The Army uses numerous adhesives and sealants, among other coating materials, that contain significant amounts of hazardous air pollutants (HAPs). This work examines laboratory and field demonstration/validation of one sealant, Torque Seal. A HAP-free alternative to Torque Seal containing ethanol as the carrier solvent has been identified. Laboratory testing including adhesion, resistance to fluids, resistance to humidity, and drying time validated that the HAP-free sealant performs very similarly to the baseline Torque Seal containing methanol (HAP). Furthermore, a demonstration study at Fort Rucker, AL, using a UH-1 helicopter rotor, shows that the HAP-free sealant performed as well as the Torque Seal.

Both the Baseline and HAP-free Torque Seal products were applied to the UH-1 helicopter rotor system on approximately 40 assemblies of nuts, bolts, and studs. On the day of application, a decision was made to place a bead of each sealant on every fastener, as seen here, for a more accurate side-by-side evaluation.
Torque Seal is used primarily to detect tampering or loosening of mechanical fasteners on military aircraft. Applied after bolts or fittings are in proper torque or position, the product gives inspectors visual evidence of any movement or tampering. Torque Seal dries to form a very brittle film that will crack, flake, or crumble when minimal force is applied. Other key product attributes include excellent adhesion to most surfaces and fast drying. The manufacturer refers to this product as an inspection seal lacquer or anti-sabotage lacquer. The fast-drying characteristic of Torque Seal is achieved by using low-boiling- point solvents as carriers, specifically ethanol and methanol. While both solvents are volatile organic compounds (VOCs), only methanol is classified by the EPA as a HAP. The reported HAP content of this material is 20% by weight. This report summarizes laboratory and field trial results for the HAP-free formulation of Torque Seal containing only ethanol, versus the current product containing ethanol and methanol.

The viscosities of the wet sealants were measured using a TA Instruments AR2000 rheometer in steady shear flow experiments using a 40-mm, cross-hatched parallel plate geometry with a peltier and a solvent trap containing ethanol, at 20 °C. Thermogravimetric analysis (TGA) was run on the samples of dried formulation solids using a TA Instruments TGA 2950. Wet formulation samples were dried in an oven until the sample weight remained constant. This dried material was pulverized in a coffee bean grinder to allow the release of any trapped solvent. Oven drying of the pulverized sample continued until the sample weight was again constant. The TGA instrument measures the sample mass as a function of temperature throughout the experiment. The percent mass remaining at the end of the run is the residual inorganic ash content. Three samples of each formulation were run to get a good measure of percent error.

The baseline and HAP-free sealants were applied to a uniform thickness of 6.5 mil on steel substrates and allowed to dry 24 hours at room temperature. Dried films were challenged by standing the panels vertically in a container of fluid so that about half of the film area was immersed. The films were examined 1 hour after removal from the fluid for blistering, delamination, and discoloration.

In the field demonstration, the HAP-free Torque Seal formulation and the baseline Torque Seal were applied side-by-side to a UH-1 helicopter rotor system. There was no perceptible difference in ease of application (spreadability, viscosity, runniness, etc.) or drying time between the two sealants. The sealants were examined for delamination and operationinduced cracking after 7, 14, 30, 60, and 90 days.

It was determined that the performance of the HAP-free formulations and current Torque Seal were similar with respect to delaminating and cracking. Both sealants adhered to clean, nongreasy surfaces longer than on greasy surfaces. As for cracking, both sealants displayed similar characteristics, cracking slightly within 60 days, then more over time as they aged, eventually maturing to a state of no additional cracks.

Laboratory testing indicated that the HAP-free Torque Seal using ethanol as the lone solvent performed similarly to the baseline product containing both methanol (HAP) and ethanol. Results were consistent across the complete test series. Testing showed little to no difference in rheology, drying time, adhesion, and weathering/exposure for the HAP-free sealant relative to the baseline product. Replacing the baseline Torque Seal with the HAP-free version will reduce HAP emissions at more than 20 installations across the Army.

This work was done by Faye R. Toulan and Felicia Levine of Data Matrix Solutions, Inc.; Leslie Hasenbein of Stanley Associates, Inc.; Genie Jones and Ivan Davis of the U.S. Army Aviation and Missile Command G-4; and John J. La Scala of the Army Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Materials category. ARL-0084


This Brief includes a Technical Support Package (TSP).
Validation of Hazardous Air Pollutant (HAP)-Free Torque Seal Inspection Lacquer

(reference ARL-0084) is currently available for download from the TSP library.

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This article first appeared in the December, 2009 issue of Defense Tech Briefs Magazine.

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