Hiding Fasteners Under Powder

Question:

A person asked in the October 2002 issue of Products Finishing how to fill over fasteners to have them hidden under a powder coating. I would like to offer some information you might find helpful for future reference in similar situations.

There are “high temperature” resistant epoxy adhesives that might be considered if the parts can be subjected to elevated temperature curing. Unfortunately, most resin systems including epoxies, polyimides, and polyphenylene oxides that are resistant to elevated temperatures also require curing or post curing at elevated temperatures. Phenolics are typical of this. About the only resin system I know that will cure at room temperature but will resist the temperatures of powder coating curing/fusing are silicones, and they obviously would not do as a filler under any subsequent coating system.

In looking at any resin system, there are two not-so-obvious things that need consideration. One is inclusion of carrier solvents that are expected to be removed by room temperature evaporation and the other is if the curing is a condensation reaction wherein a product of the curing must be removed to prevent outgassing or frothing. For something like phenol-resorcinol adhesive for wood, the porosity and water absorption allow the water reaction product to be removed from the joint. Metals obviously don’t lend themselves to that.

Epoxies are addition reaction resin systems and many that are for high temperatures are also flowing liquid at room temperature. Cured epoxies are known for their superb adhesion and hardness, especially with fillers. While epoxy-casting resins could offer similar temperature characteristics, their intended application means they are typically very fluid, while adhesive applications usually need limited flow characteristics. Those are attributes that would lend themselves to fillers or putties in the application you addressed. It is true that just about any resin system is a good dielectric, and the standard technique for making them “conductive” is addition of large amounts of conductive fillers of silver or graphite powder. Epoxies in a liquid or semi-liquid form do lend themselves to milling or mixing in such fillers, and there are some of those commercially available. Also, the aromatic amine curing agents of high temperature epoxies can be modified to substitute a portion with aliphatic amines to get gelling at low or room temperature to be followed by elevated temperature post curing to get the cross linking needed for high temperature resistance.

3M Company, Adhesives and Sealants Div., makes a line of epoxy adhesives under their EC-2214 designation. They do require curing above 200F, but remain liquid at room temperature indefinitely. It has been a number of years since I checked the properties of each of those products, but, as I recall, one or more retained at least 3/4 of the room temperature strength at 300F. That would mean sustained exposure to that temperature for 100 hours or longer. For short duration exposures of fractions of an hour such as powder coating curing, you could probably add on another 50-100F. There are other less well-known adhesive and sealant manufacturers such as Tra-Con division of National Starch who may have similar products.

I hope this is useful information and not just a rehash of what you already knew—if you knew it better than me, my apologies for the intrusion. My interest is to pass on well-known information from one technical field to another that could make the other fellow’s job much easier. D. W.

Answer:

Thank you for providing us this intriguing information. I never would have thought to use a sealant as a filler material. I wonder what kind of problems may be experienced if these sealants are applied thicker than they are normally to fill a metal defect. Will they out-gas? An interesting problem, don’t you think?

I will pass your discussion onto my readers on the basis that information like this can lead to new technology breakthroughs. Maybe we can get some one to do a study on whether or not these sealants will provide the temperature resistance to withstand powder curing and yet not out-gas when applied at thicker than usual coats to fill a substrate defect.

Until this study is done, my original advise to use Lab Metal, an aluminum filler material, is still the proven solution to filling voids or covering imperfections on metal substrates that will be subsequently powder coated.