Preparation And Properties Of Borate Ester Based Waterborne Self-healing Polyurethane Coating

As an environmentally friendly polymer material with water as a dispersing medium,Waterborne polyurethane（WPU）has been widely applied in coating,adhesive and textile fields.However,the surface and inner of the WPU coating tended to damage in the long-term use that resulted in the generation of micro cracks,which sharply decreased the protective properties,service life and aesthetic properties of coating.The self-healing coating could repair broken chemical bonds when damaged,while restored the damaged parts to original state and extend the service life of the coating.The dynamic bonds with self-healing ability included hydrogen bond,disulfide ester bond,borate bond and metal coordination bond.As a kind of dynamic covalent bond formed through the condensation reaction between boric acid and diol,borate ester bond could endow polyurethane（PU）with self-healing ability with the reversible reaction.It was widely employed to prepare self-healing PU with high mechanical strength.In this study,high strength waterborne self-healing polyurethane containing borate ester bond was prepared.The water resistance and heat resistance of waterborne self-healing polyurethane were improved by nitrogen and boron internal coordination.The primary studies were as follows:（1）In the first part,Ttriethanolamine（TEA）,Diethanolamine（DEA）and4-methylbenzeneboronic acid（HPBA）were utilized to synthesize borate-containing ester bond compounds with two hydroxyl groups（NCBC₁）and borate-containing ester bond compounds with one amino group and one hydroxyl group（NCBC₂）.The hydroxy-terminated borate bonded compound（MN）was subsequently prepared via the reaction between NCBC₂and4,4`-diphenylmethane diisocyanate（MDI）.The structures of NCBC₁,NCBC₂and MN were characterized by FTIR and ¹H NMR spectra,with the results that the designed structures were successfully synthesized.The hydrolysis yields of NCBC₁and NCBC₂were 15.8%and 18%after 24 h when molar ration of deionized water to NCBC was 20,respectively.It was found that the borate ester based compounds had an excellent hydrolysis property.The Thermogravimetric Analysis（TG）test showed the semi-decomposition temperatures of NCBC₁,NCBC₂and MN were 287^oC,293^oC and 338^oC,respectively,which indicted that borate-containing ester bond compounds possessed outstanding heat resistance.（2）In the second part,the self-healing PUs were prepared with PTMEG-1000,MDI and chain extender（MN or NCBC₁）in the molar ratio of 1:2.4:1.When NCBC₁was used as chain extender,the mechanical strength of PU was 4.87 MPa,the strength after repairing was 4.03MPa,and the self-healing efficiency was 83%.When the chain extender was MN₂（molar ratio of raw material MDI:NCBC₂=1:2）,the mechanical strength of PU was 6.06 MPa,the strength after repairing was 5.47 MPa,and the self-healing efficiency was 90%.It was significantly seen that the mechanical properties and self-healing properties of PU with MN₂as chain extender have been enhanced.The coating was obtained by dissolving self-healing PU in DMF,then applied on the surface of wood to test.The properties of the film after drying meet the application requirements,the pencil hardness of the film was 3 H,the adhesion level of film was 0,the impact resistance of film was greater than 50 cm,and the flexibility of film was 2mm.The scratches on the film surface were almost completely healed after being repaired at60^oC for 48 hours.（3）In the third part,the water-based self-healing WPU were synthetized via self-emulsification with different contents of MN₂as chain extenders.Due to the existence of borate ester bond,WPU possessed the self-healing ability（83%）and excellent mechanical properties（24.2 MPa）.The properties of the coating meet the application requirements,the pencil hardness of WPU₃film was 3 H,the adhesion level of WPU₃film was 0,the impact resistance of WPU₃film exceeded to 50 cm,and the flexibility of WPU₃film was 1 mm.Additionally,the water absorption rate of WPU₃film was 10.0%,the initial decomposition temperature of WPU was superior to 200^oC,and the semi-decomposition temperature of WPU₃reached 412^oC.The WPU film containing borate bond had superb water and heat resistance.