Environmental Engineering Reference
In-Depth Information
3.6. Processing Properties
Plastics are typically fabricated into useful articles in the molten state, thereby causing
the melt flow, or rheological, properties of a polymer to be of great importance. As discussed
above, PLAs have physical properties useful in fibers, packaging, and other applications
traditionally dominated by petroleum-based resins. Although the general literature on
polylactides is extensive, only a few articles [177-181] have considered rheological
properties. Measurements of dynamic, steady, and transient shear viscosities have been
presented and the extensional data on PLA showed a strong strain hardening behavior [181].
These studies did not, however, capture a systematic description of PLA rheology across a
broad range of stereooptical composition, as materials studied to date have usually possessed
high (>90 percent)
L-stereochemical center content
. However, a recent study [182] has
provided a comprehensive evaluation of the linear viscoelastic properties of PLA across a
wide range of molecular weights and stereochemical compositions. The rheology of blends of
linear and branched-PLA architectures has also been comprehensively investigated [180,
183]. The results suggest that excellent control over rheological behavior of PLA is possible
through blending chain architectures without compromising mechanical properties.
Mechanical properties of solution-spun [36, 37, 184, 185] and melt-spun [34, 36, 186-
189] PLA fibers have been thoroughly investigated. It has been found that these properties are
roughly equivalent to other polyesters meaning that PLA can replace textiles based on non-
renewable resources. In addition, scanning electron microscopy (SEM) [34, 37, 184, 186] and
wide-angle x-ray scattering (WAXS) [36, 190] have been useful in examining surface
structure with respect to roughness and fracture surfaces. Understanding surface properties is
important for dying and other textile finishing operations. Cicero et al. have provided a
complete characterization of the hierarchical fiber morphology from linear PLAs [35, 191],
determining thermal, mechanical, and morphological properties of the fibers and showing that
properties can be widely manipulated through a combination of processing temperature and
draw ratio (the amount of stretching the fiber undergoes). The same researchers have studied
the effects of branching on fiber properties and morphology [192] and investigated the
improvement of fiber properties specifically when thermally stabilized PLA is used [193].
These research studies provide several routes for optimizing the performance of PLA when
used as a textile fiber in place of conventional, fossil resource based polyesters such at PET.
3.7. Permeation Properties
Because of the desirability of using PLA in packaging applications, understanding the
permeation properties of PLA with respect to various gases and vapors, especially those of
interest to the food industry, is extremely important. PLA is, unfortunately, a relatively poor
barrier to water vapor and CO
2
, with a water-vapor transmission rate significantly higher than
those of PET, polypropylene (PP), or polyvinyl chloride (PVC), [194] resulting in some
limitated highly environmentally beneficial applications of PLA to packaging. For example,
PLA's high permeability to water prevents it from being used to bottle water over long
durations, despite the fact that increasing sales of bottled water have created considerable
pressure on landfill space, particularly in California.
In response, Natureworks has developed several biaxially-oriented PLA films to improve
PLA barrier properties, including two with trade names of PLA 4030-D and PLA 4040-D.
Permeation of nitrogen, oxygen, CO
2
, and CH4 in very thin (5 µm) amorphous films of
various grades of PLA (L:D ratios from 95:5 to 98:2) cast from solution have also been
