Preparation And Application Of Waterborne Self-healing Polyurethanes

During the fabrication and utilization process,polyurethanes are prone to suffer micro damage or micro cracks,leading to macroscopic damage which reduce the reliability of materials.Inspired by nature,self-healing materials have attracted much attention owing to the intriguing ability of repairing tiny damage or minor cracks in certain circumstances.Self-healing technologies can promote the reliability and durability of materials and have shown great application value in the field of polyurethane materials.Additionally,restricted by VOC emission regulations,the waterborne polyurethanes are gradually replacing solvent-borne polyurethanes.Based on the above considerations,we synthesized a waterborne self-healing polyurethane with quadruple hydrogen bonds and disulfide bonds within polymer backbones.The two-component waterborne self-healing polyurethane coatings were also prepared.The main conclusions are as follows:First,waterborne self-healing polyurethanes with different ratio of quadruple hydrogen bonds and disulfide bonds were successfully synthesized.A strong physical cross-linking network was formed by quadruple H-bonds of2-amino-5-(2-hydroxyethyl)-6-methylpyrimidin-4-ol(UPy)to improve the tensile strength of materials.The synergic effect of quadruple H-bonds and dynamic disulfide bonds also contributed high self-healing performance of 94 % at moderate temperature in combination with superior mechanical properties.The results of stress relaxation measurement indicated the activation energy was as low as 36.1 k J/mol,revealing the rapid occurrence of the reversible quadruple hydrogen bonds and dynamic exchange reaction.It was found that the self-repairing contribution of quadruple hydrogen bonds was greater compared to reversible exchanges of disulfide bonds in our studied materials.The characteristic time of the self-healing by quadruple H-bonds was also shorter than that of the dynamic metathesis reaction of disulfide bonds.Second,a waterborne hydroxyl-terminated polyurethane with quadruple H-bonds and disulfide bonds in the polymer main chains was synthesized.The two-component waterborne self-healing polyurethane coatings(2k WPU)were then successfully prepared by water-dispersible polyisocyanate AQ-298 and hydroxyl-terminated polyurethane.The effects of curing agent component on the application properties e.g.hardness,gloss,flexibility,adhesion,impact test and self-healing efficiency of 2k WPU coatings were investigated.The chemical structure of 2k WPU coatings were characterized by FTIR.The healing process of cracks was recorded by optical microscopy.The electrochemical impedance spectroscopy(EIS)using an electrochemical workstation was performed to evaluate the anticorrosion properties of coatings before and after self-healing process.The results showed that the coatings with 20 wt% of curing agent possessed good basic properties and superior self-healing performance.The pencil hardness of coatings reached HB,the cracks of injured coatings could be repaired after 10 min of hot press under 90 °C.