The TiO2 slurry for delustering polyamide, turned yellow after wet milling, the longer the milling time, more yellow of the color. We have tried a different mill, but the discoloration also exist.
Yellowing of the white mineral powder is a sign of thermal decomposition of organic matter. If you use tensides when milling or if milling takes place in a polymer container, then the yellowing of the powder can be easily explained by the gradual thermal decomposition of organic matter. Of course, prolonged grinding in an organic liquid can also lead to yellowing.
Vadim Verlotski Thank you very much for your answer. Sorry that I have decribed the question clear. We use only water to form a TiO2/water dispersion, and then milled that dispersion with zirconium beads. The beads should not contain any impurities that cause discoloration. So I am wondering that maybe the TiO2 had some kind of change during the milling. The TiO2 used for delustering polyamide ontains some surface treatments. But according the TiO2 supplier, they have not encountered this issue before. So the reason is not clear so far.
The titanium oxide powder itself may contain a minimal amount of organic matter. Try heating it in the oven to 200°C before milling. If the powder turns yellow, then the problem is in these impurities.
Actually it's the other way around: the finer the TiO2 powder, the whiter it is. This applies to all colorless powders.
The black powders become blacker as the particle size decreases.
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