When plastics parts have to be clear, standard polycarbonate and various acrylics may be come to mind. But neither of these materials families can do it all when other mechanical and physical properties matter as much as clarity. Here's a look at some clear alternatives that offer a different balance of properties than the usual see-through suspects:
Nearly Water Clear
Polysulfone is well known for its temperature and chemical resistance. But in its natural form, it traditionally has had a yellow cast that kept it out of optical and other parts that truly need clarity. Solvay Advanced Polymers has developed a way to get the yellow out of this useful polymer, which it markets under the Udel brand. According to Jamal el-Hibri, senior research associate, the company scoured its manufacturing process for impurities and also applied proprietary color reduction methods. The resulting Udel High Clarity product has a yellowness index of less than 4 for a 2.5 mm specimen, versus 12-16 for traditional PS.That's not quite good enough for prescription lenses, but it's plenty clear enough for a wide variety of medical and industrial optical components. The material, which el-Hibri describes "nearly water clear," can also serve as a better base for vibrant colors. In terms of tensile, thermal, chemical resistance, and impact properties, the material "behaves just like traditional polysulfone," says Jamal. The high-clarity material, which does carry a price premium in excess of 15 percent, may offer the biggest bang for the buck in the medical applications, since Udel holds up to steam sterilization and chemicals.
Looking Through ABS
Yes, ABS can be transparent. Just consider Terlux, a methyl Methacrylate-Butadiene-Acrylonitrile-Styrene (MABS) from BASF.This copolymer offers the key properties of impact-modified ABS--including good impact strength and chemical resistance--as well as light transmission in the 90 percent range. According to Thomas Hazenstab, BASF's styrenics manager, the material it has for years been used mostly in Europe. But starting to see an upswing in use in North America, mostly in medical applications, mostly as an alternative to polycarbonate. Hazenstab says the material does have lower impact strength than polycarbonate, but it also has a density advantage for potential cost reductions. In medical applications involving blood contact, Terlux has another advantage: "Blood platelets don't stick to it," Hazenstab says. Medical applications for the material may pick up in this country now that BASF has come out with a "Health and Diagnostics" Terlux grade that has FDA and USP Class IV approvals. Other applications have come in sports equipment, consumer products, and office equipment.
Polycarbonate Takes The Heat
GE Advanced Materials has developed a new portfolio of polycarbonate-based materials with enhanced thermal performance. Called Lexan Xtreme Heat (XHT), these new copolymers can withstand continuous use temperatures from 160 to 200 C, compared to 120 to 130 C for traditional polycarbonate. Temperature isn't the whole story, though. Jim Wilson, GE's global market director for automotive lighting, notes that there are no shortage of high temperature polycarbonates on the market, but these materials traditionally suffered from thermal yellowing and diminished flow in return for the extra heat performance. XHT resins offer improvements on both scores. According to Wilson, most hight-heat polycarbonates would lose about 15 percent of their tranmission after 1600 hours of aging at 160 C (in 2.5 mm sample). XHT loses only about 2 percent under the same conditions, he reports. In terms of heat and flow, XHT has about 20 percent more flow than comparable high-heat polycarbonates. "We've redefined the heat-flow balance," Wilson says, noting that more flow can translates into better cycle times and thinner walls. Currently, GE offers opaque, easily metallized grades available for lighting components like bezels and reflectors. Later this year, the company plans to introduce the clear versions of Lexan XHT. These grades will target automotive lighting applications, including inner lenses, but it could also go into a variety of non-automotive parts. "Whenever you want a window into heated things," Wilson says. Access GEís LiteDesign Web resource, which has information on the XHT portfolio.