Page 350 - Biodegradable Polyesters

328 13 Biodegradable Polyester-Based Blends and Composites

were studied. As the molding temperature increases, the mechanical properties
decrease, because of the deterioration of jute fiber quality. The presence of voids
and nonimpregnated regions was reduced by increasing the filling ratio, thus
resulting in an increase of mechanical properties, such as tensile strength and
stiffness [50].
Flax fibers, hydroxyvalerate (HV), and PHB [51] can be used to manufacture
composite laminates. The two sets of composites were prepared by adapting
different manufacturing methods. A set of flax/PHB/HV composites was man-
ufactured using the film stacking method, followed by compression molding.
The second set of composites was prepared using the injection molding method.
Flax/PHB/HV composites showed an increase in stiffness [51]. The composites
manufactured through injection molding had lower impact strength compared to
those manufactured by compression molding. Flax and PLA composites [44] were
manufactured using a twin-screw extruder, followed by compression molding.
The mechanical properties of flax fiber/PLA composites are favorable [44].
The strength of the composite is 50% higher than that of similar polypropylene
(PP)/flax fiber composites. The stiffness of PLA was found to increase from 3.4 to
8.4 GPa with an addition of 30 wt% of flax fibers.
PLA/hemp hybrid yarn composites could be prepared by compression mold-
ing [52–54] using a hydraulic compression molding machine. The manufactured
composites were tested to determine different mechanical properties. The wrap-
ping density of yarn with PLA was found to play an important role in affecting
the mechanical properties [52] of the composites. Composites made of PLA as
the matrix with a wrapping density of 250 showed a consistent improvement in
tensile modulus over the composites with a wrapping density of 150. The flexural
strength and the modulus of hemp-reinforced PLA composites were higher than
those of the plain PLA. The result thus shows that the wrapped yarn with PLA with
a lower wrapping density leads to lower mechanical properties in the composites.
The composites prepared from hemp/PHB showed improved ductility. Since the
composites are manufactured under low-pressure compression molding [54], the
strength of the composites can be lower than the strength of wood when the fibers
are arranged in the longitudinal direction and higher than the strength of wood
when the fibers are arranged in the transverse direction [49].
Composites made by compression molding of PLA reinforced with kenaf fibers
have good tensile strength and Young’s modulus [55, 56]. These composites are
used in structures that need relatively high tensile strength and stiffness, but
their impact strength is low. The degradation of properties, such as loss of tensile
strength and weight, were observed after composting the composites for 4 weeks.
Tensile strength and weight were decreased by 91% and 38%, respectively. PLA
reinforced with cotton fibers can be manufactured by compression molding, that
exhibits high impact strength, but low tensile strength and Young’s modulus [15].
The structural characteristics and mechanical properties of sisal fiber/PLA com-
posites have been investigated by many researchers [57, 58]. Some specific cases
from literature have revealed the suitability of sisal fiber reinforced polyester com-
posites in engineering applications [44, 59, 60]. Specifically, PLA pellets can be