Final Report: Low-to-No VOC "Chromium-Free, Permanent Primer" (CFPP) CoatingEPA Contract Number: 68D03025
Title: Low-to-No VOC "Chromium-Free, Permanent Primer" (CFPP) Coating
Investigators: Kovar, Robert F.
Small Business: Foster-Miller Inc.
EPA Contact: Manager, SBIR Program
Project Period: April 1, 2003 through September 1, 2003
Project Amount: $69,988
RFA: Small Business Innovation Research (SBIR) - Phase I (2003) RFA Text | Recipients Lists
Research Category: Nanotechnology , SBIR - Nanotechnology , Small Business Innovation Research (SBIR)
Conventional aircraft coating systems consist of topcoat, chromated primer, and chromated conversion coat layers. These coating systems typically are used for a maximum of only 6 years, mainly because of decreased topcoat performance. When this occurs, the entire coating system is removed (down to bare metal) by sanding or media blasting. An entirely new coating system then is reapplied, releasing chromate residues and volatile organic compounds (VOCs) into the environment. This is done primarily because there is no currently available method for selectively stripping the topcoat without damaging or removing the primer layer and underlying conversion coating. A top priority for the U.S. Environmental Protection Agency (EPA) is to eliminate the use of chromates from aircraft coatings because they are toxic to humans and the environment.
Foster-Miller, Inc., has demonstrated the feasibility of developing a 30+ year mission lifetime, Chromium-Free Permanent Primer (CFPP) aircraft coating system that consists of a commercially available, Boegel chromium-free sol-gel conversion coat; an abrasion-resistant, polyurethane-based permanent primer layer containing a chromium-free, zinc phosphate corrosion inhibitor that forms durable chemical bonds with both the conversion coat and topcoat; and a conventional topcoat. The low VOC spray-coatable CFPP was designed to exhibit abrasion resistance against plastic media blast (PMB) stripping that is significantly higher than the topcoat, enabling selective stripping of the topcoat to be conducted without removing or damaging the underlying primer and conversion coat layers. When the topcoat requires replacement, it is selectively stripped off using a specially developed PMB process and material. A fresh topcoat is reapplied to the original primer and conversion coat layers, leaving them in place to continue their corrosion-protective function. The CFPP avoids the hazards and expense of having to remove and reapply both primer and conversion coating each time the topcoat needs replacement, thereby eliminating chromium completely and reducing (by approximately 80 percent) the number of coating application/removal steps over the 30+ year mission lifetime of the aircraft. All CFPP and control-coated test panels were spray-coated at Dampney using Military Specification (Mil Spec) VOC levels.
The Deft commercial topcoat was only selectively removed from Deft topcoat/CFPP primer/Boegel conversion coat/aluminum substrate spray-coated test panels, leaving the CFPP primer and Boegel conversion coat layers intact. More than 80 percent of the topcoat was removed during these tests without penetrating or removing any of the primer or conversion coat layers. Further refinement of the CFPP elastomeric binder resin durometer properties in Phase II is expected to increase the percentage of selective topcoat removal to greater than 90 percent. Application of a fresh topcoat over a small percentage of residual, tightly bonded original topcoat particles is expected to produce a fully functional and durable new topcoat. In addition, the CFPP primer, which was about 5.0 mil thick to prove the principal of selective stripping, will be reduced in thickness in Phase II for aircraft applications. The current primer thickness is acceptable for building structures and industrial equipment applications.
Candidate CFPP/Boegel conversion-coated test panels without topcoat passed primer Mil Spec MIL-PRF-23377H tests for the following properties: hardness, adhesion, water resistance, solvent resistance, resistance to hydraulic fluid, resistance to lubricating fluid, and acceptable VOC level. Candidate CFPP primer coating formulations were within the Mil Spec of 2.8 lb/gal VOC (1.96 to 2.55 lb/gal) and were all spray-coated onto Boegel conversion-coated aluminum test panels.
Candidate CFPP primer/Boegel conversion-coated test panels without topcoat passed the Filiform corrosion resistance test. Traces of corrosion were detected within the crosshatch scratches of CFPP test panels, but not within chromated control panels. In Phase II, the use of Dampney dual component, chromium-free corrosion inhibitors that are expected to protect even crosshatch scratches from corrosion during the Filiform test will be evaluated. Candidate CFPP primer/Boegel conversion-coated test panels currently are undergoing long-term salt fog tests, which appear to be proceeding without evidence of corrosion thus far (1,000 hours of continuous exposure). Test results will be reported to EPA in an addendum to this Final Report upon completion of 2,000 hours of testing. Table 1 summarizes the test results obtained during this research project.
Table 1. Phase I test results for candidate chromium-free permanent primer (CFPP) coatings on Boegel conversion-coated aluminum test panels.
|Selective stripping of topcoat from Deft topcoat/CFPP coat/Boegel conversion-coated aluminum test panels||PMB directed onto topcoat at 30 or 50 psi air pressure||More than 80 percent of topcoat was removed without removing the primer layer.|
|Adhesion||Tape Test; ASTM D 3359||Adhesion Level No. 5 with nodelamination for CFPP/Boegel coatings and topcoat/CFPP/Boegel coating test samples.|
|Flexibility||Variation of MIL-PRF-23377H 4.5.5||For topcoat/CFPP/Boegel coating test samples, no change. For CFPP/ Boegel coating test samples, small cracks detected.|
|Water Resistance||Immersed in 49°C water for4 days; MIL-PRF-23377H 4.5.7||For CFPP/Boegel coating-Pass (smooth, cohesive coating after immersion). For Deft controlprimer coat/chromate conversion coat–Fail (blistering).|
|Resistance to Lubricating Oil||One-half immersed in 121°C water for 24 hours; MIL-PRF-23377H 4.5.10a||For CFPP/Boegel coating–Resistance Level No. 5 (nodelamination but detection of blister coating defects). For topcoat/CFPP/ Boegel coating test samples–Pass (no change).|
|Resistance to Hydraulic Fluid||One-half immersed in 65°C water for 24 hours; MIL-PRF-23377H 4.5.10b||For CFPP/Boegel coating–Pass.For topcoat/CFPP/Boegel coating test samples–Pass (no change).|
|Resistance to Solvent||MEK Rub; ASTM D 5402–93||For CFPP/Boegel coating–Pass. For topcoat/CFPP/Boegel coating test samples–Pass (no change).|
|Corrosion Resistance||Filiform; ASTM D 2803||For CFPP/Boegel coating–Pass (no filament corrosion). For topcoat/ CFPP/Boegel coating test samples–Pass (no filament corrosion).|
|Corrosion Resistance||Salt Spray; ASTM B 117||In progress, at 1,000 hours (halfway through test). No blistering, lifting of either coating, nor substrate pitting.|
Continuation of this work in a Phase II research project is strongly recommended based on the successful Phase I results. During this Phase I research project, commercial teaming partners were: (1) Dampney, a commercial coating/paint manufacturing company with a proprietary nonchrome corrosion inhibitor; and (2) Maxiblast, an aircraft coating removal company. These partners worked with Foster-Miller, Inc., to ensure technical success and rapid transition of the CFPP technology into the commercial sector. The enhanced environmental protection, improved performance, and low mission lifetime cost of the proposed CFPP aircraft protective coating system will encourage its immediate acceptance for military and civilian aircraft, ships, automobiles, building structures, and industrial equipment. Cost savings will be realized because of the reduction in expenses associated with repeated coating system removal/application, worker protection, worksite environmental quality and cleanup, and waste disposal.
The results of this Phase I research project will be presented at a National Meeting of the Society for the Advancement of Materials and Process Engineering as soon as patent protection has been obtained.