From: Johnny <nwaero@northwest-aero.com>
Subject: Re: Aluminum structure glue in addition to rivets?
Date: 11 Jun 1998
Newsgroups: rec.aviation.homebuilt

Service Bulletin for repairing AA-* series (bonded) aircraft specs
riveting through a high viscosity (and I mean the really nasty stuff)
polysulfide adhesive. This family of adhesive sealers comes in a
multitude of viscosities, strengths, and adhesion levels, for various
different applications.

For example, light viscosity medium adhesion stuff is used to hold and
seal windows on the heavys. Medium viscosity light adhesion is used to
seal removable fuel tank inspection covers. Medium viscosity high
adhesion is used to assemble and seal fuel tanks. And the black stuff,
the high viscosity high adhesion stuff, will hold just about anything to
anything. (use your own judgement)

There are several things you might try to prep the aluminum. All of them
should include and etch job. I have tried some test patches that were
etched and then alodyned (iridite 14), with the theory being that the
glue will stick to the alodyne really well, and the alodyned surface
will remain stable over time. Also tried the same thing with a zinc
chromate layer in there, with stability over time being the idea. I
think the adhesive stuck to the alodyne better than the chromate. It
sticks to bare really well too, but I have seen adjacent corosion
develope over time and crawl under the adhesive, lifting it.

Another thing I tried was to use an epoxy. It's not as flexible. There I
etched, alodyned (actually, I dropped them in the chem film tank at the
plating shop), then shot it with epoxy primer. Then, while still within
the recoat window of the primer, applied the epoxy (scotchweld in this
case) using the primer as an intermediate adhesion layer. Those babies
were married after that for sure.

I didn't really do any scientific testing to see how strong it was, but
the high viscosity polysulfide, over the alodyne, would not let go
before the sheet metal (.025 - .032 2024-T3) tore. In sheer, and in
peel.

The epoxy over the primer was just as strong, but if I bent the metal
back and forth I was able to get the epoxy to crack, but it still didn't
want to pop apart. Used in areas where there wasn't any real flexing
this would work well I think.

However, I wouldn't use either without the accompanying rivets. Why glue
when you are going to rivet anyway? The joint is stonger, there is no
movement, no working, the rivets don't loosen over time, they don't
crack up through primer/surfacer paint, you can laminate a flush patch
in on a bonded airplane this way and it totally undetectable after it's
painted... even years later, it's common practice in wet wing areas, the
panels don't rattle when they get time on them, used to double up areas
of high abuse like the wing walk area it makes it stiffer than it would
be without the glue, etc.

Please don't mistake any of this for a real bonding (pressure and heat
fixture) system. There's no doubt in my mind however that these methods
have been proven to increase the strength of some types of joints, and
have allowed adequate fastening with CS style reduced head rivets in
.020 skins where it would have otherwise been undoable. As for some sort
of magic glue that you can just put the whole airplane together with, I
haven't seen it yet.

In addition to calling 316 942-6211, you might try these other guys at:

ELLSWORTH ADHESIVE SYSTEMS
N117 W18711 Fulton Drive / P.O. Box 1002
Germantown, WI 53022
414-253-8600 / Fax: 414-253-8619
1-800-888-0698

Polysulfide, polyurethane, epoxy, you name it.

-j-


Mike wrote:
>
> aircraft@TopHire.com (Shaun) wrote:
>
> >Want to incrase sheer strength with minimal weight in my aluminum
> >aircraft.  Anyone know the best stuff?  What do the Airlines/military
> >use if any?  Special instructions and tools for application also
> >appreciated.   - Shaun.
> >
> Back in the 1984 issue of "Homebuilt Aircraft", Mr. Otis Holt wrote an
> article entitled "No More Rivets".  A copy of this article is
> re-produced in an excelant book by author Andrew C. Marshall,
> "Composite Basics".    This book also contains a chapter on Adhesives
> and Bonding which details exactly how to bond Aluminum to Aluminum
> using Epoxy.
>
> According to this book,  Boeing has perfected a method which works
> extreemly well, (Boeing Spec. BAC 5555), and is also very difficult
> for we home builders to copy.  Their process is way too involved to
> detail here, it involves acid etches followed by Phosphoric Acid
> Anodize (not Alodyne), and rinsing in highly filtered, de-ionized
> water.  Special corrision-inhibiting adhesive primers (which MUST be
> applied within 12 hours of the Anodizing process) are then applied to
> a precise thickness and then oven curred.
>
> However, there are many companys which simply "scuff" the Aluminum
> surface before bonding to it.  Now, if you believe what A. C. Marshall
> says in his book, these bonds Will fail; it's only a matter of time.
> Is he right???....damned it I know.
>
> Anyone else out there in RAH land care to wade in on this one???
>
> Good luck,  Mike