Alternative Aircraft Glues

by Jan Zumwalt - 2013.12.11

Introduction

"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.

Manufactures Recommendations

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 the regulators.

Several on-line sources carry it. One source in Spokane carries it for about $20 per pint, substantially higher than the highest priced epoxy.

'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.)

JOINT STRENGTH

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

Casein 
Cyanoacrylate
Epoxy
Hide Glue
Pencolite resorcinol
Phenol formaldehyde resin 
Polyurethane
polyvinyl acetate
Resorcinol-formaldehyde
Urea-formaldehyde
Weldwood phenolic resin
Weldwood Plastic Resin

Discussion


Casein

Used to make strong and robust joints in early aviation, but fell out of favor due to its susceptibility to attack by bacteria.

Cyanoacrylate

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.

Epoxy

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.

Hide Glue

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.

Polyurethane (Urethane)

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 freezes.

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.

Resorcinol-formaldehyde

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.[8] 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"[4] 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.

Urea-formaldehyde

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 Resin

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.