Лако-красочные материалы — производство

According to Pedersen and Jul-Rasmussen [2], wood chips are mixed with 20 to 25% of their weight with a 50% technical SSL and pressed at 185°C for 30 min, giving a 12-mm — thick board that must be postheated at 195°C for 80min in an autoclave. The pH value of the SSL had been adjusted to 3 by citric acid. The particleboard obtained has a bending strength of 230 kP/cm2, a tensile strength perpendicular to the grain of 5.3 kP/cm2, and a density of 0.7 g/cm3. Press temperatures may vary between 170 and 235°C and tempera­tures of the autoclave between 170 and 210°C.

High press and autoclave temperatures as well as long heating times are necessary for effective cross-linking by condensation reactions in lignin, as pointed out above. The color of the boards is dark, due to decomposition reactions and charring caused by the high temperatures, and the density of the boards usually reaches values at around 0.8 g/cm3 if the required tensile strength is to be obtained. The temperature in the core layer during pressing reaches 140°C. This may also cause condensation reactions between wood and SSL, as well as chemical and physical changes in the wood particles [4].

The relatively high dimensional stability of the particleboard toward water may be caused by these changes. Pedersen and Jul-Rasmussen (1963) found a thickness expansion after a 2-h soaking in water at 20°C of only 1.5% and 13.8% water absorbance. Open-air tests, extending over 5 years, carried out by the wood panel products laboratory of the Technical Research Centre of Finland [5] revealed that SSL boards, obtained according to the Pedersen procedure, were superior in strength and in surface properties to UF as well as to PF particleboard. Roffael [6] has shown that water absorption of Pedersen SSL particleboard at different air humidities is only about half as high as with conventional PF particleboard. Weathering for 1 year gave a nearly constant humidity at around 6% for SSL particleboard, while PF boards gave humidities between 12 and 15%. Also, after soaking the boards in water at 20°C for 24 h, the lignosulfonate boards lost only 25% of their initial tensile strength, while that of conventional PF particleboard decreased by 70%. In contrast, the mechanical strength properties of SSL particleboard were inferior to those of PF board [7].

The Pedersen procedure has been applied to mill-scale tests in Denmark, Switzerland, and Finland, but has been discontinued in all cases. One reason for this failure is the high cost caused by the two-stage heating treatment. The autoclave must consist of refined steel, due to the evolution of corrosive gases such as sulfur dioxide, causing additional high costs. Another reason was the long pressing and curing time needed for manufacture. However, one of the main reasons for the discontinuation of the procedure was the frequent fires induced by the high pressing and posttreatment temperatures [2].

As mentioned above, the condensation rate of lignosulfonates depends on the cation, with ammonium ions exhibiting the highest reactivity. Shen and Calve [8] used fraction­ated ammonium-based SSL as a binder for particleboard and found the highest reactivity, leading to the best mechanical board properties, with a low-molecular-weight fraction. Unexpectedly, the tensile strength of dry particleboard obtained with a low-molecular — weight ammonium-based SSL fraction increased with the sugar content of the SSL. The best board properties were obtained with 6% of a low-molecular-weight (0 to 5000) ammonium-based SSL fraction having 50 to 60% sugar. In this case, a pressing time of 8 min at 210°C was sufficient for manufacture of 11-mm-thick waferboard to meet the Canadian standard requirements for exterior-grade particleboard.

Obviously, the sugars take part in the condensation reactions of lignosulfonate by production of furfural. While the bending strength of dry boards increased steadily, with the sugar content of the SSL going up to 80%, the bending strength after a 2-h boiling of the boards reached a maximum at about 50 to 60% carbohydrates, indicating that the condensation between lignin and carbohydrates leads to better water resistance than that between carbohydrates only.