In his May 2009 column, Dr. Dave Dunn discusses caulking technology and the best epoxies for unique bonding. Question:
Some time ago, you
wrote that you were a big fan of silicones for exterior caulking. Is this still
the case? I really don’t like them because they’re not easy to paint.
I am still a proponent of silicones for
caulk longevity, but you are certainly correct that most silicones do not
accept paint readily. Latex caulks are a paintable alternative but do not have
the longevity or joint movement capability of silicones. Moisture-cured
polyurethane caulks have been a fast-growing product in this market and are
readily paintable after full curing, but curing tends to be slow and adhesion
on moist surfaces can sometimes be a problem.
Silane-terminated polyethers, sometimes called modified silicone (MS) polymers,
comprise a newer caulking technology of polyurethane or polyether backbones
end-capped with silyl groups that cure by reaction with atmospheric moisture,
just like silicones. These formulated sealants feature the benefits of
polyurethanes and provide better adhesion to many surfaces and faster
paintability. They also eliminate residual isocyanates and bubble formation.
I want to bond and seal some rigid
PVC-type panels to concrete that will be immersed in water. I’d like to use an
epoxy adhesive because of its moisture resistance, but I can’t get good
adhesion to the PVC. Do you have any advice?
Most rigid epoxies struggle to bond these
surfaces, and also to withstand the differences in thermal expansion and
contraction of the bonds. Try using a specially formulated flexible epoxy; good
grit blasting of the surfaces is necessary, and flame treatment of the PVC is
often very beneficial.