Finishing pretreatment: Green from start to finish

The best place to start is with the pretreatment system, a critical process in achieving higher quality and more greener finishes.

The Greener Cleaner

Alkaline cleaners, as we all know, are usually the first step in the pretreatment process. They are designed to remove the grease and oils that prevent other processing solutions from making good contact with the surface of the part you are finishing. Without a clean surface and good contact, the phosphating solution can’t do its job and you’ll end up with poor quality parts. The “right” cleaner for your shop is determined by the “clean” state in which the parts you are processing arrive. So, the “greener” cleaner is more about how you maintain it as opposed to metal treatment where the product selection (phosphate vs. non-phosphate) makes the greener choice.

Are you operating your present alkaline cleaner as green as possible? Start with controlling the concentration of the cleaner. Smaller more frequent additions control the results of the alkaline cleaner much better than large additions. Remember, you don’t want to exceed the upper concentration limit of the supplier’s data sheet recommendations.

If you do, two things can occur. First, you drag-out much more cleaner chemistry (losing money) with each part processed and second, you’re in danger of oiling/splitting out the surfactants. If the pH gets too high, the surfactants become less soluble and become water insoluble. You can’t remove oils or greases from the parts when the surfactants have split out of the aqueous solution. In fact, the surfactants can cling to the surface of your parts compounding the cleaning problems.

Using RO or DI water for the make-up and additions improves the quality of the results of the alkaline cleaner over the life of the cleaner. Proprietary cleaner chemistries have chemicals in them that hold complex calcium and magnesium and prevent the formation of small particles. These precipitates can stick to the parts causing various degrees of pitting, sand-like roughness or dull deposits. This happens most often when your water is very hard or with the hottest alkaline cleaners because they need the most water replenishment.

Are there greener cleaners? Alkaline cleaner chemistries can best be described using the “blender test.” Put alkaline cleaner containing oil into a blender and hit the on button for 30 seconds and stop. As the cleaner solution comes to a stop time how long it takes for the oil to separate from the solution. At five seconds, this would be a very fast fully splitting/displacing type of alkaline cleaner. Did it take five minutes? If so, it’s a cleaner that will not split-out in our cleaner tank while the tank is being agitated but will allow the oil to separate out in a still side tank. If it is a fully emulsified cleaner, the oil will never come out on its own. With normal usage, these emulsifying cleaners will never have “free oil” in them.

Splitting or displacing cleaners don’t need help to bring the oil to the surface of a still cooling tank were the oil can be mechanically removed.

But we can split-out the oil from a fully emulsifying cleaner by using heavier non-water miscible surfactants. When introduced into a side tank of soak cleaner with vigorous agitation and sufficient heat, the reaction begins. This works by forming a more stable micelle with the oil, thereby “stripping” the oil away from the water miscible surfactants that are in the alkaline cleaner formulation. Stopping the mixing allows the chemical splitter we added to float to the surface of the treatment tank with the oil attached, where the oily split can be skimmed. The greener cleaner is simply the one that works best for your unique dirt and soil/oil loading.

Filtration

Adding filtration technology that has a relatively short payback is even greener. Filtering the solution complements oil separation by reducing the particles in the alkaline cleaner solution.

Here are of the filter options available to today’s finisher. There is the spiral wound polypropylene centre filter or the extruded polypropylene filter, both commonly used in the finishing industry. You can remove particles from 100 to 5 microns based on requirements. Simple but effective, this system retains particles on the media pattern. It also absorbs some oil from the alkaline cleaner solution. This type of system is often used before other filtering systems (oil absorbing filters) to protect them from clogging.

Oil absorbing materials in large canisters are used with fully emulsifying cleaners to remove oils and greases while the alkaline cleaner is in use. The cleaner is pumped through a filter to capture particles, that would clog the media in the canister, and then pumped through the canister to remove the oils from the alkaline cleaner solution.

Metal Pretreatments

Iron phosphate and zinc phosphate are the traditional metal pretreatment products but they are not very environmentally friendly. Enter the extremely low phosphate and the phosphate-free metal pretreatments. These conversion coatings are quickly becoming the new industry standard.

When the green finishing movement emerged, most of the attention was focused on removal of hexavalent chromium from all phases of metal finishing including seals. Now attention has turned toward replacing the zinc and iron phosphating process with non-phosphate technologies. It’s clear that no single chemistry has emerged a clear winner. Most of the chemistry falls into one of two camps, zirconium-based and silane-based chemistries.

Zirconium based

These are new pretreatments using the “transition metals,” mostly zirconium based chemistry. This nanotechnology based pretreatment uses zirconium and other chemicals to produce a very thin amorphous coating about 15 – 80 nm thick. The surface of this conversion coating has a much finer structure than zinc or iron phosphates. This yields greater surface area which increases the paint bond strength and reduces the surface defects.

Silane based

The other emerging pretreatment technology is based on silane chemistries. These start with a silicon atom and are combined with an organic molecule to form an organofunctional silane. One end of the molecule has a group that forms a stable bond with metals and at the other end is a group that forms a stable bond with the organic components in the paint.

Ian McLeod, vice president of Vanchem Performance Chemical, says “these proprietary water soluble silanes have been commercially available since the 1970’s, but in the 1990’s, they were recognized as an environmentally friendly substitute for highly toxic chromate sealers over iron and zinc phosphate coatings, and more recently as replacements for iron and zinc phosphate coatings.”

He adds that “one of the newest and promising forms of pretreatment alternatives for both cold rolled steel and aluminum is an organofunctional silane and nanoparticle hybrid system.”

McLeod gave a presentation on the topic at the 2009 Canadian Chemistry Conference and Exhibition in Hamilton, ON. In that presentation, A New Metal Finishing Pretreatment: Waterborne Silane-Based Conversion Coating, he describes non-phosphate and non-chromate technologies and details performance testing of the technologies. You can read his paper at http://www.vanchem.com/92nd-canadian-chemistry-conference-and-exhibition-canadian-society-of-chemistry.

Both of these new pretreatment processes generate benefits over zinc and iron phosphating: Fewer tanks mean shorter lines. They offer better performance with lower coating weights. Lower coating weights operate at lower temperatures (a few degrees above room temperature) and lead to faster processing speeds. They do a great job on all common substrates, such as steel, zinc, and aluminum. With lower concentrations and little to no sludge, these lower weight coatings reduce their environmental impact. And corrosion performance meets or exceeds pretreatment specifications for painted metal substrates.

These non-phosphate chemistries are being used around the world. But a note of warning: just as there can be a wide range of differences in corrosion protection between a simple three stage iron phosphate process and an eleven stage zinc phosphate process, it’s the same with the commercial non-phosphate products. Ensure your cleaning, pretreatment and paint materials and processes are compatible and meet your finishing specifications. CM

Fred Mueller is a finishing expert and corporate quality and safety manager with General Magnaplate Corp., Linden, NJ.