Rad plastic inlet failure mode

[Doyt W. Echelberger]

Hello Doug........The effect of such a sleeve would require a 7 to 9 year
test. And after thinking about this for another day, I suspect the coolant
more than I suspect the rubber as the culprit. But the hose _is_ part of
the problem.  I think that the hose acts as an insulator to heat-soak the
distal plastic.

The sleeve you suggest might better be placed as a barrier between the
coolant and the material of the inlet, on the inner surface of the inlet.
I'll explain why I think so.

Additional factors in the system are the ethylene glycol/water solution and
the engine heat. Without these two factors, the rubber hose could probably
remain clamped onto the plastic inlet for 20 years without the inlet changing.

But, we do know that the physical properties of the distal plastic change
considerably over the seven years. Typically, changes in physical
properties can be accounted for by changes in structure and/or chemical
composition. That hard black plastic is changing into punky crumbly
flexible stuff. And the change isn't restricted to the plastic surface that
actually contacts the rubber hose. Both the hose and the plastic are more
like solids than anything else, and I would expect an interface reaction in
that situation. This degradation behaves more like a liquid that is
permeating the solid, and there is plenty of hot liquid ethylene glycol
present.

The coolant is a double alcohol and is reasonably reactive. It is a
starting material/major reactant in the synthesis of a huge number of
organic compounds, and I suspect that it is reacting with the plastic of
the inlet. Such a reaction might be catalyzed by an acidic environment,
which could develop in older coolant solutions.

A crude rule of thumb in chemical kinetics says that for every ten degree
Celsius rise in temperature, the rate of a reaction at least doubles, and
some of them triple. In any case, most chemical reaction rates are very
sensitive to temperature. Temperature differences do exist along the the
length of the inlet. The radiator half is cooler than the part under the
rubber hose.

The distal part of the inlet is rather well insulated by the rubber hose,
while the more proximal part of the inlet can radiate heat unrestricted to
the engine compartment air. The distal plastic probably runs at 190 degrees
or higher, while the proximal plastic could reasonably be ten degrees less
most of the time, especially along its outer surface.

If some slow organic reaction _is_ taking place between the plastic and the
coolant, then the elevated temperature of the distal inlet might be
sufficient to explain its degradation after about 7 years, while the more
proximal portion of the inlet retains a more structurally sound outer shell.

I will take a look at the plastic in the lumen of the stub that remains on
my old failed radiator. If the inner lining of the stub is punky and softer
than the outer shell, then the hypothesis is supported.

Doyt Echelberger
Ohio  USA

 >>>>>>>>>>>>>>>>>>>>>>>>
At 08:59 PM 7/24/2003 -0700, you wrote:
>Does it seem as though there's a incompatibility between the hose and
>plastic neck materials?  If so, would a very thin metal sleeve between the
>neck and hose perhaps help?  Just a thought....
>
>  ~ Doug
>
>
>-----Original Message-----
>From: quattro-admin at audifans.com [mailto:quattro-admin at audifans.com]On
>Behalf Of Dan Cordon
>Sent: Thursday, July 24, 2003 8:08 PM
>To: Quattro List
>Subject: RE: Rad plastic inlet failure mode
>
>
>Having just had this experience on my 200 20V, I can agree entirely.
>This is exactly the condition of my upper radiator neck when it finally
>gave away. Luckily mine happened in the driveway, not on the highway!
>
>--
>Dan Cordon
>Mechanical Engineer - Engine Research Facility
>University of Idaho
>
><snip>
>
> > Normally, the plastic is rigid and unyielding to attempts to
> > deform it by pressing on it with fingers or by gripping it with pliers. As
> > it nears the end of its useful life, the plastic becomes sort of punky,
> > softer and more rubbery and easily deformed with pressure from the pliers.
> > In the final stages of failure, one can grab the end of the inlet with
> > one's fingers and twist off a piece almost the same as tearing the corner
> > off of an asphalt roof shingle. The plastic has lost it's rigidity. All
> > this is taking place under the covering of the rubber of the upper inlet
> > radiator hose, which hides the deterioration from casual inspection. I
> > suspect that the act of tightening the radiator hose clamp at this stage
> > would accelerate the speed at which the system is moving toward
> > catastrophic failure.
> >
><snip>