forces

So I was bored and had some clear Scotch tape nearby. Decided to mess around, and now I have questions.

• What is the physics behind the force of tape adhesive?

• Why does the tape hold so much better if it is pulled parallel to the surface
plane it is on? (Excuse my poor drawing skills)

• Is there a rating for adhesion? Can I found out how "sticky" the tape is by measuring the
area of tape on a table and the force at which it breaks loose from
the table in a similar situation below?

Is this even an appropriate question? Or should I close it? It's definitely not homework.

The starting point to understand adhesion is that all materials stick together because of interatomic forces. As a thought experiment take two pieces of iron with perfectly smooth surfaces, i.e. smooth down to the atomic scale, and press them together. The join disappears and you have a single continuous piece of iron, so the adhesive force is basically the fracture strength of iron.

But in everyday life we don't see everything stick to everything else, and the reason is that real surfaces are rough on the atomic scale. So when you put you pen down on your desk the real area of contact of the pen with the desk is tiny. So although the pen adheres to the desk where it touches, it touches over such a small area that the adhesive force is negligable.

However if you put a liquid between the pen and the desk then the liquid can flow into the irregularities in the two surfaces so now you have essentially perfect contact between the pen and the liquid and the liquid and the desk. Now when you try to lift the pen you are pulling the liquid apart and this will require some force. For a thin liquid like water the force is small, but put your pen in a pool of treacle or tar and suddenly you'll find the pen sticks to the desk quite strongly.

The reason that this is relevant to Scotch tape is that Scotch tape is coated with a material called polyisobutylene and it is this that causes the adhesion just like the pool of treacle sticks your pen to the desk. Like most polymers the properties of polyisobutylene are very variable and can be changed by changing things like the degree of cross-linking. At high levels of crossing-linking and high molecular weight you get a solid rubber while at low levels of cross-linking and low molecular weight you get a thin liquid. In between you get a material which is strong but has some ability to flow, and the combination of these properties makes it extremely tacky because it can wet both surfaces but be strong enough to resist being pulled apart.

Hopefully the above has answered your first question, and it also gives you the answer to your second question. If you pull the tape sideways you are trying to break apart all the polyisobutylene on the contact area and this takes a lot of energy. When you peel the tape you're only pulling apart the polyisobutylene at a thin line where the tape is leaving the substrate so this takes much less energy.

Re your third question, I don't know whether there are industry standards for the adhesion of butyl tape, though I assume there must be. I suspect the experiment would be hard as you'd get rather variable results unless you control the experimental conditions very closely. In principle the adhesion will be related to the visco-elastic properties of the polyisobutylene, though as far as I know no-one has come with a quantitative description of the relationship.