Spray Adhesive and Extruded Polystyrene Foam Insulation

For decades I have used spray adhesive to mount layers of foam together when making terrain. Originally, I used 3M Super 77 spray adhesive, which worked wonderfully. But over the years, they "new and improved" it or otherwise changed the chemical formula such that it can attack or react with the foam. The latest change has made it even more challenging to use the product in this role, so I conducted a test with the brands of spray adhesive that were readily available at local retailers.

I purchased six brands of spray adhesive and sprayed them on samples of 1 inch thick blue Dow-Corning extruded polystyrene foam insulation.

Testing method: The adhesives were first sprayed from a distance of 2-3 inches in a heavy concentration creating a pool of liquid adhesive on the foam; then sprayed from a distance of 6-8 inches in a moist, heavy coating of adhesive; and lastly from a distance of about 12 inches in a solid but less moist light coating more typical of how I normally use the adhesive when constructing terrain.

I offer my observations below:

 Product  3M Super 77 Spray Adhesive  Krylon Spray Adhesive, All Purpose, #7010  Loctite Spray Adhesive General Purpose  Duro All Purpose Spray Adhesive  3M no. 45 General Purpose Spray Adhesive  3M Spray Adhesive for Styrofoam Brand Foam #6070
 Pool  Melts foam  Melts foam  Melts foam  Melts foam  Does not attack foam  Does not attack foam
 Heavy Spray  Melts foam  Melts foam  Melts foam  Does not attack foam  Does not attack foam  Does not attack foam
 Light Spray  May very lightly attack foam  Melts foam  Melts foam  Does not attack foam  Does not attack foam  Does not attack foam
 Spray pattern  Heavy cloud   Heavy cloud  Light cloud  Light cloud   Heavy cloud  Stringy, web-like pattern
 Longevity of Adhesion  Years, if care is taken during application, it can offer a strong, long lasting bond  Unknown, as it melted the foam test samples  Unknown, as it melted the foam test samples  Provides a weak bond, easily pulled apart, without offering adhesion across the entire surface area Seems to work well, offering reasonably strong, and long lasting (several months) bond  Works well, offers strong, long lasting bond (over two years without delamination)
 Useful for Terrain Making  Yes, with care  No  No  No  Yes  Yes

I noted the spray pattern, as the cloud of adhesive creates more overspray from farther distance, and less control when spraying. The stringy, spider web like spray of the last subject comes out of the can much thicker, is easier to aim, and results in less over spray. At first, I was concerned that the thickness of the 3M Styrofoam spray would create a thick layer of goo between the layers of foam, but it did not. I weighted the foam while the glue dried, and it final product was quite similar to foam glued with the Super 77 and 3M no.45 spray adhesive.

Longevity of Adhesion refers to the life of the bond. I have foam glued with 3M Super 77 in 1988 without the foam layers separating. The 3M Super 77 formula that was tested was purchased in February/March of 2008, and seems to be the same formula (performs the same during application) as has been marketed for the last couple of years. I have terrain using this newer 3M Super 77 formula that is a little over two years old without delamination of foam layers.

Useful for terrain making: Based on my experience, I rate the products as being useful or not for making built-up foam terrain. All of the products tested contain acetone, which by itself will reacts with the foam. In instances where the products attacked the foam, the distance from which the adhesive was sprayed radically impacted the degree to which the foam was attacked by the adhesive. Sprayed from a distance of 12 inches or more, the current 3M Super 77 spray can be used, but if sprayed too close to the foam, it will react and result in poor bonds. The 3M no. 45 General purpose spray seems to work well, but I am still curious if the bond will last years or not. In each case, the adhesive bond was stronger than the foam, resulting in the foam breaking apart, rather than the adhesive letting go, when pulling the glued sections of foam apart. Time will tell. At this point I am most comfortable using the 3M Styrofoam Brand adhesive, and then the 3M no. 45 adhesive.

Notes about other spray adhesives: I did find other spray adhesive products during my search, but discounted their use in terrain making because of warnings about use with foam, about being repositionable, or offering temporary adhesion. I tested 3M no. 78 spray adhesive for use with foam insulation board sometime ago, but found that the thick spray did not smooth out when the foam layers were pressed together, and found the bond to be weak with the adhesive failing rather than the foam. 3M Super 90 adhesive eats the foam. I have had success with a product call Florabond brand spray adhesive, terrain pieces now being three years old without delamination, but I can no longer find the product. Florabond seems to work most similar to 3M no. 45 spray adhesive with the knowledge that the bond lasts for years.

Notes about other brands of extruded polystyrene foam: Over the years I have used the blue foam from Dow-Corning for most terrain applications, but other brands of foam seem to behave in exactly the same manner as the blue foam. Note that the pink foam Owens-Corning is usually a little thicker than the same stated thickness from Dow-Corning.

Spray Adhesive Application: When applying the spray adhesive to foam, I spray an even coat of adhesive over both surfaces to be glued together, wait a brief period (varies with the brand of spray adhesive, thickness of adhesive coating, and humidity) until the adhesive gets "aggressively tacky", then carefully align the pieces of foam and press them together. I then place weights on the foam for 20-30 minutes before shaping the foam. Foam can be shaped prior to gluing, but must be carefully aligned during assembly, as the tacky adhesive can be very unforgiving.

Cutting the foam and adhesive: Once the foam is glued up and the adhesive is dry, the foam can be shaped through various methods. I have used hot wire cutters many times, and the wire will cut through and of the adhesives without too much resistance. Note that cutting across an adhesive layer with a shallow angle of approach is more likely to result in a haughtier getting "captured" in the foam layer. The cutter will slice the adhesive more readily than the foam and may leave a "step" of foam at the adhesive layer before entering the next layer of foam. I have never had any problems (other than mess) with using knives, saws, surform tools, etc with any of the foam and adhesives. If you start cutting and find the adhesive is not dry, simply place a weight on the foam and give it more time. When using a hot wire, note the harmful chemicals come off of the foam/adhesive, use proper ventilation and appropriate safety equipment.

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