Лако-красочные материалы — производство

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 PF resin. Correlation equations evaluating the chemical structures in various PF resins with differ­ent formaldehyde/phenol 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 have been developed [63]. These equations are valid only for well defined series of resins. The basic aim of such equations is the prediction of the properties of the wood — based panels on the basis of the composition and the properties of the resins used. For this purpose various structural components of the liquid resin are determined by means of 13C nuclear magnetic resonance (NMR) and their ratios related to board results, so to the strength results of the hardened resin. Various papers in the chemical literature describe examples of such correlations, in particular for UF, MF, MUF, and PF resins [63-67]. 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 [33,63]

Resin cross-linking « IB = aA/(A + B + C) + b Mo/(A + B + C)

+ cMe / (A + B + C) (1)

where the IB strength is expressed in MPa; A is the sum of the peak areas of phenolic ortho and para sites still free to react (110 to 122 ppm), B is the sum of the peak areas of phenolic meta sites (125 to 137 ppm), C is the sum of the peak areas of phenolic ortho and para sites already reacted (125 to 137 ppm), Mo is the sum of the peak areas of phenolic methylol groups (59 to 66 ppm), and Me is the sum of the peak areas of methylene bridges con­necting phenolic nuclei (30 to 45 ppm). The coefficients a, b, and c are characteristic of the type of resin and depend on a variety of manufacturing parameters. Equation (1) is one of the simpler equations of this type, the equations for UF resins in particular being in general more complex. Similar equations correlating the level of crystallinity of hardened aminoplastic resin, the IB strength of the board prepared with it, the level of cross-linking of the resin and the formaldehyde emission of the panel and resin with the 13C NMR spectrum of the liquid resin have also been presented [33, 64-67].

For certain boards, good correlation exists. However, it must be assumed that a general correlation for various resins and various panels will not exist and that maybe other correlation equations should be used. Nevertheless, these results are rather impor­tant, because they show that at least for a special combination of resin type and board type, correlation between analysis of the resin in its liquid state and the strength of the same resin in hardened form exists, and that forecast of performance can be done based on just the analysis of the liquid resin. Furthermore the various parameters corresponding to chemical and physical groups in the liquid resin will also be the decisive parameters for other resin combinations and manufacturing procedures, although the values of the coef­ficients within the individual equations might differ. However, it also must be considered that the range of molar ratio under investigation in the papers mentioned above is rather broad. At the moment it is not possible to use these equations for predictions within too narrow a range of molar ratios.