|Ask the Experts|
April 16, 2020
Conformal Coating in Nitrogen Environment
We operate a conformal coat machine using nitrogen. We are thinking of changing it back to air for cost reasons, what are the pros and cons. We are under the impression that nitrogen is the better choice for purity reasons, is that correct?
|Expert Panel Responses|
Nitrogenuse for conformal coating system: Pros:
Global Segment Manager
Air Products & Chemicals, Inc.
Conformalcoatings are basically a resin dissolved in a solvent. Most conformal coatingsutilize a flammable solvent. Once the coating is applied, the gas flow (air orN2) is needed to carry the solvent vapors from the machine/spray booth.
If airis the gas used, then the air flow must be sufficient to keep the solvent vaporconcentration below its LFL (Lower Flammability Limit). For example, the airflow for a spray booth that my employer uses is 7000 cfm. Using N2 as the sweepgas does not require as much flow since there's no oxygen in the gas flow. Hence,the LFL (also known as the LEL (Lower Explosive Limit) would not apply in a N2flow.
In addition to evaluating what air flow would be required to maintain.
Ipresume that you are using nitrogen to pressurize the pressure pot on thecoating system containing the coating and possibly power the air assist.Presumingthat this is a selective coating system.
Providingthat you filter and dry a compressed air supply you should not see anydifference.Theonly exception to this rule could be if you are using an oxygen cross linkingcoating, polyurethane with oxygen cross linking for example.
Therewould only be a negative effect here if the coating remained unused in the potfor a significant amount of time, weeks or months, this is not good generalpractice.Theonly other use of nitrogen in a coating system would be to form a blanket overa dip coating tank to reduce evaporation and solvent loss.
Without very specific knowledge of your process, including the coating materials involved, my comments must be rather general. The main benefit of nitrogen here is that it is very dry. By contrast, many factory compressed air sources contain a lot of moisture. Poor system design or maintenance can even result in water and/or oil in the lines.
If your specific process requires controlling the amount of moisture in the process, it may be as expensive to install and maintain drying and filtration systems as it is to run nitrogen for critical processes. On the other hand, if your compressed air system has effective dryers and filtration, then secondary filtration and/or drying may not be expensive, or even necessary.
Besides purity & preventing solvent loss, the main reasons for choosing nitrogen as a blanket is its inertness and to keep the system dry. Many conformal coatings can exhibit "blushing" if moisture is absorbed into the solvents and then allowed to cure. In addition, RTV silicones require a minimum amount of moisture to initiate the curing process and so must necessarily remain moisture-free prior to application.
Another con of the air is that many operators will use "shop" air instead of dry compressed cylinder air. This has the potential to introduce contaminants into the coating through entrained oils etc that should be removed through the separators normally found in an air compression system. Unfortunately, this is not always the case, generally due to lack of frequent PM on the compressor, ie blowing down the separator system.
If you must switch to air due to economics, please consider using a dedicated cylinder of what is called "5.0" compressed air for your system. It is designed for use in chromatograph systems and is extremely dry and otherwise contaminant-free.
In general we have not seen many conformal coat applications that benefit from nitrogen. So, in general, it is probably without risk to stop using nitrogen. But let's throw in the following caveats and diligence:
Heller Industries Inc.
You may use air as you have a good filtering and drying system.
Senior Manufacturing Engineer
Nitrogen is typically used to pressurize conformal coatingtanks when the material is moisture-sensitive and the factory air supply is notdry enough. The level of sensitivity to moisture varies by material and themoisture level found in house air will vary from factory to factory. Withmoisture-sensitive fluids, using moist air can cause buildup inside the fluidsupply lines. The build-up causes flow to decrease and introduces gelledmaterial particles that may enter the fluid stream and clog the applicator. Themore sensitive the material is to moisture, the more significant theconsequences.Nordson ASYMTEK Conformal Coating Applications Team
Bottled nitrogen is typically very dry and thus offers aquick fix in these situations. You can add dryers to the air, however, that istypically not as effective as using the dry nitrogen. An alternative solutionis the use of "bladder-bags" - these are air-tight bags in which the fluidsupplier packages coating materials, eliminating direct contact withpotentially moist air. You would need to request this packaging from your fluidsupplier and be sure your conformal coating system will handle the bags.
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