Natural adhesives resins

Tannin adhesives are the class of modified natural adhesives which are used and have been used industrially for more than 25 years, mainly in South Africa and Australia, but also in Zimbabwe, Chile, Argentina, Brazil and New Zealand. It is mainly the still limited raw material supply that limits their use to approximately 30,000 tons resin solids per year [16,17]. 

At the same time, natural adhesives such as casein glues, animal glue and polysaccharide gums have gradually been replaced by synthetic adhesives vinyl thermoplastic adhesives [poly(vinyl acetate)], adhesives obtained by reticulation in situ of two components (as epoxy resins),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

Certain resinous materials that act as plasticizers are well noted for increasing the tack of the formulation. Traditional tackifiers were based on naturally occurring resins such as pine tar. Today, tackifiers used in modem adhesive formulations include aliphatic and aromatic... 

A similar story of technical development, raw material cost reduction, and adhesive optimization can also be told for the amino resins, the urea and melamine polymers. Especially because of their versatile hot- and cold-curing capabilities, this development also led to the widespread replacement of natural adhesives. The rapid postwar growth of amino resins, along with phenolics... 

Thus, the picture emerges of a multifaceted wood products industry founded largely on natural adhesives but successfully weaned onto synthetic resins by a combination of low material costs and formerly unattainable performance properties. As a result of this improved performance, the development of other useful bonded wood products was heavily stimulated. 

Monosaccharides (e.g., glucose), disaccharides (e.g., sucrose), and oligosaccharides can be obtained readily from natural sources, either directly or by hydrolysis of natural carbohydrate polymers. These can be used to either modify synthetic adhesive resins or to replace them altogether. In addition, reactive derivatives could be synthesized from these compounds and used to formulate adhesive polymers. 

Another problem of a seemingly unrelated nature is the formaldehyde emission from building boards that are currently manufactured using urea-formaldehyde adhesive resins. Formaldehyde has been implicated as a carcinogen and can also cause severe upper respiratory problems and contact dermatitis in some individuals (3-5). 

The first three chapters deal with particleboard, medium density fiberboard, hardwood plywood, and softwood plywood, the four most widely used wood panel products. Chapter four compares these products with other consumer products. Chapters five through seven explain the basic chemistry of formaldehyde with cellulose and wood components and provide a current understanding of the nature of liquid urea-formaldehyde adhesive resins. The next two chapters present new analytical methods that might become useful in the future. Chapters eight and eleven through sixteen explain the complex nature of the latent formaldehyde present in the products and its correlation to formaldehyde emission from wood products. Chapters fifteen and sixteen describe currently popular formaldehyde reduction methods. The last two chapters discuss the problems involved in reducing formaldehyde emission by regulating air levels or source emissions. 

Another possibility for adhesives to be classified is the differentiation between organic compounds of natural products, so-called natural adhesives, and products resulting from targeted chemical reactions, so-called synthetic adhesives. Many substances are known from daily life, which show a natural tack, for example, tree resins, plant juices, waxes, proteins, gelatine, casein, starch. In comparison to synthetically produced adhesives they are heavily lagging behind in terms of quantity, however, they partly show excellent properties in special applications, such as casein adhesives for bottle labelling. 

Natural phenolic compounds are used as both replacements for substantial portions of synthetic phenol in plywood adhesive resins and as glue mix additives to improve performance 4 to 6% is added, based on phenolic resin solids. They bring about improvements in assembly time tolerance and flow with no significant change in adhesion. Glue mix additions of wattle tannin or other condensed flavonoid tannin extracts with or... 

UF, urea-formaldehyde resin MUF, melamine fortified UF resin MF/MUF, melamine and melamine-urea resins (MF resins are only used mixed/coreacted with UF resins MUPF, melamine-urea-phenol-formaldehyde resin PF/PUF, phenol and phenol-urea-formaldehyde resin (P)RF, resoreinol-(phenol-)formaldehyde resin PMDI, polymeric methylenediisocyanate PVAc, polyvinylacetate adhesive old nat.adhesives, old (historic) natural adhesives (e.g., starch, glutin, casein adhesives) nat.adhesives, natural adhesives (e.g., tannins, lignins, carbohydrates) inorg.adhesives, inorganic adhesives (e.g., cement, gypsum) activation activation constituents of wood to function as adhesives (i.e., lignin). 

Using different spectroscopic methods such as infrared (IR), H-NMR, C-NMR, or N-NMR, analysis of the adhesive structural compounds enables a deep insight into the structural composition of resins. These results are the basis for correlations of resin structural composition with their molar composition, their preparation procedure, and the properties of the panels produced and hence to development and production of tailor-made resins. Extensive information is available on the basic nature of resins and on the content of the various structural elements, including, e.g., data concerning the type of bridges between the monomers or the degree of branching. 

Obviously, the rheological performance of fumed silica in adhesives, resins, and paints is mainly determined by the stability of the colloidal network. This raises the question of which parameters influence the formation and stability of a colloidal silica network. In order to answer this question we have to understand a) the nature of interactions in silica/resin/solvent mixtures in terms of the silica-silica, silica-resin, and silica-solvent interactions, and b) how these interactions influence the rheological behavior of the mixtures. From this information we should be able to develop a comprehensive model that explains the rheological performance of different grades of fumed silica in different resin types. 

Condensed tannins constitute more than 90 per cent of the total world production of commercial tannins (200000 tons per year) [11]. Their high reactivity towards aldehydes and other reagents renders them both chemically and economically more interesting for the preparation of adhesives, resins and other applications apart from leather tanning. The main commercial species, such as mimosa and quebracho, also yield excellent heavy duty leather. Condensed tannins and their flavonoid precursors are known for their wide distribution in nature and particularly for their substantial concentration in the wood and bark of various trees. These include various Acacia (wattle or mimosa bark extract), Schinopsis (quebracho wood extract), Tsuga (hemlock bark extract), Rhus (sumach extract) species, and various Pinus bark extract species, from which commercial tannin extracts are manufactured. 

In addition to those given in the context of adhesively bonded joints, the following aspects, arising from the very nature of thermosetting polymers, should be recognised, especially when the adhesive/resin layer is thick ... 

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. 

Guayule bagasse has shown promise in the manufacture of particleboard and ceiling tiles, especially because it contains sufficient low molecular weight rubber and resin components to provide a natural adhesive. Additional potential uses for guayule bagasse and fiber, before or after resin extraction, include their use in building materials (108,133,134). 

Adhesive Coating of Paper and Board. Pressure-sensitive coatings, for example, for self-adhesive labels and envelopes include natural rubber-resin combinations in solvents, polyacrylate emulsions, and hot-melt adhesives. Polyacrylate emulsions and hot-melt adhesives are becoming increasingly significant because they are solvent-free. 

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