Dissertations in Forestry and Natural Sciences

Taneli Väisänen: Effects of Thermally Extracted Wood Distillates on
the Characteristics of Wood-Plastic Composites
furthermore they can change considerably during the storage
depending on the conditions.
The emissions of acetic acid and aldehydes from the WPC
were not surprising as these compounds are formed during the
thermal modification and extrusion processes (Peters et al. 2008)
and they originate primarily from the degradation of
hemicelluloses. However, the emissions of these compounds
need to be carefully monitored because they have harmful
effects on indoor air quality and they can damage human health.
Acetic acid is an irritant compound with a rather low odor
threshold (Akakabe et al. 2006). Formaldehyde and
acetaldehyde, in turn, are carcinogenic compounds with
mutagenic and irritant properties (Silla et al. 2001). The emission
rates of formaldehyde remained low during the trial; those of
acetaldehyde decreased substantially after 41 days, but no signs
of any trend-like behavior could be identified for acetic acid.
Low formaldehyde emission levels have also been reported for
wood (Roffael 2006). The proton affinity of formaldehyde is
only slightly greater than that of water, which may lead to
reverse proton transfer reactions between the protonated
formaldehyde and water molecules (Hansel et al. 1995, Schripp
et al. 2010). For this reason, it is possible that not all of the
formaldehyde was detected by PTR-TOF-MS. On the other
hand, the TD-GC-FID/MS-system has similar limitations as
described in section 3.3.1.
Hyttinen et al. (2010) obtained similar results when they
evaluated the emission rates of acetic acid in their comparison
of VOC emissions between air-dried and heat-treated wood
species. They postulated that the fluctuations in acetic acid
emissions could be caused by the diffusion of the compound
from the inside of the material to the surface if it was not evenly
distributed within the sample. A similar explanation could be
also the case for WPCs. Yrieix et al. (2010) analyzed VOCs from
wood-based panels and observed that acetaldehyde emissions
decreased by over 62% during their 25 day sampling period. In
addition, they did not detect any major changes in
formaldehyde emissions during the trial. Even though their
104 Dissertations in Forestry and Natural Sciences No 222