Wood adhesives -basics

The water resistance of the dry adhesive film obtained from emulsion adhesives, while fair, tends to be adversely affected by the presence of small residual quantities of protective colloids that are water soluble. They are used as Adhesives in the textile industry and for bookbinding and woodworking (Wood adhesives - basics). 

Reference shonld also be made to Thermal spray coatings used for adhesion and Wood adhesives - basics. 

For a general introduction, see Wood adhesives - basics. Aminoplastic resins are polycondensation products of the reaction of aldehydes with compounds carrying amine or amide groups. Formaldehyde is by far the primary aldehyde used. The name amino-plastic is traditional, but is a misnomer these are thermosetting, irreversibly hardening resins. Two main classes of these resins exist urea-formaldehyde (UF) resins and melamine-formaldehyde (MF) resins. Today, for wood adhesives, pure MF resins are not used anymore because of cost, and only melamine-urea-formaldehyde (MUF) adhesives are used. The main use of these resins is in composite products such as particle board, plywood, medium density fibreboard (MDF), oriented strand board (OSB), glu-lam/flngerjointing and furniture. 

An overview of wood adhesives is to be found in Wood adhesives - basics. Here, Hot-melt adhesives are considered. They are used in wood mainly for edge veneering and... 

Adhesive bonding of wood is very well established see Wood adhesives - basics. A relatively novel joining technique is discussed here. Thermoplastic welding techniques, which are widely used in the plastic and car industries, have recently been applied also to joining wood, by melting a thermoplastic polymer between the two wood surfaces to be joined. A variety of techniques such as uldasound, mechanical friction and others have been used to melt the thermoplastic polymer in situ. 

Adhesion between the phases is inevitably a vital aspect of all Composite materials. This article is concerned with adhesives in wood-based composite boards, which provide economic and effective ways of using a natural material. Other articles referring to these materials are Wood adhesives - basics and cross-references therein. 

Finally, a considerable amount of work has been devoted to the development of wood adhesives consisting of resins in which furfural is used in conjunction with phenol and formaldehyde or in which furfural-based diamines and diisocyanates were the basic components [27, 28]. 

D-0905. Test Method for Strength Properties of Adhesive Bonds in shear by Compression Loading (Fig. 8). A basic compression shear test which is widely used in screening wood glues. It is also used in specifications D-3110 (on nonstructural wood laminating ad hesives), D-2559 and D-3024 (on exterior and interior structural wood laminating adhesives respectively) and D-4317 (on polyvinyl acetate based wood adhesives). 

Phenolic materials go back to the patents of Baekeland and are based upon the reaction of phenol and formaldehyde to yield a lower molecular weight resinous material that can be formulated with other resins and sometimes other curatives to yield a paste adhesive or a film adhesive. When phenol and formaldehyde are reacted in the presence of an acidic catalyst with an excess of phenol versus formaldehyde, they yield what are known as novolac resins. These materials are soluble in organic solvents and do not react further with themselves thus, they need to have a curative added. That curative is most often hexamethylene tetraamine. Another type of phenolic resin can be generated with an excess of formaldehyde and under basic conditions. These materials, known as resole phenolics, will react with themselves to yield a ftilly cured phenolic and thus must be stored frozen in order to limit this reaction. Thus, resole phenolics do not need an external CTOsslinker. Resole phenolics are widely used in the binding of paper products and the bonding of wood. Adhesives based upon these materials were brittle and could only be used to bond wood, where it is still widely used. A more widely usable adhesive was developed during World War II which modified the very brittle phenolic adhesive with poly(vinyl formal) resins. These materials were... 

Urea resin adhesives, by the use of the proper hardener, may be set either by heat or at room temperature. For room temperature curing, the hardener may be ammonium chloride, together with basic materials like calcium phosphate to neutralize excess acid that might damage the wood. Cold set or room temperature set adhesives are those that set satisfactorily at 20 —30°C, whereas a hot set adhesive generally means one that is set above 99 °C. 

Because of the need for basic initiators, cyanoacrylate adhesives do not perform well on acidic surfaces, such as wood. However, the addition of sequestering agents, such as crown ethers [30], 10, or calixarenes [31], 11, and others [32] to the adhesive improves the reactivity of the adhesive on less active surfaces. 

Both the wood-based panel industry and the adhesive industry show a high commitment to and great capability towards innovation. The best evidence for this is the considerable diversity of types of adhesives used for the production of wood-based panels. Well-known basic chemicals have been used for a long time for the production of the adhesives and their resins, the most important ones being formaldehyde, urea, melamine, phenol, resorcinol and isocyanate. The greater portion of the currently used adhesive resins and adhesives for wood-based panels is produced with these few raw materials. The how to cook the resins and the how to formulate the adhesive become more and more complicated and sophisticated and are key factors to meet today s requirements of the wood-based panel industry. 

Only a small amount of work has been done up to now concerning the prediction of bond strengths and other properties based on the results of the analysis of the resin. Ferg et al. [59] worked out correlation equations evaluating the chemical structures in various UF-resins with different F/U molar ratios and different types of preparation on the one hand and the achievable internal bond as well as the subsequent formaldehyde emission on the other hand. These equations are valid only for well defined series of resins. The basic aim of such experiments is the prediction of the properties of the wood-based panels based on the composition and the properties of the resins used. For this purpose various structural components are determined by means of - C NMR and their ratios related to board results. Various papers in the chemical literature describe examples of such correlations, in particular for UF, MF, MUF and PF resins [59-62]. For example one type of equation correlating the dry internal bond (IB) strength (tensile strength perpendicular to the plane of the panel) of a particleboard bonded with PF adhesive resins is as follows [17]... 

Phenol-formaldehyde resins find numerous applications in such areas as wood composites, fiber bonding, laminates, foundry resins, abrasives, friction and molding materials, coatings and adhesives, and flame retardants (JL). From a specialty chemicals standpoint, they are also used as developer resins in carbonless papers (2.). Conventional methods of preparation involve condensation of a phenol with formaldehyde under either acidic (novolak) or basic (resole) conditions (2). Their typical molecular weight range is from 800-4000 daltons (D) and includes a wide variety of alkyl or aryl substituted phenols (A)- The... 

The variety of wood products has increased enormously over the past decades as more adhesives have been found for bonding. The most important products in terms of volume are plywood, particle board, and fiber board. But adhesively bonded products range from tiny articles of jewelry to giant laminated timbers spanning hundreds of feet (5). The modification of adhesive properties to suit the different application requirements requires a sound understanding of the basic chemistry of adhesives. 

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. 

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