Page 263 - Biodegradable Polyesters

10.1 Introduction 241

400
PTFE
350
Nylon 6,6
300
PS
PVDC PP
250 Polyester
T m (°C) 200 amides Ecoflex PLA PET
150 Aliphatic PMMA
copolyester
100
PHA PHB/V
50 Polyolefins
PCL
0
−150−125−100−75 −50 −25 0 25 50 75 100 125 150
(°C)
T g
Figure 10.6 Comparison of glass transition and melting temperatures of PLA with those of
other thermoplastics. Reproduced with permission from Ref. [33] © 2008, Elsevier.
The mechanical properties of PLA can be varied to a large extent not only in the
stereochemical architecture but also in the polyester molecular weight and molec-
ular weight distribution, the processing history, crystalline orientation, crystal-
lization degree, and so on [37, 38]. When high mechanical properties are required,
semicrystalline PLA is preferred over the amorphous PLA counterpart. Generally,
semicrystalline PLA exhibits a Young’s modulus as high as about 2–3 GPa, a ten-
sile strength between 50 and 70 MPa with an elongation at break of about 4%, and
−2
an impact strength close to 2.5 kJ m . Moreover, it has been demonstrated that
the tensile strength and modulus of PLA increases twofold when the molecular
weight raises from 50 to 100 kDa [39]. It was also highlighted that the method used
to process and shape the polymer (extrusion, injection molding, etc.) can affect the
mechanical performances. This is mainly because these techniques may decrease
the PLA molecular weight via the occurrence of thermal degradation reactions at
high temperature. For instance, annealing treatments play a decisive role in the
mechanical properties of PLA, giving rise to the crystallization of the PLA matrix
and a dramatic increase in the tensile strength of the resulting materials [40].
By comparison with commodity polymers such as polyethylene (PE),
polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS),
the mechanical properties of semicrystalline PLA appear very attractive, partic-
ularly its Young’s modulus, making it an excellent substitute for rigid commodity
polymers in short-time packaging (Table 10.1) [41]. Furthermore, its eco-friendly
profile, biocompatibility, good processability using conventional melt-processing
techniques, and relatively low cost are the main reasons for its large-scale
development.
Although PLA meets many requirements as an eco-friendly bioplastic with
attractive physical properties, which can mimic PE, PP, PS, and PET in different
types of applications such as in automotive and electronic industries, in many
cases, the practical applications of PLA have been significantly impeded by
various drawbacks such as the following: