Correlation of Three Laboratory Tests with an In-Service Test 900526
Three established and widely used laboratory tests were evaluated for their ability to distinguish between the flexibility of automotive finishes and their ability to predict in-service behavior of plastic automotive bumper/fascia parts. This evaluation was made by correlating the laboratory tests with the bumper impact test which is a representative slice of in-service behavior. The tests - mandrel bend, drop dart, and driven dart were evaluated using four currently used flexible plastic substrates and three urethane modified acrylic-polyester automotive finishes.
The drop dart and driven dart test showed a better correlation with the bumper impact test, than the mandrel bend test. The degree of correlation is explained on the basis of similarity between the stress states during impact.
The bumper impact test is not able to distinguish between the flexibility of three automotive finishes studied because they are above the minimum flexibility level required for the substrates studied. The mandrel bend test is able to distinguish between the flexibility of the three finishes.
The increasing use of flexible plastic substrates to fabricate automobile body components has created a need for exterior finishes that are adaptable to both metal and composite substrates. These coatings, referred to as “universal” coatings, have to be rigid enough to be finessed and flexible enough to pass MVSS requirements while maintaining all other properties required of automotive finishes. Currently, rigid clearcoat finishes are used on body while flexible clearcoats are used on the composite fascia. The flexible clearcoats are highly flexible even at -30°C to assure good performance in the field. Universal coatings require a balance between rigidity and flexibility to insure satisfactory field performance. If the correlation to field performance were known more realistic specifications could be established.
Currently the flexibility of painted composites is measured by bending a piece of painted substrate around a mandrel fixture. This test, known as the mandrel bend test, plays an important role in the development of flexible automotive finishes. This study was undertaken to correlate flexibility as measured by the mandrel bend test with field performance and to evaluate two other lab tests for correlation with field performance.
Test procedures, flexible composites and automotive finishes were selected which were representative of current production materials and lab procedures. The bumper impact test conducted by the ACG Group of General Motors (Inland Fisher Guide Division) is a representative slice of field performance.