I’ve noticed that adhesion failures of labels using pressure sensitive adhesives (PSAs) to painted metal surfaces such as decorative street lamp posts or even some equipment cabinet panels is becoming more common. The first suspect reasons might be application in low temperatures or a contaminated surface/poor surface cleaning. But once these issues are ruled out, what else could be at play? More frequently, manufacturers of commercial equipment are turning to powder coat paint processes vs traditional wet coat, solvent based paints. This is because power coat paints offer not only a tough, durable, attractive finish; but also can be cheaper, safer, and more environmentally safe to apply. Powder coat paints can also present challenges for adhering the labels we may be required to apply to them however.
What makes powder coat different from conventional paint?
Powder coating is applied as a dried paint in a powder form. This is different than liquid paints that are dissolved in solvents that give off VOCs as they cure and dry and that can be dangerous to health and harmful to the environment. With powder coating, the paint which is composed of very fine plastic particles is applied with a spray gun; which is similar in design to that used for wet paint coatings. But the powder is attracted to the surface to be painted by induced, opposite electrostatic charges. The powder coating is then ‘cured’ or bonded to the painted surface by baking at temperatures reaching several hundred degrees. This produces a tough, chemically resistant, fused coating.
Why does powder coat make for poor adhesion of labels?
Where adhesion of labels comes into play, it’s important to remember that powder coat is a plastic material. And many plastics are low surface energy (LSE) materials that the typical PSAs used on labels don’t stick well to.
In addition, finish texture of powder coats can be problematic. While some powder coat paints can produce a very smooth and high gloss finish, others and particularly thin powder coats can produce a textured or ‘orange peel’ like finish. High gloss finishes can affect adhesion because they can be very slick, not providing much ‘tooth’ for adhesion. But textured surfaces can provide an even greater challenge because the typically thin adhesive coating on labels is not thick enough to flow out and fill the micro-textured valleys of the surface; only contacting the high ‘peaks’ and making less than full or maximized contact with the surface.
Aside from a powder coat finish and apart from properties such as low surface energy and texture, some surfaces have contours that affect adhesion. This is because some label materials are not flexible or stretchy enough to conform; especially to surfaces having compound contours such as that pictured above.
Options to overcome these challenges
Low surface energy (LSE)
Special adhesives are available that are engineered to stick to LSE surfaces. Unfortunately these adhesives are difficult, if not impossible to source on labels made from standard material offerings. And cost would become an issue for any label manufacturer sourcing a custom coated specialty adhesive label material due to low-volume sales usage and minimum order quantity of material stock. Fortunately, these LSE adhesives are available as laminating transfer adhesives in roll form and as a product not already coated to a label stock. But they must be applied to a label stock by the label manufacturer as a separate process; which adds manufacturing cost and may present compatibility problems with some label materials.
Types of laminating transfer adhesives similar in form as those mentioned for LSE applications are available in thicker coat weights than the typical adhesive coating thickness on standard label materials. These thicker coatings provide the ability for the adhesive to flow and fill the micro-textured valleys of the textured surface; thus maximizing contact area and therefore adhesion. Again, these adhesives add complexity and cost to the manufacturing process.
As noted, some common but high quality label materials such as polyester are not at all conformable. For contoured surfaces, there are readily available, high quality vinyl label materials that exhibit the ability to stretch and provide high conformability to these curves.
Some of Electromark’s standard label materials such as polyester or Duracryl® utilize an adhesive that is engineered to provide excellent adhesion on many LSE surfaces and powder coated paints, as long as they are smooth and not textured. But for more challenging powder coat finished surfaces, Electromark also has high quality conformable vinyl label materials, and specialty thicker laminating transfer adhesive options including LSE adhesives. The key is providing full details of the surface your label will be adhered to so that we may determine and advise the best option to meet the requirements of your application.