Dear Joe,
I have a customer who wants a white powder coating - very white - and all the powder coatings I have applied are yellowish. Is there a solution?
--Aissa

Dear Aissa,
This is a common problem. While “very white” is a relative term, I think I can still provide some guidelines to allow you to provide the whitest finish for your situation.

First of all, the chemistry of the powder coating affects whiteness. For example, hybrid and especially epoxy-type powders have a tendency to yellow upon baking. Powder types less prone to yellowing include polyesters, polyurethanes and acrylics. Formulating technique also affects the yellowing of these chemistries. The choice of pigment, crosslinker and additives can significantly influence yellowing resistance. So be sure to evaluate more than one version of these types of powders.

Overbaking (at a high temperature and/or extended time) exacerbates this problem. Furthermore, the quality of your oven affects whiteness. Gas ovens cause more yellowing than electric or infrared types. Incorrectly adjusted gas/air mixtures also will cause more yellowing. It is imperative to keep your oven burner adjusted properly and to avoid overbake conditions.

If your customer requires a highly reflective white powder for an application that involves lighting fixtures, then a specifically formulated powder is required. These products will incorporate special grades of titanium dioxide (white pigment) and, quite possibly, an optical brightener and antioxidant.


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Hello Joe,
Is there a system or formula I can use to estimate powder coating service charges if I know the surface area of the part in square inches and the number of pieces to be produced in an eight-hour shift?
--Unni

Dear Unni,
I think that you are trying to determine the cost of powder per surface area covered. There is a simple conversion to calculate how many square feet a pound of powder will cover at a given film thickness. Take 192 and divide by the specific gravity of the powder. This will give you the theoretical number of square feet a pound of powder will cover at 1.0 mil thickness and 100% efficiency. Powders typically are applied at approximately 2.0 mils, and the efficiency can range from 50 to 95%.

For example: 1.0 lb of a powder with a 1.6 specific gravity theoretically will cover 60 ft2 at 2.0 mils. (192 divided by 1.6, then divided by 2.0 mils). Estimating an efficiency of 65%, you could expect to cover around 39 ft2/lb.

If you are paying $3/lb for this powder, then your material cost per square foot would be approximately $0.077. Of course this doesn’t take into account any of your overhead costs such as labor, energy, building, utilities, equipment depreciation or ancillary supplies.


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Dear Joe,
We are using tap water in our pre-cleaning and rinse bath. I have heard that we should be using DI water. What is your take?
--Craig

Hi Craig,
If you want any kind of corrosion resistance you should switch your final rinse in your precleaning system from tap to DI (deionized) water. Tap water can contain varying amounts of impurities, such as water soluble salts, that act as propagation points for corrosion.

The rinse after your alkali cleaning stage can be a tap water rinse as long as you keep it clean. This is usually accomplished by overflowing it to keep the concentration of drag-in from the previous process low.

DI water can be obtained through a reverse osmosis or ion exchange process.


Ask Joe Powder is a regular feature of Finishing Today magazine. Please send your questions to askjoepowder@yahoo.com.