For a long time humans have succeeded in joining or bonding materials together with several purposes in mind: home construction, tools, weapons, machines (probably quite rudimentary at the very beginning) and decoration.
As a result bonding techniques allowing binding together various substrates and elements have been developed. Adhesives (or glues) and sealants are one of them. Bonding by fusion and soldering is another as well as several other techniques like riveting, joining with mortise and tenon, ligaturing, using weights and counterweights, etc.
Bonding technologies were developed by humans first by being inspired by nature. For example, insects, arachnids and birds are skilled at forming composites with natural fibres and their own secretions and several molluscs attach themselves strongly to rocks. We may reasonably speculate that the stickiness of birch tree tar or of latex issued from the hevea tree, eggs albumen or starch paste were soon put to use by the early humans. It’s also likely that the adhesive properties of blood were discovered a long time ago. Based on archaeological evidence, most probably the very first bonding attempts by humans are much anterior to the Neolithic period (circa 8000 B.C.) for which the first evidence of adhesive use has been discovered.
It is commonly accepted that the chronological evolution of adhesion knowledge acquisition is as follows:
- 3 000 000 B.C.: Cell to cell bonding; mussels; spiders; bees and wasps; barnacles…
- 80 000 B.C. (Neolithic): Birch tree tar
- 5000 B.C. Mesopotamia (Babylon): Animal blood proteins, Plant resins (colophony) and Asphalt
- 3500 B.C. Egypt : Boiled Glues (Animal skins, etc.)
- 1500 B.C. Aztec : Animal blood and Natural rubber cement (Hevea Latex)
- 1841: Vulcanisation process >>> Synthetic adhesives development
- 1902: Bakelite
- 1921: Principles of Macromolecular Chemistry by Max Staudinger
- 1902-1940: Synthetic rubber (PolyButadiene); silicones; epoxy; polyurethanes…
- 1952: Cyanoacrylate (Super Glue)
Birch tree sap is an edible liquid collected from trees at the break of winter. Birch tar is not made from the sap, but from the bark heated in the absence of air, like charcoal. Black oil is collected at the bottom of the oven and cooled in water. Detailed recipes on how to make birch tar using readily available and inexpensive equipment are available on the internet. The oil should then be boiled down slowly in the open (because of the fumes) until it thickens, just before it carbonizes. After it solidifies, it can be stored indefinitely and melted just before use, just like modern thermoplastic glues commonly called hot melt adhesives.
Birch bark tar has excellent resistance to humidity: it was used to seal leather seams on moccasins and boots. Not being brittle, it also has gap-filling properties like a sealant. It was extensively used to repair pottery and affix arrow points to shafts.
In the warmer climates birch does not grow naturally and the need for alternatives developed.
There is archaeological evidence that proteins extracted by prolonged boiling and partial hydrolysis in water of skin, teeth, bones, horse hooves or tendons were used even by the Neanderthals. However, the oldest written evidence of such practices dates back to about 2000 B.C. in Egypt where their use in furniture and cabinet making probably first started.
Survival guides show how to make glue from raw hide. The hard hide must first be softened in boiling water, cut in small pieces and put back into the boiling water for several hours. After that, the remaining solids are filtered and the water is boiled down until it thickens. The thicker the solution, the faster it will dry. It can be dried up completely and if protected from humidity, it will keep for some time. Historically, hide glue was used to make composite bows.
Fish glue was invented in Ireland in the late 19th century. Until the advent of epoxy adhesives, refined fish glue was advertised extensively for affixing metals to each other, although it may have been over promoted for that use. Its major deficiency was that after drying it remained water soluble.
Albumen from egg white is used to bond gold leaf.
Casein based glues are made by precipitating casein from milk using vinegar. The curds that form are neutralized with baking soda causing them to unclump and thicken. Low fat milk works best. Casein glue can be made into a powder by drying it first by squeezing in a towel and then in air, and finally by grinding. It will keep better in that state. Before use, mix with water to a honey-like consistency and apply. Casein glue works well on woodwork.
Waste animal blood is readily available from slaughterhouses. The adhesive component is the albumen contained in blood serum. Addition of lime and alkali to albumen-water mixtures improves adhesive properties. The plywood industry is an important user of blood-based adhesives, as they are relatively resistant to humidity once hot-pressed. Synthetic glues like melamine/ formaldehyde are today more used due to a much higher resistance to humidity and regular availability. Of course due to the growing concern for sustainable and biodegradable materials, one may speculate that the market share of animal glues may increase again until a biodegradable synthetic material is available in large volumes.
In Mesopotamia, natural bitumen resurgences were used to make asphalt. The volatile components of oil evaporate and leave a sticky or solid black residue. In 6000 B.C., naturally occurring asphalt was quarried and used as mortar between building stones. Later on, bitumen was used as caulking for ships, to make jewellery and to set mosaics and weapon parts. In North America, Amerindians were using asphalt to waterproof their baskets and secure arrowheads to shafts. Nowadays, asphalt is made industrially from crude oil or from coal distillation and is still used for binding insulation panels or shingles.
Historically, the first known mineral cements were extensively used by the Romans from burnt limestone, volcanic ash and pulverized brick (from clay), calcined together and then crushed. Mineral cements harden in the presence of water due to the formation of crystalline hydrates. Mortar is a combination of cement and sand while concrete is made from cement and aggregate. Cement is nearly exclusively used in construction.
By extension, dental cements are made by mixing powder and liquid together. This is where the similarity ends. The powder is a basic metal oxide and the liquid is acidic. The metal salt formed acts as the cementing matrix.
Mineral cements are not considered further in this website since many of their properties and uses are unlike most adhesives and sealants.
Natural rubber is made by coagulating latex sap from the Hevea tree. A stable viscous liquid is formed by mixing with hydrocarbon solvent and some alcohol. The first raincoats were made by Charles Macintosh by gluing two layers of cloth together with rubber.
The accidental discovery of rubber vulcanization with sulphur in 1841 by Goodyear overcame rubber’s propensity to soften in the presence of oil or fuel or when heated. This was the first time a natural product was chemically modified to improve its properties.
Rubber cements used to repair inner tubes in tires contain cross-linking chemicals that prevent melting in hot weather.
In 1862, A. Parkes succeeded in making semi-synthetic celluloid, a mixture of nitrocellulose and of camphor. Cellulose was too brittle to be used as an adhesive.
The first totally synthetic resin was Bakelite, a thermosetting phenolic compound invented by Belgian-born Leo Baekeland in 1907. Bakelite is a hard mouldable material unsuitable as an adhesive, but the same chemistry was modified to impregnate fabrics or paper.
Nowadays particle boards and chipboards are made by binding under heat and pressure wood chips, shavings or saw dust with amino-formaldehyde resins. Since formaldehyde is classified as a potential carcinogen, any traces of unreacted residues are maintained within very strict limits.
Many new synthetic rubbers such as polychloroprene, Buna (polybutadiene) and silicones were first synthesized during and after the Second World War. Then followed epoxy resins, polyurethanes and after that methacrylate and cyanoacrylate adhesives (superglues). New drying technologies based on UV light and on electron beams were also developed to eliminate volatile solvents, requiring specially designed resins and additives.
The study of nature has revealed many examples of the use of adhesives that continue to be a source of inspiration.
Good adhesive strength is dependent on the molecular weight of the binding resin. Unfortunately, higher molecular weights increase viscosity of the resins in solution making use difficult or weakening the glue’s properties. Latexes widely used today in modern paints for example were indeed invented a long time ago by nature, which indeed has shown the way of modern lattices such as acrylic paint or glue dispersions where the resin in the form of microscopic particles is mixed with water and surfactant, just like the sap of Hevea.
In the same register, there are also several other examples of adhesives or sticky material produced by plants. Probably the most known and spectacular are the sticky glues secreted by carnivorous plants. They are made of mucilage, a gluey substance produced by nearly all plants and some microorganisms. It is a polar glycoprotein and an exopolysaccharide. Nanofibres and nanoparticles were found in Drosera mucilage, probably contributing to the observed viscosity and stickiness.
The naw (or hulusheng in China) is a mouth organ used in Northern Thailand. The reeds are glued to an emptied gourd with beeswax.
The wax used to build honeycombs in honey hives is applied as a liquid due to the warmth of the bee’s body, like a hot melt adhesive.
Wasps Nests: many social wasps make nests from materials collected nearby and then chewed and mixed with wood fibres. The nests are water-resistant due to the proline-rich nature of the mucoproteins in the saliva, similar to the wasps’ chitin exoskeleton, which dries irreversibly to a water repellent surface.
Very well know, designed for a temporary use are the swallow nests for example. They are built with earth agglomerated by saliva which contains imucin, a glycoprotein.
The dried saliva of the White-nest Swiftlet (Aerodramus fuciphagus) and of the Black-nest Swiftlet (Aerodramus maximus) is edible and is used to make bird’s nest soup, a Chinese delicacy. The swiflets deposit interwoven strands of pure salivary laminae cement secreted by glands underneath their tongues.
Termite nests are sometimes attacked by other insects. Some termite species have evolved a defence mechanism where a sticky liquid is sprayed on the attackers using a frontal secretion. The liquid rapidly hardens in contact with air trapping ants and other termites. Sometimes the spraying contractions are so violent that the termites rupture themselves (autothysis).
Barnacles are known to strongly attach themselves to hard surfaces, whether rocks or ship hulls (biofouling) or whales. Barnacles produce the most durable and toughest connection in the aquatic world. These molluscs secrete polyphenolic proteins which are now being investigated for various dental and medical purposes because of their resistance to antibodies. An enzyme, polyphenol oxidase, reacts with protein with a diphenol producing tanned (insoluble) protein.
Geckos attach and detach their adhesive toes in milliseconds while running up to 1 meter/second on nearly any surface. The adhesive on gecko toes differs dramatically from that of conventional adhesives. Conventional pressure-sensitive adhesives are difficult to remove after applied. In contrast, gecko toes bear angled arrays of branched, hair-like fibres (setae) formed from stiff, hydrophobic keratin that act as a bed of angled springs with an effective stiffness similar to that of pressure sensitive adhesives. Setae are self-cleaning and maintain function for months during repeated use in dirty conditions. Thus, gecko setae resist inappropriate bonding and are capable of easy and rapid attachment and detachment. According to scientists, engineered adhesive nanostructures inspired by geckos may become the glue of the future.
Rosin is a component of the pine tree exudation in response to a wound. Distilled rosin as such is a brittle material melting at around 80° to 90°C with some weak adhesion properties which are insufficient for practical binding usage. In mixture it can be used to protect trees from climbing insects or as sealing wax. By chemical transformations rosin derivatives enable synthetic modern adhesive formulations like the hot melts and contact adhesives widely used today, as well as for the prodution of paints and printing ink resins. Rosin as such is used to improve the adhesion of the bristles of the bow on the strings of the violin. The first utilsation of rosin for this purpose probably took place in Cremona (Italy) original town of Stradivarius.
Starch extracted from many plants such as potato, rice, tapioca and the like is also useful as an adhesive although its poor resistance to humidity and mould formation are important limitations to their wider use. Starch adhesives are used to make corrugated board and wallpaper, for example. The ancient Chinese and Egyptians were quite adept at using starch-based adhesives in paper or papyrus making. When heated in water, native starch granules hydrate (absorb water), increase in size and thicken to form a viscous liquid, gel or paste. This paste can be used to bind wood and paper together. Once dried the paste becomes relatively hard while retaining some adhesives properties. One of the drawbacks is the poor resistance to humidity. Maybe some of us have already used this glue to build kites.
- THE ART OF MAKING VARIOUS KINDS OF GLUES. By M. Duhamel Du Monceau, of the Royal Academy of Sciences (1771).