1 Feb 2015 Adhesion? It all depends
Last week someone asked if I could help with a production issue. Their product happened to need some super-light adhesion in one part of it and they couldn't think how to measure it with the apparatus in their QC department. For example, their peel tester was designed for loads much larger than the light level required for this product.
Happily I had an app ready. If you look at Weak and Strong on my Practical Adhesion site you will find the theory. To see the theory demonstrated with some kitchen scales you'll find the Peel v Lap video on my YouTube channel
In peel mode (90° is the assumption, but 180° is only a factor of 2 different) the only thing holding things together is the work of adhesion. For pure surface energy this is only 50mN/m and for the sticky note PSA used in the video it's probably 5N/m, amplified by the "dissipation" that takes place in the PSA.
In lap shear mode the work of adhesion is almost irrelevant! The reason is that the force needed to break the bond is amplified by the modulus and thickness of the sample. In the simple cases shown in the app, the peel force required to break the bond might be 1mN and the same material tested in lap mode might be 100 or 1000mN, because amplification via a modulus which might be 100s of MPa is very strong.
The video shows this in practice. The piece of stickynote stuck to a 137g metal weight managed to reduce the weight on my kitchen scales by ~40g. But in lap mode I could lift the 137g from the scales with no problem. When I have the chance to bring a 500g weight with me to an adhesive science course, I can show that the same stickynote can easily lift the 500g.
There is a slight element of cheating in the video. The theory applies to a lap joint in pure shear mode. But lap joints don't operate in shear mode, the system tends to bend to give peel failure, and in my simple experiment I cannot lift the weight before the bending becomes too severe. So out of sight of the camera I am very gently touching the weight so that it doesn't tilt and go into peel mode. Yes, I've checked that my touching does not alter the result by adding some extra adhesion! All I am doing is making sure that we see the result of pure shear versus pure peel.
Going back to the original question, it is clear that by shifting to some form of lap shear testing it should be possible to measure an "adhesion" value even with their light peel and large load cell. Quite how this will translate into product performance is a different question altogether. If the test gives numbers that reliably distinguish known good samples from known bad samples that will probably be good enough for now.
Yesterday I sent off (hooray!) the proofs and index to DesTech publishers for my book Adhesion Science: Principles and Practice. The key message running through the book is that "Adhesion is a property of the system". This simple example shows that a small change in the system, for exactly the same adhesive, can make a big difference to the measured adhesion. The book is linked to my Practical Adhesion apps so that every time you come across a major equation or complex idea in the book a click (for those using the eBook version) will take you to see how it all works out. The Practical Adhesion site is quite powerful. The book on its own is quite powerful. But my view is that the combination of the two, a book/app hybrid, is a much more potent way to learn and the way that all technical books should be in the future.