Model Forum / Radio Controlled / Air Models / February 2007

No, it is not as strong.  Is it strong enough?  Probably, unless the
application is very highly stressed.

In general, no.

On 24 Feb, 00:24, "Ook" <Ook Don't send me any freakin' spam at zootal
dot com delete the Don't send me any freakin' spam> wrote:

Forty years ago when my tech group was very involved in epoxy resin
development for home use (the stuff you buy at the hardware store or
hobby shop)  we found you could formulate epoxys with all kinds of
properties.  Our main aim was to have predictable pot life and the
fastest cure possible at the end of the pot life.  With epoxies this
is a contradictory set of requirements as anything you do to make pot
life longer tends very strongly to also make full cure a lot longer.
None the less our systems held a very substantial portion of the US
market for many years so we must have done something right.

The most critical ingredient in epoxy performance is mixing the two
parts in the right ratio and mixing them properly.  The two tube
systems were developed to allow sort of automatic dispensing in the
right ratio so the consumer did not have to use his brain.  Mixing is
a big problem.  Unlike a polyester resin where almost any half mixed
batch will cure just fine and reach very close to its design strength
poorly mixed epoxies are crap.  They will have microscopic or larger
areas that are very soft due to under cross linking.  So as a consumer
mixing properly is critical.  Our rule of thumb was to mix in a clear
container with good lighting.  Time how long it takes to mix well
enough that the refraction lines due to different optical refractive
indexes of the two parts can no longer be seen.  Then mix again for
the same length of time.  This gets the mixing down to a molecular
level.  Most epoxies are used in an under mixed state regardless of
the pot life on the label and as a result give a crappy strength bond
compared to the possible bond the product can produce if properly
mixed.

You can also easily formulate a epoxie for about any bond strength you
wish in terms of tensil strength.  There are applications where you
purposely formulate for a lower tensil because that kind of bond gives
you a bit of flex rather then being glass hard.  I have used
commercial epoxies that were formulated for extreme flex and were like
rubbers.  So it all depends on the particular formulation of epoxie
you purchased as to how strong it is after cure.  But let me be clear,
ultimate strength has very little at all to do with pot life.  I can
and have made epoxies with one hour pot life that were total rubber
after full cure and would not have glued balsa together worth a hoot.

The usual hardware store epoxies in general are all formulated to have
tensiles considerably higher then the tensil of hard woods.  Perhaps a
factor of ten higher in the most favorable grain direction to maximize
the strength of the wood.  They are also usually so hard that brittle
failure can be a problem in any impact failure mode regardless of pot
life.  The real test of any glue joint is how does it fail.  If it
fails in the glue joint the glue is the weak link.  If it fails in the
material glued the glue is more then strong enough for the
application.  In general it is close to impossible in lab testing to
get any of the commercial hardware store epoxies to fail in the glue
joint if they are properly mixed and applied to the wood while still
within pot life. Even if you are gluing a real hard wood.  Try it
yourself.  Glue two hunks of wood together end grain to end grain and
side grain to side grain, let the joint cure for at least 30 times the
stated pot life and break the joint.  Look at the break and you will
see it goes thru the wood, not thru the glue.  Even a very weak glue
like titebond will give this kind of break and it only has a small
fraction of the tensil that a typical epoxy has.

But be aware that on this group science and test results mean nothing
at all and very few will believe a word I say on the topic.  And it
makes no real difference to me if they do or do not believe.  We sold
tons of the long pot life stuff so we made money either way.
Personally I prefer 12 minute pot life on the rare occasions I use an
epoxy.  It is a lot more convenient to mix and use due to the longer
pot life.  You need to mix for about two to three minutes so with five
minute stuff you use much of the pot life mixing.  In truth I very
seldom use an epoxy as it is a poor glue for most applications vs the
other choices.  About all it really has going for it is viscosity so
it will stay someplace close to where you put it and short gel time so
it holds fairly fast.  But for most wood applications there are much
better glues such as titebond or when the application is not critical
CA.  Some of the softer CAs you can get these days are really pretty
good.  Too bad you can not get them from hobby shops.  Hobby shop CA
is pretty crappy stuff most of the time.  Epoxy is the best choice for
fuel proofing fire walls.  And it is the best choice for glass or
carbon laminates.  Polyesters are also fine in either application if
you do not mind the stink.  And they do not yellow in sunlight.  If
the wood is at all oily a polyester is better.  So like most of life
if the product is used properly and in the correct application it will
work just fine.  And if misused it will fail.  I guess that is why I
can not swap glasses with my wife and have either of us see worth a
hoot.  Right product, wrong application.