Thermoplastics modified by the addition of Nano particles point at new engineering developments that could mark a step towards environment protection

Emissions and environment protec-tion are among the biggest challenges faced by automakers today. Following calls for further cuts in CO2 emissions, lightweight engineering is assuming greater importance. Showing a way to further reduction in weight are thermoplastics modified through the addition of nano particles. Materials experts from Daimler are currently researching on ways to use such plastics for vehicle manufacture.

It is bitterly cold at the Daimler facility in Sindelfingen, Germany, by the time Erich Lehner and his colleagues from Production and Materials Technology (PWT) are ready to start the pendulum impact test. Indeed, the thermometer in the test chamber, which engineers use to investigate the material properties of fenders and other parts, shows a temperature of minus 25 degrees Celsius. In preparation for the test, a number of fenders have initially been refrigerated in a special climatic chamber to minus 40 degrees Celsius, before being mounted on a jig. A heavy pendulum is then allowed to crash into the body part. The objective is to discover how the components behave at different impact speeds.

The maltreated fenders are in fact special parts - test components that have been fabricated not from sheet steel, but from thermoplastic polymers. "The use of lightweight engineering," announces Lehner, "provides a good opportunity to halt the spiralling weight of vehicles that has resulted from the ever-increasing demands on safety, comfort and performance." Compared to metals, thermoplastics (also termed as meltable plastics) are is capable of displaying a number of advantages. They are lighter on account of their low density and therefore help reduce fuel consumption. They have superb design properties and can be processed into almost any shape by means of injection moulding. Thermoplastics are therefore ideal for making vehicle body shell parts.

Thermoplastics are already used in today's series-produced vehicles to cover bumpers and sills or to make the rubbing strips fitted to doors. As a rule, they consist of a polypropylene mixture (PP) and, given their low density, boast a relatively high stiffness-to-weight ratio. Quite often, such parts are so-called attachments, which are manufactured by automotive suppliers and then painted in the right vehicle colour.

This practice may be advantageous in terms of supplier flexibility but the painting process hikes the part cost. Not only does the supplier have to have a paint shop, the use of different processes and different materials also makes it difficult to guarantee that all parts have exactly the right colour. Explains Jens Humpenöder, a colleague of Lehner's from Daimler Research in Ulm: "Using different painting processes can result in differences in hue, gloss and surface finish. Even the tiniest difference can cause real headaches when it comes to integrating the attachments with the rest of the bodyshell."

Painting everything at the same time would eliminate this problem and save cost. The process, known as "online painting" involves subjecting all the thermoplastic parts and the steel body shell to the complete painting process - including cathodic dip painting (CDP), which provides both protection against corrosion and an undercoat for the paint and transparent lacquer above. However, the use of cathodic dip painting involves, if only briefly, temperatures of more than 200 degrees Celsius, which are dangerously high for conventional thermoplastics. Composite plastic bodyshell parts made of polyamide and polyphenylene ether (PA/PPE) that are suitable for online painting do exist. However, these parts are by no means completely satisfactory. Engineers are therefore busy cooperating with external partners from research and industry to further enhance thermoplastics components so that they will be capable of withstanding the temperatures involved in cathodic dip painting. As part of the "Leading Innovations: NanoMobile" program, which is sponsored by the Federal Ministry of Education and Research, researchers and developers are planning ways of increasing thermoplastics' resistance to thermal deformation. Other goals are to reduce thermoplastics' thermal expansion and enhance the electrical and mechanical properties.

Conventional thermoplastics already contain filler materials such as glass fibres or graphite, which affect characteristics such as expansion or electrical conductivity. As well as enhancing the quality of the thermoplastics, however, such fillers also make them heavier. In other words, they cancel out the advantages the thermoplastics would otherwise have due to their lightweight. "The big difference here is that we use so-called nano scale fillers such as 'nano clays' or 'nano tubes,'" says Lehner referring to the BMBF-sponsored sub-project "Lightweight Engineering with Thermoplastic Nano composites," which is led by Daimler. "These fillers significantly enhance the properties of thermoplastics." Nanoclays are layer silicates - clay minerals, only a few nano meters thick. And nano tubes are either carbon nano tubes (CNTs) or carbon nano fibres (CNFs) - macromolecules made of carbon atoms arranged in a hexagonal framework to form tiny but long cylinders.

To date, research and development results show that when nanoclays are added to a thermoplastic, its rigidity, and strength increase, while its density and ductility remain practically the same. "Even with small additions of just a few percent of total weight, it's possible to create mechanical properties that can only otherwise be achieved with the addition of over 30 per cent by weight of conventional fillers," says Humpenöder.

This means that thermoplastics containing nano particles are lighter than not only metals but also conventional plastic composites. The addition of nano clays also improves the surface quality and fabrication properties of thermoplastics. As a result, the wall thickness of the components can be significantly reduced, which saves further weight, or the flow distances used for injection moulding can be lengthened, which in turn lowers the costs for complex tools.

If CNTs or CNFs are used instead of nano clays, even better material properties are achieved in some areas. These tiny particles of carbon are exceptionally strong and elastic, which makes them ideal as reinforcement for plastics. They can also be used to achieve good electrical conductivity - an essential feature when it comes to cathodic dip painting (CDP).

Today, CDP-compatible thermoplastics are already expected to meet a vast range of requirements. In addition to displaying good mechanical properties, electrical conductivity and very high heat resistance, they must also exhibit low linear thermal expansion. And the trend toward using lightweight materials for body parts - for example, plastics for fenders and door panels - has raised standards even higher. Indeed, further enhancements in material quality are now essential, especially for such large-surface parts. In particular, it will be necessary to improve the mechanical properties and thermal expansion.

Following a fundamental investigation of various nano thermoplastics and their suitability for different parts, engineers in Sindelfingen have now constructed a test vehicle based on the S-Class that features lightweight fenders made of thermoplastics containing nano particles. Lehner summarizes the advantages: "One of the major benefits of nano thermoplastics compared to conventional plastic mixtures is that they only need to contain a small proportion of fillers in order to develop the required properties. As a result, these materials offer exceptionally large potential for weight savings."

"As far as future automakers are concerned, nano technology will be one of the key areas of expertise that are vital if a company is to remain competitive," confirms the Federal Ministry of Education and Research (BMBF). In line with this assessment, the BMBF is supporting research and development projects for transport-related nano technology applications as part of the "Lead Innovation: Nano-Mobile" program - especially those of the automotive industry and suppliers. The focus of the various sub-projects is on the direct benefits resulting from lower fuel consumption, enhanced driving safety and increased product durability. In addition, the joint program is intended to combine the various research activities concerned with automotive applications in the field of nano technology.

As part of its "Lead Innovation: NanoMobile" program, the Federal Ministry of Education and Research (BMBF) is sponsoring a research and development project by the name of "Lightweight Engineering with Thermoplastic Nanocompo-sites." Launched two years ago, the project is led by Daimler and brings together a total of five partners. The Leibniz Institute for Solids and Materials Research (IFW) in Dresden is using a range of chemical vapour deposition (CVD) processes to produce a whole variety of nano particles and nano structures. For example, scientists have been able to not only synthesize multi-walled and single-walled carbon nano tubes and nano fibres, but also precisely characterise them with the aid of the very latest methods from electron microscopy and X-ray analysis.

The Leibniz Institute for Polymer Research (IPF) is also located in Dresden and conducts applications-oriented fundamental research into polymer synthesis. In the lightweight engineering project, two teams are investigating how nano clays (layer silicates) and carbon nano particles can be best integrated in polypropylene plastics. For example, they are studying the internal surface tension within the plastic-nano particle mixture and methods to ensure optimal distribution of the nano particles in the molten plastic. Süd-Chemie AG, a manufacturer of organically and inorganically modified nano clays, is investigating how alterations to the particles' surface can further enhance the processing properties of molten thermoplastics.

General Electric Plastics GmbH supplies high-grade thermoplastics to the automobile and electrical industries. Areas of research here include enhanced processing methods and high-performance plastics that can be rendered electrically conductive by the addition of nanopar-ticles, thus making them suitable for the electrostatic painting processes used in automobile body manufacture. Engineers from Daimler are using demonstration parts (fuel filler doors, fenders, side body-shell panels) to investigate the suitability of various thermoplastics containing nano fillers for vehicle manufacture. Produced by means of injection moulding, the demonstration components are subject to a wide range of defined tests and trials, including impact and fracture tests, resistance to stone damage, thermal tests at changing temperatures, aging tests, climatic tests, and tests to determine the suitability of different painting processes and the components' suitability for series production.