Лако-красочные материалы — производство

Cold tack means that the particle mat has attained some strength already after the pre­press at ambient temperature, without any hardening reaction having occurred. This ‘‘green’’ strength is necessary for better handling of the particle mat during transfer on the production line. This can well be the case in multiopening presses, in special forming presses, or in plywood mills, where the glued veneer layers are prepressed to fit into the openings of the presses. At least a low level of cold tack is also necessary to avoid blowing out and loss of the fine wood particles from the surface when panels enter a continuous press at high belt speeds. On the other hand, cold tack can lead to agglomeration of fine wood particles and fibers in the forming station.

Cold tack is generated during the dry out of glue line, and reaches a maximum after a certain period of time. After this point the cold tack decreases again, when the glue line starts to dry out. Both the intensity of the cold tack as well as the optimum length of time in which it develops after application of the adhesive can be adjusted by the degree of condensation of the resin as well as by using special resin preparation procedures [33-35]. Also various additives can increase the cold tack of the adhesive resins, e. g., some thermo­plastic polymers such as poly(vinyl alcohol).

2. Isocyanate (PMDI) as Accelerator and Fortifier for UF Resins Polymeric methylenediisocyanate (PMDI) can be used as an accelerator and as a special crosslinker for UF resins. UF resins and PMDI can be sprayed separately without prior mixing onto the particles [36,37] or for improved performance the two resins can be premixed and then applied [8,38,39]. In the usual mixing procedure PMDI is pumped under high pressure into the UF resin [40,41]. Usually 0.5 to 1.0% PMDI based on dry particles is used, whereas at the same time the UF gluing factor might be reduced slightly. The specific press time is said to be reduced by up to 1 s/mm.

Addition of PMDI to UF resins with a very low molar ratio was also recommended to achieve low formaldehyde emission. The poor properties of the UF resin due to its very low molar ratio can then be improved by the addition of PMDI [42-45].