2 октября, 2015 
Malyar Hanetho [427] has discussed the experiences of the particleboard industry regarding the influence of seasonal variations of the wood quality. Some problems do occur using wood that has been harvested in the winter time and which goes into board production immediately. When these logs or chips have been stored for some time, these problems disappear. The contact angles of water and adhesive on wood are higher in the case of freshly harvested wood compared to stored chips. This means that the surface of the wood particles obtained from such a fresh wood is more hydrophobic, influencing negatively the wetting and penetration and thus the substrate gluability. It has been determined that the reason for the lower wettability of freshly harvested wood is the higher content of wood extractives. These results, however, must not be confused with the better wettability of a freshly prepared surface, independently of whether it is freshly harvested or stored wood.
Hydrophobic wood extractives and components oxidize or polymerize during storage after harvesting, as also can be seen from their lower extractibility [428]. Because of this effect the ability of wood extractives to migrate to a new surface is also reduced. Figure 8 shows this effect by plotting contact angles versus time after the
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Figure 8 Contact angles of a UF resin on the surfaces of wood particles, as a function of the contact time, hence the time elapsed after the application of the droplet. The surfaces have been cut from a freshly harvested log and from a log stored for 3 months. (After ref. 428.) Water extracts from the particles made from freshly harvested wood have higher pH values, but lower buffer capacities than the surfaces made from stored chips. The lower buffer capacity might lead to prehardening if the usual amount of hardener is used, with a consequent decrease of the board strength. |
application of the urea resin droplet onto the surfaces of freshly harvested wood and stored wood.
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