Many have found themselves frustrated while in the middle of an important project, asking: Why isn’t my tape bonding well to this Low Surface Energy (LSE) plastic?
This post explores surface energy and how it relates to adhesive bonding. It also covers the surface properties of various materials and what adhesive characteristics to consider when confronting the challenge of bonding to Low Energy Surface plastic.
Surface energy, simply stated, is the excess energy that exists at the surface of a solid. It exists because the molecules at the surface cannot interact with as many similar molecules as those in the bulk of a solid are able to do. This means they possess extra interaction energy. You can also think of surface energy as the sum of all the intermolecular forces that are acting on the surface of a material, or the degree of attraction or repulsion a material surface exerts on another material. Most people are familiar with the repulsive or attractive force between two magnets, which is a comparable concept.
In basic terms, low energy surfaces are slippery with liquids falling off them with relative ease, and high energy surfaces usually allow liquids to adhere to them well. . Surface energy is considered low when a liquid cannot simply rest on a surface or when a liquid cannot wet a surface. For example, when water is applied to a newly waxed car, the water beads up. The surface of the wax has a low energy, so the water rolls off the paint surface and cannot properly slip across and wet the car. However, when water is applied to the car before waxing, the water spreads out and flows over the surface, meaning it is a high energy surface. The water has a better affinity to the unwaxed surface.
A material’s surface energy is critical to forming a good adhesive bond. Most pressure- sensitive adhesives don’t bond well to low energy surfaces.
Polyethylene and Polypropylene are good examples of low surface energy (LSE) materials. Their surface tension is measured in dynes/cm. A dyne is the amount of force required to produce an acceleration of 1 cm/sec² on a mass of 1g. This method of measuring surface tension can help in selecting the right adhesive. Easy-to-use kits are available to measure a material’s dyne level and therefore its surface energy.
Surface tension is measured in dynes/cm. A dyne is the amount of force required to produce an acceleration of 1 cm/sec² on a mass of 1g. In simpler terms, it’s a way to measure surface tension which can help in selecting the right adhesive. Easy to use kits are available to measure a materials dyne level/surface energy.
Here’s a list of some common materials and their typical dyne levels:
- Silicone – 24 dynes
- Polyethylene – 31 dynes
- Polypropylene – 31 dynes
- Polystyrene – 33 dynes
- Nylon – 43 dynes
- Polyester – 43 dynes
- Aluminum – 500 dynes
The higher the dyne level, the higher the surface energy and the easier it is to get a good adhesive bond. As a general rule, materials with a dyne level of less than 50 are considered to have a low surface energy.
The key to bonding to low energy surfaces is to raise the surface tension of the substrate. Raising the surface tension allows the adhesive to flow or “wet out,” thereby improving the adhesive bond.
Surface tension can be increased by using various surface treatments or primers. Typical treatments you’ll encounter include flame treatment, corona treatment or a primer coating.
The right adhesive properties also help obtain a strong bond. Typically a soft, high tack adhesive with quick stick properties will “wet out” faster and more easily, increasing the surface contact and aiding bond strength. Applying heat at the point of adhesive contact can also be helpful. The heat will soften the adhesive, which improves the wet out and thus strengthens the adhesive bond. Heat is especially useful when working with firmer adhesive systems that do not wet out well.
Applying heat at the point of adhesive contact can also be beneficial. The heat will soften the adhesive which improves the wet out thus improving the adhesive bond. Heat is especially helpful when working with firmer adhesive systems that don’t wet out well.
For more information on bonding to low energy surfaces, visit the links below: