8
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
Fluorinated Naval Coatings James R. Griffith' and John D. Bultman Naval Research Laboratory, Washington, D.C. 20375
Heavily fluorinated, highly cross-linked coatings of the epoxy and polyurethane classes have been produced and are under investigation in a variety of naval applications. These coatings are tough, durable membranes with the unusual surface properties of fluorocarbon materials.
Materials which contain large quantities of fluorocarbon within the molecular structure are characterized by a number of properties which are uncommon and which make them valuable as high-quality protective coatings. Previously, however, such materials have been unavailable as general purpose coatings largely because the necessary basic organic synthesis required to produce total systems apparently had not been performed. Such systems have now been produced of the epoxy and polyurethane types which combine all of the necessary and desirable properties, and coatings are being evaluated in a variety of naval applications. It is of particular interest whether or not advantage can be taken of the special properties such as anti-stick surfaces, hydrophobicity, and low friction. CH,-CHCH,-0
'/0
PTFE contents of at least 40 vol %, substantial further reductions in water absorption can be made for the systems since PTFE absorbs less than 0.01%water by weight. A consequence of this is that coatings can be produced which very nearly preclude the presence of water a t interfaces, and the implications for corrosion control become obvious. I t is also of importance that such coatings can be produced conveniently by the use of standard coatings practices, and the absence of such convenience in virtually all previous heavily fluorinated polymeric materials has precluded many potential uses. The NRL fluorinated polyurethanes are based upon fluorinated polyols derived directly or indirectly from the fluoroepoxies. The general forrpula for the fluorinated poly01 component is represented by I.
R,-OCH,CHCH20 bH
OH I
The Nature of Fluorinated Epoxy and Polyurethane Coatings During the past few years, the basic chemistry of highquality epoxy and polyurethane materials has been generated at NRL by Griffith et al. (1970, 1971, 1972, 1975, 1976), and a wide variety of practical end items are on hand. The most interesting of the fluoroepoxy materials are the diglycidyl ethers of 1,3,5-fluoroalkylbenzene,all of which synthesized to date have been liquids in pure form.
\ I
0
The length of the perfluorinated alkyl group, Rf, may be varied at will, and the total fluorine content of the system will depend upon the size of Rf. These epoxies are effectively cured by the usual epoxy curing agents, generally amines or organic anhydrides, to produce the variety of products that can be achieved with common epoxies. However, these possess special properties relative to the common materials, and one such property of particular concern to interfacial corrosion is the very low water absorption. For example, a typical water absorption during 6 months of immersion for an anhydride-cured resin of this type is 0.35%by weight, whereas a conventional epoxy would absorb ten or more times as much. Also, there is a very effective device for further reducing the water absorption of these materials which depends upon their exceptionally low surface tensions and wetting abilities when in the liquid state. Thus, powdered PTFE is wetted and suspended by the fluorinated resins, and since these suspensions can be produced with This paper not subject to U S . Copyright.
Rf and R( (which may be the same as Rf) are heavily fluorinated, difunctional units, usually aliphatic; n is approximately 10; and the pendant hydroxyl groups provide sites for cross-linking by isocyanates to produce the final networks for the polyurethane. Hydrophobicity and Corrosion Prevention Corrosion, like combustion, requires the presence in one location of several essential ingredients, and if one of these ingredients can be completely eliminated, then the process cannot proceed. In most of the corrosion processes of concern to the Navy, water is an ingredient of major importance, and the most common means of preventing its presence on surfaces subject to corrosion is that of painting the surfaces. New types of paint are now in process which promise an order of protection to corrodible surfaces not previously attainable by practical means in general purpose coatings. These paints are very heavily fluorinated epoxy and polyurethane materials pigmented with powdered poly(tetrafluoroethy1ene), PTFE, and applied in the manner of conventional paints. There are a t least two general mechanisms by which they hinder water from reaching corrodible surfaces. First, the coating surface is not readily wetted by water, and unless the water is trapped in cup-like depressions or the object is submerged, water quickly escapes. Second, the molecular absorption of water into the polymeric film is relatively small, being as little as 15%of that of a conventional film. This is not absolute exclusion of molecular water from the corrodible surface but a definite restriction in its availability at the potential site of corrosion. The joining of two different metals or alloys for subsequent use in a saltwater environment is often unavoidable. For example, the fasteners which hold aircraft skin to the structural members cannot be of exactly the same composition as the Published 1978 by the American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev.. Vol. 17, No. 1. 1978 9
skin and substrate. Thus, the prevention of saltwater contact with such a fastener system is of major importance to prevent electrochemical action a t this point. The use of a fluoropolymer coating+ealant at this type of location to prevent water from lingering at the fastener or to prevent its significant penetration into the joined areas is a cost-effective application. In this case the coating prevents interfacial activity between the two dissimilar metals by excluding saltwater. Marine Fouling at Underwater Interfaces The underwater surfaces of ships present a number of interfacial problems of great economic significance with respect to both the preservation of the hull and to the ease with which the vessel moves through the water. The latter refers to marine fouling organisms which create a rough interface between vessel and water such that energy is wasted through ineffective motion of the water, or drag. If it were possible to produce a surface to which such organisms could not adhere with tenacity, it should he possible to remove them easily, perhaps as a result of the vessel’s motion through the water. A concept which is beine investigated to this end is the adherine of norous PTFE sceeting surfaces, such as the sides of ships, by the use of fluorinated epoxy resins as the bonding agents. At the PTFE-metal interface one has a heavily fluorinated resin which secures the PTFE by virtue of its ability to wet the PTFE surface and to penetrate into porosities. When cure is completed, the PTFE sheet is “spiked” to the surface by numerous tendrils of fluoroepoxy plastic. At the water interface, the marine organisms encounter a surface which is predominantly PTFE except for the porosities which are filled with fluorinated epoxy resin. Test specimens of this nature were placed on exposure in the marine environment in the Bay of Panama, Naos Island, Canal Zone, in October 1975, and removed for barnacle adhesion evaluation 5 months later. About 75 barnacle shells were selected for the measurements which were made in directions parallel to the sample surface or perpendicular thereto. Prior to these measurements, each of the barnacle shells was cleaned of living matter or dehris and filled with an epoxy resin to prevent shell collapse and to secure wire loops to which the force was subsequently applied. Also, the specimens were kept submerged during resin cure to prevent drying which could have affected adhesion results. For comparison, barnacles on adjacent aluminum surfaces were also dislodged with force measuring devices, and the following average results were obtained. Y
PTFE-epoxy Vertical Shear kg/cm2
3.2
4.9
Aluminum Vertical Shear 5.8
Figure 1. White fluorinated coating in a large ship bilge illustrating Problems.
.
7.7
In addition to the PTFE-fluorinated epoxy system described above, some experimental fluorinated polyurethane formulations containing varying amounts of particulate PTFE are being evaluated as antifouling coatings. Since only a few barnacle adhesion measuremenk have been made, reliable data are not yet available for these coatings. However, the average adhesion value for the collected data, obtained on five barnacle species adhering to four fluoropolyurethane substrates, is 1.1kg/cm2. A high shell fracture rate occurs during application of the pulling force, but improvements in the technique of preparing the specimens for measurement and refinements in the design of the measuring apparatus are expected to provide more reliable adhesion values for all experimental and control substrates. Although our results to date are not conclusive,it appears that substantial reductions in the tenacity of barnacle adhesion can he realized through the use of fluorinated surfaces.
Figure 2. Black fluorinated coating on external hull of a submarine at the water line. Service Tests of Fluorinated Naval Coatings A number of actual service tests on Navy vessels are underway for the purpose of assessing the practical effectiveness of fluorinated coatings. Figure 1 shows the bilge area of a large vessel which has been coated with a white fluorinated coating. The photo illustrates the very servere soiling problem found in bilges, and periodic assessments are made of the ease with which this coating can he cleaned and of protective qualities in prevention of corrosion. Figure 2 shows a black fluorinated coating on the external hull of a submarine a t the water line. This coating involves six materials variations, and it is periodically evaluated for the extent of marine fouling and ease of removal. Literature Cited Field, D.E.,GriItm. J. R., Ind. Eng Chem. Prcd. Res. Dev., 14, 5 2 (1975). Field, D. E., J. Coatings Tech., 48, No. 615. 43(1976). Griffith, J. R.. ORear, J. G., Reines, S.A.. Chemtech, 2, 311 (1972). Griffith, J. R., Quick. J. E., Ad”. Chem. Ser.. No. 92, 8 (1970). ORear. J. G., Griftith. J. R.. Reines. S. A,. J. Paint Technol.. 43, NO. 552, 113 (1971).
Presented at the Division of Organic Coatings and Plastics Chemistry, 113rd National Meeting of the American Chemical Society, New Orleans, La., Mar 22,1917.
