Figure 1: Knives, forks, and spoons made from a biodegradable starch-polyester material. Biodegradable plastics offer a promising alternative to petroleum-based plastic, but the high cost and low yield associated with their production pose significant obstacles
Living “green” has become a popular trend in the last twenty years, and reducing oil consumption remains an important goal for the sustainably-minded today. One of the top uses of crude oil is the production of plastics. In 2010, 191 million barrels of liquid petroleum gases and natural gas liquids were used to make plastic products in the United States (1). These plastics later end up in landfills, where they take years to break down. Petroleum-based plastics are becoming more expensive as oil prices continue to increase, and plastic accumulation in landfills takes up valuable space and threatens the environment. The economic and ecological drawbacks of petroleum-based plastics have pushed researchers to develop and investigate biodegradable plastics as an environmentally-friendly alternative.
During the 1920s, Maurice Lemoigne, a French researcher, discovered the first biodegradable plastic from his work with the bacterium Bacillus megaterium (2). Lemoigne isolated poly-3-hydroxybuturate (PHB), but his work went unnoticed for many years. Decades later, microbiologists in the United States and Great Britain independently discovered PHB in 1957 and 1958, respectively (2). However, research into biodegradable plastics slowed until the oil crisis of the 1970s (3). As nations faced the reality of rising oil prices, they encouraged research for synthesizing alternatives to petroleum-based products. However, when the oil crisis died down, interest in biodegradable plastics research decreased once more. In the 1990s, the research again became popular, this time in the biomedical industry. Between medical applications and fluctuating oil prices, research in biodegradable plastics has become steady and profitable. So far, researchers have invented several different types of plastics and a variety of manufacturing methods.
Polyhydroxyalkanoates (PHA)
PHB is a type of polyhydroxyalkanoate, or PHA (2). PHAs are biodegradable thermoplastics that are synthesized by many different types of bacteria. When bacteria develop in nutrient-deficient environments, bacteria create PHAs as food and energy reserves, which are then stored as insoluble granules in the cytoplasm (2,3). Depending on their molecular composition, PHAs have varying physical properties, but all PHAs biodegrade in carbon dioxide and water (3). PHAs can degrade in either aerobic or anaerobic environments through thermal degradation or enzymatic hydrolysis (3). Bacteria, algae, and fungi use extracellular enzymes to depolymerize, or break down, PHAs and, through a process known as mineralization, absorb the remaining fragments for use as minerals (4). Manufacturers can alter the properties of PHAs by changing their structure or composition. PHAs have a variety of applications in water-resistant surfaces, binders, synthetic paper, medical devices, electronic parts, food packaging, and agriculture (3). However, PHAs currently have economic drawbacks that limit their use. Until recently, PHAs have had high production costs, low yields, and limited availability (3). Consequently, they have not been able to displace the petroleum-based plastics that manufacturers can cheaply create in bulk.
One method of reducing the cost of production is to create a polymer blend 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 petroleum-based 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 acetyl-CoA 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 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).
Figure 2: In 2010, 191 million barrels of liquid petroleum gases and natural gas liquids were used to make plastic products in the United States
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 petroleum-based 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 plastics can be synthesized in bacteria or plants and have the potential to be disposed of in a way that is less damaging to the environment. Biodegradable plastics have a variety of applications, from agriculture and food packaging to biomedical devices and tableware. The major obstacles to replacing petroleum-based plastics with biodegradable plastics are high costs and low yields associated with existing methods of biodegradable plastic production. With more research into plant-based manufacturing systems, these obstacles are being overcome. Finally, the last obstacle to surmount is the proper disposal of biodegradable plastics. In order for biodegradable plastics to be effectively disposed of, the current waste management infrastructure must change, or methods with less economic and environmental costs must be developed.
The article has been edited to remove an incorrect reference in petroleum use for fuel consumption and to reflect more recent data from the U.S. Energy Information Administration.
Contact Kristen Flint at
Kristen.L.Flint.14@dartmouth.edu
References
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