Plastics, electrical properties tests

Electrical properties -of plastics [PLASTIC TESTING] (Vol 19) -of polysulfone [POLYMERS CONTAINING SULFUR - POLYSULFONES] (Vol 19)... 

Domkin, V, S. Electrical properties of plastic foams and methods of estimation. In Methods of physico-mechanical testing of plastic foams, pp. 48-53. Moscow NIITEKhIM 1976, (in Russian)... 

Water absorption 24 h—ASTM D570. The specimens for this test are 0.125 in thick and 2.00 in in diameter for molding materials. The material is submerged in water and the increase in weight is measured. This test can be performed with several procedures ranging fi om 1 h in boiling water to 24 h in water at 73.4°F. The physical and electrical properties of plastics can be affected by moisture absorption. The value for this resin, 0.22%, is quite low, too low to have a significant effect on properties. 

Standard testing of electrical properties of plastics includes dielectric strength, permittivity, dissipation factor, smface and volume resistivity, and arc resistance. Dielectric strength is the maximmn voltage required for breakdown and is determined by one of three techniques short-time, slow-rate or slow-rise, and step-by-step. The two last techniques use data from the short-time test to... 

Organizations involved in conducting and/or preparing specifications/ standards on the electrical properties on plastics include the Underwriters Laboratories (UL), American Society for Testing and Materials (ASTM), Canadian Standards Association (CSA), International Electrotechnical Commission (lEC), International Organization for Standardization (ISO), and American National Standards Institute (ANSI). 

A true material comparison is possible only when property values are determined by identical test methods under identical conditions (1). Generally speaking, physical and electrical properties of plastics and electrical insulating materials are affected by temperature and humidity. Plastic materials tested above room temperature will yield relatively higher impact strength and lower tensile strength and modulus. High humidity tends to alter the electrical property test results. Obviously, in order to make reliable comparisons of different materials and test results obtained by different laboratories, it is necessary to establish standard conditions of temperature and humidity. 

In the United States, ASTM Committee D 09, on Insulating Materials, and Committee D 20 on plastics, have the primary responsibility of issuing standards for the evaluation and specification of plastics. Pertinent ASTM test methods and specifications pertaining to the electrical properties of plastics are given in Table 1. 

To determine the electrical properties of plastics in a fashion useful for engineering purposes, it is necessary to duplicate conditions of service as closely as possible. However, these conditions are not always known, and can never be completely duplicated in the laboratory. In fact, simple rather than complex and lengthy tests are frequently justified. Electrical properties should be detei mined as a function of variables such as temperature and time. Time is an important variable in voltage degradation and moisture exposure. Properties should be given in the form of a curve where possible. Some plastics manufacturers supply such data. 

Another reason for using higher temperatures is that for an application requiring long-term exposure a candidate plastic is often required to have an RTI value higher than the maximum application temperature. The properties tested can include mechanical strength, impact resistance, and electrical characteristics. A plastic s position in a test s RTI is based on the temperature at which it still retains 50% of its original properties. 

Procedure for conditioning test specimens can call for the following periods in a standard laboratory atmosphere [50 2% relative humidity, 73.4 1.8°F (23 1°C) Adequate air circulation around all specimens must be provided. The reason for this test is due to the fact the temperature and moisture content of plastics affects different properties such as the physical and electrical properties. In order to get comparable test results at different times and in different laboratories a standard has been established. 

MPa (264 psi)J, good electrical properties, good environmental stress-crack resistance (relative to other amorphous plastics), and low flammability based on standard laboratory tests. 

ASTM Committee D-20 on Plastics has subcommittees working on testing methods for strength, hardness, thermal, optical, and permanence properties, and analytical and molding procedures. Testing methods for electrical properties are under the jurisdiction of Committee D-9 on Electrical Properties. Some methods of immediate interest to those... 

This practice covers the determination of the effect of fungi on the properties of synthetic polymeric materials. Technically, any composite material, shape and profile can be tested in accordance with this practice. Any particular property of plastics can be chosen as a readout, such as changes in optical, mechanical, and electrical properties. 

Electrical property measurement is relevant to all the polymer types, although most applications are for plastics or rubbers. Rubbers hold a particular interest because of the unique combinations of electrical properies and low stiffness that can be achieved. With respect to testing, the relative softness of rubbers and foams has to be considered when choosing electrode systems. In addition to the resistance of the polymer, there is a contact resistance at the electrode, test piece interface that can be very high, and this is a complication in testing and an important factor in the choice of electrodes. 

BS2782. Methods of testing plastics Part 2. Electrical properties. Method 230A. 

Chapter 11 contains mechanical and electrical data on hundreds of plastics in tabular form. These data are described in the standard ISO 10350-1 1998, which is titled Plastics—Acquisition and Presentation of Comparable Single-point Data— Part 1 Moulding Materials. This standard ISO 10350 identifies specific test procedures for the acquisition and presentation of comparable data for many basic properties of plastics. The properties included are often in manufacturers technical data sheets. All the data are defined by ISO standards rather than ASTM standards. Part 1 applies to unreinforced and reinforced thermoplastic and thermosetting materials. While similar ASTM standard-... 

BS 4618, Recommendations for the Presentation of Plastics Design Data. Part 2. Electrical Properties, Section 2. 3 Volume Resistivity Section 2.4 Surface Resistivity (London, 1975) BS 5102, Phenolic Resin Bonded Paper Laminated Sheets for Electrical Applications (London, 1974) BS 5762, Methods for Crack Opening Displacement Testing (British Standards Institution, London, 1979)... 

Water absorption is the percentage increase in weight of a plastic due to its absorption of water. Standard testing procedures, such as twenty-four hours of immersion, are prescribed in ASTM standards. The tests are conducted for different lengths of time and at varying temperatures, as well as with different solutions. All this seeming redundancy is important with certain plastics, since water absorption can affect mechanical and electrical properties as well as the dimensions of parts. Plastics with very low water-absorption rates tend to have better dimensional stability. 

The usual plastics are good insulators, however there are plastics that conduct electricity using certain plastics but more so by the addition of fillers such as carbon black and metallic flake. The type and degree of interaction depends on the polarity of the basic plastic material and the ability of an electrical field to produce ions that will cause current flows. In most applications for plastics, the intrinsic properties of the plastics are related to the performance under specific test conditions. 

One of the main reasons why plastics have become so widely used in electronics and electrical applications is that they are good dielectric materials with readily controllable electrical properties. However, the fact that plastics are good insulators does not mean that they are inert in an electrical field and the intrinsic properties of a material can usually be related to performance under specific test conditions. The electrical properties that have to be considered when selecting a material are the dielectric constant, the volume resistivity, surface resistivity, dissipation, power and loss factors, arc resistance and dielectric strength. When selecting a plastic for a specific application the variation of these properties with changes in the likely service environment has to be taken into account since many of them are temperature, frequency, voltage and environmentally variable. 

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