In a pressure-sensitive adhesive, three different forces are at work: tack, adhesion and cohesion. Here’s a brief definition of each and an explanation of how the different combinations between the three give rise to different kind of adhesives.
To create a high-quality pressure-sensitive adhesive, manufacturers take into considerations 3 different parameters, which relate to 3 different forces:
How do they manifest? And why are they so important?
1 – TACK
Tack relates to the speed at which an adhesive bond is created. In this sense, tack describes the behavior of the adhesive when it comes into contact with the surface.
Example: if low pressure and short contact produce a strong bond, we call it “high tack”.
2 – ADHESION
Adhesion relates to the bonding power of the adhesive on a surface. In this sense, adhesion describes the physical, macroscopically observable attraction of two bodies.
Example: if a glue sticks very well to a surface, we call it “high adhesion”.
3 – COHESION
Cohesion relates to the inner strength of the adhesive itself. In this sense, cohesion describes the degree to which the adhesive holds together by itself.
Example: if a glue is very resistant to tearing, we call it “high cohesion”.
As we told before, a pressure-sensitive adhesive in which tack is the dominant force is very different from another pressure-sensitive adhesive in which cohesion is the prevalent force. Manufacturers well know that, and use this notion in order to create different kinds of adhesive, suitable for different kind of applications.
To produce an adhesive suitable for permanent applications, you need to obtain a bond characterized by durability. So, adhesion and cohesion are predominant and generally balanced, while the speed at which the adhesive sticks to the surface is not so relevant.
To produce an adhesive which can be used for short-term applications, you need to get a perfect balance between tack and cohesion. Adhesion has a secondary role, because it is not important that the adhesive bond is extremely strong but, rather, that it develops quickly and leaves no residues.
As we say “quick”, it is immediately clear that the first force to take into account is the tack: quick applications require adhesives that stick instantly, but also firmly. So, the second force you need to consider is adhesion. Only in the end you consider cohesion, and only if the adhesive must support huge loads.
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