Test method for combustion performance of plastics - oxygen index method

　　Plastics are degraded by heating from external heat sources or fire sources in the air, producing volatile products, combustible products and thermal energy products. Under the continuous action of external heat sources, they will burn when a certain temperature is reached. Part of the heat released by combustion is absorbed by the degrading plastic through conduction, radiation and convection, and more combustible products are volatilized. The combustion process intensifies the disturbance of the air around the flame, increases the degree of mixing of combustible volatiles and air, and makes the combustion more intense.

　　The combustion of substances requires a large amount of oxygen. Different combustibles consume different amounts of oxygen during combustion. By measuring the minimum amount of oxygen consumed during the combustion of substances, the oxygen index value of the substance is calculated to evaluate the combustion performance of the substance. The oxygen index refers to the minimum oxygen concentration required for the sample to maintain stable combustion (i.e., flame combustion) in a mixed gas flow of oxygen and nitrogen under specified test conditions, expressed as the volume percentage of oxygen (i.e., the minimum volume percentage concentration of oxygen in the oxygen and nitrogen mixed gas required to maintain combustion for 50 mm or 3 minutes after the substance is ignited). It is a method of evaluating the relative combustibility of plastics and other polymer materials, as a way to judge the difficulty of burning when the material comes into contact with flames in the air. It is generally believed that oxygen index ﹤27 is a flammable material, 27≤ oxygen index ﹤32 is a combustible material, and oxygen index ≥32 is a flame-retardant material.

　　The oxygen index test method is to clamp a sample of a certain size vertically in a transparent combustion tube with a sample clamp, in which there is a mixed flow of oxygen and nitrogen mixed from bottom to top in a certain proportion. After igniting the upper end of the sample, observe the subsequent combustion phenomenon and record the continuous burning time or burning distance. When the burning time of the sample exceeds 3 minutes or the flame front exceeds the 50mm mark, reduce the oxygen concentration. When the burning time of the sample is less than 3 minutes or the flame front does not reach the mark, increase the oxygen concentration. Repeat this operation, gradually approaching the specified value from the upper and lower sides until the concentration difference between the two is less than 0.5%. Test a group of samples in different oxygen concentrations, and determine the lowest oxygen concentration when the plastic just maintains stable combustion, expressed as the volume percentage of oxygen content in the mixed gas.