Letter 23047

How to reduce paint thickness in cathodic Electrodeposition system?

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I'm working on a cathodic electrodeposition (CED) project. Currently I'm getting paint thickness in the range of 25-32 microns.I want to reduce this thickness up to 18-20 microns. CED bath parameters :: pH 5.90-6.10 Conductivity 960 micro Siemens. Temp 28-30 degree Celsius NVM(% solids) 9.43 % P/B (pigment binder ratio) 0.10 Voltage 160 V Anolyte conductivity 2500 micro Siemens I have three ways to reduce the thickness .

a. By reducing the bath conductivity

b. By reducing the anolyte conductivity.

c. By reducing the NVM.

d By reducing the voltage/current.

e. By reducing the dipping time in CED bath.

I cant reduce the dipping time in bath as i have some restriction. I cant reduce the anolyte conductivity as on reducing this fungus/bacterial growth is taking place. I cant reduce the NVM as it is already on lower side (standard is 12-14 %). My bath conductivity is already on lower side but still thickness is not coming low. I'm operating at 140 V though my standard is of 250-300 V. Please suggest me how can i reduce the thickness.

Manish C [name deleted for privacy]
- New Delhi, New Delhi, India

Manish,

Please check the pH is per your supplier's standard. Please check that the conductivity conforms to prescribed levels per supplier data sheet. Are you running an Ultrafilter and replacing the discharged permeate with DI water of 5 micro Siemens conductivity. Are the solids as prescribed.

Since we do not have an idea of the proprietary process you are using, some of these suggestions may validate the process. Hope this helps.

Regards,

Asif Nurie Atlanta Global Limited New Delhi, India

I have used electrophoretic paint in the past and found that the thickness of the cathodic electrodeposit could be controlled by either the temperature or the voltage. Changing the bath chemistry is not a wise way to proceed because you are altering an established system and unless you know exactly what you are doing, you will cause a lot of future problems. The deposition thickness is related to the bath temperature, so by reducing this and keeping all other properties constant you will reduce the thickness. The systems I have worked on are quiet sensitive to temperature, so only modest changes will alter the thickness. Alternatively, reduce the voltage; this will also reduce the thickness. I do not know what you mean by operating at 140 V although the "standard is 250-300 V", so perhaps you can elucidate. Finally, if these two changes do not reduce the thickness to your needs, you have to reduce the processing time. Assuming your system is similar to the one I have used, there is no point in reducing the current because this automatically reduces to a very low value as the deposit thickens.

Trevor Crichton R&D practical scientist - UK

Dear Asif ,

Thx for ur valuable suggestions. I m running the ultrafiltration unit and also using DI water of < 5 microsiemens conductivity. pH is in the supplier's range(5.10-6.10).but still I'm not able to reduce the thickness. I'm working with the latest CED technology . Our system is a latest one. We have four ways to reduce the thickness in CED system and which I have described in my previous query and I have tried all except reduction in dipping time. I can't go for this option. I hope that you will further give ur valuable suggestions to me.

Thanking you.

Dear Trevor,

Thx for ur valuable suggestions. Yes I am using the same system what u r using there. I am using the latest CED system which is used everywhere in this world. So assume that its the same as u have. Yes its true that standard voltage requirement is 230-250 volts but I'm operating at 160 V bcoz I thought it will help me in reducing the thickness. It's also true that my trial can damage our system and can create future problems but I love to do trial bcoz without trial we can not come to a conclusion.

As far as temperature is concerned our standard is 28-30 Degree Celsius and I have reduced it to 27 from 30 but still the thickness is coming high and I am surprised that at lower voltage(140 V),low NVM(9.41 %) and low conductivity(960 ms) , I am getting high thickness(26-32 microns). Let me reduce the temperature to 25 Deg C and then see what happens. Thx again for ur suggestion.

Regards,

Manish C
- New Delhi, India

If you are unable to control the thickness, the problem may lie elsewhere. You need to check if the voltage displayed is the same as the voltage applied on the component.

Gurvin Singh Coatec India - Mohali, Punjab, India

Dear friend,

In my opinion may the conductivity of ED bath is too low. Because I have the opposite problems from you. Since I want to raise the thickness (now is about 20 micron) but the only difference parameter was conductivity of ED Bath whereas too high in mine.

The data parameter from my process are :

1.NV 14.67%
2.Coating voltage is 140 V (range 80 to 160 V)
3.pH is 6.25 (range from 5.9 to 6.3)
4.Conductivity is about 1545 micro Siemens/cm. (standard is 1100 to 1400).
5.Ratio F1 to F2 is 1:10

I also used the latest technology of ED Paint, which is Acrylic CED-Lead Free

Henry S
- Jakarta, Indonesia

Hello sirs I humbly feel that it's really a great time to interact with you wizards. in our CED process , we r encountering a major paint problem poor E'coat deposition(dipping process) defect in the sill outer panel of the car body because of " rust " in the non-coverage area of coating . this is not occurring in the sill outer panels we use which are provided with wax holes.

I'm really baffled whether the problem is with the process parameter settings or the mechanical alignment or settings .

the process parameters settings we use in our CED process are :

%NVM = 18.7
pH = 5.94
CONDUCTIVITY = 1827 uS
P/B RATIO = 0.16-0.25
BATH TEMPERATURE = 30.7
APPLIED VOLTAGE = INLET POINT = 0 V
OUTLET VOLTAGE = 100 V
MAIN BUS = 235 V
CURRENT = INLET = 0/350 amps
MAIN = 950 amps

when we change the inlet voltage , paint defect" hash marks" start appearing in the car bodies . we r planning to increase the %NVM or the inlet voltage , WILL IT SOLVE THE PROBLEM !!!!!!
generously give me ur valuable suggestions .
looking forward with great regards ,

C Nanda K
- Chennai, India

Dear Mr.Manish,

I understand your problem.

You want to control the outer DFT without disturbing any line condition.
In this case you can cover the anode cells simply with the PVC pipe of good quality (Quality in the sense of Good Chemical Resistance).
After doing this definitely you will get reduction of 6 to 8 µ DFT.
Your throw power will not affect after doing this (Kindly check up with the paint supplier).

regards,

Amar Koppa
paints - Pune

Dear Manish,

One other way of reducing the DFT of your EDP is in choosing mineral fillers with a very fine particle size, good dispersion properties and good flow properties. It often happens that due to cost reasons, producers change the mineral filler included in the original formulation to a cheaper grade locally available. In this case, many properties of the EDP - including the DFT - are getting worse.
Especially the dispersion behavior of the fillers affect the DFT very much. If the filler is not dispersed properly, you will find agglomerates that are actually much larger than the original particle size of the filler. Also the flow properties are affected negatively by a bad dispersion.
There are only very few mineral fillers that are working properly in EDP systems, one I know is called Sillitin.

Best Regards
Christian

Christian Plein
- Neuburg, Germany

Dear Manish,
I feel sorry to say that the CED you are using have very low rupture limit.Generally rupture limit of CED is more the equal to 350 volt. No body can help you just change the paint supplier.

Alam Farrukh
- Lahore Pakistan

December 10, 2008 Dear Please check the applied voltage which is shown by the rectifier & increase the bath conductivity ,reducing the bath temp. thanks, Kuldeep Yadav - Gurgao, India March 9, 2009 Dear Manish Please check the Solvent level of the bath and co relate with the production trend . U may feeding the bath in the higher rate , that means your bath will be fresh and that's why u are getting high DFT. This is the same problem i observed in my line . I reduced Solvent level.Drained permeate at regular frequency to Keep the DFT and Finish in good condition. Thanks Srinivasan T. Srinivasan - Chennai April 9, 2009 u did not wrote abt no. of anode cells and type of anode cells. how ever reduce the number of anodes. or cover the membrane from bottom side. DINESH MAURYA - jamshedpur, India