TLDR: Adhesive, also known as glue, is a substance that sticks things together. It has been used by humans for thousands of years, starting with Neanderthals who made tar from birch bark. Adhesives have many advantages, such as the ability to bond different materials together and distribute stress evenly. They can be organized by how they bond and whether they react chemically. Adhesives can be natural or synthetic, and they are used in a wide range of applications.

Adhesive, also known as glue, cement, mucilage, or paste, is a substance that is used to bond two separate items together and resist their separation. It is a non-metallic material that is applied to one or both surfaces of the items. Adhesives have been used by humans for thousands of years, with the earliest known use dating back to Neanderthals who made tar from birch bark. The use of adhesives offers several advantages over other binding techniques, such as sewing or welding. Adhesives can bond different materials together, distribute stress evenly across a joint, and are cost-effective and flexible in design. However, there are also some disadvantages to using adhesives, such as decreased stability at high temperatures and difficulty in separating objects during testing.

Adhesives can be organized based on how they bond and whether they react chemically. They can also be categorized based on their origin, whether they are natural or synthetic. Natural adhesives are made from organic sources, such as vegetable starch, natural resins, or animal-based glues like casein. Synthetic adhesives, on the other hand, are made from organic compounds and include elastomers, thermoplastics, emulsions, and thermosets. The development of synthetic adhesives has accelerated in the 20th century, but natural adhesives are still commonly used due to their low cost.

There are different types of adhesives based on their reactiveness. Non-reactive adhesives include drying adhesives, pressure-sensitive adhesives, contact adhesives, and hot adhesives. Drying adhesives harden by evaporation of solvents, while pressure-sensitive adhesives form a bond when light pressure is applied. Contact adhesives require both surfaces to be coated and then brought together, and hot adhesives are applied in molten form and solidify upon cooling. Reactive adhesives include anaerobic adhesives, multi-part adhesives, and one-part adhesives. Anaerobic adhesives cure in the absence of oxygen, multi-part adhesives harden by mixing two or more components, and one-part adhesives harden through a chemical reaction with an external energy source.

The design of adhesive joints involves considering the material properties of the objects being bonded and the forces they will be subjected to. Fracture mechanics is often used to predict failure in adhesive joints, taking into account factors such as stress concentration and strain energy release rate. The geometry of the joint can also be modified to increase its resistance to failure, such as increasing the bonded zone and ensuring it is mainly loaded in a specific mode.

Adhesives have a limited shelf life, which can be affected by factors such as temperature, exposure to oxygen or water vapor, and the type of adhesive. Some adhesives may lose their effectiveness at high temperatures or become stiff over time.

In summary, adhesive is a substance that is used to bond two separate items together. It has been used by humans for thousands of years and offers several advantages over other binding techniques. Adhesives can be categorized based on how they bond, whether they react chemically, and their origin. The design of adhesive joints involves considering the material properties and forces involved, and adhesives have a limited shelf life.