We normally like to avoid the quips about elastomers "stretching" beyond their limits. But we won't— because that's exactly what has happened with three new elastomers that tackle the shortcomings of previous materials. One of them takes thermoplastic urethanes (TPU) into softer territory. Another strikes a better balance between clarity and hardness. And the third uses a different type of rubber to improve mechanical performance. Here's a closer look:
New Desmopan TPU grades from Bayer Plastics (www.bayerplastics.com) tackles a traditional problem with polyester-based TPU overmolding materials: They've been too hard for use in some soft-touch applications. According to Gerald DiBattista, market development specialist for Bayer's elastomers, earlier TPUs of this type had a difficult time cracking 80 Shore A. "They had a natural limit," he says.
To overcome that limit, Bayer scientists applied some proprietary twists on polyester-based urethane chemistry in order to come up with three new grades ranging from 60 to 80 Shore A. Although these new grades are softer than previous materials of the type, they retain the traditional benefits of durability and chemical resistance. "TPUs are getting better and softer," says DiBattista, who adds that the materials are being evaluated for a variety of electronic equipment and power tool applications.
See Me Through
Making olefin-based thermoplastic elastomers clear typically comes at the expense of hardness. "Harder clear grades have been a real challenge," says George Koprowicz, technical director of Star Thermoplastic Elastomers (Chicago, IL, www.star thermoplastics.com). Star recently raised the bar a bit with the introduction of a new clear 70 Shore A grade. Koprowicz describes the new grade as "ultra-clear" and cites transmission values in the neighborhood of 95%. Past grades in this hardness have generally tended to suffer from a yellowing effect that limited the clarity, he adds.
With the introduction of the new grade, Star's Optiflex line of clear TPE now ranges from 33 to 70 Shore A. Examples of uses for the clear grades include overmolding for electronics equipment, though Koprowicz adds that the hardest grade is under consideration for a confidential sporting goods application.
A Different Kind of Rubber
Dow Corning's Multibase (www.multibase.com) has come out with a new kind of thermoplastic vulcanizate (TPV) that pairs the thermoplastic phase with silicon rubber particles rather than the EPDM rubber used by comparable compounds. Called TPSiV—and pronounced "tip-siv"—the material currently comes in four flavors. "All of them address performance gaps associated with traditional organic TPVs," says Fernando Cuccioli, a global manager for Multibase.
One grade, TPSiV 1180, primarily targets extruded tubing for automotive and other applications needing enhanced chemical and temperature resistance. Based on a flexible nylon with a continuous use temperature of 284F, this material grade features a hardness of 52 Shore D. The other three grades all belong to TPSiV's 3000 Series, which consists of the silicon rubber in a proprietary polymer that can be injection molded, blow molded, or extruded.
These materials feature a hardness range spanning from 50 Shore A to 60 Shore D. Cuccioli declined to discuss the nature of the thermoplastic base or even name a comparable thermoplastic. He did claim, however, that the materials bond well to other engineered thermoplastics without use of adhesives and have good resistance to abrasion, temperature extremes, moisture, and chemicals.
Also new from Star are overmolding materials that offer adhesion to urethane substrates. These Overflex TPE grades cover a hardness range from 40 to 65 Shore A. Star is still working to quantify the adhesion values for the new materials, but Koprowicz says they offer a dramatic improvement over earlier attempts to marry TPEs with urethanes "We've gone from almost no adhesion at all to cohesive failure," he says.
|Selected Mechanical Properties
Multibase TPSiV (3000 Series)
||60 Shore A
||70 Shore A
||65 Shore A
|Tensile strength @ break
|Elongation @ break
||55%, 24 hr @ 70C
||12%, 22 hr @ 23C 70%, 22 hr @ 120C