Conductive polymer is moldable, extrudable in thermoplastic blends.

Conductive Polymer Is Moldable, Extrudable in Thermoplastic Blends

As we reported briefly in June (p. 15), what is believed to be the only commercial composition of an intrinsically conductive polymer (ICP) that is truly moldable and extrudable was unveiled first at the SPE ANTEC meeting in Montreal in May and again at NPE in Chicago in June. The development is a result of a three-way collaboration between Zipperling Kessler & Co. in Germany, Americhem of Cuyahoga Falls, Ohio, and Allied-Signal Inc., Morristown, N.J. The ICP is a doped form of polyaniline, an amorphous material that by itself is neither melt processable nor readily soluble. However, Americhem and Zipperling have learned over the past six years or so how to disperse it in selected thermoplastics to produce compounds that are conventionally processable while retaining high electrical conductivity. What's more, conductive compounds can be colored or even transparent.

Allied-Signal is the sole producer of the pure ICP. Americhem and Zipperling are willing to consider licensing their compounding technology to others for noncompeting applications. Here now are the full, publicly available details on this novel development.

PROPRIETARY MIXING METHODS

Americhem and Zipperling Kessler are both bringing to market flexible PVC and nylon-based ICP compounds, trade-named Incoblend by both firms. These are custom formulated for each customer and application. PVC blends could be used in EMI shielding, antistatic flooring, wall coverings, films, or cable jacketing. (Less satisfactory results have been obtained in rigid PVC.) An unidentified nylon composition also is being sampled for EMI shielding uses (no products are yet in commercial use). Also being developed are ICP compounds in TP polyester, ABS, and polycarbonate. ICP chemically reacts with TP urethane, losing conductivity over time, but there may be methods of stabilization to overcome this.

The intractable pure ICP takes the microscopic form of tiny beads, which have a strong affinity for each other and group together in long chains. The trick in compounding is to overcome the inter-particle attraction to separate them sufficiently to permit uninform dispersion, after which they come back together to form a conductive network within the thermoplastic matrix. This does not require any uniquely specialized equipment; however, the processing technique is critical--and patented. With their proprietary know-how, Americhem and Zipperling have been able to obtain virtually the same conductivity level as the pure ICP at only 15-25% loading by volume. But Americhem sources concede that it's a separate challenge to achieve successful results in each different thermoplastic matrix resin. Thus far, it has been easier to work with polar than with nonpolar resins (such as polyolefins).

HIGH CONDUCTIVITY POSSIBLE

Bulk conductivity can readily be achieved as high as 5 Siemens per centimeter (S/cm, which is the reciprocal of volume resistivity in ohm-cm). In some cases, it has been possible to achieve 20 S/cm and, experimentally, even close to 100 S/cm. For comparison, pure aluminum and copper have conductivities of [10.sup.4] and [10.sup.5] S/cm, respectively. According to Americhem, levels from [10.sup.-3] S/cm on up are suitable for EMI shielding. Lower levels of ICP in the blend can provide [10.sup.-5] S/cm for electrostatic-discharge (ESD) protection, and [10.sup.-9] S/cm for simple antistatic purposes.

These conductivity levels are said to be at least as good as the best that can be obtained with special carbon blacks or with metal flakes or fibers. But ICP blends offer a better balance of conductivity, processability, and mechanical properties than are obtained with conductive fillers or carbon black, especially at high loadings, says Americhem. Unlike ICP blends, conductive fillers reportedly may not be suitable for formulating conductive thin films. And ICP blends are said to give more uniform conductivity, without "holes."

In some applications, ICP blends' colorability may be an advantage over carbon black. In thin films, ICP blends can also be transparent, though with a blue-green tint. (In thicker sections and at higher ICP loadings and conductivity levels, the color is greenish-black.) But given the fairly high cost of ICP blends (see below), Americhem sources doubt that they will displace carbon black in any application where the latter performs adequately.

ICP ACTS LIKE A FILLER

The base ICP resin, Allied-Signal's Versicon Conductive Polymer, is a dark-green powder of 3 to 100 micron particle size, 1.36 specific gravity, and 3-4% moisture content. In general, the ICP in a blend acts like a particulate filler, increasing stiffness while somewhat reducing elongation. Americhem's flexible PVC compound, containing about 30% ICP for high conductivity of 1-5 S/cm, has tensile strength greater than 600 psi, elongation greater than 250%, and Shore hardness of 82A. Nylon compounds have been formulated in the 0.1-1.0 S/cm range, with tensile strength around 4000 psi and elongation over 200%.

Americhem says the blends process very much like the unfilled matrix thermoplastics. No special precautions need be taken in processing or reprocessing ICP blends. High-shear processing, like injection molding, can reduce the conductivity of finished parts by one or two orders of magnitude, apparently by changing the internal structure, or morphology, of the blend. Americhem is working on ways to minimize this effect, which reportedly does not occur under the lower shear conditions of extrusion.

Conductivity of ICP blends is said to be extremely stable over the long term and not adversely affected by environmental conditions (moisture, for instance, makes the conductivity slightly higher). High heat can cause a decrease in conductivity, owing to loss of the dopant. ICP blends reportedly are stable at 212 F for long-term service and can withstand up to 490 F for several minutes. Allied-Signal and Americhem are working to raise this to 570 F.

FUTURE POTENTIAL

Americhem and Allied-Signal see numerous potential applications for ICP and its blends. EMI shielding uses, including gaskets, are foremost. While Americhem concentrates on specialty applications, Allied-Signal will be targeting major thermoplastic resin producers and other firms that may be interested in compounding for large-volume uses.

Americhem also anticipates formulating a liquid concentrate that would be diluted to formulate ESD coatings applied by conventional roll or dip coating methods or perhaps rotogravure printing. (Such coatings would require a polymer base, involving dispersion of ICP in a thermoplastic.) Americhem already has proprietary technology for applying conductive coatings to PET film. Adhesives are also being investigated.

As higher volume usage develops, Americhem foresees that some ICP blends could be priced in the $10-50/lb range. However, initial pricing is more like $30-80/lb. (CIRCLE 10)

Pure ICP itself is available in sample quantities of 10 to 100 lb for $180/lb. For small commercial uses, certain grades will drop to around $25/lb. Allied-Signal is expected to make a decision on building a larger scale plant later this year. For samples and information, contact D.G. Frick at Allied-Signal's Buffalo, N.Y., facility; tel. [phone]. (CIRCLE 11)

PHOTO : At high loadings and in thick sections, ICP blends are greenish-black. Compounds can be colorable or even transparent in thin films. Shown here is an ICP coating on PET film.