Self-healing properties of epoxy resins with poly(ε-caprolactone) healing agent

Thermally induced healing through thermoplastic poly(ε-caprolactone) (PCL), dissolved in 12.5, 25, 37.5 and 50 wt%, respectively, in amine-cured epoxy resins (EPs) was studied on compact tension (CT) specimens. Aromatic (hydrogenated diglycidyl ether of bisphenol A-type) and aliphatic (glycerol-triglycidylether) EPs were cured with the same amine (Jeffamine D 230) to receive EPs with different glass transition temperatures (Tg). Tg values of the parent EPs were lower (Tg = 32 °C) and higher (Tg = 90 °C), respectively, than the melt temperature (Tm ≈ 60 °C) of the PCL. The curing-induced phase separation morphology of PCL was studied by light microcopy. Additional information on the phase structure was deduced from dynamic mechanical analysis. Blending with PCL reduced the Tg of the corresponding EPs. Fully broken CT specimens were repeatedly healed at 80 °C which was close to or higher than the actual Tg of the EP/PCL blend. It was found that the transition of PCL from disperse to continuous phase depends not only on the PCL amount, but also on the EP type and its curing. EP/PCL systems with semi-interpenetrating network structure (bi-continuous) exhibited markedly higher healing efficiencies compared to those in which PCL was present as disperse phase. The healing efficiency depended also on the temperature difference between the healing temperature and Tg of the EP with respect to Tm of PCL. When Tg > Tm then the related difference should be kept small, while for Tg < Tm the temperature difference should be large to support healing. Accordingly, the segmental mobility within the cross-linked EP network is a key parameter for thermal mending, too.