A Brief Review on Modification of Poly(urea-formaldehyde) Microcapsules to Improve the Performance of Self-healing Composite Systems

Nowadays, the microencapsulation of materials is used extensively in differentfields and industries including agriculture, food and pharmaceuticals, coatings,and cosmetics. One of the most important applications of microcapsules is preparationof self-healing polymer composites. Poly(urea-formaldehyde) (PUF) microcapsules havereceived a lot of attention due to their suitable properties in the preparation of thesesystems. However, in some cases, these microcapsules do not have the ability to interactproperly with the polymer matrix. As a result, they reduce the mechanical properties ofthe matrix as well as prevent the proper transfer of applied stress to the microcapsules(and thus reduce self-healing performance). Modification of microcapsules in order toreinforce the interaction of the matrix-microcapsule as well as improving their propertiesis an effective approach to decrease the mechanical properties reduction and improvestress transfer to the microcapsules. According to the literature, the modification methodsof PUF microcapsules can be classified into 4 main groups including; modificationvia silane coupling agents, modification during synthesis, modification by usingnanoparticles, and preparation of multishell microcapsules via layer-by-layer depositionmethod. In this review, microcapsules and factors affecting their effectiveness, methodsof encapsulation, and microcapsule based self-healing systems are explained briefly.PUF microcapsules and their modification methods are also discussed.