Ask Dr. Dave / Columns

Ask Dr. Dave

May 2007

Question: We bond borosilicate glass to stainless steel with a heat-cured epoxy adhesive. Our production is very slow; we get very strong bonds but sometimes see cracking of the glass. What do you suggest to alleviate this problem?

Answer: The problem with this combination of materials is that you have differing coefficients of thermal expansion between the steel, the glass and the epoxy adhesive. This can cause very large stresses in the bonded assembly, leading to the cracks in the glass that you observe. The heat curing is only making these stresses worse. You can counter this problem by trying to make the adhesive more forgiving from a thermal expansion point of view. Using a glass-based filler might help, and a slightly more flexible epoxy also might dissipate the stresses. However, the best way to improve your bonding will probably be to switch to a UV-cured acrylic adhesive. Toughened adhesives of this type have been used successfully for this kind of application for many years. Not only will they give you strong, stress-free bonds, but they will cure in seconds at room temperature and dramatically improve your productivity. You will need to invest in some UV curing and handling equipment.

Question: We want to bond aluminum to several thermoplastics and thermosets for a storm door application that will see quite high temperatures. Should we be looking at cyanoacrylates or epoxies?

Answer: A cyanoacrylate adhesive will probably give you quick, high-strength bonds, but you should be careful about the high temperature requirement. Cyanoacrylates are thermoplastic, non-crosslinked adhesives and may creep under load. It is important that you test the adhesives under the appropriate conditions. Epoxies will give you better high-temperature properties - particularly if you heat-cure them - and should perform well on most thermoset plastics. However, you should expect lower strengths than cyanoacrylates on many thermoplastics. You might consider trying a reactive acrylic adhesive for this application.

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