What are the important electrical properties of Plastics?
Commercial plastics are generally very good electrical insulators and offer freedom of design in electrical products. Electrical properties may also be changed by environmental conditions, such as moisture and / or temperature. A basic concept to remember is that electrons must be exchanged between molecules for electric current to flow through a material. Plastic molecules hold on to their electrons and do not permit the electrons to flow easily; thus, plastics are insulators.
Plastics containing oxygen and nitrogen molecules are “polar” which means that they will tend to act like magnets and align themselves in the presence of a voltage or field, the same as a needle in a compass trying to point north. Plastics not containing oxygen and nitrogen molecules such as polyethylene, polypropylene and polystyrene are nonpolar.
Volume resistivity is defined as the ratio between the voltage (DC – direct current), which is like the voltage supplied by a battery, and that portion of current which flows through a specific volume of a sample. Units are generally ohm/cm³. DC electrodes are put on opposite sides of a 1cm cube of a plastic material. When a voltage is applied, some current will flow in time as the molecules align themselves. Ohm’s Law tells us that the voltage (V) divided by the current in amps (I) is equal to the resistance or V/I = R.
The surface resistivity is the ratio between the direct current and current along the surface per unit width. Units are generally ohms. Again referring to Ohm’s Law, the surface resistivity is a measure of how much the surface of the material resists the flow of current.
The dielectric constant is the ratio of the capacitance (AC voltage) of electrodes with the insulating material between them to the capacitance of the same electrodes with a vacuum or dry air in between. The dielectric constant is a measure of how good a material works to separate the plates of a capacitor. Remember that the molecules are like little magnets and are trying to realign themselves every time the voltage (current) changes direction. Some materials do it better than others. The dielectric constant for a vacuum has a value of 1; dry air is very nearly 1 and all other materials have dielectric constants that are greater than 1. The dielectric constant for a plastic material can vary with the presence of moisture, temperature and the frequency of the alternating current across the plates.
Dielectric strength is the voltage difference (DC) between two electrodes at which electrical breakdown occurs and is measured in kilovolts/millimeter of thickness. This is an indicator of how effective an insulator the material is. The test is similar to that used for volume resistivity except the voltage is increased until there is an arc across the plates. This means that the voltage was strong enough to break down the material and allow a large current to flow through it. Again, this property can be affected by the presence of moisture and temperature.
The dissipation factor (AC) is the tangent of the loss angle of the insulating material. It can also be described as the ratio of the true in-phase power to the reactive power, measured with voltage and current 90⁰ out of phase. This is an indication of the energy lost within the material trying to realign the molecules every time the current (voltage) changes direction in alternating current. The property varies with moisture, temperature and frequency.
The arc resistance is the elapsed time in which the surface of the material will resist the formation of a continuous conductive path when subjected to a high voltage (DC), low- current arc under controlled conditions.