Bright future for automotive plastics

A luminescent polymer film that can be formed into almost any 3D shape could change the face of in-car lighting reports Jon Excell.

What do a Volkswagen engineer and a male glow worm have in common? The answer is that they’re both attracted to the luminous abdomen of the female glow worm.

However, at this point the intentions of these unlikely associates part company. For while the glow worm is driven mad with sexual desire, the engineer is intent on a host of in-car lighting possibilities offered by the creature’s luminescence.

A luminescent effect can be produced in a number of ways. In the case of the glow worm, it’s based on a chemical process, but it can also be generated electrically. Whatever the method, one thing is always the same: excited electrons discharge their surplus energy by emitting light, and in contrast to fire or an incandescent lamp, no heat is generated.

While there have been numerous attempts to commercially exploit this phenomenon, polymer expert Bayer and Swiss company Lumitec believe that their recent foray into this area represents an unprecedented success.

The companies have pooled their expertise to develop a light-emitting plastic coating that is thin, strong and flexible enough to be moulded into almost any 3D shape.

Previous attempts to successfully apply Electroluminescent (EL) films have been held back by the fact that they have been extremely hard to shape.

Dubbed smart surface technology, the innovative polymer film emits light when a small voltage is applied to it.

This phenomenon of EL is perhaps best understood if it is thought of as the opposite to a solar cell. While a solar cell converts sunlight into electrical energy, an EL system generates light when exposed to an externally applied voltage.

The film, which is less than 0.5mm thick, is made of a number of different layers. The outer skin consists of a transparent layer of the polycarbonate Makrofol. Next comes a transparent electrode made from Baytron, an electrically-conductive plastic developed by Bayer subsidiary HC Starck. This is separated from a counter electrode by an electrically non-conductive layer (dielectric) itself consisting of several layers.

When an AC voltage of around 110V is applied to the electrodes, the inorganic pigments in these non-conducting layers emit light. The system, said Eckard Foltin, head of the Creative Centre at Bayer, uses very little energy, does not give off any heat and is durable and maintenance free.

The key to the breakthrough is what Foltin called a ‘marriage’ of two technologies: EL and in-mould decoration (IMD).

In-mould decoration, also known as applique moulding, makes it possible to decorate a thermoplastic with the desired surface finish – in this case the EL film – during the injection moulding process.

Using this process, EL film is thermoformed into the desired 3D shape in a single, continuous operation and then back-injected with further polymer to give the end plastic component. This eliminates the need for subsequent coating and considerably speeds up the manufacturing process.

‘The technology not only enables us to make the films into 3D shapes and then back inject them, but it’s also easy to integrate the resultant components into surfaces,’ explained Foltin.

He is convinced that EL plastic components have a bright future in the car industry. This, he said, is down to two things – a desire within the industry to give car interiors what he calls more of a ‘homelike feeling’, and to use illuminated areas to make it easier to find your way around inside.

More significantly however, the technology offers fairly dramatic production advantages. It makes it possible to produce complete plastic components, as fully functioning modules, in one step. in terms of automation and assembly this provides huge cost and time savings.

Bayer has already demonstrated the technology on a trial luminescent heating/ventilation system car control panel.

In one continuous operation, the company back-injected a luminescent film decorated as a control panel with Bayblend, a thermoplastic commonly used in car interiors. To light the panel, all that was necessary was the installation of a small voltage transformer to convert the car battery voltage to an AC current of around 110V. In contrast, the production of a conventional back-lit panel for this application would require five or more processing steps.

An additional advantage for the designer is the low profile of the smart surface technology. While a conventional panel is fairly bulky because of the electrical demands of conventional technology, the EL surface has enabled the construction of an extremely flat component.

However, while the heating panel application is still at the prototype and test stage, the technology has already found its way on to a few production vehicles in a slightly different guise.

Using a system based on the technology, Volkswagen has equipped its new top-of-the-range Phaeton saloon with luminous rear registration plates, and BMW has illuminated the sill plates of its Z4 roadster with a bright shiny EL logo. In yet another application the tachometer dials in the Mercedes E-Class glow white without the need for bulbs, LEDs or fibre optic light guides.

Foltin is confident that further applications for the EL surface technology are just around the corner.

He explained that while the plastic film will make its first appearance in the car industry, its adaptability and ease of manufacture will soon be attracting designers in other sectors.

‘Ideas could come from anywhere as the flat dimensions of the film allow great design freedom,’ he said. ‘Thanks to its good thermoformability, even complex geometries can be made to glow, and because the luminescent film is waterproof, even underwater applications are possible.’

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