Dissertations in Forestry and Natural Sciences

Taneli Väisänen: Effects of Thermally Extracted Wood Distillates on
the Characteristics of Wood-Plastic Composites
polymer will be strong but brittle, which accounts for the high
modulus and low impact resistance (Galeski 2003).
Semicrystalline polymers have both crystalline and amorphous
regions, i.e., these polymers combine the high strength of
crystalline polymers with the flexibility of the amorphous types
(Callister 2005). In composite materials, semicrystalline
polymers are typically more efficiently reinforced by fibers than
amorphous ones because the fibers act as nucleation sites for the
crystallization process with the fiber becoming surrounded by
a finely divided microcrystalline structure, which improves the
modulus, especially the flexural modulus (Quan et al. 2005).
At the moment, PEs are the most commonly used plastics
because they are easy to produce and modify. PE (Figure 2) is a
semicrystalline polymer. The polymer chains in PE can branch
in a different manner, resulting in polymers with different
properties. HDPE has a variety of applications because of its
excellent barrier properties and resistance to different solvents.
LDPE is commonly used in containers, bottles, films, and plastic
bags because it is a flexible and tough polymer with a good
resistance to chemicals. In addition, LDPE has good electrical
properties. (Klyosov 2007, Kim and Pal 2010)
Figure 2. The chemical structure of PE.
The properties of PP resemble those of PE. PP is a
semicrystalline polymer with a methyl group (CH3) attached to
the polymer backbone (Figure 3), meaning that PP can be either
isotactic, atactic or syndiotactic. However, over 90% of
produced PP is isotactic. Like PE, PP finds its applications in
packaging, containers and films. Furthermore, PP is used in the
automotive industry and can be found in laboratory equipment
and textiles. (Kim and Pal 2010)
30 Dissertations in Forestry and Natural Sciences No 222