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10.2 Polylactide Strengthening and Strategies 253
(crazing) but also the emergence of some cavities (cavitation) and a clear plastic
deformation, which implied shear yielding of the matrix. The corresponding
amount of plastic deformation was high and effectively dissipated the fracture
energy. Therefore, the materials resulted in highly improved toughness at room
temperature as evidenced by the maximum elongation at break and impact
strength of 335% and 60.5 kJ m −2 at 30 wt% of PEBA [117]. All these examples
about toughening mechanisms within PLA are not exhaustive but strengthen
the argument that toughness of PLA is a complex function which implies all
as-described mechanisms (crazing, shear yielding, cavitation, and debonding)
and modes of fracture.
A couple of commercially available impact modifiers have recently been
launched and produced by many suppliers to provide an economic solution to
the brittleness of some materials. Accordingly, many investigations have been
made to reach the most effective impact modifier in a wide range of applications.
Commercial impact modifiers with minimal effect on mechanical performances
and related stiffness or heat distortion as well as no adverse effect on transparency,
health, or the environment are thereby targeted by companies. These impact
modifiers are either linear elastomers having low T or cross-linked core–shell
g
polymers, which typically consist of a low T rubbery core encapsulated by a
g
glassy shell that has a good interfacial adhesion with the matrix [41]. NatureWorks
has recently reported commercial toughening agents for PLA in “Technology
Focus Reports” available on their Web site [52, 118]. Accordingly, the most used
commercially available impact modifiers for PLA are listed in Table 10.2.
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Among them, Biomax Strong from DuPont Company is probably the most
investigated one with PLA. This impact modifier is reported to be EACs and is
designed to improve the toughness of PLA in packaging and other industrial
applications with minimal effect on transparency. It is claimed that the addition
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of less than 5 wt% of Biomax Strong to PLA results in a significant increase
in impact strength, together with a contact clarity similar to clarified PP (see
Figure 10.16). To support this statement, Afrifah and Matuana [116] investigated
the toughening effects and related toughening mechanisms of PLA blended with
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Biomax Strong 100. It was found that the presence of 15 wt% of Biomax Strong
100 reduced the brittle-to-ductile transition temperature of PLA. In addition,
the notched Izod impact strength of PLA increased with the impact modifier
concentrations from 17 to 88 J m −1 and 248 J m −1 at 20 and 40 wt%, respectively.
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Similarly, Biomax Strong was used by Taib et al. [115], highlighting a significant
improvement in notched Izod impact strength of the brittle PLA from 3.6 to
14 kJ m −2 and 28 kJ m −2 at 10 and 20 wt%, respectively. Finally, Murariu et al.
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[142] studied toughening effects of Biomax Strong 100 on PLA and high-filled
PLA/β-calcium sulfate anhydrite II (AII) composites. It was shown that the
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notched Izod impact strength of PLA containing 5 and 10 wt% of Biomax Strong
−2
100 increased from 2.6 to 4.6 kJ m −2 and even 12.4 kJ m ,respectively. The
addition of 5 and 10 wt% of the impact modifier to the PLA/AII (70/30 wt/wt%)
−2
composite also increased their impact strength to 4.5 and 5.7 kJ m ,respectively.