Page 351 - Biodegradable Polyesters

13.4 Composites of Biodegradable Polyesters 329
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converted into PLA films by compression molding at a temperature of 180 C
[61]. The PLA films and the sisal and nettle fibers were subjected to compression
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molding below 180 C and under a pressure of 4 MPa for 8 min. Subsequently,
after 8 min, the pressure was raised to 6 MPa for 2 min and the composites were
obtained by cooling under pressure. The measured density of the composites was
compared with the theoretically calculated densities of the composites. The dif-
ference between the actual and the theoretical density was observed owing to
the development of pores and voids in the prepared composites [61]. The pres-
ence of voids and pores results in a reduction in the strength of materials. Voids
and pores can be removed by incorporating the injection-molding technique in
the processing steps [62]. In a specific case, tests were performed on compos-
ites manufactured by injection molding to evaluate their mechanical properties.
The impact, tensile and flexural strengths, and stiffness of composites were found
to increase, compared to sisal/PLA composites manufactured using compression
molding [55].
Various other reinforcements, such as bamboo fibers, vetiver grass fibers, and
coconut fibers [63], are used to manufacture different biodegradable composites.
To manufacture these composites, the injection-molding method is commonly
used. The brittleness of PLA requires a suitable reinforcement in order to improve
its impact properties. Such an effect can be observed in composites made of bam-
boo fibers/PLA and coconut fibers/PLA. The reinforcement with bamboo fibers
exhibited significant increase in the impact properties compared to that with grass
and coconut fibers. Bamboo/PLA composites can be manufactured using the film-
stacking method followed by compression molding [64]. The energy absorption
capacity in the transverse direction was found to be better compared to those in
the other directions.
Composites can also be developed by PLA reinforced with chicken feather fibers
(CFFs). The composite was manufactured by the compression molding technique
[65]. The mechanical properties of neat PLA and CFF/PLA composites were inves-
tigated. The elastic modulus of the CFF-reinforced composites are significantly
higher than that of neat PLA.
The research of polymer composites that contain cellulosic materials has been
acknowledged as a significant area of research for over a decade. Cellulosic fibers
are being used as reinforcement in most common biodegradable polymers [66],
such as polyesters. The cellulosic fiber reinforced composites can be manufac-
tured by compression molding [53]. The composition and physical properties
of bacterially synthesized polyesters have been reviewed. Even though cellulose
fibers improved the strength and stiffness of the PHB, the composites showed
brittleness. The effect of incorporation of cellulose fibers with PHB on the tensile
modulus has been examined in [53], which revealed that the tensile modulus was
enhanced with increasing fiber content.
Natural fibers are regarded as an adaptable competitor for conventional fibers,
for example, glass and carbon fibers. One impediment is the superior strength
and stiffness exhibited by carbon and glass fibers. The recent research on natural