Synthesis And Properties Of Polymer Crosslinkers For Electrophoretic Coating Films

The cathodic electrophoretic coating occupies an important proportion in water-based environmental protection coatings,with its advantages of low VOC content,low pollution,and a recyclable coating process.Which is widely used in automobiles,household appliances,and other fields.Cathodic electrophoretic coatings mainly include epoxy resin,polyacrylate,and polyurethane systems.In the actual application process,the resin and polyisocyanate crosslinking agents are usually mixed to improve the comprehensive performance of the cured product.In this study,various structural types of closed-type waterborne polyisocyanate crosslinkers containing active epoxy groups were synthesized by adding epoxy groups to the molecular chain of polyisocyanates.The effects of various isocyanate indices(R values)and various molecular structures on the mechanical and mechanical properties of the crosslinkers were primarily investigated.(1)Linear epoxy intermediates were prepared from ethylene glycol diglycidyl ether(EGDE)and resorcinol diglycidyl ether(RDGE)with N-methylethanolamine(MEA)as raw materials,and then branched prepolymer was synthesized with hexamethylene diisocyanate trimer(HT-100),and polyethylene glycol(PEG-1000)and polyethylene glycol monomethyl ether(MPEG-1200)were used as(MPEG-1200)as hydrophilic chain extenders and methyl ethyl ketoxime(MEKO)as a sealer,a series of aliphatic closed waterborne polyisocyanate crosslinkers containing reactive epoxy groups based on HDI trimers were synthesized.The target crosslinker was confirmed by the results of the FT-IR and ~1H-NMR.The thermogravimetric analysis confirmed that the unsealing temperature of the HDI trimer-based crosslinker was 118℃.The performance of the crosslinkers was primarily examined through the addition ratio of branched epoxy intermediates.The comprehensive performance of the crosslinker shows that the RGDE crosslinker was superior to the EGDE system’s crosslinker in terms of hardness,MEK wiping resistance,and adherence to the cured coating,whereas the EGDE system’s crosslinker,was superior in terms of water dispersibility and storage stability.When the ratio of the hydroxyl group in MPEG to the total hydrophilic modifier was 0.9,the comprehensive performance of crosslinking agent was the best.(2)The branched epoxy intermediates were prepared from EGDE,RDGE,and ETA as raw materials,and then branched prepolymers were synthesized with isophorone diisocyanate(IPDI).PEG-1000 and MPEG-1200 were used as hydrophilic modifiers and MEKO was used as sealing agent.The target crosslinker was confirmed by the results of the FT-IR and ~1H-NMR.The thermogravimetric analysis confirmed that the unsealing temperature of IPDI-based crosslinkers decreased to 96℃.The comprehensive performance study of crosslinkers shows that the coating has the best performance when the ratio of the content of-NCO to the number of hydrophilic groups is 2.2.(3)RDGE and MEA were used as raw materials to prepare a linear epoxy intermediate.Then we synthesized branched prepolymer with polymethylene polyphenyl polyisocyanate(PAPI)and combined a series of aromatic closed waterborne polyisocyanate crosslinkers containing active epoxy groups in PAPI base with MPEG-1200 as a hydrophilic modifier and MEKO as a sealer.The target crosslinker was confirmed by the findings of the FT-IR and ~1H-NMR measurements.The thermogravimetric analysis confirmed that the unsealing temperature of the PAPI-based crosslinker increased to 175℃.The comprehensive properties of the crosslinker show that when the R-value of the-NCO groups present in PAPI and epoxy intermediate is greater than 4.0,the crosslinker can be disseminated steadily in both acidic and alkaline environments.The comprehensive performance study of crosslinking agents shows that the introduction of epoxy groups significantly improves the solvent resistance of the coating and to a certain extent enhances the hardness and adhesion of the coating films.