Mechanical properties of epoxy composites embedded with functionalized urea-formaldehyde microcapsules containing an oxidizable oil

Nowadays using composites reinforced by embedded microcapsules containing reactive agents are a promising approach to enhance the performance of materials. In the current study, urea-formaldehyde microcapsules (UFMs) containing an oxidizable oil phase were synthesized and were modified by 3-aminopropyltriethoxysilane (APTES). Different mechanical analyzes such as impact, compression and three-point testing were conducted. The results showed that adding a critical amount of silane (2 wt%) caused to toughen the epoxy composites. The maximum toughness in the case of UFM-5%Func, is 55% greater than the neat epoxy. The crack length was increased from 9.88 to 11.49 mm in the presence of the functionalized UFMs. SEM images of the fractured surface showed the tail and hackle markings, leading to increase the energy absorbed by the composite during crack growth, thereby increasing the resulting fracture toughness. Moreover, the functionalized UFMs were fractured and the shell of the microcapsule was left in the matrix, while in the case of non-functionalized UFMs, the microcapsule was completely gone-off the matrix. Additionally, the self-healing behavior was investigated by means of an applying a scratch, which showed that the oil is released from the UFMs. •Urea-formaldehyde microcapsules (UFMs) containing oil were synthesized and modified by a silane coupling agent.•The influence of the UFMs surface functionalization on the mechanical behavior of the composites was investigated.•The fracture surface of different composites was explored to identify the toughening mechanism.•The self-healing behavior was investigated by incorporation of a crack, showing the oil release from the UFMs.