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RE: Opacity Problem from Water-Based Coatings

Are you sure that drippage inside the oven is a cause of the opacity problem?  Dip tanks have a transfer efficiency of 95 to 99 %. Most of the paint ends up on the object being dipped.  To generate suspended solids from the dipped part you would need to hit it with a blast of compressed air.
If it is a solids problem, you could look into installing removal baffles inside the stack.  You already noted that a coating forms inside the stack.  The baffles would provide more surface area for collection.
Some other things to look for:
1) Check with the coating supplier and confirm the baking temperature.  If they try to push items through the oven faster by going to a higher temp, they may be degrading the polymer.  This volatile polymer will polymerize and form a haze upon cooling.  It might also explain the non-adherent deposit on the stack.
2) Certain solvents may polymerize and form haze.  Terpenes are known for this behavior.
3) Have you measured the relative humidity inside the oven and exhaust?  Much of the haze might be water vapor.  Possible solutions include reheating the oven exhaust prior to discharge or injecting dilution air into the base of the stack.
Hope this helps.
-----Original Message-----
From: Wayne Pferdehirt [mailto:pferdehi@epd.engr.wisc.edu]
Sent: Friday, September 28, 2001 4:09 PM
To: p2tech@great-lakes.net
Subject: Opacity Problem from Water-Based Coatings

P2 Colleagues:

I'm working with a hanger manufacturer on an air emissions problem and would appreciate any suggestions might have to offer.

They recently received a notice of noncompliance for opacity violations at the stack from their bake oven.  They use a diptank to coat hangers, which they then bake in an oven at about 350F.  Details are provided below.

DNR has indicated opacity readings of as high as 30%.  The stack, which I inspected up close on the roof, has a thick, black deposit that can be peeled from the metal.  Emissions that I saw had a blue-grey tint.

My assessment is that the problem is a combination of suspended solids primarily from drippage in the oven, along with organics.  The primary fronts of attack I have recommended are reducing drippage through a variety of means (listed below) and improving design/operation of the oven.

My questions are:
1.  Sound like anything you've tackled before?  Anything I'm missing?

2.  Do you know of any applications where powder coating is has proven economically feasible on such low-cost, non-durable products as hangers?  This would be a bigger investment than they are anticipating, but might be the better long-term investment.  I suspect but don't have good data to verify, that powder coating would be too expensive for this application.


In operation for 30 years.

Recently applied for renewal of operating permit.  DNR identified opacity as a problem on recent inspection.  Just received notice of noncompliance.  DNR says opacity is as high as 30%.

Have performed no stack testing.

DNR grabbed sample of paint for testing.

Bake oven temp:  350 F

Approx time from racking of hangers to removal:  30 min
Approx time in oven:  10 min.

Bake oven uses indirect heat.  Combustion area is under the floor of the oven.  Fired by natural gas.  Apparent main areas for introducing air to oven is at ends, where hangers enter and exit.  Oven has recirculating fans after burners.

Changed from solvent-based paints to water-based paints approx. 10 years ago.  Says they have made no significant change to operation since, except to reduce HAPs in paint.  Says smoke has always been there, but that DNR just noticed.  Said they had smoke when they used solvent-based also.

Coatings used:
        Wolke Paint Mfg.
LC HAP-Free Bakecote Blue; WC-2248-04  (mixed 5 parts paint to 1 part water)
LC HAP-free Bakecote Clear Gold; WC-2237-01  (mixed 1 part paint to 1 part water)
HAP-free Bakecote Chi-gold; WC-2255-05  (mixed 5 parts paint to 1 part water)
HAP-free Bakecote White; WC-2291-14  (mixed 6 parts paint to 1 part water)

Exhaust stack is coated with blackened hard coating that can be peeled away.  They say they remove this every few months.

They shot-blast racks to remove excess paint once per shift.


Reduce drippage in oven:
       Improve design of rack and hooks to pick up less paint in dip tank and shed paint more readily
               Air knife to blow off excess paint
       Vibrate hangers to shed excess paint
       Decrease line speed to reduce carryout from dip tank and increase drippage and drying before oven
       Increase travel time between drip tank and oven
       Adjust paint mix (viscosity and constituents) to reduce drippage

Improve design and/or operation of oven
       Improve mixing of air within oven; place baffle(s) to eliminate short circuit between fans following burners and the intake for the exhaust duct near the entrance to the oven.
       Reduce velocity / increase turbulence of air flow to reduce suspension of particulate Possibly increase temperature to improve combustion of organics (Laidlaw is concerned that higher temp will cause white hangers to lose gloss)
       Review size of oven exhaust fan; oversized for application?

If above options are inadequate, install stack combustion unit to complete combustion of organics
Consider alternative coating process; e.g., powder coat

Wayne P. Pferdehirt
Director, MEPP, http://epdweb.engr.wisc.edu/mepp/
Co-Director, PNEAC  http://www.pneac.org
Specialist, SHWEC, http://www.uwex.edu/shwec/
tel 608.265.2361
fax 608.262.6250

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