Synthesis,thermal Reversible Mechanism And Self-healing Property Of UV-curable Polyurethane Coatings Based On Diels-alder Structure

Energy conservation,emission reduction and environmental protection have become the focus of global's attention.For the question of environment pollution and energy consumption which brought by conventional solvents,restrict or prohibit of traditiona solvent coatings has became the consensus of society.UV-curable coating has drawed extensive attention as the typical representive of energy-saving and environment-protected coatings with?4E?character of low-costing,efficient,environmentally friendly and high economic efficiency.As surface coating damage is hardly avoided while highly crosslinked networks will be healed harder because its insoluble and can't melt.In order to solve the above problems,in this study,we combined the thermally reversible of Diels-Alder(DA)reaction with the energy-saved and environmental-friendly of UV-curable to design and synthesis UV-curable resin with self-healing properties,at the same time the thermally reversible mechanism and self-healing behavior is studied.The specific research content as follows:A series of polyurethane acrylate prepolymer(DAE1PU?DASiPU?DALPU and DAE2PU)with DA structure and different soft segment was prepared by diol with DA structure,isophorone diisocyanate(IPDI),different soft segment polyol(PEG1000/HPDMS1000/PLA1000/PEG2000)and hydroxypropyl acrylate(HPA).The prepolymer was characterized by Fourier transform infrared(FTIR)and ~1H Nuclear Magnetic Resonance(~1H-NMR).The UV curing behavior was studied by FTIR and the results show that each resin system could be rapidly cured and a final acrylate C=C conversion high than 75%after UV irradiation within 30 s and the DA structure can keep stable during the curing.And the result of gel content indicate that each UV-curable resin has formed highly cross-linked networks.The DA thermal reversible mechanism of UV cured polyurethane coatings was characterized by Fourier Transform Infrared Spectroscopy(FTIR)?Differential Scanning calorimeter(DSC)and dynamic mechanical analysis(DMA).The results of FTIR shown that the DA bond rupture after 30 min at the 120?,and the DA bond can rebuilt again after 2 h at the temperature of 60?.DSC results shown that during the heating progressing each coating have the endothermic peak of the rDA bond;the peak will moving to higher temperature and becoming smaller,the glass-transition temperature(Tg)move to lower temperature during the second heating progressing after the coatings kept at 60?about 6h,this results verifier that the DA bond can't rebuilt completely;during the third heating progressing the endothermic peak of the DA bond and the Tg peaks are similar to the second heating progressing reach the dynamic equilibrium of theory,those results verifier that the DA bond is repeatable thermal reversibility.DMA results shown that of the storage modulus of each UV cured coating decline to far less than 10 MPa from the progressing of 0?to 150?.Combining with FTIR and DMA results,the DA structure maintains an excellent reversible property.The Cross-linked network structure will become a linear structure with the breaking of the DA bond that will make UV cured coating have self-healing property.The self-healing performance of UV cured polyurethane coating was characterized by the optical microscope,gloss meter,pencil hardness tester.The optical microscope results shown that the DAE1PU?DASiPU and DALPU cured films can completely restored after~30 min?~200 s and~100 s respectively and can healing several times while the DAE2PU cured film was minimal healed after 60 min.The results of gloss shown that the DAE2PU cured film just can partially repair the marred area,DAE1PU and DASiPU cured film have good self-healing performance,but the rates of the repair will decline with the mar-repair cycle increase.DALPU cured film repairing efficiency higher than 90%and the value don't have any change during the three times cycles of mar-repair.The results of the pencil hardness shown that at the 60?the coatings hardness will reduced,at the 90??100??110??120?the coating hardness will further reduced shown that the r DA reaction was occured.When those coatings insulation at 60?about 2h,the hardness of those coatings will up to the same value as before.Integrated the above shown that the DALPU cured film have the best mechanical properties and repair performance in each cured film.In DALPU prepolymer,different content of dimethylol propionic acid(DMPA)was introduced to prepare waterborne polyurethane for DALWPU-3%?DALWPU-5%and DALWPU-7%.The particle size of dispersion was was analyzed by laser particle sizer,the results showed that the average particle sized decreased from 101.6 nm to15.4 nm with the increasing carboxylic acid content from 3wt%to 7wt%and the least polydispersity index(Pd I)when carboxylic acid content was 5wt%.Similarly,FTIR results show that the DA bond is thermal reversibility.And the excellent self-healing performance of each UV cured waterbone polyurethane coating was characterized by the optical microscope,gloss meter,pencil hardness tester.Thus,the combination of self repairing function and Waterborne UV curable coating is realized.