A combination of excellent mechanical, thermal and chemical properties is proving a winning solution for ULTEM resin from SABIC in lightweight spectacle frames with novel decorative effects from Zhengda Optical. The Chinese company injection molds frames in ULTEM resin, a polyetherimide (PEI) resin that can be metallized to produce highly attractive designs.
“Spectacle frames made partly or all in metal are very popular with our customers,” says Xia Guangnian, CEO of Zhengda Optical. “Being able to offer them frames in ULTEM resin that have a metal look, but which are lighter and have design features that are difficult and much more expensive to achieve in solid metal, provides us with a considerable advantage in a very competitive global market.”
Zhengda Optical treats the frames using sputtering, a PVD (Physical Vapor Deposition) process that applies a very thin layer of metal onto their surface at a temperature of around 150°C. This way, Zhengda Optical can combine the design flexibility offered by injection molding – both in terms of shapes and surface textures that can be achieved – with the metal look.
“ULTEM resin’s resistance to high temperatures enables it to resist the PVD process very easily and the metal also bonds very well to the resin,” Guangnian says. “The metal effects that we can obtain this way are really very impressive. “On top of this, the combination of ULTEM’s high strength and very good flow properties means that we can make high-performance frames that are thin and light – which makes them very comfortable to wear.”
To obtain the same look with solid metal would most likely require the use of extensive and expensive machining, which would result in frames noticeably heavier than the version made of ULTEM resin. ULTEM resin has a density of 1.27 g/cm3, which is at least six times less than that of steel.
The good mechanical properties of ULTEM resin, together with its high flow characteristics, mean that Zhengda Optical can make its frames with walls as thin as 1.2 mm. A pair of spectacle frames will typically weigh just seven to eight grams – around half that of a comparable pair in steel – making them particularly comfortable to wear.
Frames made in ULTEM resin are very stylish, but they can also take the knocks of everyday use. The high surface hardness of molded ULTEM resin means that frames can be rubbed and polished without risk of scratching. The polymer is not affected by skin creams and the like, it does not absorb moisture, it does not soften in extreme climatic conditions, and has the additional advantage of being inherently flame resistant. Frames made of ULTEM resin also show very low creep, so that they will keep a strong grip on the lenses for many years.
The material has a very high flexural modulus of 3300 MPa, which is around three times higher than a polyamide commonly used for eyewear – and which is also notably softer. In addition, ULTEM resin is very tough: unnotched Izod impact resistance is 1335 J/m, a figure that does not change down to temperatures as cold as -30°C. Spectacle frames in ULTEM™ resin show very high elastic resilience, withstanding 30,000 cycles in a bending test without showing any permanent deformation.