# FINAL REPORT

UV Curable Non-Chrome Primer and Advanced Topcoat System



## SERDP Project WP-1520

AUGUST 2010

Dr. Thomas Phely-Bobin QinetiQ North America

Distribution Statement B: Distribution authorized to U.S. Government Agencies onlyDistribution Statement B: Distribution authorized to U.S. Government Agencies only. Other requests for this docum ent must be referred to Ex ecutive Director, SERDP & ESTCP Office, 901 N. Stuart Stree t, Suite 303, Arlington, VA 22203.REVISED 03/24/06 Weapons Systems and Platforms WP-1520 

## Ultraviolet-Curable Non-Chrome Primer and Advanced Topcoat System

Background: Conventional coatings used by the Department of Defense (DoD) consist of pigments for color, appearance, and corrosion control in a polymer that serves as an organic binder. To facilitate appli cation, these substances are combined with a carrier, typically organic solvents that constitute up to 800 milligrams per liter of the coating. Before application, surfaces have to be cleaned, usually with solvents, and may be pretreated with a conversion coating, mainly chromates, to protect the surface and enhance adherence of the coatings. During application and cure, substantial amounts of vola tile organic compounds (VOC) are released. While in service, chromates and other hazardous components are released into the environment by flaking and chipping of the coating.

Objective: The objective of this project is to develop sprayable ultraviolet (UV)-curable, corrosion-inhibiting primers and high-performance topcoats that will provide superior protection to aluminum substrates even when nonchromated surface pretreatments are employed. Further, a one-coat formulation will be developed in an attempt to combine the corrosion-inhibiting properties of the primer and superior durability of the topcoat into a single coating.

Process/Technology Description: The coating system being developed is based on Foster-Miller's patented chemistry, consisting of formulations from oligomers of polyurethane (PU) and other substances that are end-capped with a vinyl functionality; reactive diluents are added to reduce viscosity. When irradiated with UV light, photoinitiators in the mixture generate free radicals that interact with the vinyl functionality, resulting in the rapid, complete reaction of all constituents, including diioscyantes, and leaving a tough, adhesive, barrier coating. Researchers plan to develop a compatible pretreatment/primer system from commercial surface preparation processes and nonchromate corrosion inhibition packages. The compatibility of the corrosion inhibitors with the coating formulations and their effect on the UV cure and coating performance will be evaluated. Topcoat development will involve modifying the backbone of the oligomers to improve performance and adding pigments to achieve the desired appearance. After selecting optimum formulations, full system testing will be performed and will include testing for compatibility with existing advanced performance coatings. The coating system will be designed specifically for aircraft interiors and target the coating performance requirements of the Joint Strike Fighter (F-35) and the Joint Unmanned Combat Air System (J-UCAS).

Expected Benefits: The environmentally benign coating system will have a long shelf life, will not require premixing, and will be applied using existing equipment. The coatings are safe to use as no vapors or diisocyanates are released during storage or application, and the wastes produced during depainting are nonhazardous. Of particular relevance to the Air Force is that the rapid UV cure will reduce the dry-to-fly time from days to minutes. In addition, the coatings can be used for rapid repairs in the field and are expected to substantially reduce aircraft maintenance and hazardous waste disposal costs. (Anticipated Project Completion - 2008)

Contact Information: Dr. Nese Orbey Foster-Miller, Inc. 195 Bear Hill Road Waltham, MA 02451 Phone: (781) 684-4170 Fax: (781) 290-0693 E-mail: norbey@foster-miller.com The UV-curable, zero-VOC, single-component coating system will have a longer shelf life.1 / 4 

### UV Curable Non-Chrome Primer and Advanced Topcoat System (WP-1520)

Objective
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.

Technical Approach
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.2 / 4 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.

Results
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 multifunctional3 / 4 backbone were synthesized. The resins were used either individually or in pairs, mixed in various proportions. A concentrated white pigment containing 60 percent TiO was dispersed in reactive 2 diluent. The TiO dispersions were stable and easily blended into the resins without settling.

2 Gloss white topcoat formulations were prepared by blending TiO dispersions into the resins, and 2 photoinitiators and reactive diluent added to produce formulations containing from 20 to 30 percent
TiO . The formulations were applied on various aluminum alloy coupons using a rubber roller or draw 2 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 TiO added had good adhesion and solvent 2 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 TiO level to 20 percent resulted 2 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 TiO produced a coating with 40 2 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.

Benefits
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 of4 / 4 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.

Points of Contact
Principal Investigator Dr. Thomas Phely-Bobin QinetiQ North America
Phone: 781-684-4022 tphely-bobin @foster-miller.com

Program Manager Weapons Systems and Platforms
SERDP and ESTCP

wp @serdp-estcp.org

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