Despite high crude-oil prices, the use of plastics in automobiles is likely to grow as the weight reduction facilitated by the material can help to save more oil than was needed to manufacture the plastic part.
“Depending on the application, the shift from metal to plastic can save up to 30% of component weight. When you consider the long utilization time of plastics in a car, plastic components save three times more greenhouse gases than produced during their life cycle,” said John Feldmann, Board Member of BASF SE (Societas Europaea) at Ludwigshafen, Germany.
Currently the average share of plastics in a passenger car is around 15%, which equals a weight share that will typically be between 150 and 200 kg (330 and 440 lb) per car, according to Rudolf Stauber, Head of Engineering Strength and Materials and spokesperson of the new material cluster, BMW Group, R&D center at Munich, Germany. New models such as Volkswagen’s new compact SUV, the Tiguan, with its 18% share of plastics, seem to strengthen the outlook of plastics being on the upswing.
However, there are a few obstacles on that path. One typical limit to using plastics under the bonnet (UTB), for instance, is the exposure to heat. Smaller engines, higher turbocharging pressures, and exhaust gas recirculation (EGR) keep raising the temperature level in the engine compartment. The problem is thermal oxidation, which can reduce the mechanical properties of thermoplastic materials to a point where they fail. The critical parameters in this context are long-term tensile and compressive strength.
“After 3000 hours of aging at 180°C, standard high-heat polyamides can typically lose up to 50% of their original mechanical strength. So far even this result was still considered to be fairly good,” said Bert Havenith, Global R&D Manager Automotive, DSM Engineering Plastics. Increasing the oxidative stability of plastics is therefore one precondition for new high-heat UTB applications.
To meet the temperature requirements of such underhood applications, DSM has further developed its existing Stanyl polyamide material to a new PA46 generation. The first available product is the Stanyl Diablo OCD2100 polyamide. The two years of development work that went into this material focused on limiting the thermal oxidation breakdown. As a consequence of studying the exact degradation mechanism, the new DSM material does not only rely on traditional antioxidants. “Instead, our technology controls the oxidation mechanism itself,” explained Willy Sour, a product development expert for the new polyamide.
“Stanyl Diablo withstands more than 3000 hours temperature exposure up to 230°C with less than 15% loss in mechanical properties,” claimed Havenith, adding that specific parameters such as the burst pressure are 100% better than is the case with standard polyamides. According to the manufacturer, weldability and long-term weld heat stability have also been improved.
Among the potential new applications for this material are plastic components on the hot side of the turbocharger. “One example would be the hot charged air duct between the turbocharger and the intercooler,” Havenith said. The DSM manager announced the first UTB series application for a passenger car in 2009.
Under the name Durethan BKV 30 HTS, Lanxess has further developed its standard PA 6 GF30 for injection molding by increasing thermal aging quality. This new glass-filled grade with high-temperature stabilization (HTS) is optimized for high impact strength. While this material is “not suited for the hot side of the turbocharger, it is a cost-efficient material option to special polyamides,” claimed Jürgen Selig, of the transport application development within the company’s semi-crystalline product business.
BKV 30 HTS will retain more than 50% of its original impact strength after 3000 h of aging at 150°C (302°F) and can take temperature peaks at up to 190°C (374°F) without losing too much of its tensile strength.
Slowing down thermal oxidation is not only an issue with stiff components: The current trend toward turbocharging necessitates air tubes with soft bellows. Currently, sequential blow molding is needed to manufacture pseudoplastic high-heat components from two polyamides of differing hardness with the harder layer preventing the soft layer of the component from collapsing in the mold during manufacturing. After conditioning, the soft-modified Durethan DP BC 6000 HTS PA 6 grade has an elasticity modulus as low as 350 MPa (50.7 ksi) but can be processed to manufacture single material charge air tubes by suction blow molding due to its high melt stiffness.