Stuck on Foam - 2009/08

There's a saying that you have to have the right tool for the job. When dealing with
kits made from balsa and light plywood, it
seems that there’s not much question about
which glues to use. Many of us have our
favorites, whether it’s CA, wood glue, or
another type, but most modelers know which
ones work.
Things seem to get more involved when it
comes to foam. We have to worry about how
the adhesive will react to the material. Will it
eat it? Add to that the different types of foam,
and the fact that some react differently can
make it a tough choice. We even need to be
conscious of the glue’s color once it cures,
especially if the adhesive joint will be visible.
I recently obtained my first EPP-foam
aircraft and had questions about what
adhesive would be best. I turned to the
Internet, where I searched for things such as
“best glue for foam” and “EPP glue.” The
more I researched my question, the more
answers I found. Unfortunately, the answers
were as varied as the foam we use to build
aircraft.
I decided that if I had questions, surely
others do as well, and that maybe an article
would help. The focus will be on two types of
foam that are commonly used in model
aircraft—Depron and EPP—and seven
adhesives: Hangar 9 30 Minute Epoxy,
Gorilla Glue, hot glue, UHU Por, Ultimate
RC Foam Glue, X-1, and Zap-O.
I am by no means an expert on foam-safe
adhesives. My goal with this article is to
provide a reference to help those who have
questions about them and the materials with
which they are used in constructing model
aircraft.
Depron: Developed in Europe for use as
thermal insulation under wallpaper and
flooring underlayment, in the US it is
commonly used for meat trays you can find at
the supermarket. Depron is available in
varying thicknesses and is specified as
insulation sheet material manufactured from
fully recycled ingredients; it is free of Freon
and halogen.
Produced in form as a foamed polystyrene
sheet, Depron does not age. Its cell structure is
built up from fine, closed cells that give the
material its smooth surface. Because this
foam has closed cells, it does not soak up
fluid, making it virtually waterproof and
requiring less glue to bond it.
Depron is easy to work with and requires
no special tools to cut it; you need only a
sharp blade. This foam can easily be sanded
and shaped, and heat and a mold are
commonly used to shape it. It can be milled,
typically with a CNC machine, to produce
feather-light aircraft.
Depron does not react well to solventbased
glues and paints; they will melt or
deform it.
Adhesive Accelerant Cure Time Clear Drying
30 Minute Epoxy Epoxy Hardener Varies (Eight hours as tested) No
Gorilla Glue Water 30 Minutes Yes
Hot Glue Heat Seconds Yes
UHU Por N/A 10 Minutes Yes
Ultimate RC Foam Glue N/A 12 Hours Yes
X-1 X-1a Activator Seconds No
Zap-O Zap Kicker Seconds No
Adhesive Depron (Weight in Ounces) EPP (Weight in Ounces)
30 Minute Epoxy 32 (Did not break) 32 (Did not break)
Gorilla Glue 18 14
Hot Glue 20 32 (Did not break)
UHU Por 16 32 (Did not break)
Ultimate RC 16 12
X-1 22 30
Zap-O 24 22
Adhesive Stress-Test Results
Adhesive General Properties
EPP: Originally designed for automotive
energy management and for use as packing
and cushion material, this foam is excellent
for shock protection.
It is made using a multistep process that
begins by combining polypropylene resin
with additives and forming individual
plastic beads that measure 4mm-6mm in
diameter. The beads are injected into a
steam chest mold, where they are fused
under steam heat and pressure.
The result is an extraordinarily durable
product that has a waxy feel on the surface.
It is environmentally friendly, since no toxic
chemicals are used in the manufacturing
process.
The same as with Depron, EPP requires
no special tools to cut it. A sharp blade or
hot-wire cutter works fine.
Forming EPP is commonly done by
bending it to shape and then wrapping the
shape with packing tape. Because of its
waxy feel, a spray-on adhesive needs to be
used to allow the tape to stick to the foam.
EPP is typically unaffected by solvent-based
glues or paints.
Testing the Adhesives: Regardless of what
adhesive you choose, follow the
manufacturer’s instructions and ensure that
there is proper ventilation in your work area.
I tested all the glues I mentioned
previously on 6mm Depron and 6mm EPP. I
selected that measurement to provide rigidity,
to allow for analysis using weights. Both
varieties of foam are available in other
thicknesses.
A butt joint was employed for testing,
because it is not only the simplest junction
to make, but it is also the weakest, because
the contact area is minimal. Since I want the
joint to either give or break, this seemed to
be the best test to prove each adhesive’s
mettle.
I cut both the Depron and EPP using a
straightedge and a razor blade. The Depron
was cut into 4 x 4-inch squares, and the EPP
was cut into 4 x 2-inch squares. I cut the
EPP smaller, because it is not as rigid as the
Depron at the same thickness.
Each end that was to be butted to the
other was lightly sanded, if needed, to be flat,
and then I cleaned all pieces with alcohol.
The joint was 4 inches long on both foams.
Once each glue seam was made, the
parts were held together tightly with blue
painter’s tape during the curing process.
Since drying times vary, all butt joints were
given 24 hours to cure before testing. Keep
in mind that this testing is done for reference
only and that every glue intersection can
vary slightly.
I followed each adhesive’s
manufacturer’s recommendations except
Gorilla Glue; it called for the joint to be
clamped. I didn’t clamp it, because I thought
it might give that glue an unfair advantage
when compared to the others in the test that
could also benefit from clamping.
To affirm that I was testing only the glue
joint, I clamped each test piece to 5/8 aircraft
plywood, 1/2 inch above and below the seam
on both sides. Then the aircraft plywood
was put in a vise, to hold it in place.
Test-weight increments were made 2
ounces at a time. For the Depron, I placed
the weight 2 inches from the glue joint.
Since the EPP was not rigid enough, I
located the weight 1 inch from the joint.
During testing, I increased the weight
after the glue intersection successfully
supported it for three seconds. The amount
of weight on the joint when it gave way was
what I recorded. I included all the results in
chart form, for easy reference.
When choosing an adhesive, there are
several things to consider. Bond strength
may be the first thing that comes to mind,
but it might not be the most important factor
for the application at hand.
If you are building in your shop, you
might be willing to allow something to cure
overnight. When you are at the field, you
may be looking for a quick repair to get your
aircraft flying again in the shortest amount
of time.
And let’s not forget about aesthetics. A
glue that dries clear could be a better choice
when building or repairing an exposed area.
This article and my EPP build gave me the
opportunity to learn more about the foams
and adhesives we use in constructing our
models. There are several other glues and
foams available, but the object was to test
some of the ones that are most commonly
used.
If you have any thoughts or insight into
either foam or adhesives, please send me an
e-mail. We might expand on this topic in a
future issue of MA. MA
Jay Smith
[email protected]
Sources:
Depron, EPP foam:
RCfOAM
(404) 556-9117
www.rcfoam.com
Hangar 9 30-Minute Epoxy:
Horizon Hobby
(800) 338-4639
www.horizonhobby.com
Gorilla Glue
(800) 966-3458
www.gorillaglue.com
UHU Por:
Hobby Lobby
(866) 933-5972
www.hobby-lobby.com
Ultimate RC Foam Glue:
Yardbird RC
(281) 394-4331
www.yardbirdrc.com
X-1:
Purimco Products
(818) 609-9629
www.purimcoproducts.com
Zap-O:
Pacer Technology
(760) 246-6462
www.zapglue.com
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