Why Do Adhesives Prefer Polymers?

The scientific name of glue in chemistry is adhesive (adhesive). Adhesive is a sticky substance that uses its sticky properties to connect two separated materials together. There are many types of adhesives, and this category is much larger than the category of glue.

In general, there are two types of adhesives, one that needs to be cured and the other that does not need to be cured, which is the so-called self-adhesive and pressure-sensitive adhesive. Here, curing refers to a process in bonding. It should be noted that many curing processes are not a transition from liquid to solid.

So, how do adhesives bond objects together?

1. First of all, you need a smooth surface. 

A long time ago, someone discovered that two objects on a smooth surface would stick together, such as two ultra-flat glass plates. This is the intermolecular force at work. A smooth surface can increase the contact area between the two objects. The smoothness we see with our naked eyes is still uneven under microscopic conditions, so the actual contact surface between the two objects is very small, but if there is a layer of water on the surface, such as a very thin layer of water on the table, then you will find that the teacup seems to be sucked. This is because the water can be distributed in those grooves that are invisible to the naked eye. The contact area between the teacup and the table is larger, and at the same time, it is the atmospheric pressure that is at work. How to eliminate the influence of atmospheric pressure? Take two pieces of hard PET plastic sheets with a certain elasticity, stick them with water, and separate them horizontally from top to bottom. The atmospheric pressure of the inner and outer layers is the same. At this time, you can see that the two pieces of plastic are separated in a Y-shaped separation instead of a V-shaped separation. The stress that causes the plastic sheet to deform comes from the intermolecular force, and the water at this time is the adhesive. Therefore, this article says that the adhesive force of the adhesive comes from the intermolecular force.

2. Water is obviously not enough as an adhesive. 

The direct problem is that water will evaporate and disappear after drying. Moreover, the force within the water molecules is too weak and it is easy to tear apart. So, would it be better if the water is frozen into ice? Yes, after freezing the water between two objects into ice, people found that the two objects can be firmly stuck together, such as the scene that often appears in the freezer of the refrigerator at home, and when people lick the iron pipe at minus 30 degrees with their tongues, the saliva on the tongue solidifies instantly, and the tongue sticks to the iron pipe. At this time, the solidified water is a very good adhesive. From the perspective of classification, this is still a hot melt adhesive, that is, an adhesive that melts at high temperature and solidifies at low temperature, but the use temperature must be below 0 degrees. In addition to water, soldering and iron welding are all based on this principle. Of course, hot melt adhesives in the general sense are much more advanced than these. This one talks about why adhesives need to be solidified.

3. The intermolecular force of water is not enough. 

Although hydrogen bonds are relatively strong, the molecular force is positively correlated with the molecular weight after all. Therefore, it is obviously more appropriate to use polymers as adhesives, and it is best to use polymers with intermolecular hydrogen bonds to strengthen the force. The ancients could not get so many polymers. The main materials they could get were starch and protein. Therefore, we can see that traditional adhesives are mainly batter, glutinous rice juice, oyster juice, etc. Another traditional polymer is inorganic silicate. Clay plus lime can build a house. This branch later developed cement. Adhesives became a discipline after the development of polymer chemistry. A large number of materials including rubber, PVA polyvinyl alcohol, EVA ethylene-vinyl acetate, PU polyurethane, etc. can be used as adhesives. Most excellent adhesives have hydrogen bonds or polar bonds. This one talks about what substances adhesives are and why adhesives prefer polymers.

4. The next question is how to solidify.

a) The method of freezing water into ice mentioned above is the principle of hot melt adhesive, but there are not many types of hot melt adhesive, because there are few polymer materials that can show fluidity at not too high a temperature, mainly block polymers such as SBS, SIS, and SEBS.

b) Therefore, a variety of curing methods have emerged, the most common of which is the solvent method, which uses solvents to turn the glue into a liquid state (in fact, to be precise, it has fluidity). This method is very common. For example, batter and cement are made with water. When the water dries, the glue solidifies. Of course, the batter and cement at this time are only partially dissolved in water, similar to the emulsion state. More are toluene, cyclohexane, etc., yes, the kind used in many furniture and clothing.

c) This method was soon discovered to be not very good, because the concentration of the solution is generally less than 20%, and a large amount of solvent is required, which wastes resources and pollutes the environment. The curing process is very slow and consumes a lot of energy. Therefore, it is necessary to find ways to increase the solid content, so emulsion-type glue began to become popular. The water in the emulsion is not a solvent but a dispersant. However, many people cannot distinguish between emulsions and solutions. I will not explain the principle here. In short, this kind of glue will cure much faster. The current mass-produced active ingredients can reach up to 60%. Many EVA emulsions and natural rubbers are cured in this way.

d) Can't we use no solvent or dispersant? Of course, the problem is how to cure pure glue. One way is to use glue with creep properties, such as natural glue. Yes, pure natural glue can be used as an adhesive. It is solid, but it has creep properties (which can be understood as the fluidity of solid state). This kind of glue that does not need to be cured is often used for pressure-sensitive adhesives, which will be discussed later. More glues use the method of adding curing agents, that is, they are liquid in themselves and some reactive substances are added. This curing method is often called vulcanization. Many two-component glues are based on this principle, and a few one-component glues are also based on this principle. Most of them use reactions with water or carbon dioxide.

e) The more alternative 502 appears here. Its curing method is polymerization. When it is squeezed out, it is a monomer, but it can quickly polymerize when it encounters air.

f) The more fashionable curing method is UV light curing and particle curing, which is to use electromagnetic waves or particles to promote its reaction. In short, there are many curing methods, and there may be more in the future.

5. Do all glues use intermolecular forces? 

At present, it is not all right. One solution is to customize the lock mode. The molecular chains grown on the two surfaces of the bond are buckled like a chain. This is a physical effect, but breaking it requires breaking the chemical bond, which is an order of magnitude stronger than ordinary glue; another way is that the two surfaces react chemically to form a chemical bond, and the chemical bond needs to be broken when it is broken. Therefore, if you want to have strong strength, you have to rely on stronger chemical bonds.

6. Finally, let’s talk about pressure-sensitive adhesive, which was once called self-adhesive. 

Pressure-sensitive adhesive is a very common adhesive in daily life, such as tape and stickers. Generally, pressure-sensitive adhesive is needed in situations where it needs to be peeled off after being glued. The important principle of pressure-sensitive adhesive is that it does not need to be cured, and the important apparent phenomenon is that it is easy to stick, but it takes a little effort to peel off. What does it mean? It may only take 1 Newton of force to stick, but it may take 10 Newtons of force to peel off. It is best to reach 20 Newtons to break the force between the adhesive layers, otherwise the glue will flow everywhere when it is peeled off (many inferior tapes are like this), and the force to destroy the object may require 100 Newtons.