Dielectric Barrier Pads & Gaskets

Dielectric barriers are electrical insulators that prevent the flow of current through them by storing a charge. Stockwell Elastomerics fabricates dielectric barriers such as pads, spacers and gaskets from solid silicone, silicone foam, and silicone-coated fiberglass materials. These materials are available as sheets, rolls, and, in the case of silicones, as moldable compounds.

Applications include high-power electronics such as switch-mode power supplies, medical X-ray and other types of non-contact imaging systems, and antenna radomes for radar systems and satellite communications. See the radome antenna blog post for a dielectric barrier case study.

Dielectric Barriers vs. Electrical Insulators

Unlike other types of electrical insulators, dielectric barriers develop an internal electric field that nullifies an externally-applied voltage. That’s because dielectric barriers have a higher dielectric constant and can be polarized while in the presence of an electric field. Dielectric constant is not the only consideration, however, and dielectric strength and breakdown voltage are important for material selection.

Dielectric Strength and Breakdown Voltage

Dielectric strength is the maximum voltage that is required to produce an electrical breakdown through a material. It is calculated by dividing the breakdown voltage by the thickness of a material and is expressed in volts per unit of thickness. Breakdown voltage is the minimum voltage that causes part of the material to experience an electrical breakdown and begin conducting electricity. This breakdown results in an electrical arc (arcing) and can be tested according to ASTM D495 for dry arc resistance.

Why is Silicone Used in Transformers, Insulators and Dielectric Barriers?

Electrical insulators were originally made from glass or ceramics.  After the creation of polymers in the mid-20^th century, they quickly began to take over as the optimal material choice.  As more advanced elastomers like silicone were commercialized, these supplanted ceramic and glass.  Silicone became the premier electrical insulation choice for various reasons.  Silicone is a thermoset rubber, so that makes it soft and tough to break.  The same cannot be said for brittle materials like ceramic and glass that can be damaged by scratching or impact.

Silicone’s special chemistry is what provides its excellent electrical insulation properties:

• High temperature resistance (400-500°F continuous, 550°F intermittent on certain chemistry)
• Fewer carbon atoms than organic polymers (arc resistance therefore is good)
• Even when burned, the resulting char is made of Si-O insulating ceramic

Dielectric Barrier Types and Specifications

Silicone rubber comes in different durometers, densities and thicknesses, which affect dielectric strength and is closely related to dielectric breakdown voltage. Dielectric breakdown voltage increases with material thickness for both AC voltages and DC voltages; however, the DC dielectric breakdown voltage is higher when the material is thinner (source: Shin-Etsu Silicones).

Stockwell Elastomerics provides customers dielectric rubber parts from liquid silicone rubber (LSR), high consistency rubber (HCR), fabric reinforced silicones, and cellular silicones.  Silicone grease and silicone oil are used as well in applications where dielectric barrier parts are not a good fit.

Because dielectric barriers are often used in applications where they are hidden from view, their physical appearance may be unimportant. Although some silicone dielectric barriers are opaque, they can also come in colors such as red, gray, and black. Color additives (known as pigments) can change a material’s dielectric properties for the better or worse. Careful selection of the color is an important consideration. There can also be difference in dielectric constant based on a silicone elastomer‘s curing mechanism (source: NIH, National Library of Medicine).

Other factors to consider when choosing a dielectric barrier include thickness as well as compressive forces available in the housing assembly. Dielectric strength is a function of thickness and the material’s dielectric constant.

While this may seem complicated, Stockwell Elastomerics’ team of Applications Engineers often work with customers to help evaluate options and design factors.

Solid Silicones and Cellular Silicones

Solid silicones such as the BISCO® HT-6000 Series from Rogers Corporation come in standard thicknesses that range from 0.010 to 0.125 in. Key specifications include:

• Dielectric strength ranges from 370 to 385 volts/mil.
• Dry arc-resistance ranges from 120 to 145 seconds.
• Dielectric constant ranges from 2.75 to 5 at 1 KHz.

Cellular silicones such as the BISCO® HT-800 Series from Rogers Corporation and Norseal F12 and F20 Series from Saint-Gobain come in standard thicknesses that range from 0.031 to 0.500 in. Key specifications include:

• Dielectric strength ranges from 65 to 75 volts/mil.
• Dry arc-resistance ranges from 125 to 175 seconds.
• Dielectric constant ranges from 1 to 1.5 at 1 KHz.

These values are approximate values. Stockwell Elastomerics has strong relationships with suppliers of silicone materials and can provide more detailed technical specifications upon request.

Fabrication and Application Methods

Stockwell Elastomerics fabricates dielectric barrier pads, spacers and gaskets using the following manufacturing methods:

• Compression molding
• Liquid injection molding (LIM)
• Die cutting
• Tool-less waterjet cutting and other tool-less cutting methods

Because metal is electrically conductive, the use of metal fasteners such as rivets is not recommended. Instead, dielectric barriers may be applied through the use of adhesive tape. Elongation can affect material thickness, however, so it’s important for engineers to consider the entire application environment, including installation, during material selection.