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The interplay of tack, cohesion, and adhesion makes pressure-sensitive adhesives what they are. We will tell you why that is so here.
It should actually work: that is, hanging up a picture with honey, since honey does not just stick to your fingers. It also sticks napkins to your finger when you want to remove the honey. It sticks! But it doesn’t bond, our experts say. That is because honey lacks a critical force that the pressure-sensitive adhesive on our adhesive tapes has.
Three forces are at work in a pressure-sensitive adhesive, so that it can really bond as part of our adhesive tape: tack, cohesion and adhesion. The tack is what happens when the adhesive first comes into contact with the surface to be glued. Tack describes how quickly an adhesive bond must be produced.
There is a high tack, for example when a very solid bond is produced with minimal pressure and extremely short contact. When paper is produced, for example, such a high tack is needed: as a machine draws in 1.9 kilometres of paper per minute, the pressure-sensitive adhesive on the adhesive tape must bond extremely quickly if it should bond the next paper roll to the end of the finished roll in a running operation. Honey also has a readily apparent tack: it glues as soon as you touch it with your finger.
The second force that acts in a pressure-sensitive adhesive is adhesion. As the translation from Latin reveals, it involves attachment. The exact definition of adhesion is: the physical attraction or joining of two substances, especially the macroscopically observable attraction of dissimilar substances. With adhesive tape, the adhesion is the bond between the adhesive and the surface. Adhesive that has high adhesion bonds especially well to substrates. In the case of honey, it is quite high, since honey sticks fairly well.
The third and last force that a pressure-sensitive adhesive must have is cohesion. Cohesion comes from Latin and means “joined.” It is the inner bond of an adhesive, the degree to which it holds strongly together by itself. High cohesion means that the adhesion is especially strong, tough and stable in itself. Then it is very resistant to tearing.
That is important, for example, if adhesive tape should hold a large amount of weight stably, since the adhesive may not rip apart. The molecules must bond strongly and “hold” firmly to each other. That is definitely not the case with honey, since honey is very liquid and rips especially easily. The cohesion of honey is very low.
So we have found the answer: honey is not an adhesive according to our definition since it lacks the inner bond, the cohesion. That means: your napkin and hands will also remain sticky in the future. But you can forget about hanging up a picture with honey.
1. Temporary applications
Tack and cohesion are tailored to the application. Adhesion is of secondary importance. Many applications require a balance between tack and cohesion. Tack guarantees instant adhesion to the substrate, while cohesion is important for removal without residue. For many temporary applications, this is more important than a high specific adhesion that would manifest itself in a high adhesion level (peel resistance).
2. Permanent applications
Adhesion and cohesion are tailored to the application. Tack is of secondary importance. Adhesives in this area are primarily characterised by a balance between cohesion and adhesion tailored to the application. What is important is the durability of the bond. Tack, the bonding power at the first moment of contact, is of secondary importance.
3. Quick stick applications
Tack and adhesion are tailored to the application. Cohesion is of secondary importance. What is important for adhesives in this area is that they stick instantly and firmly. This is why tack and/or adhesion is required. As heavy loads are not usually to be supported and there is no unconditional need for removal without residue, cohesion often plays a secondary role.
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