Page 264 - Biodegradable Polyesters

242 10 Highly Toughened Polylactide-Based Materials through Melt-Blending Techniques

Table 10.1 Comparison of PLA properties with petroleum-based commodity thermoplas-
tics. Adapted from [41] with permission from John Wiley and sons.

PLA PET PS HIPS PP
∘
T ( C) 55–60 75 105 — −10
g
Tensile strength at break (MPa) 53 54 45 23 31
Tensile modulus (GPa) 3.4 2.8 2.9 2.1 0.9
Elongation at break (%) 6 130 7 45 120
−1
Notched Izod impact strength (J m ) 13 59 27 123 27
Cost (€ per kg) 1.6–2.4 1.1–1.2 1.6–1.65 1.65–1.7 1.85–1.9
PET, polyethylene terephthalate; PS, polystyrene; HIPS, high-impact polystyrene;
PP, polypropylene.
• Hydrolytic instability: PLA readily hydrolyzes through its polyester backbone,
depending on its crystallinity, molecular weight and related distribution, mor-
phology, water diffusion rate into the polymer, and the stereoisomeric content.
This represents a serious problem in, for example, long-lasting applications [42].
• Thermal instability: Significant molecular weight degradation can occur when
∘
PLA gets held 10 C above its melting point over a long period of time, nar-
rowing its processing window [29, 43]. The thermal degradation of PLA occurs
by the hydrolysis process, unzipping depolymerization, oxidative main-chain
scission, and inter- and intramolecular transesterification reactions.
• Gas and water-permeability: PLA has insufficient barrier behavior to oxygen,
carbon dioxide, and water vapor compared to other benchmark packaging poly-
mers such as polyolefins (PE, PP) and PET [19].
• Crystallization rate: PLA crystallization rate is quite low, leading to relatively
amorphous materials under conventional processing methods [24, 44].
• Toughness: Similar to PS, PLA is a brittle material with low impact strength and
elongation at break, impeding the industrial development of PLA [45].
However, the main drawback of PLA-based materials remains their high brit-
tleness to be addressed to span PLA applications from commodity to engineer-
ing/high performance materials. This will be discussed in the following.

10.2
Polylactide Strengthening and Strategies

There are many strategies to modify the mechanical properties of PLA in an
efficient manner, including the copolymerization with other types of monomers,
stereocomplexation, incorporation of (nano)fillers, polymer blending, annealing
process, and addition of plasticizers or impact modifiers [24]. However, poly-
mer blending represents the most extensively used methodology to improve
PLA mechanical properties, especially from industrial perspectives. Blending
PLA with other polymers offers the possibility of modifying, for example, the
degradation rate, permeability characteristics, drug release profiles, thermal, and