Natural glues starch adhesives

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. 

The early adhesives were natural products (e.g. glues, starch, natural resins) but most modern adhesives are based on synthetic polymers (e.g. polyacrylates). In adhesion, two materials come sufficiently dose for strong interaction to occur. The interface is considered as the zone between the interacting substances, which is sometimes referred to as the interphase. 

Because of the hygroscopic nature of starch and fish glues and the wicking effect of wax resins, conservators have been prompted to search further for new materials to function as lining adhesives. Fortunately in the 20th C., polymer science has provided many new raw materials and processes to develop alternate techniques. 

Class II includes flexible macromolecules. They stay only in the states of liquid and solid, in order to reserve the integrity of chemical bonds. Evaporation of such macromolecules requires so high level of thermal energy that the chemical bonds are actually broken before reaching that level. The molecular flexibility in the liquid mainly comes from the internal rotation of the main-chain C-C bonds. This class includes structural materials of synthetic polymers such as Nylon, PVC, PET, and PC, adhesives such as PVA, epoxy resins and Glue 502, elastomers such as natural rubber, polyurethane, SBS and EPDM (mbber could be regarded as the cross-linked liquid polymers.), biomaterials such as celluloses, starch, silks and wools, and even bio-macromolecules such as DNA, RNA and proteins. The class of flexible macromolecules corresponds to the soft matter defined above. 

Water-based dispersions or emulsions such as polyvinyl acetate, acrylics, polyvinyl chloride and polyvinyl alcohol with plasticizers and tackifiers. In addition, this range can include urea formaldehyde and phenolic adhesives, resins, natural adhesives produced from starch, dextrin, casein, animal glues (see Polyvinyl alcohol in adhesives, Phenolic adhesives single-stage resoles. Phenolic adhesives two-stage novolacs. Animal glues and technical gelatins) and rubber latex (see Emulsion and dispersion adhesives). Solvent-free 100% solids such as polyurethane. Hot melt adhesives include Ethylene-vinyl acetate copolymers, polyolefins, polyamides, polyesters with tackifiers and waxes. More recent additions include cross-linkable systems. 

As a preservative for industrial fluids, formaldehyde (formalin and paraformaldehyde) is preferably used in cosmetic products and in polymer dispersions (natural and synthetic latex) often in combination with other microbicides. Formaldehyde is listed in the EC list of preservatives allowed for the in-can protection of cosmetics (maximum authorized concentration 0.2%, except for products for oral hygiene 0.1%, prohibited in aerosol dispensers). Percentage of use in US cosmetic formulations 0.11% paraformaldehyde. Oil, fat and wax emulsions, starch and dextrine glues, adhesive dispersions, pigment and filler slurries, thickening solutions and other aqueous formulations containing no proteins can be preserved by the addition of formaldehyde, too in order to... 

Early adhesives were based on natural products, such as starch, animal glue, natural rubber, etc. Modern adhesives are synthetic materials based on resins, synthetic polymers, epoxides, urethanes, etc. 

Solutions of natural and synthetic high molecular mass substances in water, e.g., starch, dextrins, casein, cellulose ethers, water-soluble derivatives of poly(acrylic acid), poly(vinyl alcohol), poly(vinyl pyrrolidone) (adhesive sticks). Uses paper, fiberboard. Glutins (glues of animal origin). Uses wood, paper, fiberboard, moistenable adhesive tapes. 

For example, starch is the principal water-dispersible natural polymer used industrially as an adhesive and in its many forms it is widely employed in the paper and paper products industry. Poly(vinyl acetate) and its copolymers are synthetic materials and are usually used as aqueous latices. They have found widespread used in the paper industry, where their often superior properties offset their higher price, and are used in bookbinding, milk cartons, envelopes, etc. and are also very familiar as the household white glue for woodworking. 

Although natural adhesives (animal glue, casein, starch, and rosin) are still used for many applications, a host of new adhesive materials based on synthetic polymers have been developed these include polyurethanes, polysiloxanes (silicones), epoxies, polyimides, acrylics, and rubber materials. Adhesives may be used to join a large variety of materials—metals, ceramics, polymers, composites, skin, and so on—and the choice of which adhesive to use will depend on such factors as (1) the materials to be bonded and their porosities (2) the required adhesive properties (i.e., whether the bond is to be temporary or permanent) (3) maximum/minimum exposme temperatmes and (4) processing conditions. 

About 1750, the first glue patent was issued in Britain for a fish glue. Patents were then rapidly issued for adhesives using natural rubber, animal bones, fish, starch, and milk protein (casein). By 1900, the United States had a number of factories producing glue from the aforementioned bases. 

Adhesives of natural origin were mainly used prior to the beginning of the 20th century and by early civilizations as long ago as 2000 BC. These included animal glue, casein, natural rubber and starches. Today, specially developed adhesives based on semi- or fully synthetic products are used for a wide variety of bonding applications (Fig. 8-1). 

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