Production of wood-based

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. 

For the production of wood-based panels various adhesives are in use like aminoplastic resins (UF, MU(P)F), phenolic resins (PF) or isocyanate (PMDI). The proper choice of the adhesive depends on the required properties of the wood-based panels, on the working conditions during the production as well as often on the costs for the adhesive system this not only means the net price of the adhesive but the overall costs of the gluing system including glue spread factor. 

During the production of wood-based panels, part of the adhesive penetrates into the wood surface. An overpenetration causes starved glue lines, whereas a low penetration limits the contact surface between wood and the adhesive low penetration often is the consequence of bad wetting behavior. 

Life cycle analysis and recycling of bonded wood boards also concerns the adhesive resins used, since adhesives and resins are one of the major raw materials in the production of wood-based panels. This includes, for example, the impact of the adhesives on various environmental issues such as waste water and effluent management, noxious gas emission during panel production and from the finished boards, or the reuse of panels to burn for energy generation. Furthermore, for certain recycling processes the type of resin has also a crucial influence on their feasibility and efficiency. 

Usually no hardeners are added during the production of wood-based panels (particle board, MDF, OSB, engineered wood products) using PMDI as adhesive. With special additives a distinct acceleration of the hardening reaction and hence shorter press times or lower press temperatures can be achieved [209]. This fact is especially interesting for coldsetting systems as well as for the production of particleboards. Possible catalysts are tertiary amines (e.g., triethanol amine, triethylamine, A,A-dimethylcyclohexylamine) and metal catalysts, based on organic compounds of tin, lead, cobalt, and mercury [208,215-218],... 

For the production of wood-based panels various adhesives are in use, including aminoplastic resins (UF, MUF, MUPF), phenolic resins (PF), and isocyanate (PMDI). 

Many papers deal with special wood species in the production of wood-based panels, but the total knowledge available on this subject is not really satisfactory. Neusser and coworkers [382,383] are two of the rare examples in the literature giving a broader overview on this aspect they have compared 18 different Austrian wood species by producing and testing laboratory particleboards. The test results obtained allowed adjustments for properties and density of laboratory boards. The best results were found for ash, followed by white beech and oak. However, these results may not be valid for all types of wood and all types of boards. 

Urea-formaldehyde (UF) adhesive resins as the most important type of amino resins were among the first commercially used adhesive systems and still dominate the production of wood based panels [8]. UF resins are based on the reaction of the two monomers urea and formaldehyde they consist of linear or branched oligomeric and polymeric molecules of various molar masses [9, 10]. The degree of condensation is one of the most important characteristics of condensation resins and determines several of their properties the viscosity at a certain solid content increases [11], and flowability and hence the equal distribution of the resin on the furnish (particles, fibers) and the penetration into the wood surface are hindered [12, 13]. 

Even with only the three monomers melamine, formaldehyde, and urea, a variety of different types of resins exists which can fulfill nearly all requirements given in the wood-based panels industry. This field of application involves the production of wood-based panels like particleboards, medium density fiberboard (MDF), oriented strand board (OSB), plywood, blockboards, and others. In rare cases the resins and panels are also used in the fnrnitnre industry. According to the raw materials used, various types of melamine resins are possible ... 

Melamine-based resins with extreme low molar ratios [F/(NH2)2 < 0.5] can be used as so-called formaldehyde scavenger resins (81). They are mixed mainly with UF resins during the application for the production of wood-based panels with low subsequent formaldehyde emission. 

The glues are cheap, easy to apply, and therefore suitable for the mass production of wood-based materials. 

Laminated Strand Products. The most recent developments in the family of wood-based composites are a group of laminated strand products, made with strands oriented in the long direction of the product and marketed as stmctural composite lumber. One product is made with long, narrow strips of softwood veneer. The strips or strands are about 2.5 x 13 x 600 mm (0.1 x 0.5 x 24 in.), coated with a PRE adhesive, and pressed under heat and pressure into large blocks. After the resin is cured the blocks are resawn and planed into lumber dimension stock. 

Shorter press times in a given production line for a certain type of wood-based panel can be attained, in the following ways ... 

The properties of wood-based panels are determined in principle by three influence parameters (1) wood component (2) adhesive (3) production conditions. Only if all three parameters are appropriately considered, can proper bonding results can achieved. 

D. D. Nilsson, T. Imamura, Y. Kerner-Gang, W. Trong, L. Deon, G. Proc. Symp Protection of Wood-Based Composite Products, 1988, 238-266. 

Salthammer, T., A. Schwarz, and F. Fuhrmann, Emission of Reactive Compounds and Secondary Products from Wood-Based Furniture Coating, Atmos. Environ., 33, 75-84 (1999). 

Salthammer, T., Schwarz, A. and Fuhrmann, F. (1999) Emission of reactive compounds and secondary products from wood-based furniture coatings. Atmospheric Environment, 33, 75-84. 

Wood is an important structural material consisting of a cellulosic composition with a highly porous nature. Adhesives are commonly employed to bond wood in the furniture industry. They are increasingly being used in laminating and veneering of wood-based products onto composite panels. Sealants are commonly applied to wood framing members in the construction industry. Several properties are unique to these materials that will affect their ability to be joined. 

Several chapters also demonstrate the use of smaller molecular-weight carbohydrates (i.e., monomers) in adhesives. Tony Conner and his colleagues (Chapter 25) explore the partial replacement of phenol-formaldehyde adhesives used to bond wood with various wood-derived carbohydrates. A1 Christiansen (Chapter 26) and Joe Karchesy and his coworkers (Chapter 27) investigate the very complicated chemistry and the practical application of adhesives based on the reaction of a carbohydrate with urea and phenol. Tito Viswanathan (Chapter 28) describes his attempts to utilize a very large carbohydrate waste stream, whey permeates from the processing of cheese, for the production of wood adhesives. 

Limit of formaldehyde emission of wood-based panels and finishing products... 

The necessity to achieve shorter press times is omnipresent within the woodworking industry, to keep production costs low. An increased production rate gives the chance to reduce production costs. This is only valid when the market is able to absorb such a high level of production. Shorter press times within a given production line and for certain types of wood-based panels can be achieved by, among others ... 

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