Test Method for Plastics Flammability-Oxygen-index Method
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Test Method for Plastics Flammability-Oxygen-index Method
When Plastics is heated by external heat sources or fire sources it will degrade and generate volatile products, flammable products, and thermal-energy products, if the action of external heat source is sustained, and a certain temperature is achieved it will burn. Part of the heat in its burning is absorbed by the degrading plastics though conduction, radiation, and convection, so that more flammable products  volatile out; the burning process increases the fluctuation of peripheral air, and increases the mixing degree of flammable volatile products with air, leading to more severe burning.
The burning of masses needs to consume a lot of oxygen, and different combustibles need different oxygen consumption to burn. So it is feasible to evaluate the flammability of masses based on calculating the oxygen-index values of the masses, which will be obtained through measuring the minimum oxygen consumption during the combustion of the masses. Oxygen-index refers to under the specified testing conditions the least oxygen concentration necessary for the sample to maintain steady burning (flaming burning) in the mixture of oxygen and nitrogen, which is indicated with the volume percentage occupied by the oxygen (i.e. the volume percentage concentration of the minimum oxygen in the mixed gas of oxygen and nitrogen necessary for maintaining the burning for 50mm of length or 3 minutes). It is an indicative method for evaluating the relative flammability of plastics and other polymers, and can be used to judge   the difficulty level of material’s combustion in air when it contacts with fire. Generally the materials with oxygen-index less than 27 belong to inflammable ones, the materials with oxygen-index equal or more than 27 and less than 32 belong to flammable ones, and the materials with oxygen-index equal or more than 32 belong to fire retarding ones
The procedure of oxygen-index test method is as follows: a sample with a certain size is vertically clamped within a transparent combustion cylinder with a sample clamp, in which the mixed gas of oxygen and nitrogen with a certain ratio is flowed from the bottom to the top, then observing the burning behavior after igniting the top end of the sample, and recording the time of continuous burning or the burning distance. When the burning time of the sample is more than 3 minutes or the flame front is over the marking line of 50 mm, the oxygen concentration is decreased, contrarily, if the burning time of the sample is less than 3 minutes or the flame front does not arrive the marking line, the oxygen concentration is increased, and repeating like this, so as to approach the specified value gradually from both bottom and top sides until the concentration difference of both sides is less than 0.5%. A set of samples are tested in different oxygen concentrations, when the measured plastics is just maintaining steady burning the minimum oxygen concentration is measured. And it is indicated with the volume percentage of the oxygen content in the mixed gas.   
The testing equipment for oxygen index owned by the InformationTransmissionLinesQualityInspectTestCenter, M. I. I. (the Inspection and Test Center of No 23 Research Institute, CETC) is suitable for the flammability testing of plastics, rubber, fiber, foam plastics, and various solids, and can make exact and rapid flammability testing and evaluation of many materials. At present the testing item associated with the flammability of cable and other materials is in a favorable price, so manufacturers in the industry are welcome to be our Inspection and TestCenter to test your products we will provide you with quality service. 
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