Leveraging new cable materials

Feb. 1, 2012
New engineered polymers are boosting the safety and reliability of the deceptively simple component of cable.

Silicone, modified polyphenylene ethers, halogen-free — where are new cable materials justified? Consider this example: Thousands of silicone-encased cable assemblies operate in mission-critical military applications — commercial aviation fuel control, missile gimbals and guidance, fighter-aircraft navigation and targeting, and space vehicle flight control and communication systems. Why?

Cables consisting of silicone-encased conductors are unaffected by g forces, abrasion, water, -65° to +260° C temperatures, and even flame — plus silicone jacketing functions as a shock-absorbing material by supporting encased strands. Unlike certain types of PVC, polyurethane, or Teflon-jacketed cables, silicone-encased cable also requires no clamping, because the conductors don't creep from the casing.

It's not all cut-and-dry, however: Engineered PTFE (polytetrafluoroethylene) flat and round cables, for example, are suitable for extreme environments and exhibit high flex to maintain signal integrity; unlike certain types of extruded silicone and polyurethane cables, engineers at W. L. Gore & Associates Inc., Newark, Del., point out that flat cable creates zero particles — useful in cleanrooms.

Elsewhere, it's important that cable systems meet established safety requirements in case of fire. Here, both cable carriers and the cable jacketing itself increasingly sports a halogen-free designation. Halogens — elements such as fluorine, chlorine, bromine, and iodine — are stable and present zero danger under normal operation. In a fire, however, halogenic plastics release hydrogen chloride, hydrogen fluoride, and other dangerous gases — toxic and corrosive. In addition, when the former comes in contact with water, it forms hydrochloric acid. Polyvinyl chloride (PVC), a common wire and cable insulator (due to its mechanical and electrical properties and low cost) is nearly 30% chlorine by weight. Teflon FEP and PTFE also contain significant fluorine and produce acid when burned. Ironically, halogens are often added to cable-jacket formulations because they initially retard combustion.

As one alternative, some flexible wire duct is manufactured from low-smoke, zero-halogen (LSZH) polypropylene material and carries a UL 94V-0 flammability rating and CSA approval. LSZH plastics used for cabling and cable carriers emit no dangerous gases or smokes when burned.

The only catch is that LSZH cannot be conveniently recycled or reused. For this reason, some wire features flexible Noryl insulation to provide an environmentally friendly alternative. Noryl is a modified polyphenylene ether (mPPE) thermoplastic that is more durable than PVC. mPPE contains no halogens or phthalates and meets RoHS and WEEE requirements; some industries already incorporate this alternative wire. The Toyota Tundra, for example, is fully wired with mPPE-based components, and several global PC manufacturers have set goals to replace PVC with mPPE-based insulation. Finally, the 1.03 specific gravity of mPPE is 25% to 40% lower than PVC, polyethylene, and cross-linked polyethylene insulation, for 25% weight savings and (in conjunction with suitable dielectric properties) thinner walls.

That said, where mPPE cable can be more costly, other cable chemistry still dominates extreme designs.

Consider that for continuous flexing, igus Inc., East Providence, R.I., offers twistable (torsion-resistant) hybrid, bus, motor, and fiber-optic cables for robotics. These include several with oil resistant and flame-retardant polyurethane (PUR) outer jackets; fiber-optic cable with a thermoplastic elastomer (TPE) outer jacket; and PVC-free cable.

The Lapp Group, Florham Park, N.J., also offers myriad continuous-flex cable options, including varieties that leverage polyurethane for insulation and jacketing materials — for mechanical and chemical resistance to many environments, plus flame-retardant properties. In some versions, the materials used are halogen-free and environmentally friendly; alternatively, a variety of LSZH irradiated polyolefin-insulated cables in the company's lineup — made from polyolefins such as polypropylene or polyethylene — offer other environmentally safe options.

In another innovation by the company, corrosion-resistant materials allow a new cable subcomponent to survive in applications that would prove challenging for traditional painted aluminum constructions. Here, new rectangular connector housings survive corrosive, electrically noisy or mechanically challenging environments, with nickel-plated zinc hoods and bases featuring stainless-steel hardware. An integrated brush-style cable gland allows easy termination of a cable shield with a 360° low-resistance contact area; along with the nickel-plated housing, this forms a conductive shell — or Faraday cage — around electrical contacts to eliminate electrical interference.

Stay tuned: Look for a follow-up MSD article in coming months about other connector and carrier materials, and cable designs that combat electrical interference.

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Silicone cables —

Cables for robotics —

Protective connectors —

Flexible wiring duct —

About the Author

Elisabeth Eitel

Elisabeth Eitel was a Senior Editor at Machine Design magazine until 2014. She has a B.S. in Mechanical Engineering from Fenn College at Cleveland State University.

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