Distribution of phenol-formaldehyde resin in impregnated southern pine and effects on stabilization

Two low-molecular-weight phenol-formaldehyde (PF) resins impregnated at 20% resin-solid concentration in southern pine (SP) wood and cured at 180 degrees C for 20 min were studied by various microscopic methods. The micrographs indicated that the ray tracheids of SP were the main flow path of resin, rays were reinforced by cured resin, and flow of resin into lumens was more difficult for higher-molecular-weight resin. Some PF resin deposits were found in lumens. PF resin deposits were formed to connect ray tracheids and longitudinal tracheids, resulting in interlocking bridges that possibly reduced dimensional changes of wood in the tangential direction. These resin deposits appear to be responsible for the high dimensional stability observed in this direction. The diffusion of PF resins into cell walls appears to occur through rays or primary walls, but not through lumens or the warty layer.