Silicone rubber is an elastomer composed of silicone—itself a polymer—containing silicon
together with carbon, hydrogen, and oxygen. Silicone rubbers are widely used in industry,
and there are multiple formulations. Silicone rubbers are often one- or two-part polymers,
and may contain fillers to improve properties or reduce cost. Silicone rubber is generally
non-reactive, stable, and resistant to extreme environments and temperatures from -55 °C
to +300 °C while still maintaining its useful properties. Due to these properties and its
ease of manufacturing and shaping, silicone rubber can be found in a wide variety of products,
including: automotive applications; cooking, baking,and food storage products; apparel
such as undergarments, sportswear, and footwear; electronics; medical devices and implants;
and in home repair and hardware with products such as silicone sealants.
Curing In its uncured state, silicone rubber is a
highly-adhesive gel or liquid. In order to convert to a solid, it must be cured, vulcanized,
or catalyzed. This is normally carried out in a two-stage process at the point of manufacture
into the desired shape, and then in a prolonged post-cure process. It can also be injection
molded. Silicone rubber may be cured by a platinum-catalyzed
cure system, a condensation cure system, a peroxide cure system, or an oxime cure system.
For platinum catalyzed cure system, the curing process can be accelerated by adding heat
or pressure. Addition cure system
In a platinum-based silicone cure system, also called an addition system two separate
components must be mixed to catalyze the polymers: the one component contains a platinum complex
which must be mixed with the second, a hydride- and a vinyl-functional siloxane polymer, creating
an ethyl bridge between the two. Such silicone rubbers cure quickly, though the rate of or
even ability to cure is easily inhibited in the presence of elemental tin, sulphur, and
many amine compounds. Dow Corning produced a product called Silastic Rx which is a platinum-cured
tubing used in the medical industry. This platinum-based system has many merits, such
as almost no shrinkage, high chemical resistance to aggressive components of some types of
resins, good tear strength, high degree of precision in reproduction, high dimensional
stability over time and non-deformability, high resistance to high temperatures and aging,
excellent non-stick effect and good grade. Condensation-based system
In a condensation system, also called a tin-based cure system and an RTV system, an alcoxy crosslinker
exposed to ambient humidity experiences a hydrolysis step and is left with a hydroxyl
group. This group then participates in a condensation reaction with another hydroxyl group attached
to the actual polymer. A tin catalyst is not necessary for the reaction to occur, though
it increases the rate of the reaction and therefore decreases the cure time. No mixing
is required for the reaction to take place. Such a system will cure on its own at room
temperature and is not easily inhibited by contact with other chemicals, though the process
may be affected by contact with some plastics or metals and may not take place at all if
placed in contact with already-cured silicone compounds. It may take as long as a week for
the system to cure fully, and as the system requires some water from the atmosphere in
order to cure, there is a risk of the surface layers curing enough to seal the lower layers
away from the air, resulting in a fluid pocket of uncured and uncurable silicone. Acetoxy
tin condensation is one of the oldest cure chemistries used for curing silicone rubber,
and is the one used in household bathroom caulk. The smell of vinegar in the form of
acetic acid is the usual indicator that the reaction is incomplete. Non-acid-producing
formulations also exist which have a shorter shelf-life, however, and reduced adhesion
when finally cured. Once fully cured, condensation systems are
effective as molds for casting polyurethane, epoxy and polyester resins, waxes, gypsum,
and low-temperature metals such as lead. They are typically very flexible and have a high
tear strength. They do not require the use of a release agent.
Peroxide-based system Saint-Gobain produces medical products made
of what it calls Silmedic, a peroxide-based silicone.
History The first silicone elastomers were developed
in the search for better insulating materials for electric motors and generators. Resin-impregnated
glass fibers were the state-of-the-art materials at the time. The glass was very heat resistant,
but the phenolic resins would not withstand the higher temperatures that were being encountered
in new smaller electric motors. Chemists at Corning Glass and General Electric were investigating
heat-resistant materials for use as resinous binders when they synthesized the first silicone
polymers, demonstrated that they worked well and found a route to produce it commercially.
The term "silicone" is actually a misnomer. The suffix -one is used by chemists to denote
a substance with a double-bonded atom of oxygen in its backbone. When first discovered, silicone
was erroneously believed to have oxygen atoms bonded in this way. In fact, silicone is an
inorganic polymer, and the technically correct term for the various silicone rubbers is polysiloxanes
or polydimethylsiloxanes. Corning Glass in a joint venture with Dow
Chemical formed Dow Corning in 1943 to produce this new class of materials. As the unique
properties of the new silicone products were studied in more detail, their potential for
broader usage was envisioned, and GE opened its own plant to produce silicones in 1947.
Wacker Chemie also started production of silicones in Europe in 1947. The Japanese company Shin-Etsu
Chemical began mass production of silicone in 1953. The companies mentioned above are
now still the main competitors in the oligopoly that comprises the silicone industry.
Properties Silicone rubber offers good resistance to
extreme temperatures, being able to operate normally from −100 to 300 °C. Some properties
such as elongation, creep, cyclic flexing, tear strength, compression set, dielectric
strength(at high voltage), thermal conductivity, fire resistance and in some cases tensile
strength can be—at extreme temperatures—far superior to organic rubbers in general, although
a few of these properties are still lower than for some specialty materials. Silicone
rubber is a material of choice in industry when retention of initial shape and mechanical
strength are desired under heavy thermal stress or sub-zero temperatures. Organic rubber has
a carbon-to-carbon backbone which can leave it susceptible to ozone, UV, heat and other
ageing factors that silicone rubber can withstand well. This makes silicone rubber one of the
elastomers of choice in many extreme environments. Silicone rubber is highly inert and does not
react with most chemicals. Due to its inertness, it is used in many medical applications including
medical implants. Structure
Polysiloxanes differ from other polymers in that their backbones consist of Si-O-Si units
unlike many other polymers that contain carbon backbones. Polysiloxane is very flexible due
to large bond angles and bond lengths when compared to those found in more basic polymers
such as polyethylene. For example, a C-C backbone unit has a bond length of 1.54 Å and a bond
angle of 112˚, whereas the siloxane backbone unit Si-O has a bond length of 1.63 Å and
a bond angle of 130˚.
The siloxane backbone differs greatly from the basic polyethylene backbone, yielding
a much more flexible polymer. Because the bond lengths are longer, they can move farther
and change conformation easily, making for a flexible material. Polysiloxanes also tend
to be chemically inert, due to the strength of the silicon-oxygen bond. Despite silicon
being a congener of carbon, silicon analogues of carbonaceous compounds generally exhibit
different properties, due to the differences in electronic structure and electronegativity
between the two elements; the silicon-oxygen bond in polysiloxanes is significantly more
stable than the carbon-oxygen bond in polyoxymethylene due to its higher bond energy.
(Polymax 2005) Special grades
There are many special grades and forms of silicone rubber, including: steam resistant,
metal detectable, high tear strength, extreme high temperature, extreme low temperature,
electrically conductive, chemicalacid/gas resistant, low smoke emitting, and flame-retardant.
A variety of fillers can be used in silicone rubber, although most are non-reinforcing
and lower the tensile strength. Silicone rubber is available in a range or
hardness levels, expressed as Shore A or IRHD between 10 and 100, the higher number being
the harder compound. It is also available in virtually any colour, and can be colour
matched.
Applications
Once mixed and coloured, silicone rubber can be extruded into tubes, strips, solid cord
or custom profiles according to the size specifications of the manufacturer. Cord can be joined to
make O-rings and extruded profiles can be joined to make seals. Silicone rubber can
be moulded into custom shapes and designs. Manufacturers work to set industry tolerances
when extruding, cutting or joining silicone rubber profiles. In the UK this is BS 3734,
for extrusions the tightest level is E1 and the widest is E3.
Becoming more and more common at the consumer level, silicone rubber products can be found
in every room of a typical home. It is used in automotive applications, many cooking,
baking, and food storage products, apparel including undergarments, sportswear, and footwear,
electronics, to home repair and hardware, and a host of unseen applications.
Freeze-tolerant solar water-heating panels exploit the elasticity of silicone to repeatedly
accommodate the expansion of water on freezing, while its extreme temperature tolerance maintain
a lack of brittleness below freezing and excellent tolerance of temperatures in excess of 150 °C.
Its property of not having a carbon backbone, but a chemically robust silicon backbone instead,
reduces its potential as a food source for dangerous waterborne bacteria such as Legionella.
Non-dyed silicone rubber tape with an iron-oxide additive is used extensively in aviation and
aerospace wiring applications as a splice or wrapping tape due to its non-flammable
nature. The iron-oxide additive adds high thermal conductivity but does not change the
high electrical insulation property of the silicone rubber. This type of self-amalgamating
tape amalgamates or fuses to itself, so that when stretched and wrapped around cables,
electrical joints, hoses and pipes it bonds into a strong seamless rubbery electrically
insulating and waterproof layer, although not adhesive.
With the addition of carbon or another conductive substance as a powdered filler, silicone rubber
can be made electrically conductive while retaining most of its other mechanical properties.
As such it is used for flexible contacts which close on being pressed, used in many devices
such as computer keyboards and remote control handsets.
Self-healing Recently, silicone rubber formed the matrix
of the first autonomic self-healing elastomer. The microcapsule-based material was capable
of recovering almost all of the original tear strength. Additionally, this material had
improved fatigue properties as evaluated using a torsion-fatigue test.
See also Injection molding of liquid silicone rubber
Forensic engineering Forensic polymer engineering
Medical grade silicone RTV silicone
References Informative Source about Properties of Liquid
Silicone RubberInformative Source about Properties of Liquid Silicone RubberInformative Source
about Properties of Liquid Silicone RubberInformative Source about Determination of Cure Kinetics
of Liquid Silicone Rubber Using DSC DataInformative Source about Liquid Silicone Rubber vs High
Consistency RubberInformative Source about Liquid Silicone Rubber for Medical Applications
Further reading Brydson, John Plastics Materials, Butterworth,
9th Ed Lewis, PR, Reynolds, K and Gagg, C Forensic
Materials Engineering: Case Studies, CRC Press