Wood-based panel products, adhesives

Progress in research and development in the wood-based industry and in the adhesive industry has shown many successes during the last decades. On the other hand, many industrial requirements still require considerable and important developments in this area. The main driving forces today are cheaper , quicker and more complex . The first two are caused by the heightened competition in the above-mentioned industries and the attempt to minimize costs while maintaining a certain level of product quality and performance. The key word more complex stands for new and specialized products and processes. Adhesives play a central role in wood-based panel production. The quality of bonding, and hence the properties of the wood-based panels, are determined mainly by the type and quality of the adhesive. Development in wood-based panels, therefore, is always linked to development in adhesives and resins. 

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. 

Wood-based panel products are usually bonded with synthetic adhesives based on condensates of phenol, resorcinol, urea, or melamine with formaldehyde. Particleboards and fiberboards can also be bonded with mineral binders like cement or gypsum. Wood adhesives derived from natural products have more... 

Lines of demarcation between hardwood plywood, softwood plywood and certain other wood based panel products have become less distinct in recent years. One of the most important distinctions in respect to formaldehyde emission potential is that softwood plywood is typically bonded with phenol-formaldehyde while hardwood plywood is typically bonded with urea-formaldehyde. Phenol-formaldehyde adhesives are more stable and have less tendency to emit formaldehyde than do urea-formaldehyde adhesives. Some important features of hardwood plywood ... 

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

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. 

Adhesives and resins are one of the most important raw materials in wood-based panels. Thus, each question concerning the life cycle assessment and the recycling of bonded wood panels does bring into question the adhesive resins used. This includes, for example, the impact of the resin on various environmental aspects such as waste water and effluents, emission of noxious volatile chemicals during production and from the finished boards, or the reuse for energy generation of wood panels. The type of resin has also a crucial influence on feasibility and efficiency for several material recycling processes. 

Emission of volatile noxious chemicals from wood-based panels during their production can be caused by chemicals inherent to wood itself, like terpenes or free acids, as well as by volatile compounds and residual monomers of the adhesive. The emission of formaldehyde as well as free phenol effluents is a matter of concern. 

Adhesives based on isocyanate (especially PMDl, polymethylene diisocyanate, more exactly polymeric 4,4 -diphenylmethane diisocyanate) have been used for more than 25 years in the wood-based panel industry [88], but still have a low market value in the wood-working industry compared to systems based on UF-, MUF- or PF-resins. The main application is the production of waterproof panels, but also the production of panels from raw materials that are difficult to glue, like straw, bagasse, rice shells or sugar cane bagasse. They can be used as adhesives for wood-based products like particleboard, oriented strandboard (OSB), laminated strand lumber (LSL), medium-density fiberboard (MDF) or... 

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. 

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. 

Table 2 summarizes the general parameters of importance for wood adhesives. Research and development in adhesives and resins are mainly driven by the requirements of the bonding and production processes and by the intended properties of the wood-based panels. These requirements are summarized in Table 3. 

The various aminoplastic resins are the most important class of adhesives in the wood-based panels industry, especially for the production of particleboards and medium density fibreboard (MDF), and partly also for oriented strandboard (OSB), plywood, block-boards, and some other types of wood panels. They are also used in the furniture industry as well as in carpenters shops. 

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],... 

The main parameter for the application of tannins as adhesives for wood-based panels is the content of reactive polyphenols and the reactivity of these components towards formaldehyde. Tannins can be used as adhesives alone (with a formaldehyde component as crosslinker) or in combination with aminoplastic or phenolic resins. These resins can react chemically with the tannin component in a polycondensation reaction, form only two interpenetrating networks, or both. The simplest adhesive mix formulation consists of the tannin solution and powdered paraformaldehyde as crosslinker [283]. The addition of paraformaldehyde can cause in the short term a relatively high level of formaldehyde emission. Glue mixes using paraformaldehyde for the production of particleboards with low formaldehyde emission are described and used industrially [284]. In the literature a large number of papers describe the combinations of tannins with synthetic resins (Table 14). 

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]. 

This paper is an overview of European standards dealing with adhesives for wood-based products. No standard exists in Europe for the assessment of adhesives for wood-based panels except for the evaluation of the bonding queility of plywood panels. However, many standards exist and are described here for the assessment of adhesives for stmctiual purposes such as glulam beams or non-structural purposes such as glued components for joinery. 

The current European standards for wood-based panels deal with evaluation of properties of end-products such as internal bond strength or bending strength. In case of plywood, the bonding quality can be evaluated by tensile shear testing according to EN 314-1 (2005) [1]. These properties are fully linked to adhesives used and process parameters. Nevertheless, no standard exists in Europe for the assessment of adhesives for wood-based panels. 

For wood-based panels, European standards are focused on general properties of end-products. The only standard really linked to adhesion is EN 314-1 (2005) [1] which defines a test method for evaluation of the bonding quality of plywood panels. 

A considerable variety of thermosetting and thermoplastic resins are used to bond a multitude of wood products. Today, at least 80% of wood products are bonded in some manner. The quality of wood bonding is determined by three main parameters (1) the wood, especially the wood surface, including the interface between the wood surface and the bond-line (2) the applied adhesive (3) the working conditions and process parameters. Good-quahty bonding and adequate properties of the wood-based panels can be attained only if each of these three parameters contributes to the necessary... 

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. 

---