Note: This is the second article of a four-part series covering automotive plastics lightweighting (1) trends, (2) material advances, (3) process technologies and (4) applications.
Automotive lightweighting with alternative plastic materials will remain an important technology trend into the foreseeable 2020-2025 time period. The lightweighting potential of every vehicle component is currently under investigation, and advanced plastics and plastic composites offer significant lightweighting potential.
Fiber-reinforced plastic composites are typically 25-35 percent lighter than steel parts of equal strength. Low and ultra-low density sheet molding compound (SMC) advances are also facilitating weight reductions.
New material uses to reduce weight will include:
- Greater use of engineered plastics and composites in car body panels
- Long and continuous fiber technology for structural parts
- More use of carbon fiber-reinforced plastic for structural and other parts as lower cost composites are developed
- Polycarbonate and acrylic as glazing, including car roofs and rear ends
- Advanced nylons in under-the-hood applications
- Foaming and glass-bead technology to reduce part density
- More use of plastic-metal and organic hybrid technology
- Advances in thin-gauge, high-performance steel
- Growing use of aluminum and magnesium metals
The challenges for materials in the automotive drive and under-the-hood technology include high temperatures, moisture and vibrations. In order to be able to withstand such an environment over the service life of a car, the plastics used often have to be specially equipped.
One material making its importance felt under the engine hood is the hydrolysis-resistant thermoplastic polyester polybutylene terephthalate (PBT). BASF's new Ultradur HR PBT grades that combine high levels of hydrolysis resistance with flame retardance and laser transparency for automotive drives and other under-the-hood applications have been introduced.
Contact with water — even in the form of atmospheric moisture — leads in the case of polyesters to hydrolytic splitting of the polymer chains, and therefore to a weakening of the material, especially at elevated temperatures. Ultradur incorporates highly-effective additives that greatly retard the hydrolytic degradation and can therefore considerably extend part life.
Additives normally used to improve the hydrolysis resistance also tend to increase melt viscosity. Ultradur HR is optimized so that melt viscosity remains as stable as possible, even with long residence times — ideal conditions for stable, problem-free processing.
Ultradur B4330 G6 HR also has a much higher resistance to alkaline media that cause stress cracks. Damage due to stress cracks propagates along the emerging microcracks. This also rapidly leads to a macroscopic fracture. Chiefly at risk are plastic parts that are in direct contact with metal.
BASF
PBT GF30 test bars on a bending jig after wetting with caustic soda solution. The material without improved stress crack resistance breaks quickly (top); Ultradur B4330 G6 HR proves resistant for a considerably longer time (bottom).
Another material making a splash in weight savings and reliability is the new polyether ether ketone (PEEK) resin from Victrex Plc. The inclusion of high-modulus fibers (HMFs) into high-flow PEEK material offers the strength and stiffness necessary to displace metals such as steel, aluminum, titanium, brass and magnesium in automotive and industrial applications. New Victrex PEEK HMF polymers are based on the Victrex 90-Series formulation used to mold tough, thin-walled parts.
The new material provides mechanical strength and stiffness for high-temperature applications typically requiring metals. In addition to lowering part weight, the ability to consolidate parts with more functional designs can help shorten cycle times and reduce production costs because of the efficiencies gained through injection molding.
Victrex Plc
Victrex PEEK HMF (blue) weight-reduction metal comparisons.
The PEEK HMF polymer has been tested in hydraulic fluid, fuel, oil, grease and lubricants. It also has a long-term temperature use of 260 degrees C. PEEK HMF polymer is being used to replace metal brackets, clips, fasteners and other secondary structures.
When it comes to renewable-content plastics, wood-cellulose composite car parts are making inroads into automotive lightweighting. Weyerhaeuser has invented an efficient, practical method for incorporating cellulose fiber into thermoplastic composites that can replace a variety of fiber-reinforced and fiber-filled thermoplastics in applications including automotive parts.
Ford is looking to make extensive use of the new cellulose-fiber reinforcement composite. The key attributes of Weyerhaeuser's "Thrive" composites are:
- Neutral color readily absorbs colors/dyes for full color flexibility
- Odorless (passes stringent automotive interior tests)
- Lower specific gravity than glass fiber by 6 percent
- Excellent moldability with lower cycle time
- Attractive system economics
- Excellent tensile and flexural properties
- Consistent quality
- Globally sustainable supply
Ford is using Thrive cellulose-reinforced polypropylene (PP) for the center console armrest of its 2014 Lincoln MKX. Ford has been using the material in several prototype parts, but this is the first application in a production vehicle. Weyerhaeuser collaborated with auto parts supplier Johnson Controls in developing the armrest.
Ford & Weyerhaeuser
Weyerhaeuser Thrive cellulose-reinforced PP center console armrest in the Ford 2014 Lincoln MKX.
Ford believes the high level of performance provided by the cellulose fibers also makes the composites good candidates for automotive exterior and under-the-hood applications. Weyerhaeuser and its lead users have conducted an extensive number of commercial injection-molding trials for undisclosed targeted automotive applications.
Weyerhaeuser development partner Interfacial Solutions has developed proprietary technologies to improve moisture resistance of high natural-fiber-content composites at room and elevated temperatures. When composites with high-fiber content are in humid environments, they can absorb as much as 10 percent of their mass in moisture. High-moisture content reduces the compound's dimensional stability, color fastness and microbial resistance.
Interfacial Solutions has also developed a moisture scavenger technology that eliminates the need for predrying natural fibers and other moisture-sensitive plastics prior to processing.
Other Thrive applications include office furniture, household goods/kitchenware, small and large appliances, industrial goods, consumer personal goods, and building and construction goods. Thrive is available in multiple base polymers such as acrylonitrile butadiene styrene (ABS), low- and high-density polyethylene (LDPE and HDPE), polypropylene (PP), and Polyvinyl chloride (PVC).
Ford and Essentium Materials developed a coir (coconut) fiber-reinforced composite part for the trunk load floor for the Ford Focus Electric in production for two years. Ford and Essentium Materials are developing another coconut-coir composite application expected to be introduced next year.
With an average diameter of 250 microns, coir is much larger than most other natural fibers, making it inherently stiff and ductile. Beyond good flexural modulus, coir also has among the highest lignin content of all natural fibers, which helps make it resistant to burning, microbial attack and moisture uptake swelling. It is broadly available and exhibits consistent properties regardless of plant species, country of origin, soil and weather conditions and time of harvest.
In summary, weight-reducing material selection is also impacted by:
- Material weight-saving potential per vehicle produced
- Cost per unit of weight saved
- Material availability in quantities required for series volume production
- The drive for lightweighting results in materials technology increasingly being considered as part of the initial design
Researchers from MIT and GM have developed a tool for estimating secondary mass savings potential early in the vehicle design process. Using the tool early in the process — before subsystems become locked in — maximizes mass savings.
Secondary mass savings (SMS) are mass reductions that may be achieved in load-bearing vehicle parts when the gross vehicle mass (GVM) is reduced. Mass decompounding is the process by which further reductions are identified via secondary mass savings that result in further reduction of GVM.
Maximizing SMS is a key tool for maximizing vehicle fuel economy, but can be difficult to achieve given the current design process.
