Ethan Clarke
The resistivity of plastic, or more specifically, its volume resistivity, is a measure of a plastic material's ability to oppose the flow of electric current. It is a fundamental property for applications requiring electrical insulation or controlled conductivity. Volume resistivity is measured in ohm-centimetres (Ohm-cm) or ohm-metres (Ohm-m). The higher the volume resistivity, the better the electrical insulation properties. The volume resistivity of plastics varies depending on the specific type of plastic, with values ranging from 10^10 Ohm-cm for cellulose acetate to 10^19 Ohm-cm for polystyrene. Factors such as voltage, electrification time, temperature, humidity, and the presence of fillers can influence the volume resistivity of plastics.
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What You'll Learn
Volume resistivity testing
The volume resistivity of a polymer material refers to its capacity to resist or oppose the flow of electric current through a cubic volume. This property is measured in ohm-meters (Ohm-m) or ohm-centimetres (ohm-cm), with the Greek letter rho (ρ) used to denote it. Plastics typically exhibit volume resistivities ranging from 1010 ohm-cm for cellulose acetate to 1019 ohm-cm for polystyrene.
The test procedure involves applying a known voltage to the material and measuring the resulting current. By doing so, the volume resistivity of the material can be calculated. This test is particularly useful for thin films, and employing a four-point probe configuration helps minimise errors caused by contact resistance.
The volume resistivity of plastics is influenced by various factors, including temperature, relative humidity, voltage application duration, and the presence of fillers. For instance, insulation resistance decreases with increasing temperature and humidity. Voltage application duration also impacts volume resistivity, with longer applications resulting in higher resistivity values. Additionally, fillers in the polymer can affect volume resistivity, depending on their type and quantity.
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Factors affecting volume resistivity
The volume resistivity of a polymer material is its ability to oppose the flow of electric current through a volume of the cubic specimen. It measures a material's inherent electrical resistance to current flow throughout its bulk, making it a fundamental property for applications requiring electrical insulation or controlled conductivity.
- Temperature: Insulation resistance falls as temperature increases. For instance, PS has high insulation resistance at room temperature, but above 80°C (176°F), its resistance becomes unsatisfactory. Polymers like PTFE and PCTFE are more suitable at higher temperatures.
- Relative Humidity: Insulation resistance decreases with an increase in humidity. Plastics with high water resistance are less affected by high humidity.
- Voltage: Higher volume resistivity is achieved with the longer application of voltage.
- Presence of Fillers: The type and amount of filler in a polymer affect its volume resistivity.
- Additives: For specific applications like ESD (electrostatic discharge) protection or EMI (electromagnetic interference) shielding, plastics with lower resistivity values are intentionally created through additives like carbon black or metal fibres.
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Plastic as a class of polymeric material
Plastic is a synthetic or semi-synthetic material composed primarily of polymers. They are derived from crude oil and natural gas, which are hydrocarbon materials. When refined, these materials are split into different fractions that vary based on molecular size and structure. One such byproduct is naphtha, which is used to produce plastics.
Plastics are a subset of polymers, which are materials composed of repeating chains of individual atoms or molecules. Polymers can be naturally occurring, such as cellulose, latex, and rubber, or synthetic, like nylon, polyethylene, and polypropylene. The terms “polymer” and “plastic” are often used interchangeably, but there are differences between the two. Polymers exhibit the same structures as plastics, but not all polymers are considered plastics. These differences include recyclability, flexibility, and strength.
Plastics are known for their plasticity, which allows them to be molded, extruded, or pressed into various solid forms. This adaptability, coupled with their low weight, durability, flexibility, chemical resistance, low toxicity, and low-cost production, has led to their widespread use globally. The world's first fully synthetic plastic was Bakelite, invented in 1907 by Leo Baekeland, who coined the term "plastics."
Today, dozens of different types of plastics exist, including polyethylene, commonly used in product packaging, and polyvinyl chloride (PVC), utilized in construction and pipes due to its strength and durability. Plastics can be classified as thermoplastics or thermosets, depending on their reversible or irreversible chemical processes during heating. Thermoplastics, such as polyethylene, polypropylene, polystyrene, and PVC, can be molded repeatedly without undergoing chemical changes when heated. On the other hand, thermosets, also known as thermosetting polymers, can only take shape once. After solidification, they permanently retain their shape and decompose when reheated instead of melting.
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Measuring volume and surface resistivity
The volume resistivity of a polymer material is its ability to oppose the flow of electric current through a volume of the cubic specimen. It measures a material's inherent electrical resistance to current flow throughout its bulk, making it a fundamental property for applications requiring electrical insulation or controlled conductivity. The SI unit of volume resistivity is ohm-meter (Ohm-m) or ohm-centimetre (Ohm-cm).
The volume resistivity of plastics ranges from 10^10 ohm-cm for cellulose acetate to 10^19 ohm-cm for polystyrene. Materials with volume resistivity below 10^5 Ohm-cm are considered conductive, while those above 10^9 Ohm-cm are considered electrical insulators.
To measure volume resistivity, a standard-size specimen is placed between two electrodes. A voltage is applied for a specified period, typically 60 seconds, and the resistance is measured. The resulting current is then used to calculate the volume resistivity.
Surface resistivity, on the other hand, is the resistance to leakage current along the surface of an insulating material. It is measured in Ohms or Ohms per square, depending on the electrode configuration. The test procedure is similar to that of volume resistivity, with a voltage applied across the surface of the insulator sample, and the resultant current measured.
The testing environment's conditions, especially humidity and temperature, can significantly impact the results. Therefore, these conditions should be kept constant when comparing multiple tests.
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ASTM standards for testing
The resistivity of a plastic material refers to its ability to oppose the flow of electric current through a volume of the material. It is a fundamental property for applications requiring electrical insulation or controlled conductivity. ASTM International has developed ASTM D257, a test method to measure the resistivity of insulating materials, which is critical for establishing surface and volume resistivity. This test provides two basic surface and volume resistance values, which are essential during material selection for electrical applications. It helps assess the insulating properties of materials used in electrical components, ensuring safety and performance in electrical systems.
ASTM D257 plays a vital role in insulation testing, influencing material properties, and ensuring electrical safety. The test provides a reliable method for evaluating the resistivity of insulating materials, contributing to quality control, and material selection. Calibration and maintenance of testing equipment, such as the ASTM D257 fixture, are crucial to ensure accurate and reliable measurements.
ASTM International also offers a range of other ASTM standards that are relevant to plastics and polymers. These standards cover various aspects, including chemical composition, physical and mechanical properties, thermal properties, and performance. For example, ASTM D635 focuses on flammability testing, while ASTM D2583 covers Barcol hardness testing.
Additionally, there are ASTM standards for specific applications, such as ASTM D150 for the dielectric constant and dissipation factor, and ASTM D149 for dielectric strength. These standards help ensure the safety and performance of plastic materials in different contexts. Overall, the ASTM standards provide comprehensive guidelines for testing and evaluating the properties of plastics, contributing to quality control, and ensuring compliance with specifications.
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Frequently asked questions
Plastic is not a single material, but a class of polymeric materials, so it does not have a single value for resistivity. The volume resistivity of plastics ranges from 10^10 ohm-cm for cellulose acetate to 10^19 ohm-cm for polystyrene.
Volume resistivity is a measure of a material's inherent electrical resistance to current flow throughout its bulk. It is calculated using the ratio of the potential gradient to the current in a material with the same density.
Volume resistivity is important for applications requiring electrical insulation or controlled conductivity. It is used as a quality control measure and a material selection criterion, helping plastic manufacturers ensure consistent electrical performance and maintain compliance with safety standards.
Volume resistivity is affected by the presence of fillers in the polymer, voltage, electrification time, and environmental factors such as humidity and temperature.
