Testing Coefficient of Friction of Non-Skid Coatings

Meghan asks:

I work at a facility that produces both lithographic and flexographic printed boxes with aqueous non-skid coating applied. We have the ability to test the slide angle with an incline plane device or the COF with a horizontal plane device. We are experiencing discrepancies between the two methods. I thought either method was interchangeable and that the results should correlate with each other. Do you have any insight on this or any opinion about which method is better or more reliable? Thank you very much!

On both testing machines, we test with the fiber direction across the machine direction. For the incline plane, a 2 pound sled is used. And for the horizontal plane, a 3 pound block is used.

While TAPPI Test Methods T-815 and T-816 both mention in the introduction that the two methods should be similar for uncoated surfaces, they do have the caveat that treated surfaces will likely provide different result depending on the method used.

Now, if we go back Mr. Lynam’s physics class, (please don’t cringe) you are really performing two different tests, right. As you said in your question, you’re testing for slide angle… and… Coefficient of Friction (COF).

The conventional way to test the COF is on a flat plane. This measures static or kinetic friction, or how much energy is required to move the sheet with a side force (as pushing or pulling sheets across the floor or each other). Once you’re on an incline, you’re introducing vector forces that will have and affect the outcome of your COF test.

When you use the inclined machine, you are measuring, as you stated, slide angle friction. This is how far a sheet can be tipped/tilted before gravity overcomes friction and the sheet begins to slide down the incline.

-Ralph

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