Glossed Over

GE’s Lexan SLX/substrate combo not only eliminates the paint shop but achieves a higher gloss factor than paint.

A robotic laser cutter trims the panels after the Lexan and substrate have been molded together.

The market called and GE Plastics listened. When GE asked automakers what type of innovation they wanted one of the answers was a weather resistant material that doesn’t need to be painted yet is still glossy. GE’s answer to their wishes is Lexan SLX — a film that is added on top of a thermal formed substrate. Developed at the company’s research and development center in Schenectady, N.Y., the Lexan SLX /substrate combo is designed to take paint out of equation but still allows for the glossiness paint provides.

According to information from GE, most automotive paints will register about an 85 or 90 unit reading on the Gardner scale (also referred to gloss units). GE claims to have achieved between 105-110 gloss units with their substrate/Lexan SLX film combination. Bob Johnson, Lexan SLX film product manager, says that while this is impressive the true selling point was not just the initial gloss. “The big selling feature is that it retains that gloss after a long period of time outdoors,” Johnson says. “Which many plastics do not.”

According to GE, the gloss remains above 95 gloss units after three years of simulated weather testing. This retention is very important; since it illustrates that the Lexan SLX film/substrate combination not only starts at a significantly higher gloss level than paint and remains higher even after years of simulated weathering. The weather test was also conducted on the Lexan SLX film alone and according to GE’s results the change in gloss was almost immeasurable.

Another benefit to the Lexan SLX film on top the other layers was the lack of flow lines. GE noted that while some injection molded pieces will display flow lines, due to the manufacturing process the Lexan SLX film is much smoother thanks to extrusion forming. The combination of those two processes made a bit more sense and not only for aesthetic reasons, says Johnson.

“We’ve always had a couple of objectives with polymer development. We’re always looking for chemical resistant materials. We’ve been looking for durable outdoor polymers. It’s not injection moldable which makes the film approach more sensible.

Because you can simply lay the film over the injection molded materials,” says Johnson. Since gloss is not the only concern in replacing steel painted body parts with plastics, the substrate/Lexan film combination also had to fulfill other duties.

“In the vertical body panels like quarter panels or door skins it is more than adequate to use standard injection molder thermal plastic at about 1/8 inch thick or so. Saturn has been doing that for over ten years. It gives you enough rigidity that it won’t flap around in the wind but it’s nice and flexible and will bounce back after minor collisions. Horizontal body panels are tougher because there are a lot more requirements on modules and horizontal body panels. OEM’s are working on a glass filled thermoset polyurethane. This is not a product we make at the moment.”

GE is working towards another combination that would feature their film over the top of a material that will meet the requirements for a horizontal body panel. It is not available yet.

GE is in deep developmental programs using the SLX film on exteriors. Johnson says that the first automotive application of the company’s substrate/Lexan SLX combo will be unveiled at the NPE 2003 plastics show in Chicago in mid-June.

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How Gloss is Measured

So how is gloss measured?

BYK Gardner manufactures gloss measurement systems and described the measurement process as such: Using reflectometers, reflected light of a surface is measured in an angle range which is limited by aperture dimensions. The light source is projected over the sample surface onto the opening of aperture 2. A photoelectronic detector measures the light passing through the aperture.

Different angles, or geometries, are better for measuring different gloss values and surfaces. For example, if you are measuring low gloss surfaces (0 to 30 gloss units) the 85 degree meter will provide you the most accurate readings. If you are measuring semi-gloss surfaces (30 to 70 gloss units) the 60 degree meter is best. And for high gloss surfaces (over 70 gloss units) the 20 degree meter is best. There are also ‘specialty’ angles. For example the 45 degree meter is used for the ceramic and film industries, and the 75 degree meter is used for the paper and vinyl industries. According to GE, a 60 degree meter is used for automotive purposes.