of PHAs with renewable materials, such as starch or cellulose, a technique that requires a smaller amount of PHA per unit (4). The blends still have properties that are easy to modify, providing a viable, less expensive alternative to pure PHAs (4). Blends also degrade better than PHAs without renewable materials. When PHAs are blended with hydrophilic polymers, more water can penetrate the plastic and increase the efficiency of degradation (5). The sole caveat to using blends is that when creating the blends, manufacturers must mix the biodegradable substance with the renewable sources thoroughly to avoid having small starch or cellulose particles that interrupt the plastic’s biodegradation and harm the environment (4).
Synthesis of PHA with Plants Production costs are major impediments to the marketability of biodegradable plastics. Researchers have experimented with various methods of production, but one of the most promising techniques is cultivating PHAs in transgenic plants. Synthesizing PHAs through bacterial fermentation costs five times as much as the production of petroleumbased plastics because of low yields per bacterium (6). By using transgenic plants to produce PHAs, net yields increase at a lower cost. The plants can grow PHAs by redirecting cytoplasmic acetyl-CoA present in the plant to produce PHB (6). However, redirecting cytoplasmic acetylCoA stunts plant growth and negatively affects yield of both PHB and the plant itself (7). To avoid this side effect, researchers have instead targeted PHB production to specific areas of the plant with existing high levels of acetyl-CoA, such as chloroplasts (7). When synthesized in this manner, PHB yields make up 15% of the plant’s dry weight, dramatically lowering the cost of production (6).
Polylactide (PLA) Another popular biodegradable plastic is polylactide, or PLA. PLA is a synthetic polyester that biodegrades within a year, decaying much faster than conventional petroleum-based plastics (8). The creation of PLA involves bacterial fermentation, similar to the fermentation in the synthesis of PHAs. This fermentation creates lactic acid, which is then polymerized (5,8). Manufacturers use PLA because its method of synthesis is more economical than the 14
synthesis of other biodegradable plastics. Scientists can already produce lactic acid inexpensively, so the cost of producing PLA is less than the cost of producing PHAs (8). Furthermore, PLA is biocompatible and can be utilized in biomedical applications, such as in medical plates and screws that can be degraded and absorbed by the body (8). However, PLA exhibits several physical and mechanical properties that make it more difficult to use than PHAs or other plastic options for applications outside of biomedicine. PLA is brittle, thermally unstable, and hydrophobic (4). PLA degrades by hydrolysis with no need for external enzymes, but creates a large build-up of lactic acid during degradation, which can cause problems in biomedical applications (4,8). Like PHAs, PLA has a variety of physical and mechanical properties that can be changed by altering its chemical structure and molecular weight (8). Manufacturers can also blend PLA with renewable polymers to alter its properties and lower production costs (4).
Environmental Effects Researchers have worked on developing biodegradable plastics with the hopes of bettering the environment, but the production methods and applications of biodegradable plastics could still be detrimental to environmental and human health. The waste management infrastructure currently recycles regular plastic waste, incinerates it, or places it in a landfill (9). Mixing biodegradable plastics into the regular waste infrastructure poses some dangers to the environment. Biodegradable plastics behave differently when recycled, and have the potential to negatively influence to human health. To be effective in food packaging, plastics must exhibit gas permeability, chemical resistance, and tensile strength (10). If the food packaging materials are recycled, their physical properties could change, allowing degraded chemical compounds and external contaminants to enter the food (10). On top of that, plastics contaminated with food are difficult to recycle, and blended plastics sometimes leave behind starch residues that can further contaminate the recycling process (9,10). Another option for biodegradable plastic waste is incineration with energy capture, so that the energy that goes into producing the plastic could be reclaimed during decomposition. However, incineration of biodegradable plastics does
not create any more energy than petroleumbased plastics, so the environmental effects of the two are roughly equivalent (9). The third option is landfilling biodegradable plastics. However, when biodegradable plastics decompose, they produce methane gas, a major contributor to global warming (9). While methane gas could be collected and used as an energy source, capturing that energy would be another expense, and some of the gas would still escape. Thus, the biodegradable nature of these plastics poses economic and ecological problems in the current waste management infrastructure. To assess the environmental costs and benefits of biodegradable waste, James Levis and Morton Barlaz, researchers at the Department of Civil, Construction, and Environmental Engineering at North Carolina State University, developed an equation for the “global warming potential” of waste. Their equation considers landfill construction, operations, cover placement, gas management, maintenance, and monitoring (11). With this equation, researchers can estimate the emissions released during the production and disposal of a biodegradable substance and compare the figures to the estimated emissions for producing and disposing petroleum-based substances. The substance with lower overall estimated emissions is considered to be “better” for the environment. In their study, Levis and Barlaz used a hypothetical biodegradable polymer and found that petroleum-based polymers may have a less negative impact on the environment than biodegradable plastics (11).
Conclusion Biodegradable plastics offer a promising alternative to petroleum-based plastics. While petroleum-based products use oil in their manufacturing and take up space in landfills, biodegradable
Image courtesy of Trevor MacInnis. Retrieved from http://commons.wikimedia.org/wiki/ File:Fuel_Barrels.JPG (accessed 20 February 2013)
Figure 2: In 2006, 331 million barrels of petroleum were used for making plastic. DARTMOUTH UNDERGRADUATE JOURNAL OF SCIENCE