The two major considerations when comparing the published dielectric strength of different materials are:
a) the thickness of the test specimen
b) the test temperature.
The test results can be drastically different if these two conditions are not identical for the products being compared.
Normally the standard test calls for a specimen thickness of 1/8" (125 mils) tested at room temperature (20ºC) and the result is expressed in volts/mil. Occasionally it is necessary to generate application specific test results at some other temperature and these are the results shown on technical data sheets.
The following rules of thumb are applicable to Epoxy and Polyurethane compounds:
- Unfilled materials (materials that do not contain any fillers such as silica) almost always are higher in dielectric strength than those containing fillers.
- A thinner test specimen will yield higher results and visa versa.
- The higher the test temperature the lower the volts/mil.
- Rigid materials yield higher volts/mil than softer products.
- Flexible materials typically fall between 350 - 400 volts/mil.
- Rigid materials are usually between 450 - 500 volts/mil.
Other than the usual mechanical causes, such as sharp edges or insufficient distances between the components, partial discharge may be the result of improper handling during the encapsulation process. The vapor pressure of the epoxy or urethane components could also play a role in causing corona.
By definition, corona occurs when a gaseous substance is ionized and becomes conductive. This occurs frequently if there are air bubbles or vapors are trapped in the casting. It can also occur immediately at the interface between insulating layers with drastically different dielectric properties.
It is very important to remove as much air from the epoxy or urethane mixture before the encapsulation process. At the same time, it is also important to control the vacuum level while de-airing the mix so as not to "strip" vapors that could be trapped in the form of bubbles in the casting as the material solidifies. It is best to first thoroughly de-air the mix on its own, pour the material into the mould and de-air again all the wile controlling the amount of vacuum to minimize vaporizing.
Further to this, since the location of any trapped bubbles in relation to energized components will have a major impact on partial discharge levels, the part design is critical. Smooth, rounded surfaces instead of sharp undercuts are preferable to aid with the de-airing process. The application of air pressure to the surface of the casting during the gellation process will compress the trapped gases and significantly raise the voltage required to ionize them thereby reducing the amount of corona in the casting.
Some processes, where the products are encapsulated under vacuum, employ a dielectric gas which is allowed to enter the vacuum chamber instead of air as the vacuum level is adjusted. In this way, any remaining air is mixed with the dielectric gas raising its ionization potential.
For best results:
- Eliminate sharp corners, undercuts and cavities.
- Select a product with the correct vapor pressure.
- De-air the mix before pouring.
- Do not use excessive vacuum to severely strip the ingredients.
- Pour and de-air again.
- Apply pressure during gellation.
The following table contains the benefits offered by Polyurethane Compounds when compared to Metal or Plastic products.
|
Urethane Vs. Metal |
Urethane Vs. Plastic |
Urethane Vs. Rubber |
|
Lightweight |
High Impact Resistance |
High Abrasion Resistance |
|
Noise Reduction |
Elastic Memory |
High Cut & Tear Resistance |
|
Abrasion Resistance |
Abrasion Resistance |
Superior Load Bearing Capacity |
|
Less Expensive Fabrication |
Noise Reduction |
Thick Section Molding Without a Curing Gradient |
|
Corrosion Resistance |
Variable Coefficient of Friction |
Colorability |
|
Resilience |
Resilience |
Oil Resistance |
|
Impact Resistance |
Thick Section Molding |
Ozone Resistance |
|
Flexibility |
Lower Cost Tooling |
Radiation Resistance |
|
Easily Moldable |
Low Temperature Resistance |
Broader Hardness Range |
|
Non-Conductive |
Resistance to Cold Flow (or Compression Set) |
Castable Nature |
|
Non-Sparking |
Radiation Resistance |
Low Pressure Tooling |
Non-sag epoxy systems are said to be thixotropic. Certain applications require the epoxy or urethane compound not to run off the surfaces being coated. An extreme example of a thixotropic compound would be shaving cream. No matter where it is applied, it will not run off the surface.
Thixotropic materials are greatly effected by shearing forces. High shear, such as high speed mixing or high pressure dispensing, will usually destroy thixotropic properties.
The degree of thixotrophy is indicated by the thixotropic index, usually shown on technical data sheets for materials developed for this purpose. The higher the index the better the material will cling to a given
The dielectric strength, which is the measure of the ability of a cured epoxy or polyurethane to withstand voltage, is directly related to the amount of impurities contained within the specimen being tested. As a rule, the thicker the specimen under test, the lower the volts/mil results will be because the thicker sample will contain more impurities per unit volume.
Epoxy and urethane products containing fillers will exhibit lower volts/mil values because additional impurities are contained in the fillers. Similarly, unfilled epoxies contain less impurities and will yield higher volts/mil figures.
The typical values for epoxies range from 425 volts per mil (0.125 in. thickness) to 1800 volts per mil (0.001 in. thickness).
It must be considered that the volume and the amount of impurities within do not follow a direct relationship. For example, doubling the volume of the epoxy will not double the amount of impurities contained within the specimen. Although the volts per mil will slightly decrease over thicker specimens of cured epoxy, the overall withstand voltage will increase considerably. Overall, increasing the material thickness will allow for higher operating voltages.