Research on preventing marine biofouling organisms 

with PTFE film and potential mechanism

Abstract: Offshore wind power is an important development direction of wind power generation. However, the monopile steel structures of offshore wind power are immersed and eroded by seawater for a long time and its surface is easily adsorbed, fixated and corroded by marine organisms, leading to thickening of the monopile steel structure diameter and internal corrosion of monopile steel, which can seriously shorten the service life of offshore wind power equipment. How to prevent the corrosion of seawater and marine organism on the offshore wind turbine pile is important for the development of wind power industry. The polytetrafluoroethylene (PTFE), known as “the king of plastic”, is a kind of tetrafluoroethylene monomer polymerization of polymer, which has been widely used in national defense, atomic energy, petroleum, radio, electricity, machinery, chemical industry, etc. In the present study, two kinds of PTFE with low surface energy were prepared by hot extrusion method, named as highdensity PTFE film (HDPTFE) and lowdensity PTFE film (LDPTFE). Results showed that the density of HDPTFE and LDPTFE was 2.3 g·cm-3 and 1.6 g·cm-3, respectively. The crystallinity of HDPTFE and surface energy of HDPTFE was 89.0% and 16.4 mN·m-1, and these of LDPTFE were 79.0% and 22.7 mN·m-1, respectively. The HDPTFE and LDPTFE were treated with nitrogen, and then physically laminated with R4088 tape (3M, USA) to produce PTFE composite films with selfadhesive properties (i.e. HDPTFE film and LDPTFE film). The peeling strength of R4088 tape with HDPTFE, polyester film and epoxy resin were more than 1 000 gf, which had strong bonding strength and could be firmly stuck on the surface of steel pipe pile. After 40 000 wear resistance tests, 14 400 h salt spray test and 1 000 h rain erosion test, the surface of HDPTFE film had no roughness or damage, neither change in the appearance or change in mechanical properties. After 15 h of high and low temperature aging resistance experiments, 5 000 h artificial climate aging resistance and ozone aging resistance experiments, the tensile strength retention rate of HDPTFE remained as 99.9% and the elongation retention rate was more than 99.0%, indicating that HDPTFE had excellent property of weather resistance. In addition, no small molecules were resolved from HDPTFE film, suggesting that HDPTFE had no adversely affects on the seawater environment. Then, HDPTFE and LDPTFE films were immersed in seawater for 3 years. We found that LDPTFE film were swelled and peeled off, while HDPTFE film maintained its original surface morphology structure, which was attributed to the high density, low surface energy and high crystallinity of HDPTFE. The reason for the better resistance of seawater corrosion of HDPTFE than that of LDPTFE might be: 1) The density and crystallinity of HDPTFE were higher than that of LDPTFE; 2) The high density and crystallinity of HDPTFE blocked the swelling that caused by the wetting and penetration of seawater. While, the seawater could penetrate slowly into the inside of LDPTFE that caused the crack between the layers of LDPTFE, which finally led to the rupture and fall off of LDPTFE. Further, HDPTFE film with selfadhesive property was selected to be wrapped on the surface of wind turbine steel pipe pile (monopile), and the results of marine organism adhesion and attachment on the surface of monopile with HDPTFE film (T5 monopile) or without film (T4 monopile) were compared after 9 months of actual operation at sea and subsea photography. We found that the attachment of marine microbial on the surface of T4 monopile was obvious, which mainly were Balanus, Mytilus edulis and calcareous curing hard layerhard layer. In contrast, there were small number of marine microorganisms on the wavefacing surface of T5 monopile and the surface of  HDPTFE remained smooth and flat. For marine microorganism there were 23 Balanus  on the surface of HDPTFE per m2, but no Mytilus edulis on it. The waveback surface of T5 monopile was more suitable for the growth of marine microbial and could also form hard layer that caused by death of fouling organisms. However, the marine organism and hard layer on the waveback surface of T5 monopile were easier to fall off, compared with that of T4 monopile. We concluded that: 1) With large impact of seawater on the wavefacing surface of HDPTFE film, the marine organism could only adhere and temporarily immobilize on the surface of HDPTFE, but no calcification layer was formed; 2) With small impact of seawater on the wave back surface of HDPTFE film, the marine organisms could adhere, immobilize, grow, reproduce, die and calcify on the surface of HDPTFE. However, due to the excellent corrosion resistance and low surface energy of HDPTFE, the adhesion between marine organisms and HDPTFE was weak, and the HDPTFE could not be corroded, directly resulting in the falling off of the calcification layer of marine organisms; 3) The falling of the calcification layer on the HDPTFE was random and the area of calcification layer on the HDPTFE deceased as time extended; 4) The marine organisms could normally adhere, grow, reproduce, die and calcify on the surface of monopile without HDPTFE and the calcification layer did not fall off and increased with time extended, finally leading to more and more marine organism accumulate on the surface of monopile. Thus, the mechanism of HDPTFE material against marine fouling organisms was proposed to be a repeated cycle of “adsorption, temporary fixation, shedding, secondary adsorption (reagglomeration)”, and there was no permanent fixation of marine fouling organisms. Although HDPTFE material did not inhibit the adsorption, growth and proliferation of adhesive marine organisms, the fouled marine organism calcification layer would be actively peeled off and shed due to the low surface energy and excellent corrosion resistance of HDPTFE, and the selfshedding of marine fouled organisms was more obvious under the impact of seawater. In conclusion, the HDPTFE has excellent weather resistance and can be used as a protective material against marine life fouling for offshore wind power steel pipe piles, which provides a new method for antimarine life adhesion and fouling of static and mobile platforms at sea, with extremely important theoretical and experimental guidance.

Keywords: polytetrafluoroethylene (PTFE); membrane materials; marine biofouling organisms; bioadhesion