Flame retardancy process can be characterized either in gas phase, by investigating present pyrolysis species, or in solid phase, by studying the morphology and composition of the char layer. There are numerous macro and micro fire characterization methods. Limiting oxygen index (LOI), UL-94, cone calorimetry, microscale calorimetry, and thermogravimetric analysis (TGA) are of the most common fire characterization methods.
LOI is one of the primary methods that has been used for many years to investigate the relative flammability of materials. Material with LOI less than 21% can burn easily whereas materials with LOI greater than 21% exhibit reduced flammability after removal from ignition source. LOI requires a cost-effective setup and small sample size. However, due to the high oxygen index simulation and small scale input heat, it is not very suitable for determining the real extent of fire performance.
UL-94 tests have been considered for measuring burning rate and characteristics of plastics. UL94 vertical test is widely used for determination of ignitability and flame spread rate of plastic materials. In this test, the specimen is burnt using specific flame conditions for a certain time period. The time required for the fire to be extinguished (after-flame removal) is an indication of fire retardancy properties of the specimen.
As one of the most important fire characterization methods for polymeric materials, cone calorimetry measures the reducing oxygen concentration in the combustion gasses of a specimen subjected to a given heat flux. After exposing the sample to a conical radiant electrical heater, the combustion is triggered by an electric spark. Variety of data, such as, heat release rate (HRR) as a function of time, peak of heat release rate (pHRR), total heat release rate (THR), and mass loss rate (MLR) can be reported by this test.
Microscale calorimetry test provides a rapid, cost-effective method for academic and industrial polymer research. Studies suggest the potential of this method for quantitative determination of full-scale fire performance for milligram samples. Samples are thermally decomposed at a constant heating rate. Then, combustion products from solid state pyrolysis are removed using nitrogen and then mixed with excess oxygen to ensure complete combustion. The oxygen exhaustion is measured by analyzing the remaining oxygen/nitrogen mixture after the elimination of combustion volatiles from the gas stream. The heat release rate and other flammability parameters, such as, heat release capacity and char yield are among the outcomes of microscale calorimetry.
TGA measures the changes in weight of materials as a function of temperature based on ASTM E1131 and ISO 11358 standards. Weight loss at onset temperature (Tonset) and percentage of the weight that remains at the end of the heating process (including mass of char residue) are obtained by TGA. Materials with higher Tonset produce less fuel for the combustion and therefore have better flame retardancy properties.
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