Another major concern in using organic materials in space is outgassing and degradation in a thermal-vacuum environment. Outgassed products can condense on optical lenses and instruments or create a corrosive ambient for electronics. The main mode of outgassing consists of the volatilization of low-molecular-weight, high-vapor-pressure species. These may consist of resins and hardeners that have not completely reacted due to insufficient cure or use of nonstoichiometric amounts of reactants. Additives such as plasticizers, flame retardants, diluents, and solvents are also vulnerable to outgassing. In some cases, outgassing may be due to sublimation which is the vaporization of a solid without going through the liquid state, followed by redeposition from the gaseous state onto a cooler surface. At sufficiently high temperatures, all polymers decompose by the rupturing of atomic and molecular bonds and by the release of smaller more volatile fragments.
Outgassed products from plastic materials constitute serious contaminants in both unmanned and manned spacecraft because of one or more of the following reasons; they may:
• Be flammable.
• Be noxious or toxic to astronauts.
• Interact with volatiles from other materials to yield even more toxic or flammable products.
• Corrode thin-film metallizations on electronic devices.
• Deposit onto slip rings, switches, relays, and electromechanical parts causing high contact resistance, electrical opens, or high noise levels.
• Deposit on windows, optical lenses, and display units decreasing their transparency and reflectance.
Because plastics outgas and degrade to varying degrees in a high vacuum space environment, special formulations, curing conditions, and testing must be performed. In addition, for manned spacecraft, the out-gassed products must not be toxic. Outgassing data must be known before specifying an organic material for space use. Fortunately, extensive tests have already been performed and data are available from NASA and from numerous contractors and universities who have conducted programs for the Space Shuttle, Space Station, Skylab, and numerous satellites. For example, NASA-SP-R0022A, MSFC-SPEC-1443, and ASTM-E-595 all describe the test methods for total mass loss (TML) and collected volatile condensible material (CVCM) when exposed to a thermal-vacuum environment.[80] Requirements in these specifications are 1% maximum TML and 0.1% maximum VCM (Volatile Condensible Materials) after the sample has been heated for 24 hours at 125°C and 10-6 torr. In a Marshall Space Flight Center specification (MSFC-SPEC-1443), there is also a uv reflectance measurement test for condensibles on a mirror. According to this specification, the reflectance of a mirror coated with aluminum magnesium fluoride or a chromium-plated collector held at 25°C should not change more than ± 3% at 200 nm. A fourth measurement of water vapor regained (WVR) is optional. Water vapor regained is the weight gained by the sample after the vacuum bake exposure on allowing the sample to reabsorb moisture in a 50% RH ambient at 25°C for 24 hours. Thus, in specifying conformal coatings for space applications, it is not sufficient to specify only MIL-I-46058C since many of the coatings, polyurethanes, and acrylics, in particular, may outgas over 1% TML, depending on their degree of cure and formulation ingredients. With the imposition of the NASA requirements, suppliers have developed many space-grade coatings that meet or exceed all the requirements. For example, a polyurethane that passes the NASA outgassing requirements and has been used successfully for many years as a PCB conformal coating in space applications is Uniroyal’s Solithane 113, an isocyanate resin cured with a polyol (C113-300).
Several simple processes, such as preconditioning the liquid resins and hardeners prior to cure or after curing, may be used to reduce the amount of outgassed material. With the solvent-less coatings, volatiles may be removed by evacuating the resin and hardeners separately at elevated temperature using a rotating evaporator. This relatively inexpensive equipment continuously rotates the liquid resin producing thin layers and exposing a large surface area to vacuum. Low-molecular-weight species vaporize and condense in a second chamber that is cooled (Fig. 4.38).
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