Automotive Biobased
Automotive Biobased polymers for automotive safety components By Udo Gaumann Plastic Competency Management Global Engineering Airbag Takata AG, Aschaffenburg, Germany Fig. 1: exploded view of airbag and steering wheel Emblem Cover Cushion Retainer Bezel Steering wheel Fig. 2: deployed airbag. For a lot of people biobased polymers are sufficient for packaging or plastic parts with low technical requirements. For them it will be astonishing that biobased polymers (that are either partially or entirely based on renewably-sourced raw materials) are fully able to fulfill the high requirements of automotive safety components like airbags and steering wheels. The increased pressure on fossil-oil based products motivated Takata, of Aschaffenburg, Germany, and a global leading supplier of airbags, steering wheels and vehicle safety systems, to look more closely into the use of bio-based polymers in automotive safety components as an alternative to traditional polymers. Several years of development work ended in a complete and fully functional steering wheel including driver airbag. To achieve this target the available biopolymers were benchmarked according to technical requirements and the most promising materials were chosen. Following this the components were tested in compliance with the specifications of the automotive industry to verify the material limits in steering wheels and airbags. In November 2012 the fully functional airbag/steering-wheel prototype received the 7 th Global Bioplastics Award. Component overview Emblem The emblem, which shows the carmaker’s logo, is important for brand consciousness. OEMs demand high technical performance and appearance for this part. It has to fulfill surface appearance tests, scratch resistance, resistance to certain substances (e.g. sweat, glass-cleaner, stain remover, sun cream, etc.). Usually the emblem is located in the centre of the airbag cover. During deployment of the airbag in case of an accident the emblem 14 bioplastics MAGAZINE [01/13] Vol. 8
Automotive PROPERTY Testing method Unit Hytrel DYM250S BK497 Hytrel RS renewably-sourced Melting point ISO 11357 °C 219 220 Melt flow rate ISO 1133 @ 2.15 kg/240 °C g/10 min 15 16 Density ISO 1183 kg/m 3 1.16 1.16 Tensile properties @ 23 °C ISO527 – 5A bar Tensile strength MPa 20 20 Elongation at break % 365 375 Tensile modulus MPa 188 193 Tensile properties @ –40 °C ISO527 – 5A bar Tensile strength MPa 39 28 Elongation at break % 244 247 Tensile modulus MPa 440 406 Tear strength @ 23 °C ISO 34A N/mm² 49 47 Charpy impact strength ISO 179 1eA @ 23 °C kJ/m 2 63 64 @ –40 °C kJ/m 2 76 72 has to suffer high dynamic deformation within the complete working temperature range (-35°C to +85°C). Due to this the emblem needed an excellent ductility to pass deployment tests in the airbag system without damage. A biobased polyesterelastomer blend (Hytrel ® RS by DuPont) was used as the basic material for the emblems. This material has a good elasticity to assure a good deployment performance on one hand, and on the other hand at +85°C the emblem must still be rigid enough. After injection moulding a metallic layer was applied by PVD technology as a substitute for the chrome plating. The combination of TPC-ET and PVD coating passed the OEM requirements. As an additional feature chrome was eliminated as critical substance. Cover The airbag cover is a highly sensitive component. First of all it has to meet the safety requirements during its whole lifetime. And as a visible component, it has to fulfill the highest demands in terms of surface appearance. At deployment the airbag cover has to open within milliseconds, along the defined, integrally-moulded tear seam. During deployment there should be no fragments breaking off from the cover. In the past Takata used, for serial applications, the standard TPEs Hytrel DYM 250 or DYM 350, which have been specially developed for this application to show a specifically optimized balance between stiffness and low temperature ductility. For the biobased airbag cover, DuPont developed a grade of Hytrel RS that is more or less a biobased copy of the DYM 250 airbag cover grade in terms of its properties. Tests showed only a minimal difference between the conventional and the new, renewably-sourced grade of Hytrel RS, which is based on 35 % renewably-sourced content (cf. bM issue 01/2011). Table 1. Comparison of basic material properties of Hytrel DYM 250 and its equivalent renewably-sourced grade of Hytrel RS (Source DuPont) According to the standard material testing procedure, Takata tested processability, paintability, outgassing, odour, ageing and airbag deployment reliability. During airbag deployment, the most critical test, carried out at 85 °C and at –35 °C, shows similar opening forces and inflation times within the OEM-specified requirement of 3 to 5 ms. Even after ageing (UV radiation, high temperature ageing, humidity ageing, …) the covers passed the test programme without any abnormality. Cushion The challenge for a cushion is to dissipate the high dynamic energy of the passenger during an accident, without harming the occupant. Here a defined elasticity and high strength from the cushion fabric is required. For current production PA66 and PET fabric are being used. After a market screening of potential materials Takata chose Sorona ® EP by DuPont. Sorona is a biobased PTT (polytrimethylene terephthalate see page 46) as the fabric raw material. But particularly the weaving of test fabric requires, due to the high throughput during production, a high expenditure of time and money. So it was decided to compare only the mechanical properties of Sorona EP yarns to PET fabric yarns. The tenacity of PTT fibers is approximately 10% lower compared to PET fibers. The elasticity however, is three times higher. Due to this excellent overall mechanical performance the engineers at Takata are confident that PTT is suitable as a cushion material. bioplastics MAGAZINE [01/13] Vol. 8 15
