ELECTRODEPOSITION COATINGS history- why Electro-deposition- types of Electro-deposition- comparative study of CED and AED
Electro-priming of motor vehicle began in the late 1960s & within few years almost all mass-produced vehicles in industrialised countries were primed in this manner.
The main reason for the universal and rapid acceptance of this process despite its higher initial installation, running & material costs were:
- Uniform film thickness in the range of 15-35 ยต
- Greater operating flexibility
- Superior mechanical & corrosion performance properties
- Greatly reduced solvent emission & hazard
- Highly efficient with upto 95%of the coating material being utilised.
Until mid 1970s all the electro primers used by the automotive industry were of the anodic type & resin systems included maleinised oils, polyesters, epoxy ester, or poly-butadiene polymers combined with specific anticorrosive pigments. Now company's also do the cathodic version of the electro-priming.
WHY ELECTRO-DEPOSITION ?
- Protection of automobile body shell from corrosion.
- Automation and faster production rate.
- Uniform paint film.
- High degree of paint utilization.
- Better finish and appearance.
- Less labour intensive.
- Free from fire hazards and environment friendly.
Main Purpose: Corrosion Protection
Rough Comparison of Salt spray corrosion protection with other coatings
1. Edge protection, due to proper coverage of CED.
2. Superior Throwing power due to high film resistance
3. Less consumes of primer of external less thick 10 micron dry film thickness (dft)
in box section areas.
4 Low RA value film, smooth coating, imparts better DOI Wet look to composit sys.
5. Less L effect same dft on vertical & horizontal surfaces
Typical electro-deposition operating parameters
- Superior Corrosion Resistance even at lower DFT.
- Higher Throwing Power i.e., more coating in box sections.
- Better wet adhesion.
- Better mechanical & Chemical properties.
- Less baking temperature & time than AED.
Flow Chart of CED Process
Raw materials of CED BathPigment: TiO2, Carbon black Extenders: Bismuth hydroxide, China Clay, Baryates.
Binder: Epoxy resin
Neutralizing agent: Acetic acid, citric acid, Formic acid.
Cross linkers: Isocynate cross linkers 1) Diphenylmethane diisocynate (MDI) 2) Isophorone diisocynate (IPDI) 3) Tetrametyl xylene diisocynate (TMXDI) 4) Hexamethylene diisocynate (HMDI)
Throwing Power of ED Primer
It is power of ED primer to reach to nooks & corners of the object to be coated.
It is measured by various methods as follows :
1.Tube method:
- Mixture of ED Primer is taken in a glass beaker, 10mm dia MS tube, 300mm long, is dipped in ED Primer fully, but 1inch, Top end is kept out in air.
- One MS strip is of same length in inserted in the tube.
- It is connected to one electrode of Rectifire.
- Another strip is immersed in the same beaker as a another electrode.
- Recommended DC voltage is applied for the process time (say 6min), after that the strip inside the tube, is taken out, rinsed and baked at scheduled temp. Say180DC/10 min.
- ED DFT on the strip is measured & plotted Vs distance from the Lower end, where it is usually 25 micron, distance up to which 'ED DFT on the strip' is 6microns, is the Throwing Power of ED Primer
2. Box method
3. Practical method
References
1. Fettis, Gorden. Automotive Paints And Coatings. United Kingdom : VCH Publishers, 1994.
2. D¨ossel, Hans-Joachim Streitberger and Karl-Friedrich. Automotive Paints and Coatings. s.l. : WILEY VCH Verlag GmbH & Co.KGaA, Weinheim, 2008.
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