Physics Of Adhesive Tape

Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106

2017-11-06 07:46:18



Every one of us might have come across with the use of adhesive tapes in our day to day life but without bothering to know about the science behind this adhesion. It had long been theorized that it should be possible to create sheets of carbon atoms only one atom thick, and that they would have some pretty amazing properties. Since then, scientists had been trying to isolate these sheets, known as graphene. In 2010, with the use of these adhesive tapes and lead of pencils, scientists were able to isolate graphene for the first time, a super-material that could be used to make ultra strong and light materials, revolutionize electronics and build better solar cells.

Bothering to understand how adhesives work, and why they fail, we're not alone and this question has taxed some of the world's best minds since ancient times. Even after all these years, scientists still don't fully understand how gluey substances make one thing stick to another. According to historians and archeologists, adhesives have been used for thousands of years-probably since Stone Age cave dwellers first applied bitumen (a tarry substance used to surface highways) to stick flint axe heads to the tops of their wooden hunting spears. In ancient times, people made their glues from whatever they found in the world around them-such things as sugar, fish skins, and animal products boiled in water. We still use some of these natural adhesives today, though we're much more likely to use artificial adhesives made in a chemical plant. It's obvious modern glues are chemical products from the horrible names they have—polyvinyl acetate (PVA), phenol formaldehyde (PH), ethylene vinyl acetate (EVA), and cyanoacrylate ("super glue") to name just four. Many modern adhesives are called synthetic resins for no good reason other than that resin (a gooey substance found in pine trees and other plants) was one of the first widely used adhesives.

Forces make things stick

People stick to Earth's surface even though the planet is rotating at high speed, and even there's no glue on the soles of our feet. The reason is simply that gravity bonds us to the planet with enough force to stop us whizzing off into space. But gravity isn't enough to keep us permanently in place. If we supply bigger forces, for example by using our muscles to move our legs and jump in the air, we can "unstick" ourselves and go somewhere else. Gravity tries to pull the water down to the bottom of the glass, and sooner or later it usually wins, but two interesting things try to stop it. First, water molecules (two atoms of hydrogen and one atom of oxygen joined together) naturally stick to one another, so they clump together in big droplets on the window. The forces that make them do this are called cohesive forces (and the process involved is called cohesion). Second, the water droplets also stick to the glass without any help or glue. Different forces are at work here known as adhesive forces (the sticking process is called adhesion). Now the cohesive forces must be bigger than the adhesive forces or the water wouldn't form droplets at all. Instead, it would just spread out in a very thin layer on the glass—much as oil does when spread on water. But the adhesive forces are still pretty strong: some of the water droplets that stick to window are surprisingly big.

Adhesion of tapes

Here, for the sake of curious readers, efforts are made to highlight the physics of adhesive tape so that they can understand how these tapes work and can be made to work more effectively. The simplest answer that I can give to the question is that pressure-sensitive adhesives (which are polymers) are 'tacky' or 'sticky' because they are essentially very high viscosity liquids that also have some elastic characteristics--in technical terms; they are 'viscoelastic. This property means that they exhibit some of the characteristics of liquids, and so they will 'wet' a surface to which they are pressed. But then, because of their elasticity, they will resist separation when stressed. Thus, 'stickiness' is strictly a physical (viscoelastic) phenomenon, not a chemical one. There are two fundamentally different components of tape's sticky nature; adhesion and cohesion. Adhesion is the binding force between two different materials, whereas cohesion is the binding force between two similar materials. When two materials are brought into contact with each other, the surface molecules interact, giving rise to attractive forces that may be physical, chemical or electrostatic (corresponding to adsorption, covalent bonding or van der Waals forces, respectively). When the molecules are similar, as in the case of two 'glue molecules,' the cohesive force causes the glue to stick to itself. When the molecules are dissimilar, as in the case of a glue molecule and a molecule of the substrate (the surface the glue is sticking to), the adhesive force holds the glue to the substrate. Hence, the 'stickiness' of tape is caused by a combination of the molecular forces of the glue material sticking to itself as well as holding onto the substrate.

There is no chemical bonding or reaction between a pressure-sensitive adhesive and the substrate, the surface to which it bonds. A pressure-sensitive adhesive is a sticky, viscous, liquid like material that adheres to a surface using only pressure. To function well, it should have good adhesion to a surface and good cohesion, or internal strength. For good adhesion, it is important that the adhesive can readily flow out on the surface. The degree, or freeness, of the flow often determines the intensity of adhesion. Degree of flow depends on the difference between the surface energy of the material and the surface energy of the adhesive. Pressure-sensitive adhesives tend to flow out on materials having a high surface energy--for instance, metals, glass and plastics such as acrylic, polycarbonate and nylon. Pressure-sensitive adhesives tend not to flow out as easily on low surface-energy materials such as polyethylene, polypropylene and Teflon. The starting point to understand adhesion is that all materials stick together because of inter-atomic forces. 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 we put pen down on our 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 negligible. However if we put a liquid between the pen and the desk then the liquid can flow into the irregularities in the two surfaces so now have essentially perfect contact between the pen and the liquid and the liquid and the desk. Now when we try to lift the pen we are pulling the liquid apart and this will require some force. For a thin liquid like water the force is small, but put our pen in a pool of treacle or tar and suddenly we’ll find the pen sticks to the desk quite strongly.

For each different glue and each different surface we use it on, scientists think a combination of different factors are at work holding the two together. But the plain truth is: no-one exactly what's going on in every case. One of the main factors is called adsorption. When we spread adhesive, it wets the surface we apply it to. Lots of very weak electrostatic forces between the glue molecules and the molecules in the surface (called van der Waals forces) hold the two things together. For adhesives to work well like this, they have to spread thinly and wet the surfaces very well. There's no actual chemical bond between the glue and the surface it's sticking to, just a huge number of tiny attractive forces. The glue molecules stick to the surface molecules like millions of microscopic magnets. In some cases, adhesives can make much stronger chemical bonds with the materials they touch. For example, if we use certain glues on certain plastics, the glue and the plastic actually merge together to form a very strong chemical bond-they effectively form a new chemical compound at the join and the process is called chemisorption. Absorption and chemisorption are chemical connections between the glue and the surface. Glues can also form physical (mechanical) bonds with the surface they're sticking to. Suppose the surface is porous (full of holes). The glue can seep into those holes and grip through them, like a climber's fingers grabbing holes in a rock face. That's called the mechanical theory of adhesives. Another theory of how glues work suggests the adhesive can diffuse into the surface and vice-versa, with molecules swapping over at the join and mingling together and is called the diffusion theory.

Adhesives are designed to work when they leave the tube—and not before. Different adhesives achieve this in different ways. Some are dissolved in chemicals called solvents that keep them stable and non-sticky in the tube. When we squeeze them out, the solvents quickly evaporate in the air or get absorbed by the surfaces we're sticking to, freeing the adhesives themselves to do their job. Plastic modeling glue works like this. It contains molecules of polystyrene in an acetone solvent. When we squeeze the tube, the glue spurts out and we can usually smell the very strong acetone as it evaporates. Once it's gone, the polystyrene molecules lock together to make strong chemical bonds. Glue doesn't smell when it's dry because all the solvent has vanished into the air. Some glues (such as synthetic, epoxy resins) have to be mixed together before they work. They come in two different tubes, one containing the synthetic resin and the other containing a chemical that makes the resin harden. The two chemicals are useless by themselves but, mixed together, form a tough, permanent adhesive.