From small sticky tape on your desk to powerful industrial adhesives used in aircraft construction, adhesives are a constant presence in our daily lives. But an interesting question that we pay less attention to is how does glue stick? Why is it difficult to separate two surfaces when we glue them together with glue? What forces are involved, and what materials are adhesives made of? At first glance, it may seem that glue just dries and sticks two objects together, but in fact, behind this simple process lies a complex science of chemistry, physics, and even materials science. In this article, we want to examine the mechanism of adhesion in simple but scientific language and understand the secret of the power of adhesives.

What is glue and what is it made of?

Adhesives are substances that can create an adhesive force or bond between two surfaces. They are generally made up of two parts:

• Polymer base (the main adhesive substance)
• Additives (to improve properties, such as flexibility, heat resistance, or drying speed)

Many adhesives are based on polymers such as polyvinyl acetate (PVA), epoxy, silicone, or polyurethane.

These polymers have long molecular chains that can interact with molecules on the surface of objects and form bonds.

How does glue stick? (Scientific mechanism of adhesion)

In answering the question of how glue sticks, we must mention several types of bonds and forces that act simultaneously.

Adhesives usually cause bonding in three ways:

🔸 A) Physical Adhesion

In this type, the Van der Waals force between the glue molecules and the surface of the object causes adhesion.

Van der Waals forces are weak, but because they act over a large contact area, their sum can create a large force.

For example, when liquid glue is spread on a surface, its molecules come close to the molecules of the object and physical forces are established between them. The closer the contact and the cleaner the surface, the greater this force.

🔸 b) Chemical Adhesion

In many industrial adhesives, actual chemical bonds are established between the adhesive molecules and the surface.

For example, in epoxy adhesives, the adhesive molecules react with active groups on the surface (such as OH or COOH groups) to form covalent bonds.

This type of bond is very strong and makes it resistant even to heat and humidity.

🔸 C) Mechanical Interlocking

When the surface of the object is rough (for example, wood or rough metal), the adhesive penetrates into its pores and, after drying, locks into the surface texture.

In fact, the adhesive physically gets stuck in the surface and acts like a hook or a barb. This mechanism is one of the main reasons for the strength of construction adhesives and epoxy.

Ancient times; the first natural adhesives

The oldest evidence of the use of adhesive dates back to about 200,000 years ago!

Archaeologists in Germany have found the remains of birch tar, which Neanderthals used to glue spearheads to wood.

This can be considered the first “industrial adhesive” of mankind.

Later, ancient civilizations such as the Egyptians, Sumerians and Romans used various natural compounds:

• Plant gum to glue papyrus and wood
• Animal protein (collagen) from skin and bone to make furniture and instruments
• Wax and bitumen to seal boats and pottery

In ancient Egypt, even mummies were held together with the help of natural adhesives!

19th century; the beginning of polymer chemistry

With the development of chemistry, scientists realized that adhesion was actually due to molecular structure and intermolecular forces.

In this era, natural adhesives such as gum and gelatin were improved, but the real revolution occurred in the 20th century. Lets see How Does Glue Stick?

Smart and nano-adhesives

Today, the science of adhesion has reached a very advanced level.

Using nanotechnology and biotechnology, scientists have created adhesives that:

• Activate or deactivate at specific temperatures
• Remove without leaving a residue
• Absorbed in the human body (in medical surgeries)
• Made from renewable and biodegradable materials

Some adhesives are even inspired by animal behavior, such as adhesives that mimic the structure of lizard hair and can be used multiple times without losing their adhesive strength.

The Role of Surface Energy and Wettability

To understand how glue sticks, we need to understand the concept of surface energy.

Every solid surface has surface energy; that is, its tendency to make contact with another material. If the surface energy is high, the adhesive spreads more easily on it and forms a better bond.

A phenomenon called wettability is also important here. Wettability is the ability of a liquid (liquid adhesive) to spread on a solid surface.

Clean, dry, and rough surfaces usually have better wettability. For this reason, cleaning the surface of grease and dust is a key step when gluing.

Factors Affecting Adhesion Strength

The ultimate strength of an adhesive depends not only on its type; it also depends on the environmental conditions and how it is used. How Does Glue Stick?  The most important factors are:

▪ Temperature

Increasing temperature usually softens the adhesive polymer and reduces adhesion, except in heat-resistant adhesives.

▪ Moisture

Water can prevent bonding, especially in water-based adhesives. However, some adhesives (such as silicone) require moisture to activate.

▪ Pressure

When we apply appropriate pressure when gluing, the air between the surface and the adhesive is expelled and molecular contact increases.

▪ Drying time

If the drying time is not observed, the bonds will not be fully formed and the connection will remain weak.

Types of Adhesives and How They Work

🔸 Natural Adhesives

include animal gums and glues (gelatin, starch, dextrin). These adhesives mostly use physical and mechanical forces.

🔸 Synthetic Adhesives

are made based on synthetic polymers such as polyvinyl acetate (PVA), epoxy, and polyurethane.

These adhesives often also use chemical bonds.

🔸 Pressure-Sensitive Adhesives

like everyday adhesive tapes that stick to the surface with pressure without drying or chemical reaction.

Here, van der Waals forces and the viscoelastic property of the adhesive play a major role.

🔸 Hot-Melt Adhesives

melt at high temperatures and, after cooling, solidify and firmly bond two surfaces together.

This type is widely used in packaging, shoemaking, and electronics.

🔸 Epoxy adhesives

consist of two components: resin and hardener. Once combined, a chemical reaction occurs, forming a very strong adhesive.

🔸 Silicone adhesives

are based on silicone and are very resistant to heat, water and UV rays. They are great for caulking and the construction industry.

 

The science behind adhesive drying

Drying means that the adhesive changes from a liquid state to a solid and forms a stable network of molecules.

The drying method varies depending on the type of adhesive:

• Evaporative adhesives: dry by evaporation of the solvent (such as water-based adhesives).
• Reactive adhesives: harden through a chemical reaction between the two components or with moisture in the air (such as epoxies).
• Hot melt adhesives: solidify when cooled.

In each case, the goal is for the adhesive molecules to lock together stably between the two surfaces after spreading, creating the adhesive force.

Adhesion on a Microscopic Scale

To better understand how glue sticks, we need to examine the contact surface on a microscopic scale.

Seemingly smooth surfaces actually have many ridges and depressions. The liquid glue penetrates these cavities and, after drying, forms a “lock and key” structure.

At the same time, physical and chemical forces also act between the glue molecules and the surface.

As a result, separating two surfaces requires the simultaneous breaking of several types of bonds, which is what makes adhesives so strong.

Why doesn’t glue stick to some surfaces?

If you’ve ever seen glue stick poorly to plastic or Teflon, it’s because these materials have low surface energies. In such materials, the glue molecules can’t make effective contact.

To solve this problem, industries use methods such as surface roughening, plasma treatment, or the use of primers to increase surface energy and improve adhesion. Now we know How Does Glue Stick?

Adhesives in life and industry

Adhesives today play a vital role in various industries:

• In construction for installing ceramics, glass, and insulation
• In automotive for connecting parts and sealing
• In medicine for making special dressings and bio-bands
• In electronics for assembling parts and preventing vibration

In all these applications, a scientific understanding of how adhesives adhere helps to select the right type of adhesive.

The future of adhesives; from nano to biodegradable

New research is moving towards the production of smart and biocompatible adhesives.

Scientists are designing adhesives that:

• Activate at a specific temperature
• Separate spontaneously after a while
• Are made from natural and recyclable materials

Some of these adhesives are even inspired by the mechanisms of animals, such as lizards or clams, which can hold surfaces together with simple physical force.

Conclusion

The main secret to how glue sticks is a combination of physics, chemistry, and surface engineering. Adhesives connect two objects by penetrating the surface of objects and establishing various bonds. Surface energy, cleanliness, temperature, and the type of polymer are among the factors that affect the ultimate strength of the adhesive. Understanding this process is not only scientifically interesting, but also important in industry and everyday life. Every time we glue a piece of paper or fill a gap, we are actually using knowledge that comes from decades of research in the field of materials and molecules.