Ook: See this thread for a full discussion:
It's under "epoxy xtrength". Despite the typo, the content of
the thread addresses your question.
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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
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.
The two tube dispensers (Devcon) are good. Hobby-Poxy used to cure to a
somewhat stiff rubbery feeling joint.
Your lack of appreciation for your fellow modelers is puzzling. Many are
quite bright and eager to learn from professionals. Every group of humans
has its not so bright component. Including chemists, engineers, etc. After
all, even Bush holds a Masters Degree, so merely having credentials no
longer means anything. I doubt that it ever did.
I use epoxy for sealing and fuel proofing mostly. I do agree that most of
the pot life of 5-minute epoxy is consumed in the mixing process. Most times
the results of a thorough mixing of 5-minute epoxy is satisfactory, strength
wise, plus it fills voids. I'm in the habit of mixing fibers in my epoxies
as I feel the epoxy lacks strength otherwise. Yes, the fibers decrease pot
I use CA for tacking while building and aliphatic resin glues for the
majority of building. I am used to aliphatic's characteristics and can
assemble things rather quickly with this type of glue. The fact that it is
water soluble provides an endearing property. It seems to have a very long
shelf life, is inexpensive and is available everywhere. As you pointed out,
the majority of glues available are stronger than the wood that is being
glued together. Even aliphatic resin (Titebond/Carpenters Glue).
Some of the folks in the hobby have moved on to the new (to me) foaming
glues. I haven't seen the need to move and adapt. My old ways work at my
working pace and are not critical when it comes to jigging and compressing
the parts together. Maybe that should have been pressing the parts together.
We are fortunate to have such as you contributing to our collective
knowledge base - even if you think we are stupid. Who knows? Maybe we are.
| 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.
Say something crazy, and we won't believe it without some pretty
serious proof. But as for this post, I don't really see anything
really crazy except for this claim.
Saying `people will say I'm wrong, but I'm not!' is best reserved for
posts where people will say you're wrong. I don't think this is such
a post. Quit trying to be a martyr!
| You need to mix for about two to three minutes so with five minute
| stuff you use much of the pot life mixing.
Indeed. Which is the main reason I really prefer not to use 5 minute
epoxy -- you don't have enough time to work. Usually the strength
isn't an issue, because it's already way stronger than anything you're
gluing together unless you mess up the mixing.
| In truth I very seldom use an epoxy as it is a poor glue for most
| applications vs the other choices.
People might disagree with this statement, but the rest of your post
is pretty much spot-on.
| 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.
So, which CA do you suggest? Or, where do you get this good CA?
I've found `Wal-Mart' epoxies to be comparable to hobby store epoxies,
but I've found standard consumer CA to be a good deal inferior to
hobby store CA. So what's better than hobby store CA glue?
Doug McLaren, firstname.lastname@example.org
`Mate, this parrot wouldn't VOOM if you put four million volts through
Yeehah, someone who has a clue about adhesives...
I also spent several years manufacturing structural adhesives and a bit of
adhesive formulating under my belt, and I agree heavily with the technical
issues presented here. One spot where you run into quibbles in discussions
about hobby epoxy systems is the general quality of the epoxy systems
offered on the consumer market. There is really little to no theoretical
reason for a highly reactive system with short pot life/cure time to suffer
in properties in comparison to systems with longer pot lives/cure times. It
has more to do with the resin chemistry, the additives/fillers, and the
curative family chosen. That said, so many of the epoxy products offered at
retail are little more than downpacked straight epoxy resin, which btw is
reasonably fine for laminates such as glass or carbon fibre/epoxy matrices,
but sucks the proverbial donkey balls as a structural adhesive. "However",
given that we are most often bonding substrates with mediocre mechanical
properties, such as lite ply, balsa, spruce plywood, whatever, as long as
the epoxy is, as you mentioned, thoroughly mixed, it will usually provide a
bond of reasonable strength that matches or exceeds the substrates involved.
A "real" structural epoxy is more than downpacked Epon 828 or Dow 362 or
whatever - it is a system consisting of a combination of one or more of, and
perhap other materials, epoxy resin, reactive or non-reactive diluents,
structural fillers, wetting agent, rubber modifier, etc.
Even with longer pot life consumer systems, many people have a lack on
understanding of the importance of proper mix ratio and thorough mixing, and
have a hard time differentiating between catalytic cure, such as in
polyester or vinyl ester systems, and addition reactions such as in epoxies.
I continually hear people say things like "yeah if you want your epoxy to
cure faster add more curative"... no no no no no! You need to match the
number of reactive sites on the resin to those on the hardener. and
depending on what you buy the manufacturer may specifiy mixing by volume
(common in consumer systems) or by weight (far better, and more common in
industrial systems but not prevalent). I think the "Part A" and "Part B"
nomenclature was/is an attempt to remove focus from just the resin side, but
over time due to the large number of "resin" and "hardener" systems out
there "B" has automatically and irrevocably become "hardener" in many
people's minds. Yes, it is a curative, but it and the resin are a
team...they work best together in the right ratio. I understand that part of
this misunderstanding is because for one resin, there are oodles of curative
choices that sometimes demand different ratios, but that is the nature of
the beast.. the reactivity is primarily manipulated through the curative
side and the number of reactive site sper molecule and the molecular weight
vary between choices. Epoxy curative is not a catalyst where you vary the
addition level to manipulate cure time. It is an addition reaction - A + B ABABABAB etc, and there is one ratio whre each reactive site is satisfied
with one of opposite gender. (p.s. don't get into oddball systems that don't
apply to consumer and most commericial adhesive systems please...).
Another area people is mixing - some think "thorough mixing" involves
stirring madly for 15 seconds. Nope. One minute or more of quality mixing is
needed, and that invloves scraping down the walls and bottom of the
containers to give every molecule of the curative the best chance of meeting
an available molecule of the resin. Think "micro" versus "macro".
Aliphatic resins like Titebond, carpenter's glue etc. are far, far better
than most people give them credit for, and I use the mextensively in model
construction, usuallt reserving CA for tacking bits together while the
aliphatic glue dries . They penetrate the substrate, are flexible and
therefore "tough", reducing stress risers at brittle interfaces like most CA
joints, distributing loads better. Epoxy should be reserved for composite
laminates, filled structural adhesives, fuelproofing and the like. CA is
really, honeslty, a very, very mediocre adhesive for most applications -
consumer CA systems are generally never modified for elongation and
flexibility, and there are really only a limited number of CA systems in
commercial adhsvvie catalogs that offer flex and elastomeric behaviour, but
they do exist. CA is convenient, little else. It cures in a heartbeat, which
satisifies most people's impatience, and beacause so many model sturctures
are overbuilt or use material with mediocre sturctural propeties, you can
"get away" with CA many times. But most systems are brittle and reduce the
potential strength of many structures. An understanding of adhesive
properties and the substrates involved can go a long way to making better
Thank you very very much for that..it explains some less than ideal
epoxy joins I have had in the past.
I totally concur in the use of e.g. PVA for most model work..I generally
use CA exclusively, but have had extreme problems with it not curing
fast except when used with porous balsa, and its gap filling abilities
are fairly poor, it's brittle, and when used with ply, failures are not
uncommon. It is also a bastard to sand and the drips ruin anything they
touch that can't be soaked in acetone.
I tried aliphatic, but that too seemed to leave a brittle residue..the
last model was made almost entirely with simple white PVA, and frankly,
it was ideal. Some areas were tacked with CA first before applying the
white glue though. I just used more pins and had to plan the build to
allow an hour or three for stuff to dry out.
Epoxy I use only with glass cloth to reinforce metal to wood joins.
The one unreinforced join I made with epoxy - a brass tube slide bolt
glued to a hatch - failed before the model even got COVERED. Should have
used JB weld etc. Or mixed as you say, I guess.
I would say a lot of people's satisfaction with CA comes from the fact that
a lot of our airframe structures are overbuilt, not that this is a bad
thing, but the convenience factor of CA then outweighs the negatives and you
end up with a structure that works, which in the vast majority of cases is
"okay". No arguing instant cure rocks for the impatient like me. But for
sure, many joints are substandard no matter how hard you try to do a good
job with the CA. Plywood/hardwood joints generally stink because the CA
barely penetrates the surface, and as such the stresses are concentrated on
the surface of the exterior lamination which to me nearly defeats the
purpose of using ply or hardwood (hard as in harder than balsa) in the first
Brass is a pig to bond. Rough up the surface like crazy, and if you can put
a strip of glass cloth over it to the substrate life will be better.
Plenty, I do not recommend it, EXCEPT when you are brushing a thin coat on
say an engine compartment for fuelproofing. IPA has a failry low vapor
pressure and takes a while to evaporate. In a thin film such as fuel
proofing, chances are good that the IPA will have a chance to flash off
leaving pure epoxy behind. But if any kind of cross-section is involved,
such as most stuctural bonds, IPA will be left suspended in the epoxy matrix
and will negatively affect it's mechanical properties. I would lean towards
MEK as a brushing solvent myself, as it is a very effective solvent and
flashes off quite a bit more quickly. And I would totally stay away from
attempts to thin the epoxy for mixing with any kind of solvent. There are
specific diluents formulated for epoxy systems, both reactive and
non-reactive, but they aren't available in an 8 ounce bottle at the glue
store, and these are generally formulated into an adhesive system by the
adhesive manufacture who, you would hope, has a competent formulator on
As an example of solvent problems, at work I regularly need to apply a thin
layer of rubber to surfaces we are working with to act as an intermediary
bonding layer. This is not epoxy, but the scenario is totally relevant. I
cut it with about an equal part by weight of MEK, basically "to taste" i.e.
I am happy with the viscosity. It is then rolled or bnrushed on. The thin
film surfaces cure to a high quality film with no residual solvent, even in
a fairly reactive (fast cure) blend. However, the material left over in the
mixing cup, which may be 1/4" deep, eventually cures into a miserable chunk
of weak rubber, easily torn, breaks into chunks, elongation is ruined,
nothing compared to the virgin material if allowed to cure in a similar
cross-section. It is ridden with residual MEK and continues to shrink over
time as the remaining MEK tries to worm it's way out to atmosphere etc.
Addition of solvent ruins the physical properties of the cured joint.
In fairly fresh cured joints the solvent acts as in internal lubricant
and make the cured resin more flexable but at the same time dashes the
tensil strength. After the joint has stood for a period of months the
solvent will diffuse out and evaporate and the cured resin will become
exceedingly brittle and have even less tensil properties. You can
often crush it between your finger nails if there was much solvent
used. So if you want the cured product to have any kind of physical
strength do not ever add any solvents.
There are uses where such additions are important and desireable. But
those uses have nothing to do with mechanical strength of the cured
product. Off hand I can not think of any such uses in model building
with the possible exception of laminates where the only purpose of the
laminate is cosmetic. In such cases it does make the stuff very easy
to sand after the solvent is fully evaporated due to the brittleness.
When I do laminates they are not cosmetic so I never thin the resin.
Well, I add methanol to epoxy under 2 circumstances with no obvious ill affects :
first is when I fuel proof the inside of a fuselage and the firewall etc,
second is in laminating to slow down the speed of the cure.
I have not seen any obvious adverse strength effects from this but I haven't
done any comparative testing. Painting the inside of the fuse with thinned
epoxy adds strengh in addition to the fuel proofing effect because it helps fill
any deficiency in the gluing that ARTFs sometimes suffer from. Strength is
probably not the ultimate consideration here, but in laminating it's obviously
much more important. Does anyone have any manufacturers' data on how much
ill-effect 10% of methanol has on standard laminating epoxies ?
I once asked Fbiretech what was in the epoxy thinners they sell and they said it
was methanol and that it is preferable to acetone which is "a bit aggressive to
epoxy resin when it is in its pre mixed state". They didn't mention any adverse
effects on strength but I didn't specifically ask that question.
I only use 30 minute and 1 hour epoxy and I don't know if it is any different in
this regard from the 5 minute stuff. Probably not.
Just my experience...
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