Environmental Engineering Reference
In-Depth Information
In 75% of cases, wood fibres are the preferred filler for thermoplastic matrices
(their high availability may account for this). The resulting composite material is
internationally known as WPC, which stands for wood-plastic composites. They
have been produced industrially since the 1980s and the market has increased in
the last decade, especially in the United States, reaching 700,000 metric tons with
an 11% increase rate per annum. In Europe, the WPC market is considered as
emerging. The best estimations indicate that it reached 100,000 metric tons in
2005 [48]. In America, more than 50% of WPC is used for parquets and decking
made with polyethylene. In Europe, the automobile sector is the dominant con-
sumer, using polypropylene-based composites. For these two types of WPC, the
most studied parameters are: fibre/matrix adhesion; filler content; granulometry
of the fibres; extrusion parameters; and durability of the WPC when exposed to
water, sunlight, fungi and insects.
The adhesion between the fibre and the matrix is by far the crucial parameter of
the composite materials. The mechanical properties of the WPC depend greatly
on it and on the compatibility of the filler and the matrix. The only practical way
to improve the adhesion is by adding a coupling agent to the formulation, that is,
a molecule capable of establishing bonding between the filler and the matrix.
Three kinds of bonding are involved: covalent, hydrogen and non-polar interac-
tions. They all contribute to the reduction of the natural incompatibility between
the hydrophilic fibres and the hydrophobic matrix. The most used coupling agent
for polyolefins-based composites is maleated polypropylene (MAPP). It is pre-
pared from polypropylene (PP) and maleic anhydride (Figure 6.27). The anhy-
dride function reacts with wood fibres and the attached PP moiety is fully
compatible with free PP.
The addition of MAPP at around 1-2% ensures the perfect covering of the
wood fibres by polypropylene or polyethylene. The micrographs of Figure 6.28
clearly show the lack of adhesion between the fibres (in dark colour) and the PP
matrix (in light colour). The mechanical properties are increased by at least 30%
when a coupling agent is used.
(a)
(b)
Fibre
O
O
OH
OH
O
O
O
O
Figure 6.27
(a) Maleated polypropylene and (b) the adduct formed with vegetable fibres by
covalent bonding.
