An Accelerated Test Procedure for the Evaluation of Acid-Etch Resistance of Coatings

Acid etch, or environmental etch, is a defect that can occur in any exterior paint finish.1-9 However, it is a major concern for the automotive industry. The result of acid etch may appear as rings on a car's finish. These rings may seem like water spots, but in actuality, damage has been done to the finish, so washing does not remove the rings. Certain geographical areas are affected more than others - areas adjacent to heavily industrialized cities usually have high incidents of acid etch complaints.

Historically, isocyanate-crosslinked coatings outperformed melamine-crosslinked coatings for acid-etch resistance. However, melamine-crosslinked coatings10 are still in use because they are more cost effective and have better mar resistance.11, 12

This article details an accelerated acid-etch test. A similar test has been mentioned by Gregorovich and Hazan.1 They compared the acid-etch test to actual Jacksonville test data. It has been the automotive industry that has primarily studied and tried to eliminate this paint defect.

This relatively simple test yields a pair of values, the minimum spot temperature (MST) and the scaled acid value. These values are used to rank the severity of the acid damage to a coating. The test procedure uses a BYK-Gardner gradient oven. In addition, steel panels (4" x 22.5" x 0.032") are required (these are the standard size panels used in the gradient oven). To make the comparison of coatings in this brief study easier, one acrylic polyol and either an isocyanate resin or a melamine resin were used as the crosslinker.

Procedure

1. Prepare a panel for testing by coating a film on a 4" x 22.5" x 0.032" steel (polished) panel using the formulation and cure parameters for the specific material. Sometimes a post cure time period is recommended before the acid-etch test is run.

2. Set the gradient oven for a 30-minute time cycle using a temperature range of 35-75?C as a continuous gradient. The oven has 45 individual temperature zones.

3. Gradient oven supplies include pre-pasted paper strips with evenly spaced measuring points numbered from one through 45 over a 20" length. These correspond to the 1-45 gradient temperature zones. Affix the strip along the length of the panel near an edge. Directly opposite each number on the strip, use a pipette to dispense approximately two drops (0.4 grams) of a 10% sulfuric acid solution (or other solution desired). Move the panel with the drops gently onto the preheated gradient surface for the 30 minutes.

4. At the end of the 30 minutes, remove the panel and obtain the printout of the actual temperature of each measuring point. Rinse off any remaining etching material with tap water. Using a facial tissue, wipe off any excess water. Spray the panel with "Super-Clear lens cleaner" from AOSafety Products and wipe it off using a clean tissue.

5. Examine the panel under a good light source. Tilt the panel back and forth and note the temperature corresponding to the first visually etched spot, no matter how faint. Record the temperature for that point from the printout. This value is the minimum spot temperature (MST). To make the faintest rings visible, exhale, as if cleaning glasses, over the rings where "grade 1" damage starts. (This spot may disappear from visual sight in a short period of time.) Grade each using a scale of 1 to 5, with 5 being the worst:

6. Sum all the grades. This value is the total acid effect. A maximum of 225 is possible if all spots etch through to the bare metal. To express acid resistance on a scale from 0.0 to 1.0, take this total number and divide it by the maximum (i.e., 97/225), and subtract the result from 1.0. This gives a scaled acid value [1.0-(97/225)]=0.57. A zero rating would be equal to a panel receiving a total acid effect of 225 (total failure) and a 1.0 rating would be equal to a panel not being affected by the acid at all.

It is possible to do multiple tests of several solutions at the same time, noting which row of spots corresponds to each solution. Extra care is needed when the panel is placed into the oven so that the different solutions do not mix together. The figure shows an ordered series of test panels. The most affected panels are on the right.

Results and Discussion

Higginbottom, et al. compared the effect of the type of crosslink functionality, hydroxyl groups vs. carbamate groups, in melamine-containing coatings.2 They used an acrylic polyol and derivatized it to form an acrylic carbamate. The resulting coatings were examined and selected data is presented in Table 1.

The data shows that this type of modification can produce a large enough effect that causes the scaled acid value to increase from 0.56 to 0.69. A minimum spot temperature (MST) of 50+ ?C and a scaled acid value of between 0.65 to 0.85 are common for coatings considered fairly resistant to the acid etch phenomenon.2

In this study, the acrylic polyol used was Macrynal SM 515/70BAC.13 Three variables were examined. The DFT (dry film thickness), the ratio of melamine resin to acrylic resin, and the crosslinker type were examined. The coating formulations and the acid etch results are shown in Table 2.

The acid-etch values found for the 80/20 (acrylic/melamine) ratio coating indicate that this ratio produced a more acid resistant coating than the 75/25 ratio coating. Notice that the 2 mil coating did not significantly improve the acid-etch resistance as compared to the 1 mil coating. This demonstrates that as little as a 1 mil coating can have good acid-etch resistance. When the two different crosslinkers were compared, the melamine-crosslinked coating produced a slightly better acid-etch-resistant coating.

This is not a surprising result. Melamine systems can be made to produce coatings with similar acid-etch resistance as isocyanate systems. The choice of the vehicle binder resin seems to be the key in producing a coating with improved acid etch resistance when using a melamine crosslinker. Acrylic resins modified with alkoxysilane groups1,3 or with carbamate functionality2 are two technologies used to increase acid-etch resistance.

Conclusion

A simple procedure for testing the acid-etch resistance of coatings was presented. As with most accelerated test procedures, this test should only be used as a screening tool. This article also mentioned ways to improve the acid-etch resistance of coatings. Obviously, all the properties of a coating must be considered before a paint system is chosen. Isocyanates were once the preferred crosslinker system used to produce coatings with good environmental etch resistance. Today, when suitable vehicle resins (or auxiliary crosslinkers) are used, a melamine resin is a viable crosslinker candidate for use in acid-etch resistant automotive coatings.

References
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3. Hazan, I. "Low VOC-Super High Solids Clearcoats", Proceedings of the 7th Annual ESD Advanced Coatings Technology Conference, Detroit, Sept. 28-29, 1998.
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13. A styrene-acrylic polyol, Solutia Inc.