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How Green are Green Plastics?
Scientific American Aug00
It is now technologically possible to make plastics using green
plants rather than fossil fuels. But are these new plastics the
environmental saviors researchers have hoped for?
Driving down a dusty gravel road in central Iowa, a farmer gazes
toward the horizon at rows of tall, leafy corn plants shuddering
in the breeze as far as the eye can see. The farmer smiles to
himself, because he knows something about his crop that few people
realize. Not only are kernels of corn growing in the ears, but
granules of plastic are sprouting in the stalks and leaves.
This idyllic notion of growing plastic, achievable in the foreseeable
future, seems vastly more appealing than manufacturing plastic
in petrochemical factories, which consume about 270 million tons
of oil and gas every year worldwide. Fossil fuels provide both
the power and the raw materials that transform crude oil into
common plastics such as polystyrene, polyethylene and polypropylene.
From milk jugs and soda bottles to clothing and car parts, it
is difficult to imagine everyday life without plastics, but the
sustainability of their production has increasingly been called
into question. Known global reserves of oil are expected to run
dry in approximately 80 years, natural gas in 70 years and coal
in 700 years, but the economic impact of their depletion could
hit much sooner. As the resources diminish, prices will go up--a
reality that has not escaped the attention of policymakers. President
Bill Clinton issued an executive order in August 1999 insisting
that researchers work toward replacing fossil resources with plant
material both as fuel and as raw material.
With those concerns in mind, biochemical engineers, including
the two of us, were delighted by the discovery of how to grow
plastic in plants. On the surface, this technological breakthrough
seemed to be the final answer to the sustainability question,
because this plant-based plastic would be "green" in
two ways: it would be made from a renewable resource, and it would
eventually break down, or biodegrade, upon disposal. Other types
of plastics, also made from plants, hold similar appeal. Recent
research, however, has raised doubts about the utility of these
approaches. For one, biodegradability has a hidden cost: the biological
breakdown of plastics releases carbon dioxide and methane, heat-trapping
greenhouse gases that international efforts currently aim to reduce.
What is more, fossil fuels would still be needed to power the
process that extracts the plastic from the plants, an energy requirement
that we discovered is much greater than anyone had thought. Successfully
making green plastics depends on whether researchers can overcome
these energy-consumption obstacles economically--and without creating
additional environmental burdens.
Traditional manufacturing of plastics uses a surprisingly large
amount of fossil fuel. Automobiles, trucks, jets and power plants
account for more than 90 percent of the output from crude-oil
refineries, but plastics consume the bulk of the remainder, around
80 million tons a year in the U.S. alone. To date, the efforts
of the biotechnology and agricultural industries to replace conventional
plastics with plant-derived alternatives have embraced three main
approaches: converting plant sugars into plastic, producing plastic
inside microorganisms, and growing plastic in corn and other crops.
Cargill, an agricultural business giant, and Dow Chemical, a top
chemical firm, joined forces three years ago to develop the first
approach, which turns sugar from corn and other plants into a
plastic called polylactide (PLA). Microorganisms transform the
sugar into lactic acid, and another step chemically links the
molecules of lactic acid into chains of plastic with attributes
similar to polyethylene terephthalate (PET), a petrochemical plastic
used in soda bottles and clothing fibers.
Looking for new products based on corn sugar was a natural extension
of Cargill's activities within the existing corn-wet-milling industry,
which converts corn grain to products such as high-fructose corn
syrup, citric acid, vegetable oil, bioethanol and animal feed.
In 1999 this industry processed almost 39 million tons of corn--roughly
15 percent of the entire U.S. harvest for that year. Indeed, Cargill
Dow earlier this year launched a $300-million effort to begin
mass-producing its new plastic, NatureWorksTM PLA, by the end
of 2001 [see Gruber interview].
PRODUCTION AND ENERGY DEMANDS
PLANT-BASED PLASTICS
PHA (grown in corn plants) = 90 m/kg* of plastic
Corn stover grown, harvested and delivered to factory
Plastic extracted from stover using solvents
Solvents distilled and separated from plastic
PHA (bacterial fermentation) = 81 m/kg of plastic
Corn or other plants grown, harvested and delivered to factory
Plants processed to yield sugar
Sugar fermented into plastic inside bacteria
Bacterial cells opened; plastic separated, concentrated and dried
PLA = 56 m/kg of plastic
FOSSIL FUEL-BASED PLASTICS
Energy* Raw Materials*
PE 29 81
PET 37 76
NYLON 93 142
* m/kg = megajoules per kilogram of plasticOther companies, including
Imperial Chemical Industries, developed ways to produce a second
plastic, called polyhydroxyalkanoate (PHA). Like PLA, PHA is made
from plant sugar and is biodegradable. In the case of PHA, however,
the bacterium Ralstonia eutropha converts sugar directly into
plastic. PLA requires a chemical step outside the organism to
synthesize the plastic, but PHA naturally accumulates within the
microbes as granules that can constitute up to 90 percent of a
single cell's mass.
In response to the oil crises of the 1970s, Imperial Chemical
Industries established an industrial-scale fermentation process
in which microorganisms busily converted plant sugar into several
tons of PHA a year. Other companies molded the plastic into commercial
items such as biodegradable razors and shampoo bottles and sold
them in niche markets, but this plastic turned out to cost substantially
more than its fossil fuel-based counterparts and offered no performance
advantages other than biodegradability. Monsanto bought the process
and associated patents in 1995, but profitability remained elusive.
Many corporate and academic groups, including Monsanto, have since
channeled their efforts to produce PHA into the third approach:
growing the plastic in plants. Modifying the genetic makeup of
an agricultural crop so that it could synthesize plastic as it
grew would eliminate the fermentation process altogether. Instead
of growing the crop, harvesting it, processing the plants to yield
sugar and fermenting the sugar to convert it to plastic, one could
produce the plastic directly in the plant. Many researchers viewed
this approach as the most efficient--and most elegant--solution
for making plastic from a renewable resource. Numerous groups
were (and still are) in hot pursuit of this goal.
In the mid-1980s one of us (Slater) was part of a group that isolated
the genes that enable the bacteria to make plastic. Investigators
predicted that inserting these enzymes into a plant would drive
the conversion of acetyl coenzyme A--a compound that forms naturally
as the plant converts sunlight into energy--into a type of plastic.
In 1992 a collaboration of scientists at Michigan State University
and James Madison University first accomplished this task. The
researchers genetically engineered the plant Arabidopsis thaliana
to produce a brittle type of PHA. Two years later Monsanto began
working to produce a more flexible PHA within a common agricultural
plant: corn.
So that plastic production would not compete with food production,
the researchers targeted part of the corn plant that is not typically
harvested--the leaves and stem, together called the stover. Growing
plastic in stover would still allow farmers to harvest the corn
grain with a traditional combine; they could comb the fields a
second time to remove the plastic-containing stalks and leaves.
Unlike production of PLA and PHA made by fermentation, which theoretically
compete for land used to grow crops for other purposes, growing
PHA in corn stover would enable both grain and plastic to be reaped
from the same field. (Using plants that can grow in marginal environments,
such as switchgrass, would also avoid competition between plastic
production and other needs for land.)
The Problem: Energy and Emissions
Researchers have made significant technological progress toward
increasing the amount of plastic in the plant and altering the
composition of the plastic to give it useful properties. Although
these results are encouraging when viewed individually, achieving
both a useful composition and high plastic content in the plant
turns out to be difficult. The chloroplasts of the leaves have
so far shown themselves to be the best location for producing
plastic. But the chloroplast is the green organelle that captures
light, and high concentrations of plastic could thus inhibit photosynthesis
and reduce grain yields.
The challenges of separating the plastic from the plant, too,
are formidable. Researchers at Monsanto originally viewed the
extraction facility as an adjunct to an existing corn-processing
plant. But when they designed a theoretical facility, they determined
that extracting and collecting the plastic would require large
amounts of solvent, which would have to be recovered after use.
This processing infrastructure rivaled existing petrochemical
plastic factories in magnitude and exceeded the size of the original
corn mill.
Given sufficient time and funding, researchers could overcome
these technical obstacles. Both of us, in fact, had planned for
the development of biodegradable plastics to fill the next several
years of our research agendas. But a greater concern has made
us question whether those solutions are worth pursuing. When we
calculated all the energy and raw materials required for each
step of growing PHA in plants--harvesting and drying the corn
stover, extracting PHA from the stover, purifying the plastic,
separating and recycling the solvent, and blending the plastic
to produce a resin--we discovered that this approach would consume
even more fossil resources than most petrochemical manufacturing
routes.
In our most recent study, completed this past spring, we and our
colleagues found that making one kilogram of PHA from genetically
modified corn plants would require about 300 percent more energy
than the 29 megajoules needed to manufacture an equal amount of
fossil fuel-based polyethylene (PE). To our disappointment, the
benefit of using corn instead of oil as a raw material could not
offset this substantially higher energy demand.
Based on current patterns of energy use in the corn-processing
industry, it would take 2.65 kilograms of fossil fuel to power
the production of a single kilogram of PHA. Using data collected
by the Association of European Plastics Manufacturers for 36 European
plastic factories, we estimated that one kilogram of polyethylene,
in contrast, requires about 2.2 kilograms of oil and natural gas,
nearly half of which ends up in the final product. That means
only 60 percent of the total--or 1.3 kilograms--is burned to generate
energy.
Given this comparison, it is impossible to argue that plastic
grown in corn and extracted with energy from fossil fuels would
conserve fossil resources. What is gained by substituting the
renewable resource for the finite one is lost in the additional
requirement for energy. In an earlier study, one of us (Gerngross)
discovered that producing a kilogram of PHA by microbial fermentation
requires a similar quantity--2.39 kilograms--of fossil fuel. These
disheartening realizations are part of the reason that Monsanto,
the technological leader in the area of plant-derived PHA, announced
late last year that it would terminate development of these plastic-production
systems.
The only plant-based plastic that is currently being commercialized
is Cargill Dow's PLA. Fueling this process requires 20 to 50 percent
fewer fossil resources than does making plastics from oil, but
it is still significantly more energy intensive than most petrochemical
processes are. Company officials anticipate eventually reducing
the energy requirement. The process has yet to profit from the
decades of work that have benefited the petrochemical industry.
Developing alternative plant-sugar sources that require less energy
to process, such as wheat and beets, is one way to attenuate the
use of fossil fuels. In the meantime, scientists at Cargill Dow
estimate that the first PLA manufacturing facility, now being
built in Blair, Neb., will expend at most 56 megajoules of energy
for every kilogram of plastic--50 percent more than is needed
for PET but 40 percent less than for nylon, another of PLA's petrochemical
competitors.
The energy necessary for producing plant-derived plastics gives
rise to a second, perhaps even greater, environmental concern.
Fossil oil is the primary resource for conventional plastic production,
but making plastic from plants depends mainly on coal and natural
gas, which are used to power the corn-farming and corn-processing
industries. Any of the plant-based methods, therefore, involve
switching from a less abundant fuel (oil) to a more abundant one
(coal). Some experts argue that this switch is a step toward sustainability.
Missing in this logic, however, is the fact that all fossil fuels
used to make plastics from renewable raw materials (corn) must
be burned to generate energy, whereas the petrochemical processes
incorporate a significant portion of the fossil resource into
the final product.
Burning more fossil fuels exacerbates an established global climate
problem by increasing emissions of greenhouse gases, such as carbon
dioxide [see "Is Global Warming Harmful to Health?"
by Paul R. Epstein]. Naturally, other emissions associated with
fossil energy, such as sulfur dioxide, are also likely to increase.
This gas contributes to acid rain and should be viewed with concern.
What is more, any manufacturing process that increases such emissions
stands in direct opposition to the Kyoto Protocol, an international
effort led by the United Nations to improve air quality and curtail
global warming by reducing carbon dioxide and other gases in the
atmosphere.
The conclusions from our analyses were inescapable. The environmental
benefit of growing plastic in plants is overshadowed by unjustifiable
increases in energy consumption and gas emissions. PLA seems to
be the only plant-based plastic that has a chance of becoming
competitive in this regard. Though perhaps not as elegant a solution
as making PHA in plants, it takes advantage of major factors contributing
to an efficient process: low energy requirements and high conversion
yields (almost 80 percent of each kilogram of plant sugar used
ends up in the final plastic product). But despite the advantages
of PLA over other plant-based plastics, its production will inevitably
emit more greenhouse gases than do many of its petrochemical counterparts.
The Answer: Renewable Energy
As sobering as our initial analyses were, we did not immediately
assume that these plant-based technologies were doomed forever.
We imagined that burning plant material, or biomass, could offset
the additional energy requirement. Emissions generated in this
way can be viewed more favorably than the carbon dioxide released
by burning fossil carbon, which has been trapped underground for
millions of years. Burning the carbon contained in corn stalks
and other plants would not increase net carbon dioxide in the
atmosphere, because new plants growing the following spring would,
in theory, absorb an equal amount of the gas. (For the same reason,
plant-based plastics do not increase carbon dioxide levels when
they are incinerated after use.)
We and other researchers reasoned that using renewable biomass
as a primary energy source in the corn-processing industry would
uncouple the production of plastics from fossil resources, but
such a shift would require hurdling some lingering technological
barriers and building an entirely new power-generation infrastructure.
Our next question was, "Will that ever happen?" Indeed,
energy-production patterns in corn-farming states show the exact
opposite trend. Most of these states drew a disproportionate amount
of their electrical energy from coal--86 percent in Iowa, for
example, and 98 percent in Indiana--compared with a national average
of around 56 percent in 1998. (Other states derive more of their
energy from sources such as natural gas, oil and hydroelectric
generators.)
Both Monsanto and Cargill Dow have been looking at strategies
for deriving energy from biomass. In its theoretical analysis,
Monsanto burned all the corn stover that remained after extraction
of the plastic to generate electricity and steam. In this scenario,
biomass-derived electricity was more than sufficient to power
PHA extraction. The excess energy could be exported from the PHA-extraction
facility to replace some of the fossil fuel burned at a nearby
electric power facility, thus reducing overall greenhouse gas
emissions while producing a valuable plastic.
Interestingly, it was switching to a plant-based energy source--not
using plants as a raw material--that generated the primary environmental
benefit. Once we considered the production of plastics and the
production of energy separately, we saw that a rational scheme
would dictate the use of renewable energy over fossil energy for
many industrial processes, regardless of the approach to making
plastics. In other words, why worry about supplying energy to
a process that inherently requires more energy when we have the
option of making conventional plastics with much less energy and
therefore fewer greenhouse gas emissions? It appears that both
emissions and the depletion of fossil resources would be abated
by continuing to make plastics from oil while substituting renewable
biomass as the fuel.
Unfortunately, no single strategy can overcome all the environmental,
technical and economic limitations of the various manufacturing
approaches. Conventional plastics require fossil fuels as a raw
material; PLA and PHA do not. Conventional plastics provide a
broader range of material properties than PLA and PHA, but they
are not biodegradable. Biodegradability helps to relieve the problem
of solid-waste disposal, but degradation gives off greenhouse
gases, thereby compromising air quality. Plant-based PLA and PHA
by fermentation are technologically simpler to produce than PHA
grown in corn, but they compete with other needs for agricultural
land. And although PLA production uses fewer fossil resources
than its petrochemical counterparts, it still requires more energy
and thus emits more greenhouse gases during manufacture.
The choices that we as a society will make ultimately depend on
how we prioritize the depletion of fossil resources, emissions
of greenhouse gases, land use, solid-waste disposal and profitability--all
of which are subject to their own interpretation, political constituencies
and value systems. Regardless of the particular approach to making
plastics, energy use and the resulting emissions constitute the
most significant impact on the environment.
In light of this fact, we propose that any scheme to produce plastics
should not only reduce greenhouse gas emissions but should also
go a step beyond that, to reverse the flux of carbon into the
atmosphere. To accomplish this goal will require finding ways
to produce nondegradable plastic from resources that absorb carbon
dioxide from the atmosphere, such as plants. The plastic could
then be buried after use, which would sequester the carbon in
the ground instead of returning it to the atmosphere. Some biodegradable
plastics may also end up sequestering carbon, because landfills,
where many plastic products end up, typically do not have the
proper conditions to initiate rapid degradation.
In the end, reducing atmospheric levels of carbon dioxide may
be too much to ask of the plastics industry. But any manufacturing
process, not just those for plastics, would benefit from the use
of renewable raw materials and renewable energy. The significant
changes that would be required of the world's electrical power
infrastructure to make this shift might well be worth the effort.
After all, renewable energy is the essential ingredient in any
comprehensive scheme for building a sustainable economy, and as
such, it remains the primary barrier to producing truly "green"
plastics.
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Further Information:
Polyhydroxybutyrate, a Biodegradable Thermoplastic, Produced in
Transgenic Plants. Y. Poirier, D. E. Dennis, K. Klomparins and
C. Somerville in Science, Vol. 256, pages 520-622; April 1992.
Can Biotechnology Move Us toward a Sustainable Society? Tillman
U. Gerngross in Nature Biotechnology, Vol. 17, pages 541-544;
June 1999.
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Author: http://engineering.dartmouth.edu/thayer/faculty/tillmangerngross.html
The Author
TILLMAN U. GERNGROSS and STEVEN C. SLATER have each worked for
more than eight years in industry and academia to develop technologies
for making biodegradable plastics. Both researchers have contributed
to understanding the enzymology and genetics of plastic-producing
bacteria. In the past two years, they have turned their interests
toward the broader issue of how plastics manufacturing affects
the environment. Gerngross is an assistant professor at Dartmouth
College, and Slater is a senior researcher at Cereon Genomics,
a subsidiary of Monsanto, in Cambridge, Mass.
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