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Composite elements produced using the Baypreg® F polyurethane spray system are very light but still strong and rigid. Bayer MaterialScience has significantly extended the capabilities of this construction method, for which many production-line applications already exist in the automotive industry. The project was also facilitated by a development cooperation agreement with the polyolefin foam manufacturer Trocellen.
(Published on November-December 2008 – JEC Magazine #45)
MARCOS RAMOS BOSCH, HEAD OF BAYSYSTEMS COMPOSITES, POLYURETHANES BUSINESS UNIT, BAYER MATERIALSCIENCE AG
The automotive industry is responding to spiralling fuel prices and rapid climate change – and their consequences in terms of energy and the environment – with a number of different strategies. One of these strategies involves reducing the weight of various modules and sub-assemblies. Bayer MaterialScience AG has been quick to react to this situation and has developed, either on its own or in cooperation with experienced partners, several production-ready solutions based on polyurethane. These form part of BaySystems®, the umbrella brand for the company’s worldwide polyurethane systems business.
For example, door liners made of natural fibres can be produced with Baypreg® F, a two-component polyurethane spray system. The liners weigh 30-40% less than their counterparts reinforced with, e.g., glass fibre. Production is simple and efficient. Flax or sisal mats are sprayed with the polyurethane system, placed in a mould and compressed to the shape of the door liner. As is essential for this application, the material is very stiff and has good impact resistance. It also scores points for safety. Should the material be destroyed in a serious collision, it breaks without leaving any sharp edges.
The polyurethane spray system can also be used to produce sandwich elements that are lightweight but nevertheless very stress-resistant. They can be used as trunk floors, spare wheel covers or sliding roof cassettes. The components consist of a honeycomb core made of cardboard, overlaid top and bottom with an outer layer of glass fibres. This sandwich is treated in a similar way to that described for natural fibre composites (Fig. 1). It is sprayed on both sides with Baypreg® F, placed in a mould and pressed at around 130°C. The polyurethane spray system reacts at these temperatures and foams slightly, bonding the components to each other strongly and durably. This results in components that weigh less than three kilograms per square metre, i.e. up to 80% lighter than equivalent components made of conventional materials such as wood or sheet metal.
The German supplier, Ideal Automotive GmbH, uses this process to manufacture the trunk floor of the Volkswagen Tiguan (Fig. 2). The part is approximately 80 cm deep, 1 m wide and 2 cm thick. Its fibre-reinforced facings (0.4 to 0.6 mm thick) provide very high flexural strength. Their modulus of elasticity is around 13,000 megapascals, so that they experience only moderate deformation even under high loads.
Sliding roof cassettes must retain their shape even when exposed to high temperatures and major temperature fluctuations, otherwise they would be unable to slide properly along the guide rails. Sandwiches based on the polyurethane spray system have already proved they can meet these requirements very well in various production-line applications.
New thermoplastic honeycomb core
A new process variant was recently added to the range of applications for polyurethane sandwich composites. The lightweight core elements used so far were mostly made of paper honeycomb, but a special thermoplastic honeycomb made from polycarbonate was developed for this purpose. The resulting polyurethane composites are insensitive to damp and moisture. They are therefore suitable for exposed applications inside and outside the vehicle bodywork such as backrests, seat shells and wind deflectors for trucks. The materials and process know-how for the new composites have been optimized, so that BaySystems is in a position to enter into application-specific cooperative development agreements with automotive system suppliers and OEMs.
Composites with a soft-touch surface
The capabilities of the polyurethane sandwich lightweight construction process have now been considerably extended to include a number of new alternatives. Lightweight trunk floors for cars can be manufactured in a single processing step and produced directly with a coloured, textured soft-touch surface. An abrasion-resistant polyurethane coating is first sprayed into a compression mould with a textured bottom section using the inmould- coating process (IMC), followed by a layer of the elastic, wear- and tear-resistant polyurethane elastomer Baytec®. The above-mentioned polyurethane sandwich combining paper honeycomb, glass fibre mats and the spray system serves as the backing layer. With this version of the process, the laborious task of covering the trunk floors with fabric becomes redundant, leading to considerable cost savings. The resulting soft-touch surface has a high-quality appearance, is robust, and easy to maintain (Fig. 3).
Polyurethane sandwiches combined with Trocellen foam
There is also another way to produce a soft-touch surface. Together with Trocellen®, a global supplier of cross-linked polyolefin foams, Bayer MaterialScience has developed sandwich composites that also have a marked sound-insulating effect. To manufacture these composite parts, a layer of closed-cell Trocellen® vertical polyethylene foam is placed in the mould, followed by the sandwich consisting of the honeycomb core and glass fibre mats sprayed with the polyurethane system. The entire composite is then compressed, whereby the polyurethane bonds the individual components permanently and durably to each other. Special surface treatment of the foam ensures excellent adhesion to the polyurethane sandwich. The high heat resistance and good resilience of the physically crosslinked foam means that it can also be used in a press. No additional adhesive is needed, not even when the composite is provided with a decorative facing layer like a highly scratchproof PVC surface (Fig. 4).
Acoustic measurements clearly illustrate the sound-insulating effect of the composites. Figure 5 shows the measurements obtained on the window test rig for various composite structures. The simple sandwich combining paper honeycomb, glass fibre mats and the polyurethane system is compared with composites that have been additionally provided with one or two foam layers. Thus, the foam absorbs sound by an additional 4 dB.
Such PE/PU composites can be used to produce large yet lightweight, rigid mouldings that are also pleasant to touch and have good sound-insulating properties. This combination of properties makes them particularly suitable for use in trucks, for example for the driver's cab trim.
Innovative polyurethane roof module concept
Bayer MaterialScience is also working on alternative roof module concepts, in which polyurethane sandwiches based on Baypreg® F play an important role. These modules should be 25% lighter than conventional sheet steel roof modules. The concept is based on the Multitec® polyurethane spray system that is sprayed into an open compression mould. This solid, non-reinforced topcoat determines the surface quality of the component. A light honeycomb core and two outer layers made of glass fibre mats are then combined to produce a semi-finished sandwich with an edge that has no core layer. The whole package is sprayed with the Baypreg® F two-component polyurethane system, and cut long glass fibres are additionally applied to the core-layer-free edge of the glass fibre mats. Finally, the sandwich composite system is compressed. The edge reinforced with glass fibre mats/long glass fibres gives the roof module high torsional stiffness. It also allows the integration of inserts so that add-on parts such as handles can be assembled more easily. The central honeycomb sandwich section saves weight and ensures high flexural strength. The IMC process could conceivably be integrated into the manufacturing process, thus allowing the production of components that have already been treated with a primer or base coat.