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I have become a coro addict, and I would like to take this opportunity to offer some insight as to what is happening chemically during the flashing, or flame treating of the surface of coroplast in order to get good adhesion with various types of adhesives.

Back in 1966, immediately after obtaining my mechanical engineering degree, I accepted a position with one of the largest producers of high density polyethylene and in the world. My first assignment was in their plastics development laboratory working on the development of a flame treatment process for a proposed high density polyethylene automotive fuel tank to facilitate the application of an epoxy barrier coating to prevent the transmission of gasoline vapors through the fuel tank walls into the atmosphere. Automotive emissions legislation was just beginning to be discussed at that time.

Polyethylene and Coroplast (which is made from polypropylene) are first cousins chemically, and are members of a family of polymers known as polyolefins. Olefin means "lack of affinity," meaning that these materials are, for all practical purposes, chemically inert. Nothing will adhere to them in their natural state, because nearly all of their covalent bonds are saturated.

In order to get anything to adhere to a polyolefin, you need to create chemically active bond sites on the surface of the polymer that the adhesive can chemically bond to. One way to do this is to flame treat the surface with a stoichiometric air/fuel mixture. This means that there is sufficient oxygen present in in mixture to completely burn all of the fuel molecules present. We explored many different fuel/air mixtures and found the stoichiometric ratio to be optimal. If you are using a Bernz-O-Matic propane torch, simply adjust the flame to get a good blue cone, with no orange flame present. The outer cone should be almost clear. Go to your local hardware store and pick up a fan-shaped flame spreader used for burning off house paint to give you a nice 2-inch wide flame pattern.

Hold the torch about 4 inches above the surface to be treated and move steadily at about one to one and one half feet per second. You will see a blush pattern spread out about two inches to either side of the flame in the reflected light from your shop lights. Wait about 10 seconds for the surface to cool and make a second pass to the side of the first pass, if the area you are gluing is wider than the flame pattern.

One test to see if the flame treatment has worked is to "water wet" the surface. Untreated polyolefins have little effect on the surface tension of water, therefore, water will bead up on untreated surfaces. Treated surfaces, with their available covalent bond sites, will attract the polar water molecules and the water will spread out evenly on treated areas. Try it. Hold the untreated surface under a faucet and see how the water beads up. Then dry the surface and flame treat it. Repeat, holding the test piece under the faucet and notice the difference. In the lab we used a carbol fuscia dye to see the results. The treated areas would turn purple because the dye molecules had bonded to the treated areas.

Do not touch the treated area. The oils present in your skin will destroy the monomolecular thickness of the treated surface. Chemically, what you are doing is forming carbonyl (pronounced "carbon-eel") groups on the surface. These groups contain a hydroxyl radical, which forms the bond with the adhesive molecules.

During my employment in this capacity, I searched the organic chemical library for an organic compound containing metallic atoms in the polymer that could be blended into the base polyolefin polymer to create a bondable polymer that would eliminate the need for flame treating, but I couln't find anything at that time. Perhaps such a material exists now, but flame treating still works.

Hope this helps. One last thing. There are no manufacturing oils used in the extrusion process. The material is nothing but polypropylene polymer, which possesses a natural "oily" feeling to the touch, a hallmark of all of the polyolefin family of polymers. When extruding Coroplast, the only thing the polymer sees is the pressure exerted on the liquified polymer from the extrusion screw which rotates in an electrically-heated cylinder plasticizing the resin pellets before the melt passes through the die, and the cooling temperature of the extrusion die which brings the polymer back to the solid state in the form of Coroplast sheet.

Please accept my sincere apologies if my remarks have offended anyone by this. Tatoo and Kraut and all of the other SPADers have done all of us a tremendous service by selflessly sharing their wealth of knowledge and their designs. I just wanted to set the record straight as to what is actually happening chemically when we flash this material. Additional comments or questions are always welcome.

Al Mangani
JG 26
AMA 22336