Material Innovations

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Cool Polymers conductive plastic

Spot-heat sources were applied to the center of a CoolPoly panel (left) and a standard plastic panel (right) to demonstrate the difference in how each polymer handles the heat.

Although plastics are the material of choice for many applications, standard plastics do not possess the needed conductivity to provide thermal solutions in an unmodified state. Heat build-up from microelectronic and other heat-generating functions that rely on plastics to conduct heat often leads to component failure. Thermally conductive polymers take heat away from a "hot spot," spreading it through a component to more efficiently dissipate the heat into the surrounding environment. The trend toward developing smarter, smaller, and lighter products has driven the development of these polymers.

Cool Polymers, Inc., a division of Chip Coolers, has developed CoolPoly, a thermally conductive polymer that the company claims will provide automotive engineers with a plastic material for solving heat-transfer problems. According to the company, applications for the product include fluid/air or fluid/fluid heat exchangers, passenger interface components, electronic enclosures, electric motor encapsulation, coil bobbins, and sensors, among others. CoolPoly can replace metals and ceramics in some applications, and commodity or engineering plastics in other applications.

CoolPoly is made through a patent-pending process that combines a base resin (most commodity and engineering plastics can be used) with proprietary fibrous and particulate-reinforcement fillers to tailor material properties to custom performance requirements. All fillers are non-metallic; some CoolPoly grades are based on carbonaceous and ceramic materials. Additional additives such as UV inhibitors, heat stabilizers, and lubricants can be used for specific applications.

For a demonstration, engineers at Cool Polymers applied 5-W spot-heat sources to the center of a 76 x 76 x 3 mm (3 x 3 x 0.125 in) CoolPoly thermally conductive panel and a standard plastic (polypropylene) panel that, like most plastics, is a thermal insulator. The CoolPoly panel conducted the heat, generating a more isothermal profile, while the standard plastic panel created a hot spot. The maximum temperature difference on the CoolPoly panel from the center temperature to the outer edge temperature was 4°C (7.2°F), while the standard plastic produced a 24°C (43°F) difference. Additionally, the temperature at the edge of the standard plastic was equal to the ambient temperature, indicating that no thermal energy had been transferred to the edge.

- Jean L. Broge

Solvay's polyolefin SEQUEL

The covers for the three roof pillars of the 2001 Chrysler Sebring and Dodge Stratus (shown) are injection molded of SEQUEL 2325 engineered polyolefin from Solvay Engineered Products.

For the 2001 model year, DaimlerChrysler completely redesigned cars in its entry-level midsize lineup. The new Chrysler Sebring and Dodge Stratus have been fitted with an optional side-curtain airbag that will deploy from the headliner and the C-pillar cover, and all three roof-pillar covers have been designed to meet federal safety regulations for head impact. Providing impact resistance for the A-, B-, and C-pillar trim parts in this redesigned environment is SEQUEL 2325 engineered polyolefin, one in a series of materials designed by Solvay Engineered Polymers specifically for automotive interior applications that involve energy management. Venture Industries injection molds the parts at its Harper plant in Detroit.

The flow characteristics of the 2325 polyolefin present processing benefits in terms of tooling, machinery, and productivity. The new material permits an integral rib design to be molded into the back of the A- and B-pillar covers to manage the energy of a head impact. The rib pattern is accommodated without sink marks, and the surface of the integrally colored parts has a low-gloss appearance.

The underside of a B-pillar cover for the two DaimlerChrysler sedans shows the integral rib pattern designed for energy management.

The side-curtain airbag in the two vehicle models is tethered behind the A-pillar cover, and there is a deployment door attached to the C-pillar trim that impacts the cover during inflation. Several materials including ABS, polypropylenes, and alternative TPOs failed to meet the impact requirements of the new sedans; the new material would have to interface with the airbag deployment at temperatures ranging from -30 to +85°C (-22 to +185°F). The SEQUEL 2325 polyolefin exhibited ductile behavior across the required temperature range and had the highest impact resistance of any material in the 2300 series of interior materials, with a typical instrumented-impact value of 27 J (20 ft•lb). Its flexural modulus of 1200 MPa (174 ksi) makes it the most flexible material in the series.

For the roof-pillar covers in the new sedans, Venture drew on its polyolefin experience for the interior trim in DaimlerChrysler's full-size sedans. Its engineering team was confident that they could mold the parts in color and consistently meet the basic appearance requirement of less than 2° of gloss. The parts are molded in light natural beige and medium sandstone, and the surface is finished in DaimlerChrysler's Rochester grain.

The A-, B-, and C-pillar covers (left to right) are molded in two low-gloss colors by Venture Industries at its Harper plant in Detroit.

The roof-pillar covers have nominal wall thicknesses ranging from 2.5 to 3.5 mm (0.1 to 0.14 in), qualifying as "thinwall" applications. Because of the 2325 polyolefin's flow properties, Venture is able to use two-cavity molds to produce matching pairs of parts on 635-t (700-ton) presses. Savings derive both from lower tooling cost and from reduction in cycle time, compared to single-cavity molds running a traditional, low-flow engineering resin.

The Computer-Aided Engineering Group at Solvay modeled prospective designs for the parts and properties for the new material prior to pilot production. A high degree of correlation was achieved between predicted and actual test results for compliance with federal head-impact regulations, helping to reduce the time and effort required before the parts were tested and approved by DaimlerChrysler.

- Jean L. Broge