Microcracks are easily generated inside polymers and polymer composites during their service in structural applications.Development and coalescence of microcracks would bring about catastrophic failure of the materials and then reduce their lifetimes.Therefore

early sensing

diagnosis and repair of microcracks become necessary for removing the latent perils.In this context

the materials possessing self-healing function are ideal for longterm operation.Attempts have been made to develop solutions in this filed

among which the route based on microcapsulation seems to be promising.Self-repairing polymer composites containing microencapsulated healing agent have attracted more and more research interests.The present article analyzed the existing problems and proposed the criteria for manufacturing qualified microencapsulated healing agent.Accordingly

microcapsules containing epoxy resin were successfully fabricated by in situ polymerization in an oil-in-water emulsion with poly(melamine-formaldehyde) (PMF) as the shell material and high-activity lowviscosity diglycidyl tetrahydro-o-phthalate (DTP) as the core substance.The typical synthesis procedures are described below.About 50 g DTP was added into 250 mL aqueous solution of 2% styrene-maleic sodium (SMS).The mixture was vigorously agitated by a homogenizer for 10 min at a selected rate and then two drops of 1-octanol were added to eliminate surface bubbles of the emulsion.The pre-condensate of 12.5 g melamine and 27.1 g formaldehyde was prepared at 70～75℃ for 30 min

and the pH value of the solution was kept at about 9～10 by adding triethanolamine.Subsequently

the pre-condensate solution was added to the above emulsion with 450 r/min continuous mechanical stirring by a two-bladed stirring paddle while the pH value of the solution was adjusted by adding citric acid.Eventually

the reaction mixture was cooled down to room temperature.The resultant slurry was neutralized by 2% sodium carbonate solution

and then diluted with deionized water.The deposit of microcapsules was separated through a Buchner funnel and rinsed with deionized water and then vacuum dried.The synthesis method was improved by optimizing the processing parameters.The appropriate reaction time was about 30～60 min.Further extension of time had little influence on the products.The reaction temperature should be set at about 50～70℃.The pH value of about 3.2 was appropriate and had complex effect on the properties of the products.The appropriate core/shell mass ratio was about 2∶1～3∶1.The size and size distribution of microcapsules were mainly determined by the dispersion rate and the emulsifier content.Having been encapsulated

the core material DTP kept the same activity as its raw version.The microcapsules’ shell wall was relatively thin (ca. 0.2 μm)

which assured sufficient core content (83.2 wt%) even if the microcapsules were very small (ca. 6.7 μm).Moreover

the microcapsules were proved to be strong enough to survive handling during manufacturing self-healing composites.Having been preembedded in cured epoxy

the microcapsules had satisfied affinity to the matrix.They can be readily ruptured releasing DTP fluid as expected upon damaging of the composite

and activity of the released core material was the same as the raw material.Besides

the microcapsules had good sealing ability and high stability.They can be stored at room temperature for long time and could bear moderate or high temperatures applied for curing composites.The present work provided a new simple approach for mass-production of epoxy loaded microcapsules

which can be used to manufacture self healing composites or other self-curing stuffs like single constituent epoxy adhesive and self-locking threaded fasteners.