Abstract
The mechanically-enhanced urea-formaldehyde (UF) microcapsules are developed through a multi-step in situ polymerization method. Optical microscope (OM) and field emission scanning electron microscope (FE-SEM) prove that the microcapsules, 147.4 μm in diameter with a shell thickness of 600 nm, are well-formed. From 1H-nuclear magnetic resonance (1H-NMR) analysis, we found that dicyclopentadiene (DCPD), a self-healing agent encapsulated by the microcapsules, occupies ca. 40.3 %(v/v) of the internal volume of a single capsule. These microcapsules are mixed with EPDM (ethylene-propylene-diene-monomer) and Grubbs’ catalyst via a solution mixing method, and universal testing machine (UTM) tests show that the composites with mechanically-enhanced microcapsules has ca. 47% higher toughness than the composites with conventionally prepared UF microcapsules, which is attributed to the improved mechanical stability of the microcapsule. When the EPDM/microcapsule rubber composites are notched, Fourier-transform infrared (FT-IR) spectroscopy shows that DCPD leaks from the broken microcapsule to the damaged site and flows to fill the notched valley, and self-heals as it is cured by Grubbs’ catalyst. The self-healing efficiency depends on the capsule concentration in the EPDM matrix. However, the self-healed EPDM/microcapsule rubber composite with over 15 wt% microcapsule shows an almost full recovery of the mechanical strength and 100% healing efficiency.
Highlights
With advances in materials research and technology, efforts to overcome the limitations of existing materials and industries are gradually increasing
The microcapsules had a rough and uneven surface (Figure 2b), which is a common feature found in UF microcapsules [12]
The calculated self-healing efficiency of the EPDM/microcapsule rubber composites with 5, 10, 15, and 20 wt% microcapsule loading were 72.0, 93.0, 98.3, and 97.4%, respectively (Figure 7c). These results revealed that the wounded EPDM/microcapsule rubber composites fully restored their intrinsic mechanical strength by the self-healing reaction when the composites included the microcapsule over 10 wt%
Summary
With advances in materials research and technology, efforts to overcome the limitations of existing materials and industries are gradually increasing. Microcapsules synthesized by the interfacial polymerization method are known to be mechanically stable These microcapsules have the disadvantage of being less efficient to induce self-healing because they include self-healing agents and materials used for shell formation [17]. Despite the relatively lower mechanical stability of the shell, the self-healing microcapsules manufactured by the in situ polymerization method are most commonly used because of the high purity of the contained-core material, which leads to high self-healing efficiency [18]. As mentioned earlier, these UF microcapsules have exhibited a poor mechanical stability because the shell of the UF microcapsule was very thin compared to the size of the capsule Due to these limitations, many problems occur, such as easy breakage when introducing a microcapsule into a matrix, or core healing material leaking during storage [22]. Considering the impact of rubber materials on the industry, it is anticipated that this study will serve as a cornerstone for the application of self-healing rubber to various industrial fields such as automobile, aviation, space, shipbuilding, et cetera
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