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Heat and Chemical Resistant Silicone Rubber
Silicone materials exhibit unique properties based on their chemical structure. But those properties turned out to be less than ideal for insulation. In less than sixty years manufacturing grew to more than 1 billion kg of silicone resins made by Rochow's process. But only a small fraction, about 10%, of that production ended up in the small, efficient motors. Silicone polymers failed to have the properties suitable for widespread insulation uses.
The biggest issue for efficient motors was the insulation for magnet wire, the copper wire windings that wrap the rotating electromagnetic coils. The rotating coils in a generator produce the current that can be sent to motors. In a motor, the process is reversed and the current causes the rotor to rotate and produce work. General Electric's engineers could develop a long list of ideal properties for the magnet wire insulation. They called this insulation, wire enamel. Their ideal wire enamel should have as many of the following properties as possible:
Mechanical Electrical Thermal Chemical Environmental
|1. Extensible |
5. Fatigue Resistant
6. Craze Resistant
7. Easy to Apply to Wire
|1. High Electric Strength |
2. High Breakdown Strength
3. Resists Tracking
4. High Resistivity
5. Low Permittivity
6. Low Loss Angle
|1. Low Thermal Conductivity |
2. Low Thermal Expansion
3. Low Flow
4. Flame Resistance
5. High Melting Point
6. Low Outgassing
|1. Chemical, Solvent, Oxygen Resistant |
2. Does Not Effect Other Adjacent Materials
3. Electrochemical Resistance
|1. Low Gas Permeability |
2. Low Porosity
3. Low Water Permeability
4. Low Moisture Absorption
5. Resistance to UV Light
At least two dozen factors needed consideration. Some were more important than others. In a wire enamel, resistance to the UV rays of the sun would be less likely to cause a problem than, let's say, low resistance to electrical current flow. Before the fact, before Eugene Rochow had his magical laboratory day in 1940, no one could predict the inventory of properties that silicone might have. But with silicones in hand, GE's scientists and engineers sensed trouble.
They could make silicone resins and impregnate them into glass tape with excellent overall properties. But wire enamels made from silicones gave real problems. Wire enamels had the required heat resistance, but they were soft, not hard. The enamels were easily cut by adjacent wire. Silicones were porous to gases and water.
Many of these poor properties were unexpected and sent the scientists in two directions. First to understand, if they could, why the silicones failed and to try to improve those properties. Second the scientists renewed their quest for other compositions that would work as wire enamels.
Wire Coating at General Electric
Silicones Under the Monogram, H. A. Liebhafsky, John Wiley and Sons, NY, 1978, p.27
GE's first approach, to try to fix the silicones, did not work very well. But their scientists became aware of a Canadian research development, a resin called polyvinyl formal. GE licensed the Canadian development. It certainly was not GE's first choice to use technology developed by others, but what was more important was finding the best wire enamel they could.
This material met the test of the broad range of properties and by the early 1940's, this resin, now called Formvar® by GE, became the magnet wire enamel of choice. The one property that Formvar® did not have was the extremely high heat resistance of the silicones. It was only a Class F, 150 degrees Celsius insulation, not the Class H, 180 degree insulation that silicones could achieve, in theory. But the enamel applied so easily, performed so well and could be coated in such thin layers that smaller, more efficient motors and generators came forward.
From the beginning, GE scientists saw that silicones would have thousands of uses beyond electrical insulation. They succeeded in opening up many opportunities. Scientists from around the world began to look at other polymers that would show the complex sets properties required for our modern world. And the chemists and engineers developed marvelously improved materials.