Are there any issues with using aluminum versus steel panels to do system sprayouts? We use a polyester epoxy blend powder.
--Paulo Dos Santos
The panel type shouldn't make much of a difference. Remember that aluminum is a better heat conductor so the metal heats up faster than steel. The mass of the panels will also affect the heat-up rate - obviously heavier panels heat up more slowly. You should also ensure that the panels are always clean, oil- and dust-free.
Our facility is in Houston. I understand that “fish eyes” are usually caused by oil, grease, and other contaminants that are incompatible with the coating, but it appears that humidity also can cause these crater-like defects to occur. We store our powder at temperatures below 74ºF. In discussions with some powder suppliers about product shelf life and storage, there seems to be a general consensus that if a powder sprays, it is okay even if it is beyond the 12-month “shelf life.” Since powder is hygroscopic and it is not stored in airtight, sealed boxes, is the humidity likely to contribute to coating issues, particularly in older, stored coatings?
--Brian Gurney, S&S Technology
First of all, it’s important to define our terms. Fish eyes, a coater’s worst nightmare, are relatively large, crater-like disruptions in the powder coated surface. They’re caused by a large differential in surface tension between the powder and the contaminant. Contaminants that cause fisheyes can be silicones, acrylic powders, greases, lubricants (including WD-40), hand creams, antiperspirants, forming oils and metal processing fluids, to name but a few. Moisture in powder causes pinholes, which look like sharply defined microholes in the surface. Pinholes can be numerous and are usually fairly evenly spaced apart.
Powder shelf life can span years under the proper conditions. Virtually all powder suppliers deliver their products in lined containers. The lining is typically a 4-mil-thick polyethylene bag which, when sealed, is impervious to atmospheric humidity. Powder only attracts moisture if it is exposed to a humid environment over a period of time. This can happen if containers are left open and also in the application area (e.g., hoppers, reclaim systems, etc).
If you feel that you are observing fish eyes, water probably is not the culprit. Your air supply may be contaminated. This can be the incoming air to the booth or the compressed air used for your application system. Make sure the compressed air is being properly dried and filtered prior to entering your application system. You might also need to increase the capacity of your dryer; in the summer, the dryer is taxed more by the increased ambient humidity.
If you still are experiencing fish eyes and the compressed air appears clean, then start looking for extraneous contaminants like those mentioned above. If you feel you are experiencing pinholes, then moisture may be the source. The moisture may emanate from incompletely dried parts or improperly stored powder.
I have been told by my distributor that many powders contain an overcure protection additive. What is this additive, and is there a way to find it on an MSDS?
Overcure is a problem with hybrid and epoxy-based powder coatings. The polyester and polyurethane types inherently have fairly good overcure protection to about 425ºF. Hybrids and epoxies have epoxy resin as part of the composition. Epoxies are great for hardness, scratch resistance, flexibility and chemical resistance, but they’re lousy for overcure and weatherability. They gradually yellow with extended bake cycles or excess heat. The additive your supplier is referring to is commonly known as an antioxidant. A pinch of this in an epoxy-containing formula provides some overbake resistance. It has to be incorporated as the powder is being made, so don’t try to add some to an already made powder. Good formulators always add an antioxidant to hybrid and epoxy powders.
MSDSs (material safety data sheets) are required by law to list hazardous ingredients. Antioxidants at the levels used in powders aren’t considered hazardous, so they don’t make it onto the MSDS. The best way to determine overcure performance is to check out the product data sheet (sometimes called a technical data sheet) associated with the material in question. Your supplier should furnish one at your request. Most data sheets will contain a reference to the overbake performance. If it doesn’t, ask the supplier directly. If you don't get a satisfactory answer, you may want to consider switching to a more forthcoming supplier.
What is the best “everyday” solvent? We do a complete pretreat and also media blast, but what should we use as a touch-up prior to powdering for items that might have gotten dirty after the pretreat or blast process? We have been using denatured alchohol. Are there any other solvents that we can use safely, economically and effectively?
--Chuck Bell, Atlantic Powder Coating
I’ve always been a methyl ethyl ketone (MEK) kind of guy. MEK is relatively inexpensive, fairly non-toxic and probably one of the best working solvents for a powder coating operation. That being said, I thought I would perform a little research on what constitutes the best choice of solvent for an operation such as yours. As a business person, you want the best performance at the most reasonable cost. As a conscientious corporate citizen, you need to consider the impact that a process decision has on your operators and the environment.
My criteria for “best” solvent were effectiveness, cost, flammability, toxicity and whether it is a volatile organic compound (VOC). VOCs create air pollution when released into the atmosphere. Here’s a summary of what I found:
- Denatured Alcohol: Decent effectiveness, but not the strongest of the bunch. Relatively economical, fairly flammable, low toxicity. It’s considered a VOC.
- Lacquer Thinner: Strong solvating ability. More expensive, very flammable, can cause neurological damage and is considered a VOC.
- Mineral Spirits and Naptha: These petroleum distillates are decent solvents but not the strongest. They’re inexpensive, somewhat flammable, relatively non-toxic and are considered VOCs.
- n-methyl Pyrrolidone: Decent solvating ability. Expensive, high boiling (less flammable), medium toxicity and is considered a VOC but evaporates slowly. I’ve found that it leaves a greasy residue that needs to be removed.
- Xylene and Toluene: Strong solvents, inexpensive, flammable with moderate toxicity. Both are considered VOCs.
- DiBasic Esters (DBE): Moderate solvating ability, expensive, low flammability and considered a VOC. Leaves a residue.
- d-Limonene: A citrus-based solvent with decent solvating ability but not the best. More expensive, flammable, relatively non-toxic and is considered a VOC.
- MEK: Very good solvating ability, reasonable in price, flammable, low toxicity and considered a VOC.
- Acetone: A very good solvent, reasonable in price, flammable, low toxicity and not considered a VOC.
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