Dissertations in Forestry and Natural Sciences

1 Introduction
The finiteness of crude oil reserves is globally recognized, and
therefore, new raw material alternatives are being sought from
renewable sources (Najafi et al. 2010). Wood is an inexpensive
and abundantly available material that possesses suitable
characteristics for multiple applications, such as in the
construction industry (Clemons 2008). On the other hand, the
combination of wood with the commodity plastics, adhesives,
and other substances provide unique properties that cannot be
achieved with either wood or plastic products on their own.
Thus, wood-plastic composites (WPCs) are ecological, durable,
and recyclable materials (Kim and Pal 2010). In WPCs, the wood
fibers are surrounded by a continuous polymer matrix, and the
compatibility between these two constituents is typically
enhanced by incorporating coupling agents and other additives
into the composite. WPCs can be created with a wide range of
performance levels, and therefore, they have many applications
not only in decking, and fencing, but also in more sophisticated
manufacturing, e.g., in the car-making industry (Klyosov 2007,
Faruk et al. 2012).
Even though the use of WPCs is becoming more and more
common, at present, these materials cannot be used in
applications where high mechanical strength is required. This is
mainly due to the weak bonding between the hydrophilic wood
fibers and the hydrophobic polymer matrix (Gao et al. 2008,
Yuan et al. 2008, Butylina et al. 2011). Moreover, wood fibers
contain a large amount of hydroxyl groups that can form
hydrogen bonds with water molecules. Hence, WPCs are
susceptible to water absorption that induces thickness swelling
and the creation of microcracks in the material (Li et al. 2014),
which increases the risk of mold growth.
Many different approaches have been examined to eliminate,
or at least reduce, these limitations in the present generation of
Dissertations in Forestry and Natural Sciences No 222 19