Conventional coatings used for Department of Defense (DoD) vehicles and equipment contain several components that are being discontinued due to their adverse health and environmental effects. These include volatile organic compounds (VOCs), which constitute up to 800 mg/L of the coating formulation, and hexavalent chromate anticorrosion pigments. VOCs and chromates are also used to prepare the substrates prior to coating. These hazardous substances are released into the environment throughout the life cycle of the coating from coating manufacture to surface preparation, application, service life, and decoating. Containing these releases with control equipment, hazardous waste disposal sites, and monitoring instrumentation is expensive and imposes an added logistics burden on the military.
The National Emissions Standards for Hazardous Air Pollutants for Aerospace Manufacturing and Rework Facilities severely curtails or eliminates the use and release of hazardous substances in coatings. The coating industry has since produced a variety of products that are essentially free of hazardous substances, but these new materials generally do not meet the rigorous and unique DoD performance requirements. Consequently, coating the large fleet of U.S. military aircraft and other vehicles continues to cost the DoD hundreds of millions of dollars each year in the procurement, use, and disposal of toxic and hazardous materials.
The objective of this program is to develop a family of advanced coatings that meets current and anticipated environmental regulations and at the same time simplifies and quickens the coating application process for military applications.
Although previous versions of Foster-Miller's (FM) ultra-violet (UV)-curable polyurethane formulations have been shown to have superior adherence and barrier properties and meet many of the performance specifications for aircraft coatings, several issues have to be addressed for the coatings to succeed in the intended application. A key challenge is that the coatings have not yet been used as primers, and incorporation of anticorrosion pigments into the formulation will impede the UV cure process and possibly have an adverse effect on other properties. In addition, although strongly adherent and durable, the polyurethane vinyl dioxolane (PUVD) resins do not meet the new Mil-Spec requirements for flexibility. Further, a pigment has to be introduced into the topcoat to match the specified Federal color code, and may also hinder penetration of UV irradiation during cure. Finally, the viscosity of the PUVD resins has to be reduced to a level suitable for DoD HVLP spray equipment.
This program for the development of an advanced primer and topcoat from FM’s baseline PUVD resins included:
- Incorporating anticorrosion and color pigments and keep them stably dispersed in the formulations.
- Modifying the polymer backbone chemistry to enhance its flexibility.
- Introducing additional reactive diluents into the formulation to reduce its viscosity.
- Increasing the depth of cure by judicious selection of pigment type and concentration as well as use of state-of-the-art photoinitiators and UV equipment.
Making other changes to the resin chemistry and the formulation as necessary to produce coatings that are sprayable, cure rapidly, and meet all relevant Mil-Spec standards.
Primer formulations with a range of pigment concentrations were prepared by blending anticorrosion pigment dispersions into the baseline formulation, and adding the photoinitiators and other additives, as required. The primers were then applied to aluminum alloy test coupons, cured, and subjected to a preliminary set of performance and corrosion tests. Final versions of the primer contain three blends of Sartomer reactive diluents SR489, SR506 and SR351, and could be sprayed at pressures as low as 60 psi to produce smooth coatings 0.6 to 1.0 mil thick. All formulations showed good adhesion, fluid resistance, impact resistance, solvent resistance, and low-temperature flexibility. It was found that increased amounts of SR351 in the formulation increased the solvent resistance of the coating, but decreased pigment leaching, implying less corrosion protection.
Several formulations were developed for UV-curable primers without solvents. These were successful adhering to Sulfuric Acid Anodized (SAA) panels and in corrosion protection to those panels. The basic PUVD resin system using reactive diluents for sprayability were found not to provide corrosion inhibition to Chromate Conversion Coating (CCC) panels. The SAA panels have inherently more corrosion protection relative to CCC panels. This general conclusion is made under the assumption that the high degree of functionality of the resulting system tended to prevent mobility of the corrosion inhibitors. It is thought that more use of exempt solvents rather than reactive diluents may eventually work through increased mobility with the basic FM PUVD resin system. The SAA panels were inherently more resistant to corrosion.
Much of the topcoat development was directed toward enhancing the flexibility of the baseline PUVD formulation to meet the 40 percent elongation requirement. To this end, two new resins having polyether modified backbones of different chain length, and a third resin with a multifunctional backbone were synthesized. The resins were used either individually or in pairs, mixed in various proportions. A concentrated white pigment containing 60 percent TiO2 was dispersed in reactive diluent. The TiO2 dispersions were stable and easily blended into the resins without settling.
Gloss white topcoat formulations were prepared by blending TiO2 dispersions into the resins, and photoinitiators and reactive diluent added to produce formulations containing from 20 to 30 percent TiO2. The formulations were applied on various aluminum alloy coupons using a rubber roller or draw rod. Thicker coatings, up to ~2mils were cast, and in other cases the coating was applied in multiple thin layers to obtain the desired thickness.
As expected, the baseline resin with 30 percent TiO2 added had good adhesion and solvent resistance, but its GE Impact flexibility was only 0.5 percent. By using various combinations of the polyether-modified resins and alternative reactive diluents, the flexibility was progressively increased to 10 percent but with some loss of solvent resistance. Reducing the TiO2 level to 20 percent resulted in an increase in flexibility to 20 percent but, as a result of the lower opacity, the color of these resins was unacceptably sensitive to coating thickness. Further trials with blends of the polyether resins and the multifunctional resin in formulations containing 30 percent TiO2 produced a coating with 40 percent flexibility and good solvent resistance. The investigation then focused on increasing the depth of cure to avoid the cost and time of applying the coating in multiple layers. The approach previously developed by Foster-Miller of using pairs of photoinitiators with different absorption spectra, namely one with a longer wavelength response for in depth cure, and one with a shorter wavelength response for surface cure, was further pursued. Several photoinitiators were investigated, with those having adsorption spectra extending into the visible light range proving most useful. To overcome compatibility and other issues, different reactive diluents had to be used in combination with the selected photoinitiator pair. It was found that good cure to a depth of ~ 2 mil could be obtained with these photoinitiators and a resin consisting of a ~70:30 mixture of the polyether and original PUVD resins, combined with a reactive diluent mix and a small amount of adhesion promoter. General results found that formulations had general poor adhesion to CCC substrates while having generally good adhesion to SAA substrates. The color matching was generally fair with poor gloss being achieved.
The combination of a chromate and VOC free substrate-preparation and coating system is expected to meet the needs of a growing market that is being driven by increasingly stringent environmental legislation. Although the UV-cure process limits the system to use by professionals, potential applications are extensive. The baseline PUVD resin has previously been used to produce low-observable coatings and one-coat repair formulations for military aircraft, and the availability of the high-performance primer and topcoat developed here will extend this market segment to virtually all the aerospace needs of the Air Force, Navy, commercial airlines, and NASA. In addition, the coating system can be relatively easily tailored to meet specifications for the ground and marine vehicles/equipment of all branches of the military. Other market segments include industrial coatings in chemical and mechanical plant, as well as civil structures including bridges.