Dissertations in Forestry and Natural Sciences

Taneli Väisänen: Effects of Thermally Extracted Wood Distillates on
the Characteristics of Wood-Plastic Composites
However, no conclusions can be drawn on the effects of
hardwood distillate on water absorption of UFs because the
granules could not be successfully impregnated with the
distillate. As expected, and as shown previously by Ndiaye et
al. (2013), water absorption of the composites increased as a
function of fiber content. However, water absorption did not
consistently decrease after the addition of distillate-treated LGs.
An incompatibility of the raw materials may explain the
inconsistent results as the wood-based fillers used in the
composites are chemically different. Moreover, the distillate
may be more effective with WPCs that contain thermally
modified wood fibers as the reinforcing material.
The results from study I indicated that the extent of water
absorption of LG was considerably lower than that of UF50 even
though they possessed the same fiber content. This finding is in
accordance with other studies, and it can be attributed to the fact
that thermally modified wood fibers are less hydrophilic than
pure cellulose fibers because of the degradation of
hemicelluloses and other changes occurring in the structure of
cellulose during the thermal modification process (Balasuriya et
al. 2002, Ayrilmis et al. 2011). Water absorption of UF50 was
approximately 0.63%, which was in agreement with the values
found in the literature; typical water absorption (24 h) for WPCs
with a wood fiber content of 50–65 wt% has been reported to be
in the range 0.7–2.0% (Klyosov 2007). According to the
manufacturer (UPM), water absorption (ISO 62, 24 h) of UF40 is
0.66% whereas water absorption of UF50 is 0.90%. The
considerably lower water absorption of the UFs used in this
thesis may be due to the differences in the manufacture of the
sample. Moreover, it was unclear whether UPM used similar
samples in their tests.
Water absorption of LG can be further decreased by adding
hardwood or softwood distillates; this was examined in studies
I, III, and IV. As discussed in the previous section, the distillates
may fill the pores and voids in the WPC, and affect the fiberpolymer
interface or interphase. Therefore, the penetration of
water molecules into the hydrophilic wood fibers may be
102 Dissertations in Forestry and Natural Sciences No 222