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,” Dr. Maurice 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 Isolate IT! 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 are 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 Sorbothane with different levels of hardness.