Flame treatment is the application of a gas flame to the surface of a material to improve adhesion.
Polyolefins, especially polyethylene and polypropylene bond poorly, because they consist of long non-polar molecules. Without special treatment, adhesives, ink, and other coatings cannot be applied to these materials. By rapidly applying intense heat to a surface, molecular chains are broken and polar functional groups are added. Flame treatment also burns off dust, fibers, oils, and other surface contaminants.
Flame treatment is faster than corona treatment, but requires more complicated and expensive equipment.
Fuel gas is pre-mixed with air, and the resulting flame is directed at the surface to be treated. The flame has a light blue inner cone, called primary, and a dull blue outer cone, called secondary. The hottest part of the flame is the primary, which may be up to 1800 °C. The tip of the inner cone (primary) contains an assortment of highly reactive chemicals, including unburned oxygen, carbon dioxide, water vapor, and free radicals atomic oxygen (O1) and hydroxyl (OH).
To treat a surface quickly and evenly, many gas jets, similar to the one on the right, are lined up in a long row on a single burner. The material being treated is quickly passed directly in front of or under the inner cones. The surface is in contact with the flame for less than a second. The surface is treated quickly, without time for the material to melt.
The reactive chemical species in the gas flame break the long-chain molecules in the plastic material, and attach themselves to the break points, resulting in polar point charges on the surface. On a microscopic scale, the surface also becomes rougher.
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