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Bend Test Failure

Could pretreatment or tri-cat phosphate be causing e-coat failure in a mandrel bend test?

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Q. We have a problem with our e-coat failing our mandrel bend test. We use a cathodic e-coat and a 13-stage pretreatment with conditioner and zinc phosphate, alkaline cleaner, heated stages 1 and 2, oil skimmer, etc. A cleaned down panel with e-coat only has passed the test. Does this mean the pretreatment is causing the failure? Could it be the tri-cat phosphate?—H.T.

A. It definitely could be the zinc phosphate.

The mandrel bend is a very interesting physical test for multilayer systems, because the bend radius of the external layer is greater than that of the internal layers. Because of the differential pulling forces between the multiple layers, they generate great stress and thus have a tendency to produce disbondment between the layers.

Typically, the more finishing layers, the more difficult it is to pass the mandrel test. In your example, the e-coat-only panel passed, and the phosphate plus e-coat failed (one layer versus two layers). For multilayer systems such as galvanized, zinc phosphate, electrocoat and topcoat (four layers), the mandrel bend test is very difficult to pass unless flexible coats are used.

In my experience with the mandrel bend test on zinc phosphate and electrocoat, the most important contributing factors are:

1. Maintain low phosphate weight and small crystal size. The lower the weight and crystal size, the better, while still maintaining the minimum requirements for corrosion performance. Iron phosphates perform better on mandrel bending than zinc phosphates because of their lighter coating weights.

2. Maintain low cure conditions for the electrocoat, especially temperature, although time also impacts flexibility. The higher the cure temperature, the less film flexibility. Un-flexible films typically perform poorly in mandrel testing as the films get harder and more brittle.

3. Maintain low electrocoat film thickness. The lower the film thickness, the better, while still maintaining minimum requirements. Thicker films increase the differential bend radius between layers, thus increasing the chances for mandrel bending failures. 

4. Maintain or use electrocoats with low p/b ratios of more resin and less paste. The resin is more flexible than the pigments, therefore resin-rich systems perform better in mandrel bending.

Keep in mind that galvanized steel, because of the additional zinc layer between the metal and the electrocoat, tends to perform worse than steel when everything else remains the same. Again, the more layers and the greater their thickness, the worse the performance. Other factors such as type of resin (epoxy or acrylic) and type of the electrocoat may also contribute to poor mandrel bending. 

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