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 Adhesive or glue is a compound in a liquid or semi-liquid state that  bonds items together.   Adhesives may come from either natural or synthetic sources. Some modern adhesives are extremely strong, and are becoming increasingly important in modern construction and industry. The types of materials that can be bonded using adhesives is virtually limitless, but they are especially useful for bonding thin materials. Adhesives usually require a controlled temperature and temperature to cure or set.  They can be electrically and thermally conductive or non-conductive. is a solution used to stick or holds materials together. 

Natural adhesives

Natural adhesives are made from inorganic mineral sources, or biological sources such as vegetable matter, starch (dextrin), natural resins or from animals e.g. casein or animal glue.  They are often referred to as bio-adhesives.  One example is a simple paste, made by mixing flour and water.

Synthetic adhesives

Elastomers, thermoplastic, and thermosetting adhesives are examples of synthetic adhesives.

Drying adhesives

These adhesives are a mixture of ingredients (typically polymers) dissolved in a solvent.  White glue and rubber cements are members of the drying adhesive family.  As the solvent evaporates, the adhesive hardens.  Depending on the chemical composition of the adhesive, they will adhere to different materials to greater or lesser degrees.  These adhesives are typically weak and are used for household applications.

Contact adhesives

Contact adhesives must be applied to both surfaces and allowed some time to dry before the two surfaces are pushed together.  Some contact adhesives require as long as 24 hours to dry before the surfaces are to be held together.  Once the surfaces are pushed together, the bond forms very quickly, hence, it is usually not necessary to apply pressure for a long time. so there is no need to use clamps.

Natural rubber and polychloroprene (Neoprene) are commonly used contact adhesives.  Both of these elastomers undergo strain crystallization.  Contact adhesives find use in laminates, such as bonding Formica to a wooden counter, and in footwear, such as attaching an outsole to an upper.

Hot adhesives

Hot adhesives, also known as hot melt adhesives, are simply thermoplastics. They are applied hot and harden as they cool. These adhesives have become popular for crafts because of their ease of use and the wide range of common materials to which they can adhere. A glue gun, pictured right, is one method of applying a hot adhesive. The glue gun melts the solid adhesive and then allows the liquid to pass through the "barrel" of the gun onto the material where it solidifies.

Paul E. Cope is reputed to have invented thermoplastic glue around 1940 while working for Procter & Gamble as a chemical and packaging engineer.  His invention solved a problem with water-based adhesives that were commonly used in packaging at that time.  Water-based adhesives often released in humid climates, causing packages to open and become damaged.

UV and light curing adhesives

Ultraviolet (UV) light curing adhesives, also known as light curing materials (LCM), have become popular within the manufacturing sector due to their rapid curing time and strong bond strength.  Light curing adhesives can cure in as little as a second and many formulations can bond dissimilar substrates and withstand harsh temperatures.  These qualities make UV curing adhesives essential to the manufacturing of items in many industrial markets such as electronics, telecommunications, medical, aerospace, glass, and optical.  Unlike traditional adhesives, UV light curing adhesives not only bond materials together but they can also be used to seal and coat products.

Pressure sensitive adhesives

Pressure sensitive adhesives (PSA) form a bond by the application of light pressure to marry the adhesive with the adherend.  They are designed with a balance between flow and resistance to flow.  The bond forms because the adhesive is soft enough to flow (i.e. "wet") the adherend.  The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond.  Once the adhesive and the adherend are in close proximity, molecular interactions, such as van der Waals  forces, become involved in the bond, contributing significantly to its ultimate strength.

PSAs are designed for either permanent or removable applications.  Examples of permanent applications include safety labels for power equipment, foil tape for HVAC duct work, automotive interior trim assembly, and sound/vibration damping films.  Some high performance permanent PSAs exhibit high adhesion values and can support kilograms of weight per square centimeter of contact area, even at elevated temperature.  Permanent PSAs may be initially removable (for example to recover mislabeled goods) and build adhesion to a permanent bond after several hours or days.

Removable adhesives are designed to form a temporary bond, and ideally can be removed after months or years without leaving residue on the adherend.  Removable adhesives are used in applications such as surface protection films, masking tapes, bookmark and note papers, price marking labels, promotional graphics materials, and for skin contact (wound care dressings, EKG electrodes, athletic tape, analgesic and transdermal drug patches, etc.).  Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion and generally can not support much weight.

Pressure sensitive adhesives are manufactured with either a liquid carrier or in 100% solid form.  Particles are made from liquid PSAs by coating the adhesive and drying off the solvent or water carrier.  They may be further heated to initiate a crosslinking reaction and increase molecular weight.  100% solid PSAs may be low viscosity polymers that are coated and then reacted with radiation to increase molecular weight and form the adhesive; or they may be high viscosity materials that are heated to reduce viscosity enough to allow coating, and then cooled to their final form. 

Plastic wrap displays temporary adhesive properties as well.

A glue gun is an example of hot adhesive.
A glue gun is an example of hot adhesive.