An adhesive is a compound that adheres, or bonds, two items together. Adhesives may come from either natural or synthetic sources. Some modern adhesives are extremely strong, and these are becoming increasingly important in modern construction and industry.
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History
The first adhesives were natural gums and other plant resins or saps. It was believed that the Sumerian people were the first to use adhesives, until it was discovered that Neanderthals as far back as 50,000 years ago made adhesives from birch bark. The discovery of 6000-year-old ceramics brought evidence to archaeologists about the first practical uses and ingredients of the first adhesives. Most early adhesives were animal glues made by rendering animal products such as horse teeth. During the times of Babylonia, tar-like glue was used for gluing statues. The Egyptians made much use of animal glues to adhere furniture, ivory, and papyrus. The Mongols also used adhesives to make their short bows, and the Native Americans of the eastern United States used a mixture of spruce gum and fat as adhesives to add waterproof seams in their birchbark canoes.
In Medieval Europe/Eurasia, egg whites were used as glue to decorate parchments with gold leaf. Holland, in the early 1700s, founded the first ever glue factory. Later, in the 1750s, the British introduced fish glue. As the modern world evolved, several other patented materials, such as bones, starch, fish, and casein, were introduced as alternative materials for glue manufacture. Modern glues have improved beyond recognition. Such improvements are noticeable in their flexibility, toughness, curing rate, temperature, and chemical resistance.
Categories of Adhesives
Natural Adhesives
Natural adhesives are made from inorganic mineral sources, or biological sources such as vegetable matter, starch (dextrin), natural resins, animal skin, and bioadhesives. A simple paste can be 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. Glues such as white glue, and rubber cements, are members of the drying adhesive family. As the solvent evaporates, the adhesive hardens. Depending on the chemical composition, different drying adhesives will adhere to different materials to greater or lesser degrees. These adhesives are typically weak and are used for household applications. Some intended for use by small children are now made non-toxic.
Contact Adhesives
A contact adhesive is one that 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 that 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, for example, attachment of an outsole to an upper.
Hot Adhesives (Thermoplastic Adhesives)
Also known as "hot melt" adhesives, hot adhesives are thermoplastics; they are applied hot and then 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 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 he was working for Procter & Gamble as a chemical and packaging engineer. His invention solved a problem with the 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.
Reactive Adhesives
A reactive adhesive works either by chemical bonding with the surface material or by in-situ hardening as two reactant chemicals complete a polymerization reaction. They are usually applied in thin films. Reactive adhesives are less effective when there is a secondary goal of filling gaps between the surfaces. These include two-part epoxy, peroxide, silane, metallic cross-links, or isocyanate. Such adhesives are frequently used to prevent loosening of bolts and screws in rapidly moving assemblies, such as automobile engines. They are largely responsible for the quieter-running modern car engines.
Pressure-Sensitive Adhesives
Pressure-sensitive adhesives (PSAs) 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); they build adhesion to a permanent bond after several hours or days.
Removable adhesives, one type of PSA, 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 products that will have contact with the skin (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 cannot support much weight.
PSAs are manufactured either with a liquid carrier or in 100% solid form. Articles are made from liquid PSAs by coating the adhesive and drying off the solvent or water carrier. They may be further heated to initate a cross-linking 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.