| Since the ministry of environmental protection issued the n13th five-year plan for the prevention and control of volatile organic matter pollution" in 2017,the environmental protection of wood coating is extremely urgent,and the research and development of highly efficient and environmentally friendly wood coating has become a hot topic in coating research.Ultraviolet(UV)curing coating has fast curing rate,high efficiency,short construction period,low cost of coating equipment and low volatile organic compound(VOC)emissions,has been one of the developing direction of environmental coating,but waterborne UV wood coating has incomplete curing,poor heat resistant and water resistance,low hardness and so on defects.In this paper,UV curing castor oil-based PUD resin was synthesized with isophorone diisocyanate(IPDI),six methylene diisocyanate(HDI),castor oil(CO)as main monomers,2,2-dihydroxy methyl propionic acid(DMPA)as hydrophilic chain extender,and it was blocked with the hydroxyl ethyl methacrylate(HMEA),Chemical groups,emulsion particle size,stability of emulsion and the effect of different DMPA,CO content were studied.The results showed that the emulsion had good storage stability and no precipitation or delamination in 90 days.The experimental results showed that the synthesized PUD contained-NHCOO-structure through the infrared spectrogram,and unsaturated double bonds in the HEMA and castor oil was eonnected with the long chain of polyurethane which achieved the expected purpose.The double bond conversion rate of the film reached 90.1%at the 20s of UV curing,and the change was not significant after 20s.When DMPA content was 6.5%and CO content was 14.8%,the emulsion was stable and the coating had good heat resistance and mechanical properties.Secondly,in order to improve the curing efficiency of the coating,the active monomers pentaerytol triacrylate(PETA)and dipropylene glycol diacrylate(DPGDA)were used to modify castor oil-based waterborne polyurethane,and the effects of the ratio of different PETA to DPGDA on the curing speed and properties of the coating film and paint film were investigated.Results show that the PETA/DPGDA monomers can significantly speed up the system of UV curing process,time of UV curing raised to 20 seconds,and the active monomer can enhance the hardness of coating,the film hardness of ratio of 9:1 reached 4H.Finally,In order to make up for the defect of incomplete UV curing,the air curing system was introduced into the system,and the drying agent was used to accelerate the oxidation of active methylene in castor oil to provide auxiliary curing effect for the shadow part of paint film that cannot be irradiated by UV light,so as to make up for the deficiency of single UV curing and make the system fully cured.The results showed that the UV/air curing film had a gel rate of 95.3%and water absorption rate of 12%,while the UV curing film had a gel rate of 93%and water absorption rate of 14.5%.The tensile strength,elongation at break and elastic modulus of UV/air curing films were 13.06mpa,22.53%,and tensile strength,elongation at break and elastic modulus of UV curing films were 7.29mpa,32.85%respectively.The hardness of UV/air and air/UV dual curing film reached 4H,adhesion were 0;The hardness of UV curing reached 3H,adhesion was 1.The water resistance,mechanical properties,film hardness,adhesion of UV/air and air/UV dual curing system were better than traditional single UV curing process,can be better applied in the field of wood coatings. |
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