Natural glues casein glue

At the same time, natural adhesives such as casein glues, animal glue and polysaccharide gums have gradually been replaced by synthetic adhesives vinyl thermoplastic adhesives [poly(vinyl acetate)], adhesives obtained by reticulation in situ of two components (as epoxy resins), represent very important materials in this field. 

Consolidation by organic materials. More commonly, organic polymers have been applied for consolidation of paint layers. Natural polymers were most frequently applied in Europe, including animal glue, casein, egg yolk... 

The viscosity and consistency of casein glues can be altered substantially by reaction with most of the classic protein denaturants such as sulfur compounds, formaldehyde donors, and complexing metal salts [6,56]. One or more of these are frequently used as manufacturing control to offset the natural variability of casein and produce glues of uniform properties. The water resistance of cured casein glues is also improved by moderate denaturing. 

Adhesives as materials can be classified in a number of ways such as chemical structure or functionality. In this book, adhesives have been classified into two main classes natural and synthetic. The natural group includes animal glue, casein- and protein-based adhesives, and natural rubber adhesives. The synthetic group has been further divided into two main groups industrial and special compounds. Industrial compounds include acrylics, epoxies, silicones, etc. An example of the specialty group is pressure-sensitive adhesives. 

Today for the assembly gluing of wood, for gluing veneers, plastic sheets, and films, and for the manufacture of wood-based materials (chipboard, plywood, hardboard, profiles), synthetic adhesives are used almost exclusively. The traditional use of adhesives based on natural products (glutin and casein glues) is confined to a few special cases only (for example, violin making). 

Non-metallic. Wood—this material has already been discussed in connection with structural adhesives. Where both loads and conditions are less demanding, the available adhesives multiply. Polychloroprene and polyvinyl acetate, the latter as an aqueous emulsion, are widely used, the former accommodating a wider range of acceptable substrates when joining wood to some other material. Casein glues can be formulated to give water resistance and are stiU used for the manufacture of plywood, web-strengthened flat doors, and beams for interior use. Natural rubber latex forms with casein a valuable adhesive with... 

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. 

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). 

Natural products (animal glue, starch, soya, blood glue, casein, cellulose derivatives)... 

Over the centuries the natural organic materials that have best met these requirements are proteinaceous materials (egg, animal glue and casein or milk), polysaccharide gums... 

The major synthetic adhesives used for bonding wood include urea, phenol, and melamine formaldehyde resorcinol formaldehyde, phenol resorcinol, and polyvinyl acetate emulsions. More recently one-component, moisture cured polyurethane adhesives have become popular for bonding wood. Natural adhesives such as casein and animal glues are also often used for general-purpose wood bonding. Epoxies have been used for certain specialized wood joining applications such as when wood is bonded to metal substrates. 

Remarkably, gelatin was not the only substrate explored for use with dichromates egg albumen, agar, casein, fish glue, shellac, and starch have all been tried and used. In modem times, synthetic polymers such as poly(vinyl alcohol), poly(vinyl pyrrolidone), and poly(vinyl butyral) have, to a significant extent, displaced the above natural materials for use with dichromates only gelatin and albumen have remained in use until quite recently. A typical modern recipe for dichromated albumen is given in Table 6.1. 

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. 

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. 

Casein is the main protein of milk. Salzberg notes that the adhesive nature of the casein curd was recognized in ancient Egypt. It has been identified as the glue in ancient museum pieces and in wood backs and frames holding price-... 

Natural animal (beeswax, casein), vegetable (gum, wax, dextrin, starch) and mineral- (amber, paraffin, asphalt) based glues. Commonly low strength applications such as paper, cardboard (packaging) and wood. 

Casein, animal and fish by-products, blood and soy are all used to make glues. Natural macromolecules in their formulations - long protein chains - perform the same role of polymers in synthetic adhesives. However, the proteins have to be processed, dried, and compounded for the intended application. Specific hydrolysis reactions, at controlled pH and temperature, denature the proteins and change their three-dimensional structure to adjust the molecular weights. 

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