Dissertations in Forestry and Natural Sciences

2 Wood-plastic composites
By definition, composite materials are formed by combining
two or more constituent materials that have substantially
different chemical or physical properties (Callister 2005). As a
result, the individual components remain distinct within the
finished material, and thus composites can possess properties
that cannot be achieved with the individual constituent
materials. Composites can be classified into particle-reinforced,
fiber-reinforced, and structural composites. There are many
well known composite materials, e.g., metal and ceramic
composites, cements, concrete, and reinforced plastics.
In materials science, a fiber is commonly defined as a
substance that has been drawn into a long and thin filament, i.e.,
the aspect ratio, which is defined as the ratio of fiber length to
diameter, is at least 100 (Callister 2005). However, the term fiber
may also refer to the spindle-shaped cells within the wood
material (Clemons 2008), and in the case of natural fiberpolymer
composites, fiber can be defined as an object with an
aspect ratio greater than one (e.g. Stokke et al. 2014). Thus, from
the viewpoint of materials science, some WPCs can be classified
as particle-reinforced composites although they are commonly
referred to as fiber-reinforced composites. In this thesis, the
definition of a fiber is adopted from the terminology used for
natural fiber-polymer composites.
WPCs are fiber-reinforced composites produced by mixing
wood components and molten thermoplastics. In a WPC, a
polymer forms a continuous matrix that surrounds the
reinforcing wood components. The low price and high stiffness
of wood makes it an attractive reinforcement for the commodity
plastics. Since the processability of WPCs is similar to the
plastic, there are several appropriate manufacturing
technologies available for WPCs. Although the majority of WPC
products are extruded, injection and compression molding are
Dissertations in Forestry and Natural Sciences No 222 23