Environmental Engineering Reference
In-Depth Information
examined [195]. Notably, changes in polymer chain branching and L:D ratios had no effect
on the permeation properties of PLA with respect to small gases.
4. Research Priorities
4.1. Development of LCIA Tools
One of the greatest challenges facing the production of truly environmentally-benign
plastic materials, PLA and otherwise, is the evaluation of net environmental impacts,
beginning with feedstock production (agriculture or collection of biomass wastes), including
processing steps (production of lactide and subsequent polymerization) and ending with the
emissions resulting from biodegradation. Specific processes chosen at each stage, particularly
concerning conventional vs. sustainable methods, are likely to have dramatic impacts on the
net environmental profiles of individual materials, yet the tools with which to evaluate these
differences are not yet fully refined [196]. These metrics are needed urgently, both to guide
research and development of bioplastics and to advocate use of the truly environmentally
beneficial materials. Consequently, a top priority in the development of environmentally
benign plastics is the continuation of efforts to develop tools and standards within the context
of LCIA that will make comparisons transparent and meaningful.
4.2. Improvement of Physical Properties
Presently, PLA and other biopolyesters suffer from two important deficiencies that limit
their use. The first of these is their relatively low heat distortion temperatures, and the second
is their relatively high permeabilities toward a number of substances, particularly water. As
current, best-available LCIA analyses have indicated that PLA is indeed environmentally
benign, continued research into biological, chemical, and physical transformations of PLA-
based materials to improve these properties is warranted. In particular, nanocomposite
technologies (Chapter III.D.) hold promise of improving both temperature distortion and
permeation characteristics, as they have in conventional plastics, and should be investigated.
Microcomposite technologies are related, already well-established approaches to achieve
similar improvements in conventional plastics. In addition, plant microparticles derived from
waste agricultural residues and simple grasses can be used directly as microparticles,
providing both economic and environmental advantages [197, 198]. Alternatively, blending
and trans-reacting PLA-based plastics with starch- or triglyceride-based materials (Chapter
III.D.) may improve performance while maintaining biodegradability, with the result that
these techniques also deserve further investigation [199-205]. Recently, copolymerization of
cellulose acetate with PLA has demonstrated that the heat distortion temperature can be
increased [206]. In this interesting case, both constituents of the plastic material come from
renewable resources. This suggests that copolymerization of PLA, especially with other
polymers based on renewable resources, can provide a viable route towards improved
performance.
4.3. Exploration and Development of New Polyesters
A recent comprehensive study by the DOE [207] has identified 12 promising low
molecular-weight materials that can be produced by fermentation in commercial quantities
