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Forums - Modelling / Construction |
The construction forum is for the discussion of techniques on the construction phase of AFV modelling and the tools and materials used. |
| Topics | 1894 |
| Messages | 8734 |
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| Subject: | Re: what's polyethylene and polypropylene plastic? |  |
| Date: | Apr 19, 2001 |
| From: | Jeff Nelson | |
Jack -
I don't know if this is the information you asked for - but since process engineering and injection molded plastics is my career, here is my answer:
Polyethylene is a simple polymer based on the "ethylene" molecule. That is, a molecule with two carbon atoms that aren't completely hydrogenated. (The two carbon atoms are double bonded)
Part of everybody's problem, (mine included - see my post "fixing" stuff), is that there are basically two methods to manufacture polyethylene, and both result in what is referred to as "polyethylene" - yet each method gives the polymer different densities, crystallinity, etc. The small molecule size (comparatively) results in fairly high polymer densities. This makes a plastic that is hard to degrade. Since "most" model glues work by "degrading" (ie melting) the plastic, this doesn't work well with polyethylene. Or polypropylene for that matter, since that polymer only has three carbon atoms. These two plastics have very dense polymer chains, yet are very flexible (soft) due to the small molecule size. Plastic bags are mainly made of these two materials, very good tensile strength and good pliability.
On the other hand - good old polystyrene!
The base molecule for polystyrene is styrene (aka vinylbenzene, or phenylethylene). The base styrene molecule has no less than 8 carbon atoms and FOUR double bonds between those carbons. Not to mention a benzene ring. This molecule is "attacked" far more easily by organic solvents (glue) than the tighter chains of the smaller polymers. There are more places to "attack and degrade", as every double bond is a target. Not to mention that the benzene ring is very bulky (so to speak) and doesn't allow for very tightly packed polymer chains. GLUE WORKS - YEA!
For people that aren't into organic chemistry:
Imagine two chains made out of metal.
1. Chain #1 has very small links made out of thick metal. Each link is almost round on this hypothetical chain.
2. Chain #2 has long skinny links, each one is almost rectangular. (At least 5 times longer than it is wide)
Both chains are made out of the same metal. Which one would be stronger? - More limber?
ANSWER - chain #1 (Polyethylene, Polypropylene)
Hope that helps :)
Jeff
References:
I had to look some stuff up to be "sure"! I hadn't had my organic text book open in 5 years. Ha Ha
It's the 6th edition "Organic Chemistry" by Morrison and Boyd.
Pages of interest (or not)
356, 551, 553, 557, 558, 1088 |
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