Outgassing? How can Alpha Wire help?

Posted on Wednesday, July 16, 2014 by Alpha Wire

Dave Watson, Director of Engineering, also known as Alpha Wire's resident wire and cable expert was prompted with the question below regarding outgassing.  Check out Professor Watson's explanation!


Q.  I just saw something on “low outgassing wire and cable."  What is outgassing? What do you do different in these cables? What are the applications?


A. To answer this, let’s take a trip to a new car lot.  Don’t you just love the smell (aroma, if you will) of a new car?  Well a good bit of that smell comes from the plastics that are used for interior components (dashboard, door panels, seats etc.).  If you have ever had the good fortune to purchase a new car, you may have noticed that within a few months, the inside of windshield gets a bit hazy and needs to be cleaned.  That is evidence of outgassing. Outgassing can be defined as the release of a liquid or solid substance into the atmosphere.  So in the case of our new car, substances that are ingredients in the interior component plastic parts are outgassing into the cabin.  This is perfectly normal, this is not evidence of the plastics degrading, but it is a nuisance.

As you might gather, different plastics have different outgassing performance.  PVC’s tend to be poor performers while Fluoropolymers (FEP, PTFE etc.) tend to be the very best performers.  A very handy resource is the NASA Outgassing Database - http://outgassing.nasa.gov/cgi/uncgi/search/search_html.sh.  By no means is this all inclusive or very specific on what product was actually tested, but it can be used to get general information on how different materials perform.  Some of the plastics that are newer on the market aren’t in this database.  In order to combat the lack of newer substances in the database, we’ve contracted a 3rd party test lab to conduct the testing to ASTM E 595-07.  This test method gives 3 different test values; TML (Total Mass Loss), CVCM (Collected Volatile Condensable Material) and WVR (Water Vapor Recovered). These are all measured as percentages and lower values are better.  More on this later.  Some example data is shown below. So the direct answer to your second question is that for cables where low outgassing is a requirement, we select insulation and jacketing materials that are known to have good outgassing performance.

 

TML (%)

CVCM (%)

WVR (%)

EcoCable Insulation & Jacket

3.05

1.26

0.06

EcoWire & EcoFlex Insulation

1.95

0.84

0.07

EcoFlex Jacket

1.82

0.34

0.12

PVC (NASA Data)

13.98 to 30.67

7.23 to 12.72

0.07 to 0.40


Now for the Application question.  The fact that this is a database that NASA administers should give a huge clue on the origins on low outgassing applications.  It turns out that the vacuum of outer space is the environment that induces the greatest outgassing.  Thus, the earliest applications were spacecraft, satellite and high altitude aircrafts.  Bringing things down to earth, there are many applications that involve both expensive optics and wire & cable.  Go back to the new car example and the windshield I started this blog with.  If you are building an electron microscope that includes a very, very expensive lens and lots of wire and cable within the instrument, outgassing is a concern.  Broadly speaking, low outgassing cables would be desirable where applications require cable not to contribute to any airborne (atmospheric) contamination to the environment where they are installed. This leads to the question, in outgassing, how low is low?  Historically, a TML of 1.00% or less and a CVCM of 0.10% have been considered the levels for aerospace applications.  But the vacuum and temperatures of outer space that drive these values are probably overkill for terrestrial applications. So the outgassing level requirements would tend to be application dependent.  

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