What Does Tack Really Mean And Is It Important To Me?
“I want your tackiest adhesive”; “this isn’t very tacky”; “xyz is a very high tack adhesive tape”.
We use the words “tack” and “tacky” all the time, but what do they really mean?
Pressure-sensitive adhesives defined as permanently tacky adhesives that form a bond using just(relatively light) pressure. They don’t require heat, they don’t need a drying time or setting time, they don’t need activating by water or solvent. Tack describes how easily such an adhesive will form a bond. So a high tack adhesive will form a bond under very light pressure.A low tack adhesive will require quite firm pressure to form a bond. Tack is not about the strength of the bond that is formed. Adhesion and cohesion measure bond strength,tack does not. It is only about how easily the bond is formed.
Is It Important To Me?
Tack is important in a number of situations:
⦁ When it is difficult to apply firm pressure by hand because the surfaces are large - such as bonding mirrors to furniture. The rigid mirror spreads the load, so the pressure is reduced.
⦁ When the pressure can only be applied from one side and the material on the other side is fragile or will deform - such as applying glazing bars to windows on-site.
⦁ When firm pressure cannot be guaranteed because the assembly work is contracted out, or performed in unsupervised conditions
⦁ When application conditions may be cool. All adhesives lose tack as the temperature drops, and will require firmer pressure than in warm conditions. But take care and read the data sheet carefully or take advice - some adhesives are very tacky at room temperature but lose tack quickly at lower temperatures.
⦁ When the material being bonded is slightly rough. A high tack adhesive will wet out better on rougher materials. Low tack adhesives generally require smoother surfaces. But note - coatweight is also important in bonding to rougher materials.
How Is It Measured?
This is the difficult one, because there is no really good, reliable measurement of tack. Several methods are in use:
Rolling Ball Tack measures how quickly a tape will stop a steel ball, rolled down a specified incline onto a horizontal sample of tape. So it is an inverse measure - a low result means high tack. Unfortunately, it doesn't measure tack at all. A coating of pure rubber that will not bond to anything except itself, will stop a rolling ball very quickly. In my early days in the industry, we used rolling ball tack on our packaging tapes. A value below 60mm indicated that the tape would perform properly on automatic carton sealing machines. Rolling Ball Tack certainly measures something - possible related to the modulus of elasticity. But it doesn't measure tack.
Probe Tack (Polyken Probe Tack) uses a special machine that was designed to quantify "thumb tack". A probe is brought into contact with the tape sample, pulled away, and the force measured. Speed, force and time in contact can all be varied. The method is not widely used because it requires special equipment and it has not been incorporated in standard test methods. We have used a similar approach for research purposes, by developing special software with a tensile tester. The number of variables makes it difficult to define a test regieme. Disconcertingly, we could prove that HSA (a very hgh shear but low tack adhesive) was our highest tack adhesive!
Loop Tack is probably the most widely used method. This uses a tensile tester to bring a loop of tape, adhesive side out, down on to a horizontal steel test panel and then peeling if off. This is essentially an adhesion test, but performed using low application pressure and a very short dwell time. One problem with this test is that while the application pressure is low,it depends on the tape construction. So the same adhesive coated on tissue, a this film, a thicker more rigid film and a foam, will give different results. Another problem is that the short dwell time an low pressure introduce variability. This test may be the closest to measuring tack, but arguably, giving a loop tack figure offers false precision.
Thumb Tack is purely subjective and of course, is subject to interpretation. Nevertheless, we find that a panel of people will rank several adhesives quite consistently. So with one slight modification - finger rather than thumb! - and with good attention to cleanliness, this is the method we most commonly use. Not perfect, but better than the other methods.
For more information, please do contact us on our main line 01628 642800, or drop us an email to sales@technibond.co.uk with your requirements where we will be happy to discuss