Novel decorative effects produced by multilayer sheet.

After two years of testing and development, Dow Plastics, Midland, Mich., has revealed more details on its novel Polymeric Reflective Material (PRM), a thermoplastic extruded sheet that contains no metal yet offers reflective properties that mimic chrome plating or aluminum vacuum metalization. Dow supplied a preview of this technology two years ago.

Gregg Motter, commercial development manager for Dow's New Ventures Development, says PRM is a multilayer sheet of polycarbonate and acrylic, produced by a proprietary extrusion process. He declines to reveal specifics of the extrusion process, but notes that PRM can range from 650 layers (providing 40% reflectance that simulates iron) to 1300 layers (with 55% reflectance that simulates brushed aluminum, steel or platinum), or even 2600 layers (70% reflectance simulates gold, chrome plating or bright copper).

Dow is targeting thermoforming applications for PRM sheet in durable-goods markets such as automotive, appliances and home furnishings. PRM is being produced at a Dow research facility and is not yet a commercial product. Though Dow declines to comment, it's believed that a prototype automotive lighting application utilizing PRM may be introduced early next year.

Though PRM technology theoretically can utilize various combinations of optically clear thermoplastics, Motter says Dow has chosen the PC/acrylic combinatin for its balance of processing, temperature and mechanical properties. PC and acrylic also have a desirable degree of difference in their index of refraction, which accounts for the metallic-like reflectance of PRM. Motter points out that as the difference in the index of refraction between the polymer layers increases, fewer layers are needed to attain the necessary level of reflectivity.

A VARIETY OF EFFECTS

While this alternating-layer "core" is the reflective mechanism of the sheet, the skin or cap layers produce additional optical effects that allow PRM to provide a family of market-tailored products. Various combinations (clear, colored or black) of top and bottom skin layers can create a metallic appearance, pearlescence, a lustrous "wet look," or other special optical effects that combine simultaneous light transmission and reflection (a phenomenon Dow calls "transflection").

Besides thermoforming, PRM can also be compression molded. Dow envisions that the material would be supplied only in sheet form; it cannot be offered as pellets, because the reflectance properties depend on the proprietary extrusion process to form flat, uniform resin layers with a controlled interface.

Motter says PRM already has undergone extensive production testing in several commercial thermo-forming shops, demonstrating its ability to be processed by existing forming and trimming methods and with existing tooling.

PRM sheet processes similar to ABS with a melt strength suitable for drape, vacuum, pressure, plug-assist, compression, billow, and snap-back thermoforming, according to Motter. He says PRM can achieve an impressive 3:1 draw ratio.

The material has a heat-distortion temperature of about 285F at 264 psi, with impact and mechanical properties similar to those of polycarbonate. PRM thermoforming trim scrap can be reprocessed as a PC/acrylic blend that provides a gray, pearlescent look.