Generally speaking, the flame retardant grade is determined based on the flame retardant performance of the material under specific conditions. For plastic materials, such as PVC, the flame retardant rating is usually evaluated according to international standards (such as UL 94), which take into account the performance of the material in terms of flame contact, flame spread and dripping. The flame retardant rating of wires is usually determined based on specific industry standards (such as UL 1581). These standards may pay more attention to the performance of the material in electrical equipment, such as heat resistance and arc resistance.
What is UL94？
Flammability UL94 rating is the most widely used flammability performance standard for plastic materials. It is used to evaluate the ability of a material to extinguish after being ignited. There are many evaluation methods based on burning speed, burning time, anti-drip ability and whether the dripping beads burn. Many values can be obtained for each material being tested based on color or thickness. When selecting a material for a product, its UL rating should meet the thickness requirements of the wall portion of the plastic part. The UL rating should be reported together with the thickness value, reporting the UL rating without thickness is not sufficient.
The plastic flame retardancy ratings increase progressively from HB, V-2, V-1, V-0, 5VB, to 5VA:
HB: The lowest flame retardancy rating according to UL94 standard. Requires the burning speed to be less than 40 millimeters per minute for samples with thicknesses of 3 to 13 millimeters, or less than 70 millimeters per minute for samples thinner than 3 millimeters, or extinguishing before reaching a 100-millimeter mark.
V-2: After two 10-second burning tests, the afterflame and afterglow must extinguish within 60 seconds. Droplets from the burning sample can ignite cotton.
V-1: After two 10-second burning tests, the afterflame and afterglow must extinguish within 60 seconds. Droplets from the burning sample cannot ignite cotton.
V-0: After two 10-second burning tests, the afterflame and afterglow must extinguish within 30 seconds. Droplets from the burning sample cannot ignite cotton.
5VB: After five 5-second burning tests, the afterflame and afterglow must extinguish within 60 seconds. Droplets from the burning sample cannot ignite cotton. Block-shaped samples are allowed to be penetrated by the flame.
5VA: After five 5-second burning tests, the afterflame and afterglow must extinguish within 30 seconds. Droplets from the burning sample cannot ignite cotton. Block-shaped samples are not allowed to be penetrated by the flame.
Test Methods
UL94 commonly used flammability tests include the following two test methods:
1. Horizontal combustion measurement method for materials classified as UL94HB.
2. Vertical burning test method for materials classified as UL94V-0 ULV-1 ULV-2.
The burning modes for wires in UL1581 are as follows:
VW-1: Vertical burning test (UL wire burning rating).
FT1: Vertical burning test.
FT2: Horizontal burning test.
FT4: Vertical burning test.
FT6: Horizontal burning and smoke emission test (FT-class burning ratings are CSA wire burning ratings).
Among these levels, VW-1 is equivalent to FT1, FT2 is the lowest level, and the flame retardancy levels in ascending order are FT6, FT4, VW-1, FT1, and FT2. Although W-1 and FT1 are the same level, VW-1 is stricter than FT1.
Criteria for judgment:
A. The burning mark (parchment paper) should not be charred by more than 25%.
B. The afterflame time for 5 consecutive 15-second burns should not exceed 60 seconds.
C. Burning droplets should not ignite cotton.
VW-1 requires compliance with A, B, and C; FT1 only requires compliance with A and B.
For the wire industry:
UL94’s V-2, V-1, V-0, 5VA, 5VB assess the materials used for wires. During testing, standard samples are made separately from the wire and are not labeled on the wire itself.
UL1581’s VW-1, FT-1, FT-2 assess the wires themselves. After passing the test, corresponding ratings can be labeled on the wire.
Fire resistance for wires in UL1581 and UL94 are completely different. An insulation material may pass UL94’s V-0 but may not pass UL1581’s VW-1/PT1.
There are significant differences between the VW-1/PT1 test and the V-0 test in UL94:
1. Flame height and temperature are different.
2. Different methane flow rates are used during testing.
3. Different methane back pressures are applied.
4. The volume of the combustion chamber differs: VW-1 requires greater than 4 cubic feet, while V-0 only requires greater than 0.5 cubic feet.
5. The number of burning cycles is different.
6. In the burning results: V-0 requires recording the residual burning time, while W-1 does not.
However, the test conditions and methods for the 5V level burning in UL94 and VW-1 burning are similar and relatively close.
Note: UL1581 and UL94 differ only in test methods. Both sets of testing equipment must adhere to ASTM5025 and ASTM5027 designs.