Impact tests stop paints from cracking

We all know that the right paint can make a product more aesthetically pleasing and more desirable. That's certainly true for cars and trucks. But nothing detracts from a beautifully painted bumper or quarter panel more than a dent.

There's no way to guard against the impact hazards that an automobile can encounter—shopping carts, flying pebbles, and even telephone poles—but with the proper impact testing, automakers and suppliers can help prevent these hazards from marring their vehicles.

This fixture holds the specimen during an impact test. Courtesy of Instron.

Red Spot's lab manager, Jeff Adler, says Ford Motor Co. requires that its coatings be tested using methods specified in ASTM D3763 (Ref. 1). For these tests, Red Spot creates a set of five unpainted specimens and another set of five painted specimens. The engineers then test the unpainted specimens to establish a baseline or reference value.

Next, they test the painted specimens and compare the impact retention of the painted specimens to the values for the unpainted specimens. Adler notes, "We see a wide variety of impact retention changes. I have seen impact retention changes as low as 10% and as high as 200%." The Ford specification is 90% for Lexan and other polycarbonate plastics and 60% for ABS plastics.

"For GM," Adler says, "we test to one of their internal standards. Those tests give us information on the traits of a given coated plastic and the reasons for failure, whether it be ductile or brittle, and whether or not it can withstand temperature extremes."

During the tests, the tester captures, plots, and analyzes the entire impact event. With this data, Red Spot can pinpoint the ductile-to-brittle transition point and can also study ductility, incipient damage, maximum load, and absorbed energy.

The impact test methods give Red Spot a greater understanding of the real-world properties and characteristics of the paints and coatings they manufacture. And this means that shopping carts and telephone poles have a smaller impact on the plastic bumpers found on today's cars.