Dissertations in Forestry and Natural Sciences

Wood-plastic composites
of lubricants in WPCs is less than 5%, but it is also dependent
on the type of the polymer matrix. For example, the lubricant
content level for a HDPE-wood composite (wood content
50–60%) is usually 4–5% whereas a similar composite composed
of PP as a polymer matrix instead of HDPE is typically
manufactured with 1–2% of lubricant. (Sherman 2004)
Light stabilizers and colorants (pigments) are also added to
WPCs to improve the resistance against color fade and UV
degradation, and to provide the desired appearance (Sherman
2004, Clemons et al. 2013). The amount of pigments in WPCs
must be 1–3% or even higher to avoid color staining from the
wood. Biocides, such as zinc borate, protect composites against
fungal and microbial attacks and maintain their surface
appearance. They also reduce the moisture absorption. Flame
retardants suppress the production of flames and therefore,
prevent the spread of fire.
2.2 PROPERTIES
There are two common reasons to add wood to polymers: 1) to
lower the price of the final product and 2) to reduce the
dependency on mineral oil based products (Kim and Pal 2010).
This, however, means a compromise as the properties of wood
and plastics are altered, i.e., WPCs possess rather different
characteristics. For example, WPCs absorb less water than wood
but have higher tensile strength than plastics. WPCs have
therefore found use in multiple applications. The low density
and good processability of WPCs are favored in automobile
industry, for instance. On the other hand, WPCs are widely
used in building products, such as siding and decking because
of their low water absorption and good creep performance.
However, the properties of WPCs are highly dependent on the
product formulation, manufacturing, and the quality of the raw
materials.
Dissertations in Forestry and Natural Sciences No 222 33