Understanding Surface Energy

Surface energy…what is it? When talking about UL or other compliance labels, you’ll hear people sometimes say things like, “The product material has a low surface area.” or “This adhesive only works on high surface energy applications.” What are they talking about and how important is it?

First off what is surface area? It relates to the intermolecular bonds that occur across the surface of a solid. Is your brain is screaming: English please? Mine too. Let’s think of it this way–within the context of label, surface energy measures how well a given application surface bonds with adhesives. The higher the surface energy of a material, the easier things stick (i.e. stainless steel); the lower, the harder it is (i.e. Teflon).

As a general rule, adhesive works by forming tight bonds with the application surface. The more it can spread out a bond with the surface, the better. As an example, consider a freshly waxed car. When it rains, water beads up into small droplets. In a rough approximation, this is the equivalent of adhesive on a low surface energy material–the adhesive sticks in certain areas but can’t form a complete bond. Now as the wax is removed over time, water starts to flatten out and pool up into larger clumps. This is more like adhesive on a high surface energy material–the adhesive can get a nice, full, complete bond.

Why do you care about this? Well, when it comes to UL labels, you want your labels to stay where you stuck them. Depending on where your label is going (and the surface energy of the item), you may need different formulations to achieve the desired performance results you are looking for

To give you a feel for relative surface energy, here is a breakdown of some common materials. Bigger numbers have higher surface energy. Smaller numbers have lower.

Application Surface
Copper
Stainless Steel
Aluminum
Zinc
Tin
Glass
Lead
Phenolic
Nylon
Polyester
Epoxy Paint
Polyurethane Paint
ABS
Polycarbonate
Rigid PVC
Acrylic
PVA
Polystyrene
EVA
Polyethylene
Polypropylene
Polyvinyl
PTFE Flouropolymer (Teflon)
mJ/m²
1103
1100-700
840
753
526
500-250
458
47
45
43
43
43
42
42
39
38
37
36
33
31
29
28
18

 

Photo courtesy of Geert Orye

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