| Summary: | The potential of alkylammonium compound (AACs) as wood preservatives, was
recognized during the 1970s. Two of the problems associated with AAC-treated wood,
particularly treated with didecyldimethylammonium chloride (DDAC), are severe surface
degradation and discoloration compared to ammoniacal copper quaternary ammonium
compound (ACQ) and chromated copper arsenate (CCA) treated wood. The key
objective of this study was to identify the mechanistic pathways by which DDAC affects
wood weathering.
An FTIR technique was developed and employed to study the photodegradation of
wood. The compositional changes in wood sections of various thickness following
DDAC and ACQ treatment, and either natural weathering or UV irradiation were
examined using FTIR spectroscopy. The studies on DDAC and ACQ fixation suggested
that interaction between the preservative and lignin had taken place and that these
preservatives favored fixation to lignin over cellulose. The natural weathering and
artificial UV irradiation studies showed that DDAC accelerated wood weathering via
enhancing wood photodegradation (mainly delignification), and that ACQ slowed wood
photodegradation as demonstrated by inhibiting the formation of carbonyl groups and
delignification.
The effect of DDAC, ACQ, and CCA treatments and their retention on wood
photodegradation during weathering was investigated using FTIR and Ultraviolet-Visible
reflectance spectroscopy. Delignification, demethoxylation, and cellulose degradation
were accelerated by DDAC treatment, but slowed by both ACQ and CCA treatments.
ACQ treatment also inhibited carbonyl formation. Those effects were enhanced by higher
preservative retentions. Weight loss of the weathered samples corroborated the FTIR and
UV-Vis results. The comparison of the photodegradation of methylated and
unmethylated wood demonstrated that both fixed and adsorbed DDAC must be involved
in the photodegradation mechanism and that the fixation reaction of DDAC to lignin was
one of the principal factors affecting wood photodegradation.
The studies using lignin model compounds in solution revealed that DDAC had
no influence on the major photoreactions taking place in the solvent employed. The lack
of significant DDAC-lignin interaction in solution most likely explains why the solution
photochemistry failed to provide useful evidence of the pathway.
The ESR studies on the free radical formation and decay in wood revealed that
DDAC functioned as photosensitizer. The fixed DDAC was essential for sensitization to
occur while the adsorbed DDAC accelerated this process. === Forestry, Faculty of === Graduate
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