Preparation Of Fluorinated Polyurethane And Its Anti-Cavitation Erosion/Corrosion Properties

The ship propeller is facing severe cavitation erosion and corrosion damage in the marine environment,which will not only reduce the service life of the propeller,but also reduce the propulsion efficiency of the propeller.Coated with an organic coating on the surface of the propeller is currently a more widely used means of protection.At present,the anti-cavitation/corrosion organic coatings that can be applied on a large scale are mainly polyurethane（PU）,but there is a problem of insufficient durability.Although the traditional PU is able to absorb and buffer the repeated erosion of cavitation micro-jet and shock wave,there are-CH₂OH hydrophilic groups on its molecular chain,resulting in poor water resistance,which seriously reduces the service life of PU coating.The introduction of fluorinated groups with low surface energy into PU can improve the hydrophobicity of PU,thereby extending its service life.Most of the traditional fluorinated polyurethanes（FPU）are synthesized by means of end-group grafting or soft chain modification of fluorinated small molecules.However,the content of fluorinated small molecules introduced by these methods is limited,which cannot further improve the hydrophobic properties of PU.Therefore,in this paper,the hydroxyl-terminated liquid fluoropolymer（LFH）with high fluorine content（M_w～2000）was prepared by the pyrolysis carboxyl-hydroxylation process of fluororubber,and a series of FPU with different LFH contents were prepared.At the same time,the cavitation resistance and corrosion resistance of the PU and FPU were studied.Based on the FPU,the superhydrophobic protective coating with stronger hydrophobicity and long-term stability under water was further prepared.The main research content of this paper is as follows:In the study of carboxyl-hydroxylation process of fluororubber,it was found that when the mass ratio of KOH and H₂O₂ was 1:5.6,and the molar ratio of carboxyl to reducing agent was COOH:Li Al H₄=1:4,the molecular weight and molecular weight distribution of LFH were low（M_w～2000,D=1.50）,and the comprehensive performance was the best.Then,FPU was successfully prepared with diphenylmethane diisocyanate（MDI）as hard segment,LFH as the first soft segment and polytetrahydrofuran glycol（PTMG）as the second soft segment.The thermal stability,mechanical properties and water resistance of FPU with different fluorine contents were compared and analyzed.The results showed that with the increase of fluorine content,the hydrophobicity of FPU increased first and then decreased.When the content of the LFH was 20%,the hydrophobic effect was the best,and the thermal stability was the best.The tensile strength was up to 37.64 MPa,and the elongation at break was 750%.The tensile strength did not change significantly after immersion in deionized water（DW）and artificial seawater（ASW）for 30 days.After soaking in DW and ASW for 150 days,the water absorption rate was only 0.63%and 0.73%,indicating that the FPU has good water resistance.The cavitation resistance of FPU was tested according to ASTM G32-2010 ultrasonic cavitation method.The results show that the surface of the FPU coating with a fluorine content of 20%（FPU-20）remains intact after 10 h of cavitation,and the cavitation resistance of the PU coating is improved by 60.36%,which is expected to be applied as a cavitation resistance coating to the surface protection of overcurrent parts such as marine propellers.In addition,in order to further improve the hydrophobicity of PU coatings,we combined the excellent comprehensive properties of FPU-20 and the rough surface of the arc-sprayed aluminum（Al）coating to carry out a study of superhydrophobic polyurethane-based composite coatings.Firstly,Al coating was prepared by arc spraying technology,and then Al-FPU superhydrophobic coating was prepared by adjusting the thickness of Al coating and the concentration of FPU-20.The results show that with the concentration of the FPU-20 increase,the hydrophobicity of coatings with different surface structures shows a tendency to increase first and then decrease.When the concentration of the FPU-20 is 2%and the thickness of Al coating reaches 70μm,the Al-FPU coating achieves superhydrophobicity,with the maximum contact angle of153°and the rolling angle of 16.5°.At the same time,the corrosion resistance of the coating was studied by electrochemical workstation and neutral salt spray test.The results showed that the corrosion potential of the Al-FPU coating was-0.748 V,the corrosion current was 1.238×10^-4 A·cm^-2,and the R_pu and R_ct values of the coating were 210.1Ω·cm² and 5963Ω·cm²,which showed that the corrosion resistance of the Al-FPU coating was better than that of the Al coating and the Al-PU coating.Moreover,the Al-FPU coating still has good gloss and surface integrity after salt spray test.The above results show that Al-FPU coating is expected to be used as superhydrophobic material for marine protection.