Preparation and characteristics of crosslinked fluorinated acrylate modified waterborne polyurethane for metal protection coating

The use of coatings to protect metals from corrosion is currently a common practice. In this study, a double bond-terminated waterborne polyurethane prepolymer was firstly synthesized from polycaprolactone polyol, toluene diisocyanate, dimethylolpropionic acid and hydroxyethyl methacrylate. Then, waterborne polyurethane prepolymer was modified with styrene, butyl acrylate, glycidyl methacrylate, and dodecafluoroheptyl methacrylate though soap-free emulsion polymerization method in order to obtain the crosslinked fluorinated acrylate modified waterborne polyurethane (CFWPUA). The CFWPUA emulsion was used as the matrix resin to prepare the coating with a bar coater, which possesses the characteristics of external hydrophobic surface constructed by long fluorinated side-chains and internal double cross-linking system. The results of infrared spectrum analysis proved the successful synthesis of modified waterborne polyurethane. The energy-dispersive X-ray spectroscopy results showed that the long fluorine side-chains partially migrate to the surface of the coating during the curing process, which leads to the occurrence of fluorine enrichment on the surface. When the glycidyl methacrylate content is 5 wt% and the dodecafluoroheptyl methacrylate content is 7.5 wt%, the water absorption rate of the membrane reduces to 7.9%, the water contact angle of the coating reaches 102.5°, and the gel content is 91%, which proved the successful construction of the double cross-linking system and prevents the penetration of water molecules. The pencil hardness of CFWPUA coating can reach 3H and flexibility and adhesion properties are at the highest level. Most importantly, in the electrochemical impedance spectroscopy test, the modified waterborne polyurethane coating effectively protects Q235 metal substrate from corrosion in the 3.5 wt% NaCl solution, the impedance modulus of CFWPUA coating could be maintained at about 107 Ω·cm2 for a long time, the resistance of CFWPUA coating slowly decreased from 2.124 × 107 Ω·cm2 to 9.605 × 106 Ω·cm2 within 10 days of immersion, two-time constant was generated after 12 days of immersion, which is slightly superior compare to commercial waterborne fluorocarbon coating. The modified waterborne polyurethane coating prepared here shows a potential practical application in the field of metal anti-corrosion.