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Model Airplane News, Feb 2004

Everything you ever wanted to know about glue but were afraid to ask

A great variety of adhesives is available for modelers, and some work better with certain materials than others do. Certain glues are better suited to specific materials and structures within our various models. Balsa and plywood kits, fiberglass and composite aircraft and even the easy-to-assemble ARF kits all require different types of glue to complete their construction. Let's see what's out there!

GLUE TYPES

Glues are divided into two groups: single-part (you simply apply the adhesive directly from the bottle to the parts being glued) and two-part (two separate ingredients must be combined before the glue is applied).

Polyvinyl acetate (PVA), aliphatic resin, polyurethane, cyanoacrylate (CA) and epoxy are among the most popular, but there are, of course, many other types of structural adhesives available.

PVA and aliphatic resin have been around the longest. PVA (white glue) has been in use since the late '40s; Elmer's is the most common brand. Although it will bond any moisture-free, oil-free, porous materials, it has characteristics that make it less than ideal for RC modeling. PVA stays relatively soft and plastic, so structural joints secured with it will move over time. Its lack of moisture resistance makes it unsuitable for exposure to water. White glue is, however, a good adhesive for gluing small foam models together-if they won't be exposed to moisture.

Aliphatic resin (Titebond) was introduced in the early 1960s; Titebond-II is a more recent development. Aliphatic resin is good for almost anything we want to glue on a model. It's very water-resistant and sandable, and it has good gap-filling strength, fatigue- and shock-resistance.

It can be used to join small pieces of wood with little or no clamping pressure. Glue joints can be handled after 15 minutes, but they must be left overnight for the bond to reach full strength. After a reasonable shelf life of about two years (usually), it starts to thicken when it reaches the end of its useful life. Aliphatic resin has a relatively short working time-about 15 minutes-during which you can shift the parts into position. It can be thinned slightly and cleaned up with water, and it's the least expensive glue. Its only negative is that it dries from the evaporation of its water base and tends to shrink slightly (just as PVA does).

CYANOACRYLATE (CA)

CA glues are reactive monomers and are available in various viscosities: thick, medium and thin. They are compounds that form chemical links (that is, they polymerize) when spread or pressed into a thin film. The very small amount of moisture that's present on most construction surfaces acts as a catalyst for the adhesive to cure. When properly applied, CA forms a bond that is several times stronger than the materials that it joins. Too much moisture, however, will degrade the strength of the bond. The speed at which the bond is formed is the greatest advantage CA offers; you can build a model in hours instead of days and instantly repair damage at the field. When used with an accelerator (kicker,) thick CA can quickly fill in gaps caused by damaged or missing pieces. CA glues are also fuelproof.

CA USES

CA is great for general balsa and plywood parts construction. Because of its gap-filling properties, thick CA should be used on parts that don't fit together perfectly and when you join two different materials such as balsa and carbon fiber. Thin CA should be used on parts that fit tightly together but not to glue balsa sheeting together. The joint it forms is much harder than the surrounding balsa, and this makes it very difficult to sand the sheeting smooth; aliphatic resin would be best to use here.

CA STORAGE

CA's relatively long shelf life can be further extended if you store unopened bottles in a freezer or refrigerator. You should allow the glue to return to room temperature before you use it. Store open bottles in a cool dry place that isn't exposed to direct sunlight. Don't store your bottle next to your accelerator; over time, even the faintest of fumes will cause the CA to thicken and cure.

UNCLOGGING

To prevent clogs in the applicator tip, keep it away from any surface that has been sprayed with accelerator. When you finish using the glue, lightly tap the bottom of the bottle on the workbench to knock the glue in the applicator tip back down into the bottle. Give the bottle a little squeeze to blow air through the nozzle to clear it out completely, then wipe the tip clean with a cloth. If the tip does become clogged, don't use a metal pin to clear the tip. This will only push the clog down deeper into the bottle, and it will clog up again when you use the glue. Cut the clogged tip off the bottle, or use a thin, 1/16-inch bit to drill out the clog and form a new hole. A better way to prevent a clogged tip is to use an applicator tip extension. If it becomes clogged, you can remove the extension, clear it with a thin wire and then replace it on the bottle.