Solid State NMR Investigation of Coating Materials Prepared Using the SNAP Methodology
EMSL Project ID
2597
Abstract
The aging U.S. Air Force legacy aircraft fleet has a well-documented corrosion problem. Current coating systems generally incorporate a chromated conversion coating surface treatment and a solvent-borne epoxy-polyamide primer that is pigmented with a strontium chromate inhibitor. While this system is effective at reducing corrosion, toxic Cr6+ is present in both the surface treatment and coating. The U.S. Department of Health and Human Services has recently released the 9th Report on Carcinogens (January 2001) in which hexavalent chromium compounds, including the most widely used chromate corrosion inhibitor, strontium chromate, are listed as known human carcinogens. The EPA, OSHA and the FDA all have restrictive exposure limits on hexavalent chromium compounds. Both environmental and toxicity issues are compounded by the fact that this coating system is replaced every 6-8 years on average. The size of the aging legacy fleet of AA2024-T3 skinned aircraft is rather large and the current trend is to greatly extend these aircraft projected lifetimes, as much as four times the original estimate. Given the environmental impact of the de-paint/paint cycle, the Air Force has a specific need for a chromate free surface treatment and primer coating system that will provide adequate corrosion protection. In order to address these needs, the Air Force Research Laboratory has begun to develop new coating systems with chemically tailored surfaces and interfaces may provide an effective approach for attaining effective corrosion prevention without incorporation of heavy metal inhibitors. The Self-assembled NAnophase Particle (SNAP) coating process is hybrid sol gel coating process that is designed to provide superior adhesion between the primer coating and substrate and, if possible, chemically alter the surface of the substrate, rending it less susceptible to corrosion. Since AA2024-T3 surfaces are primarily composed of mixed aluminum oxides, we have investigated the use of compatible sol-gel techniques to produce very thin films that are capable of covalently bonding to the oxide. Silanes and other silicon-based sol-gel films are widely used as adhesion promoters for glass substrates, and Al-O-Si bonds have been observed for silane coatings providing a covalent adhesion component. SNAP solutions are prepared by drop wise addition of GPTMS and TMOS to aqueous solutions of dilute acetic acid. The solution is allowed to react for three days and a multifunctional amine crosslinking agent is added. Panels can then be dip coated to generate the SNAP film. Work is underway or has been completed using TOF/SIMS (Dr Mike Donley with EMSL collaboration), liquid NMR, infrared spectroscopy, and Raman spectroscopy to characterize the SNAP solution and the resultant coating with a without the crosslinking agent. Solid state 13C and 29Si will allow us to probe the species present in the SNAP solid state material to determine chemical species and degree of crosslinking. Some initial experiments were conducted to investigate the solid state properties of these materials using solid state NMR. In these studies the aqueous solution was evaporated to leave the solid organosilicate. One experimental difficulty that was identified was the concentration of the acid species as the water evaporated. This lead to difficulties due to both silicon and epoxy chemistry that could occur under these more acidic conditions. We propose to freeze dry SNAP solutions that have been prepared with different ratios of TMOS and GPTMS, while varying the reaction time and the presence of the crosslinking agents. By performing solid state 13C and 29Si NMR on these samples we will gain additional insight on these unique organosilane materials.
Project Details
Project type
Capability Research
Start Date
2002-09-01
End Date
2004-09-03
Status
Closed
Released Data Link
Team
Principal Investigator