Why do some glues fall off easily, while others are as good as welding?

Why can glue be used to stick things together? Of course, it is because of "stickiness", but why does it stick?

Although they are all glue, different glues have very different viscosity. Some glues fall off as soon as they are torn, such as the transparent tape we often use, while the viscosity of some glues is comparable to welding. Once they are stuck, they cannot be removed no matter what, such as 502, welding agent, and strong tape used for plugging leaks. In fact, the reason why "glue" can be used to stick things is essentially because of a certain force at work, and the reason why different glues have different viscosities is because the force that causes the viscosity is not the same. To make this problem simple and easy to understand, we still need to start with the nature of matter. From a macroscopic perspective, different substances have completely different structures, but from a microscopic perspective, all substances are composed of atoms.

An atom is a very small microscopic structure, which mainly consists of two parts, namely the nucleus and the electrons outside the nucleus.

Electrons are negatively charged, while atomic nuclei are positively charged. When the two are combined, atoms show electrical neutrality. A material structure larger than an atom is a molecule, which is formed by the combination of atoms. So what force is used to combine two different atoms to form a molecule? It is a "chemical bond". The so-called chemical bond, in simple terms, is a kind of binding force generated by two atoms by exchanging electrons with each other. Whether it is an atom or a molecule, in order to maintain stability, one factor is crucial, and that is electrostatic balance. That is to say, the positively charged atomic nucleus and the negatively charged electrons in a molecule should be able to achieve a perfect balance, so that an uncharged atom or molecule can be obtained.

Although molecules are generally electrically neutral, this is not always the case.

The electrons in a molecule are not static, they are always in random motion, and this motion results in more electrons in one part of the molecule and fewer electrons in another part. At this time, the molecule becomes like a magnet with distinct magnetic properties, one part is positively charged and the other part is negatively charged. Therefore, the positively charged part of one molecule will attract the negatively charged part of another molecule, and the two will be adsorbed together, or bonded together. In 1857, physicist Van der Waals was the first to discover this intermolecular interaction force, so it is also called "Van der Waals force".

Although van der Waals forces can combine two molecules together, they have harsh conditions, that is, the two substances must be close enough to fit completely together.

How can we achieve this? Even if we put two seemingly extremely flat objects together and press them hard, they are still far apart at the microscopic level, and to make them fit together completely, we need a medium, that is "glue". Even if the object looks smooth on the surface, it is uneven at the microscopic level, and "glue" is essentially a soft viscoelastic fluid substance. When it comes into contact with an object, it will quickly flow into various holes and cracks on the surface of the object to fill them up. At this time, the "glue" and the surface of the object can be said to be completely fitted, so the van der Waals force begins to play a role, and the molecules are tightly adsorbed together. This is the internal principle of why glue can stick to things.

The glue mentioned here is just ordinary glue, such as transparent tape. Although this kind of glue can stick to things, we all know that it will fall off once it is torn. Why?

Because the van der Waals force is a weak interaction force, this force itself is not very strong. So why are some glues very viscous, comparable to welding? In these super glues, it is not the van der Waals force that plays a role, but another more powerful force, hydrogen bond. The so-called hydrogen bond is the force that attracts and binds the hydrogen atom in one molecule to the oxygen atom in another molecule. Although different atoms in a molecule share electrons, some atoms always like to eat more and occupy more, such as oxygen atoms, which will pull all the electrons to their side, while hydrogen atoms can easily lose electrons.

Oxygen atoms like to monopolize electrons, while hydrogen atoms easily lose electrons, so oxygen atoms carry negative charges and hydrogen atoms carry positive charges.

So when two molecules meet, the positively charged hydrogen atoms in one molecule will adsorb to the negatively charged oxygen atoms in the other molecule. This force is extremely strong, and this is the hydrogen bond. On the surface, hydrogen bonds seem to be similar to van der Waals forces, but in essence, hydrogen bonds are strong interaction forces, so the force is much greater than van der Waals forces. The main components of those highly sticky glues are polymers containing hydrogen bonds. When they are attached to the surface of an object and solidified, the two objects can be tightly bonded together and are difficult to separate. Different glues cure at different speeds, so some glues are good to stick to, while some glues require a longer wait.

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