Page 290 - Biodegradable Polyesters

268 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques

of the optimum particle size for toughening PLA that changes the deformation
mechanism of the matrix from crazing to shear yielding, has been identified.
Consequently, suitable particle size is the precondition for a highly crystalline
matrix to work effectively in the toughening because only small particles are
effective in toughening by triggering the shear yielding needed for effective energy
dissipation, whereas large particles are only capable of toughening amorphous
matrix effectively by initiating multiple crazing of the matrix. Accordingly, the
increase of PLA crystalline content is viewed as a promising way to enlarge plastic
deformation throughout the matrix, thus causing high-impact energy dissipation.

10.4
Conclusions

The environmentalconcernsand ashortageofpetroleum resourceshavedriven
efforts toward the mass production of biodegradable/renewable materials.
Among many different bio-based and biodegradable plastic materials, PLA is cer-
tainly the most promising of polymers with good mechanical properties (similar
to commodity plastics) for industrial applications (packaging, automotives, and
electronics devices … ). However, PLA suffers from restrained properties limiting
its development for new fields of application, showing its limited toughness as
the main key parameter to be improved for its industrial implementation.
Among strategies developed to overcome this limitation, the addition of
impact modifiers is viewed as the most economic and practical strategy to
allow the toughening of materials. However, the overall material toughness and
related toughening mechanisms are mainly governed by a variety of factors such
as the composition and crystallization behavior of the matrix, the relaxation
behavior of the dispersed phase, the rubber particle size and size distribution, the
rubber–matrix interfacial adhesion, or the blend morphology. As a result, it is
only when most of these criteria are well controlled that high energy-dissipative
mechanisms and ultimately improved toughness can be reached.
Interestingly, many approaches can tailor the effectiveness of impact modifiers
through the fine-tuning of these criteria and especially the morphology of the
resulting rubber-toughened blends. Among them, the addition of nanoparticles
as compatibilizers, the addition of nucleating agents, and the formation of inter-
penetrating networks certainly represent the most attractive approaches to impart
high toughening effect while maintaining the inherent rigidity of PLA.

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