Лако-красочные материалы — производство

Not much work has been done up to now concerning the prediction of bond strengths and other board properties based on the results of the analysis of the adhesive resin in its liquid state. What has been investigated and derived up to now are correlation equations that correlate the chemical structures in various UF resins having different molar ratios F/U and different types of preparations with the achievable internal bond strengths of the boards as well as the formaldehyde emission measured after resin hardening.

The basic aim of such experiments is the prediction of the properties of the wood — based panels, hence of the adhesive resin in its hardened state, based on the composition and the properties of the liquid resins used before their hardening. For this purpose various structural components were determined by means of NMR spectroscopy and the ratios of the amounts of the various structural components were calculated, for example:

(i) for UF resins:

free urea related to total urea

methylene bridges with crosslinking related to total sum of methylene bridges

sum of methylene bridges in relation to sum of methylols

(ii) for MF resins:

unreacted melamine to monosubstituted melamine unreacted melamine to total melamine

number of methylene bridges in relation to the number of methylol groups degree of branching: number of branching sites at methylene bridges in relation to total number of methylene bridges

(iii) for MUF resins:

sum of unreacted melamine and urea to sum of substituted melamine and urea

number of methylene bridges in relation to number of methylol groups or to the sum of methylene bridges and methylol groups

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].

For certain boards, some good correlations exist. Even these equations, however, cannot predict all properties for all types of UF resins. This is because it must be assumed that a general correlation for various resins and various panels cannot exist. Other corre­lation equations might have to be used sometime. However, the types of equations that have already been published describe how a universal equation for this task might look. Only the coefficient need to be changed from case to case. These results are of some importance, because they show that at least for a certain combination of resin type and board type, correlations do indeed exist. It will be the task for chemists and technologists to evaluate in further detail all possible parameters as well as their influence on the performance of the resins and the wood-based panels. It can also be assumed that the various parameters mentioned above will also be decisive for other combinations, even if the numerical values of the coefficients within individual equations might differ. The range of the molar ratio under investigation in the papers mentioned above was rather broad.

It would not appear to be possible to use these equations for predictions within narrow ranges of molar ratio, e. g., the usual range of an E1 UF resin with approximate F/U = 1.03 to 1.10. A method showing how different resin preparation procedures, for equal molar ratio resins, can be included in these correlation equations also needs to be developed.