Page 147 - Biodegradable Polyesters

References 125

5.7
Conclusions

PLA is a versatile biodegradable and compostable polymer, which can be
processed using conventional production equipment. Commercial PLA grades
are copolymers of L-and D-lactic acid, and a proper variation of L-/D-lactic
acid ratio allows production of different resin grades for processing into a wide
spectrum of products. Since the raw material for PLA is based on agricultural
feedstock, the increased demand for PLA resins is expected to yield a positive
impact on the global agricultural economy. Nevertheless, there are a number of
areas which still need to be improved, especially in applications where PLA is
intended to be used as a substitute for existing thermoplastics.
One of the main drawbacks of PLA is its slow crystallization rate, which
largely limits the actual range of possible replacement of nonbiodegradable and
noncompostable polymers. As L-lactic acid is usually the main component in the
commercial PLA grades, the minor D-lactic acid units act as a noncrystallizable
comonomers that reduce the crystallization rate. Crystal polymorphism of PLA
is also affected by comonomer concentration, which in turn affects material
properties.
A peculiar consequence of the stereoisomeric nature of lactic acid is that
the PLLA and PDLA homopolymers can co-crystallize in the form of a stere-
∘
ocomplex. The stereocomplex has a melting temperature 50 C higher than
the respective homopolymers, and is a very efficient nucleating agent for PLA.
Other additives have been suggested as possible nucleators to fasten the onset
of crystallization of PLA; however, to date, talc appears the most efficient one in
terms of cost/properties ratio. The combined addition of plasticizers able to favor
crystal growth also permits a sizeable increase in the crystallization rate, which at
the current state of research still remains too low for a large-scale substitution of
nonbiodegradable commodity polymers. Further improvements in this direction
are thus needed to further expand the application range of PLA and open the way
to increased utilization of renewable resources and development of sustainable
products.

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