Environmental Engineering Reference
In-Depth Information
The degradation of polymeric materials in the environment is a critical area of concern
due to the large quantity of plastics generated. Each year, over 70 million tons of polymers
are produced which end up in landfills [14, 15]. Breakdown of PLA in the environment can
occur biotically or abiotically [16]. In the absence of sufficient microbial activity or oxygen,
hydrolysis becomes the predominant pathway for degradation, while in aerated composting
environments, biotic processes can degrade PLA rapidly (within weeks) and completely [17,
18]. Regardless of the process, degradation is also subject to the form of the plastic and purity
of the PLA used [19]. As an alternative to biodegradation, waste PLA can be recycled into
lactic acid, which can then be reformed into lactide and repolymerized [16, 17].
To assess the environmental impact of PLA synthesis and use, a comprehensive LCA
enabling apples-to-apples comparisons with petrochemical-based thermoplastics was recently
undertaken (Figure 8) [20]. Among the most notable benefits of PLA shown were reductions
in both fossil-fuel use and global warming potential, even assuming use of fossil-based
energy sources for agriculture and processing. Compared to most traditional hydrocarbon-
based polymers, PLA uses 3 0-50 percent less fossil-fuel energy and results in lower CO2
emissions by 50-70 percent. Whereas conventional thermoplastic polymers require oil as
their source of monomers and additional fossil fuels for processing, solar energy provides
approximately one- third of the gross energy requirement in PLA production; in addition,
processing of oil into conventional plastics releases even greater amounts of CO2 than does
PLA production [20]. On October 11, 2005 Natureworks announced that they would purchase
renewable energy certificates to offset the fossil energy being used in the production of PLA.
By doing so, they claim to be producing the first-ever greenhouse gas-neutral commercial
plastic (see www.natureworks.com).
PLA1 represents present technology; PLA Bio/WP is the projection for the production of PLA from
agricultural waste using wind power
Figure 8. Life cycle analysis results for energy content of various thermoplastic polymers [20].
3. State of the Science
3.1. Physical Properties
The bulk properties of PLA are greatly affected by the molecular weight of the polymer,
the chain architecture (branched vs. linear), and the degree of crystallinity in the polymer [21,
22]. The amount of crystalline character within a type of PLA, in turn, is determined by the
