Metal finishing Q&As since 1989

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White dirt in mica based solvent borne base coats

August 1, 2008

We are facing problem of White dirt in mica based solvent borne base coats.

Our base coats are applied with electrostatic Robotic bell aplicator.

Application solids :-16-18 %
Turbine speed:-40000 to 55000 rpm
Operating voltage:-60-80KV
Flow rate variation:-60ml/min-500ml/min
Shaping air variation:-40NL-500NL.
Velocity of paint in circulation:-0.3-0.4m/sec.

We have done the cross section analysis of the defective components we found that it is cluster of Titanium dioixide also as we face this problem only in Mica based base coat.

hence we doubt on the mica based base coat.

We really don't know how the clusters are getting formed in our painting process

We would like to have some light on the formation of clusters in the system.

regards,

Abdul qureshi
paint Industry - India

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August 2, 2008

I hesitate when answering this type of question because you have to think that the possible cause of the defect is the material. It sounds like the manufacturer is using mica based upon a patented process of depositing Titanium Dioxide onto mica. Therefore is the defect due to:
a) Hard settling of the mica constituent and upon remixing at you plant, un-dispersed mica particles result. Answer, ask mf to add rheology agent.
b) Is the velocity of the paint mix too high, causing impact to take place ò use bigger diameter?
c) Poor mixing at the manufacturer? Discuss your problem and evaluation with mf.
d) The mica being supplied to the manufacturer does not possess or up to the quality required by this type of coating? Change mica supplier?
To test the quality or evaluate if the plant is the cause -flow the ready to use paint onto a glass plate and view the dried out finish. Don't forget to turn the glass over to prevent contamination from air-borne particulates.
Please let me know if this has helped.

Terry Hickling
Birmingham, United Kingdom