Written by Jody Muelaner
Adhesives are a broad category of materials used to bind surfaces, also known as glues and cements. Joining materials using an adhesive is called bonding. Several adhesives are used to bond different types of material under different conditions. They may be referred to as glues or cements.
Compared to other methods of joining — such as welding or mechanical fastening — bonding can reduce the time and expense of assembly while providing improved distribution of stress. The reliability and durability of adhesives have advanced significantly. However, under certain conditions, adhesives may not provide as reliable of a joint as other forms of fastening.
Adhesives are also typically more difficult to disassemble than mechanical fasteners.
Some common types of adhesives include:
- Emulsion adhesives: such as polyvinyl acetate (PVA) are water-based and form a bond as the water evaporates. They are commonly used to join wood, paper, and fabric.
- Multi-component adhesives: are supplied as two or more substances that, when mixed together, polymerize to form a thermoset plastic. This typically creates an extremely strong bond with excellent gap filling, as well as resistance to moisture and heat. Examples include epoxy and polyester resins.
- Cyanoacrylate adhesive: is supplied as a liquid containing monomers. When exposed to normal atmospheric humidity the monomers quickly polymerize to form a bond between surfaces in close contact. A popular brand of cyanoacrylate adhesive is Superglue.
- Polyurethane adhesive: also cures to form a polymer when exposed to humidity. It is easily applied and creates strong bonds between most materials both porous and non-porous. It foams and expands during curing to fill gaps although bond strength is then reduced. It is widely used within the construction industry as both an adhesive and a gap filler.
- Pressure-sensitive adhesives: flow into the surfaces being bonded and as they get very close together, molecular interactions create a strong bond. They are used on tapes and stickers.
- Contact adhesives: are applied to both surfaces and dried before the surfaces are brought together, rapidly forming a strong bond. This can create a strong bond between both porous and non-porous materials, provided a close interface can be achieved. Once the surfaces are brought together the bond is formed very quickly removing the need for clamping but making repositioning difficult.
- Hot-melt adhesives: are thermoplastics that are melted, typically using a glue gun, and applied in a molten state. As they cool they solidify forming a bond.
- UV light curing acrylic adhesives: can cure within one second to form a very strong and durable bond between many different materials. They are widely used in manufacturing processes.
- Solvent cement: is a mixture of resin and solvents that is used to join polyvinylchloride (PVC) parts such as pipes and fittings. It actually dissolves the two adjacent surfaces and causes them to re-polymerize into a single piece of plastic. It therefore effectively performs a chemical weld between the parts.