Boedeker TECH Talk Episode 15 | Measuring Surface Resistivity of ESD Thermoplastics

In this episode of our TECH Talk series, we cover the correct method for measuring the surface resistivity of ESD thermoplastics. We cover common mistakes and best practices for correctly measuring surface resistivity. Intro: Electrostatic static dissipative or anti-static thermoplastic materials are typically used in some very critical applications where consistency and reliability in electrical flow control are crucial. These ESD materials are typically used to prevent the build-up of static electricity. They are just conductive enough to bleed away static charges yet insulative enough to protect their applications from larger current flows. When testing ESD or anti-static thermoplastic materials, critical surface resistivity measurement errors are often made, which yield inaccurate results. This may lead engineers and designers to determine that their ESD materials may not be performing as expected when, in fact, they very well may be performing as intended. This video is intended to show the proper way to measure the surface resistivity for these ESD materials to ensure that accurate results are obtained. To start, let's first consider traditional plastic materials. Plastics are typically insulators that resist an electrical current's flow. The flow of electrical current across the surface of a material is measured in ohms/square area. A fully insulative plastic will have a Surface resistivity measurement of greater than 10^12 ohms/sq. Conductive plastics are also available, which occur when a certain amount of conductive filler, such as carbon fibers or metal powders, is incorporated into the plastic matrix. Conductive materials allow for the flow of electrical current both through them and across their surface. A conductive plastic will have a surface resistivity of less than 10^4 ohm/sq. The more conductive a material, the more readily the current will flow across the material's surface, and the lower the surface resistivity measurement. ESD plastics are materials that fall between being insulative and being conductive. Users of ESD materials are typically looking for these materials to have a surface resistivity somewhere between 10^4 and 10^12 ohms/sq. Many people try to measure their ESD materials using simple electrical voltmeters using point-to-point probes. The results are incredibly variable and completely inaccurate. These types of meters will yield inaccurate results for ESD materials. These meters are accurate for fully homogeneous materials….like fully insulative plastic or a fully conductive metal plate. However, critical to understand is that ESD or static dissipative plastics are not considered fully homogeneous. Therefore, when using such a point-to-point probe, your probe may or may not be in contact with conductive elements of the polymer. For measuring ESD plastics that are considered non-homogeneous, using an area-to-area measurement is essential to use a device that records resistivity. The proper testing standard for measuring static dissipative planar materials is ANSI/EOS/ESD S11.11. This test method ensures the accuracy of surface resistivity results by using an area-to-area probe under the correct conditions. The standard calls for using a ring tester comprised of two concentric rings made of conductive materials that make contact with the material under test. The test probe weighs 5 lbs to ensure good solid contact with the material is made…..and the test uses a consistent 100 volts. Using this ESD S11.11 test method will ensure accurate resistivity readings. Of course, other lower cost, more portable probes also measure an area-to-area resistivity that has been found to yield acceptable results. One popular device is a Pinion Meter. The Pinion Meter uses 30 volts and has round area-to-area surface probes that require good contact with the test material. Of course, there are times when small machined parts must be measured. These smaller surface areas and non-planar part geometries make them difficult to accurately measure and almost impossible to measure using either a Ring tester or a Pinion Meter. In these cases, I have seen smaller area-to-area probes utilized on the market. However, ensure they are marketed for and suitable to meet the ANSI ESD STM test methods. If you have any questions about anti-static plastics, including how to accurately test with them, or for assistance in selecting the most appropriate ESD material for your application, don't hesitate to contact Boedeker Plastics. Contact us at https://www.boedeker.com/Contact-Us or 1-800-444-3485 One of our Technical Service Engineers will be happy to support your efforts.