Ask Dr. Dave

Ask Dr. Dave

In his August 2008 column, Dr. Dave discusses the best way to seal porous metal castings and powdered metal components, and the difference between one- and two-component structural adhesives.

Question: What is the best way to seal porous metal castings and powdered metal components?

Answer: The modern way of doing this is to use either heat-cured or room-temperature-cured, methacrylate-based impregnation sealants. These are low-viscosity sealants (typically less than 30cps) that are impregnated into pores by submersing the parts in the sealant, applying a vacuum to remove the air from the pores and then restoring atmospheric pressure, whereby the sealant penetrates into the evacuated pores. Following the impregnation, the sealant cures to a polymeric resin and seals the porosity. Most sealants are cured using heat via an oven or hot water at 85-95°C. However, an alternative system uses the so-called “anaerobic” cure system where the resin cures inside the pores at room temperature. Anaerobic sealants require exposure to atmospheric oxygen via an aeration grid between vacuum cycles; cooling of the sealant to maintain stability; and adjustment of the reactivity of the resin by chemical additions. The equipment for heat-cured sealants is much simpler, with no cooling, aeration or chemical additions, and this system is usually the most cost-effective solution for most sealing problems. However, there are instances where anaerobic sealants are most effective, such as in heat-sensitive electrical components and highly porous powdered metal components, where a heat cure sometimes causes “bleed-out” of sealant due to the differences in thermal expansion coefficients of the sealant and the metal.

Question: As a manufacturing engineer in the auto industry, I use a lot of structural adhesives. However, my suppliers always want me to use two-component structural adhesives that require complicated mixing and application equipment. Why can’t I use one-component adhesives?

Answer: Two-component structural adhesives such as epoxies or urethanes are used for fast cure speeds, good gap-filling ability, and high performance. Unfortunately, the trade-off with two-components is the need for mixing equipment. The only one-component alternatives are heat-cured epoxies, reactive hot-melt urethanes or toughened cyanoacrylates. The epoxies require heating for cure cycles ranging from 30 minutes to several hours, whereas the reactive hot-melt urethanes are applied in molten form and fixture almost instantly but require hours or days to fully cure through reaction with atmospheric moisture. Toughened cyanoacrylates are quite different from common cyanoacrylate-based super glues and are true structural adhesives. However, they suffer from limited ability to fill gaps and very high costs, making them suitable only for small-area bonding.


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