Thermoplastic weather seals pick up speed

Thermoplastic automotive weather seals reached an important milestone last year. Though thermoset rubber and some PVC still hold sway with more than 90% of global sealing applications, thermoplastic vulcanizates (TPV) have now penetrated into every variety of automotive weather seal. "No one vehicle has gone entirely to thermoplastics, but we now have commercial applications on every seal on different vehicles throughout the world," reports Zev Gurion, automotive vice president at Advanced Elastomer Systems, the maker of Santoprene TPV.

The drive toward TPV weather seals kicked off in the early 90's with the limited replacement of EPDM rubber in static seals, such as the ones that encapsulate rear windows and quarterlights. "These were the easiest ones to replace," notes AES technology manager Colleen McMahan. Next came some of the semi-dynamic seals, such as belt-line and glass-channel seals. Over the past few years, thermoplastics have gone into the more technically challenging dynamic seals, including foamed seals for trunks and hoods as well as door seals.

In one recent example, a Santoprene TPV grade went into a rear-door trim seal on Toyota's "bB" SUV for the Japanese market-which has led the decade-old push into thermoplastic seals. At 800g, this coextruded Santoprene-and-metal seal weighs about 30% less than the traditional PVC seal and can be recycled within an olefin recycling stream, according to Masaaki Saito, automotive director for AES in Japan. In another recent application, Fiat picked a Santoprene TPV for the hood seal of its current Punto model. Made by Italy's Ziliani SpA, this coextruded seal combines foamed and hard grades of Santoprene to take the place of an EPDM and metal carrier. Recent Audi and BMW models have also made use of thermoplastic seals, adds Jurgen Glockler, AES' worldwide director of automotive weather seals.

Beneficial properties. The driving force behind all the interest in TPV weather seals can be found in the potential for weight and cost reduction. As McMahan points out, the TPV grades used in weather seals have a specific gravity of less than 1.0, compared with 1.25 to 1.30 for EPDM. In equivalent profiles, the TPV version would weigh 25 to 30% less. Add the thinner walls made possible by TPV's higher flexural modulus, and the weight savings can total 35 to 40%, McMahan says, noting that a typical passenger car today carries between 8 and 12 kg of EPDM seals.

And even though Santoprene can cost about 15% more than EPDM on a per-pound basis, it nonetheless offers a potential for cost reduction. Part of the savings comes from that ability to use less material. But TPVs have other characteristics that can add up to 20 to 30% lower system cost, according to McMahan. These include:

Barriers remain. For all these advantages, TPV weather seals still have a few hurdles to clear. For one, the supply chain has yet to gear up for thermoplastics, and the bulk of sealing system suppliers remain heavily invested in thermoset rubber.

For another, TPV weather seals don't achieve their full potential as a drop-in replacement for EPDM. In fact, without design changes, they run the risk of compromising sealing performance. "TPVs really require a redesign of the profile," Gurion admits. Much of the need for redesign is driven by property differences with EPDM: TPVs, for example, tend to have a higher flexural modulus and a lower compression set than EPDM. And the thermoplastic material also shows a greater initial loss in elastic recovery than EPDM-though it stabilizes over time at a higher level than EPDM, according to Kenneth Kear, a senior design engineer at AES.

"But all these property differences can be designed around, so that the sealing performance of TPV meets or beats that of EPDM," Kear concludes.

For more information about thermoplastic elastomers from AES: Enter 533