Ask Dr. Dave
May 1, 2007
May 2007Question: 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
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
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.