Posted by CGarbee on Tuesday, May 03, 2005 at 9:01AM :
In Reply to: Anybody up on their paints? posted by DaveL(inPA) on Tuesday, May 03, 2005 at 1:32AM :
The system used to test paint hardness is based upon the Mohr scale and not the Rockwell scale. Hardness has been defined as the ability of substances to abrade or indent one another and the original Mohr scale of hardness was based on a list of 10 minerals (ranging from talc to diamond) arranged in an order based on their ability to scratch one another. Talc was given number one with diamond being number ten (on a ten point scale).
With the Mohr scale, you scratch the item to be rated with various items of known hardness in order to compare your item to the standard. Ie, if a diamond scratches it, but nothing else does, then you say that it has a harness of ten, equal to the diamond. If steel scratches it, but aluminum does not, then the hardness lies in between aluminum's number and that of steel...
For paints, a "scratch" test using a pencil of known hardness is used to determine how hard the paint is. The test method is described in ASTM D3363.
Keep in mind that the hardness rating for a coating is just a small portion of the determining factors for how well it works... You also need to look at UV resistance, adhesion to the substrate, porosity, and a host of other things... Also, the pencil test is more of an indicator in some folks minds as there are more "rigid" test methods available...
The folks at ecology coating appear to be working on improving the packing of paint solids by using different particle gradations (big and small solids) in order to improve the density of the final coating.
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