Adhesive and sealant troubleshooting tips from Dr. Dave Dunn. Question
: Do you know of any
publications that address the effects of moisture and alkali on polyurethane
adhesive used as part of roofing systems on concrete sub-surfaces? Are there
any guides that discuss acceptable moisture and alkali pH levels?
: Polyurethanes are known for their
excellent adhesion, flexibility, high cohesive strength, low-temperature
performance and fast curing. However, most adhesives used in this particular
application are one-part, moisture-cured adhesives, and there are certain
precautions you should take.
I am not aware of any truly objective publications that address this issue, but
personal experience suggests that making sure the concrete is in the right
condition and choosing a well-formulated adhesive are two keys to success. It
makes a difference if you are bonding to fresh or old concrete. Most
polyurethane adhesive manufacturers will recommend that you bond to fresh
concrete that has cured for a minimum of 28 days; presumably, this is to ensure
that the concrete has completely cured and dried. However, complete dryness can
never be assured, particularly if you are bonding to old concrete.
To get maximum adhesion to a damp surface, you need to work with your adhesive
supplier to ensure that you get a product that cures slowly enough to allow the
adhesive to wet the surface, but fast enough that moisture on the surface will
not cause foaming of the adhesive and affect its physical properties.
Alkalinity should not be a problem for most adhesives.
: I am using an RTV silicone to seal
electrical switches from moisture, but I seem to be getting some failures due
to silicone creeping or condensing on the contacts. Is silicone the right
product for this application?
: I have addressed similar problems in
previous issues of ASI
, and I certainly think that
a silicone is the best sealant for this type of application. Your problem is
likely caused by your use of a silicone that contains a small amount of
volatile contaminants that can condense on the metal surfaces. These materials,
called cyclic siloxanes, are normal byproducts of silicone manufacturing and do
not cause problems in most applications. In the early days of using oxygen
sensors in auto exhaust manifolds, these volatile silicones were a major source
of failures until the problem was identified. You should ask your supplier for
an electronic-grade or "low-volatile" silicone in which these
contaminants have been stripped out.