Definition and Advantages of Adhesive
- Adhesive is a non-metallic substance used to bond materials together and resist separation.
- Adhesives offer advantages over other binding techniques such as sewing, mechanical fastenings, or welding.
- Adhesives can bind different materials together.
- Adhesives distribute stress more efficiently across a joint.
- Adhesive processes are cost-effective and easily mechanised.

Disadvantages of Adhesive
- Adhesives have decreased stability at high temperatures.
- Adhesives may be relatively weak when bonding large objects with a small surface area.
- It can be challenging to separate objects bonded with adhesives during testing.
- Adhesives may not be suitable for certain applications that require high temperature resistance.
- Some adhesives may not provide sufficient bonding strength for large-scale projects.

Classification of Adhesives
- Adhesives can be classified as reactive or non-reactive based on whether they chemically react to harden.
- Adhesives can also be classified based on their physical phase or whether they are natural or synthetic.
- Reactive adhesives chemically react to harden and form a strong bond.
- Non-reactive adhesives do not chemically react and rely on physical processes for bonding.
- Adhesives can be classified as liquid, solid, or semi-solid based on their physical phase.

Historical Use of Adhesives
- The earliest known use of adhesives dates back approximately 200,000 years ago when Neanderthals used tar from birch bark to bind stone tools to wooden handles.
- The Greeks and Romans made significant contributions to the development of adhesives.
- Adhesive use became more widespread in Europe during the period AD 1500-1700.
- The development of synthetic adhesives accelerated rapidly in the 20th century.
- Adhesive innovation continues to the present day.
- Ancient humans used various adhesive techniques, such as using plant gum and red ochre as an adhesive in South Africa approximately 70,000 years ago.
- Prehistoric tribes buried their dead with broken clay pots repaired using tree resins around 6,000 years ago.
- Bituminous cements were used to fasten ivory eyeballs to statues in Babylonian temples around 4,000 BC.
- The discovery of the Tyrolean Iceman or Ötzi revealed the use of pitch as an adhesive approximately 5,200 years ago.
- Ancient Egyptians used animal glue, casein-based glues, and starch-based pastes for bonding various materials.

Types of Adhesives
- Non-reactive adhesives: Drying adhesives harden by evaporation of solvents or water. Solvent-based adhesives and polymer dispersion adhesives are examples. Used in woodworking, packaging, fabrics, and engineered products.
- Pressure-sensitive adhesives: Form a bond by applying light pressure to marry the adhesive with the adherend. Used in safety labels, foil tape, automotive assembly, and Post-it notes.
- Contact adhesives: Form high shear-resistance bonds with rapid cure time. Applied in thin layers for laminates and footwear. Require drying time before surfaces are pushed together. Clamps are usually not needed due to rapid bond formation.
- Hot adhesives: Thermoplastics applied in molten form and solidify on cooling. Commonly used in crafts. Glue guns are commonly used to apply hot adhesives.
- Natural adhesives: Made from organic sources such as vegetable starch and natural resins. Used in various applications like paper sack production and wood joining. Animal glues traditionally used in bookbinding and specialist applications.
- Synthetic adhesives: Made from organic compounds. Examples include epoxy, polyurethane, and cyanoacrylate. Widely used in various industries.
- Industrial adhesives: Used in automotive, construction, electronics, and packaging industries. Provide high strength and durability. Can bond different materials like metals, plastics, and composites.
- Pressure-sensitive adhesives: Adhere to surfaces with light pressure. Used in tapes, labels, and sticky notes. Provide instant adhesion and easy removal.
- Structural adhesives: Provide high strength and load-bearing capabilities. Used in bonding large structures like aircraft and bridges. Can replace traditional mechanical fasteners like screws and rivets.
- Biomedical applications: Thiolated polymers form chemical bonds with proteins. Used in drug delivery systems and tissue engineering. Adhere to biological surfaces like mucus glycoproteins and integrins. Offer potential for targeted drug delivery and wound healing.