Thermoplastic Composites for Body-In-White Application Development
A Collaborative Approach Between DuPont and Renault
In a bid to slow climate change, nations around the world are setting stringent new targets for reductions in vehicle CO2 emissions. In the U.S., for example, the Corporate Average Fuel Efficiency (CAFE) standards are expected to reduce per kilometer CO2 emissions on a fleet-average basis from 200g (2009) to 105g by 2020. The European Union’s current emissions targets require a fleet average of just 95g of CO2 per kilometer by 2021, down from 154g in 2008. And China’s stated goal is to drive down CO2 emissions from a fleet average of 194g/km in 2009 to 107g by 2020.
Aerodynamics, powertrain innovations, regenerative braking, and improved tires are just a few of the technological fixes enabling automakers to rein in the pollution their vehicles produce. But there is another design strategy getting serious attention these days: reducing vehicle weights, also known as “lightweighting.”
There are three technical paths to reducing vehicle weight:
- Reducing part thickness
- Reducing surface area
- Reducing component density
The first two are engineering opportunities; the last is a material opportunity. For years, carbon steel was the material of choice for automakers, and with good reason. Steel is abundant, design-friendly, and offers good crash resistance. But conventional steel is much denser than its alternatives, and with the pressure of new sustainability goals mounting, automakers must become more creative in their materials selection.
Some of the materials now under study for reducing the weight from vehicles include:
High-strength steels: The improved strength-to-weight ratio of advanced high-strength steel (AHSS) makes it one option for vehicle lightweighting.
Aluminum: At two-thirds less dense than steel, aluminum has seen notable use in vehicles like the Audi A8, the world’s first fully aluminum body in white design, and recently in Ford’s popular F-150 truck, where it allowed the automaker to trim 350 kg from the vehicle’s overall weight. However, aluminum is more expensive than steel on a weight-specific basis and requires specialty training and techniques for repair.
Magnesium: This adaptable metal can be cast into mechanical or body parts. It is lighter than steel (-75%), titanium (-60%), and aluminum (-33%), and boasts greater per unit strength than any of them. On the other hand, magnesium presents sourcing, sustainability, and cost concerns because it is mined from just a few regions worldwide using very resource-intensive processes.
Thermoplastic Composite Materials: This final class of materials shows great promise. A thermoplastic composite consists of a glass, carbon, or natural fiber reinforcement within a polymer matrix. Thermoplastic composites compare favorably with metal in terms of weight, stiffness, dimensional stability, and crash resistance. A global scaling-up of structural polymer production capacity is currently underway, which will lead to greater availability, uniformity, and much lower costs. Finally, many thermoplastic composites are suitable for both pre-consumer and post-consumer recycling.
The EOLAB Example
Thermoplastic polymers recently showed off their advantages for vehicle lightweighting in the design of EOLAB, an ultra-low emissions prototype car from Renault, unveiled in 2014. DuPont Performance Materials was one of the partners selected to participate in the EOLAB project. In addition to the steel, aluminum, and magnesium that went into EOLAB’s multi-material shell, several of the car’s structural components were made from DuPont™ Vizilon® thermoplastic composite (TPC).
The EOLAB car included four parts designed with DuPont™ Vizilon® TPC, which, in comparison to steel, yielded the following weight savings:
Cross beams: 2.1kg
B Pillar: 2.1kg
Central floor pan: 16.5kg
Rear floor pan: 3.2kg
Apart from its use as a lightweighting tool, DuPont™ Vizilon® TPC displayed several other merits in the EOLAB project, including:
Improved part integration: In EOLAB’s floor pan, DuPont™ Vizilon® TPC allowed a high degree of parts integration and reduced the number of parts required by more than half. In the car’s lower windshield structural cross beam, DuPont™ Vizilon® TPC enabled a total parts reduction from 10 to 6.
Dimensional stability: In the EOLAB floor pan, DuPont™ Vizilon® TPC showed little variation in part stiffness from -40°C to 90°C. For EOLAB’s lower windshield structural cross beam, ultrasonic welding allowed the fabrication of a twin-shell design that offered a more than 500% gain in torsional stiffness and a more than 150% improvement in flexural stiffness.
High damping coefficient: DuPont™ Vizilon® TPC demonstrated improved noise, vibration, and harshness-reduction qualities versus steel.
Integrated functions: For EOLAB’s windshield beam, DuPont™ Vizilon® TPC was overmolded with polyamide 66, which allowed the seamless integration of multiple functions, from wiper motor support and water gutters to HVAC ports and a hood grommet groove.
Crash resistance: Many in the industry are under the impression that composites are brittle and therefore unsuitable for crash-relevant parts. On the contrary, a structural beam made from thermoplastic composite can resist as much deformation as a steel beam without breaking. Thermoplastic composites also show excellent recovery and less permanent deformation after impact than steel.
In the EOLAB project, DuPont™ Vizilon® TPC also demonstrated an excellent ability to withstand electrocoating requirements. Moreover, in independent tests, DuPont™ Vizilon® TPC has shown superior long-term heat aging, making it suitable for engine compartment use. Finally, DuPont™ Vizilon® TPC has a predicted high recyclability, given that thermoplastic composite sheet products are fully recyclable both in production and at end of life.
Lessons for the Road Ahead
In short, the Renault EOLAB project proved that thermoplastic composites offer a cost-effective, affordable, and high-performance replacement for metals in auto design and manufacture. Thermoplastic composites enable significant weight savings and in many cases present better functional integration than metal and crash-resistance.
Still, to exploit the considerable advantages thermoplastic composites offer, automakers will need to alter their engineering mindsets. Getting the most out of thermoplastics composites will require direct design for these materials rather than looking to them as one-for-one replacements for metal components. Fortunately, with its advanced computer-aided engineering and analysis capabilities, DuPont is already working with automakers and OEMs around the world to bring thermoplastic composites into the mainstream.
With thermoplastic composite production rising around the world, there’s reason for optimism in the auto industry. Thermoplastics now offer a desirable and cost-effective replacement for steel—and are serving as a valuable new ally in helping automakers meet emissions targets.