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Smaller Can Be Bigger
Tweaks to polymer chemistry and tiny tooling developments yield improved plastic parts
Joseph Ogando -- Design News, January 10, 2005
With engineering plastics, even the tiniest changes to polymer chemistry and processing technology can have a huge influence on what plastics can do. Just look at the following three developments. One promises to make data storage more effective, while another has produced a new elastomer with a useful balance of properties. On the processing front, a novel molding technology relies on nanoscale surface features in the mold cavity to improve the optical properties of plastic parts.
Holographic Data Storage
The growing need for secure, reliable, fast, low-cost data storage technologies has prompted researchers at Bayer Material Science to come up with a system that allows data to be stored on plastic film as holographic images.
The company's developmental system relies on a patented family of photoaddressable polymers (PAPs). Typically acrylic-based with azobenzene and liquid crystal side chains, the PAPs undergo changes in their optical properties when exposed to polarized laser light of a predetermined wavelength. As Bayer physicist Rainer Hagen explains, these laser-sensitive side chains align themselves perpendicularly to the light source and stay that way even after the light goes dark. "Those areas of the polymer become highly birefringent," he says. And the interference pattern that defines a holographic image is in essence written onto the polymer in the form of localized changes in birefringence.
Bayer researchers have also developed prototype hardware for writing and reading the images. Hagen says the hardware resembles the lasers used to create and read holograms for decades, except that the laser wavelengths and filter designs had to be carefully matched to the laser sensitivity of the PAP. He won't reveal much more about the system, noting the security of the holographic data storage resides in the hardware used to encode and read the images. "You have to know what conditions created the image in order to read the image," he says.
Most researchers have viewed holographic data storage as a solution for large-scale data storage. And one day it may be. "The roadmap for the technology is for terabytes in the same area that might hold gigabytes today," says Robert Kumpf, Bayer's vice president of future business.
Yet these large-scale systems remain years away, while Bayer has more immediate plans for its holographic technology. The company has already been able to apply PAP, which is heat stable to about 130C, to polycarbonate carrier films that can be incorporated into bank cards containing financial information or identification cards that could hold biometric data. The same technology could also be used for smart labels. In these implementations, the PAP would hold not just the few kilobytes possible with magnetic strip technology but "several megabytes of data in a square centimeter," says Hagan.
New Elastomer
Some engineers may not care exactly how the chemists make plastics. But the details can make a difference. Consider, for example, the latest elastomeric materials from Dow Plastics. Called Versify elastomers, they owe their existence to proprietary catalyst technology that enabled the company's chemists to combine ethylene and propylene in one copolymer.
The implications of this marriage boil down to desirable properties. "The beauty of the material is the combination of properties. You get the best of both worlds," says Didier Leroy, an r&d leader who helped develop the new elastomer. He goes on to cite a balance of properties that includes low modulus, good optical properties, heat resistance, elastic recovery, adhesion to polyolefins, tolerance for high filler loadings, and a soft touch feel. According to Leroy, the actual properties of each individual Versify grade can be tailored to various applications—because Dow can optimize the proportion of the two monomers to arrive at a different profile of properties.
Suitable for extrusion, injection molding, and other plastic processes, Versify could fit into a variety of applications. Some of them involve flexible and rigid packaging. The company is also looking at that material for consumer durables, wire-and-cable jacketing, and tubing. It could also serve as an overmolding material or gasketing too.
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| New photoaddressable polymers from Bayer Material Science may make holographic data storage a reality--at first in cards capable of storing financial or biometric data. |
Nanoscale Tooling Surfaces
The optical properties of molded plastic parts come down partially to the plastic itself, but attention to molding conditions and tooling have every bit as much of an impact. A new molding and tooling technology from Germany's AlCove Surfaces promises to dramatically improve the light transmittance of clear parts made from transparent thermoplastics such as polycarbonate or acrylic.
The company does so by treating an aluminum mold cavity with an electro-chemical etching process that leaves behind nanoscale "pillars" on the tooling surfaces. According to Norbert Beyer, Alcove's managing director, the etching process can produce a range of pillar heights. "We've found that a height of 200 nanometers is best for optical purposes," he says.
At that height, he continues, the pillars do a good job at blocking the reflections that would occur with a planar surface. The resulting upswing in light transmittance has been dramatic. Beyer reports that transmittance on polycarbonate improved from about 88 to 90 percent on an untreated sample to about 93 percent on sample structured on one side to as much as 98 percent for sample structured on both sizes.
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| AlCove's mold-surface structuring creates anti-reflective surfaces on transparent molded plastic parts, enabling dramatic improvements in light transmission. |
The structuring method works best when used in an injection-compression process—whereby the plastic resin is shot into a slightly gapped mold, which then squeezes shut to complete the filling. And Zoellner says the process needs very careful control of molding temperatures to ensure that the part replicates the tooling surfaces.
The technology so far seems robust. "We haven't seen any washout of the nano-structuring in the mold even after 1,500 or more shots," reports Olaf Zoellner, manager of injection molding technology for Bayer Material Science, which has been using Alcove's patented technology in conjunction with polycarbonate. He adds that the method has "a lot of potential" for a range of polycarbonate optical parts such as lenses, heads-up displays, and light guides.
It's worth noting that AlCove's technology may do more than produce antireflective surfaces for visible light. By varying the topography of the structured surface, the company's engineers have achieved the same effect for infrared light, according to Beyer. And they have been able to use nano-structuring techniques to impart a "lotus effect" or resistance to the accumulation moisture and dirt particles.
| Web Resources | ||
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Alcove: http://rbi.ims.ca/4385-524 |
Bayer: http://rbi.ims.ca/4385-525 |
Dow: http://rbi.ims.ca/4385-526 |
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