapsules and 2 wt% latenthardener, an impressive 111% healing efficiency was reported however the approach failed to qualify asa true autonomous healing system. The obvious reason is the inherent need to heat the samplesbetween 130 o C – 170 o C to initiate the curing of the epoxy resin. To make this approach fullyautonomous, the imidazole-metal complex hardener was replaced with low-temperature mercaptanhardener 34 in conjunction with a tertiary amine catalyst, encapsulated in microcapsules and embeddedalong with epoxy filled microcapsules in an epoxy matrix 35 . The microcapsules used for bothcomponents, were based on the melamine-formaldehyde due to instable nature of mercaptan hardener.The benzyldimethyl amine catalyst was infiltrated into mercaptan filled microcapsules in a secondarystep, since being basic in nature the catalyst couldn’t be incorporated during the microencapsulationprocedure which is performed in acidic medium. A healing efficiency of 104.5% was reported forspecimens containing 5 wt% (2.4 wt % each) capsules at 20 o C (Figure – 1.11a). It was observed thatmaximum healing occurred when the ratio of two microcapsules set is near 1:1. A minimum of 12 hours13