Electronic Restoration >> Silicone Outgasses

Silicone outgasses and is a concern even in a cable configuration where the outer jacket can protect the system from small amounts of enclosed outgassing material. Current product typically contains relatively large quantities of silicone. The Gore-Tex® construction has not yet been Silicone Outgasses evaluated. 

The task of stripping polyamide coated fiber is not trivial. Polyamide coating can be stripped only with hot sulfuric acid for space flight applications; hot sulfuric acid can be dangerous and is not portable. Mechanical stripping methods are not recommended for flight cable assemblies because they can introduce surface flaws to the glass fiber, Silicone Outgasses compromising long-term reliability.

Connector manufacturers have tried to circumvent this problem by offering a ferrule hole size that will accommodate the unstripped polyamide coating's outer diameter. This approach has not been accepted readily by the industry because it makes the connectors considerably more expensive Silicone Outgasses and forces the user to commit to a non-standard configuration. 

When used in single mode applications, where perfect alignments are crucial to limiting the insertion losses, Silicone Outgasses the polyamide coating still must be removed before terminating the fiber due to the tighter tolerance constraints of single mode connectors. Hermetic polyamides over amorphous carbon coatings are susceptible to failure. 

The failure mode for one loose tube cable configuration was Silicone Outgasses determined to be caused by method of extrusion. The full analysis is contained in reference 7, the "ISS Fiber Optic Failure Investigation Root Cause Report". There have been no reports of hermetic polyamide coated optical fiber failures in tight tube configurations. 

Currently, nonhermetic coatings are recommended over hermetic when feasible. CABLES The standard cable construction includes the coated fiber, which is supplied by a fiber manufacturer and has a unique part number, a tight or loose tube buffer, woven Silicone Outgasses or spiraled strength members, and an outer jacket. 

Each cable component performs an important function in the overall mechanical reliability of the terminated fiber. The coating, which is physically in contact with the fiber, greatly affects optical performance. Part number traceability is limited to the finished cable Silicone Outgasses and the coated fiber. The performance and manufacturability of cables are heavily dependent on the materials used. 

Most optical cable jackets considered for use in space flight are extruded Teflon or Tefzel. The materials currently in use for loose tube buffers are Teflon, Tefzel, Kynar (PVDF) or Hytrel, tight fiberglass or Kevlar (Aramid). Properties of these Silicone Outgasses materials are outlined in Table 1. A validation test should be conducted on connector ferrule hole dimensions and tolerances prior to procurement. 

This test ensures that the fiber will be reliably centered in the hole after termination. This test is particularly important when Silicone Outgasses inspecting nonstandard ferrule sizes. The ferrule sizes that are commonly used are 125 or 140 microns. An excessive amount of epoxy will be required if the ferrule hole is too large for the bare fiber outer diameter. 

The terminated fiber will not mate properly to another if a mismatch is created due to non-concentric Silicone Outgasses fiber placement within the ferrule. Terminated fiber end faces must be visually inspected at 200X to ensure that the position of the fiber is central to the hole of the ferrule. 

MATERIAL OUTGASSING Selection Silicone Outgasses and use of non-metallic materials in cables shall be traceable to test reports with acceptable levels of TML and CVCM as listed in NASA reference publication 1124 (outgassing data for selecting spacecraft materials) or NASA MSFC Handbook 527 (Materials Selection List for Space Hardware Systems).

However, materials listed as acceptable in these publications may have been baked out for evaluation, Silicone Outgasses and the user may have to repeat the same processing. This processing usually consists of a vacuum bakeout of 125 C and a vacuum of 10 -6 TORR for 24 hours. Acrylate optical fiber coating does not pass the ASTM 595E vacuum outgas test. 

However, it can be used in a non-outgassing cable configuration. Other materials that are prone to outgassing in a vacuum environment are the epoxies used for terminating assemblies, connector boot materials, cable configuration materials, Silicone Outgasses and all plastic connector parts. In the past Kevlar was considered to be a contamination concern for assemblies; it will absorb 3% of its weight in moisture.

Any absorbed water shall be driven off with a thermal preconditioning step prior to termination. At present Silicone Outgasses there are only a few epoxies qualified for space flight cable assemblies. Goddard Space Flight Center uses Tra-Con Bipax and Tra-Bond BA-F253 for fiber optic space flight cable assemblies using the Johanson FC connector. 

EPO-TEK 353ND is used for termination of the Diamond AVIMS. Two epoxies from AngstromBond were recently tested for their outgassing characteristics both inside Silicone Outgasses and outside of a cable configuration. The two test epoxies used in the experiment were the Angstrom Bond AB9119 and the Angstrom Bond 9112 fiber optic epoxies. 

The cable test showed inconclusive results.The ASTM 595E test conducted on the epoxies alone showed that the AngstromBond AB9119 passed while the AngstromBond 9112 did not [8]. TERMINATIONS The termination process must be well understood Silicone Outgasses and controlled in order to reduce the chance of inducing surface crack growth in the fiber. 

Crack growth can be induced during any step of the termination process. The proper epoxies and strain relief materials must be used to avoid radial or Silicone Outgasses lateral stress on the fiber. Crack growth can be a longterm failure mechanism that is difficult to simulate in the laboratory. All terminated cable assembly preparation work must be performed using space flight procedures including those found in NASA-STD-8739.5, 

Fiber Optic Terminations, Cable Assemblies, and Installation. Termination techniques that are appropriate for highly reliable assemblies in space flight environments are well understood Silicone Outgasses and practiced by the APIM laboratory. Code 562's past experience has been that it is very time consuming and difficult to ensure that commercial vendors fabricate assemblies in compliance with NASA-STD-8739.5.

Due to this experience, in-house fabrication is recommended. All cable must be thermally preconditioned prior to termination. During termination chemical stripping is used to remove optical fiber coatings. Performing a strippability validation test is recommended for flight fiber to ensure that space flight termination procedures can be accomplished. Other methods of stripping fiber are available, Silicone Outgasses but none at present have been approved at GSFC for space flight terminations.


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