33 34
Frequently
Asked
Questions
What does RTV mean?
RTV stands for Room Temperature
Vulcanization (cure). Despite the
low-temperature connotations
conveyed by this name, RTV
silicones consist of both Room
Temperture Cure and Heat Cure
grades.
What is the cure mechanism
of a condensation cure
product?
Condensation cure silicone
products cure when exposed to
atmospheric moisture. Moisture
in the air is generally required to
cure (or vulcanize) condensation
cure products. The cure process
begins from the outer surface,
and therefore time is required
for complete cure. The cure
time is affected by the reaction
mechanism and viscosity of the
material. Generally, at 25C and
50%RH, condensation cure RTV
silicones cure through in 24 to 48
hours. Full physical properties
may take up to 7 days to develop.
What is the depth (bead
thickness) limit for a
condensation cure grade?
For 1-part, condensation
cure products, the depth
(bead thickness) limitation is
approximately 6mm (1/4").
For 2-part, condensation
cure products, the depth
(bead thickness) limitation is
approximately 25mm (1").
Can I accelerate the cure
time of a 1-part product?
Condensation cure silicone cure
rates depend on humidity, silicone
thickness, and to a smaller degree
heat. Increasing the relative
humidity around the silicone or
reducing the thickness of the
material will reduce the time to cure
the material. Increased heat (not
over 50C) will somewhat reduce
cure time but as mentioned will do
so to a much smaller degree than
humidity or thickness.
What is the cure
mechanism of an addition
cure product?
Addition cure silicone RTV
products may be 1 or 2-part
and cure when exposed to heat.
Although some heat cure products
can cure at room temperature,
higher heat greatly accelerates the
cure. 1-part heat cure products
typically have an inhibitor in the
formulation that stops the product
from curing until an activation
temperature, greater than room
temperature, is achieved and the
inhibitor is driven off and the cure
reaction is allowed to proceed.
What does "tack free time"
mean?
Tack free refers to the amount of
time it takes for a condensation
cure silicone product to form a
cured outer layer (the cured outer
layer is not tacky like uncured
material).
What is "mix ratio"?
Mix ratio is a term used to state
the amount of each material to
be in a multi-component material.
The mix ratios for 2-part products
are described on the individual
product data sheets and are
given as a ratio by weight of each
material.
What does "pot life" or
"work life" mean?
The amount of time after a 2-part
grade is mixed with its curing
agent that it will remain useful or
pliable.
How do I remove silicone?
Before it is cured: use a putty knife
to remove any of the uncured
paste. Wipe the area clean with
isopropyl alcohol to remove any
leftover residue. After it is cured:
First mechanically remove as
much of the silicone as you can
with either a knife or a razor. A
solvent (mineral spirits, toluene,
xylene, acetone) can them be
used to remove any oily residue or
any remaining silicone, It may be
necessary to soak the silicone in a
solvent overnight to break it down.
Can I thin a silicone?
Silicone can be thinned using a
solvent in which the silicone is
miscible, generally an aromatic
solvent such as toluene or xylene.
As always, be sure to follow the
producer's instructions when
using solvent products and always
use in a well-ventilated area. The
shrinkage of the silicone and the
cure time will increase with the
addition of solvent. Alternative
suggestions would include non-
reactive fluids or an RTV with a
lower viscosity.
What can I do to improve
the adhesion of the silicone
adhesive to my parts?
The first step to good adhesion
is to have clean surfaces for the
silicone to bond to. For difficult-
to-bond-substrates, Momentive
Performance Materials offers a
number of primers that can be
used to improve and maximize
adhesion.
How do I ensure that air
is removed from 2-part
grades?
If you are hand mixing, air may
become added to the material
during the mixing process.
Vacuum de-airing is most effective
in removing air prior to use.
Automated mixing equipment
that utilizes a static mixer can
eliminate the need to de-air prior to
dispensing.
On complex high-density
electronics, air can sometimes be
trapped under components during
the potting process. Where this is
a concern, potting under vacuum
or vacuum de-airing after potting
can remove the trapped air. An
alternate approach may be to use
a grade with a low viscosity and
longer potlife and to cure at lower
temperatures (if heat-cure grade),
allowing entrapped air to escape
prior to the cure of the material.
What is cure inhibition, and
how do I prevent it?
Cure inhibition is a phenomenon
that may be observed in addition-
cure grades. These materials
use a platinum catalyst to drive
the curing reaction. Surfaces
containing water, sulphur,
nitrogen compounds, organic
metal compounds, or phosphate
compounds, may inhibit cure.
Cure inhibition is characterized
by a gummy or sticky appearance
of the silicone at the interface
between the silicone and
offending substrate. Inhibition
can be prevented by application
of a barrier coat, cleaning of
the offending material prior to
application of the silicone material,
replacing the offending material
with a suitable alternative, or
selection of a condensation cure
grade.
Other Electronic
Solutions from
Momentive
Performance
Materials
Provides detailed information
on silicone materials used
for thermal management
applications in electronics and
micro-electronics. Includes
SilCool* grease & adhesives, and
conventional grades for adhesion,
encapsulation and potting.
Provides opto-electronic
solutions for LED Packages and
Assemblies. Includes InvisiSil*
LED encapsulants, Glob Top,
Lens fabrication materials, Die
Attach adhesives, and Dot Matrix
assembly materials.
Dispensing Equipment Examples
Tube Type Dispensing Unit
Cartridge Type Dispensing Unit
Cartridge Air-Gun Dispensing Unit
Small Can Pump and Dispensing Unit
Pail Can Pump Unit
T h e r m a l M a n a g e m e n t S i l i c o n e s
f o r E l e c t r o n i c s
S i l i c o ne M a t e r i a l S o l u t i o n s f o r
L E D P ac k a g e s a n d A s s e m b l i e s