In this study, it has been conducted the microscopic observation of glue line in sixteen tropical woods glued with phenol-formaldehyde resin adhesive.
Micrographs of the glue line of the examined species are given in Photo. 1 through 16. Species of wood, appearance of wood section, appearance of glue line (range of thickness, air pocket content, percent of vessel along glue line in linear measurment), bond quality, wettability (C. W. A. H.) and specific gravity are given in Table 1.
The results obtained are as follows :
(1) On the thicker glue line, more air pockets are found in all species.
(2) Most of the air pockets observed in the glue line of Rose wood and Zebra wood appeared on the top of the wood rays and a part of the pockets are always contacted to the wood rays. (Photo. 1 and 2)
It was observed that the glue also penetrated into the wood rays to contact with the air pockets, but the amount the glue penetrating into the rays touching with the air pockets was less than that penetrating into the rays that had no pockets to contact with.
It is considered to be a very interesting phenomenon.
(3) In all species, when the depth of opening vesseles in the external glue line is large, the glue lines are delaminated at the opposite wood face, while in shallow case, they are delaminated at the vesseles.