Лако-красочные материалы — производство

In 1965, W. Arnold [2] found that the pressing time of SSL particleboard obtained with sulfuric acid as catalyst (see the Shen procedure) can be reduced by 50% and the specific gravity of the boards by 7 to 10% if 10 to 30% of the SSL-blended wood chips are replaced by UF-blended wood chips. However, at pressing times of 0.6 to 1 min/mm board thickness, the pressed particleboard still has to be posttreated at high temperatures to meet German standards for mechanical strength properties. Again, the necessary posttreatment of the boards has hindered the practical application of this finding. On the other hand, small amounts (up to 10% of the UF resin) of SSL improve the cold adherence of blended UF particleboard (Schmidt-Hellerau, 1973, 1977) [2]. This has found practical applications in current industrial use in some western European particleboard mills.

Roffael [6] has shown that 20% of the UF in the surface layers of UF particleboard can be replaced by ammonium-based lignosulfonate without significantly worsening the mechanical board properties. The release of formaldehyde decreased only slightly, which was attributed to the reduction of UF resin rather than to a reaction of formaldehyde with the lignin. The binding of formaldehyde by lignin in UF particleboard is claimed in three Japanese patents [2], together with other patents dealing with lignin UF formulations as wood adhesives. Other improvements achieved by lignosulfonate in UF resins are decrease of adhesive viscosity, increased wettability of wood particles, and improved water resis­tance of finished boards.

According to a recent report [20], substitution of up to 15% of the UF adhesive in particleboards by SSL does not cause major impairment in particleboard properties. This can be seen from the properties of 17.7-mm-thick one-layer particleboard obtained with 8% UF binder (formaldehyde/urea = 1.27), replaced partially by 10 to 30% magnesium — based SSL. The adhesive contained 0.5% paraffin emulsion and 3% ammonium chloride, and the pressing time was 10 s/mm at 200°C [2]. It is obvious, however, that substitution of 20% or more of the UF binder by magnesium-based SSL worsens both the strength and water resistance of the boards, while the gelling time (pot life) of the adhesive is increased. When three-layer 20-mm particleboard was manufactured with 15% of calcium-based (A), sodium-based (B), or ammonium-based (C and D) lignosulfonate and 85% UF binder, with pressing times of 9 s/mm at 200°C and 10.5% adhesive in the surface layer and 8.5% in the core layer, the board properties were different.

At board densities of about 0.7 g/cm3, the bending and tensile strengths of the UF-lignosulfonate boards are not decreased compared to boards prepared with 100% UF binder, while the percentage of swelling is increased. The formaldehyde release is considerably decreased by ammonium (C and D)- rather than by calcium — and sodium — based SSLs, indicating that the ammonium ions react with formaldehyde under the con­ditions existing in the boards, but not the lignin. In the case of calcium — and sodium-based SSLs, the reduction in formaldehyde release lies between 10 and 18%, which corresponds to the amount of SSL in the UF-SSL formulation.

In a patent by Edler (1978) [2] it has been claimed that about 33% of UF binder in particleboards can be replaced by ammonium-based SSL if certain conditions are main­tained. First, the UF resin should have a relatively high number of methylol (CH2OH) groups, characterized by a Witte number of 1 to 1.8, preferably 1.6, which leads to better compatibility between UF and SSL. Second, the concentration of ammonium ions has to be adjusted to 0.2 to 4%. The ammonium ions react with free formaldehyde, forming less reactive hexamethylenetetramine, which leads to excessive sulfonic acid groups in lignin. If the ammonium-ion concentration is higher than 4%, based on dry lignosulfonate, the acidity becomes too high, resulting in very fast curing. The latter causes soft board sur­faces and diminished strength properties. On the other hand, if the ammonium ion con­centration is below 0.2%, the curing time becomes too long. The properties of four types of particleboard, obtained with three different types of adhesive—A having a Witte number of 1.58, B of 1.50, and C of 1.02, the latter prepared using only UF resin, while types A and B contained 33% lignosulfonate and 67% UF—show pressing times of about 15 s/mm at 160°C. Wood chips consisting of 67% pine and 33% Douglas fir gave different results. The mechanical strength properties of boards obtained with resins of types A and B (33% lignosulfonate) show no major impairment compared with those of conventional UF boards (resin type C). Significant improvements in the water resistance are gained, due to the polyphenolic structure of lignin [2].