milled , hydrolysis com-> starches-*

milled , hydrolysis com-> starches-*










fermentation d-glucose fermentation

d-glucose dimerization

polylactic acid melt polymerization H3C, melt polymerization H3C,

FIGURE 7-12 The synthesis of polylactic acid.

The environmental advantages of PLA over petroleum-based polymer include the following:

• It is made from annually renewable resources (corn, sugar beets, and eventually waste biomass).

• Production of PLA consumes 20-50% less fossil-fuel resources than do petroleum-based polymers.

• It uses natural fermentation to produce lactic acid. No organic solvents or other hazardous substances are used.

• It uses catalysts, resulting in reduced energy and resource consumption.

• High yields of over 95% are obtained.

• The use of recycle streams help to reduce waste.

• PLA can be recycled—converted back to its monomer via hydrolysis, then repolymerized to produce virgin polymer (i.e., closed-loop recycling).

• PLA can be composted (it is biodegradable); complete degradation occurs in a few weeks under normal composting conditions.

Another environmental consideration of PLA is that the plants, such as corn, used to produce the polymer consume atmospheric carbon dioxide, thus reducing the concentrations of this greenhouse gas. When PLA biodegrades, it releases this carbon dioxide back into the atmosphere in amounts approximately equal to the carbon dioxide absorbed by the plants used to produce it, thus making PLA in theory carbon neutral. However, fossil fuels are required during the production of PLA. Life cycle assessment studies (see Chapter 16) indicate that PLA requires 25-55% less fossil-fuel-supplied energy than do petrochemical polymers.

PLA can be used to produce products that are currently made from petroleum-based polymers such as cups; rigid food containers; food wrappers and bags; bags for refuse; furnishings for homes and offices (carpet tile, upholstery, awnings, and industrial wall panels); and fibers for clothing, pillows, and diapers. Small purveyors of natural foods have been using PLA packaging for several years. In 2005 PLA received a significant boost when Wal-Mart announced plans to use 114 million PLA containers a year. According to the company, this will save 800,000 barrels of oil annually.

Biodegradable polymers produced from renewable resources help to reduce our consumption of oil and have the potential to offer significant environmental and economic advantages over petroleum-based polymers. However, we must remember that even production of chemicals from annually renewable resources such as biomass does not offer a complete solution to energy and environmental problems. Growing crops, whether they are used to produce food or chemicals, require fertilizers and pesticides. Energy is needed to plant, cultivate, and harvest; to produce, transport, and apply fertilizers and pesticides; to make and run tractors; to transport seeds, biomass, monomers, and polymers. Use of land to produce crops for chemicals, and more significantly for bio-based fuels, also removes land that could be used to produce food and feed, and it increases the price for food.

Continue reading here: The Extent and Potential Consequences of Future Global Warming

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