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Single-Component Polysiloxane Topcoat for Navy Surface Ships

From JPCL November 2016

By Erick Iezzi and James Tagert, U.S. Naval Research Laboratory
Figure 1 Fig. 1: “Pinking” of an LSA haze gray silicone alkyd on the topside of a Navy surface ship. Figures courtesy of NRL.
Silicone alkyds have been the predominant exterior topside coating for U.S. Navy surface ships for over 50 years1. These single-component (1K) haze gray colored coatings were originally specified under TT-E-490E2 (enamel, silicone alkyd copolymer, semigloss) when first introduced, but have since been qualified to the performance requirements found in MIL-PRF-24635E3 (Coatings Systems, Weather-Resistant, Exterior Use). Silicone alkyd topside coatings have proven to be a user-friendly technology because they are an “all-in-one-can” system that does not require the addition of components or a catalyst before application. They have an indefinite pot-life in a closed can, are free of hazardous isocyanates, and will crosslink (i.e., cure) even under the most adverse conditions. 
 
Silicone alkyd coatings are based on alkyd polymers that have been reacted with silicone intermediates4. Coatings that contain silicone (i.e., polysiloxane) provide enhanced weatherability due to the presence of silicon-oxygen (Si-O) bonds, which have a greater bond strength than the carbon-carbon (C-C) bonds found in all-organic coatings, such as epoxies. Although haze gray silicone alkyds demonstrate improved exterior performance compared to straight alkyds5, they nonetheless lose their gloss, chalk and/or color fade to a light gray within 9-to-12 months after application on Navy ships, and silicone alkyds formulated with low-solar-absorbing (LSA) pigments (i.e., titanium dioxide, yellow iron oxide, red iron oxide and copper phthalocyanine blue) have been observed to color-shift towards a pinkish hue after six months of exposure (Fig. 1). Silicone alkyds also provide a three-fold longer recoat window than straight alkyds5, yet their surface hardness is reduced due to a lower crosslink density.  As a result, running rust, soot and other contaminants will easily stain the surface of these Navy topcoats and produce an unsightly appearance. To address these performance issues, sailors are continually over-coating weathered and/or stained silicone alkyds with fresh silicone alkyd, which perpetuates a cycle of topside painting and increased maintenance costs to the Navy (Fig. 2). For years, silicone alkyds have provided the Navy with adequate performance. However, the need to maintain visual camouflage and aesthetics for longer periods of time, in addition to reducing maintenance costs as ship life cycles are extended, has meant that silicone alkyds can no longer provide the performance that is required for Navy ship topside coatings.
 
Figure 2 Fig. 2: Sailors roll-apply fresh silicone alkyd.
Two-component (2K) polysiloxane topside coatings have been qualified to MIL-PRF-24635E requirements by several manufacturers during the past decade.  These topcoats are hybrid cure coatings based on the reaction of organic functional groups (i.e., epoxies with amines or acrylates with amines) and moisture-curable alkoxysilanes that hydrolyze and crosslink to form polysiloxane linkages6. Haze gray 2K polysiloxanes have demonstrated a significant enhancement in color and gloss stability compared to silicone alkyds, in addition to providing greater cleanability and hydrocarbon resistance. However, these 2K coatings require the mixing of components before application, which can result in insufficient cure and/or reduced performance if the coatings are not mixed at the proper ratios or for the specified lengths of time. These systems also have a limited pot-life once mixed, which can be troublesome for sailors attempting to paint on hot days. 
 

Single-Component Polysiloxane Topcoat

The U.S. Naval Research Laboratory (NRL) has recently developed novel moisture-curable alkoxysilane-terminated N-substituted urea polymers for use in single-component polysiloxane topcoats for military assets7, 8.  As a haze gray topcoat for Navy ships, the coating provides significantly greater exterior durability and resistance to contaminants compared to qualified silicone alkyds, in addition to demonstrating greater color stability and flexibility compared to qualified two-component polysiloxanes. The 1K polysiloxane offers applicators, especially sailors, a user-friendly system for painting ship topsides, in addition to reducing the generation of hazardous waste by providing for a longer pot-life compared to 2K systems.
 

Experimental Procedure

Figure 3
Fig. 3: A comparison of flexibility between NRL’s 1K polysiloxane, a qualified silicone alkyd and various 2K polysiloxane topside coatings after being bent over a ¼-inch cylindrical mandrel.
NRL’s LSA haze gray single-component polysiloxane topcoat was formulated to provide a system that contains less than 250 g/l of volatile organic compounds (VOCs); is free of hazardous air pollutants (HAPs) and isocyanates; possesses a flash point (Fp) of greater than 100 F; can be brushed, rolled or sprayed; and will cure over a wide range of temperatures and humidity to provide a semi-gloss finish. The coating is currently being qualified to MIL-PRF-24635E, Type V (high-durability coating), Class 2 (Semigloss), Grade B (LSA pigments) performance requirements, which include laboratory tests and active ship demonstrations. For all tests, the 1K polysiloxane was applied by roll, spray or drawdown bar to achieve a dry film thickness (DFT) of 2-to-4 mils, then allowed to cure at ambient conditions (72 F, 50-to-60-percent RH) for 14 days before testing occurred. Adhesion tests were performed over MIL-DTL-24441, Type IV epoxy, MIL-PRF-23236, Type VII high-solid epoxy and other primers according to MIL-PRF-24635. Adhesion was also tested over weathered silicone alkyds according to ASTM D33599. Solvent resistance was tested in accordance with ASTM D540210, whereas flexibility was tested over sanded 3-by-6-by-0.025-inch 3003-H14 aluminum panels according to ASTM D52211. Accelerated weathering was tested in accordance with ASTM G155 and ASTM G15412,13, and atmospheric exposure (i.e., outdoor) testing was conducted on panel racks in Key West, Fla. Qualified LSA silicone alkyds and 2K LSA polysiloxane topcoats were applied at similar DFTs to the 1K polysiloxane for comparative testing.
 

Results

The haze gray 1K polysiloxane topcoat demonstrated excellent adhesion to a variety of 24-hour- and seven-day-cured primers, showing an X-Cut rating of 5A in all cases. The 1K also demonstrated a 5A rating over both weathered silicone alkyds and weathered silicone alkyds that were sanded and solvent wiped. In addition, 1K polysiloxane possesses excellent hydrocarbon resistance and passes over 100 double-rubs with a methyl ethyl ketone (MEK)-soaked rag without marring, whereas silicone alkyds cured for the same period will only resist 25-to-50 double rubs before rubbing through the coating. However, even though the 1K polysiloxane possesses greater solvent resistance than silicone alkyds it remains highly flexible, unlike the two-component polysiloxanes. Figure 3 shows a comparison of flexibility between NRL’s 1K polysiloxane topside coating, a qualified silicone alkyd, three qualified 2K polysiloxanes and a non-qualified 2K polysiloxane after being bent on a ¼-inch cylindrical mandrel at room temperature. As can be seen in the photo, all 2K polysiloxanes cracked after partial bending on the mandrel, whereas the 1K polysiloxane and silicone alkyd did not. Even after being heated at 140 F for 24 hours the 1K polysiloxane was able to pass a conical mandrel bend test without cracking. 
 
Figure 4 Fig. 4: Color change after atmospheric exposure of low-solar-absorbing haze gray topcoats.
With regard to color stability, the 1K polysiloxane demonstrated a color change (ΔE) of less than 1.0 after 2,000 hours of accelerated weathering in a Xenon Arc Weatherometer (WOM) chamber, in addition to a color change of 0.08 after 300 hours of exposure in a UV-B (313 nm bulb) chamber.  Furthermore, exposure of the 1K polysiloxane, a qualified Type II and Type III silicone alkyd, and two qualified 2K polysiloxanes on an atmospheric exposure rack in Key West, Fla. for a period of 1.7 years showed that the 1K polysiloxane retained greater color stability with a color change of only 0.92 (Fig. 4).
 
During the past two years, multi-gallon quantities of the haze gray 1K polysiloxane have been scaled-up by a coating manufacturer for topside demonstrations aboard active Navy ships. These include applications aboard the USS Winston S. Churchill (DDG-81), USS Chosin (CG-65), USS Cowpens (CG-63), and the USS San Antonio (LDP-17) from 2014 through 2015, in addition to applications on the USS Laboon (DDG-58) and USS Gunston Hall (LSD-44) in 2016. In November of 2014, the 1K polysiloxane was roll-applied by Ship’s Force on approximately 400 square feet of the 03 Level Starboard bulkhead aboard the USS Cowpens. The substrates were an abraded and solvent-wiped silicone alkyd and a 24-hour-cured MIL-PRF-23236, Type VI/VII high-solids epoxy primer. Figure 5 shows the bulkhead immediately after the 1K polysiloxane was roll-applied and began to cure. The 1K coating was inspected in June of 2015, and after seven months of service the color change (ΔE) was approximately 0.6 and the loss of gloss was 8 percent. It should be noted that these topside demonstrations are inspected by NRL every three-to-six months, or when ships are available in port.
 
Figure 5 Fig. 1: “Pinking” of an LSA haze gray silicone alkyd on the topside of a Navy surface ship. Figures courtesy of NRL.
 

Conclusions

NRL has recently developed a novel moisture-curable, haze gray single-component (1K) polysiloxane topcoat for Navy surface ships. The topcoat contains reduced levels of VOCs, is HAPs-free and provides excellent hydrocarbon resistance, flexibility and weatherability, especially when compared to silicone alkyds. The topcoat has also demonstrated excellent performance when applied on active surface ships. The 1K polysiloxane provides applicators with a “user-friendly” system that eliminates the measuring and mixing of multiple components, eliminates potential curing and performance issues associated with incorrectly mixing components, in addition to providing for a longer pot-life and reducing the amount of hazardous waste generated from unused mixed materials.  JPCL
 

Acknowledgments

The authors would like to thank the Naval Sea Systems Command (NAVSEA) Paint Center of Excellence (PCoE) and the Office of the Secretary of Defense (OSD) Corrosion Policy and Oversight programs for supporting this research. The authors would also like to thank NRL co-workers, NAVSEA 05P2, GK Consulting, Corrosion Control Services, and NAVSEA’s Corrosion Control Assistance Team (CCAT).
 

ABOUT THE AUTHORS

Figure 1
 
Erick Iezzi   Erick Iezzi is a Ph.D. organic chemist with over 10 years of experience developing novel molecules and coating technologies. At NRL, he is the principal investigator of several basic and applied research programs and his primary areas of research are the synthesis of novel small molecules, oligomers and polymers, the formulation of thermosetting coatings, the study of coating surface interactions with liquids and coating degradation due to weathering and corrosion, and mechanical property testing of coatings. Several of Iezzi’s technologies are currently being demonstrated on Navy surface ships and submarines. 
 
James Tagert   James Tagert is a materials research engineer working at the Naval Research Laboratory and has over 10 years of experience working in the coatings industry. He graduated from the University of Maryland in 2004 earning a Bachelor of Science degree in mechanical engineering and is a member of both the American Society of Naval Engineers and NACE International. Tagert has worked at NRL since 2008 supporting U.S. Navy research and engineering programs related to materials science with an emphasis on the development and transition of advanced coating systems.

 

References

  1. Witucki, G. L. “The Evolution of Silicone-Based Technology in Coatings,” Dow Corning Corporation, 2003.
  2. TT-E-490E Federal Specification, “Enamel, Silicone Alkyd Copolymer, Semigloss (For Exterior and Interior Use),” September 25, 1975.
  3. MIL-PRF-24635E Performance Specification, “Coating Systems, Weather-Resistant, Exterior Use,” September 15, 2009.
  4. Hiles, H. C.; Golding, B.; Shreve, R. N. “Copolymerization of Alkyd Silicones for Coatings,” Ind. Eng. Chem., 1955, 47, 1418-1424.
  5. Androit, M., et. al.  “Silicones in Industrial Applications,” Dow Corning Corporation, 2007, pp. 30.
  6. Witucki, G. “Next Generation Polysiloxane Hybrid Coatings,” Polymers Paint Colour Journal, November 2012, 22-24.
  7. Iezzi, E. B. “Single-Component Moisture-Curable Coating Based on N-Substituted Urea Polymers with Extended Chains and Terminal Alkoxysilanes,” U.S. Patent 9,139,753, 22 September, 2015.
  8. Iezzi, E. B. “Single-Component Coatings Having Alkoxysilane Terminated N-Substituted Urea Resins,” U.S. Patent 8,133,964, 13 March, 2012.
  9. ASTM D3359, “Standard Test Method for Measuring Adhesion by Tape Test.”
  10. ASTM D5402, “Standard Practice for Assessing the Solvent Resistance of Organic Coatings Using Solvent Rubs.”
  11. ASTM D522, “Standard Test Methods for Mandrel Bend Test of Attached
  12. Organic Coatings.”
  13. ASTM G155, “Standard Practice for Operating Xenon Arc Light Apparatus for Exposure of Non-Metallic Materials.”
  14. ASTM G154, “Standard Practice for Operating Fluorescent Ultraviolet (UV) Lamp Apparatus for Exposure of Nonmetallic Materials.”
 

Tagged categories: Coating Materials; Coating performance; Erick Iezzi; Features; Marine; Marine Coatings; Performance testing; Polysiloxane; Single component coatings; Topcoats; U.S. Navy

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