Low-Temperature Pretreatment


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Q. With the increasing prices for energy we want to reduce our manufacturing costs by reducing our zinc phosphate pretreatment bath temperatures.

What kind of trouble can we expect if we reduce the temperatures of our pretreatment baths? I heard lowering the temperature of the zinc phosphate bath will cause a great amount of sludge. D.N.


A. Gone are the days when pretreatment chemicals ran at three-digit temperatures. Most of the energy is consumed in pretreatment processes by heating the water in aqueous chemical baths. You didn’t say how hot your chemical baths were, but some of the newer pretreatment chemicals are formulated to run at 80–85°F. I used to say that in order to remove oily soils, you had to move them. To move them, you had to reduce their viscosity by using aqueous alkaline cleaners at three-digit temperatures. Properly formulated and controlled cleaner solution baths containing the right surfactants, will remove most oily soils at two digit temperatures.

It’s been said that low-temp zinc phosphate processes will produce more sludge than hot processes. This myth arose from attempts to run chemicals formulated for high-temperature use at low temperatures. The newer low-temperature zinc phosphate process chemicals are formulated to produce low sludge. Furthermore, since there is little scaling of the heating coils, tank maintenance is greatly reduced. This is because the heating coils do not have to get very hot using the low-temperature chemicals.

The problems you may encounter are: 1) Difficulty in removing heavy oily soils because of being unable to reduce their viscosity at the lower temperature and shorter time cycle (dwell time) on a high-speed line, and 2) The shorter dwell time on a high-speed line may not produce a high enough coating weight to meet your specifications. 

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