Question about epoxy resin chemistry..

Still more on epoxy resins....

I downloaded the Canard Pusher text files, and started searching for
"Epoxy", this patch in particular caught my attention:

MATERIALS SUBSTITUTION - Those of you who receive "Sport Aviation" may
have noticed an article in the January issue by Hans Neubert inferring
that VariEzes could be built from commercial weave 181 and 143
fiberglass and any one of a number of commercial epoxy systems.
Particularly disturbing was the author's inference that our
distributors were merely pouring the low-cost, highly-toxic 815 resin
into containers with a different label. We are not concerned that
VariEze builders are being misled, as they have been kept aware of the
reasons we had to resort to special formulations and cloth weaves.
"CP" 10 (Oct '76) describes the problems we encountered trying to use
181/143 cloth and attempting to use Shell 815 resin systems. It also
describes our fight to reduce the materials' cost. As we discussed in
"CP" 12, development of the resin/hardener system after the first
vendor went out of business, was a difficult, time-consuming task.
This required five months of testing over 20 different formulations
from three different vendors to develop a system that would meet our
requested specifications of (1) min SPI of II, (2) heat distortion,
(3) odor, (4) room temperature physicals, (5) fuel and foam
compatibility (6) moisture absorption (7) cure time at 65 degrees and
95 degrees (8) exotherm with micro mix, in insulated foam core, (9)
mix ratio, (10) viscosity, and (11) shelf life. It is of interest to
note that one of the larger formulators in the Los Angeles area was
unable to develop a system to simultaneously meet the requirements,
and Applied Plastics required over a dozen different system variants
to arrive at the solution. I have asked Applied Plastics, the RAE
formulator, to respond to the Neubert article. Their comments follow:
"I believe it to be fairly well known that in early 77 the R A resin
system was changed from the "Lambert Blend" to epoxy resin and
hardeners manufactured by Applied Plastics Co. Inc, 612 E. Franklin,
El Segundo, Calif. Applied Plastics is a resin manufacturer supplying
the aircraft and aerospace industry world-wide and have been
manufacturing chemical intermediates for more than twenty-five years.
Explaining the varied technical capabilities, unique abilities of our
chemists and our extensive quality control department would be time
consuming and may indicate an attitude of DEFENSE while to the
contrary we are offensively incensed by this article which in our
opinion does not take sufficient regard for the personal safety of
fellow EAA builders and of others who might follow these suggestions.
The statement that R A resins after March are the familiar Shell Epon
815, only further substantiates our feeling that substituting resin
systems without laboratory evaluation by QUALIFIED INDIVIDUALS can be
extremely dangerous. Our laboratory testing showed the use of B.G.E.
resins in this construction to cause extensive foam damage. Let me
state here that Applied Plastics does not now, and never has, supplied
Shell 815 or Shell 828 as R A resin. The cost comparison example
which suggests you buy 828 by the drum is an extremely hazardous
recommendation, in our opinion. The pages that address themselves to
epoxy hardener substitutions are equally as potentially hazardous.
The suggestion that would encourage a homebuilder to handle materials
such as DETA and TETA are in our opinion reckless and unnecessary.
The following hardeners were not usable because of the safety problems
they represent; also our knowledge of working with these materials
showed them to be too exothermic when used in foam cores with
microballoons: AEP, TETA, SHELL U, SHELL T. Shell A was never
considered as it is a system that requires heat curing and not too
many have autoclaves or walk-in ovens, not to mention the added
responsibilities heat-curing systems require to make good laminates.
Shell T has been discontinued for well over a year. The chart which
showed the above curing agents along with Versamid 140, which is a
high viscosity material, and then suggested that you thin with toluene
or alcohol can again produce a hazardous situation, hazardous when
working with them as well as hazardous from the standpoint that you
change the physical properties of the hardeners when you cut them with
solvents. When solvents are trapped in laminates you will also get
long term degradation of the laminate, and foam core damage. The
development of the RAE slow hardener to meet the requirement of
adequate cure and yet not result in exotherm damage deep in a foam
core, required several months testing. More than a dozen different
systems were tested, not merely to optimize the pot life but in fact
to achieve a system that provided the homebuilder with a safe
structure. Both the fast hardener and the slow hardener required
numerous tests to meet the specifications. Often we would find
material that would be excellent in all properties but then would fail
to meet the all important heat resistance test. This requirement
alone excluded most of the common base hardeners referred to in the
article by Hans Neubert. Let me add that Applied Plastics sells most
of its materials in drum quantities and would be most pleased to pass
on savings through the distributors to builders who have the resources
and the equipment necessary to handle five-hundred-pound drums.
Finally, let me reiterate that from the beginning our intention was to
provide safety and safety at a moderate cost; in our opinion the R A
resin systems are sold at a very low cost for formulated systems."
I have invited Mr. Neubert to our shop for a first-hand demonstration
of what happens when the VariEze layups are attempted with 181 and 143
cloth. While they perform nicely when used in vacuum bag operations
they present unacceptable problems when used in most of the Eze
hand-layups. We made many measurements of this when we originally
attempted to use them. They can more than double some lay up times
due to the frustration of chasing wrinkles and bubbles, they cannot be
flagged for spar caps, they hide air making inspection more difficult
and less reliable, they draw in air after a moderate pass with the
squeegee, their ability to conform to compound curves is less than BID
and UND, the difficulty in determining major fiber orientation leads
to errors, and they require more resin to wet out for the
inexperienced laminator, resulting in a weight increase.
Summarizing, we have spent a considerable amount of effort developing
methods and materials to make it possible for the homebuilder to do
what the aviation industry cannot yet do - that is, build a safe
all-composite airplane. To allow the average individual to be capable
of this task did require other than the commercially available
materials. A VariEze with engine and radio will cost from $5000 to
$10,000. Saving 3% of that by substituting unsafe structural
materials is foolish."

So, you can forget what I suggested about using Versamid 140 for a
hardener, it appears as if it will make the mixed resin too viscous
for hand layups. And don't use Epon 815.

This pitch about RAE epoxy (no longer available) is also interesting
(it's actually the portion of CP 10 referred to above):

"Next, we ran into several problems with the epoxy. Its toxicity was
quite high (SPI-4), mix ratio at 12 percent was very critical and we
were certain we would need two pot lives due to the exotherm damage we
found on our high temperature insulated tests. Thus, due to the high
ventilation and skin protection requirements and uncertainty of local
availability of the required hardener systems, the viability of the
project was in doubt. About that time we met with several composite
engineers working in the advanced composite development department of
a large aerospace corporation. We had a meeting at their facility and
described the entire VariEze structure to them and discussed with them
the epoxy problems we were experiencing. They were anxious to not
only solve our problems but also to suggest that recent developments
of elastomeric - modified epoxy systems would greatly add to the
fatigue life and peel strengths in our structure. We initially tried
a commercially available system but found the work-ability poor due to
higher viscosity and the pot life still not optimum. What followed
was a long series of testing numerous variations, attempting to
optimize the formulation of the epoxy system. Building components for
N4EZ (the homebuilt prototype), samples for strength, environmental
and exotherm tests, gave us a good basis to evaluate the system for
not only physical properties but also for work-ability. The result
was a system that was not only less toxic (SPI - 2) but also had
considerably better fatigue and peel strengths (data are shown in an
article in the July issue of "Sport Aviation")."

This looks like it would be the July 1976, or perhaps 1975 issue. So,
at least THIS epoxy is NOT a STRAIGHT Bisphenol-A resin.

Going back to your original posting, I noticed you were looking for
TWO hardeners, and you wanted the same strength properties in both a
room temperature cure and an elevated temperature cure. I think I've
demonstrated I'm a dangerous amateur in this field, but what little I
DO know suggests that this is impossible. You might be able to
approach the properties of the high temperaure cure hardener by
post-curing the room temperature layups.

Good luck, and I hope I've been of SOME help.