Formulations and Field Performance of Fluorinated Polyurethane

Chapter 22

Downloaded by NORTH CAROLINA STATE UNIV on September 9, 2012 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch022

Formulations and Field Performance of Fluorinated Polyurethane Coatings

Robert F. Brady, Jr.

Materials Chemistry Branch, Naval Research Laboratory, Washington, DC 20375-5342

Heavily-fluorinated polyurethane coatings have been formulated and tested in demanding marine applications. In addition to the chemical and weather resistance typical of polyurethane coatings, these coatings have an anti-adhesive surface which permits the facile removal of water, ice and soil. This paper describes the chemistry of the polyol resins, the formulation of these resins into tough, anti-adhesive polyurethane coatings, and performance of these coatings in rigorous marine and industrial applications.

A continuing basic research program on the synthesis offluorinatedpolymers has yielded a series of tough, chemically-resistant polymers with unusual and useful properties. The polymers are soluble in common solvents and can react with the biuret trimer of hexamethylene diisocyanate to form a series of polyurethane coatings. These coatings are applied by conventional brush and spray techniques and cure at room temperature to form tough, uniform, integral films. The coatings may be pigmented to any desired shade with inorganic or organic pigments or employed as clear films. In fact, because the low surface energy of the polymer is comparable to that of poly(tetrafluoroethylene) (PTFE), up to 38 volume percent of finelypowdered PTFE can be incorporated as a pigment to form homogeneous paints having high fluorine content and extreme resistance to adhesion and penetration. Coatings based on this chemistry are being used in specialized defence applications (7). Several of these uses which are relevant to industrial applications will be described. Tests of these coatings in US Naval vessels and shore facilities began in 1977 and continue to the present day. Each test has a unique objective, and these new materials have performed successfully in a broad spectrum of applications, demonstrating their endurance and desirability. The coatings offer retention of appearance properties, ease of maintenance, protection of 282

U.S. government work. Published 1998 American Chemical Society

In Organic Coatings for Corrosion Control; Bierwagen, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

283 the substrate from corrosion, long service life, and superior resistance to heat, actinic radiation and chemicals.


Background Fluoropolymers are among the most chemically inert of organic compounds. They demonstrate outstanding thermal stability and resist oxidative attack. These properties can be attributed to the presence of thefluorineatom (2): •The carbon-fluorine bond energy (540 kJ mol* in fully-fluorinated aliphatic ÇF CF hydrocarbons) exceeds that of the carbonH O - C ^ ^ ^ r — C-OH hydrogen bond (435 U mol" in aliphatic hydrocarbons). C F 3 l ! ^ GF3 •Carbon—carbon backbone bonds are strengthened whenfluorineis attached to the 1 backbone. Carbon—carbon bond energies are ÇF CF 406 kJ mol" in perfluoroethane and 368 U mol in ethane. C-OH HO-C •The covalent atomic radius of I I CF fluorine (7.2 nm) is nearly twice that of CF hydrogen (3.7 nm). Afluorineatom screens the polymer backbone from attack without more effectively than a hydrogen atom, CF CF without introducing steric strain to the backbone. H O ^ K ^ ^ T O H CF CF Compared to an unfluorinated analog, a polymer containingfluorinehas a lower equilibrium moisture absorption, a lower dielectric constant, a lower index of refraction leading to intriguing optical properties, and frequently increased thermal stability. 1

3

3

1

1

Ο

3

1

3

3

3

3

3

3

3

Afluorinatedpolymer also has a low surface energy. Pioneering studies by Zisman (J) demonstrated that the surface HOCH 2CF2CF2CF2CH2OH energy of a polymer is determined by the functional groups on the polymer surface. Zisman showed that substitution of fluorine causes the surface energy to decrease in the HOCH2CH2CH2CH2OH order -CH - > -CH > -CF - > -CF . This is readily observed in the surface energies of 6 polymers with surfaces composed or ordered single groups such as poly (ethylene) (33.7 Figure 1. Diols used in the mJ m ), poly(dimethylsiloxane) (21.2), synthesis of thefluorinatedpolyoh. poly (tetrafluoroethy lene) (18.6), and poly[di(3,3,3-trifluoropropyl)siloxane] (6.0). 2

3

2

3

2



284 Griffith was first to apply these findings to the synthesis of fluorinated polyol resins suitable for the production of coatings with low surface energy (4). He used a number of fluorinated diols (Figure 1) in his work. The aromatic diols 1 and 2 were synthesized by the reaction of benzene with two moles of hexafluoroacetone using the method of Farah et al. (5); the reaction product contained 90% of 1 and 10% of 2. The unsaturated diols 3 and 4 were obtained in yields of 82% and 4%, respectively,fromthe reaction of propene with hexafluoroacetone at 170 °C for 48 hours (6). 2,2,3,3,4,4-Hexafluoropentanediol (5) was prepared by the reduction of diethyl perfluoroglutarate with lithium aluminum hydride (7). Butane-l,4-diol (6) is available from commercial sources. The polyols are synthesized by refluxing the mixture of diols 1 and 2 with an equimolar amount of either the mixture of diols 3 and 4, or 5, or 6 with epichlorohydrin and a large excess of sodium hydroxide in a solution of acetone containing a small amount of water. The reaction is monitored by gas chromatography, and reflux is discontinued as soon as the starting diols have disappeared. The resulting viscous polymers are washed with water until free of base and dried at 120 °C, producing light amber solids in yields of 87 to 97 percent. The generic structure of the polyol is shown in Figure 2, and properties of the neat polyols are given in Table I. The products are dissolved methyl isobutyl ketone at 50 weight percent and filtered, and the solution is used directly in coatings. Formulation of Coatings We have formulated fluorinated polyurethane coatings containing powdered PTFE which exhibit not only the hardness and toughness of conventional polyurethane coatings, but also the low surface energy and easy cleanability of PTFE. Formulations containing up to 38 percent by volume of PTFE have been successfully tested infieldapplications. Our standard coating contains 24 volume percent PTFE; fluorine comprises 41.7% of the dry weight of this coating. Clear coatings in flat, semigloss, or gloss finished can be formulated with these fluorinated resins, and conventional pigments can be used without difficulty. Basic formulations for fluorinated polyurethane coatings were devised using the polyol of Figure 2. The biuret 8 of hexamethylene diisocyanate (HMDI) shown in Figure 3, made by the reaction of three moles of HMDI with one mole of water, was chosen as the curing agent in order to maximize the resistance of the coating to chemicals and to weather. Titanium dioxide was used as a hiding pigment, and dibutyl tin dilaurate (DBTDL) was used at levels of 0.01 to 0.07 percent by weight of the curing agent to catalyze the cure. In early work the solvent consisted of a 20:20:60 volume percent blend of ethyl acetate, methyl isobutyl ketone (MIBK), and ethylene glycol monoethyl ether acetate, but we no longer use this solvent blend. Current formulations use a 50:50 volume percent mixture of η-butyl acetate and xylene, or methyl isobutyl ketone alone. All solvents are urethane-grade.



285

Figure 2. The structure of thefluorinatedpolyols. Rj is a 90:10 mixture of diols 1 and 2; R is as shown in Table I; R may be either R or R . 2

3

1

2

Table I. Properties offluorinatedpolyols R 3

2

Fluorine content, %

& 4 (82:4) 5 6

48.84 44.22 34.09

1

Equivalent weight 700 579 487

hydroxy or epoxy equivalent weight, based on two of each functional group.

Formulated binders such as these have surface energies of about the same magnitude as poly(tetrafluorœthylene). PTFE can be easily incorporated into the liquid coatings but, because it contributes no hiding in this resin, it must be considered as an extender pigment. Coatings containing up to 38 percent by volume of PTFE can be made, but these are somewhat soft and easily marred. We have found that 24 percent by volume is the optimum level for whiteness, hardness, and chemical resistance. A formulation for a typical fluorinated urethane coating filled with 24 volume percent of PTFE is given in Table II. Sources of ingredients used in the formulation are shown in Table III, and the ranges of composition acceptable for manufactured coatings are given in Table IV. This formulation is normally applied to a dry-film thickness of 75 jim (0.003 inch); at this spreading rate, 270 square feet may be covered with one gallon of paint. Thefluorinatedurethane is the topcoat in a three-coat system. After abrasive blasting of the steel, a primer and intermediate coat are applied. Both contain polyester polyols and aliphatic polyisocyanates. The time between blasting and priming must be less than four hours; times between the prime and intermediate coats, and between the intermediate and top coats must be less than sixteen hours. When these intervals are observed, no difficulties with the adhesion of the topcoat have been experienced. Thefluorinatedtopcoat may even be overcoated with itself; procedures customary for overcoating fully cured crosslinked coatings {i.e., epoxies or urethanes) must be observed.


286

H N-C-O O-C-l

N-C-O


8

Figure 3. The curing agent used in the formulation of thefluorinatedpolyurethane coating.

Applications Tank Linings (8). An organic lining in a steel fuel storage tank helps to maintain the purity of the fuel and helps to prevent minor fuel leaks. Large tanks always contain water at the bottom of the tank which accumulates naturally from condensation. The lining also forms a barrier to prevent this water from corroding the bottom of the tank. Fluorinated urethane coatings are being used as linings in shore-based petroleum storage tanks containing between 10,000 and 12,000,000 US gallons of fuel. The costs of emptying and cleaning these tanks, preparing their surfaces for coating, and applying three or more coats of a lining system are so great that the cost of the topcoat is not an overriding consideration. Significant savings can be achieved by using linings with very long service lives. Tests of fluorinated urethane tank linings began in 1978 at the US Naval Air Station in Norfolk Virginia. By comparison with other linings a service life of 30 or more years is anticipated for these materials. Fluorinated linings also expedite required periodic cleanings. The surface of the fluoropolymer permits most cleaning jobs to be performed with high pressure water hoses alone. No detergent is needed and oily waste may be processed in an oil-water separator, eliminating the high cost of waste water disposal. Linings of this type have been installed in fuel storage tanks at US Naval Air Stations in Norfolk, Virginia, Patuxent River, Maryland, Corpus Christi, Texas, Koshiba, Japan, Hakozaki, Japan, and Pearl City, Hawaii. They have been certified by the US Navy as their standard lining for fuel storage tanks, and are now being used in new tanks in Craney Island Virginia and Pensacola Florida. Ship Hulls. Although the coating which never fouls is not yet a reality, significant strides have been made in the formulation of low surface energy coatings which are easy to clean, and in the demonstration of their performance on various US Naval vessels (9). Fluorinated polyurethane coatings have been extensively trialed as non-toxic foulant release coatings for ships' hulls. These coatings are intended to


287 facilitate the release of barnacles, grass, dirt, algae and other fouling from hulls by permitting the formation of only weak bonds to the surface. These bonds are usually broken by the weight of the fouling or by the motion of the ship through the water.

Table II. Composition of PTFE-pigmentedfluoropolyurethanecoating 1


Ingredient

Weight

Component A Poly(tetrafluoroethylene) Titanium dioxide Fluoropolyol resin Methyl isobutyl ketone Dibutyl tin dilaurate solution J

3

4

5

Component Β Biuret of hexamethylene diisocyanate solution

6

1

Vdume

248.5 98.3 224.3 324.0 5.3

13.15 2.99 16.38 48.59 0.79

160.0

18.10

Measuring weight in pounds produces 100 gallons of coating; measuring weight in grams produces about one Liter of coating. Pigment-grade poly(tetrafluoroethylene), less than 6 microns average particle size. American Society for Testing and Materials, Specification D-476, Type IV. Different fluoropolyols have different equivalent weights, depending on composition (see Table I). This formulation is based on the copolymer having a hydroxy equivalent weight of 579. A solution of 1.7 percent by weight of DBTDL in MIBK. A solution of 75 percent by weight of the biuret of HMDI in a solvent containing equal weights of η-butyl acetate and xylene.

2

3

4

5

6

Table III. Sources of ingredients in thefluorinatedurethane coating

Titanium dioxide

PTFE

Company) Dibutyl tin dilaurate Biuret of hexamethylene diisocyanate

Ti-Pure R-960 (Ε. I. duPont de Nemours & Company) Kronos 2160 (Kronos, Inc.); formerly Titanox 2160 (NL Industries) Tioxide UF02 (Tioxide Specialties, Ltd.) Tiona RCL-6 (SCM Chemicals) Polymist F-5A (Allied Signal) TL-102 (Liquid Nitrogen Processing) SST-3 (Shamrock) Teflon MP 1200 (Ε. I. duPont de Nemours & M&T Chemicals, Inc.; Aldrich Chemical Company Desmodur N-75 (Bayer) Luxate HB 9075 (Olin Chemicals)



288 Early work with sheets of PTFE was discouraging, for the PTFE accumulated all types of marine fouling with astonishing speed {10). This is due to the porosity of PTFE: marine adhesives invade cavities in the surface and cure inside them, creating a secure mechanical interlock even in the presence of chemical incompatibility (77). However, thefluorinatedurethane binder fills these cavities and creates a smooth, low-energy surface which resists but does not escape attachment of fouling organisms. Tests of the various formulations have included conventional laboratory performance tests as well as the evaluation of static panels immersed at Chesapeake Beach Maryland, Key West Florida, and at Naos Island in the Bay of Panama. Trials of the fluorinated hull coating began in 1977, when a tugboat at the Norfolk, Virginia Naval Base was coated (72). This coating lasted until the boat was destroyed in an accident in 1989. During this time the coating did indeed accumulate fouling. Cleaning, if accomplished at 4- to 6-week intervals during the fouling season, could be accomplished by water from a standardfirehose, but left to accumulate, fouling became quite difficult to remove. The coating was rugged and sturdy, and effectively protected the hull from corrosion during its 12-year lifetime. Subsequently, the coating was applied to a 65-foot patrol boat and to a patrol hydrofoil vessel. Again, the coating proved durable and effective when cleaned regularly, but it did not remain free of fouling.

Table IV. Quality control test ranges for the PTFE-pigmentedfluoropolyurethanecoating in Table 11 Component A min max Grind, Hegman Viscosity, Krebs Units Pigment, percent by weight Volatiles, percent by weight Nonvolatiles, percent by weight Weight per gallon, pounds Drying time, tack-free, hours Flash point [Seta], °C

5 35 39.8 35.9 63.9 11.0

-85 40.0 36.1 64.1 11.2

Mixture A and Β min max

-

32.7 34.0 65.8 10.5

—

—

—

—

82

—

—

32.9 34.2 66.0 10.7 4 79

The introduction of foulant release coatings to the operating Navy involves several new concepts. Sailors are accustomed to having unfouled underwater hulls, and tend to dismiss the idea of a coating that will not resist fouling. In addition, an easily-cleanable coating has no value if it is not cleaned regularly; so to make the use of this coating realistic and practical, it has been necessary to implement a regular program of underwater hull maintenance wherever this coating is used.



Figure 4. A new tank at Patuxent River Naval Air Station, Maryland, lined with the NRLfluoropolymeranticorrosive coating


290


Submarine Radome Coatings. Films of water on antenna housings interfere with the transmission and reception of signals. Antennas must be retuned as water drains from their surface, and a strong and steady signal is obtained only when water is absent. Antennas on submarines are therefore more effective when they have a coating that allows water to drain from the surface rapidly. A standard submarine radome is about 22 feet long and has an oval crosssection with chords of 22 and 14 inches. The radome is raised and lowered against high density polyethylene bearings which are frequently contaminated with sand, and the long axis is vertical during use. The previous coating, an amidoamine-cured epoxy, retained a uniform film of water when new and was easily abraded. An unpigmented fluorinated polyurethane coating causes water to bead and run down the side of the radome, and has proven to be practical and effective in this application. It allows a one-third reduction in the time needed to achieve a constant signal because of its superior water-shedding properties, and delivers an extended lifetime due to its excellent abrasion resistance. Collection, Holding and Transfer (CHT) Tanks. The system aboard US Naval ships for accumulating, transporting, storing, and discharging septic waste is called the CHT system. Septic waste is contained aboard ship in CHT tanks until it can be discharged. The periodic opening and cleaning of these tanks is an unusually distasteful chore. Fluoropolymer coatings extend the interval between cleanings, reduce the time needed for cleaning, and preserve the steel walls and floor of the tank from corrosion. Fluoropolymer coatings have been trialed in septic tanks in the USS MCCANDLESS (FF 1084) and the USS MANITOWOC (LST 1180). The former ship is a Knox-class frigate with a standard displacement of 3,000 tons and a crew of 282; the test coating was installed in a cubic tank measuring about five feet on a side. The latter ship is a Newport-class tank landing ship with a standard displacement of 4,800 tons and a crew of 253; the tank with the test coating measured about 30 χ 15 χ 6 feet. On each ship the service life, corrosion protection, and cleanability of the fluorinated urethane coating very substantially exceeded that of the epoxy—polyamide coating it replaced. Anticorrosive Coatings for Ships' Bilges. Fluoropolymer coatings have demonstrated extended service life in the bilge of the aircraft carrier USS FORREST AL (CV 59). Test areas were cleaned by hand using a wire brush or a chemical cleaner; half of each section was primai with an epoxy-amidoamine primer, and half was left unprimed. The fluoropolyurethane coating was applied by brush to a dry film thickness of 5 mils over each of the four test areas. The coating was inspected regularly for 8 years, during which time all of the coatings remained intact and tightly bonded to the steel. At the final inspection, it was estimated that less than 5 percent of the coating had been removed, all of this apparently by mechanical damage. During its service life the coating was easily cleaned by hand scrubbing with detergent and water.


291


Future Work The special properties of these fluoropolymers suggest many applications. The resins are hydrophobic and oleophobic, and thus produce coatings and composites that can be easily cleaned. They offer promise as flame-resistant coatings because of their low heat-release characteristics. Fluoropolyols with low molecular weights react through their epoxy groups to form tough polymers useful as laminating resins, solution adhesives, and conformai coatings. The polymers also have a low refractive index which makes them especially attractive for applications in optics and electronics. The market for anticorrosion coatings in ship and shore applications will provide the initial impetus for sales of this coating while secondary markets develop. Conclusions Fluorinated polyols have been synthesized from a range of aliphatic and aromatic fluorinated diols. These polyols react with aliphatic polyisocyanates to form urethane coatings which retain their appearance, are easy to maintain, protect the substrate from corrosion, and provide superior resistance to heat, ultraviolet radiation and chemicals during a long service life. Seventeen years of tests in the marine environment have proven the outstanding endurance and cost-effectiveness of these coatings. References 1. 2. 3.

4. 5. 6. 7. 8. 9. 10. 11. 12.

Griffith, J. R.; Brady, Jr., R. F. Chemtech 1989, 19, 370-373. Brady, Jr., R. F. Chemistry in Britain 1990, 26 (5), 427-430. Zisman, W. A. In Contact Angle, Wettability, and Adhesion. Advances in Chemistry Series 43, American Chemical Society, Washington, DC, 1964, 1-51. Field, D. E . ; Griffith, J. R. Indus. Eng. Chem., Prod. Res. Dev. 1975, 14, 52-54. Farah, B. S.; Gilbert, Ε. E . ; Sibilia, J. P. J. Org. Chem. 1965, 30, 998. (1965). Urry, W. H.; Niu, J. H. Y.; Lundsted, L . G. J. Org. Chem. 1968, 33, 2302. McBee, E. T.; Marzluff, W. F.; Pierce, O. R. J. Am. Chem. Soc. 1952, 74, 444. Brady, Jr., R. F.; Griffith, J. R.; Thomas, R. Navy Civil Engineer 1993, 23 (2), 23-25. Brady, Jr., R. F.; Griffith, J. R.; Love, K . S.; Field, D. E. J. Coatings Technology 1987, 59 (755), 113-119. Saroyan, J. R.; Lindner, E . ; Dooley, C. Α.; Bleile, H. R. Indus. Eng. Chem., Prod. Res. Dev. 1970, 19, 123-128. Brady, Jr., R. F. Nature 1994, 32 (2), 23-25. Griffith, J. R.; Bultman, J. D. Naval Engineers Journal 1980, 92 (2), 129.


Formulations and Field Performance of Fluorinated Polyurethane

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