Most plastics are carbon-based materials and will burn and give off gases and smoke when subjected to a flame. Plastics are excellent fuels but are generally classed as ordinary combustibles and fall into the same category as wood, leather and many other common materials.
All of these materials will degrade at very high temperatures into volatile and gaseous combustion products.
Testing for Flammability：
As with any testing, the tests for flammability are designed for the laboratory and quality control.
In real fires plastics may behave significantly differently and the results of laboratory testing cannot predict the actual performance of a particular plastic or product.
Limiting Oxygen Index (LOI)：
The limiting oxygen index is a measure of the percentage of oxygen that has to be present to support combustion of the plastic- the higher the LOI the lower the flammability. Air contains approximately 21% oxygen and therefore any material with an LOI of less than 21 will probably support burning in an open-air situation.
In the LOI test, a candle like sample is supported in a vertical glass column and a slow stream of oxygen/nitrogen mix is fed into the glass column. The sample is ignited with a flame and burns downward into unheated material. The oxygen/nitrogen ratio can be varied and the test records the minimum concentration of oxygen (as a percentage) that will just support combustion.
Limiting Oxygen Index The results for the test will vary considerably with the composition of the specific plastic being tested. The LOI value is a basic property of the plastic but tells us nothing about how the plastic will react to burning in an open atmosphere.
Underwriters Laboratory (UL94)：
UL testing is a method of classifying a material’s tendency to either extinguish or spread a flame once it has been ignited and although originally developed by UL, it has now been incorporated into many National and International Standards (ISO 9772 and 9773). The testing involves applying a flame to a sample in various orientations and assessing the response of the material after the flame is removed. Materials that burn slowly or selfextinguish and do not drip flaming material will rank highest in the UL classification
Horizontal Burning Test (94HB)：
This is the easiest test to pass and materials that pass the vertical burning test will usually be acceptable for applications that require a HB r only.
The test involves applying a flame to one end of a horizontal bar of the plastic for 30 seconds or until the flame front reaches the specified mark. If burning continues then the time taken to reach the second mark is measured. If burning stops then the time of burning and the damaged length are measured.
A material will be classified 94HB if the time taken to reach the second mark is greater than the specified minimum or if burning stops before the second mark.
Vertical Burning Test (94V)：
This is a more demanding test than the HB test because the specimen is tested in the vertical orientation and the material burning at the lower end of the sample preheats the material in the upper areas of the specimen. A test flame is applied to the lower end of the test specimen and the material is classified according to the table below. UL94V requires materials to be self-extinguishing to pass the test.
Other UL Tests (94VTM, 94-5V, 94HBF and radiant panel)：
UL94 contains a range of tests for a variety if materials and scenarios such as 94VTM for thin materials and surface burning but these are less commonly used than the 94HB and 94V test methods.
The flammability and burning response of plastics is a complex subject and these properties vary greatly with the type of plastic and the detailed additives used in the plastic. This paper can only touch on some of the most important topics for the designer and engineer in an effort to give some broad guidance.
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