10 October 2012

Silicones






The invention:

Synthetic polymers characterized by lubricity, extreme
water repellency, thermal stability, and inertness that are
widely used in lubricants, protective coatings, paints, adhesives,
electrical insulation, and prosthetic replacements for body parts.

The people behind the invention:
Eugene G. Rochow (1909 - 2002 ), an American research chemist
Frederic Stanley Kipping (1863-1949), a Scottish chemist and
professor
James Franklin Hyde (1903- ), an American organic chemist





Synthesizing Silicones

Frederic Stanley Kipping, in the first four decades of the twentieth
century, made an extensive study of the organic (carbon-based)
chemistry of the element silicon. He had a distinguished academic
career and summarized his silicon work in a lecture in 1937 (“Organic
Derivatives of Silicon”). Since Kipping did not have available
any naturally occurring compounds with chemical bonds between
carbon and silicon atoms (organosilicon compounds), it was necessary
for him to find methods of establishing such bonds. The basic
method involved replacing atoms in naturally occurring silicon
compounds with carbon atoms from organic compounds.
While Kipping was probably the first to prepare a silicone and was
certainly the first to use the term silicone, he did not pursue the commercial
possibilities of silicones. Yet his careful experimental work was
a valuable starting point for all subsequent workers in organosilicon
chemistry, including those who later developed the silicone industry.
On May 10, 1940, chemist Eugene G. Rochow of the General
Electric (GE) Company’s corporate research laboratory in
Schenectady, New York, discovered that methyl chloride gas,
passed over a heated mixture of elemental silicon and copper, reacted
to form compounds with silicon-carbon bonds. Kipping
had shown that these silicon compounds react with water to form
silicones.
The importance of Rochow’s discovery was that it opened the
way to a continuous process that did not consume expensive metals,
such as magnesium, or flammable ether solvents, such as those
used by Kipping and other researchers. The copper acts as a catalyst,
and the desired silicon compounds are formed with only minor
quantities of by-products. This “direct synthesis,” as it came to be
called, is now done commercially on a large scale.


Silicone Structure

Silicones are examples of what chemists call polymers. Basically, a
polymer is a large molecule made up of many smaller molecules
that are linked together. At the molecular level, silicones consist of
long, repeating chains of atoms. In this molecular characteristic, silicones
resemble plastics and rubber.
Silicone molecules have a chain composed of alternate silicon and
oxygen atoms. Each silicon atom bears two organic groups as substituents,
while the oxygen atoms serve to link the silicon atoms into a
chain. The silicon-oxygen backbone of the silicones is responsible for
their unique and useful properties, such as the ability of a silicone oil
to remain liquid over an extremely broad temperature range and to
resist oxidative and thermal breakdown at high temperatures.
Afundamental scientific consideration with silicone, as with any
polymer, is to obtain the desired physical and chemical properties in
a product by closely controlling its chemical structure and molecular
weight. Oily silicones with thousands of alternating silicon and
oxygen atoms have been prepared. The average length of the molecular
chain determines the flow characteristics (viscosity) of the oil.
In samples with very long chains, rubber-like elasticity can be
achieved by cross-linking the silicone chains in a controlled manner
and adding a filler such as silica. High degrees of cross-linking
could produce a hard, intractable material instead of rubber.
The action of water on the compounds produced from Rochow’s
direct synthesis is a rapid method of obtaining silicones, but does
not provide much control of the molecular weight. Further development
work at GE and at the Dow-Corning company showed that
the best procedure for controlled formation of silicone polymers involved
treating the crude silicones with acid to produce a mixture
from which high yields of an intermediate called “D4” could be obtained
by distillation. The intermediate D4 could be polymerized in
a controlled manner by use of acidic or basic catalysts. Wilton I.
Patnode of GE and James F. Hyde of Dow-Corning made important
advances in this area. Hyde’s discovery of the use of traces of potassium
hydroxide as a polymerization catalyst for D4 made possible
the manufacture of silicone rubber, which is the most commercially
valuable of all the silicones.


Impact

Although Kipping’s discovery and naming of the silicones occurred
from 1901 to 1904, the practical use and impact of silicones
started in 1940, with Rochow’s discovery of direct synthesis.
Production of silicones in the United States came rapidly enough
to permit them to have some influence on military supplies for
WorldWar II (1939-1945). In aircraft communication equipment, extensive
waterproofing of parts by silicones resulted in greater reliability
of the radios under tropical conditions of humidity, where
condensing water could be destructive. Silicone rubber, because
of its ability to withstand heat, was used in gaskets under hightemperature
conditions, in searchlights, and in the engines on B-29
bombers. Silicone grease applied to aircraft engines also helped to
protect spark plugs from moisture and promote easier starting.
AfterWorldWar II, the uses for silicones multiplied. Silicone rubber
appeared in many products from caulking compounds to wire insulation
to breast implants for cosmetic surgery. Silicone rubber boots were
used on the moon walks where ordinary rubber would have failed.
Silicones in their present form owe much to years of patient developmental
work in industrial laboratories. Basic research, such as
that conducted by Kipping and others, served to point the way and
catalyzed the process of commercialization.



                     

                                                                   Eugene G. Rochow






Eugene George Rochow was born in 1909 and grew up in the
rural New Jersey town of Maplewood. There his father, who
worked in the tanning industry, and his big brother maintained
a small attic laboratory. They experimented with electricity, radio—Eugene put together his own crystal set in an oatmeal
box—and chemistry.
Rochow followed his brother to Cornell University in 1927.
The Great Depression began during his junior year, and although
he had to take jobs as lecture assistant to get by, he managed
to earn his bachelor’s degree in chemistry in 1931 and his
doctorate in 1935. Luck came his way in the extremely tight job
market: General Electric (GE) hired him for his expertise in inorganic chemistry.

In 1938 the automobile industry, among other manufacturers,
had a growing need for a high-temperature-resistant insulators.
Scientists at Corning were convinced that silicone would
have the best properties for the purpose, but they could not find
a way to synthesize it cheaply and in large volume. When word
about their ideas got back to Rochow at GE, he reasoned that a
flexible silicone able to withstand temperatures of 200 to 300 degrees
Celsius could be made by bonding silicone to carbon. His
research succeeded in producing methyl silicone in 1939, and
he devised a way to make it cheaply in 1941. It was the first
commercially practical silicone. His process is still used.
After World War II GE asked him to work on an effort to
make aircraft carriers nuclear powered. However, Rochow was
a Quaker and pacifist, and he refused. Instead, he moved to
Harvard University as a chemistry professor in 1948 and remained
there until his retirement in 1970. As the founder of a
new branch of industrial chemistry, he received most of the discipline’s
awards and medals, including the Perkin Award, and
honorary doctorates.



See also : Buna rubber ; Neoprene ; Nylon ; Plastic ; Polyethylene ; Silicone Wikipedia

6 comments:

Neoprene Rubber said...

Silicone polymers has great chemical and mechanical properties. It is resistant to rain, snow, humidity,etc. At -100 to 316 degrees C, the silicone polymers wont crack or brittle. And this polymer is flexible in nature and strong in insulation.

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