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Biotechnology is the key to better plastics
Improved catalysts – both synthetic and natural – will enhance the production of new and improved plastics. Tom Shelley reports
New catalysts can impart engineering properties to low cost polymers, while biotechnology could lead to lower cost plastic manufacture and entirely new materials.
Polyolefins can be given engineering performance by re-arranging their structures. Engineered bacteria or plants can be made to perform difficult chemical syntheses and the way is open to commercial production of materials based on spider silk.
Better commercial products are the goal, and the new products and processes are expected to yield commercial benefits in the short, medium and longer terms. Applications are expected in the traditional plastics fields: packaging, engineered parts, fibres and many more.
The patent filed for ‘Versipol’ catalysts has 582 numbered claims, making it the largest patent ever filed by DuPont. More applications will follow. Versipols imbue polyolefins, such as polyethylene, with higher properties. Don von Schriltz, DuPont’s global technology director for engineering polymers, says the company has long been trying to achieve low temperature, low pressure coordination of ethylene with polar molecules such as acrylates. The new catalysts come from an initial discovery made in the laboratories of Professor Maurice Brookhart at the University of North Carolina.
The basic catalysts are positive ions of nickel or palladium in the middle of a cluster of organic groups. They can be used to produce polyethylenes ranging from very low density polymers, full of branches on branches, to very high density products. Higher polymerisation temperatures give lower molecular weights but increased branching. Hydrogen can be used as an independent means of controlling molecular weight, so it is possible to produce product with a wide combination of properties.
A particular strength of the catalysts is their ability to copolymerise ethylene with polar monomers, in such a way that an appreciable number of branches terminate with polar groups. The benefit for users is that it may be possible to tailor adhesion characteristics, so that tie layers are no longer needed in coextruded films. A barrier to grease or oxygen could be built in, making the material suitable for packaging films. Polar monomers are poisons to present day metallocene or Ziegler-Natta catalysts.
Back to
basics
The
other area of development is to use biotechnology. DuPont started as a
cellulose-based materials company, using wood as a feedstock. Now, it
seems, it is returning to its roots, with a bioprocess to make a polyester
intermediate from corn-derived glucose, and plans to make materials derived
from nature, with or without its help.
One project aims to reduce the steps needed to make dodecanedioic acid, (DDDA), as used in nylon 6,12. At present it is made in four steps: butadiene is converted to cyclododecatriene, then to cyclododecene, then to cyclododecyl alcohol and finally to DDDA. DuPont scientists have now genetically engineered a novel biocatalyst from bacteria which makes DDDA from dodecane in just one step. Biocatalysts generally function at ambient temperatures and pressures and are highly specific in their function. They achieve high yields – 98 per cent in the case of a DuPont process to produce an intermediate for a herbicide as opposed to 20 per cent using manganese dioxide.
There are already pathways in natural organisms to convert glucose to glycerol using enzymes. DuPont is working with Genencor International, and has genetically engineered and patented organisms that can catalyse the entire process from glucose (a natural sugar) to 1,3 propanediol, a key intermediate in the manufacture of polyesters.
The conversions occur at high yields. One bioengineer once told Eureka that it was possible to find a bacterium which could perform almost any kind of downhill conversion process in an average cubic metre of soil. The only problems, he said, were to isolate the right one and then enhance its performance.
Another approach might be to use engineered bacteria to go all the way, directly producing plastics based on proteins. The approach is to design the protein polymer and translate this into a DNA sequence. The sequence is then cloned and inserted into a plasmid vector for incorporation into a suitable host bacterium, typically E. Coli. Usually only a small fraction of the total cells becomes the protein of interest, so considerable purification is required to remove cell debris and other proteins. Once the pure protein is obtained, it is dissolved in a solvent such as hexafluoro isopropanol, and spun into fibre.
One of the target proteins is spider silk. Scientists across the world have long been trying to emulate the production of spider silks, because of their immense strength and elasticity. DuPont will not be drawn on how strong their artificial spider silk is, except that it is "in the typical range of textile fibres". The company is also interested in elastin, the load bearing protein in tendons, ligaments and arterial walls; keratin, the protein in hair and fingernails; and collagen, the protein responsible for a lot of the suppleness of skin and toughness of bones. Von Schriltz points out that fingernails are not dissimilar to ‘Delrin’ acetal polymer.
All these protein polymers have various soft segments. Von Schriltz says the good news is that some of the properties are readily understood and altered by amino acid sequence changes. The bad news is that nature uses "a lot of spinning and other forming tricks" to get the full development of physical properties. Spiders, for example, have spinarettes which have an almost perfect parabolic entrance to the spinning orifice, which they can vary in order to accommodate slight changes in properties. Pushing material through a machined mechanical shape is liable to lead to shear-induced crystallisation of the polymer molecules and a consequent jam up. For this reason, the company says: "We are still measuring the amount we make in grammes rather than tonnes."
Commercially mass produced artificial spider silk looks to be way in the future. But in the mean time, DuPont has just announced the launch of ‘Zypan’, which it believes to be the lowest price, highest quality polyaniline in the world. The manufacturing process involves the use of an enzyme catalyst extracted from soybean husks. Polyanilines are particularly used in anti-static applications because their electrical conductivity.
If, as some futurists in DuPont predict, oil feedstock costs rise sooner than generally expected, biologically derived and produced polymers for mechanical engineering could become commercially attractive quite soon. The company would then be basing its production on cellulose and other natural products just as it did when it was started.
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Design
Pointers Bioengineering could reduce the eventual cost of many plastics, though there may be some differences between biologically derived mass produced plastics and those made from oil feedstocks using traditional chemical methods Commercial production of plastics based on biological materials – such as spider silk – is now possible, although commercial production still looks to be some way of
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