Natural product adhesives

Adhesives made from Natural Products. The first adhesives developed by humans were based on naturally available materials such as bone, blood, milk, minerals, and vegetable matter. Beginning with the commercial development of Baekeland s phenolic resin adhesives by the General Bakelite Co. around 1910, synthetic adhesives began to replace natural product adhesives for existing applications. The use of adhesives by industry began to grow and diversify over the... 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Health and safety information is available from the manufacturer of every adhesive sold in the United States. The toxicology of a particular adhesive is dependent upon its components, which mn the gamut of polymeric materials from natural products which often exhibit low toxicity to isocyanates which can cause severe allergic reactions. Toxicological information may be found in articles discussing the manufacture of the specific chemical compounds that comprise the adhesives. 

Terpene-based hydrocarbon resins are typically based on natural products such as a-pinene, P-pinene, and ti-limonene [5989-27-5] which are obtained from the wood and citms industries, respectively. These resins, which were originally the preferred tackifiers for natural mbber appHcations, possess similar properties to aHphatic petroleum resins, which were developed later. Terpene-based resins have been available since the mid-1930s and are primarily used in the adhesives industry. 

Wood (qv) is arguably the oldest building material used by humans to constmct their dweUings. It is a natural product obtained from trees, used in both stmctural and decorative appHcations. The chemical composition of wood is largely cellulose (qv) and lignin (qv). Today there are a variety of composite or reconstituted wood products, such as plywood, particle board, wood fiber boards, and laminated stmctural beams, where small pieces of wood or wood fiber are combined with adhesives to make larger sheets or boards (see Laminates). 

Adhesives. Clays, especially kaolin and attapulgite, are widely used in various adhesive formulations. Adhesives (qv) containing clays can be derived from natural products such as starch or protein, or be whoUy synthetic, eg, latex, hot melt, emulsion, etc. 

Pyridine is a polar, stable, relatively unreactive liquid (bp 115°C) with a characteristic strong penetrating odor that is unpleasant to most people. It is miscible with both water and organic solvents. Pyridine was first isolated, like pyrrole, from bone pyrolysates. Its name is derived from the Greek for fire (pyr) and the suffix idine used to designate aromatic bases. Pyridine is used as a solvent, in addition to many other uses including products such as pharmaceuticals, vitamins, food flavorings, paints, dyes, rubber products, adhesives, insecticides, and herbicides. Pyridine can also be formed from the breakdown of many natural materials in the environment. 

Closure liners of pulpboard or cork, unless specially treated with a preservative, foil or wax coating, are often a source of mould contamination for liquid or semi-solid products. A closure with a plastic flowed-in linear is less prone to introduce or support microbial growth than one stuck in with an adhesive, particularly if the latter is based on a natural product such as casein. If required, closures can be sterilized by either formaldehyde or ethylene oxide gas. 

Whereas the range of uses to which resin exudates and tar-pitch extractives has been emphasized, some of these functions could have been plausibly served by a range of other natural products. For example, knowledge of other prehistoric adhesives could be biased by the poor survival of protein-based (such as fish, bone and horn) glues. However, it should be remembered that these substances... 

Many seemingly simple adhesive applications are actually complex. The labels on commercial dry cell batteries can contain over a dozen layers each present for a specific purpose. Although price is a major consideration, ease of application is another. Thus, while many naturally derived adhesives are less expensive, synthetic materials may be chosen because of ready application and consistency of the end product. 

Natural-product-based structural adhesives include protein-based adhesives, starch-based adhesives, and cellulosics. 

Enzymatic development of functional polymers for application as coating or adhesive materials, for example, has attracted much scientific interest over the years. A number of natural products ranging from plant to animal polymers are being actively investigated. In this section, the ability of heme peroxidases to mediate the production of such polymers is revised. 

The objective of this symposium was to include research on a broad range of natural products directed to a wide variety of bonding applications. The speakers described research on adhesive polymers derived from lignins, tannins, carbohydrates, terpenes, and proteins for applications as diverse as tire-cord bonding and eye surgery. 

Wood-based panel products are usually bonded with synthetic adhesives based on condensates of phenol, resorcinol, urea, or melamine with formaldehyde. Particleboards and fiberboards can also be bonded with mineral binders like cement or gypsum. Wood adhesives derived from natural products have more... 

Formulations for particleboard and plywood adhesives based on combinations of diisocyanates and compounds from renewable resources like tannins, starch, and proteins have been developed and tested at the Fraunhofer-Institute. All in all, the results of gluing tests indicated the potential for using diisocyanates combined with natural polymers for adhesive purposes where each natural product used alone will fail. More attempts will be necessary to find precise mixtures to produce successful adhesives for panel products. The development of adequate particleboard and plywood glue formulations based on diisocyanates and natural polymers has to be accompanied by more sophisticated technologies than those used in conventional production processes. The results presented here may give some hints on how these technologies can evolve. 

Another, and perhaps the most important, obstacle to increased use of adhesive polymers based on natural products relates to our inadequate knowlege of the fundamental chemistry of these systems. A large body of empirical formulation work is generally required in the development of new adhesives, and success often rests on how well that work is done. However, knowledge of the structures and reactions of these polymers is just as important and usually re-... 

Hot-melt adhesives are usually used for box-sealing applications for coated paperboard products. Migration of mineral hydrocarbons from these adhesives into foods was investigated in a survey. No mineral hydrocarbons were detected but several other substances were present in the adhesives. These were proposed to be natural resins such as polyterpene resins, tall oil rosin esters, and sterol-like natural products.Examples of each of these natural products are described in ref. 13 as being used for hot-melt adhesives. 

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