Since the introduction of PTFE in 1960s, there are many kinds of resins in China, but there is still a certain gap between PTFE and foreign advanced products in terms of quality and processing performance. Driven by innovation and development, ptfe new products are developing rapidly to the middle and high end. As we all know, ptfe has many excellent properties, such as high temperature resistance, low temperature resistance, climate resistance, high insulation, high lubrication, wear resistance, corrosion resistance, non-adhesion and non-toxicity and so on. Facing different customers, we usually process PTFE into different forms of products, such as PTFE rods, PTFE tube, Teflon plate, Teflon film, PTFE tape and so on. In order to ensure that these different forms of products can ensure its inherent excellent performance, we need to have a quantitative index to meet the needs of each method of use. This requires the performance testing of these molded products. Evaluate products by inspection and testing. In addition, in the development of new products at the same time, also need to conduct performance testing products. Therefore, the performance test of ptfe products can not only control the processing technology and product quality, but also have important significance for the research of new products.
Performance test of ptfe products
According to the national polytetrafluoroethylene product inspection standard, at present many manufacturers by reference to the following standards for testing.
QB/T 3624-1999 teflon pipe
QB/ t3625-1999 teflon board
Q B/T3626 -- 1999 teflon bar
QB/T 3627-1999 teflon film
QB/T 4876-2015 ptfe turning film
Some manufacturers have adopted new standards, such as QB/T 4041-2010 ptfe bar (instead of QB/T 3626-1999 ptfe bar), QB/T 4877-2015 ptfe pipe (instead of QB/T 3624-1999 ptfe pipe), etc., but the main test items are basically the same. Mold products inspection and testing items are: relative density, tensile strength, elongation at break, breakdown voltage, etc. The main test items of push products are: relative density, tensile strength, elongation at break, breakdown voltage. The main testing items of film and tape products are: tensile strength, elongation at break, dielectric constant, tangent of dielectric loss Angle, breakdown voltage.
1. Relative density
1) definition: the ratio of the mass of a substance in a certain volume to the mass of a reference substance at the same temperature. When the reference material is water, it is called the relative density.
2) the relative density of ptfe products is a very important physical quantity. It reflects the physical structure and quality of PTFE products. The pressing molding technology has a great influence on the relative density.
3) test method: according to GB1033 test method for density and relative density of plastics, the test temperature was 23℃±2℃, and the reference material was distilled water.
2. Tensile strength and elongation at break
1) tensile strength is the maximum tensile stress borne by the sample until fracture in the tensile test.
Elongation at break is the ratio of the increment of the distance between standard lines and the initial standard distance when the sample breaks in the tensile test, expressed as a percentage.
2) test items: the tensile strength and elongation at break of ptfe products can be judged as unqualified products no matter which of these two indexes does not meet the specified technical assessment indexes. The test results of tensile strength and elongation at break of ptfe products are relatively high. The test results of tensile strength and elongation at break are also higher for high crystallinity products. However, it has a higher elongation at break. In practical applications, such as ptfe as wear-resistant products, like a number of gaskets or supports, the general requirements of the crystallinity of its products is higher. In terms of test items, it has a relatively high elongation at break. Russia, for instance, the standard Г OCT1007-80 "teflon technical requirements" in specific provision: the tensile strength is not less than 24.5 MPa, the elongation at break is not less than 350%, the index than the HG/T 2902-1997 molding the elongation at break of polytetrafluoethylene resin standard: not less than 250% of the index is much higher. The density and crystallinity of polytetrafluoroethylene products are relatively high.
In another case, the product reaches a lower crystallinity by controlling the temperature drop of the product. The result of this test is high tensile strength, low elongation at break. This is a hardened product. PTFE quenching products are commonly used as mechanical seals.
Polytetrafluoroethylene (ptfe) films have different properties due to their different molecular orientation. Generally, the greater the longitudinal molecular orientation of ptfe film, the higher its longitudinal tensile strength and the lower its longitudinal elongation at break, and the lower its corresponding transverse tensile strength and the higher its transverse elongation at break. Therefore, the vertical and horizontal technical requirements must be considered comprehensively when designing the production process. For example, the Russian standard ty301-05-49-90 stipulates that the longitudinal tensile strength of PTFE calendered belt should not be less than 12.7mpa and the longitudinal elongation at break should not be less than 100%, while the transverse tensile strength should not be less than 1.5mpa and the transverse rupture elongation should not be less than 600%. The calendered belt is very suitable as the covering material of wire and cable, and it has its own self-adhesive energy, so it does not need to be sintered again. Ptfe - calendering tape is mainly used as cover material for wire and cable abroad. Of course, ptfe calendered tape is not equal to the ptfe calendered tape defined in QB/ t3628-99 "ptfe raw tape for thread sealing".
3) test method: through tensile test, static tensile load is applied to the sample along the longitudinal axis to destroy it. The tensile strength and elongation at break of the sample are obtained by measuring the destructive force of the sample and the elongation between the sample marks.
3. Voltage resistance and breakdown voltage intensity
1) definition: the withstand voltage is the highest voltage that the sample does not break down under the specified test conditions when the specified voltage and time are applied to the test.
Breakdown voltage intensity refers to the breakdown voltage per unit thickness when the sample is broken down. KV/mm, or MV/m. This quantity is sometimes referred to as dielectric strength or electrical strength.
2) test item description: ptfe is a material with excellent insulation performance. However, it is also an insulator within a certain voltage range. With the increase of applied voltage, the insulation performance will gradually decline. When the voltage rises to a certain value it becomes locally conductive and the material is broken down. As insulation material, it is very important to test the voltage resistance or breakdown voltage of ptfe products. In the actual test, ptfe board is usually only tested for voltage resistance. Microtubes and films. Current standards require breakdown voltage strength testing.
The voltage resistance or breakdown voltage intensity of ptfe products is directly related to the processing environment of the products. Under the same conditions, if the processing environment is not strictly controlled, the voltage resistance or breakdown voltage intensity of the products can not meet the technical requirements. Therefore, the production site of ptfe products must meet the specified environmental requirements. Environmental controls must be in place.
The breakdown voltage strength of ptfe film is related to its directivity. The higher the orientation (that is, the orientation of the molecule), the higher the breakdown voltage intensity of the film. When users need voltage resistant film products, when other conditions are certain, the general use to improve the orientation of the film. As for the orientation of the film, it can be determined by exploration. The control of film orientation is an important parameter in ptfe film processing. Most of the coating materials used in the wire and cable industry in China are teflon semi-directional film, which is determined by the production needs of the wire and cable industry. Different manufacturers have different processes and require different directionality, generally between 1.4 and 2.0.
The relationship between the breakdown voltage performance of ptfe film and tensile strength and elongation at break is as follows: high breakdown voltage strength means high tensile strength and low elongation at break. For example, the tensile strength of the thin film used for ct should be more than 70MPa, and the breaking elongation should be less than 60%. This kind of ptfe film can only meet the requirements of users with the performance of breakdown voltage.
3) test method: test according to GB1408 "test method for power frequency and electrical strength of insulating materials".
4. Tangent value of dielectric constant and dielectric loss Angle
1) by measuring the tangent value of dielectric loss Angle and the dielectric constant, the factors affecting dielectric loss and dielectric constant can be further understood, providing a basis for improving the properties of materials. The permittivity is the ratio of capacitors of the same size made from insulating materials and vacuum materials. (the relative permittivity of air is 1.00053 at standard atmospheric pressure. Therefore, in fact, capacitors with air as the reference can be used to determine the relative permittivity with sufficient accuracy.) The polarity of polymer material can be determined according to its dielectric constant. In general, substances with dielectric constant greater than 3.6 are polar substances. Substances with dielectric constant in the range of 2.8~3.6 are weakly polar substances. A nonpolar substance has a dielectric constant less than 2.8.
The tangent value of dielectric loss Angle, also known as dielectric loss Angle tangent, refers to the energy consumed by the dielectric to convert electrical energy into heat energy (in the form of heating) per unit volume in unit time. Delta is the dielectric loss Angle, which is the physical quantity representing the dielectric loss when the electric field is applied. The dielectric loss Angle is the Angle between the current vector and the voltage vector flowing through the dielectric under the alternating electric field. The tangent of the excess Angle between the applied voltage and the current of the same frequency. The tangent value of the dielectric loss is equal to the energy of the dielectric loss per period divided by the energy stored in the dielectric during the period.
2) the test shows that dielectric constant and tangent of dielectric loss Angle are two important indexes of insulation materials. As capacitors, large dielectric constants are required to reduce the size of the element. As a component in the network, a small dielectric constant is required to reduce the capacitance between conductors and thus reduce the charging times. The dielectric constant is related to the density of the dielectric material. For the same material, the dielectric constant is low when the density is low. A capacitor plate is filled with a material with a dielectric constant of PI. The dielectric has the property of making space larger or smaller than it really is. For example, when a dielectric material is placed between two electric charges, it reduces the force acting on them as if they were moved away.
The tangent value of dielectric loss Angle. The polymer materials are mostly of good insulation materials, and the insulation material itself is not expected to lose much energy when used, so the dielectric loss factor can be measured to evaluate the dielectric energy loss of the material itself. In industry, polymer materials with low dielectric loss factor are often used as insulation materials. Due to material in electric field under the action of internal fever, in general, the internal heating materials under high frequency high voltage will increase rapidly, often requires a small dielectric loss tangent value, so why teflon wire and cable is better than that of PVC wire and cable, in addition to the teflon wire and cable heat level is higher than PVC wire and cable, another reason is the teflon wire and cable dielectric loss tangent value is far smaller than the PVC wire and cable, higher safety and reliability in use. The tangent of dielectric loss Angle characterizes the ratio of dielectric loss energy to its storage energy in each cycle.
3) test method: according to GB1409 test method for relative dielectric constant and tangent factor of dielectric loss Angle of solid insulation materials at power frequency, audio frequency and high frequency.
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