| Dow Corning brand addition cure gels and elastomers cure
rapidly and uniformly at widely different temperatures, in thick sections, and
under conditions of air exposure or in total confinement. They can be cured in
modules or parts made of a variety of materials and in contact with most
common electronics materials. |  |
There are certain situations, however, where the cure reaction cannot
proceed normally. These conditions occur when materials called inhibitors are
present. In this inhibited area (usually less than 0.02 inches thick) the
elastomer remains in its liquid state even though the cure schedule has been
completed. This liquid material will remain liquid regardless of any subsequent
attempts to convert it to a hard, dry mass.
Inhibition can be overcome in some instances. Pre-baking the unit at
the highest tolerable temperature can help remove volatile chemicals causing
the inhibition. In many applications, the use of a primer can act as a barrier
coating against these inhibiting materials. In other cases, the use of higher
or longer cure temperatures may be enough to overcome mild inhibition.
Causes of Inhibition
 | Inhibition is caused by the contamination of addition cure
materials with trace quantities of certain types of chemicals. The catalyst is
a chemical containing platinum atoms, which are not only affected by
working-time control agents, but can also be poisoned by contaminants. These
chemicals interfere with the cure reaction and thus prevent conversion of
material to a solid. Extremely small quantities of inhibitors or contaminants
may be sufficient to produce this effect. Fortunately, only a small number of
material types can cause inhibition. However, unlike a working-time
control agent, once a contaminant attaches to the platinum, any negative
effects on the cure are permanent. |
Certain materials, chemicals, curing agents and plasticizers can inhibit the
cure of addition cure adhesives. Most notable of these include:
-
Organotin and other organometallic compounds
-
Silicone rubber containing organotin catalyst
-
Sulfur, polysulfones or other sulfur-containing materials
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Amines, urethanes or amine-containing materials
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Unsaturated hydrocarbon plasticizers
-
Some solder flux residues
 | If a substrate or material is questionable with respect to
potentially causing inhibition of cure, it is recommended that a small-scale
compatibility test be run to ascertain suitability in a given
application. A portion of the part containing the suspect material can be
placed in a clean dish. The dish is then processed by the desired cure
method. A control dish without the part can be compared to the dish with
the suspect part. The presence of liquid or uncured product at the
interface between the questionable substrate and the cured gel indicates
incompatibility and inhibition of cure. |
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