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Coverstory Finding of

Coverstory Finding of research project between Takata-Petri and DuPont: No technical limitations to the use of renewably-sourced TPC-ET for the production of airbag covers (development model pictured) A Bio-Cover for the Airbag Article contributed by Udo Gaumann, Takata-Petri, Aschaffenburg, Germany Thomas Werner, DuPont, Neu-Isenburg, Germany Table 1. Comparison of basic material properties of Hytrel DYM 250 and its equivalent renewably-sourced grade of Hytrel RS PROPERTY Testing method Unit Hytrel DYM250S BK497 Hytrel RS renewablysourced 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 38 Elongation at break % 244 247 Tensile modulus MPa 440 406 Hardness, Shore D ISO 868 49 47 Charpy impact strength ISO 179 1eA @ 23 °C kJ/m 2 63 64 @ –40 °C kJ/m 2 76 72 Engineering polymers that are either partially or entirely based on renewably-sourced raw materials provide a fully-functional alternative to their fossilfuel based counterparts. This is confirmed by testing conducted by the tier 1 automotive supplier Takata-Petri AG in cooperation with the material supplier DuPont on an airbag cover made from a renewably-sourced grade of thermoplastic elastomer. In light of the automotive industry’s efforts to increase the use of bio-based materials, Takata-Petri, a global leader in the production of steering wheels and vehicle safety systems, is actively seeking new alternatives to traditional polymers. Within the area of airbag systems, it is the airbag cover that lends itself the most to this challenge. It brings with it a complex set of requirements, including the requirement that it breaks open almost instantly when the air bag inflates within milliseconds after an impact. For a number of years the company has been using engineering polymers from DuPont for this application. It is for this reason that it also turned to the material producer for assistance in its quest to find more environmentally-neutral alternatives. Acting as a pioneer in this area, DuPont currently offers the broadest range of renewably-sourced engineering polymers. Takata-Petri’s requirements for any potential replacement materials were clear: the properties and processing performance should be at least equal to, if not better than, those of the conventionally-used material. Renewably-sourced TPC-ET as an alternative? DuPont was very early in its research into the use of renewable resources as the basis for polymer production. One result of this research was the commercialization as early as K2007 of a series of renewably-sourced engineering polymers including DuPont Hytrel ® RS (RS: Renewably Sourced). This thermoplastic polyester elastomer (TPC- 12 bioplastics MAGAZINE [01/11] Vol. 6

Coverstory ET) contains a renewably-sourced polyether diol as its soft segment. The hard segments of Hytrel RS consist of polybutylene terephthalate (PBT), as is the case with the purely fossil-fuel based Hytrel. Internal testing by the producer showed the material to have comparable base properties to its conventionally-produced counterpart. At the same time, Life Cycle Assessments (LCA) revealed it to have considerably improved behavior with regard to CO 2 emissions und the use of non-renewable energy. DuPont therefore suggested that the polymer specialists at Takata-Petri test the new Hytrel RS grade for its potential use in airbag covers. A ‘replica’ of Hytrel DYM 250 The airbag cover is a highly sensitive component for a number of reasons. Not only must it meet exacting safety requirements, but, as a visible component, it must also fulfill the highest demands in terms of its surface appearance. Amongst the safety aspects is the defined breaking open of the airbag cover, within just a few fractions of a second, along the designated, integrally-molded tear seams when the airbag is deployed. When doing so, there should be no risk at all of any fragments breaking off from the cover, even at the lowest of ambient temperatures. For serial applications, Takata-Petri uses the hitherto standard TPEs Hytrel DYM 250 or DYM 350, which have been specially developed for this application to exhibit a specifically optimized balance between stiffness and low temperature ductility, yet differ, amongst others, with regard to their e-modulus. As part of the cooperation described in this article, DuPont was able to modify a previously-developed grade of Hytrel RS in such a way that it corresponds to the DYM 250 grade in terms of its properties. Tests carried out at DuPont of the basic mechanical properties revealed, even in this special case, only a minimal difference between the conventional and the new, renewably-sourced grade of Hytrel RS, which is based on 35 % renewably-sourced content (table 1, images 1 and 2). Proven practicality Using the results of the standard material testing carried out at DuPont as a basis, Takata-Petri was also able to establish a match in those properties relevant to the application. Areas of investigation included processability, paintability, outgassing and behavior during airbag deployment. Processing behavior during injection molding was largely identical for both materials. Image 3 shows the pressure versus time plots recorded at the nozzle tip during the timedistance-controlled mold filling process (holding pressure: pressure-controlled). At constant machine settings and the same shot weight, there are almost identical curves, which demonstrates that this Hytrel RS grade, in the eyes of the processor, can be used without any problems as a drop-in replacement product for the fossil-fuel based grade. Image 1. The comparison of the shear stiffness of fossil-fuel based and renewably-sourced Hytrel, dependent on testing temperature, reveals an almost complete correlation. Shear Stiffness [MPa] 10000 1000 100 Modulus Comparison 10 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 Temperature (°C) Hytrel DYM250 Hytrel RS Image 2. The comparison of the force versus time paths of fossil-fuel based and renewably-sourced Hytrel during an instrumented impact penetration test at –70 °C reveals no significant variations. Force (N) 5000 4000 3000 2000 1000 0 -1000 -2000 -3000 Instrumented impact penetration test @ -70°C Hytrel DYM250 Hytrel RS 0 1 2 3 4 5 6 7 8 9 10 Time (ms) Image 3. The differences in the pressure versus time plots for the fossil-fuel based and renewably-sourced Hytrel are within the tolerance limits for charge fluctuations. Pressure (bar) 1000 800 600 400 200 Hytrel DYM250 Hytrel RS 0 0 1 2 3 4 5 Time (s) bioplastics MAGAZINE [01/11] Vol. 6 13

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