Certain additives (and pigments) may contain particles that can block screens used in production equipment. Especially within PP fibre production very fine screen packs are used to prevent blocking the hundreds of dies in a spinning machine.
Calcium stearate is the key acid scavenger used within polyolefins. In some grades the size and content of by-products such as calcium hydroxide and calcium carbonate may lead to blocking the screen pack and extra work for the fibre producer.
Norner offers a test where 2.0% calcium stearate is mixed with 0.1% antioxidant and 97.9% PP powder. The PP powder is a very pure PP specially selected for filter test.
The normal test is using a screen pack with 1400 mesh (ca. 10-11 microns) as the finest screen. The filter test result is a measure of the pressure difference divided by kg of PP tested.
Some customers prefer a tougher test with 5 microns as the finest screen. Such test is also offered by Norner.