from the print-out. 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:

grade 1 - barely visible
grade 2 - complete circle visible, but not filled in
grade 3 - totally visible and filled circle
grade 4 - blistering evident
grade 5 - removal of film to bare panel

• 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.

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

Acid etch, or environmental etch, is a defect that can occur in any exterior paint finish. 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 coatings are still in use because they are more cost effective and have better mar resistance.

The automotive industry has primarily studied and tried to eliminate the paint defect caused by acid etch. An accelerated acid etch test using the BYK-Gardner gradient-oven facilitates evaluation of different coating systems.

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. 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

• 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.
  
• 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.
  
•  Label the panels with the supplied paper strips which are numbered from 1 - 45 corresponding to the 45 gradient temperature zones. 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.
  
• At the end of the 30 minutes, remove the panel and obtain the print-out 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.
  
• 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