Dissertations in Forestry and Natural Sciences

Taneli Väisänen: Effects of Thermally Extracted Wood Distillates on
the Characteristics of Wood-Plastic Composites
and cellulose. Lignin is covalently linked to hemicellulose
molecules, increasing the mechanical strength of the cell wall.
Lignin is a non-polar hydrophobic polymer whereas cellulose
and hemicelluloses are hydrophilic (Thomas et al. 2011). In the
pulp industry, lignin is normally removed from the pulp
(chemical pulp) when manufacturing bleached writing paper,
because lignin is responsible for the yellowing of paper with age
(Ek et al. 2009). Since lignin yields a considerable amount of
energy when burned, it is considered as a potential alternative
to fuels derived from non-renewable sources. In addition, the
pyrolysis of lignin yields chemical compounds that are thought
to be potentially useful in many fields of applications (Lora and
Glasser 2002). For instance, guaiacol, which is a thermal
degradation product of lignin, has smoky sensory notes and it
can be used as a flavorant (Goldstein 2002, Dorfner et al. 2003).
In addition to lignocellulose, wood contains small amounts
(3–10%) of other organic components (Pettersen 1984, Rowell et
al. 2013, Stokke et al. 2014). Wood extractives include simple
sugars, fats, waxes, resins, proteins, terpenes, and gums. The
extractive compounds are crucial components of the defense
system of the tree, but they also act as energy reserves and
support tree metabolism (Clemons 2008). There are also trace
amounts (about 1%) of inorganic ash in wood (Rowell et al.
2013).
2.1.2 Polymers
A variety of thermoplastic or thermosetting polymers can be
used as the matrix material in WPCs (Clemons 2008). However,
the low thermal stability of wood limits the polymers to those
that have adequately low processing temperatures. The thermal
degradation of wood components begins at approximately
120 °C, and major changes take place at over 200 °C. Thus,
polymers which have a processing temperature lower than
200 °C need to be used in WPCs (Godavarti 2005). The most
common polymers include PE, PP, and PVC that are
thermoplastics.
28 Dissertations in Forestry and Natural Sciences No 222