Surface Energy: The Basis of Bonding

When it comes to pressure-sensitive adhesives, there are two driving principles behind adhesion: surface energy and rheology. In this post, we will discuss surface energy, how it influences adhesion and how low-surface energy adhesives work.

Surface energy at-a-glance

Surface energy is the amount of excess energy present at the surface of a solid material. To visualize this, picture a cube. At the center of the cube, each individual molecule is surrounded by other molecules in the x, y and z planes. These molecules interact with one another and are balanced in their interactions. These bonds and interactions are called cohesive strength. But at the surface, the molecules aren’t completely surrounded and are only interacted on by the molecules adjacent to and below them. This causes the molecules’ interactions to be imbalanced, which results in unrealized energy known as surface energy. 

The magnitude of these intermolecular forces/interactions is proportional to surface energy. Polar and ionic materials, where electrons are not shared evenly, have higher molecular forces and thus higher surface energy. Nonpolar materials, where electrons are shared evenly, have lower molecular forces and low-surface energy (LSE). Below is a table of common materials and their surface energies: 

 

Surface tension at-a-glance

Surface tension functions like surface energy but for liquids. Surface tension is the tendency of fluid surfaces to shrink into the minimum surface area possible, such as when a bead of water forms on a freshly waxed car. Conversely, surface tension can also be viewed as the energy required to increase the surface area of a liquid, which is known as wet out.

How does this apply to pressure-sensitive adhesives and bonding? Surface energy as it relates to adhesion is the force of molecular attraction between two materials. For an adhesive to form a bond, it must be able to wet out the surface it will adhere to. This is especially true for pressure-sensitive adhesives, which are part solid (visco-elastic) and rely on pressure to flow and wet out the surface. If there isn’t enough surface energy to break the surface tension, then the adhesive will not properly flow and wet out the surface. This is not an issue on high surface energy materials, such as metals and glass. But it could become an issue on lower surface energy materials, such as polypropylene and polyethylene. This can be seen in the following diagram:

                                           

 

 

How do LSE adhesives work?
LSE adhesives are usually acrylic adhesives that have been modified to lower their surface tension in order to adhere to low-surface energy materials. When it comes to adhesive bonding, it’s important that the material you are bonding to have greater surface energy than the adhesive you are using. Looking at the chart above, you can see that rubber adhesives have an energy of 28 dynes/cm, which is below most of the common materials used in applications. A typical straight acrylic, however, has an energy of 39 dynes/cm and is much higher than plastics such as polypropylene and polyethylene. This is why straight acrylics do not adhere well to these plastics and must be modified to lower their surface energy to be able to wet out and adhere to these materials. Release liner coatings are designed to have very low surface energies (20 dynes/cm), which is why they are capable of releasing from acrylic and rubber adhesives.

 

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