Abstract:
The importance of wood bonding increased in the last decades due to the
increased usage of wood composites whose performance depends to a large extent on
the adhesive penetration and subsequent bonding of the adherends. The presented
research used XMT (x-ray microtomography) to perform a non-destructive, threedimensional
analysis of the adhesive bondline and wood-structure of Southern yellow
pine, Douglas-fir and yellow-poplar samples. A phenol-formaldehyde adhesive was
used. The sodium hydroxide catalyst was replaced with rubidium hydroxide during
resin formulation. This was done to improve the image contrast. The reconstructions
of the wood structure of Southern yellow pine showed tracheids, rays, fusiform rays,
resin canals and pits. On the Douglas-fir sample tracheids, pits and rays were
displayed clearly. The yellow-poplar images showed vessels, fibers, bordered pits,
scalariform sieve plates and rays.
The renderings of the adhesive-bondline of Southern yellow pine proved the
dominant role of tracheids for the adhesive flow and showed rays as a secondary
pathway of adhesive flow. The results revealed no adhesive flow occured through
bordered pits, while simple pits permitted some adhesive flow through ray
parenchyma. The results for Douglas-fir showed a similar result; the tracheids were
the predominant path of adhesive penetration, while rays played a secondary role and
no adhesive flow through the pit aperture was visible. The adhesive flow through the
microstructure of yellow-poplar wood occured mainly through vessels and also
through rays, but no adhesive flow through the pits was directly observed. The
segmentation of the images in three phases: void space, cell wall substance and
adhesive, enabled the calculation of the effective bondline thickness based on the
adhesive, as well as the volumetric measurement of all three elements and their share
on the sample volume.
Subsequent experiments showed that the exposure of the Southern yellow pine
and yellow-poplar bondlines to cyclic moisture did not cause delaminations or cracks.
However, there were some indicators that the samples experienced some irreversible
swelling.
Finally, the generation of 3-D animations of wood samples and bondlines of
Southern yellow pine, Douglas-fir and yellow poplar was achieved. This animation
was a way to present the results of this research in a quick and accessible way that
illustrates the three-dimensional microstructure of a wood-adhesive bond.
