Subject:
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Re: A question of humidity...
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Newsgroups:
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lugnet.general
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Date:
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Fri, 16 Jul 2004 17:53:11 GMT
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Viewed:
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1275 times
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In lugnet.general, John Barnes wrote:
> Quite a lot of materials are "waterproof" while still able to absorb a small
> amount of water. I believe plastic molecules are quite large whereas water
> ones a quite tiny.
All organic compounds are, since they all include hydrogen, oxygen, and carbon,
and many plastics are organic in nature. Many others (like ABS) are also
combinations of two or more other plastics, which further complicates things.
But that's not what causes them to be hygroscopic. The space between the
molecules is where the water molecules squeeze in.
There are three basic types of plastic. Crystalline plastic is that which has a
true crystalline structure, and it always has a true melting point (in fact,
it's the only type of plastic that actually melts like frozen water). Amorphous
plastic looks like spaghetti strands under extreme magnification, as the
molecular chains kinda wrap around each other in very random ways.
Semi-crystalline plastic includes both types of structure, basically having a
crystalline structure that forms at or below a certain temperature, and an
amorphous "spaghetti" structure that flows somewhat like chewing gum. ABS is a
semi-crystalline material, and the randomized component to its structure is
probably at least partly responsible for the fact that it has gaps where water
molecules will fit.
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Message is in Reply To:
 | | Re: A question of humidity...
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| In lugnet.general, David Koudys wrote: (major snip> (...) What an interesting post. It had never crossed my mind before, but the circuit board assembly places which use infra-red ovens to melt solder on circuit boards to allow the surface mount (...) (20 years ago, 16-Jul-04, to lugnet.general)
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