POLYMER FLAMMABILITY (PART 2)
LIMITED OXYGEN INDEX
The oxygen index represents the minimum level of oxygen in the atmosphere which can sustain flame on a thermoplastic material.
A sample is positioned vertically in a tube and a mixture of oxygen and nitrogen is forced upward. The sample is ignited at the top. The concentration of oxygen is adjusted until the sample just supports combustion. The Limited Oxygen Index is defined as the minimum concentration of oxygen in which the material will burn for three minutes, or can keep the sample burning over a distance of 50mm. The higher the value the less likelihood of combustion.
There are two applicable standards – ASTM D2863 and ISO 4589-1, -2. Both are similar.
GLOW WIRE (IEC 695-2-1)
The glow wire test simulates what may occur if the material is exposed to sources of heat, such as overloaded resistors or glowing elements.
A sample is held vertically for 30 seconds with a force of one Newton against the tip of an electrically heated glowing wire. To evaluate the likely spread of fire either a layer of the sample material or a single layer of tissue paper is positioned underneath the sample. After withdrawing the sample, the time for extinguishing flames, plus the presence of any burning drops are recorded. The sample is considered to have passed if one of the following requirements is met:
- There is no flame and no glowing
- If there are flames or glowing of the sample, or the surroundings and layer below, extinguish within 30 seconds of the removal of the glow wire, and the surrounding parts and the layer below have not burned away completely. When a layer of tissue paper is used, there shall be no ignition of this paper nor scorching of the pine wood board
Actual parts or enclosures are tested in a similar way. Temperature of the glow wire is dependent on how the finished part will be used:
- Attended or unattended
- Continuously loaded or not
- Used near or away from a central supply point
- In contact with a current carrying part or used as an enclosure or cover
- Under less or more stringent conditions
The temperatures used range from 550˚C to 960˚C.
NEEDLE FLAME (IEC 695-2-2)
This test simulates the effect of small flames that may result from faults within electrical equipment. The test procedure is similar to the Glow Wire test. The flame is applied to the sample for a specified period of time, usually 5, 10, 20, 30, 60 or 120 seconds.
The sample is considered to have passed if one of the following requirements is met:
- The sample does not ignite
- If flames or burning/glowing particles falling from the sample do not spread fire to the surrounding parts or to the layer placed below the sample, and if there is no flame or glowing of the sample at the end of the application of the flame
- The duration of the burning is less than 30 seconds
- If the extent of burning specified in the relevant specification has not been exceeded