A proprietary visco-elastic polyurethane offers design
engineers unique opportunities to dampen vibrations and noise in mechanical
systems. Sorobothane is a thermoset, polyether-based polyurethane solid that
flows like a liquid under load while retaining excellent memory.
Viscous materials (liquids) deform under load and transmit
force in all directions. They do not recover their shape when the load is
removed. An elastic material deforms under load and returns to its original
shape after the load is removed. The energy from a Sorbothane deflection is
converted into a small amount of heat, which dissipates from the material.
The result is a unique system to absorb and deflect shock.
"Industrially we look for applications requiring shock and
vibration isolation," says Bob Boyd, an owner and former president of Sorbothane. "We're all over the map.
Stereo people use it for isolators on
turntables for high-end players. We have isolated disc drives in laptop
computers. We are still in the Wilson
baseball glove and we do a lot of medical applications." Shipping problems on
medical scanners dropped from as high as 12 percent to less than one percent
after using Sorbothane as part of the shipping material.
Invented in England
Sorbothane was invented by Dr. Maurice Hiles in England
with funding from the British National Research Development Corp., which subsequently
formed B.T.R. Development Services to commercialize the technology. In 1982
Sorbothane, Inc. was formed to take over the Sorbothane business in the U.S. Other
independent companies sold products made from the same technology in the United Kingdom and Japan.
Dr. Hiles received the John W. Hyatt Award in 1992 from the
Society of Plastics Engineers. The award is given annually for benefit to
society. His research into the energy dissipation properties of human soft
tissue disclosed a structure very similar to an interpenetrating polymer
network. This led to his synthesis of the first commercial simultaneous
interpenetrating network, now called Sorbothane. Original use focused on shoe
inserts.
Dr. Hiles was professor of biomaterials science at the University of Akron from 1980 to 1983 and holds
eleven U.S.
patents, primarily covering energy-absorbing compositions.
In one
of his early patents, Dr. Hiles said his polymer production method comprises
reacting a slightly branched polyol of low molecular weight with a relatively
small amount of an isocyanate. He said the elastomer should be stable at
temperatures of from -40C to +100C – which in fact remains one of Sorbothane's
strongest competitive advantages.
"Say someone in Phoenix has a laptop and it hits the ground.
And then the same thing happens in Minnesota
in the dead of winter. Sorbothane has the advantage of a larger window of
performance than other materials," says Boyd.
Heavy on polyol
Most urethanes are created by reacting equal amounts of
polyol and isocyanate. The balance is tipped ten to one in favor of the polyol
to produce Sorbothane. As a result, the polymer has a minimum of chain
branching. "The resulting solid polymer behaves like a quasi-liquid, being
readily deformed by an applied force and slow to recover, although in the
absence of such a force it takes up a defined shape and volume," Hiles said in
his patent.
Curiously, Hiles proposed the material for automotive
bumpers. That goal was never achieved, although Hiles was correct that metal
bumpers were going to be replaced with polymers. Less expensive thermoplastic
olefins filled the role in part because they could be mass produced with
injection molding machinery.
The Sorbothane urethanes are cast, a time-consuming batch process.
Sorbothane primarily uses book molds, a closed casting process. Skilled mold design is required to avoid air
entrapment. In an interesting twist, a
two-pour system is sometimes employed. In one stream, a rigid polyurethane is
metered into a mold, followed by a stream of Sorbothane. The two layers are
chemically bonded as they react together in the mold.
In another construction, called "constrained," a layer of
Sorbothane is bolted between pieces of steel, producing a solid,
vibration-damping base for machinery.
There a few key parameters engineers need to consider when
designing with Sorbothane.
"We want to know the weight, the frequency and the
temperature (in the proposed application) so we can tailor the material and put
a sufficient amount of material under the base so you are carrying the load and
won't get creep," says Boyd. "And then we can tailor the hardness so that you get
the maximum performance."
The polyol formulation is a constant, but the amount of
isocyanate can be varied to produce Sorbothanes with different levels of
hardness.
Shore "00" scale
Engineers may be
surprised to learn that the hardness of Sorbothane is measured in the Shore "00" scale.
"If you take a hard rubber, it's generally measured on the D
scale," says Boyd. "Soft vinyls measure
on the A scale. A low durometer on the A scale would be 30. A high durometer
for Sorbothane on the 00 sale would be 70. That 30 and 70 equate. So our
hardest version tends to be the softest version of what most people would make
on the A scale. We're down there with foams, but we are not a foam. We are a
solid material. We have no cell structure."
Sorbothane parts are normally cast between 30 and 70
durometer on the Shore "00" scale. Sorbothane can be cast as low as
20 durometer (limited strength) and as high as 80 durometer (limited
visco-elastic properties).
For obvious reasons, Sorbothane is a specialized business,
and operates with annual sales under $10 million. One strength is its diversity
of markets and applications.
One
interesting new application is a top covering for rails used to transport
performers in the new Cirque du Soleil in Macau.
Sorbothane keeps noise to a minimum.
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