-----Original Message-----
From: <Gonaj@aol.com>
To: <Tomsaudi@aol.com>; <larryhoy@prodigy.net>; <mrazor@mis.net>;
<mgs@autox.team.net>
Date: Sunday, December 31, 2000 7:46 AM
Subject: Re: Let It Snow!
>In a message dated 12/31/00 3:07:23 AM Central Standard Time,
>Tomsaudi@aol.com writes:
>
><< I raised the top, but by then
> it was too cold for me to latch it closed. Ugh! >>
>
>When I put my MGB up for the winter I deliberately left the top clips
loose.
>Thought that the rubber gasket will last longer if I let it relac rather
than
>keeping it compressed all winter.
>
>George
>
You've actually discovered a standard elastomer test called compression set.
The sample is compressed 25% and held at a temperature for a time.
Difference in sample heights is reported as a percentage. Since technically
all elastomers are liquids, albeit very viscous liquids, they will flow to
fill the space they are contained within. Foams tend to be worse in
compression set due to the gas in the cells which provides little structural
value. Compression set is a logarithmic effect. It tapers off as a
function of time.
To really keep those gaskets like new you need to put the car in a nitrogen
filled bag to eliminate ozone, Move the bag and car to a dark
ave( ultraviolet). Then move the bag, car and cave to Antarctica (heat
degradation). It also helps to keep oil off most rubber since it causes
swell and softens the rubber. That is why most elastomers use oils in their
formulations. Elastomers with low oil swell numbers tend to contain
fluorine or are silicones which for cost and other reasons have limited use
in automotive markets. About the only place you will find fluoroelastomers
on most LBC's is the valve stem seals, there the performance justifies the
cost.
Stretch chem is my life.
Bill
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