Alternative Aircraft Glues
by Jan Zumwalt - 2013.12.11
"Glues" are professionally refereed to as "adhesives". There use
in aircraft can be broken into two topics: use in certified aircraft
and alternative products for use in amateur built aircraft.
Sensei recommends resinorcol glue for props.
However, Weldwood Resinorcol glue is unavailable at any price in
California from retailers or wholesalers. Something in it excites
Several on-line sources carry it. One source in Spokane carries
it for about $20 per pint, substantially higher than the highest
'Plastic Resin' is certified. If you're doing a repair on a certified
airframe, even something relatively minor like mending an aileron,
you owe it to the customer (and the next mechanic down the line) to
use only certified materials. Check the specs. Most of the adhesives
available today are NOT certified for use on airplanes.
Why The Debate?
Like most conflicts, the Glue Wars are spawned by ignorance. The War
begins when someone says they use Brand X glue. No matter what type it
is, someone is sure to declare THEY use Brand Y because it's STRONGER.
All of which may be true, except for the part about the ignorance.
Some builders find the 5:2 ratio of 'Plastic Resin' to water to be a
challenge. Others find the 10:1 ratio of FPL-16A beyond their abilities and
even the 1:1 of T-88 something of a trial. Aerolite is too viscous; Hughes
too runny; Excel One can't be any good because it's too easy to use (!!).
Or their shop is too something to use Brand X (hot, cold, damp, dry,
dusty... you fill in the blank). The reasons a builder selects a particular
glue are as varied as the builders themselves but once they settle on what
works best for them, an odd thing happens.
With one airplane's-worth of experience under their belt the typical
homebuilder becomes an expert on his particular choice of glue, wood and
fabric. The sad part is that they often say as much :-)
The truth is, ALL modern adhesives are STRONGER THAN THE WOOD. There
may be some differences in resistance to moister, but by the time
moisture effects the glue, the airplane is probably un-airworthy anyway.
I recommend you get good at using a couple types of glue. Keep a good
notebook with pressure times and temperature. Go build yourself an
airplane. Go fly. And leave the the ignorant talk to the hanger fliers.
(It takes two to start a war.)
The following data from "Fine Woodworking, August 2007" compares
different glues to Polyvinyl Acetate (PVA)
Type I PVA glue (Waterproof) 100%
Slow-set epoxy 99%
PVA glue 95%
Liquid hide glue 79%
Hot hide glue 76 %
Polyurethane 58 %
Selecting The "Right" Glue
Glues are usually not select for strength as much as they
are selected for other qualities such as ease of mixing, pot life,
sand-ability, color and tinting, etc.
If the glue needs to be waterproof Urethanes or epoxy is the best choice.
Common brands and types that we are most likely to select from are
Phenol formaldehyde resin
Weldwood phenolic resin
Weldwood Plastic Resin
Used to make strong and robust joints in early aviation, but
fell out of favor due to its susceptibility to attack by bacteria.
This glue is used mainly for small repairs, especially by wood turners.
Bonds instantly, including to skin. Cured CA is essentially a plastic
material. Versions are available that are able to wick into tight
joints but bond with reduced strength because much drips out and
much soaks into the wood leaving very little on surface for the
bond), or thicker formulations (gel) which can fill very small
gaps, do not flow out of the joint, and do not soak so quickly
into wood. Thinner cyanoacrylate glue does not bond more quickly
nor form shorter polymer chains than the gel version when used
on wood. Cyanoacrylate is stiff but has a low shear strength
(brittle) thus normal wood bending can break the bond.
Usually a two part mix system, cures under a wider range of
temperatures and moisture content than other glues, does not
require pressure while curing - clamping actually weakens
bond, and has good gap-filling properties - near perfect
joints with very small gaps actually produce weaker bond.
Bonds to most cured wood glues (except PVA). Two part epoxy
adhesive is very resistant to ultraviolet light and salt water,
most epoxy is heat resistant up to 350 °F, the formulations
containing powdered metal and rubber or plasticizers are very
tough and shock resistant. Epoxy can trigger long-term
sensitivity (allergies) from overexposure, and is often expensive.
Is unacceptable for aircraft because it softens and comes apart
if heat is applied.
Phenol formaldehyde resin
This is primarily an industrial adhesive used for making plywood.
It is cured at elevated temperature and pressure.
Bond to textile fibers, metals, plastics, glass, sand, ceramics, and
rubber, and wood. When exposed to moisture, helps cure the adhesive.
Interaction with Douglas-fir and yellow birch can diminish strength.
Varies in strength to water saturation. Best when 70 to 100 psi
pressure is applied for about 24 hours. On the practical side, once
you've opened a container of urethane glue it is going to set-up,
sooner or later, due to the air introduced into the container.
Polyvinyl Acetate - waterproof (PVA)
Often called "white glue" (similar to Elmers). Type I is
"water proof". Tensile strength 3750 psi, good for exterior/exposed
wood, water proof, weatherproof, can be sanded, unaffected by
finishes, water cleanup. Long shelf life but may be ruined if it
Aliphatic Resin (Yellow Glue)
Commonly known as "yellow glue" or "carpenter's glue" is a improved
form of white PVA glue. It has been modified to make it stronger and
more moisture resistant. Requires good clamping pressure (70-100psi)
Designed to work on porous materials only. Most are water resistant
but not water poof. Can Be Sanded, Unaffected By Finishes, Water
Cleanup. Long storage life but is ruined if it freezes.
This is a very strong and durable (resisting immersion in boiling water, mild
acids, salt water, solvents, mold, fungus, ultraviolet, etc.). It must be mixed
before use (liquid resin and powdered catalyst). It has a dark purple
color which may be objectionable in some uses. Toxic. For many years,
the Federal Aviation Administration (FAA) has stated that "Resorcinol is
the only known adhesive recommended and approved for use in wooden
aircraft structure and fully meets necessary strength and durability
requirements" for certificated aircraft, however in fact the vast
majority of wooden aircraft built in recent decades (mostly amateur-built
aircraft) instead use other types of adhesives (primarily epoxy resin
systems) which offer greater strength and even more importantly, much
less criticality in perfect application technique.
A powdered wood glue activated by mixing with water. Forms a bond
stronger than the wood itself. Low effective cost, low cure
temperatures, resistance to microorganisms and abrasion, and light
color. It does not creep, and can be repaired with epoxy. Can rapidly
deteriorate in hot, moist environments.
Phenolic resins are mainly used in the production of circuit boards.
They are better known however for the production of molded products
including pool balls, laboratory counter tops, and as coatings and
adhesives. In the form of Bakelite, they are the earliest commercial
synthetic resin. Glass phenolics are particularly well suited for use
in the high speed bearing market. Phenolic micro-balloons are used
for density control. Snooker balls as well as balls from many
table-based ball games are also made from phenol formaldehyde resin.
The binding agent in normal (organic) brake pads, brake shoes and
clutch disks are phenolic resin. Synthetic resin bonded paper, made
from phenolic resin and paper, is used to make counter tops.
Be aware that the present-day Weldwood 'Plastic Resin' is NOT the original
formulation. After Weldwood was acquired it's new owners gave up the aircraft
certification as a means of lowering the production cost. I've not found any
loss of strength in the new stuff, based on glue-block shear tests, but I've
had two bad batches in which the glue-block AND the component being glued
simply came apart. Since that happened, any time I use 'Plastic Resin' I mix
a small test batch from each new container and do a few glue-blocks.
You will also find that the plastic containers they now use can not be
re-sealed; once opened, the powder will solidify in the container in a
few weeks. The need to buy the stuff in small containers, immediately
prior to use, and to then test it before use, is the main reason I've
started using urethanes for most chores. But I do not know of a urethane
I could recommend for propellers.