Preparation And Performance Of Self-healing Epoxy Resin Matrix Composites Using Microcapsules Coated By Melamine Urea Formaldehyde Copolymer

The micro-cracks of composite have obvious impact on the safe use of materials, therefore, it is important to resolve and repair the internal micro-cracks of materials. Microcapsules and latent curing agent were implanted into matrix material by micro capsulation technology. When the micro-cracks were produced in matrix materials, the microcapsules ruptured and cross-linked with hardener to achieve self-healing effect.A binary self-healing system with E-51 ethoxyline resin and latent hardener of phthalic anhydride was used. The key of achieving self-healing was to synthesize epoxy microcapsules with regular morphology and uniform particle size. By the situ polymerization, the microcapsules of E-51 epoxy resin core were prepared with urea-melamine-formaldehyde copolymer as the wall material. The effects of process parameters on the surface morphology and properties of microcapsules were investigated to optimize the synthesis condition and obtain the best scheme. The results show that the optimal molar ratio of urea, melamine and formaldehyde is 5: 1: 13 for preparing MUF prepolymer. To obtain microcapsules with uniform size and dense spheres, the thickness ratio of core and wall is 1.2:1 by sodium dodecyl benzene sulfonate as emulsifier with mass fraction of 1% of core material, and the stirring rate is 600r/min with acidification pH of 4.2 and acidification time of 2.5h. The effects of stirring rate on particle size and performance of microcapsules were investigated. The median diameter of microcapsules is 22.48 um with uniform particle size and little adhesion, which meets the experiment requirement. The particle size distribution and the size of microcapsules were determined by laser particle size distribution analyzer, and the structure of microcapsules was analyzed by infrared spectroscopy. The formation and the surface morphology of microcapsules were observed by scanning electron microscopy and optical microscopy, and the thermal stability of microcapsules were determined by comprehensive thermal analyzer. The results show that the prepared microcapsules are uniform spheres with dense surface and moderate intensity, and the thermal stability is good with less adhesion and good boiling resistance.The curing mechanism of acid anhydride for epoxy resin was analyzed. The microcapsules with various contents and the latent curing agent were implanted into epoxy resin matrix material, and the self-healing efficiency was determined. The results show that the implantation of microcapsules decreases the impact strength of matrix material. When the added latent curing agent content is less than 2%, the impact strength is increased with latter decreasing. When the contents of the microcapsules and the curing agent are 4% and 1%, respectively, the impact strength of self-healing composites reaches the maximum value of 15.63KJ/m2. The self-repairing efficiency of the matrix material is increased with the content increasing of microcapsule and curing agent. By scanning electron microscope, the polymer particles can be observed on the sample cross section after self-healing process, which states that polymer is generated after microcapsules rupture, crosslink and solidify to verify the self-healing function of the prepared composite materials.