Wood-Based Panels

Wood-based panels such as particleboard, oriented strand board and medium density fibreboard eonsist of particles of widely varying shape and size bonded together with an adhesive system. 

The variable shapes and sizes of the wood elements contribute significantly to the particular properties in any panel and in the way it can be used. These range from veneer sheets for plywood and LVL (Chapter 11), to individual fibres and fibre fragments used in fibreboards. The shape of the particles has become increasingly important as producers seek to maximize product properties while minimizing 

Over the last 20 years, environmental concerns have become an important consideration in adhesive formulation and use. Firstly, in the plant the adhesive may require particular handling or the use of protective equipment. Secondly, volatile emissions arising from adhesive reactions both in the hot press and subsequently when the panels are in service are subject to tight regulatory control. In particular the reduction formaldehyde emissions from wood-based panels has been a major objective in adhesive development over the last few years, both because formaldehyde-based adhesives are the major type used and because these have been implicated in environmental and health concerns. 

For plywood and LVL the objective in pressing the panel is to create only enough pressure necessary to form good bonds. However, the average density of the panel in the cases of OSB, particleboard and MDF is well above that of the wood from which the particles are derived. In the case of these panels, strength properties are as much 

The method of assembly of the particles ahead of the press can also be used to manipulate the properties of the composite. For example, it is possible to use different particles (or adhesive systems) in different layers of the panel. Indeed, this approach allows composites with totally different structures in different layers of the panel to be produced. Such a product is Triboard produced by Juken Nissho in New Zealand, where a product with MDF faces and a strand core is produced in a single pressing operation. 

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Epoxy resins are also used in special appHcations, such as an overlaying procedure requiring a durable, heat-resistant bond of a difficult-to-bond overlay on a wood-base panel substrate. Metal sheets used as overlays, for example, often require an epoxy adhesive. 

Spelter, H., McKeever, D. and Durbak, 1., Review of Wood-Based Panel Sector in United States and Canada, General Tech. Rpt. FPL-GTR-99, U.S. Department of Agriculture, Forest Service, Madison, Wl, 1997. 

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. 

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. 

The quality of bonding and hence the properties and the performanee of the wood-based panels and beams are determined by three main parameters (1) the wood, espeeially the wood surface, including the interface between the wood surface and the bond line (2) the adhesive (3) the working conditions and process parameters. 

Good-quality bonding and sufficient properties of the wood-based panels can be attained only if each of these three parameters contributes to the necessary extent to the bonding and production process. 

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

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. 

Actual regulations cone, the subsequent formaldehyde emission from wood-based panels (Germany) according to the German Regulation of Prohibition of Chemicals (former Regulation of Hazardous Substances)... 

Molar ratios F/U and F/(NHt)2, respectively, of pure and melamine-fortified UF-resins currently in use in the wood-based panels industry... 

The deterioration of a bond line can occur due to (1) the failure of the resin (low hydrolysis resistance, degradation of the hardened resin causing loss of bonding strength) (2) the failure of the interface between resin and wood surface (replacement of secondary forces between resin and reactive wood surface sites by water or other non-resin chemicals) (3) the breaking of bonds due to mechanical forces and stresses (the influence of water will cause swelling and therefore movement of the structural components of the wood-based panels). 

Correlations between the composition of aminoplastic resin and the properties of wood-based panels... 

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]... 

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... 

PMDI is produced on an industrial scale by the phosgenation of diamin-odiphenylmethane. Structure and molar mass of PMDI depend on the number of aromatic rings in the molecule. For PMDI the distribution of the three monomeric isomers has a great influence on the quality, because the reactivities of the various isomers (4,4 -, 2,4 - and 2,2 -MDI) differ significantly. The greater the portion of the 2,2 - and 2,4 -isomers, the lower is the reactivity. This can lead to different bonding strengths as well as to residual isomers in the produced wood-based panels. 

Usually no hardeners are added during the produetion of wood-based panels (PB, MDF, OSB, engineered wood produets). With speeial additives, an aecel-eration of the hardening reaction and hence shorter press times or lower press temperatures ean be aehieved. This is espeeially interesting for eold-setting systems as well as for the produetion of partieleboards. Possible catalysts are those traditionally used for the promotion of urethane formation [17,143],... 

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. 

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. 

Hence the wood, especially the wood surface (including the interface to the bond line), plays an crucial role for the quality of bonding and hence for the quality of the wood-based panels. Low or even no bonding strength can be caused by unfavorable properties of the wood surface, e.g. due to low wettability. 

The strength of a bond increases with the wood density in the region of approx. 0.7 to 0.8 g/cm Above this density, a decrease of the bond strength occurs. Performance and properties of wood-based panels are strongly influenced by the properties of the used wood. The anisotropy as well as the heterogeneity, the variability of various properties and the hygroscopicity have to be taken into account. Also the orientation of the wood fibers bonding solid wood has to be considered. 

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. 

European Norm EN 120, Wood-based panels. Determination of formaldehyde content. Perforator extraction method, 1995. 

Driljie, R.M., FAO-Bericht World Consultation on Wood Based Panels, New Dehli, 1975. 

Forest products industries know that temperature increases in piles of sawdust and bark. In pulp and paper mills, self-heating develops in amassed tree chips. Paper rolls stacked hot tend to self-heat, as occasionally do stored bales of waste paper. The wood-base panel products particleboard, hardboard, and fiberboard self-heat after being stacked too hot in the factory. Where in structures the framing lumber, wood-base panels, and lignocellulosic insulation is heated by items such as steam pipes, temperatures tend to rise above that of the heat source. 

Lignocellulosics reach temperatures around 80 C in many ways besides. For example, planer shavings and peat insulation around hot pipes or in walls of dry kilns easily attain 100 C, and forest product industries stack still hot wood-base panels at temperatures around 80 C. In many cases the temperatures of the hot materials later on rose above the initial 80 or 100"C, first to levels of smoldering combustion, and finally to those of open flames. In air-exposed and ventilated materials oxidation could cause the heating above 80 or 100 C, but inside tight packs of panels pyrolysis must have been the heat source. 

Standard practice for the determination of VOCs (excluding formaldehyde) emissions from wood-based panels using small environmental chambers under defined test conditions... 

CEN EN 120 Wood-based panels. Determination of the formaldehyde content. Extraction method called the Perforator method . ... 

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