Post 224 ⇒ by Gautam Shah →
NATURAL ADHESIVES (Bio-adhesives)
Bio-adhesives are comparatively a recent nomenclature to distinguish a variety of Adhesive materials that are of natural origin. These materials are often only ‘primarily processed’ and so are believed to retain their natural characteristics. Synthetic adhesives pose environmental concerns of volatile organic compound (VOC) emissions, safe disposal and recycling problems (very long life cycles). The health and safety issues relating to hazardous ingredients, in foods, medicines and bio-implants, cause concerns.
Bio-adhesives like other natural materials tend to be bio-compatible, and so there is increased commercial interest in them. There use in certain applications such as for biomedical and topical uses such as the bonding of skin and body tissues are being studied. The uses of bio-adhesives are being assessed, as food additives and supplicants, due to low level of toxicity. Secretions by microbes, marine molluscs and crustaceans, are being researched with a view to applications in bio-mimicry.
In 2005, researchers from the University of California, Santa Barbara (UCSB) discovered that the glue used by the phragmatopoma to build its protecting tube was made of specific proteins with opposite charges. Those proteins are called polyphenolic proteins that are used as bio-adhesives. They succeeded in obtaining the sequence of these adhesive proteins and described the detailed mechanisms by which the adhesive sets. Inspired by these results University of Utah researchers reported in 2009 that they succeeded in duplicating the glue that the worms secrete and use to stick sand grains together underwater. The typical amount of glue that the worm produces at once is approximately 100 picoliters, requiring 50 million to fill a teaspoon. They believe the glue to have applications as a bio-compatible medical adhesive, for instance to repair shattered bones. >>> from Wikipedia: http://en.wikipedia.org/wiki/Sandcastle_worm
Adhesives are primarily bonding materials, but many such substances are also used as thickening and homogenising substances in foods and medicines and as fragrance giving gums. Bio-adhesives consist of a variety of substances, but proteins and carbohydrates form a large section.
Natural adhesives or bio-adhesives are of three broad classes: Plant materials and exudates, such as starches, plant saps and resins, natural rubber, gum arabic, colophons, oils like linseed oil, and wax like carnauba wax, proteins like the soybean, and carbohydrates like starch. Animal products like casein, milk proteins, wax, glue and other gelatinous substances, shellac. Mineral products like Pozzolana and other natural cement products, silicates, asphalt, bitumen pitch. To these can be added, a range of modified natural products, typically various derivatives like a chlorinated rubber, cyclized rubber and rubber hydrochloride.
● Natural adhesives of vegetable origin include plant exudates such as: Gum arabic, colophon; Oils and waxes like linseed oil, carnauba wax; Proteins like the soybean, and carbohydrates like starch.
● Natural adhesives of animal origin include various glues and other gelatinous substances, milk casein, shellac and bees wax.
● Natural adhesive like mineral substances include silicates, Pozzolana, asphalt, bitumen etc.
● Natural elastomers are natural rubbers and their various derivatives like a chlorinated rubber, cyclized rubber and rubber hydrochloride.
● Proteins glues such as a gelatin, and carbohydrates such as starch, have been used as glues by man since very ancient times. Postal stamps have natural wet-able glue despite synthetic alternatives. There are several applications where cooked starch or flour adhesives are used, such as for book binding, corrugated board, paper-bag production, paper tube winding, kite thread colouring, and wallpaper fixing. Casein glue is mainly used to stick labels on bottle labels. Animal glues are traditionally used for leather work, bookbinding and wood joinery.
● Animal Glues: The term animal glue is usually confined to glue prepared from mammalian collagen, the principal protein constituent of skin, bone, and muscle. When treated with acids, alkalies, or hot water, the normally insoluble collagen slowly becomes soluble. If the original protein is pure and the conversion process is mild, the high-molecular weight product is called a gelatin which is used for food or photographic products. The lower-molecular weight material produced by more vigorous processing is normally less pure and darker in colour and is called animal glue. Animal glue traditionally has been used in wood joining, book bindery, sandpaper manufacture, heavy gummed tapes, leather shoes and luggage items and similar applications. Joints with animal glue work well in dry to moderately moist weather, but with high humidity (80 % or more) growth of micro organisms weakens the adhesive. In spite of its advantage of high initial tack (stickiness), animal glue is being replaced by synthetic adhesives.
● Protein Glues: Casein glue is made by dissolving casein, a protein obtained from milk, in an aqueous alkaline solvent. The degree and type of alkali, influences behaviour of the product. In wood bonding, casein-glues generally are superior to animal-glues in moisture resistance and ageing characteristics. Casein also is used to improve the adhering characteristics of paints and coatings (calcimine, distemper coatings). In wood bonding, casein-glues generally are superior to animal-glues for moisture resistance and ageing characteristics. Casein is now getting replaced by urea formaldehyde resins.
● Blood Albumin Glue is made from serum albumin, a blood component obtainable from either fresh animal blood or dried soluble blood powder to which water is added. Addition of an alkali to albumen-water mixtures improves adhesive properties. A considerable quantity of glue products from blood was used in the plywood industry.
● Starch and dextrine: Starch and dextrine, are extracted from corn, wheat, potatoes, tamarind or mango seeds or rice. They constitute the principal types of vegetable adhesives that are, soluble or dispersible in water and obtained from plant sources throughout the world. Starch, and dextrine, glues are used in corrugated board, packaging, and as a wallpaper adhesive.
● Natural Gums: Natural gums, are extracted from their natural sources, also are used as adhesives. Agar-agar, Calcium alginate, a marine-plant colloid (suspension of extremely minute-particles), is extracted by hot water and subsequently frozen for purification. Algin, is obtained by digesting seaweed in alkali and precipitating either the calcium salt or alginic acid. Gum arabic is harvested from acacia trees that are artificially wounded to cause the gum to exude. In India Babool, Neem, provide such gum exudates (Hindi-Gund), some of which are also used in sweets, and Ayurvedic-herbal preparations. Another exudate is natural rubber latex, which is harvested from Hevea trees. Natural gums are used chiefly in water-re-moistenable products.
● Bituminous Adhesives: Bitumen and coal tar derivatives are available as hot melt or softening, emulsion and solvent diluted materials. The hot melt or softening materials have a tendency to run at high temperatures. Bituminous materials are used for fixing waterproofing felt and roof insulation boards.