Composite Pipe Repair

[Gray E Fowler]

I did not mean to leave everyone hanging, but I had to go...more on this 
subject.  Concerning composite built pipes of any brand. First the main 
problem with the pipes made from epoxy is not the fact that the resin 
matrix is epoxy, but more from the epoxy that was chosen. Most of the 
pipes that I have seen are made by taking commercially available carbon 
fiber braid and conforming it down  to a male mandrel. This braid is dry 
and needs to be wetted out with epoxy. Since most of our Pattern equipment 
is produced by innovative small business guys, working out of their garage 
or something, most of the epoxies that could survive are not feasible to 
use in this method of producing the pipe. The choice is limited by two 
factors, one-the need for the resin to be liquid at room temperature, Two- 
the need for the resin to "set" at room temperature, then maybe postcure 
after room temp gel. These two factors force the manufacturer to use an 
epoxy resin similar to the adhesive I mentioned earlier, Hysol 9394, 
supposedly a  350F system. Technically speaking, this is and aromatic 
amine cured epoxy. The temperature resistance of the epoxy comes from the 
use of the aromatic amine. Two problems that relate to aromatic 
amine....they are soilds at room temperature and they really need elevated 
temperaures to cure-hence violating the fab needs stated earlier.  So 
Hysol 9394, and other "liquid" aromatic amines usualy have copius amounts 
of regular primary amine, which are liquid and cure easily at room 
temperature---and have low temp resistance. 
So at 350F these epoxy systems work, albeit at dramtically reduced 
strength (75% reduction), and that is one reason for crushed inlets. Even 
at about 400F the epoxy does not burn but has little to no strength. Along 
comes the hot day and slightly lean run, the exhaust temp goes up and 
bye-bye inlet tube. Long aluminum headers and rich runs protect the inlet 
from that temperature. As you can see these type of epoxy systems are 
absolutely borderline effective. That is why half of you guys say never 
had a problem and the other half burn up your inlet tube.
Concerning the post directing people to sources of 600F epoxy.....Sorry no 
such thing- period. Look closely at the web site info. No structural data, 
because these are potting compounds, AND guess what...they need high 
temperatures to cure. As a potting compound these materials can take short 
duration spikes up to 600F and not burn up, but the will be soft rubbery 
gels with zero strength, which works for those applications because it is 
a potting compound only.

The only way to get high temp properties from an epoxy is use an epoxy 
that cures at high temp, but this has problems of its own. Applying-Using 
an aromatic amine cured epoxy at 250F is just like using a liquid epoxy at 
room temperature......only the humanoid operator has cooling problems at 
this condition so this is not feasible (without an asbestos suit). 
Attempts to wet out braids with viscous epoxy/A-amine goo with a heat gun 
will work, but as the temperaure is raised to cure at 350F, all the 
trapped air expands creating voids in the laminate and leaks in the 
pipe....bad-bad. Curing under a vacuum enhances the expansion of trapped 
air BUT if you can get it all out the no pinhole leaks.....but there is no 
way to really do this perfect everytime.

Epoxy of any type WILL NOT absorb oil. But pinhole leaks, microcracks and 
sanded outter surfaces exposing the fiber will. Pinholes come from the 
process problems mentioned above, microcracks come from stress at 
temperature, and sanding the outside of the pipe is a cosmetic REQUIREMENT 
from all you totally anal retentive pattern types that gotta have a "Good 
Lookin" pipe......one word here-function! Hats off to Bolly! they use a 
high temp Bismaleimide resin (I think) AND they do not sand the outside of 
the pipe. Granted, it looks like something that shoulda gotten flushed, 
but it does not burn out, and it does not absorb oil. It is also for this 
reason heavier by a couple of ozs.


The answer.....do it the way the B-2 was done.....epoxy-aromatic amine (or 
bismaleimide or cyanate ester)  prepreg cured in an autoclave.....Now can 
you afford this? Perhaps.  To avoid blatant advertising I will stop here. 
Anyone interested please email me offline at  Aeroslave at comcast.net.


Gray Fowler
Principal Chemical Engineer
Composites Engineering
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