When selecting an Engineering Plastic for insulator use the term “dielectric strength” is often of great consideration. But be aware, the term “dielectric strength” has multiple meanings.
In physics, the term “dielectric strength” has three meanings:
1) In an insulating material it is the maximum electric field strength that the insulating material can withstand without breaking down or failing and losing it’s insulating properties.
It can also mean…
2) For a given configuration of dielectric material and electrodes, dielectric strength is the minimum electric field that causes a breakdown of the properties.
3) The maximum electric stress the dielectric material can withstand without breakdown
Overall, dielectric strength is considered to be a natural property of a given material. It’s just a part of what it is. When a material’s dielectric strength is pushed past it’s limit the electric field of the material frees bound electrons. If the applied electric field is sufficiently high an event called an avalanche breakdown occurs. This happens very, very fast (within nanoseconds) and basically it causes the material to severely degrade or even completely destroy it’s insulating properties.
What to look for in an Insulator Material
When looking at Engineering plastics there are several things that will affect the material’s dielectric strength. When an insulator is subjected to increasingly high voltages, it eventually breaks down and allows a current to pass. The voltage reached before break down divided by the sample thickness is the dielectric strength of the material, measured in volts/mil. It is generally measured by putting electrodes on either side of a test specimen and increasing the voltage at a controlled rate. Factors that affect dielectric strength in applications include: temperature, sample thickness, conditioning of the sample, rate of increase in voltage, and duration of test. Contamination or internal voids in the sample can also have an affect on the outcome of testing.
Another way to Look at Dielectric Strength…It is…
directly proportional to the thickness of the material
inversely proportional so it decreases as operating temperature increases
inversely proportional to an increase in frequency
inversely proportional to an increase in humidity
To see how dielectric strength compares let’s take a look at some common insulator materials:
(All Dielectric Strengths are given in MV/m)
DuPont™ Vespel® – 700
Ultem PEI – 830
PEEK – 480
Cast Nylon – 500
The Ultimate Insulator Material
When looking at materials that provide a high dielectric strength many engineers look to DuPont™ Vespel® This line of materials has been designed into many Electronics applications as an Insulator material due to their high dielectric strength combined with other properties.
Scientific Instrumentation, Semiconductor Manufacturing, Analytical Equipment, Ultrasonics, Communications, and Avionics are just a few of the places where you will find Vespel. Recently DuPont extended their Vespel® line to include SCP-5000, with even higher dielectric strength than standard SP-1.
Do you have more questions about choosing the right Engineering Plastic based on electrical properties, feel free to call me directly. I’d be happy to assist.
Sales and Marketing Manager, DuPont™ Vespel®
ThyssenKrupp Materials NA
AIN Plastics Division