Adhesive

Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation.
The use of adhesives offers certain advantages over other binding techniques such as sewing, mechanical fastenings, and welding. These include the ability to bind different materials together, the more efficient distribution of stress across a joint, the cost-effectiveness of an easily mechanized process, and greater flexibility in design. Disadvantages of adhesive use include decreased stability at high temperatures, relative weakness in bonding large objects with a small bonding surface area, and greater difficulty in separating objects during testing. Adhesives are typically organized by the method of adhesion followed by reactive or non-reactive, a term which refers to whether the adhesive chemically reacts in order to harden. Alternatively, they can be organized either by their starting physical phase or whether their raw stock is of natural or synthetic origin.
Adhesives may be found naturally or produced synthetically. The earliest human use of adhesive-like substances was approximately 200,000 years ago, when Neanderthals produced tar from the dry distillation of birch bark for use in binding stone tools to wooden handles. The first references to adhesives in literature appeared approximately 2000 BC. The Greeks and Romans made great contributions to the development of adhesives. In Europe, glue was not widely used until the period AD 1500–1700. From then until the 1900s increases in adhesive use and discovery were relatively gradual. Only since the 20th century has the development of synthetic adhesives accelerated rapidly, and innovation in the field continues to the present.

History

The earliest evidence of human adhesive use was discovered in central Italy when three stone implements were discovered with birch bark tar indications. The tools were dated to about 200,000 before present in the Middle Paleolithic. It is the earliest example of tar-hafted stone tools.
An experimental archeology study published in 2019 demonstrated how birch bark tar can be produced in an easier, more discoverable process. It involves directly burning birch bark under an overhanging rock surface in an open-air environment and collecting the tar that builds up on the rock.
Although sticky enough, plant-based, single-component adhesives can be brittle and vulnerable to environmental conditions. The first use of compound adhesives was discovered in Sibudu, South Africa. Here, 70,000-year-old stone segments that were once inserted in axe hafts were discovered covered with an adhesive composed of plant gum and red ochre as adding ochre to plant gum produces a stronger product and protects the gum from disintegrating under wet conditions. The ability to produce stronger adhesives allowed middle Stone Age humans to attach stone segments to sticks in greater variations, which led to the development of new tools. A study of material from Le Moustier indicates that Middle Paleolithic people, possibly Neanderthals, used glue made from a mixture of ocher and bitumen to make hand grips for cutting and scraping stone tools.
More recent examples of adhesive use by prehistoric humans have been found at the burial sites of ancient tribes. Archaeologists studying the sites found that approximately 6,000 years ago the tribesmen had buried their dead together with food found in broken clay pots repaired with tree resins. Another investigation by archaeologists uncovered the use of bituminous cements to fasten ivory eyeballs to statues in Babylonian temples dating to approximately 4000 BC.
In 2000, a paper revealed the discovery of a 5,200-year-old man nicknamed the "Tyrolean Iceman" or "Ötzi", who was preserved in a glacier near the Austria-Italy border. Several of his belongings were found with him including two arrows with flint arrowheads and a copper hatchet, each with evidence of organic glue used to connect the stone or metal parts to the wooden shafts. The glue was analyzed as pitch, which requires the heating of tar during its production. The retrieval of this tar requires a transformation of birch bark by means of heat, in a process known as pyrolysis.
The first references to adhesives in literature appeared in approximately 2000 BC. Further historical records of adhesive use are found from the period spanning 1500–1000 BC. Artifacts from this period include paintings depicting wood gluing operations and a casket made of wood and glue in King Tutankhamun's tomb. Other ancient Egyptian artifacts employ animal glue for bonding or lamination. Such lamination of wood for bows and furniture is thought to have extended their life and was accomplished using casein -based glues. The ancient Egyptians also developed starch-based pastes for the bonding of papyrus to clothing and a plaster of Paris-like material made of calcined gypsum.
From AD 1 to 500 the Greeks and Romans made great contributions to the development of adhesives. Wood veneering and marquetry were developed, the production of animal and fish glues refined, and other materials utilized. Egg-based pastes were used to bond gold leaves, and incorporated various natural ingredients such as blood, bone, hide, milk, cheese, vegetables, and grains. The Greeks began the use of slaked lime as mortar while the Romans furthered mortar development by mixing lime with volcanic ash and sand. This material, known as pozzolanic cement, was used in the construction of the Roman Colosseum and Pantheon. The Romans were also the first people known to have used tar and beeswax as caulk and sealant between the wooden planks of their boats and ships.
In Central Asia, the rise of the Mongols in approximately AD 1000 can be partially attributed to the good range and power of the bows of Genghis Khan's hordes. These bows were made of a bamboo core, with horn on the belly and sinew on the back, bound together with animal glue.
In Europe, glue fell into disuse until the period AD 1500–1700. At this time, world-renowned cabinet and furniture makers such as Thomas Chippendale and Duncan Phyfe began to use adhesives to hold their products together. In 1690, the first commercial glue plant was established in The Netherlands. This plant produced glues from animal hides. In 1750, the first British glue patent was issued for fish glue. The following decades of the next century witnessed the manufacture of casein glues in German and Swiss factories. In 1876, the first U.S. patent was issued to the Ross brothers for the production of casein glue.
The first U.S. postage stamps used starch-based adhesives when issued in 1847. The first US patent on dextrin adhesive was issued in 1867.
Natural rubber was first used as material for adhesives in 1830, which marked the starting point of the modern adhesive. In 1862, a British patent was issued for the plating of metal with brass by electrodeposition to obtain a stronger bond to rubber. The development of the automobile and the need for rubber shock mounts required stronger and more durable bonds of rubber and metal. This spurred the development of cyclized rubber treated in strong acids. By 1927, this process was used to produce solvent-based thermoplastic rubber cements for metal to rubber bonding.
Natural rubber-based sticky adhesives were first used on a backing by Henry Day in 1845. Later these kinds of adhesives were used in cloth backed surgical and electric tapes. By 1925, the pressure-sensitive tape industry was born.
Today, sticky notes, Scotch Tape, and other tapes are examples of pressure-sensitive adhesives.
A key step in the development of synthetic plastics was the introduction of a thermoset plastic known as Bakelite phenolic in 1910. Within two years, phenolic resin was applied to plywood as a coating varnish. In the early 1930s, phenolics gained importance as adhesive resins.
The 1920s, 1930s, and 1940s witnessed great advances in the development and production of new plastics and resins due to the First and Second World Wars. These advances greatly improved the development of adhesives by allowing the use of newly developed materials that exhibited a variety of properties. With changing needs and ever evolving technology, the development of new synthetic adhesives continues to the present. However, due to their low cost, natural adhesives are still more commonly used.

Types

Adhesives are typically organized by the method of adhesion. These are then organized into reactive and non-reactive adhesives, which refers to whether the adhesive chemically reacts in order to harden. Alternatively they can be organized by whether the raw stock is of natural, or synthetic origin, or by their starting physical phase.

By reactiveness

Non-reactive

Drying

There are two types of adhesives that harden by drying: solvent-based adhesives and polymer dispersion adhesives, also known as emulsion adhesives. Solvent-based adhesives are a mixture of ingredients dissolved in a solvent. White glue, contact adhesives 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.
Polymer dispersion adhesives are milky-white dispersions often based on polyvinyl acetate. They are used extensively in the woodworking and packaging industries. They are also used with fabrics and fabric-based components, and in engineered products such as loudspeaker cones.

Pressure-sensitive

Pressure-sensitive adhesives form a bond by the application of light pressure to bind the adhesive with the adherend. They are designed to have a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow to 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 temperatures. Permanent PSAs may initially be removable 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, barcode labels, price marking labels, promotional graphics materials, and for skin contact. Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion, and generally cannot support much weight. Pressure-sensitive adhesive is used in Post-it notes.
Pressure-sensitive adhesives are manufactured with either 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 initiate 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. Major raw material for PSA's are acrylate-based polymers.