Silicone Materials Series
Troubleshooting Silicone Materials Series:
Silicone Inhibition
Silicones are formulated to cure with a number of catalysts. Each catalyst
requires a certain set of conditions to cure, and these conditions are
typically included on the product profile or data sheet. Silicones can
contain chemical entities, known as cure inhibitors, which are added by
the manufacturer to adjust pot life (working time) and curing time. Again,
the cure conditions provided by the manufacturer account for these inhibitors
as a way to achieve complete cure.
Material preparation, device components and/or assembly processes can, at times,
compromise the cure of silicones. These “poisoning” effects can range
from a slight surface tack to a complete cure failure. Some inhibition can be
overcome, while other types permanently arrest cure. Tin-catalyzed acetoxy cure
adhesives are inhibited by isopropyl alcohol (a cleaning solvent often used in
medical device manufacturing). Once the alcohol is removed, the cure proceeds.
The platinum catalyst used in addition-cured silicones is susceptible to
poisoning. Although there are many substances that can cause
cure poisoning, the following list includes many of the most common poisons:
- Sulfur-containing materials
- Rubber
- Latex
- Neoprene
- Buna N
- Natural rubber
- Polysulfides
- Sulfur compounds
- Organotin containing materials
- Condensation-cured silicones
- Acetoxy-cured silicones
- Oxime-cured silicones
- Plasticizers
- Other
- Plasticized polyvinyl chloride
- Plastisols
- Adhesive tapes
- Some amino groups
Curing a small amount of silicone in contact with a questionable material
can be an effective way to evaluate potential poisoning effects.
The next troubleshooting article will focus on some process diagnostics
that can be useful in determining the source of the inhibition.
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New Products & Developments
Phosphor-Filled Gels
NuSil will be highlighting it phosphor-filled silicones at Photonics
West in San Jose. For those unable to attend the show, the “Optical
Gel Encapsulants Designed for Phosphor Dispersions” handout will
be available for download from the Photonics West website
after the Jan. 22-24 show.
Among the highlights is NuSil’s developmental phosphor gel, GEL8-9617-30.
It is a pre-blend of LS3-3354 and 11% phosphor. GEL8-9617-30 will be a
great alternative to those who are looking for encapsulating applications
requiring a soft gel with dispersed phosphor. Because the GEL8-9617-30
uses LS3-3354, it still has the high refractive index of 1.54 and a 4,000cP
viscosity at its uncured state.
In addition to the developmental product, NuSil offers its standard optical
gels designed for phosphor dispersions. For a high-purity, optically clear,
soft, silicone gel that has medium viscosity, check out LS-3354 and LS3-3354.
They cure at room temperature in only 24 hours and have a refractive index
of 1.54. While both offer good UV resistance, the LS3-3354 also has improved
adhesion without the use of primer.
LS-3354 and LS3-3354 are great for potting, encapsulating, backfilling
and dampening applications that require a soft gel with optical clarity.
Their medium viscosity makes them excellent for dispersing phosphors to
obtain high brightness white light.
Another option for an optical gel is LS-3351, a two-part, 1:1 mix ratio,
encapsulation gel with a 1.51 refractive index. Its uses vary from protection
of sensitive photonics assemblies to mechanical and thermal shock, dust,
and ambient atmosphere, as well as LEDs and display elements. It is ideal
for phosphor mixing.
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