LED Headliner

The competition in car manufacturing is tremendous. The main field of competion are of course technology, design and price. Automotive headliner composites are visible for the driver, they absorb sound and they could emmit light. Consequently headliner composites can be used to gain competitivenes in all three of the mentioned fields. This article is summarizing the option of emmitting light through the composite of the headliner.
INTRODUCTION
The Dictionary of Automotive Terms defines headliners "as fabric or vinyl upholstery on the interior of the roof of a vehicle". 's definition is a bit more informative: "A headliner is a composite material that consists of a face fabric with nonwoven or foam backing that is adhered to the inside roof of automobiles or yachts". This definition reflects the fact that modern headliner consist of multilayer composite materials which bring together multiple functionalities, i.e. the requested look, haptik, stiffness and soundreduction needed in cars. One additional functionality could be the use of heating headliners. But for interior trim component, lighting is a functionality, which is getting more important today. A well known starlight Headliner was produced 2009 for the Rolls-Royce Phantom Sedan. Its starlight consisted of multiple illuminating glass fibers, all connected separately. In other words, the Lighting functionality was not part of the initial composite layers. Automotive companies, have realized the integration of electroluminescent light panels as dome light behind fabrics. Today, surface mount technology (SMT/SMD) may use 0603 or 0402 SMD LEDs (< 0,3 mm thickness) enabling the production of flexible electronic circuits below 1 mm thickness. As headliner interlayers, they are nealy as thin as electroluminescent panels mentioned above, but they may work with simple 12 VDC without need for electroluminescent inverters. The literature, describes precisely the hot vacuum-lamination of these thin LED Interlayers between glass and polymer composites. A LED Headliner describes a composite Headliner specification containing such an for lighting purposes.
MATERIAL SPECIFICATIONS
Purchasers of middleclass automobiles also have preferences for vehicle exteriors with hidden or concealed light sources as a distinguishing decorative appearance for the vehicle. Big efforts are done to provide a lighting arrangement for the interior of an automotive vehicle in which the light source, such as LED, is recessed into the interior structure of the vehicle and covered with a layer of fabric. The light shall shine through the interior trim of the vehicle. Providing invisible light sources when not in use, but enough light through the fabric/foam layer when illuminated, is the main goal of any lighting Headliner.
Since 2006, the Stella Consortium, a group of 12 companies and universities to develop stretchable electronics mounted on fabrics. This solution may open an option for electronics mounted in the last layer of headliners.
LED Headliner consist of multiple layer composite material containing light emitting diode (LED) mounted on electro conductive Films. These films based on conductive printed or transparent conductive polymers or paper will substitute wires and transport electricity into the roof area of the vehicle, where other overhead systems may be powered. LEDs may be integrated in light guide plate, if their thickness and their weight will not affect the glueing and laminating process of the headliner. Thermoplastics like Polyester and Polypropylene are already in use in Headliner composites for optimised head impact countermeasure (HIC),. LED Film embedded in LED Headliner may be perforated to enable a well define placement of these films on or to mounting elements within the composite. Perforation may contain Thermoplastic or Adhesive, in different shapes. Example of a shape would be the one of a connecting plug, which is present on both sides of the LED film. This type of connecting plug will reduce vibrations within the LED Film and the LED headliner, which is supposed to absorb noise within its composite.
Topview LED will produce small lighting area, starlike, in the Headliner fabric. Sideview LED, with a beam parallel to the substrate surface, will enable bigger area as well as pseudo-homogeneous lighting if used in bigger quantities. The best pseudo-homogeneous lighting is achieved by embedding side view LED films into transparent polymeric layers, which will act as lightguide materials.
Special interlayers of electroconductive LED Films based on flexible conductive polymers, on flexible printed circuit board or on transparent conductive metals & oxides sputtered on polymer films which will be laminated between some layers of the composite Headliners.
New automotive composite Headliner base material are disclosed by many different companies, allows sound waves originating within a vehicle to penetrate deeply into the headliner where a significant portion of their energy is absorbed rather than reflected back into the vehicle. Consequently, modern headliner composites will often consist of following layers:
9. Release Sheet
8. Scrim Layer
7. Upper porous fibrous layer
6. Upper adhesive layer
5. Closed cell PU - foam
4. Lower porous fibrous layer
3. Lower (sound permeable) adhesive open cell PU barrier
2. Sound permeable cover stock foam layer
1. Coverstock material layer
Internal sound waves within the car will enter the coverstock material layer (1). Sound will diffuse to the upper adhesive layer and it will be reflected by the scrim layer(8). But it will be absorbed all the way through. In the case of LED Headliners one solution would be to laminate the polyester LED Film to the upper adhesive layer (7):
10. Release Sheet
9. Scrim Layer
8. Upper porous fibrous layer
7. Upper adhesive layer
6. Polyester LED Film (full area or perforated)
5. Closed cell PU - foam
4. Lower porous fibrous layer
3. Lower (sound permeable) adhesive open cell PU barrier
2. Sound permeable cover stock foam layer
1. Coverstock material layer

The Polyester LED Film (6) may also be perforated to allow the upper adhesive layer to connect to the closed cell PU foam (5). One possible production process for producing a composite headliner will comprise: Applying adhesive to the different surfaces of the foam cores; placing a fibrous layer to the adhesive coated surface of the foam cores; placing a scrim layer next to the lower porous fiber layers; preparing a coverstock sheet comprising a coverstock material layer and an adhesive barrier; and applying heat (100 - 160 °C) and pressure to the layered materials with a hot press. The pressures and temperatures of the lamination process mentioned above are quite similar to glass laminating temperatures. Consequently not only flexilble printed circuit boards, but also LED embedded films, which are also laminated as inserts at 120 °C to produce transparent LED embedded glass are integrated in LED Headliner lamination process.
REFERENCES
simple:Printed circuit board

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