Investigation of Residue and Coating Stoichiometry on 120-mm Combustible Cartridge Cases

摘要

An investigation was conducted to determine the cause of coating residue found in the test gun chambers during qualification firing of 120-mm combustible cartridge case (CCC) ammunition for the MlAl/A2 main battle tank. The CCC is coated with a clear epoxy- polyamide topcoat and a highly pigmented aluminum epoxy basecoat. A laboratory analysis was performed on actual residues; parts of the CCC retain samples of actual coatings and laboratory control samples. Since there are six different resins used in this two-component coating system, production errors and deviations from correct mixing prior to application could have caused the residue. Any variation from the specified stoichiometry could also result in films with lower cross-link densities and lower glass transition temperatures (Tg). Rounds fired at 120 C have more residues than identical rounds fired at ambient temperature; additional weeks of cure lower the amounts of residue. A differential screening calorimeter (DSC) analysis of the various samples revealed that a suspect lot of ammunition with higher levels of residue had lower Tg values. Fourier-transform infrared (FTIR) was effective in monitoring the degree of cure of the epoxy coating system. Analyzing the residue from actual firings showed similar features. Koenig hardness values showed variations caused by dry film thickness, but also showed insignificant changes caused by transformations in the stoichiometry of the clear topcoat from 52.5 to 45 phr (pounds per hundred of resin).