POLYMER FLAMMABILITY (PART 1)
UL 94 ratings are the most widely accepted standards covering flammability of plastic materials. These are intended to indicate a material’s ability to extinguish a flame once ignited. Several ratings can be applied depending upon the performance, and each material may receive different ratings based upon thickness and/or colour.
HORIZONTAL TESTING (UL 94HB)
A test sample is supported in a horizontal position and tilted at 45 deg. A flame is applied to the end of the sample for 30 seconds or until the flame reaches the 1” (25mm) mark. The flame is then removed. If the sample continues to burn, the time for the sample to burn between the 1” (25mm) and 4” (102mm) marks is recorded. If the sample stops burning before reaching the 4” (100mm) mark the time of burning and the length of damaged material between the two marks is recorded.
Three specimens are tested for each thickness. Requirements:
- Samples must not have a burn rate greater than 1.5” (38mm) per minute for thicknesses between 0.12” (3mm) and 0.50” (12.5mm) and 3” (76mm) per minute for thicknesses less than 0.12” (3mm)
- Samples must stop burning before the flame reaches the 4” (102mm) mark
Where flammability is a safety requirement, HB materials are not normally permitted. Generally, HB classified materials are not recommended for electrical applications except for mechanical and/or decorative purposes.
VERTICAL TESTING (V-0, V-1, V-2)
A sample is supported vertically and a flame applied to the bottom for 10 seconds. The flame is removed until the flaming stops at which point the flame is applied for a further 10 seconds. Two sets of five samples are tested. A specimen of cotton is placed below the test sample.
Requirements:
V-0
- Samples must not burn with a flame for more than 10 seconds after either test flame application
- Total flaming combustion time must not exceed 50 seconds for each set of 5 samples
- Samples must not burn with flaming or glowing combustion up to the specimen holding clamp
- Samples must not drip flaming particles that ignite the cotton
- No sample can have glowing combustion remain for longer then 30 seconds after removal of the test flame
V-1
- Samples must not burn with a flame for more than 30 seconds after either test flame application
- Total flaming combustion time must not exceed 250 seconds for each set of 5 samples
- Samples must not burn with flaming or glowing combustion up to the specimen holding clamp
- Samples must not drip flaming particles that ignite the cotton
- No sample can have glowing combustion remain for longer than 60 seconds after removal of the test flame
V-2
- Samples must not burn with a flame for more than 30 seconds after either test flame application
- Total flaming combustion time must not exceed 250 seconds for each set of 5 samples
- Samples must not burn with flaming or glowing combustion up to the specimen holding clamp
- Samples can drip flaming particles that ignite the cotton
- No sample can have glowing combustion remain for longer than 60 seconds after removal of the test flame
VERTICAL TESTING (5V, 5V-1, 5V-B)
This is the most severe of all the UL tests with testing carried out on both bar and plaque samples. A bar sample is supported in a vertical position and a flame applied to one of the lower corners at a 20 deg angle. The flame is applied for 5 seconds and then removed for 5 seconds. This procedure is repeated 5 times.
The same test is carried out for plaques except the sample is mounted horizontally and a flame applied to the centre of the lower surface. Requirements:
5V
- Samples must not have flaming or glowing combustion for more than 60 seconds after the five flame applications
- Samples must not drip
- Samples must not be destroyed in the area of the flame
5V-A
- Samples must not have flaming or glowing combustion for more than 60 seconds after the five flame applications
- Samples must not drip flaming particles that ignite the cotton
- Samples must not exhibit burnthrough (a hole)
5V-B
- Samples must not have flaming or glowing combustion for more than 60 seconds after the five flame applications
- Samples must not drip flaming particles that ignite the cotton
- Plaque samples may exhibit burnthrough (a hole)li>
VERTICAL TESTING OF THIN MATERIALS (VTM-0, VTM-1, VTM-2)
There is also a test for materials that are thin or too flexible for ordinary vertical testing. The material is formed into a cone and tested in a similar way to the vertical testing of a bar. As this is not relevant to injection moulding thermoplastics we shall not go into more detail.
