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۱٫ Introduction

The self-healing materials, inspired by biological systems, exhibit the ability to repair minor damage without the need for detection or any type of manual intervention. Self-healing materials incorporated in various polymeric products are increasingly used in structural applications in the space, automobile, defense and construction industries. These smart materials can significantly increase the lifetime and safety of polymer-based products and components [1–۴]. While several extrinsic (capsule-based, vascular) and intrinsic self-healing strategies have been investigated, self-healing based on microencapsulated healing agents is more likely to be successful in the near future because microcapsules can easily be incorporated into the polymer matrix using existing blending techniques without changing the original chemical structure of the polymers. In addition, this self-healing can heal large-volume cracks [5–۸]. Typically,mechanicaldamage inamaterial ruptures the microcapsules and releases healing agents into the damaged area. Then,the crack is repaired by polymerization via the reaction between different agents. Recently, several efforts have been made to apply self-healing materials to automobile interiors and exterior components such as plastic panels or paint because emotional engineering has become importantfor customer satisfaction [9–۱۱]. For example, if an automobile user encounters a scratch in the skin layer ofthe instrument panel (which consists of an outermost skin layer, interlayer and core layer), the visual and tactile emotion of the user may worsen. Among various polymeric materials in automobiles, PU resin consisting of a urethane bond ( NH COO ) has mainly been used for the skin layer because it can decrease the scratch probability due to its intrinsic elasticity [12]. However, external damage such as scratches on the PU-based skin layer is still unavoidable, which may degrade the user’s emotional state. To protect the PU layer from scratches, two approaches have been explored. The first is enhancing the scratch resistance of the skin layer by using nanosized ceramic particles to improve the hardness of the clear coating and the second is recovering or healing the scratches by enhancing the elastic property of PU resins [13–۱۶]. However, the first approach is not effective once a scratch has occurred, and the second approach may also be limited depending on the extent of the scratch. Therefore, the incorporation of microcapsule-based selfhealing materials into PU resin may be a better way to minimize the presence of scratches and subsequently prevent customer dissatisfaction. PU-based skin layers are produced by either reaction injection molding (RIM) or powder slush molding (PSM) processes, which typically involve high shear stress and/or high temperature [17,18]. Furthermore, in order for self-healing microcapsules to incorporate into PU resin, a severe washing and subsequent drying process should be conducted because moisture (water) can react with PU resin and generate urethane foam with a rough surface. Therefore, self-healing microcapsules must have adequate mechanical and thermal properties to withstand the molding and washing/drying processes, which can be achieved by increasing the shell thickness or introducing inorganic materials into the shell [19–۲۳]. In this study, a PU-based matrix with a self-healing capability was prepared by incorporating two different microcapsules into the matrix for future application in the skin layers of instrument panels in automobiles. One microcapsule was filled with poly (dimethyl siloxane) (PDMS) and platinum catalyst (Pt) blend, and the other microcapsule was filled with a crosslinker. To improve the mechanical and thermal properties, additional urea-formaldehyde (UF) layers were deposited on the newly-prepared poly(ureaformaldehyde) (PUF)-based microcapsules, and the microcapsule size was tuned. After analysis of various microcapsule properties, the self-healing capacity of PU matrix was evaluated.

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