Preparation And Properties Of Intrinsic Self-healing Waterborne Polyurethane Cathodic Electrophoretic Coatings

The cathodic electrophoretic coating is one kind of water-based coating,which has been widely used in the automotive industry,home appliances,metal parts and other industries due to its decorative and protective functions.If the coating could have self-healing ability after being damaged,it would be benefit for reducing the cost of maintenance and prolonging the service life of the substrate.At present,studies concerned about self-healing cathodic electrophoretic coatings is rare,and most of them are focused on external aided self-healing.Compared with the external self-healing,which has a limited number of healing times and a more complicated construction process,the intrinsic self-healing has the advantages of fast selfhealing speed,more times of healing,and relatively mild healing temperature,showing a wider application prospect.With the aim to prepare intrinsic self-healing cathodic electrophoretic coatings,this thesis took advantage of the "tailoring" properties of polyurethane materials and introduced a self-healing structure into the molecular chain of polyurethane.Intrinsic selfhealing waterborne polyurethane cathodic electrophoretic coatings with single and dual interactions were designed and prepared.All the specific research content and results are as follows:(1)Design,synthesis,and characterization of intrinsic self-healing waterborne polyurethane cathodic electrophoretic coatings based on disulfide bonds.An intrinsic selfhealing cationic waterborne polyurethane emulsion(SCWPU)containing disulfide bonds was synthesized using isophorone diisocyanate and polycaprolactone diol as raw materials,Nmethyldiethanolamine as hydrophilic chain extender,and 2,2-diaminodiphenyldisulfide(APDS)as self-healing extender.The effects of APDS content and polyurethane hard segment content(HSC)on the self-healing properties of SCWPU were systematically investigated.The results showed that with the increase of APDS content,the self-healing efficiency increased at first and then decreased,and the self-healing efficiency decreased with the increase of HSC.When the APDS and HSC contents were 2 wt.% and 50 wt.%,the prepared S2-CWPU/50 had the best performance,and its self-healing efficiency could reach 97.2% after self-healing at room temperature for 24 h.And the electrophoretic coating prepared by S2-CWPU/50 has good adhesion and pencil hardness up to 3H.The cross scratch on the 8μm thick coating disappeared after 24 h of healing at room temperature,indicating good healing ability of the coating.(2)Design,synthesis,and characterization of dual intrinsic self-healing waterborne polyurethane cathodic electrophoretic coatings based on hydrogen bonds and disulfide bonds.A dual intrinsic self-healing waterborne polyurethane cathodic electrophoretic coating(SCWPU-TA)containing hydrogen bonds and disulfide bonds was prepared by simply mixing SCWPU with tannic acid in the synthesis process of SCWPU.The effects of tannic acid on the hydrogen bond index and the degree of microphase separation of SCWPU-TA were systematically investigated by using variable temperature infrared and Gaussian peak fitting technology.The results showed that the enhanced hydrogen bonds of SCWPU-TA disassociated and re-associated after being heated and cooled,respectively,indicating temperature reversible.With the increasing of tannic acid content,the content of ordered hydrogen bonds in SCWPUTA gradually increased,the hydrogen bonding index increased,and the degree of microphase separation increased.The results of stress-strain test showed that the addition of tannic acid significantly improved the mechanical properties of SCWPU-TA.T The comprehensive performance of SCWPU-TA2% film is the best,the maximum stress reaches 11.05 MPa,and the self-healing efficiency is 83.24% at 65℃ for 4 h.(3)Design,synthesis,and characterization of dual intrinsic self-healing waterborne polyurethane cathodic electrophoretic coatings based on coordination bonds and disulfide bonds.During the synthesis process of SCWPU,another ligand chain extender 2,6-pyridinedimethanol(pdm)with pyridine structure was added,and dual intrinsic self-healing waterborne polyurethane cathodic electrophoretic coatings(SCWPU-pdm:Fe)based on the coordination effect between Fe(III)and pyridine structure as well as disulfide bonds were prepared.The successful preparation of SCWPU-pdm:Fe was confirmed by infrared and ultraviolet spectroscopic analysis.The results of DSC and XRD showed that SCWPU-pdm:Fe had partially ordered crystallization,and the maximum stress of SCWPU-pdm:Fe-31 was 4.80 MPa under the combined action of coordination bonds and disulfide bonds.The self-healing efficiency of SCWPU-pdm:Fe was 95.73 %.By adding a small amount of ferric sulfate hydrate aqueous solution,the conductivity of the cathodic electrophoretic coating was significantly improved,the adhesive strength of the cathodic electrophoretic coating was increased,and the coating surface became more uniform and smooth.