:29. Apr. 2024
:Fabio TombolatoConductive polymers are already utilized in diverse industries and research continues in this area, with new applications emerging as our understanding continues to improve. A series of ESD standards for conductive polymers have been developed to ensure the safety and performance of these materials. In this article, we delve into the most widespread standards and the test methods utilized by the producers and compounders of polymer goods to meet evolving industry requirements. We also share practical experience and expertise from OCSiAl as the supplier of advanced conductive solutions for the polymers market
The importance of conductive polymer composites cannot be overstated; with applications across industries as diverse as manufacturing, electronics, healthcare, and aerospace, these materials are indispensable. When using most polymers, there is the risk of electrostatic discharge (ESD), which can be detrimental, damaging sensitive electronic components and even posing a hazard to safety. These risks can be mitigated by using anti-static or conductive polymers, and industry standards have been developed to regulate the use of conductive polymers in ESD-sensitive applications.
Electrical conductivity (σ) is a measure of the ability of a material to conduct an electrical current. The units of conductivity are Siemens per meter (S/m). The Siemen, which is the unit of conductance, is the reciprocal of the Ohm, the unit of resistance. The units of conductivity are sometimes given as mhos/meter or millimhos/meter. Resistivity (ρ) is the inverse of conductivity (ρ = 1 / σ).[1]
ESD standards describe the required level of conductivity as resistivity values. Depending on the intended application of the material in question, a specific value of either surface or volume resistivity may be required. Volume resistivity measures a material’s ability to conduct electrical current through its bulk and is expressed in ohm-centimeters (Ω·cm),while surface resistivity measures the resistance to leakage current along the surface of a material and is expressed in ohm/square (Ω/sq).
The higher the surface/volume resistivity, the lower the leakage current and the less conductive the material is.[2]
The required level of resistivity and its measuring method are defined by the industry standard applied by an end-customer. Various anti-static and conductive agents are available to guarantee a certain level of resistivity. Produced by OCSiAl, graphene nanotubes, also known as single wall carbon nanotubes, are universal additives that allow the full range of resistivity, spanning anti-static, static dissipative, and conductive. But in contrast to standard conductive additives, single wall carbon nanotubes do not compromise on other important properties of materials, such as mechanical, elastic, aesthetic, and processing properties.
Surface resistivity characterizes the electrical resistance of a fixed surface length of an insulating material. Being independent of physical dimensions like thickness or diameter, this measurement exclusively gauges the surface’s resistivity, necessitating only one physical measurement. Consequently, the assessment of surface resistivity is made between electrodes, along the insulator material’s surface.
IEC 61340-2-3:2016 is an international standard that provides guidelines for measuring the surface resistivity and surface resistance of materials used in ESD control applications. It’s important in industries where controlling static electricity is critical, such as electronics manufacturing and handling, to help minimize the risk of electrostatic damage to sensitive electronic components.
The following ESD standards can be considered as analogues to IEC 61340-2-3:2016:
The standard electrode for testing in accordance with this method consists of a central disk surrounded by a concentric conductive ring, with both in contact with the tested planar sample surface. The total mass of the electrode assembly is 2.5 kg. The environmental temperature requirements are 23°C ± 3°C.
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The most common industry setups for measuring surface resistivity according to these ESD standards are:
The material sample is placed onto the insulating support with the surface to be tested facing up. The electrode is then positioned onto the approximate center of the specimen or at least 10 mm away from the edges. A test voltage of 100 V or 500 V is applied for 60 seconds, and a surface resistance value in ohm (Ω) is indicated on the display of the measuring equipment that should be recalculated into surface resistivity values (Ω/sq).
ASTM D257 is an ASTM International standard titled “Standard Test Methods for DC Resistance or Conductance of Insulating Materials” that provides guidelines and methods for measuring the surface resistance or surface resistivity, and the volume resistance or volume resistivity, of insulating materials, including plastics, rubber, and other nonconductive materials. Various electrode systems are described in the standard, such as circular, strip, and taper pin electrodes. A conductive paint parallel two-point electrode system for surface electrical resistivity measurement is described as an example.
The method that OCSiAl utilizes is well suited for material samples with uneven/non-planar surfaces. As an example of resistance meters, an APPA 605 or Tera Ohmmeter TO-3 can be used. In this case, conductive copper/silver paint electrodes are used, with two conductive parallel lines of 10 cm length and 0.1 cm minimum width made with the paint. The distance between the electrodes should be 1 cm.
A test voltage of 100 V or 500 V is applied and the surface resistance value (Ω) is directly indicated on the measuring equipment’s display, which must then be recalculated into surface resistivity (Ω /sq).
A common way to measure the resistivity of certain thin, flat materials is using the four-point probe technique. Four equally spaced probes are brought in contact with a material, a direct current (DC) is applied between the two outer probes and a voltmeter measures the voltage difference between the two inner probes. The resistivity is calculated from geometric factors, the source current, and the voltage measurement.[3] It’s important that testing is done in controlled environmental conditions to ensure that temperature and humidity don’t impact the electrical properties of the materials.
The ANSI/ESD STM 11.12 standard provides guidelines and methods for measuring the volume resistivity of static dissipative planar materials. It exists to ensure that materials used in ESD-sensitive environments have the correct volume resistivity characteristics, preventing the buildup and discharge of static electricity. It is therefore crucial for organizations involved in electronics manufacturing and the handling of sensitive electronic components.
The following ESD standards can be considered as analogues to ANSI/ESD STM 11.12:
• IEC 61340-2-3:2016 2-3: Methods of test for determining the resistance and resistivity of solid materials used to avoid electrostatic charge accumulation
• ASTM D257 Standard test methods for DC resistance or conductance of insulating materials
• ISO 14309 Rubber, vulcanized or thermoplastic — determination of volume and/or surface resistivity
• ISO 2878
• EN 1149-2: 1997 Protective clothing — electrostatic properties —Part 2. Test method for measurement of the electrical resistance through a material (vertical resistance)
• EN 16350:2014 Protective gloves. Electrostatic properties, etc.
For measuring the volume resistivity according to these methods, OCSiAl utilizes a high-resistance meter called a Tera Ohmmeter TO-3 and an AE 30-ANSI electrode, with a total mass of the electrode assembly of 2.5 kg. The counter electrode is placed on an insulating support and the sample placed above the electrode. A concentric ring electrode is positioned above the sample and a test voltage of 100 V or 500 V is applied. The test conditions are 23°C ± 3°C.
Volume resistivity is then calculated according to the following formula:
ASTM D4496-21 measures DC resistance or conductance of moderately conductive materials. It is useful for the comparison of materials, as a quality control test, and for specification purposes.[4] It’s typically used for materials like plastics, elastomers, and composites that have moderate electrical conductivity.
ASTM D4496 outlines a specific method for measuring volume resistivity, which involves preparing a test sample of the material with specific dimensions, placing two electrodes in contact with the sample, applying a known voltage across the electrodes, measuring the resulting current passing through the material, and calculating the volume resistivity using Ohm’s law.
OCSiAl measures volume resistivity according to the “Cabot test method” based on ASTM D4496, whereby a piece is cut from the sample at both ends with a knife to obtain a smooth surface at the edges. Both ends are painted with conductive copper paint and the resistance is measured with a multimeter, for example an APPA 605.
Recommended test conditions are 23°C. A test voltage of 100 V or 500 V is applied, and then calculations are made for volume resistivity, in which the measured resistance is corrected for the dimensions of the piece (length, width, and thickness).
ASTM D991 outlines a specific method for measuring the volume resistivity of electrically conductive and anti-static rubber products. It has been noted as useful and accurate for measuring anti-static products.[5]
The following ESD standards can be considered as analogues to ASTM D991:
• ISO 3915-1981 Plastics – measurement of resistivity of conductive plastics
• ISO 1853 Standard test method for rubber property – volume resistivity of electrically conductive and anti-static products
• ASTM D4496 Standard test method for DC resistance or conductance of moderately conductive materials
ASTM D991 outlines a specific 4-point probe method for measuring volume resistivity, in which a test sample of a rubber product with specific dimensions is prepared, four electrodes are placed in contact with the sample’s surface, and a current is applied through the specimen after connection to a DC source, which is adjusted so that the power dissipation in the specimen between the electrodes is approximately 0.1 W. As soon as the current has stabilized, after a maximum time of 5 s, a measurement is taken of potential difference across the potential electrodes and the current through the electrodes. Volume resistivity is then calculated using a formula based on the measured voltage, current, and sample dimensions. As an example of an electrode assembly, an ETS Model 831 Test Fixture or Four-pole electrode VE-D991 can be used.
OCSiAl uses a 4-probe electrode to measure ranges below 107 Ω·cm. To ensure accurate measurement, there needs to be no interference from contact resistance and the contact area. For elastomers with a volume resistivity of 0.1–108 Ω⋅cm, OCSiAl utilizes a stable source of DC potential that can be adjusted to limit the power dissipated in the specimen between electrodes to approximately 0.1 W, a precision milliammeter, a galvanometer with a sensitivity of 1 μA or less per scale division in a null-voltage circuit, and an electrostatic voltmeter with a DC resistance greater than 19 TΩ or an electrometer such as a multirange voltmeter with an input DC impedance greater than 0.1 TΩ.
A material sample with a width between 10 and 150 mm, length between 70 and 150 mm, and thickness between 2 and 6.6 mm is carefully placed in the electrode assembly. Once the current has stabilized, the potential difference across the potential electrodes and the current through the current electrodes to the nearest 1% of the respective values can be measured, along with the thickness and width of the sample. These measurements should be made on three samples. Then, calculations can be made.
The above standards provide guidelines and testing methods to assess the electrical properties of conductive polymers, ensuring they meet specific ESD control and safety requirements. Compliance with these standards protects electrical components and devices from damage and also promotes the reliable and safe use of conductive polymers, which is beneficial for diverse industries, enhancing performance and durability.
However, when measuring resistivity, it should always be noted that surface resistance or conductance cannot be measured accurately, only approximated, because some degree of volume resistance or conductance is always involved in the measurement. The measured value is also affected by any surface contamination. Surface contamination, and its rate of accumulation, is affected by many factors including electrostatic charging and interfacial tension. These, in turn, affect the surface resistivity. Surface resistivity or conductivity is considered to be related to material properties when contamination is involved but is not a material property of electrical insulation material in the usual sense.[6]
Comprehensive video guidance on various resistivity measuring methods according to ESD standards can be found here: https://youtu.be/4cgU9mkHiKo?feature=shared
EPA (United States Environmental Protection Agency), https://www.epa.gov/environmental-geophysics/electrical-conductivity-and-resistivity#:~:text=Resistivity%20(ρ)%20is%20the%20inverse,%2C%20permeability%2C%20and%20the%20saturation
https://www.intertek.com/polymers/testlopedia/surface-and-volume-resistivity-astm-d257/
https://www.tek.com/en/documents/application-note/materials-char_apps_guide
https://www.parker.com/static_content/parkerimages/chomerics/Test%20Report/Volume_Resistivity_EN.pdf