The term "Marshall Convergent Coating-1" ("MCC-1") denotes an improved formulation and a concomitantly improved method of spray deposition of a cork-and-glass-filled epoxy ablative thermal-insulation material. MCC-1 has been used on the space shuttle solid rocket booster and on some Air Force and commercial rockets, and at least one aircraft manufacturer has expressed interest in commercial applications of MCC-1.
MCC-1 was developed to replace an older formulation and spray deposition process that entailed four major disadvantages: (1) hazardous solvents were used; (2) the older process was a batch process and, as such, was limited by a pot life; (3) it was necessary to sand each substrate prior to spray deposition of the material; and (4) the material tended to come off during flight and/or splashdown.
In comparison with the older formulation and process, MCC-1 is environmentally friendly because it does not involve the use of any solvents. The MCC-1 process is a continuous (albeit interruptible) rather than a batch process: the ingredients are mixed only during spraying for immediate application on demand; consequently, the process can be started or stopped at will and pot life is no longer an issue.
The system of equipment used in MCC-1 for the space shuttle includes the following:
- Pots for holding and subsystems for delivering the wet and dry ingredients, which are the epoxy resin, epoxy catalyst, ground cork, and micron-size glass spheres;
- A Convergent Spray Technology (CST™) nozzle, which is designed specifically for use in this process;
- Process-control and data-acquisition equipment;
- A spray-cell with environmental control system;
- A robot that moves the CST nozzle; and
- A turntable on which the substrate to be coated is placed.
The epoxy adhesive is atomized in a flow of air and sprayed by use of the nozzle. The ground cork and glass micro-spheres are delivered via an eductor air line, mixed in a cyclonic mixer, then injected into the adhesive spray plume outside the nozzle. The composition of the sprayed and deposited material is controlled by regulating the rates of flow of the individual ingredients. The pattern of deposition of the material on the substrate is controlled by regulating the trajectory of the robot and the speed of the turntable. Once the spraying process has been completed, the deposited material is cured by gradually heating it to a temperature between 112 and 200 °F (between 44 and 93 °C) and holding it at that temperature for at least 9 hours.
In comparison with the older batch process, the mix-and-spray-on-demand MCC-1 process generates significantly less waste and thus necessitates less cleanup. Unlike in the older process, it is not necessary to sand the substrate prior to spraying. The flatwise tensile strength of the MCC-1 deposited mate-rial is superior to that of the material deposited by the older process. In addition, thus far, little or no falloff of MCC-1 material in flight or splashdown has been observed.
This work was done by Carl N. Lester of Marshall Space Flight Center and Samir V. Patel formerly of USBI. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category. MFS-31295