28 сентября, 2015 
Pokraskin With regards to their chemical structure, the higher-molecular-weight, organic base materials used in natural adhesives can be classified as:
• Proteins: elastin, fibronectin, keratin, collagen
• Polysaccharides: cellulose, starch, gum arabic
• Polyphenols: lignin, urushiol
• Lipids: terpenes, terpene resins
Proteins represent the most important structural component of living cells in both the animal and plant kingdoms. They are high-molecular-weight, colloidal natural substances composed of a large number of different alpha-amino acids as building blocks, with long chains of amino acids being linked via peptide bonds. Proteins — or macropeptides — are composed of more than a hundred amino acids. Collagen, which is an important protein for the manufacture of adhesives, is the main component of the supporting connective tissue of tendons, ligaments, cartilages and organic bone substance. When boiled with water to which acid has been added the collagen is converted into ‘bone glue’ that disperses as a colloid. Further purification of the bone glue leads to the production of colorless gelatin, which has strong swelling capacity in aqueous media and jellifies upon cooling. Casein, another protein used in the manufacture of natural adhesives, exists in milk as its calcium salt. In this type of protein, phosphoric acid is bound to a free OH-group like an ester. A third protein, albumin, is found in the serum of the blood.
Polysaccharides form the second group of natural base materials for the manufacture of adhesives. Besides cellulose, which is important in the manufacture of natural adhesives, starch in particular is used to manufacture the
dextrin adhesives that are used for many different applications (see Section 8.5). Gum arabic, which is also known as ‘acacia gum’ or ‘Sudan gum’, is an interesting historical example of a polysaccharide used in adhesive manufacture. Gum Arabic, which is the alkaline salt of arabic acid obtained from the barks of Acacia arabica, is soluble in warm water but insoluble in alcohol. Gum arabic is applied as an aqueous solution and then allowed to dry; it can then be reactivated (i. e. reconferred with its bonding capacity) by the re-addition of water. In former times, gum arabic was used as a moisture-activated adhesive for stamps; in fact, its slightly sweet taste was easily recognized is still appreciated by philatelists today. Although gum arabic has lost its commercial importance as an adhesive, it is still available as paper glue, with an odd trading pattern. One interesting use of gum arabic is as a binder in the manufacture of pharmaceutical tablets, with an annual world consumption of 50 000 tonnes.
During recent years, natural phenols such as lignin (obtained during the course of wood processing) have become less important in the manufacture of adhesives. Another important group is that of lipids — or rather lipoids, a sub-group of lipids — especially in the form of terpenes and terpene resins. In a broader sense, rosin belongs to this group, as does natural rubber, which is a polyprene. Lipids are the glycerin esters of even-numbered fatty acids. The so-called ‘waxes’ also belong to this group, and are still often used as additives in modern adhesives. Waxes are fatty acids esterified with higher monovalent alcohols. In a mixture with natural rubber, rosin was the first really suitable PSA for the manufacture of adhesive tapes. In a broader sense, shellac belongs to the group of terpenes; this is secreted by the Indian lacquer sign louse, and is essentially composed ofaliphatic and aromatic hydrocarbon acids. Shellac can be dissolved in alcohol, or used as hot-melt adhesive with melting temperatures ranging from 65 to 85 °C. In former times, shellac was used for fastening ruby bearings to watch movement plates.
Among the combinations of the four groups of raw materials listed above is celluloid, a mixture of nitrile-cellulose and camphor (a terpene), and a material with which we are still familiar today. Celluloid is composed of subsequently nitrated natural cellulose plastified with camphor, and today is no longer used as adhesive but rather as a precious raw material for combs. As celluloid derivatives are pleasant to touch, they are also often used to make the cases of high-quality fountain pens.
Marine glue was used for the sealing of ships made from wood (caulking), and for the bonding of different materials. The glue was a mixture of rubber, asphalt and kerosene, and sometimes contained shellac; a typical formulation comprised (by weight) 100 parts of rubber which was dissolved (for two weeks) in 120 parts of kerosene. Molten bitumen (20 parts) was then incorporated into this solution. A variation was to dissolve 50 g of fragmented rubber in 200 g carbon disulfide, and 130 g of organic shellac in 300 ml of high-proof alcohol, and to mix both solutions to obtain a glue. It was essential to store this marine glue in the absence of air, and to apply it hot. The glue adhered extremely well to wood, stone, glass and metal.
Sealing wax is another traditional formulation of a natural hot-melt adhesive. In former times, this was composed of rosin, shellac, turpentine and cinnabar, but today it is a mixture of shellac, cinnabar, chalk, gypsum and turpentine. It may be astonishing
to find sealing wax listed here as a natural adhesive, but as a very good hot-melt adhesive with very pleasant flow characteristics it can be used for the bonding of glass, making possible the formation of vacuum-sealed joints (up to 10~6 Torr).
The adhesives that can be produced from natural raw materials include:
• solvent-containing contact adhesives, such as shellac and alcohol
• dispersion adhesives, from Ficus benjamina
• water-activated solid adhesives, such as rubber/dextrin, gum arabic
• hot-melt adhesives, such as rosin, amber, shellac
• aqueous structural adhesives, such as casein, blood albumin, glutine, but without chemical crosslinking (structural viscosity)
• aqueous structural adhesives, such as glutine/formalin, with crosslinking
• pressure-sensitive adhesives (PSAs), such as rubber-rosin
• detachable adhesives (barnacles)
At this point it becomes clear that virtually all types of adhesive discussed in Chapter 4 can be produced (if desired) from pure natural raw materials. For example, solvent — containing contact adhesives do not exist naturally but can be produced by dissolving shellac in alcohol; antique restorers use this type of adhesive to reassemble broken turtleshell. It is easily possible at home to obtain a natural dispersion adhesive by cutting a bough from the popular indoor plant, Ficus benjamina, and then removing from the scissors (with quite some effort) the rubber dispersion that has leaked from the bough, after any water has evaporated. Besides gum arabic (see above), rubber dextrin mixtures are water-reactivated solid adhesives that are still used today as fastening adhesives for envelopes (see Section 8.5). As mentioned above, natural hot- melt adhesives for the assembly of watches are composed of shellac; likewise, rosin can be used as a hot-melt adhesive. For many years, the optical industry used Canada balsam as a binder for lenses, because it is crystal-clear and does not impair the refractive properties of the interfaces of various glasses. Today, pitch or bitumen are still used in some sectors of the building industry to join roofing felts. Waxes that were once used to adhere gold to wooden statues in Ancient Egypt have now lost their importance as adhesives, but are still used as additives. Amber, when used as an adhesive is a curiosity; it is still mined on a large scale and is noted for its very high melting point of 380 °C.
The details of detachable adhesives are discussed in Section 8.15; an example is the adhesive that barnacles use to attach themselves to a ship’s hull, the high strength of which makes them very unpopular with sailors [42-45].
5.9.3
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