 |
What makes a tape stick ?
Why does one adhesive results in a good bond, whilst the other adhesive hardly sticks to the surface? And why is the surface to which the tape is to be applied that important?
Good contact
There is no chemical bonding or reaction between a pressure-sensitive adhesive and the substrate. A good bond requires a good contact between the adhesive and the surface to be bonded – this is known as wetting. Wettability depends on one specific property of the surface: surface energy, often referred to as surface tension.
Energy as a key
Every surface has its surface energy. Steel, for example, has a high surface energy, while plastics have a lower energy. Good surface wetting occurs when the surface energy of the substrate is higher than the surface energy of the adhesive being applied. On a high surface energy material the adhesive can flow or “wet out” more easily to assure a stronger bond.
 
Why wax your car?
A good example is a car that has been waxed. The wax creates a low surface energy. When rain falls on the waxed car, then the rain droplets will bead up (form near-spherical droplets). The water droplets that land on an unwaxed car behave differently. They tend not to form droplets, but will spread out on the surface. This is high surface energy allowing the water to flow.
| |
Upgrade your energy level
In summary: the higher the surface energy of the substrate, the easier it is to make things stick to it. If the surface energy of the substrate is low, it is difficult to find an adhesive with a lower surface energy and sticking is difficult. LSE (low surface energy) materials, such as polyethylene (PE) and polypropylene (PP), are becoming more popular especially in the automotive industry and represent a real challenge for PSA suppliers.
Opposites attract - Theory
Adhesion is the molecular force of attraction between two different materials (like adhesive and substrate), whereas cohesion is the binding force between two similar materials. When an adhesive is brought into contact with a substrate, we observe the following phenomena.
1. The adhesive molecules at the surface experience an attractive force which tends to pull them into the adhesive liquid.
2. The adhesive molecules at the surface are being attracted by the substrate
The adhesive forces between the adhesive and the substrate will compete with the cohesive forces of the adhesive liquid. The “inward” molecular attraction forces, which must be overcome to increase the surface area, are called the "surface tension".
Complete wetting occurs when molecular attraction between the liquid and solid molecules (adhesive forces) is greater than that between similar liquid molecules (cohesive forces). Liquids with strong cohesive bonds and weaker adhesive forces will tend to bead-up or form a droplet when in contact with the substrate.
| |