Panel various wood-based, formaldehyde

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]... 

The higher the molar ratio F/U, the higher is the content of free formaldehyde in the resin. Assuming stable conditions in the resins, which means that, e.g., post-added urea has had enough time to react with the resin, the content of free formaldehyde is very similar even for different manufacturing procedures. The content of formaldehyde in a straight UF resin is approximately 0.1% at F/U = 1.1 and 1% at F/U = 1.8 [19-21]. It also decreases with time due to aging reactions where this formaldehyde reacts further. Table 5 summarizes the various influences of the molar ratio F/U on various properties of wood-based panels. Table 6 summerizes the influence of the molar rations F/U and F/(NH2)2>... 

These ratios then are correlated to various properties of the wood-based panels, e.g., internal bond strength or subsequent formaldehyde emission. Various papers in the literature describe examples of such correlations and present workable predictive equations. For UF resins Ferg [30], Ferg et al. [99,100] for MF resins Mercer and Pizzi [101] for MUF resins Mercer and Pizzi [102], Panamgama and Pizzi [103]. 

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 ... 

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