Capsule-based healing systems in composite materials: a review

Abstract Composites are used in a variety of applications due to their excellent properties. However, structural polymers are sensitive and susceptible to thermal and mechanical damage in form of micro-cracks, which are onset to grow deep within the structure where detection and repair are practically impossible. To overcome these problems, broad range of self-healing structures have emerged. This technology has led to an increase in the material’s lifetime and safety while reducing the repair and replacement costs. Capsule-based healing systems are a well-known technology that has many uses in smart protective coatings, dental composites, concrete components, and generally for polymer and fiber-reinforced composites. This article summarizes the research work on the capsule-based self-healing system over the last two decades. In this regard, after a brief introduction, various types of microencapsulation-based methods used in healing systems are classified. After explaining the manufacturing process of capsules, parameters affecting the microencapsulation quality particularly, agitation rate, core to shell weight ratio, monomer viscosity, solvent property, reaction time, temperature, pH, and U/F ratio are explained in detail. Finally, the most common healing efficiency evaluation methods are described. This review provides the reader with an overview of achievements to date, and insight into future development for industrial and engineering applications. Graphical Abstract Abbreviations 2MZ-Azine 2,4-diamino-6[-2-methyl-imidazolyl(1)]-ethyl-cis-triazine 2PhI 2-phenyl Imidazole BGE N-butyl Glycidyl Ether CAI Compression After Impact CB Carbon Black CC Compliance Calibration CNS Calcium hydroxide (Ca(OH) ) Nano-spherulites CNTs Carbon Nanotubes DCB Double Cantilever Beam DCM Dichloromethane DCPD Dicyclopentadiene DGEBA Diglycidyl Ether of Bisphenol A DTHP Diglycidyl Tetrahydro-o-Phthalate EDA Ethylenediamine ENB Ethylidene Norbornene EPA Ethyl Phenyl Acetate FCG Fatigue Crack Growing FRP Fiber-Reinforced Polymer GHS Globally Harmonized System of the Classification and Labeling of Chemicals GO Graphene Oxide HGFs Hollow Glass Fibers IPDI Isophorone Diisocyanate MBT Modified Beam Theory MCC Modified Compliance Calibration MF Melamine-Formaldehyde MWCNT Multi-Walled Carbon Nanotube NaCMC Carboxymethyl Cellulose O/W Oil-in-Water PA Phenyl Acetate PAA Phthalic Anhydride PAANa Sodium Polyacrylate PCL Polycaprolactone PCP Polycyclopentadiene PDA Polydopamine PDMS Poly (Dimethyl-Siloxane) PEA Polyetheramine PhCl Chlorobenzene PMCs Polymer Matrix Composites PMMA Poly (Methyl-Methacrylate) PMUF Poly (Melamine-Urea-Formaldehyde) PU Polyurethane PVA Polyvinyl Alcohol ROMP Ring-Opening Metathesis Polymerization SEM Scanning Electron Microscope SENB Single-Edge Notched Bending SIFs Stress Intensity Factors SWCNT Single-Wall Carbon Nanotube TDCB Trapped Double Cantilever Beam TGA Thermogravimetric Analysis UF Urea-Formaldehyde UFM UF Microcapsules W/O Water-in-Oil W/O/W Water-in-Oil-in-Water Nomenclature Healing efficiency Critical stress intensity factor (Mode I) Crack length Crack opening displacement Specimen length Specimen width Specimen thickness Critical fracture load Young's modulus Compliance Critical energy release rate (Mode-I) Internal work (Strain energy) Number of Fatigue cycles