Page 273 - Biodegradable Polyesters

10.2 Polylactide Strengthening and Strategies 251

an impact strength of 41 kJ m −2 at 30 wt% of PU [104]. In the same way, Ojijo
et al. investigated PLA/poly(butylene succinate-co-adipate) (PBSA) blends in the
presence of TPP as a reactive compatibilizer. They found that the poor interfacial
adhesion in the neat blends led to premature interfacial failure and hence rapid
void propagation, while moderately good interfacial adhesion in the compatibi-
lized blend delayed the occurrence of matrix deformation through a debonding
process with fibrillated ligaments at the interface [105]. Reactive blending was
also described by Oyama [106], leading to supertough PLA-based materials. This
strategy improved not only the dispersion of the second component but also
the bonding between the particles and matrix to expect combination of crazing
and shear yielding. Additional investigation through the PLA/poly(butylene
succinate) (PBS)[107], PLA/PBAT [108], PLA/PE [109], and PLA/poly(butylene
carbonate) [110] proposed interfacial debonding and matrix yielding as the
toughening mechanisms responsible for the improved toughening performance.
Ma et al. [111] also revealed that internal rubber cavitation in combination with
matrix yielding are the dominant mechanisms for the PLA/ethylene-co-vinyl
acetate copolymer blends, reaching 83 kJ m −2 with a rubber content as high as
30 wt% (Figure 10.13).
Kang et al. used a new bioelastomer to promote toughness improvement
(impact strength of 10.3 kJ m −2 and elongation at break of 300% at 11.5 vol%)
within the resulting PLA-based materials upon a large-scale matrix deformation
induced by cavitation of rubber particles [112]. In addition, Liu et al. found that
debonding mainly occurs around the relatively large particles, together with fib-
rillated crazes and absence of cavitation when blended with an ethylene/n-butyl
acrylate/glycidyl methacrylate terpolymer (EBA–GMA) [113, 114]. However, the
addition of a zinc ionomer of ethylene/methacrylic acid copolymer (EMAA–Zn)
within the PLA/EBA–GMA blends gradually turned the morphology into
the so-called “salami-like” phase structure, which provides a low cavitation
resistance coupled with suitable interfacial adhesion. Internal cavitation of the
dispersed particles followed by matrix shear yielding was thereby predominant
and the material resulted in an optimum impact strength (860 J m −1 at 15 wt%
EBA–GMA and 5 wt% EMAA–Zn) [113] (Figure 10.14).

Cavities in
rubber

Cavitation in rubber
500 nm 5 μm

Figure 10.13 Morphology of the PLA/EVA (80/20 wt/wt%) blends after impact testing (a)
and necking (b). Reproduced with permission from Ref. [111] © 2012, Elsevier.