Date: Thu, 3 Feb 2011 12:39:44 -0500
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Bad plastic(s)
In-Reply-To: <57E18C09-C598-4C5F-A6BD-A408F0E4421C@ocotillofield.net>
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At 11:13 AM 2/3/2011, Mark L. Hineline wrote:
>I'm inclined to go with Karl's idea that the heat makes it worse. But
Heat makes most things worse. A rule of thumb I remember is that
around room temperature, every 10C rise in temperature will double
the speed of a reaction.
All non-rigid plastics contain "plasticizers," some of the more
flexible ones like vinyl upholstery a third of their weight. It
slowly evaporates - that's the nasty film that magically appears on
the inside of your windshield and the plastic slowly gets less and
less flexible and also smaller and smaller. That's what leads to the
linear shrinkage of the plastic furniture trim in Westys.* High
ambient temps increase the evaporation rate. I'm sure this is a
major cause of embrittlement leading to failures at stress concentrations.
*In '82 I bought (in the face of all common sense) a '78 Plymouth
Horizon. From a Plymouth dealer in New England, though I don't know
where it had been before. All of the plastic door trim had shrunk
several inches in length, and the steering wheel had shrunk to the
extent of a 1/4 wide crack at the exact bottom, exposing the steel
core. I'll spare you the rest of my Horizon rant.
VW has used a lot of ABS plastic (acrylic-butadiene-styrene) which
has excellent impact resistance but is rated poor for UV resistance -
so it's not terribly surprising that grilles and such on the outside
where they're exposed to UV are getting brittle. However it has good
ozone resistance. The Japanese mfrs seem to have gone heavily in the
direction of softer plastics containing I'm pretty sure some amount
of polyethylene (polythene in UK) which seems to have held up much
better. Likewise their engineering resins for mechanical parts like
steering column stalks contain some ingredient that adhesives don't
like to stick to, and have proven very durable.
That being said, those door handles you mention are of a related
polyethylene-containing compound, and the ones in Millee's Sally - a
New Mexico van - look awful. They work fine but they're obtrusively
scratched up. I've never seen a Toyota from New Mexico.
Their mechanical designers have been behind the curve IMO in
designing mounting tabs and mounting surfaces to minimize mechanical
stress, which is something that any plastic hates. *Most* mechanical
engineers took a long time,* i.e. this was still a noticeable problem
for example in small electronics up into the '90s, in developing
fasteners and specifying post dimensions that would not lead to
splitting of mounting posts in ABS/styrene and similar plastics. A
split post can usually be laid to the mechanical engineer.
*I once read a long and fascinating article in Plastics yearbook,
saying in detail that it was long past time that engineers stopped
treating plastic like metal and started actually learning about
it. That would have been mid '80s I think.
Many people do not realize that you only get to cut a thread in
plastic once. You *may* get away with doing it a second time, but
the third time you will probably lose. Usually this means the thread
will strip out, but if the plastic has become brittle (and depending
on the fastener) a post might split. It's very important to reinsert
screws into the same thread that was originally cut, by reversing
rotation until the screw drops into the thread and by being very
conscious of any additional effort over what was required to remove
the screw, and backing out and restarting if it's
encountered. Sometimes it's too late, the original thread can't be
found and you just have to keep going gently and pray.
Likewise it's a great temptation to overtighten fasteners on mounting
tabs for the instrument panel, heater box and such.
And as Ben points out, kicking the stuff rarely helps it.
That being said, I've simultaneously owned Vanagons (total 3) and
Toyotas (total 4 Corollas, 1 Camry and 1 H***da Accord) covering the
same ages, and except for the vinyl dash pad the Toyotas simply do
not have the inside plastic issues that the Vanagons do, nor do the
outside fittings lose finish and become brittle the same way. Again,
this is in New England, a fairly benign climate for plastics.* OTOH
the Corollas all had/have sealed front running lights with a little
rubber vent hose, and every one of those little hoses from '84 to '95
has rapidly perished. Every maker has their blind spots. In the
case of the '98 Accord that would be their million-dollar
electronically controlled transmission that nobody outside Japan (and
maybe in, for all I know) can fix. Shudder.
*The '84 Vanagon was in Scotland until '92. Miserable climate for
rust, but not bad for plastic, I think. Interestingly the rubber
connector boots had perished by '92 much more than I expected and
much more than my '89's have up to the present.
Yours,
David
