Silicone elastomers have been employed for many years in
medical, aerospace, electrical, construction, and industrial
applications. Good resistance to compression set, flexibility over wide temperature ranges,
a wide range of durometers, inert and stable compounds are among the reasons for their popularity today.
Silicone rubbers are synthetic polymers with molecular
structure consisting of a giant backbone with alternating silicon and oxygen atoms.
There are two popular catalyst systems used to cross-link silicone polymers: peroxide
(free-radical) systems and platinum (addition-cure) systems.
First generation silicones used peroxide as the catalyst to
initiate curing of the silicone. However, the peroxide reaction
leaves a residue in the rubber that can deposit a powder
on the part surface if not removed through a postcure process.
Though peroxide is still used, addition-cured,
platinum-catalyzed silicones are used nowadays on a large scale
because of their faster cure rates, lack of peroxide, and availability
in an injectable, liquid form.
The strong silicon-oxygen chemical structure of silicone gives
the elastomer its unique performance properties, including
biocompatibility, superior temperature and chemical resistance, good
mechanical and electrical properties, and natural clarity or
Mechanical Properties. Silicone rubbers have high tear and tensile strength,
good elongation ( up to 1300%) and flexibility, low compression set, and a durometer range
of 5 to 80 Shore A.
Electrical Properties. Silicones exceed all comparable
materials in their insulating properties as well as in their
versatility for electrical applications. They are nonconductive and
can maintain dielectric strength in temperature extremes far higher
or lower than those in which conventional insulating materials are
able to perform.
Chemical Resistance. Silicones resist water and many
chemicals, including some acids, oxidizing chemicals, ammonia, and
isopropyl alcohol. Concentrated acids, alkalines, and solvents
should not be used with silicones. However some grades have poor hydrocarbon,
oil and solvent resistance.
Heat Resistance. Silicones can withstand a wider
range of temperature extremes than nearly all other elastomers,
remaining stable through temperature variations from –50° to 250°C.
They can be sterilized via steam autoclaving, gamma or E-beam irradiation,
and other methods.
Gas permeability. Silicone rubbers exhibit high gas permeability, allowing for this material to be used in medical applicattions
such as oxigen permeable membranes.
Biocompatibility. In extensive testing, silicone rubbers
have exhibited superior compatibility with human tissue and body
fluids and an extremely low tissue response when implanted, compared
with other elastomers. Odorless and tasteless, silicones do not
support bacteria growth and will not stain or corrode other
Silicone elastomers are typically molded by three
main methods: liquid injection molding (LIM), transfer molding, or
compression molding. In designing for the molding process,
designers should take into account the material shrinkage rate,
which can range from 2 to 4% depending on the type of silicone.
During molding, the three variables that must be controlled are
temperature, pressure, and time. The temperature must be high enough
to minimize cure times, yet low enough to prevent scorching of the
elastomer. The pressure selected must allow for complete mold
filling while permitting the venting of all the air, and must be
optimized to prevent voids and flash. As in most molding, precise
timing of all functions is critical for the production of
consistently high-quality, fully cured parts.
©2004 Trinom Prod Proiect