Lightweighting Auto Interiors: Advanced PP for New Uses

Lightweighting is an ongoing need in the automotive industry, and plastics continue to play a key role for innovations in internal combustion engine (ICE) vehicles as well as electric vehicles (EVs). Whether the goal is to shed weight to improve fuel economy or to compensate for heavy battery packs, automakers (OEMs) and suppliers are subject to ever-increasing market demands as well as requirements. The latest and perhaps most market-disrupting development among them — the Biden administration’s carbon reduction targets — affect both the ICE and EVC worlds.

Plastics, therefore, remain front and center in the search for new and better lightweighting solutions. Today, plastics represent about 50 percent of the volume of the vehicle but only account for 10 percent of total weight. Even after decades of innovation, however, there remain challenges and new frontiers to cross. Polypropylene resins, whether used in compounds or pure (i.e.,“neat”) resin form with no fillers, provide a prime example of these challenges and new frontiers.

For this discussion, we will focus on interior challenges and innovations.

Impact copolymer, evolved

Automotive interiors present a particular challenge to lightweighting with neat resins, which are used for trim pieces and panels. However, they cannot always meet the demands of more challenging applications, which we discuss below. Factors such as material performance, manufacturing/machineability, look-and-feel, cost, and other factors must be balanced to allow for a lightweighting substitution.

Polypropylene offers low density, hence low weight, especially relative to compounds such as thermoplastic olefin (TPO). Specifically, polypropylene impact copolymers (ICPs) are widely used for many trim pieces and panels. They still, however, face performance challenges that limit applications involving large and complex parts.

This may be changing with the development of an ICP with greater ductility and other properties that help in replacing compounds. Ironically, it was first used in compounds. A case in point: A large, global compounder used it to replace 50 percent of its additives amid rising prices. The benefits included immediate cost savings; increased impact resistance by 20 percent (importantly at low temperatures); and “helps with appearance and shortens the molding cycle time,” says a senior product manager at the compounding facility, shown below. (Click here to view a video case study on the material used: Achieve Advance PP8285E1 from ExxonMobil.)

Image courtesy ExxonMobil Company

OEMs meet the ‘door panel’ challenge

The new ICP holds promise for automakers and their Tier 1 partners who have searched for a suitable polypropylene to broaden interior trim and panel applications to a greater weight-saving opportunity, door panels. Design engineers have for more than 30 years found ways to replace TPO door panels, wrestling with challenges in such as ductility and dimensional stability or shrink (i.e., gap tolerance).

The new advanced ICP can reduce the amount of wrestling needed by design engineers to allow for large, lighter weight panels that perform well enough to cost-effectively replace TPOs. Features include:

• Lower density and, in turn, lower weight relative to compounds found in automotive interiors
• Higher ductility, which improves not just flex but consumer aesthetics that may foster more material substitutions
• Improved coefficient of linear thermal expansion (CLTE), which reduces design re/engineering effort when substituting higher-density parts made from compounds
• Enhanced molding due to a higher melt index of 30 (i.e., melt-flow rate flow rate of 30 g/10 min) vs. the industry standard of 20. This allows molding at lower pressures; reduces material tonnage; and reduces mold flashing for higher finished product quality.
• A 35% improvement in impact resistance over standard ICPs
• A 20 percent increase in scratch and mar resistance over standard ICPs

Consider how these attributes may finally open new applications for door panel lightweighting. Weight-wise, ICP savings for five-pound doors on a sedan using would total about 1.5 pounds — less than many sport-utility vehicles and trucks. That weight reduction becomes 150,000 pounds per 100,000 vehicles. Now consider: The US auto industry sold nearly 15 million cars and light trucks in 2020, according to Statistica. It’s hard to imagine how much additional savings in weight, vehicle efficiency, and carbon footprint would be achieved if just half of those cars used the new, lighter-weight material for door panels, among other applications.

Taking the advanced ICP challenge

The history of automakers’ efforts to use ICPs for lightweighting large and complex parts — think door panels — has been a hit and miss proposition. Some tried those years ago and abandoned them; others returned to the material for value-oriented vehicles. Today, OEMs are scattered across both camps. However, the new grade has made an enormous difference in the industry and more door panels are indeed rolling off production lines using the new and advanced grade.

You can download data sheets (including this one), but beware; test methods vary from vendor to vendor, so it’s hard to conduct a true “apples to apples” comparison. For instance, data for cold-temperature impact testing has been particularly thorny. Therefore, a real-world test is where the proverbial rubber meets the road: Get pellets in the hands of suppliers and molders; mold the material into parts to see how well the material performs.

This discussion provides one example of the advancements in high-performance, weight-saving automotive plastics. Another recent innovation in the Achieve Advanced polypropylene family is a “Foamable” material for more efficient thermal and sound-dampening performance, durability, recyclability and affordability. There are of course many options for the use of plastics in automotive interiors and beyond that are designed to offer high-performance, lightweighting and compliance.

About the Author

Brian Dujardin, Customer Application Development with ExxonMobil Company, has been with the company for 23 years. His experience includes OEM and supplier support, problem-solving, and process optimization; assembling multi-lot certifications for polypropylene and polyethylene materials; and injection molding prototyping, mold making, and production.