Conference Review

18th International SAMPETechnical Conferenc~

Henry Brown with Stuart LeeSociety for the Advancement of Material and Process Engineering

8 bFigure 1. SEM photographs of plasma-exposed Teflon show the physical effects whichoccur as a result of reacting with atomic oxygen. Compared with unexposed Teflon (a),samples exposed for 30 minutes (b) showed the greatest change in surface appearance.

The technical conferences of theSociety for the Advancement of Ma­terial and Processing Engineering(SAMPE) are intended to dissemi­nate current data on materialsprocesses to engineers and scientistsas well as to archive data forpublication. Annually, the SAMPEconference features a differenttheme, and this year's conference ex­amined materials in space.

UV radiation and vacuum outgas­sing effects on materials from thespace environment have been exten­sively evaluated. An unexpected en­vironmental effect, however, is nowknown to be caused by atomicoxygen, which is the primary con­stituent of the atmosphere at spaceshuttle orbital altitudes (200-300km). Chemiluminescence, seen visu­ally from the space shuttle (space­craft glow), is a related phenome­non caused by the reaction of atom­ic oxygen with oxidizable materials.

A paper by Lubert Leger, JamesVisentine and Beatrice Santos-Masonof NASA examined degradation con­cerns associated with long servicelife space station development. Ear­ly experiments indicate that atomicoxygen effects are far more detri­mental than ultraviolet radiation.Coating technologies include a prom­ising technique of wrapping graphite/epoxy structural elements with alu­minum foil, which is then chemicallyconverted by anodizing to providethe desired optical properties forthermal control of the system. Inanother theoretical discussion, LarryTorre and Gary Pippin of Boeing hy­pothesized a mechanism by whichatomic oxygen recombines withpolymers, contributing significantlyto the degradation ofthose materials.

Teflon was originally consideredan excellent thermal control mater­ial. The space effects observed onTeflon samples taken from shuttleflights , however, have proven other­wise . If the sample is on thespacecraft's "leading edge" (with re­spect to flight direction), it is de­graded to a much rougher surfacewhen examined under a microscope

(Figure 1). There is severe mass loss,degradation of thermo-optical proper­ties and a decrease in tensilestrength. In fact , once degradationis initiated, it continues for weeksafter the radiation source is removed.

Metals (except carbon, silver andosmium) and ceramics are unaffect­ed by atomic oxygen. All polymersare affected, but degradation gener­ally stops when the radiation sourceis removed. Since space stations aredesigned for a 30-year life , provi­sions must be made to protect ex­posed polymers from oxidation.

Aluminum tubes are five t imesheavier than equivalent graphite!epoxy tubes, and the cost of protect­ing epoxy resin is therefore worth­while. Epoxy erosion on the forwardside of a tube is estimated to be.034 to .132 inches during a 30-yearlife . Current thought involves pro­tecting the epoxy with a thin alumi­num sheath or plating.

Concerns over thermoset resinbrittleness and water intrusion intocomposite matrices continues. Ther­moplastic polymers, such as PEEK,have been evaluated for severalyears as a solution to both problems.However, these polymers are notgenerally selected for applications.None of the presentations explained

the rationale for this lack of atten­tion, but the obstacle apparently in­volves a cost-effective method of"prepregging" resin onto fibers.

A newer approach for matrix prob­lems is IPNs (InterpenetratingNetworks). IPNs are two or moremutually soluble liquid polymerswhich are cured together to form atangled network of the components.

An interesting, and somewhatcontroversial, paper by Susan Krol­ewski and Dr. Timothy Gutowski ofMIT featured an economic model ofadvanced composite fabrication pro­cesses for aircraft. They concludedthat pultrusion and filament wind­ing are cost-effective, automatedprocesses. However, hand lay up ismore cost effective than automatedprepreg cutting followed by robotictransfer or automated tape laying,or an y of these automated steps com­bined with hand lay up. Their pa­per argues that the low volume inaerospace cannot support singlepurpose , automated equipment.Flexible manufacturing centers aretherefore necessary before significantcost reductions in fabrication arepossible.

If you want more Information on this subject,please circle reader service c:ardnumber 51.

JOURNAL OF METALS· December 1986 23