Author: Jay Smith
Edition: Model Aviation - 2009/08
Page Numbers: 28,29,30,31
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Stuck on Foam

by Jay Smith

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, there isn't much question about which glues to use. Many of us have our favorites—CA, wood glue, or another type—but most modelers know which ones work. Things get more involved when it comes to foam. We must consider how the adhesive will react to the material. Will it dissolve or deform the foam? Add to that the different types of foam, and the fact that some react differently, and choosing the right adhesive can be tough. We also 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 which adhesive would be best. I turned to the Internet, searching for things such as "best glue for foam" and "EPP glue." The more I researched, the more answers I found—and the more varied the answers were. I decided that if I had questions, others probably do as well, and that an article might help. This article focuses on two commonly used foams—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 not claiming to be an expert on foam-safe adhesives. My goal is to provide a practical reference for modelers building with these materials.

Foams

Depron

Depron was developed in Europe as thermal insulation for under wallpaper and flooring underlayment. In the U.S. it is commonly used for supermarket meat trays. 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 as a foamed polystyrene sheet, Depron does not age. Its cell structure is built from fine, closed cells that give the material a smooth surface. Because Depron has closed cells, it does not soak up fluids, making it virtually waterproof and requiring less glue to bond it.

Depron is easy to work with and requires no special tools to cut—only a sharp blade. It can be sanded and shaped easily; 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 solvent-based glues and paints; solvent-based products can melt or deform it.

EPP

EPP (expanded polypropylene) was originally designed for automotive energy management and for packing/cushion material. It is excellent for shock protection. EPP is made by forming individual plastic beads from polypropylene resin, adding additives, and injecting the beads into a steam chest mold where they are fused under heat and pressure. The result is an extraordinarily durable product with a waxy-feel surface. EPP is environmentally friendly because no toxic chemicals are used in the manufacturing process.

Like Depron, EPP requires no special tools to cut—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 surface, either a spray-on adhesive must be used or the surface must be scuffed to give it some tooth. EPP is typically unaffected by solvent-based glues or paints.

Adhesive General Properties

  • 30 Minute Epoxy
  • Accelerant: Epoxy hardener
  • Cure time: Varies (eight hours as tested)
  • Dries clear: No
  • Gorilla Glue (Fast Cure)
  • Accelerant: Water (requires a small amount of moisture to activate)
  • Cure time: ~30 minutes (as tested)
  • Dries clear: Yes
  • Notes: This is a polyurethane adhesive that expands as it cures; clamping is normally recommended. Once cured, it is sandable and paintable.
  • Hot Glue
  • Accelerant: Heat
  • Cure time: Seconds
  • Dries clear: Yes
  • UHU Por
  • Accelerant: N/A (contact adhesive)
  • Cure time: 5–10 minutes before joining; full process ~10 minutes
  • Dries clear: Yes
  • Notes: Apply evenly to both parts, let dry for five to ten minutes, then press surfaces together for a few seconds to bond.
  • Ultimate RC Foam Glue
  • Accelerant: N/A
  • Cure time: ~12 hours
  • Dries clear: Yes
  • X-1
  • Accelerant: X-1a activator
  • Cure time: Seconds (with activator)
  • Dries clear: No
  • Zap-O
  • Accelerant: Zap Kicker
  • Cure time: Seconds (with kicker)
  • Dries clear: No

Notes on epoxy: Epoxy is a thermosetting polymer formed from the reaction between resin and hardener. Epoxies are classified by working time once mixed; curing time is the amount of time until the adhesive reaches full strength.

Testing the Adhesives

Regardless of which adhesive you choose, follow the manufacturer's instructions and ensure proper ventilation in your work area.

I tested all seven adhesives on 6 mm Depron and 6 mm EPP. I selected 6 mm to provide rigidity and to allow analysis using weights. Both foams are available in other thicknesses.

Test method:

  • A butt joint was used because it is the simplest and typically the weakest joint due to minimal contact area. I wanted the joint to give or break to test each adhesive's strength.
  • Depron samples were cut into 4 x 4-inch squares. EPP samples were cut into 4 x 2-inch rectangles because EPP is less rigid at the same thickness.
  • Each mating end was lightly sanded if needed to be flat, then all pieces were cleaned with alcohol.
  • The joint length was 4 inches for both foams.
  • After applying adhesive, parts were held together tightly with blue painter's tape during curing. All joints were allowed to cure 24 hours before testing.
  • I followed each adhesive manufacturer's recommendations, except Gorilla Glue called for clamping—I did not clamp it in order to keep the test conditions comparable across adhesives.
  • To ensure I was testing only the glue joint, each test piece was clamped to 5/8-inch aircraft plywood positioned 1/2 inch above and below the seam on both sides. The plywood assembly was placed in a vise to hold it steady.
  • Test weights were added in 2-ounce increments. For Depron, weights were placed 2 inches from the glue joint. For the less-rigid EPP, weights were placed 1 inch from the joint.
  • During testing, I increased the weight after the glue intersection supported it for three seconds. The weight at which the joint failed was recorded.

Keep in mind this testing is for reference only; every glue intersection can vary slightly.

Test Results (weight supported at failure)

  • 30 Minute Epoxy
  • Depron: 32 oz (did not break)
  • EPP: 32 oz (did not break)
  • Gorilla Glue
  • Depron: 18 oz
  • EPP: 14 oz
  • Hot Glue
  • Depron: 20 oz
  • EPP: 32 oz (did not break)
  • UHU Por
  • Depron: 16 oz
  • EPP: 32 oz (did not break)
  • Ultimate RC Foam Glue
  • Depron: 16 oz
  • EPP: 12 oz
  • X-1
  • Depron: 22 oz
  • EPP: 30 oz
  • Zap-O
  • Depron: 24 oz
  • EPP: 22 oz

Choosing an Adhesive

When choosing an adhesive, consider more than just bond strength. Other factors include:

  • Cure time: If you build in your shop you might allow something to cure overnight; at the field you may need a quick repair.
  • Activation method: Some adhesives need moisture, heat, or an activator.
  • Finish: A glue that dries clear might be preferred for exposed areas.
  • Compatibility: Avoid solvent-based glues and paints with Depron; EPP is typically more tolerant but may need surface scuffing for some adhesives.

This testing and my EPP build helped me learn more about the foams and adhesives used in model construction. There are other glues and foams available; the goal here was to test several commonly used options.

If you have thoughts or insight into foams or adhesives, please send me an e-mail. We might expand on this topic in a future issue of MA.

Jay Smith [email protected]

Sources

Transcribed from original scans by AI. Minor OCR errors may remain.