Page 288 - Biodegradable Polyesters

266 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques

Neat PLA Nucleated PLA
50 μm 50 μm

Figure 10.23 Comparison of crystal growth between neat PLA (a) and PLA containing EBH
as a nucleating agent (b). Reproduced with permission from Ref. [199] © 2012, John Wiley
and sons.
simultaneously showed improved modulus and impact strength, which are related
to high crystallinity and small spherulite size of nucleated PLA (Figure 10.23). In
a similar way, the combination of talc and N,N’-ethylenebis(stearamide) (EBS)
nucleating agents was thought by Harris and Lee [44] to achieve both reduced
cycle times and reduced energy requirements in the injection molding process of
PLA. As a result, the materials with higher crystallinity showed correspondingly
increased mechanical properties and improved heat distortion temperature
(HDT), and allowed for a decreased processing time. Similarly, Kawamoto et al.
[200] found that the molding cycle time of PLA was less than 3 min in the
presence of octamethylenedicarboxylic dibenzoylhydrazide (OMBH). Directly
related to this enhanced PLA crystallization, excellent physical and mechanical
properties were achieved as evidenced by a flexural modulus of 4.1 GPa and
−2
an Izod impact strength of 7.9 kJ m . Accordingly, the utilization of effective
nucleating agents makes it possible to extend the application range of PLA as
automotive parts and electric appliances.
Surprisingly, it was evidenced that crystallization of the PLA matrix also plays
a significant role in the toughening of immiscible PLA/impact modifier blends.
For instance, the notched impact strength of untreated PLA/ethylene-glycidyl
methacrylate (EGMA) blends increases to reach an impact strength over 50 times
−2
higher than that of the neat PLA (i.e., 72 kJ m ) after annealing [106]. The anneal-
ing effect was also investigated on the mechanical properties of PLA/metallocene
polyethylene blends containing an ethylene-glycidyl methacrylate-vinyl acetate
compatibilizer [201]. Tensile results showed that the synergistic effects by com-
patibilization and annealing treatment improves the tensile strength and Young’s
modulus, by up to 38% and 62% increase, respectively. In addition to tensile
properties, a brief impact test was also conducted, reaching a slight increase for
annealed samples. This was ascribed to the increased density of entangled PLA
chains. In addition, Ran et al. [202] investigated the crystallization morphology
and tensile properties of the annealed PLA/poly(ethylene glycol-co-propylene
glycol) blends to highlight that larger crystallinity level characterized by fine
spherulites had a positive effect on the tensile properties of the material. Many