Powder Clinic: Adhesion of Powder to Brass
Adhesion of Powder to Brass
Q. I’ve heard from a few vendors that brass presents challenges with adhesion, whereas steel and aluminum do not. What is the reason for this?
A. Metals have different oxide layers, porosity, hardness/softness, reactivity and other properties. Steel is easy to convert using phosphate or other chemical treatments so that the surface is receptive to bonding with an organic coating. Aluminum has a tight oxide layer, but it can also be chemically altered to promote adhesion. Galvanized and most other metals can be cleaned or abraded for good adhesion. Brass is a soft, porous metal that cannot be phosphated or converted, and bonding depends a lot on the cleaning. Polishing or mild abrasion can help with adhesion, but chemical bonding is often not reliable. Brass parts can be coated with powder, but some caution is advised on development of a reliable preparation method.
Powder Coating Issues on Aluminum
Q. I am the process engineer at a company that makes surgical tools. We currently use powder coating in our day-to-day operation. One of the projects I am working on is trying to get powder coating to adhere to aluminum. If we introduce heat above 550°F it makes the aluminum soft, and this becomes an issue. We have tried different heat settings with different times. Is there a powder coat material out there that works on aluminum with low heat?
A. First, adhesion—preparation of the aluminum is the key to adhesion. You need to use a chrome or non-chrome product to alter the normal oxide layer at the surface. There are a number of chemical products for this purpose. You can find a good list of suppliers at PFonline.com/suppliers.
Second, cure temperature—why are you working at such a high temperature? 550°F is an extremely high temperature, and is too hot for aluminum. It is not necessary with any standard organic powder material. A standard powder will cure in the range of 350°F to 400°F if given enough time. Maybe you are short on time in the oven; you may need to add to the oven to reduce that temperature. If you need to go lower than 350° for a tempered aluminum, then you can get down to around 250°F for epoxy or 325°F for a polyester. Again, contact suppliers for a lower temperature product and do some testing to determine the best fit.
To summarize, use a conversion coating to treat the aluminum for adhesion and use a lower temperature cure powder and lower cure temperature.
Q. We coat very large parts with automatic and manual equipment. We get a lot of impact fusion and the transfer efficiency is not as good as we think it should be. These problems seem to be related to the powder particle size, as we have a lot more trouble when reclaimed powder is mixed in with fresh. Particle analysis of our reclaim mixture shows a higher number of smaller particles so we think this is the problem. Are there “normal” levels of finer particles that we should expect in the fresh and reclaim materials? How can we fix this?
A. Yes, there are average fine particle percentages you can expect to see in the fresh powder and the reclaim blend.
When powder coatings are manufactured, dry-blended materials are melted in an extruder. The extruder uses heat and pressure to combine the dry ingredients into a homogenous mixture that will exit the extruder in a semi-liquid condition with the appearance of taffy. The mixture is cooled and flattened onto a belt. A chipper is used to crack the cooled sheet of powder into small pieces. These chips of powder are then put into a mill to be ground into dust. The blades in the mill and rotational speed control the average grind size. When the powder exits the mill, the particles above a certain target size are caught in a screen and returned to the mill for further reduction. The particles below a certain size are removed using a cyclone separator. The particles that fit into the intended range of size are captured, boxed and shipped. The process is good, but not infallible. Some smaller particles are able to pass the cyclone and get into the box. The percentage of particles under 20 microns could be as low as seven percent or as high as 13 percent. The percentage under 10 microns is usually around five to seven, and the average particle size is usually between 30 and 60 microns.
As powder is used in the spray operation, the particles can break down. Smaller particles with low mass are less likely to apply well. The low mass makes these particles more susceptible to reclaim airflow and the lower overall charge in them makes them less likely to be attracted to the grounded part. In a cartridge module reclaim system, these finer particles will begin to accumulate and drive down the average particle size in the module and feed hopper. As the fines accumulate, the application efficiency will decline.
The most important thing to understand is that you must try to consume reclaimed powder at least as fast as you create it. That means that your overall transfer efficiency (TE) should be a minimum of 50 percent at all times. Why 50 percent minimum? Because if half of the powder goes onto the parts and half goes to reclaim at all times you can avoid accumulating powder in the reclaim system. What will happen if the TE drops below 50 percent? You will be putting more powder into reclaim than you can consume, and you will accumulate a high number of fines.
The concentration of particles below 20 microns in your powder blend can reach 20 percent and still work fairly well. Above 20 percent, efficiency begins to decline until the powder is too fine to use. The usual reaction is to increase the powder flow rate to get coverage, thereby compounding the problem by further reducing the particle size and reducing the percentage that applies to the part.
Work harder on your first pass transfer efficiency (FPTE). If you are accumulating fines rapidly, the problem is not so much the powder as excess overspray. Things that can affect FPTE are number of guns, gun position, voltage/amperage adjustment, flow rate, part grounding and the spray environment (heat, humidity, airflow). Ask your powder equipment supplier or powder material supplier or both to help you with adjustment and training. Set a goal of 50 percent FPTE as a minimum, and develop standard settings for all of your guns. Consume your reclaim as fast as you generate it. If the powder efficiency drops to a level that tempts you to increase the flow rates, empty the module and reintroduce that powder to the system in small amounts. If that does not work, you are better off to throw it out than to run at a low TE. In the longer run, running higher flow rates will cost more than occasionally throwing out some bad powder.
Infrared cure is gaining increased attention from coaters as a result of shorter cure cycles and the possibility of smaller floor space requirements when compared to convection oven curing.
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