Ask Dr. Dave / Columns

Ask Dr. Dave

January 2008

Question: We build several hundred structural steel components every year that weigh 1,300 lbs and are made to put formed channels in compression. It takes over 4 hours to weld the structural members in place.  We use mild steel for the structure and we would not be resistant to special preparation steps to get good adhesive bonding to the substrate. Could you recommend a suitable adhesive to use in outdoor (though painted and protected from sunlight) applications?   We have a 3/16” steel deck supported on seven gauge-formed channels that we drive heavy equipment on.  The structure looks like it would be ideal for adhesive use, as we are putting virtually no joins in tension and very little in shear.

Answer: It sounds like you need a high-performance, two-component epoxy for this application. One that is somewhat toughened would be ideal to resist the impacts and vibrations from the heavy equipment. An alternative would be a reactive acrylic adhesive; this type of adhesive might require less surface cleaning than an epoxy.

Question: I am a mechanical design engineer with a curtain wall designer/manufacturer.  I am looking for an alternative to our current method of wet-applied silicone to seal butt joints in aluminum extrusions. Are there any pre-applied sealants that could be used that are similar to thread sealants? Could assembly pressure/clamping break microspheres the same way as pre-applied threadlocking products that I have used in the past?

Answer: Adhesives or sealants containing microencapsulated activators are attractive in that they can be pre-applied and form a dry-to-touch system that can be stored for future use. Unfortunately, I do not know of any products where microcaps can be broken and mixed well in a face-to-face bonding situation. A threaded assembly is required to achieve this. It might be possible to apply a water-based, pre-applied acrylic sealant to your parts, but in this case you would not get adhesion to the second surface when you assemble the joint. Have you considered using a contact adhesive as an alternative? This could be applied in latex form and dried on both sides of the joint, and would bond to itself on contact. Contact cements are normally based on natural rubber or polychloroprene rubber. If you merely want to increase your speed of production with your current product, then you might consider a fast-curing, two-component silicone. This will be more expensive than a one-component RTV type, but the increase in productivity will probably outweigh the extra cost.


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