254 Introduction to Renewable Biomaterials
Table 8.11(Continued)
Density
(g cm−^3 )
Elongation
(%)
Tensile
strength (MPa)
Young’s
modulus (GPa)
Polyetherimide 0 105 2.8
Nylon 1.12 29 66 3.5
Polyethylene (HDPE) 0.95 30 28 1.1–1.2
Polypropylene 0.9 200 35 1.5–2
Polystyrene (high impact) 1.05 15 35 3–3.5
Epoxy resin 1.3 3–4 32 4–6
Wambuaet al. 2003 [34]. Reproduced with permission of Elsevier.
are one of the commonly known families of natural fibre–thermoplastic composites.
Another interesting class of materials is characterized by thermoplastic composites
with cellulosic fibres derived from wastes such as newspapers, additivated paper, paper
board and paper mill sludge. These two classes of composites are described in detail.
The main drawback of composites with natural fillers is the poor compatibility
between hydrophilic fibres and hydrophobic matrices: the different nature between
filler and matrix causes the formation of fibre–fibre interactions instead of fibre–matrix
ones. As a consequence, formation of agglomerates and lower dispersion of natural
fibres is possible. These two problems, bad mixing and weak interface, will render low
mechanical properties to a composite [39].
The most popular methods to improve fibre–matrix interface are the introduction of
a coupling agent [40–43] as well as physical and chemical treatments [44, 45].
Natural fibres present drawbacks, such as interface problems with traditional poly-
olefins, low temperature processes, moisture absorption and swelling fibres, supply and
demand cycles based on product availability and harvest yields, quality variations based
on growing sites and seasonal factors, but their advantages are much more interesting
and will lead to new materials taking care of our planet.
8.3.1 Wood–Plastic Composites
WPCs were born in Italy in the 1970s and then developed as commercial products in
North America in the 1990s. By the start of the twenty-first century it spread to India,
Singapore, Malaysia, Japan and China [46]. The introduction of WPCs in the decking
market (Figure 8.10) was mainly responsible for its growth. A direct result of success in
the decking market is that products are now being developed and introduced for new
exterior applications such as railing, fencing, roofing and siding [47].
The WPCs are defined as thermoplastics reinforced with wood or other natural fibres,
principally produced from commodity thermoplastics such as polyethylene, polyvinyl
chloride or polypropylene [48]. The matrix chosen for WPC production is influenced
by limited thermal stability of wood: only thermoplastics that melt or can be processed
under 200∘C are commonly used [49]. These kinds of composites are generally
manufactured by typical techniques of plastics industry: first extrusion compounding
followed by profile extrusion or injection moulding. [50]. New production processes
have been developed in order to improve the amount of fibre in composites avoiding
the formation of agglomerates. An example is turbo-mixing process (Figure 8.11)
