Unique Urethane Dampens Shocks and Noise

DN Staff

August 13, 2009

4 Min Read
Unique Urethane Dampens Shocks and Noise

A proprietary visco-elastic polyurethane offers designengineers unique opportunities to dampen vibrations and noise in mechanicalsystems. Sorobothane is a thermoset, polyether-based polyurethane solid thatflows like a liquid under load while retaining excellent memory.

Viscous materials (liquids) deform under load and transmitforce in all directions. They do not recover their shape when the load isremoved. An elastic material deforms under load and returns to its originalshape after the load is removed. The energy from a Sorbothane deflection isconverted 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 andvibration isolation," says Bob Boyd, an owner and former president of Sorbothane. "We're all over the map.Stereo people use it for isolators onturntables for high-end players. We have isolated disc drives in laptopcomputers. We are still in the Wilsonbaseball glove and we do a lot of medical applications." Shipping problems onmedical scanners dropped from as high as 12 percent to less than one percentafter using Sorbothane as part of the shipping material.

Invented in England

Sorbothane was invented by Dr. Maurice Hiles in Englandwith funding from the British National Research Development Corp., which subsequentlyformed B.T.R. Development Services to commercialize the technology. In 1982Sorbothane, Inc. was formed to take over the Sorbothane business in the U.S. Otherindependent 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 theSociety of Plastics Engineers. The award is given annually for benefit tosociety. His research into the energy dissipation properties of human softtissue disclosed a structure very similar to an interpenetrating polymernetwork. This led to his synthesis of the first commercial simultaneousinterpenetrating network, now called Sorbothane. Original use focused on shoeinserts.

Dr. Hiles was professor of biomaterials science at the University of Akron from 1980 to 1983 and holdseleven U.S.patents, primarily covering energy-absorbing compositions.

In oneof his early patents, Dr. Hiles said his polymer production method comprisesreacting a slightly branched polyol of low molecular weight with a relativelysmall amount of an isocyanate. He said the elastomer should be stable attemperatures of from -40C to +100C â€“ which in fact remains one of Sorbothane'sstrongest competitive advantages.

"Say someone in Phoenix has a laptop and it hits the ground.And then the same thing happens in Minnesotain the dead of winter. Sorbothane has the advantage of a larger window ofperformance than other materials," says Boyd.

Heavy on polyol

Most urethanes are created by reacting equal amounts ofpolyol and isocyanate. The balance is tipped ten to one in favor of the polyolto produce Sorbothane. As a result, the polymer has a minimum of chainbranching. "The resulting solid polymer behaves like a quasi-liquid, beingreadily deformed by an applied force and slow to recover, although in theabsence of such a force it takes up a defined shape and volume," Hiles said inhis patent.

Curiously, Hiles proposed the material for automotivebumpers. That goal was never achieved, although Hiles was correct that metalbumpers were going to be replaced with polymers. Less expensive thermoplasticolefins filled the role in part because they could be mass produced withinjection 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 airentrapment.  In an interesting twist, atwo-pour system is sometimes employed. In one stream, a rigid polyurethane ismetered into a mold, followed by a stream of Sorbothane. The two layers arechemically bonded as they react together in the mold.

In another construction, called "constrained," a layer ofSorbothane is bolted between pieces of steel, producing a solid,vibration-damping base for machinery.

There a few key parameters engineers need to consider whendesigning with Sorbothane.

"We want to know the weight, the frequency and thetemperature (in the proposed application) so we can tailor the material and puta sufficient amount of material under the base so you are carrying the load andwon't get creep," says Boyd. "And then we can tailor the hardness so that you getthe maximum performance."

The polyol formulation is a constant, but the amount ofisocyanate can be varied to produce Sorbothanes with different levels ofhardness.

Shore "00" scale

Engineers may besurprised 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 Dscale," says Boyd. "Soft vinyls measureon the A scale. A low durometer on the A scale would be 30. A high durometerfor Sorbothane on the 00 sale would be 70. That 30 and 70 equate. So ourhardest version tends to be the softest version of what most people would makeon the A scale. We're down there with foams, but we are not a foam. We are asolid material. We have no cell structure."

Sorbothane parts are normally cast between 30 and 70durometer on the Shore "00" scale. Sorbothane can be cast as low as20 durometer (limited strength) and as high as 80 durometer (limitedvisco-elastic properties).

For obvious reasons, Sorbothane is a specialized business,and operates with annual sales under $10 million. One strength is its diversityof markets and applications.

Oneinteresting new application is a top covering for rails used to transportperformers in the new Cirque du Soleil in Macau.Sorbothane keeps noise to a minimum.

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