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The Science Behind Adhesives

Ever wondered how the two halves of your sandwich stick together? The science behind adhesives is fascinating and dates back into prehistory, when cave dwellers would use bitumen to stick flints to spears.

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In fact, some of the natural glues derived from fish skins, sugar and animal by-products are still in use today. However, as adhesive technology has developed, modern glues are more likely to be chemically derived.

Using Force to Make Things Stick

But knowing the strange-sounding chemical names for adhesive products like metal bonding adhesive is one thing. Knowing how they work to stick things together is quite another.

The answer is forces. Just like gravity sticks us the surface of a globe spinning in space, cohesive forces act to stick things together. For example, next time it rains watch the way raindrops clump together because of cohesion and then stick to your window because of adhesion. These same forces are at work in glue.

Cohesion and Adhesion

In any metal bonding adhesive like there are three different forces at work – adhesive forces that stick the metal being stuck to the glue, cohesive forces that stick the two glued surfaces together and then the force that sticks the glue to itself.

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This is where your sandwich comes in. The filling acts as a kind of glue, sticking the two halves of your sandwich together. But a sandwich also teaches us about why an adhesive bond can fail. Peel your sandwich apart and you’ll find filling on both surfaces. The adhesive forces prove stronger than the cohesive ones – and a bonded joint can sometimes fail because of lack of cohesion.

So How Does Glue Stick?

Think about water and iron. Both exhibit extraordinarily strong cohesion when adhering to each other, but you’d never dream of using them as a glue because they have no adhesive forces.

Substances that can be used as glues combine four forces in a variety of ways:

– Adsorption to spread thinly and wet the surface to be bonded
– Chemisorption, which creates a new and stronger bond
– Mechanical forces that grip surfaces together
– Diffusion of molecules across the surface to be bonded

It’s this ability for glues to work in different ways to create different kinds of bonds that makes them so versatile whatever the material you want to glue.

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