256 Introduction to Renewable Biomaterials
[51, 52]: the only source of heat is generated by kinetic energy of rotating blades thanks
to friction (Figure 8.9), using the same idea of working as screw extrusion, but in this
case we can reach 3000–5000 rpm. After the charge is melted, the samples are produced
through hot compression.
One of the biggest problems of WPCs, because of their different nature, is low
compatibility between hydrophilic filler and hydrophobic matrix. Many researches
have been conducted in order to face this problem. The introduction of a coupling
agent is one of the most common methods adopted. A variety of coupling agents used
in WPC exist such as acrylates, amides and imides, anhydrides, chlorotriazines and
derivatives, epoxides, isocyanates, organic acids, monomers, polymers and copolymers,
inorganic agents and organic–inorganic agents (silanes, titanates).
Polyolefins grafted with maleic anhydride as coupling agents have received attention
because they allow improving mechanical properties of wood fibre–polyolefin com-
posites. The maleic anhydride mechanism can be exposed through two phenomena:
formation of bonds between the hydroxyl groups of natural fibres and carbonyl groups of
maleic anhydride, and blending of polymer chains of coupling agent with the composite
matrix [53]. Figure 8.12 shows an example of a polypropylene-grafted-maleic anhydride
(MAPP) polyolefin, and Figure 8.13 shows the mechanism of coupling agent action.
The main effects of maleic anhydride are improving tensile and flexural modulus as
well as tensile and flexural strength. However, an optimization of its concentration has
to be performed [53–55].
CH 3
CH 3
O O O
m n
Figure 8.12Polypropylene-grafted-maleic anhydride.
Compound Composite
(a) (b)
OH
OH
OH
OH
Plant Polymer Coupling
agent
Figure 8.13Working mechanism of polyolefin-grafted-maleic anhydride as coupling agent (elliptical
shape is the maleic anhydride graft): (a) compound formation because of the weak interaction
between plant fibres (hydrophilic) and polymers (hydrophobic); (b) composite formation thanks to the
interaction of MAPP maleated graft with plant fibres and polymeric MAPP chains mixing with polymer
matrix. Ashori 2008 [37]. Reproduced with permission of Elsevier.
