Glue it to it

To All:

A few days ago I responded to Lou's question about the cold weather
performance of T-88...


http://groups.google.com/group/rec.a...40a97c3fdfa121

....where in closing I said, "Marvelous as epoxies are they don't even
come close to resorcinol when it comes to durability."

Boy, did I catch hell :-)

One feller politely provided me with a table of specifications for
various epoxies showing their shear strength, modulus of rupture and
modulus of elasticity. At least half of the figures were better than
the specs for Weldwood Resorcinol.

Another less polite message informed me I didn't know what I was
talking about. It seems this particular person had been using T-88
since it was introduced '...during the Second World War..' and
stated it was the best glue ever invented.

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Allow me to repeat the obvious:

Bum Dope can get you killed.

At least half of the information on the internet is incorrect to some
degree.

Anyone offering an opinion should be willing to back it up, if not with
facts, at least with the basis for their opinion.

So here goes :-)

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'Epoxy' as a chemical term dates from the early 1900's with the
first patents applied for in Germany during the 1930's. During World
War II some epoxies were developed for various roles - - adhesives,
insulators, paints and so forth - - but commercial exploitation of
epoxies was not seen on any scale until the late 1940's with the
development of methods for making the stuff from natural gas.

With regard to T-88, as best I can recall it didn't arrive on the
scene until the early 1970's, not '...during the Second World War.'
I'll be more than happy to admit my error here if someone can offer
a better reference. (I'm old. I fart a lot and forget things.)

As I said in my response to Lou, epoxies are marvelous stuff as
adhesives but when it comes to their durability they don't do as well
as plain old-fashioned resorcinol. At least, not if we are talking
about gluing wood, and specifically so when the task is gluing birch
plywood to softwood stringers. Shear strength remains high but peel
strength declines with age. Fortunately, it's rare for a
birch-spruce joint in an aircraft structure to be subjected to a
delaminating load. Something else would have to fail first. (Hint:
Given enough time, simply allowing moisture to collect inside the
structure would be sufficient.)

This comes under the heading of Old News since the problem has been
around as long as epoxies themselves. But in the late 1960's there
were indications that certain chemical 'coupling agents' could
facilitate a CHEMICAL bond between an adhesive and whatever it was
applied to. This created an enormous amount of interest because it
hinted we might be able to GLUE metals together and end up with a joint
equal to welding, riveting, brazing and so on.

As you may have noticed, that hasn't happened. Yet. At least, not
with metals on an industry-wide scale. But it HAS happened with wood
and some plastics.

In 1995 a fellow named Charles B. Vick from the Forest Products
Laboratory presented a paper in Portland describing tests with a
coupling agent called HBR (hydroxymethylated resorcinol) as a means of
enhancing the adhesion of epoxy to wood. The results of the tests were
so promising that it lead to the academic equivalent of dancing in the
streets (which means more studies. :-)

The key point here is that the tests compared epoxies, enhanced or
otherwise, to plain old-fashioned resorcinol, which I believe justifies
my statement to Lou.

In the decade since Mr. Vick presented his paper on epoxy and HBR he
has completed a number of other studies, read with the keenest interest
by folks selling epoxy glues. And by guys like me who occasionally
trust their lives to an assemblage of glued joints. In responding to
Lou's message I felt I was doing what any other homebuilder would
have done had he gotten to the keyboard first. But if I erred in doing
so, please have the courtesy to state your correction publicly since
doing so privately makes it little more than a personal attack and
contributes nothing to the sum store of information.

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As I've mentioned in earlier articles ('Glue Wars,' etc.) ALL
modern adhesives are stronger than the softwoods normally used for
airplane construction. That means the question of strength drops out
of the equation when selecting what we're going to use to gluit toit.
While epoxies are the near-ideal adhesive for airplanes the gritty
reality is that it's more expensive than other equally useful glues.
Such considerations fly right over the head of someone who thinks a
thirty thousand dollar kit is 'inexpensive' and it's $25,000.00
engine 'affordable' but the hundred dollar difference between an
epoxy from Aircraft Spruce and a few pounds of Weldwood 'Plastic
Resin' from the local Borg is a CRITICAL factor to the tens of
millions of Americans who can simply no longer afford to be a part of
the General Aviation community.

-R.S.Hoover