Paul,
It may be sooner than that. Look at
http://www.cotronics.com/vo/cotr/ea_ultratemp.htm . Three types of
epoxy are
listed there with 600K and 700K operational limit. I checked one of
them. It
is an epoxy with a ceramic filler. It will not cure at room
temperature, it
requires oven curing. After curing it looks like a black brick and
it seems
to be hight temp resistant. But I did not make any measurements of
strength.
Cotronics might have all that info.
Jerzy
Yes but how will we know for sure until ten years go by?
Paul Lamar
Krasinski wrote:
Paul,
There are two main terms in decomposition of plastics.
One is thermal excitation of molecule to an excited state with a higher
reactivity where it can react with oxygen, water, or
break apart, and the rate of decay is given by the Arhenius law.
According to Arhenius the decay rate ~ exp(E/kT), where E is
the energy of the excited reactive state, and T is temperature. Only
the excitation energy of the material and temperature
define the decay rate here, and the lifetime can be extended by reducing
temperature, which is not very practical. However, if
the stuff survives high temperature that means E is very large, and the
lifetime at room temperature would be huge due to the
exponential form.
The other term is photodecomposition, and that one is probably faster
for most of plastics, we all know what happens with
plastics left outside in the full sun light. This term is manageable.
Just block UV and photodecomposition is stopped. So with
a proper selection of adhesive in terms temperature resistance (i.e. in
terms of E), and proper blocking of UV it is probably
not a very bad prognosis even for 10 years.
Jerzy
I am so disappointed in the strength of the plastics I see in ten year
old cars that I will believe it when I see it ten years from now if I am
still
alive :)
Paul Lamar
I beg to differ on the topic of heat and UV being the only thing that will
cause plastic to decay. What about chemical attack? Molded in stress, stress
corrosion, moisture contamination, dielectric breakdown. I design plastic
parts for a living. I don't know who the hell Arhenius is, but to me that
just looks like a standard reaction rate equation applicable to just about
all reactions. It is exponential, but where is the intercept? At zero
Kelvin? What is that little constant k? This is far too general a form of
this equation to be making application specific conclusions.
I can tell you first hand that plastics are great when applied properly, but
contrary to what the polymer sales guy and the MBA bean counter would like
you to believe, they are not capable of replacing metal. I know all about
the various additives, new super resins, etc. The reality is these super
exotic polymers are too expensive to be used anywhere except where they must
be used for some very exotic design reason. (Probably a bad design to start
with, otherwise you wouldn't have to use this exotic resin that costs more
than copper, when aluminum would have worked just fine) Most all structural
parts wind up being polypropylene, ABS, Polycarbonate, or Nylon, or some
blend of these. You can add fillers to increase strength like glass fibers
or mineral fill (talc). All of these plastics have their weak and strong
points and proper application. A high heat environment involving solvents,
especially the ones you are liable to find in gasoline as additives, along
with moisture, oil, and oil additives, coolant, coolant additives, ozone,
various acidic substances as well as alcohols, (fuel, windshield cleaner)
etc. Is not a very good place to put plastic. If you put plastic parts in a
confined space with the right container of solvent and simply open the lid
of the container, you will come back and find that the plastic has crazed
and cracked and lost all structural integrity in a few hours, or min. It
all depends on the particular solvent and its affinity for the particular
polymer chains in the particular plastic. What that little equation fails to
state is that some solvents will attack the plastic even at low temperature.
Also how the part was processed and what the molded in stresses are and how
the virgin resin has been degraded during the manufacturing process. It is
not as simple as a three variable equation. Sounds nice in a text book
though.
Beware the Giant equation with many terms of questionable accuracy raised to
many powers. Also beware the oversimplified equation. You can probably make
this equation work by measuring E, k, and T for a SPECIFIC case. Do you have
good data on what these are for various plastics with various solvents? Is
it good within 5%, 10%, 30% for all the solvents or combinations seen in an
underhood application, who knows? What happens when you have a 30% error
for the value of a constant and place it in an exponential equation? Pure
science meets engineering.
Monty
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