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Plastics with a Bluetooth bite
Bluetooth and other wireless communications standards call for a materials-selection strategy centered on electrical properties
Joseph Ogando, Materials Editor -- Design News, July 2, 2001
Wireless technology will soon create a world in which nearly everything computes. The technology prophets call it "pervasive computing." And they're not just talking about cell phones and handheld digital assistants, both of which can already send data through the ether.
No, the wireless future will allow nearly all kinds of electronically-enabled products to communicate with short-range radio waves. Even products previously thought of as the strong, silent types will electronically bend the ear of any nearby device that cares to listen. Cars, for example, will speak to their owners' cell phones and to nearby PCs. Stoves, dishwashers, and refrigerators may wirelessly exchange digital pleasantries across the kitchen.
Bluetooth, the most dominant of the emerging communications technologies, already has more than 2,000 electronics, automotive, and appliance manufacturers collaborating on a wireless standard that will let their products communicate via 2.4 GHz radio waves.
Whether the idea of pervasive computing sounds too good to be true or like too much of a good thing, it will certainly change both the electrical and mechanical design of many products as engineers add embedded antennas where there once were none. And for this reason, Bluetooth also promises to change the role of thermoplastics in electronic devices, according to Steve Maki, technology manager for RTP Co., a custom compounder in Winona, MN.
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Longer wavelengths mean larger antennas. So encapsulating antennas in plastics cann enable size reductions by reducing a signal's wavelength before it reaches the metal antenna. |
Plastics today function mostly as cladding for electronic devices, so their mechanical properties reign supreme. In a wireless world, though, plastics will increasingly be picked for their ability to enhance wireless performance—a task that requires plastic compounds with finely-tuned electrical properties. "Traditional plastics might work in wireless devices," Maki says. "But the best performance will come from materials optimized for Bluetooth and other wireless standards."
Catching the waves. Summing up a basic materials selection strategy for wireless devices, Maki advises engineers to pay closer attention to dielectric constant and loss factor. These two electrical properties govern how Bluetooth's electromagnetic waves pass through plastic components to embedded antennas. Materials for enclosure components—such as housings, frames and bezels—should exhibit a low dielectric constant and low loss factor, which together will leave the signal relatively unchanged as it passes through the plastics. Of the two, Maki argues that loss factor matters the most because it has the greatest effect on the signal strength. "When the loss factor is zero, signal strength is unaffected," he says, adding that higher loss factors result in diminished signal and the conversion of electromagnetic energy into unwanted heat.
Materials for encapsulated metal antennas, another promising Bluetooth application, also need a low loss factor. Yet a relatively high dielectric constant will enable reductions in antenna size. As Maki explains, bumping up the dielectric constant of the plastic surrounding a metal antenna will decrease the electromagnetic wavelength that dictates antenna size. He cautions, however, that too high a dielectric constant reduces the range of frequencies that the antenna can handle. For Bluetooth's 2.4 GHz, he says, the upper limit for dielectric constant would be around 9.
"To be ideal, the material has to have a dielectric constant and loss factor that are stable over temperature, humidity, and frequency," Maki says. In RTP's electrical tests, the least stable plastics have been those that absorb moisture. "Water has a very high loss factor," he notes. RTP's list of the most stable plastics includes syndiotactic polystyrene, olefins, and fluoropolymers.Fill it up. Given that unmodified plastics don't often go into electronic devices, picking the right base material is only half the battle. "Fillers also contribute to the electrical performance," Maki says, explaining that compounds for Bluetooth devices, like those for traditional electronics applications, still need fillers to optimize mechanical and physical properties. Flame retardants, it seems, don't pose much of a problem. "We can make these compounds flame retardant with very little effect on dielectric properties," he says.
But the fillers that impart strength, stiffness, and lubricity are another story because their electrical properties vary so widely. PTFE, for example, has a dielectric constant of 2.10 at 1 MHz and a loss factor of 2.1 at 1 MHz 10-4, while a common glass-fiber reinforcement has a dielectric constant of 5.34 and a loss factor of 110 at the same frequencies. "Doped ceramics," those oxides and zirconates used to drive up the dielectric constant nantenna-encapsulation materials, cover an even bigger scope. Their dielectric constants range from 15 to 10,000 and loss factors from 1 to 4,000.
For all their advantages, fillers can cause some of the difficulties in creating Bluetooth compounds. An exponential relationship between filler loadings and electrical properties means that these compounds require very precise formulations. And fillers tend to change electrical properties in what Maki describes as "unpredictable ways." Skillful blending of plastics and fillers can offset these problems. "The trick is not just selecting the right plastic," says Maki, "but the right combination of plastics and fillers."
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Engineering plastics from RTP Co.:
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| Comparing Bluetooth compounds | ||
|---|---|---|
| Syndiotactic polystyrene | Polyolefin | |
| Filler | Glass fiber | Glass fiber |
| Dielectric constant @ 2.4 GHz | 3.1 | 2.9 |
| Loss Factor @ 2.4 GHz | 0.001 | 0.001 |
| Specific gravity | 1.25 | 1.13 |
| Tensile strength, psi | 17,500 | 12,000 |
| Elongation, % | 1.5 | 2.5 |
| Notched IZOD impact, ft-lbs/in | 1.5 | 1.9 |
| Mold shrinkage, in/in | 0.003 | 0.003 |
| HDT @ 246 psi, F | 460 | 295 |
| Electrical properties of engineering plastics | ||
|---|---|---|
| Polymer | Dielectric constant @ 1 MHz | Loss factor @ 1MHz x 10-4 |
| PP | 2.20 | 4.4 |
| ABS | 2.40 | 170 |
| SPS | 2.50 | 31 |
| PC | 3.00 | 290 |
| PBT | 3.10 | 620 |
| PEEK | 3.25 | 98 |
| PES | 3.54 | 200 |
| Source: RTP |
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