Pictures of the Future    Fall 2007

Plastics: A Growing Field

Life is good. Take Paracoccus denitrificans, for instance. This round, purple, single-celled organism has an unharried existence that consists of breaking down organic residue in wastewater or soil. But in times of stress, when key trace elements required for cell division become scarce, it can respond by stockpiling reserves made of plastic. It does so by converting excess carbohydrates into fatty acids, which it joins together into long molecules, ultimately creating polyhydroxybutyric acid (PHB), which collects in bacterial cells as small, hard globules. PHB is a polymer similar to the solid plastic polypropylene that is used in many areas, ranging from food packaging to textiles.

PHB, which is produced by many types of bacteria and is biodegradable, is a coveted raw material. That’s why materials researchers from Siemens Corporate Technology (CT) and BASF AG are also interested in it. The two organizations are working together with other partners in the "BioFun" and "BioPro" projects funded by the German Ministry of Food, Agriculture and Consumer Protection. Their goal is to develop high-quality plastics from renewable raw materials and identify the most promising possibilities for their application.

Up until now, bioplastics have been used mainly in packaging and non-durable products such as disposable dishes, as many of these plastics are biodegradable. A major boom in demand for such materials began in 2006, according to the European Bioplastics Association. This rising popularity was brought about by greater environmental awareness on the part of consumers, a growing interest in sustainable development among companies, and higher raw material and energy prices. The Association believes bioplastics have the potential to account for five to ten percent of the plastics market in the near future; at the moment, they account for only around 0.1 %.

Limitless Quantities. The key benefit offered by eco-plastics is that their production requires practically no fossil fuels. Moreover, their disposal releases only about the same amount of CO₂ absorbed by the plants that are consumed by the bacteria that produce the plastics in the first place. Bioplastics are also interesting from an economic perspective because the base products for their production—sugar and starch—are available in virtually limitless quantities. In addition, high oil prices have significantly narrowed the price gap between bioplastics and petrochemical plastics. For years, Japanese electronics companies in particular have been attempting to manufacture durable products made of bioplastics. Sony, for example, has marketed a Walkman with a housing made of polylactic acid (a biopolymer), and NEC and Motorola have used the same material for cell phone casings.

Such bioplastic products remain the exception to the rule, however, in part because polylactic acid turns soft at temperatures above 50 °C, at which point it begins to deform. "PHB, on the other hand, has some decisive advantages when it comes to demanding applications," says Reinhard Kleinert, general project manager at Siemens CT in Berlin. For one thing, PHB can withstand temperatures of up to 120 °C, and the material can also be processed with the same machines used for conventional polypropylenes.

The BioFun project focuses on electronic products, whereby the most important aspects involve mechanical properties such as flexibility, resistance to impact, and the adhesion of the surface. "As an electronics manufacturer, we know exactly what these materials need to be capable of," Kleinert explains. "Our involvement in BioFun enables us to ensure at an early stage that the new materials being developed have the right properties." Raw materials specific to certain regions can be used. For instance, P. denitrificans cultures bred in tanks at the SIAB research institute in Leipzig are being fed glycerin, a wax-like liquid by-product of rapeseed oil-to-biodiesel manufacturing. In Europe alone, it is expected that by 2010, 300,000 t more glycerin will be produced than the global cosmetic, luxury foods and pharmaceutical industries can use. If BioFun researchers have their way, the excess glycerin will be used to make plastics.

Firm and Elastic. Before such plastics can be manufactured in quantity, their production processes, which include everything from cleaning raw glycerin and fermentation in a bioreactor to extraction of PHB from bacterial cells, will have to be simplified. "Up until now, a lot of energy has been required for these steps," comments environmental engineer Cornelia Petermann from Siemens CT, whose job is to draw up ecological balance sheets that take into account the energy consumed during production and the environmental compatibility of additives. Petermann believes a great deal of energy can be saved through material and heat recycling.

Chemists are also working on an optimal composition for such plastics. The demands placed on electronic products mean that associated PHB mixtures need to be thoroughly examined. For instance, researchers at Siemens CT are examining how long different PHB variants remain firm and elastic, and whether or not protective coatings or special additives prevent them from decomposing in hot-humid environments—a problem shared by all polyesters. BioFun researchers have now succeeded in improving PHB’s elasticity by mixing it with a biodegradable, petroleum- based plastic produced by BASF.

Scientists are also examining the extent to which PHB may be suitable for use with mechatronic systems, since PHB surfaces could be metalized, in which case they could perform the functions carried out by normal conductor paths. "You could then mount electronic components directly on the PHB housing’s metal coating," says Kleinert. This would eliminate the need for conventional circuit boards, thus conserving space and materials. Naturally, one of the most important criteria here is price. "For our plastics to have a chance on the market, they can’t be any more expensive than established products," Kleinert explains. "They also have to be of equal or better quality."

While researching his Master thesis at Siemens Medical Solutions, environmental engineer Stefan König discovered that fibers made from renewable raw materials could be used to reinforce conventional plastics, as natural fibers significantly improve the latter’s mechanical properties. Moreover, tests with plastics containing a portion of renewable raw materials revealed that they were able to meet the most stringent demands for flame resistance, such as those required for paneling components in large medical devices. "The ideal situation would be to reinforce biopolymers with natural fibers," says König. "There are already such reinforced materials today that contain only a few petrochemical raw materials." Obviously, the results of the BioFun project are set to produce exciting developments for years to come.
                                                                                                                        Ute Kehse