Materials: Impact modifier helps TPOs balance stiffness, toughness

Dow Chemical (Midland, MI) is in the process of commercializing a new impact modifier for rigid thermoplastic olefins (TPOs) that will increase their flexural modulus without affecting low-temperature impact resistance, allowing designers to downgauge parts without sacrificing mechanical properties. In a paper presented at the Society of Plastics Engineers’ Automotive TPO Conference, Dow’s Kim Walton said the new impact modifier, which will join the Engage family of products, showed 15-20% greater impact efficiency than best-in-class commercial ethylene/1-octene copolymers, with equivalent low-temperature high-speed dart impact results and 15% higher modulus as SEBS block copolymers in TPOs at equivalent levels.

In late October, Dow Chemical Market Manager Dave Mitchell told MPW that the latest member of the Engage family had completed internal testing, and was currently being validated at several automotive customers, with commercialization expected in the fourth quarter. Walton and Mitchell said the impact modifier’s higher level of performance is primarily derived from improvements in its compatibility with polypropylene (PP), allowing what they call “optimum dispersion” and a greater balance in stiffness/toughness properties.

At the SPE event, and in other communications with automotive customers, Mitchell and Walton said that lightweighting is top of mind for many in the sector, driven by heightened fuel-efficiency standards. In internal simulations, Dow has found that for certain parts, a TPO with its new Engage impact modifier could downgauge by 5-10%. “In real terms,” Mitchell said, “you think about bumper fascia, which have an average weight of 25-30 lb; if you can get to a 10% reduction, you’re talking about 3 lb of material, and that’s a significant change for the industry. The industry sees every pound as a pretty significant shift.”

In addition to allowing lightweighting, Dow also believes the new impact modifier could allow greater adoption of plastics in a vehicle exterior due to its effect on TPOs’ coefficient of linear thermal expansion (CLTE). “I think the other trend we see in the marketplace is the desire to use plastics in more places,” Mitchell said. “One of the major things folks are doing is beginning to look at is, ‘How do I wrap further around the car?’”, targeting parts like rear fenders, which traditionally have been steel. “This product has a reduction in CLTE as well, which we think will also help with design and let our customers get into the different areas, get into the rear portion of the car.” Matching steel’s CLTE and allowing better fit/function has been a barrier to plastics in some automotive exterior components. —Tony Deligio