Originally Published MPMN September 2001

PRODUCT UPDATE

Conductive Urethanes Retain Key Properties

Materials combine static dissipation with toughness and elongation

Enhancing certain properties in a polymer traditionally has meant accepting trade-offs in other areas. Selectively enhanced polymers (SEPs) promise to change that equation. The technology has been applied to the formulation of conductive urethanes.To promote static dissipation in urethanes, suppliers typically add a compound that affects the material's other properties, or they may use an external static dissipation system whose effectiveness erodes over time. By leveraging SEP technology, one company has developed a line of conductive urethanes that provide static dissipation without a loss in flexibility or other mechanical properties. This technology and other developments in medical-grade materials are discussed in this section.

Inherently nonconductive, thermoplastics must be modified to dissipate static charges that can damage electronics. Traditionally, urethane fabricators have promoted electrical dissipation by introducing internally compounded additives, such as carbon. The trade-off has been a decrease in material toughness, elongation, and other key mechanical properties.

To address this issue, Foster Corp. has created SEP conductive urethanes that combine static dissipation with the flexibility and mechanical properties of urethane. To achieve this result, Foster uses a blend of highly structured carbon additives and compounding techniques.

"Static charges can be an engineer's worst nightmare, since they jeopardize sensitive electronic components," says company president Larry Acquarulo. "Conductive materials give these charges a path to dissipate without destroying the product. Yet the sacrifice in toughness and flexibility from traditional carbon-filled polymers is unacceptable for belts, tubing, and other flexible components."

The company's SEP conductive urethanes exhibit as much as 300% elongation, but are specifically designed for applications with 105 (omega)/sq surface resistivities. The combination of properties enables optimal consolidation of components, increasing design flexibility and reducing overall product costs, according to the company. The SEP urethanes can be processed using standard extrusion, molding, and other manufacturing techniques. Applications include tubing, film, sheet, and injection-molded components.

Polyurethane lines offer strength and heat resistance

Urethanes offer a combination of properties such as toughness, a soft feel, and ease of fabrication, as well as resistance to microbes, skin oils, moisture, and a range of chemicals that make them suitable for use in demanding medical environments. They can be sterilized by most common methods, including gamma, with little effect on performance.

A line of polyurethanes from Stevens Urethane combines the properties of rubber and plastic to offer high performance and reliability for applications including burn blankets, breast prostheses, wound dressings, cold packs, and tubing. The company provides three materials with varying degrees of hardness. MP-1880 is a general-purpose urethane that features an 87 Shore A hardness rating, good abrasion resistance, and a 60°–200°F maximum service temperature range. MP-1882 is a softer, more pliable grade that is soft to the touch; the 82 Shore A hardness material is suitable for applications involving long-term skin contact. MP-1890, with a 90 Shore A hardness rating, is designed for applications in which greater temperature parameters and puncture resistance are required, such as cold packs. The urethanes are available in 0.001–0.125-in. thicknesses, in widths starting at 5 in., and in a range of colors, opacities, and surface textures. Tubing comes in 1/16–1/4 in. ID and 1/8–1/2 in. OD with 1/32–1/8 in. wall thicknesses.

Vantico Inc. has added two new polyurethane compounds to its Parts-in-Minutes line. The compounds are designed for quick molding of tough prototypes and short-run parts. The RP 6462 R/H Fast is a high-impact-strength, high-heat material that is intended for rapid, efficient molding of polyethylene-like parts. RP 6463 R polyurethane resin along with slow or fast hardeners is formulated for the rapid production of clear, thermoplastic-like prototypes and parts with good UV stability.

RP 6462 R/H Fast can be dispensed by means of automated meter/mix equipment. Jet-black in color, the material gels in less than 50 seconds and can be demolded in about 5 minutes. After a room temperature cure, the polyurethane exhibits 2.3-ft lb/in. notched Izod impact strength, 194°F heat deflection, and 120,000-psi flexural modulus. Mechanical properties can be enhanced with a postcure process.

RP 6463 R is suited for parts from 1 in. thick with a fast system to 2–3 in. thick with a slow system. The room temperature demold time ranges from 4 to 16 hours, depending on the hardener or hardener combination that is used. Once cured, 136° to 152°F heat deflection temperatures and 1-ft lb/in. notched Izod impact strength can be achieved. The molded products have a flexural modulus in the 240,000-psi range and 79 Shore D hardness.

Laminated COC provides a high moisture and oxygen barrier

A cost-effective cycloolefin copolymer (COC) laminated film offers high moisture and oxygen barrier properties for blister packaging applications. The Pentapharm COC SD/03 is a rigid vinyl/COC/PVDC laminated film that seals to common vinyl and PVDC lidding materials. Available from Klöckner Pentaplast of America Inc., the coextruded film has a low specific gravity, which results in higher thermoforming yields. In addition, the film has the thermoforming capabilities of vinyl as well as good optical clarity. Pentapharm films run on standard thermoform tooling. "The material benefits medical manufacturers by offering them a material that has both oxygen and moisture barriers for any specialized products," says communications and media manager Nancy Ryan. "There is no other rigid film on the market that offers this."

Katherine Sweeny

Copyright ©2001 Medical Product Manufacturing News