Polyhydroxyalkanoate-based Multiphase Materials - Polyhydroxyalkanoate (PHA) Based Blends, Composites and Nanocomposites - page 131

PHA for biocomposite elaboration is its polar character. PHA shows
better adhesion to ligno-cellulose fibres compared to conventional poly-
olefins. 63 We can find a lot of papers based on PHA-biocomposites. The
addition of cellulose fibres and different fillers has often been proposed as a
solution to increase the mechanical performance and toughness of PHB and
PHBV. 63-69
In terms of crystallization and thermal behaviour, no significant effect of
cellulose on PHB crystallinity was reported. A slight increase of T g and a
delay in the crystallization process were observed. 65 The presence of cellu-
lose fibres also increases the rate of PHBV crystallization, due to a nucleating
effect, while thermal parameters, such as crystallinity content, remained
unchanged. Studies on the crystallization behaviour of PHB/kenaf fibre
biocomposites showed that the nucleation by kenaf fibres affected the
crystallization kinetics of the PHB matrix. 67 Differences in the effect of cel-
lulose fibres on the crystallization process have been attributed to the lignin
content at the surface/interface of the cellulose fibre.
The increase of HV content, the addition of compatibilizers and the rise of
fibre content on PHA-based composites influenced the mechanical per-
formance of the corresponding biocomposites.
For PHBV, the presence of HV led to a reduction in the stiffness but to
increased elongation at break compared to PHB. In reinforced PHBV, a
50-150% enhancement in tensile strength, 30-50% in bending strength and
90% in impact strength have been reported. 66 The varying HV content in
PHBV copolymers improved the toughness of the composites based on
natural fibres and increased the ductility, but lowered the crystallization
rate. It has been suggested, however, that the combination of coupling
agents and HV units improved the storage modulus and led to a reduction
in the tan d, 63 due to an improvement in the interfacial bonding between
PHA and the fibres and an increase in transcrystallinity near the fibre
interfaces.
The addition of cellulose fibres led to some improvements in tensile
strength and stiffness, but the composites remained brittle. 64 At low content,
the incorporation of cellulose fibres lowered the stiffness, however, higher
amounts of cellulose fibres greatly improved the mechanical properties
of PHB.
For biocomposites based on cellulose fibres and PHB, the effects of fibre
length and surface modification on the tensile and flexural properties have
been investigated. The results on PHB reinforced with straw fibres have been
published. 65 The fracture toughness values of composite materials con-
taining 10-20 wt% straw fibres were higher than those of pure PHB, while
biocomposites containing 30-50 wt% straw fibres presented about the same
values as neat PHB.
With the addition of interface modifiers, the interfacial shear strength was
also improved. 70 PHB containing wood flour and plasticizers presented a
modest increase in tensile strength, while some improvement in terms of
thermal stability was demonstrated.

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