Study On Damage Indication And Repair Performance Of Epoxy Polymer Materials

Epoxy-based polymers have been widely used in fields of adhesives,coatings and composites,because they are with excellent mechanical,chemical and thermal properties,as well as easy molding and low cost.However,they are susceptible to damage and form many micro-cracks in their internal structures under external stress due to the lack of effective energy dissipation from the three-dimensional(3D)networks of epoxy polymers,which lead to a sharp decrease in their sustainability,safety and lifetime.Thus,exploring self-healing polymeric materials with damage indication is in high demand.In this paper,epoxy-amine polymers with damage indication and self-healing ability were prepared and the properties of damage indication and self-healing were studied.Based on the principle of amino-probe and the color change phenomenon,the damage indication ability of 2’,7’-dichlorofluorescein(DCF)for epoxy-amine polymer was identified and the indication mechanism was explored.The results showed that DCF could be dissolved in most of the healants and coexisted with them.Also,DCF could react fastly with amine curing agent.The color of DCF solution changed from light yellow to bright red when equivalent molar diethylenetriamine(DETA)was added,because amine group could capture hydrogen atoms of hydroxyl group on DCF molecules and make DCF molecules isomerized from lactone structure into quinone structure.When the epoxy matrix was damaged,the DCF solution was flowed out quickly and penetrated into the cracks in the epoxy matrix.Then DCF reacted with residual amines in the cracks,and the cracks showed a red color,indicating that damage had been occurred in the epoxy polymer.Meanwhile,the swelling content of the epoxy polymer had a greater influence than that of the residual amine on the damage indication ability.On the basis of the damage indication,an aliphatic amine latent curing agent(ALCA)with fast curing ability was screened and a repair system was prepared with glycidyl methacrylate(GMA)monomer.The healing property and mechanism of GMA/ALCA system were studied.The results showed that the GMA/ALCA healing system was stable at room temperature and reacted quickly at 80-120℃ to form a transparent epoxy polymer.The best curing condition for GMA/ALCA system was heated at 120 ℃ for 20 min.GMA could react with ALCA in a wide range of mixing ratios by the ring-opening addition and nucleophilic addition reaction,and the crosslinked epoxy polymers with better mechanical properties were formed,which were much higher than that of the epoxy/amine polymer.A maximum shear strength of 12.24 MPa was obtained when the weight ratio of GMA and ALCA was 100:50.The healing efficiency determined by tension experiment was above 95%and a toughness surface of the healed crack was observed.In order to endow epoxy polymer with repeated self-healing properties,an epoxy monomer and a series of epoxy polymers with thermal reversibility were prepared by embedding furan-maleimide reversible covalent bond into the epoxy polymers.The crosslink density,glass transition temperature and the mechanical properties of the epoxy polymers could be modified by changing the molar ratios of furan and maleimide group(F/M).The results showed that the glass transition temperature decreased with increasing the F/M molar ratio and a maximum shear strength of 1.80 MPa was achieved at the molar ratio of F/M=1.5.The repeated self-healing property was confirmed by the lap shear experiment and the optical observation.The healing efficiency was improved as the molar ratio of F/M increased and the healing temperature was gone up.The recyclability was confirmed by heating experiments and thermal gravity analysis.From the practical application,the epoxy polymer with damage indication and self-healing ability was studied.The mixture solution of DCF,DGEBA and EPA was encapsulated by poly(urethane/urea-formaldehyde)resin,and the spherical and stable microcapsules with particle sizes of 40-530μm and shell thicknesses of 200-850 nm were manufactured.By incorporating different contents of the microcapsules into the epoxy polymer,the epoxy composites with damage indication and self-healing functions were prepared.The addition of microcapsules with healing agents did not deteriorate the mechanical properties of the epoxy composites.The healing efficiency was above 70%by filling 20-30 wt%of the microcapsules into the epoxy polymer.