Patrick Gruber,vice president of technology for Cargill Dow,answers questions about his company's new plant-derived plastic.
How will NatureWorks™ PLA compete with petrochemical plastics?
NatureWorks™ PLA combines several attributes into a single family of plastics. Its glossiness and ability to retain twists and folds better than its petrochemical counterparts, for example, appeal to companies that are developing PLA for candy wrappers and other kinds of consumer packaging. PLA also offers fabric manufacturers a natural fiber that can compete with synthetics, such as nylon, in both performance and ease of processing. Overall, industry sources have identified several billion pounds of market potential for PLA in areas such as ap-parel,activewear,hygiene products,carpet fibers and packaging.
What are the environmental advantages of PLA?
Because we use plant sugar rather than fossil fuels as the raw material for PLA, its production consumes 20 to 50 percent fewer fossil resources than do conventional plastics. PLA can be broken down into its original chemical components for reuse, or it can be recycled. One of our customers already plans to use PLA in recyclable carpet tiles. PLA will also biodegrade,much in the way that paper does, in municipal composting facilities. For these reasons, PLA will reduce society's dependency on fossil fuels while providing products that fit current disposal methods.These clear environmental benefits of PLA are a bonus—we believe that people will buy this plastic primarily because it performs well and can compete with existing technologies.
Do these benefits offset the fact that the energy required to produce PLA is greater than that needed to produce some petrochemical plastics?
It is important to realize that our PLA-manufacturing technology is only 10 years old and has yet to profit from the nearly 100 years during which petrochemical-plastic manufacturing has been improving.Even our first manufacturing facility, now being built in Nebraska, will use only 40 percent of the fossil-fuel energy that is required to power the production of conventional nylon.As our scientists and engineers optimize the production of PLA, we expect to reduce the energy requirements of our second and third manufacturing facilities, targeted for construction as early as 2004, by as much as 50 percent.
Do you plan to address what Gerngross and Slater call "the environmental shortcomings"of PLA?
Yes. Not only are we developing production methods that require less energy, we are also investigating more efficient ways to generate energy, including co-generation and use of renewable fuels such as plant material, or biomass. We are also pursuing alternative raw materials for PLA. Using fermentable sugars from corn stover would allow a second crop to be harvested from the same land used to grow corn grain. PLA can also be derived from wheat, beets and other crops best suited to particular climates.
CANDY WRAPPERS are just one of the products that companies plan to manufacture from Cargill Dow's new plant-based plastic when it hits the market in late 2001.
CANDY WRAPPERS are just one of the products that companies plan to manufacture from Cargill Dow's new plant-based plastic when it hits the market in late 2001.
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, on page 50]. 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 powergeneration 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. E3
We did not immediately assume that these plant-based technologies were doomed forever. 1
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.
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.
Energy Information Administration, U.S. Department of Energy (www.eia.doe.gov).
Association of Plastics Manufacturers in Europe (www.apme.org).
Early Daysin the
Life of a Star
To make a star, gas and dust must fall inward. So why do astronomers see stuff streaming outward?
by Thomas P. Ray
Go out on a winter's night in the Northern Hemisphere and look due south around midnight. You will see the constellation of Orion the Hunter, probably the best-known group of stars after the Big Dipper. Just below Orion's Belt, which is clearly marked by three prominent stars in a line, is the Sword of Orion, and in the center of the sword is a faint fuzzy patch. This region, the Orion Nebula, is a giant stellar nursery embracing thousands of newborn stars.
Orion is a convenient place to study the birth of stars because it is relatively close by—a mere 1,500 light-years away—and has a good mix of low- and high-mass stars. It also contains a vast quantity of gas and dust in the form of a so-called molecular cloud. Such clouds are known to provide the raw material for new stars. What is now happening in Orion probably replicates what took place in our part of the galaxy five billion years ago, when the sun and its planets first came into being.
Understanding how stars and planets form is one of astronomy's quintessential subjects yet, until recently, one of the most poorly understood. Twenty years ago astronomers knew more about the first three minutes of the universe than they did about the first three billion days
STELLAR BIRTHING GROUND in the Orion Nebula (opposite page) has given rise to hundreds of new stars. Surrounding it is an invisible but immense molecular cloud—a million suns' worth of our solar system. Only in the past decade have they started to get answers. Infant stars, it turns out, look like scaled-down versions of the heart of a quasar, with powerful jets of material flung outward by sweeping magnetic fields. These stellar fountains of youth not only make for spectacular pictures but also help to resolve paradoxes that have long dogged astronomers.
The theory of how stars and planets form has a venerable history. Just over 200 years ago French mathematician Pierre-Simon Laplace put forward the idea that the solar system was created from a spinning cloud of gas. He proposed that gravity pulled most of the gas to the center, thereby creating the sun. At the same time, some of the material, because of its spin, could not be absorbed by the young sun and instead settled into a disk. Eventually these dregs became the planets. According to modern numerical simulations of the process, once the spinning cloud starts to collapse, it proceeds quickly to the formation of one or more stars, a protoplanetary disk, and a leftover envelope of gas (individual atoms and molecules) and dust (clumps of atoms and molecules) [see "The Early Life of Stars," by Steven W. Stahler; Scientific American, July 1991].
Laplace's model was not universally accepted. Rival theories, such as the idea that the planets were made of material torn from the sun by a passing star, were openly considered up to a few decades ago. The uncertainty was mainly observational: testing the model was well be-
From Mud to a Star
A star begins to coalesce when a disturbance, such as a nearby supernova explosion,causes a cloud of gas and dust to collapse.
Gas and dust clumps at the center,surrounded by an envelope of material and a swirling disk.Mag-netic forces direct jets along the axis.
Material continues to rain onto the disk. Roughly a tenth of it streams out in an uneven flow,shoving aside ambient gas.
yond the astronomical capabilities of, say, 30 years ago, for two reasons. First, the leftover cloud of gas and dust blocks our view of the very region that must be studied. Second, protoplanetary disks subtend minute angles on the sky: if the distance between the sun and Pluto (six billion kilometers) is representative of the scale of the disks, conventional ground-based telescopes can resolve them to a distance of only 200 light-years. Simply building bigger telescopes does not help, because the blurring of detail occurs in the atmosphere.
Theoretical problems also stymied astronomers. Sunlike stars at the youthful age of 100,000 years rotate once every few days and are four or five times bigger than the mature sun. As such stars contract, they should spin faster, just like ice skaters pulling in their arms. Yet the sun has evidently slowed down, currently taking a month to rotate once. Something must have drained away its angular momentum. But what?
Another puzzle is how molecular clouds survive for as long as they do. Gravity is trying to force them to collapse, and without support they should implode within about a million years. In practice, however, clouds seem to have endured for a few tens of millions of years. What holds them up? Thermal pressure is woefully inadequate because the clouds are far too cold, just 10 or 20 kelvins. Turbulence might do the trick, but what would generate it? In giant molecular clouds such as Orion, winds and shock waves produced by embedded massive stars would stir things up, but many smaller, sedate clouds have no massive stars.
The first observational obstacle yielded in the late 1970s, when astronomers began to observe star-forming regions at wavelengths that penetrate the dust shroud. Although dust grains absorb visible light, they have little effect on wavelengths that are much bigger than the grains, which are about one micron across. Studying regions such as the Orion molecular cloud at millimeter wave-lengths—a previously unexplored part of the spectrum sandwiched between the infrared and radio bands—astronomers identified dense, cold clumps typically measuring a light-year across. Such clumps, known as molecular cores, contain as much as a few suns' worth of gas and quickly became identified with Laplace's spinning clouds.
As is often the case in astronomy, new mysteries immediately emerged. Although a few of the molecular cores seem to be in the process of collapsing, most of them are stabilized by means that are not entirely understood. What triggers their eventual collapse is equally uncertain, but it may involve some outside push from, for example, a nearby supernova explosion. The biggest conundrum of all concerns the direction in which material is moving. According to Laplace's hypothesis, stars arise from gravitational accretion, so astronomers expected to see signs of gas plummeting toward the cores.
To their astonishment, they discovered that gas, in the form of molecules (as opposed to atoms or ions), is actually moving outward. Usually two giant lobes of molecular gas were found lying on either side of a young star. These lobes, typically a few light-years in length, have masses similar to or even larger than that of the young star itself, and they move apart at speeds of tens of kilometers per second [see "Energetic Outflows from Young Stars," by Charles J. Lada; Scientific American, July 1982].
The molecular lobes bear a strange resemblance to the vastly larger lobes of hot plasma seen near active galaxies such as quasars. Astronomers had known for years that jets produce these lobes. Squirting outward at velocities close to the speed of light, jets from active galaxies can stretch for many millions of light-years [see "A New Look at Quasars," by Michael Disney; Scientific American, June 1998]. Might a miniature version of these jets also drive the molecular lobes in star-forming regions?
This idea harked back to a discovery in the early 1950s by astronomers George H. Herbig and Guillermo Haro. Herbig, then working at Lick Observatory in northern California, and Haro, at Tonantzintla Observatory in Mexico, independently found some faint fuzzy patches in Orion. Now known as Her-big-Haro objects, these small clouds were initially thought to be sites of star formation. (Some popular astronomy books repeat this erroneous theory even to the present day.) In 1975, however, Richard D. Schwartz, then at the University of California at Santa Cruz, realized that the spectrum of a Herbig-Haro object closely resembles that of the material left over from a supernova. From the Doppler shifting of the spec
Disk material agglomerates into planetesi- The high pressure and temperature at the center mals. The envelope and the jets dissipate. of the star trigger nuclear fusion. The planetesi-By this point,one million years have passed. mals have assembled into planets.
tral lines, he found that Herbig-Haro objects are moving at speeds up to a few hundred kilometers per second.
That is slower than the motion of a typical supernova remnant, but Schwartz reckoned that the principles are the same—namely, that the Herbig-Haro objects are heated gas flowing away from a star. The heat, as in supernova remnants, comes from the motion of the gas itself; shock waves convert some of the bulk kinetic energy into thermal energy and then into radiation. Schwartz's idea gained further support when astronomers looked at photographs of Herbig-Haro objects taken a number of years apart. They were indeed moving. By extrapolating backward in time, astronomers deduced the source of Herbig-Haro objects. Invariably it was a star only a few hundred thousand years old.
Verification of this connection came with another technological revolution: the charge-coupled device (CCD), the light-sensitive chip found in camcorders and digital cameras. For astronomers, CCDs offer greater sensitivity and contrast than the traditional photographic plates. In 1983 Reinhard Mundt and Josef Fried of the Max Planck Institute for Astronomy in Heidelberg, Germany, made the first CCD observations of stellar jets. Subsequent work by Mundt, Bo Reipurth of the European Southern Observatory in Santiago, Chile, and others (including me) showed that jets from young stars stretch for several light-years. They are closely related to Herbig-Haro objects. In fact, some such objects turned out to be nothing more than the brightest parts of jets. Others were discovered to be bow shocks caused by jets as they plow their way supersonically through ambient gas, like the shock wave that surrounds a bullet zinging through the air. The jets typically have a temperature of about 10,000 kelvins and contain 100 atoms per cubic centimeter—denser than their surroundings but still thinner by a factor of 10,000 than the best vacuum available in labs on the earth. Near the star the jets are narrow, opening with an angle of a few degrees, but farther from the star they fan out, reaching a diameter wider than the orbit of Pluto.
How are the jets and Herbig-Haro objects, which are mostly made up of atoms and ions, related to the molecular flows? When molecular flows were first discovered, researchers suggested that they might consist of gas that had been accelerated close to the young star. But this idea had its difficulties. Molecular flows, even those associated with low-mass stars, often contain several solar masses of gas. If this amount of material had to be gravitationally sucked in before being accelerated away again, star formation would be an extremely inefficient process. A more persuasive explanation is that a molecular lobe consists of ambient gas that got in the way of the jet and was accelerated.
None of these observations got to the heart of the matter:
the disk around the nascent star. Astronomers had long been gathering circumstantial evidence for disks. In the early 1980s the Infrared Astronomical Satellite discovered that many new stars had excess infrared radiation over and above what should be produced by the star alone. Warm dust in a disk seemed the most likely source. Around the same time, millimeter-wave telescopes began to measure the mass of gas and dust around these stars, typically finding 0.01 to 0.1 solar mass—just the right amount of material needed to form planetary systems. In the mid-1980s Edward B. Churchwell of the University of Wisconsin and his colleagues observed the Orion Nebula at radio wavelengths. They found sources comparable in size to our own solar system and suggested that they were clouds of hot gas that had evaporated from a disk.
Sighting the disks themselves, however, ran up against the second observational obstacle: their comparatively small size. For that, astronomers had to await the clarity afforded by the Hubble Space Telescope and by ground-based instruments equipped with adaptive optics. In 1993 C. Robert O'Dell of Rice University and his collaborators observed Orion with Hubble and finally saw the disks that Laplace had predicted [see illustrations on page 43]. Their material, where buffeted by the intense radiation and winds from nearby massive stars, was seen to be evaporating. O'Dell christened these disks "proplyds," for protoplanetary disks. The name may actually be a misnomer, because these disks will evaporate within a million years, probably before planets can form.
Mysterious though their detailed mechanisms may be, jets always involve the same basic physical process: a balance of power between gravity and angular momentum. Gravity tries to pull matter toward the center of mass, but because of centrifugal forces,the best it can do is gather material into a swirling disk. Narrow streams of gas shoot out along the axis of rotation, the direction in which matter can most easily move.The escaping matter carries away angular momentum, thereby allowing less footloose matter to settle inward.
WILLIAM B. SPARKS Space Telescope Science Institute
In the core of the active galaxy Messier 87,the driving force is thought to be a black hole a billion times more massive than the sun.
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