There are several factors that are important in the construction and effectiveness of adhesive tapes, some of the most important ones are adhesion, cohesion, and tack. So what is Adhesion, cohesion and tack? And why are they important?
Adhesion
Adhesion is when a bond is made between two dissimilar materials i.e., bonding of adhesive tape to substrates. The process of adhesion is caused by molecular interactions between the substrate surface and the adhesive. It takes place between weak intermolecular interactions and strong chemical bonds.
Cohesion
Cohesion is the bonding of particles within the adhesive that holds the adhesive mass together. Cohesive force is due to:
- The chemical bonds within the adhesive polymers.
- The chemical bonds resulting from crosslinking of the polymer.
- The intermolecular interactions between the molecules in the adhesive.
- The mechanical adhesion between various molecules in the adhesive.
Tack
Tack is known as the initial stickiness of the adhesive. It indicates how quickly a bond is formed when the adhesive is brought into contact with a surface with very slight pressure. Tack is sensitive to a variety of factors, including temperature, processing, and the properties of the adhered surface etc.
Shear
Shear adhesion gives the holding time of the adhesive. Shear adhesion is the ability of a tape to resist the static forces applied in the same plane as the backing.
Relation between tack, Adhesion & cohesion!
The adhesive & cohesive forces work in tandem and are the reasons for the sticking of tape to the substrate (stickiness). These 2 forces ensure:
- How two things (carrier & substrate) can be firmly connected to one another.
- How quickly, firmly, and long it takes for things to adhere to each other.
Other than cohesion and adhesion, tack also plays an important role in the system:
- Tack and cohesion give short and reversible bonding, so for temporary application, these play a major role.
- Cohesion and adhesion provide permanent bonding.
- Adhesion and tack provide quick stick bonds.
Good adhesion and strong bonding are due to cohesion of the adhesive. The cohesion forces of an adhesive are based on molecular interactions. Adhesives are polymers, that having high molecular chain length, which are entangled together. The tangled-up molecules make the inner cohesive bond in adhesives. Inner cohesive strength is directly related to shear.
Why do different bonds fail?
Generally you can have 2 types of failures: Adhesive Failure and Cohesive Failures:
Adhesive failure: Adhesive failure, also known as delamination, is the most common form of bond failure. The adhesive cleanly pulls away from one substrate and stays attached to the other.
Cohesive failure: Cohesive failure can be defined as the physical separation of the adhesive material. Rather than delaminating from the applied surface, the adhesive “splits,” leaving adhesive residue on both the application surface and the backing substrate.
Why does the failure occur?
- Contaminants from poor cleaning or substrate surface preparation processes prior to coating
- Inappropriate use of adhesion promotors.
- Surface separation of adhesive due to the improper mixing of chemicals within the adhesive that makes it more cohesive.
- Incorrect way of application
Sometimes the reason for PSA failure is simple: the adhesive that was chosen is incapable of meeting the necessary functional requirements. This is preventable by choosing the right PSA tape.
So before suggesting the tape be sure you are aware of these.
- Where and how will the tape be used?
- What kinds of stress will the PSA tape be subject to?
- What kinds of environmental factors will the adhesive be subject to?
- How long do you need the adhesive to last?
- What substrate(s) do you need the adhesive to stick to?
