Optimizing Paint Durability, Part II
It is not uncommon for military tactical systems to spend anywhere from 24 to 30 months in harsh marine environments. Consequently, the vast majority of once new equipment is in need of selective overhaul. Part two of this article discusses production methodologies for improvement of paint durability on these systems, with an emphasis on pretreatment and topcoat...
What is a Military Paint Finish?
Applicators have a variety of options when it comes to a paint finish on ferrous tactical and combat vehicles. Pretreatment can range from a surface blast (lowest level of performance) to a zinc phosphate pretreatment (highest level). Abrasive blasting is limited to relatively thick gauges of steel to preclude deformation in the blasting process. Paint can be applied by conventional means or electrostatically to provide enhanced edge coverage. Primers range from sprayed epoxy primers (lowest level) to epoxy-urethane electrocoat primers (highest level).
The benchmark for the pretreatment/primer system is the ability to withstand a 336-hr neutral salt spray after being scribed per ASTM D1654 to the substrate. Primed test coupons are scraped with a metal putty knife after a 24-hr recovery period to quantify scribe creep/paint delamination from a paint film defect and rust in the field. To date, only paint systems involving a zinc phosphate pretreatment have been able to meet this performance objective. Electrocoat primer systems are held to a significantly higher performance benchmark (ability to resist 1,000 hr of neutral salt spray).
When it comes to topcoats, two aliphatic polyurethane options are available. There is a 1K system and a 2K system. The former relies on humidity in the air for curing, and is the preferred topcoat by most applicators due to ease of use. Both primer and topcoat have unique military performance requirements imposed, which preclude "commercial equivalents" from being used. Some of the newer Army systems mandate galvanized steel, zinc phosphate pretreatments and electrocoat primers. Many of the Army systems, however, are legacy systems whose designs were finalized 15 or more years ago and are still being manufactured to the standards of that era.
Improving the Pretreatment
A durable paint finish that can resist paint film defects requires a strong foundation. A clean surface followed by a zinc phosphate pretreatment or one of the new dry-in-place pretreatments provides the ultimate receptor for the primer. Two seemingly "invisible" aspects of this process need to be addressed —the seal rinse and the final water rinse prior to primer application.
The best overall performance is obtained with hexavalent chromium-based seal rinses. Although the functional mechanism is still controversial, it works. Many platers who provide zinc phosphating services are familiar with controlling this heavy metal in their effluent. Most paint applicators, however, do not want the environmental burden.
Many non-chromium-based seal rinses will also meet the performance requirements. One approach that does not work is what might be best described as "hot dog technology." These are rinses based upon the salt, sodium nitrate. This is an excellent preservative for hot dogs but does nothing but promote osmotic blistering when used in conjunction with a pretreatment system. Considering the fact that paint finish societies such as the Steel Surfaces Painting Council (SSPC) go to great lengths to remove all residual salts prior to painting, the fact that some applicators were rinsing parts in salt water was unanticipated. The process controls within the pretreatment specification, TT-C-490E, provide an effective screening tool for selecting an appropriate seal rinse. Regrettably, some applicators were improperly interpreting the requirements. Process control samples are only meaningful if they duplicate production variables. When process control coupons receive 3–4 mils of primer but production receives 1.0–1.5 mils, salt spray test results are moot. Other salt spray coupons were found unscribed, overcoated with polyurethane topcoat, or not scraped after salt spray exposure. These alterations to the required protocol give a false positive when testing.
If you only process coil or sheet goods (no chemical drag-out between pretreatment stages) or live in a soft water region like Camden, CT (where the conductivity of the water is 100 µS/sq cm), the purity of your rinse water is not an issue. For everyone else, adding a clean water (reverse osmosis or deionized) rinse prior to painting will provide a significant improvement to long term paint performance. The benefit of this rinse is often not evident when conducting acceptance testing but is readily apparent when test coupons are tested to failure. A DI rinse is standard practice with electrocoat primers to minimize contamination to the paint tanks. The primary advantage of this additional operation is to remove/dilute unreacted chemicals from previous steps that are often trapped in the recesses of production parts. The other advantage is that it mitigates hard water deposits and other conductive salts which contribute to osmotic blistering. Applicators in Indiana where the water conductivity measurements are 670 +µS/sq cm and Ft Worth, TX (1,050 + µS/sq cm) are especially aware of these problems.
The Topcoat Contribution
Army topcoats were developed for military unique performance requirements: visual and spectral camouflage, chemical/live agent resistance and resistance to the decontamination solvent DS2. Automotive-type paint performance is secondary (i.e. nice-to-have but not mandated). Conventional wisdom dictates that for maximum exterior durability you need a high gloss (low pigment volume concentration-PVC) resin system that is UV stable, resistant to moisture permeation (void free) and flexible. The Army topcoats are flat gloss with a high PVC. The green color has a higher PVC than the tan. Considering the fact that the product literature alludes to "excessive porosity"2 in some of the topcoats and some topcoat formulators' technical bulletins warn of "blistering under hot and humid conditions," the benefit of the topcoat to corrosion resistance wasquestionable.
To determine the basic film formation properties of the 1K and 2K topcoats, the acceptance protocol followed by the epoxy primer and commercial polyurethane primers was duplicated—a 336-hr neutral salt spray over zinc phosphated test panels with waxed edges. Four paint applicators and one paint manufacturer applied the topcoats to sets of test panels. The topcoats were cured identical to production painted parts. All testing was conducted at the U.S. Army Test Laboratory at Rock Island, IL.
In all instances, the 2K urethane provided a void free, quality finish. The 1K urethanes were found to be extremely porous (rust spots) with the green being significantly more porous than the tan (see Figures 1 and 2). After the first phase of testing, it was unknown as to whether all the applicators had water contaminated paint delivery systems or if there was a problem with the formulation. Pigments used in the formula can contain adsorbed moisture. If there is an insufficient quantity of a moisture scavenger such as isophorone diisocyanate in the formula, the adsorbed moisture will present itself as porosity.3 In the second phase, curing methods, flash off times and reducing solvents were varied, but yielded the same results. Increasing the film build to five mils significantly reduced—but did not totally eliminate—the voids. The most effective means of creating a continuous urethane barrier was to double coat the test panels with a partial cure between coats.
The production methods witnessed at all OEMs thus far indicated that topsides of combat and tactical systems receive two to three coats of topcoat. This approach effectively eliminates the porosity potential in the 1K urethane topcoats and improves the corrosion resistance of the paint finish. Underbody components, however, receive only a single coat of topcoat. This may explain the dramatic difference in corrosion resistance noted between topside and underbody components after "sea duty." Failure to adopt a double coating practice when using these 1K urethanes can have serious consequences to long term paint performance. The pores in the topcoat provide a direct pathway for UV to degrade the epoxy primer.
The 2K urethane provides a much more corrosion resistant coating (all other performance features excluded) than the 1K urethane. If only a single coat topcoat is to be applied, it should be the 2K system to maximize corrosion resistance. The dramatic difference in salt spray resistance between the 1K and 2K urethanes can account for the difference in corrosion resistance between adjacent underbody components as these parts are often supplied by different vendors.
Coating technologies such as electrocoat and powder make it physically impossible to create the problems noted with wet-on-wet and high temperature forced curing which can thin and perforate the coatings. Corners need complete epoxy primer coverage to resist corrosion of the substrate. The reliance on topcoat alone to protect a steel substrate does not work when the topcoat is not designed for that function.
If you are purchasing a simple, easily replaceable commodity, visual paint inspection for gross defects is adequate. When a paint finish takes on a potential life support mission such as camouflage and live agent resistance where soldiers lives can be compromised, higher standards must apply. That simple door panel might not appear that it deserves a premium paint finish, but it may end up as a part of the armor solution in a ballistic door. A loss in material thickness due to corrosion from a faulty paint finish will compromise that armor system; it may no longer defeat that bullet. America's finest are being placed in harm's way. Please give that soldier the paint finish that he/she deserves.