Plastic Panels

Despite plastic production cars being more than 50 years old, car body manufacture is still dominated by metals – steel and, to a lesser extent, aluminium. However, plastic body components have the potential to reduce vehicle mass considerably. Oddly enough, one of the problems in the widespread introduction of plastic panels is painting them... This article, sourced from Daimler, describes some of that company’s research into plastic body shell panels.

It’s bitterly cold by the time Erich Lehner and his colleagues from Production and Materials Technology (PWT) at the Damler plant in Sindelfingen, Germany, 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, as is still customary in automobile manufacture, but rather from thermoplastic polymers. As a result, they are substantially lighter.

“The use of lightweight engineering,” says 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.”

The rigorous use of lightweight materials is therefore also one of the approaches that Daimler engineers have been using to reduce fuel consumption and CO2 emissions. In fact, the company has been investing a lot of time and money to achieve this objective for many years now.

Body Shells

Thermoplastics - in other words, meltable plastics - display a number of advantages over metals.

For a start, they are lighter on account of their low density, and therefore help reduce fuel consumption. Secondly, they have superb design properties and can be processed into almost any shape by means of injection moulding. They are therefore ideal for making vehicle bodyshell parts. In fact, 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. Often, such parts are so-called attachments, which are manufactured by automotive suppliers and then painted in the right vehicle colour.

Although this practice brings all kinds of advantages in terms of supplier flexibility, the painting process also makes the parts considerably more expensive. After all, each supplier must have its own paint shop. The use of different processes and different materials also makes it difficult to guarantee that all parts have exactly the right colour.

Jens Humpenöder, a colleague of Lehner’s from Daimler Research in Ulm, explains the implications in more detail: “Using different painting processes can result in differences in hue, gloss and surface finish,” he says. “Even the tiniest difference can cause real headaches when it comes to integrating the attachments with the rest of the bodyshell.”

One easy way of getting around this problem would be to paint everything at the same time - a method that also saves costs. Known as “online painting,” this process involves subjecting all the thermoplastic parts and the steel bodyshell 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 - temperatures that are too 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. Daimler 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.

[It would seem that other car manufacturers currently using plastic body panels colour them separately – Ed]

Researchers 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 light weight.

“The big difference here is that we use so-called nanoscale fillers such as ‘nanoclays’ or ‘nanotubes,’” says Lehner referring to his Lightweight Engineering with Thermoplastic Nanocomposites project. “These fillers significantly enhance the properties of thermoplastics.”

Nanoclays are layer silicates - clay minerals, only a few nanometers thick. And nanotubes are either carbon nanotubes (CNTs) or carbon nanofibers (CNFs) - macromolecules made of carbon atoms arranged in a hexagonal framework to form tiny but long cylinders.

Bigger weight reductions with nanoparticles

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 percent by weight of conventional fillers,” says Humpenöder.

This means that thermoplastics containing nanoparticles are lighter than not only metals but also conventional plastic composites. The addition of nanoclays also improves the surface quality and fabrication properties of thermoplastics. As a result, the wall thicknesses 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 nanoclays, even better material properties are achieved in some areas. These tiny particles of carbon are exceptionally strong and elastic, which makes them ideal as a 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 nanothermoplastics 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 nanoparticles.

Lehner summarizes the advantages: “One of the major benefits of nanothermoplastics 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.”

See also A New Word to Know: Nano-Composites!