Extinguishing the Confusion: UL94 V vs UL94 VTM
While there are many standards that can be followed for flame retardancy, the UL94 standard is a common one used in the plastics industry. UL94 isn’t just one test, but rather multiple tests that cover a wide range of sample types. Commonly, plastic products may have a UL94 rating, such as UL94 V-0 or UL94 VTM-2. What do these mean? Let’s take a look at how UL94 V differs from UL94 VTM and what these ratings mean.
Below in Table 1, the two methods are compared to highlight the differences in the test method.
TABLE 1:
| UL94 V | UL94 VTM | |
|---|---|---|
| STANDS FOR… | VERTICAL | VERY THIN MATERIAL |
| Purpose | To evaluate the flammability response to small-scale testing of polymeric materials, used as parts in appliances and devices, when subjected to an open flame or radiant heat source | |
| Sample size | 125 mm x 13 mm, ≤ 13 mm thick | 200 mm x 50 mm, ≤ 3 mm thick |
| Number of samples | 2 sets of 5 | 2 sets of 5 |
| Sample positioning | Suspended from a clamp | Rolled around ½” mandrel and taped on one side, then tape-side suspended from a clamp |
| Conditioning | 48 hrs/ 23°C/50% RH (set 1) 168 hrs/70°C + 4 hrs desiccation (set 2) |
48 hrs/ 23°C/50% RH (set 1) 168 hrs/70°C + 4 hrs desiccation (set 2) |
| Height of blue flame | 20 mm | 20 mm |
| Angle of the flame | 0° | 45° |
| Number of flame applications | 2 | 2 |
| Duration of each flame application | 10 seconds | 3 seconds |
| Bottom edge distance from burner | 10 mm | 10 mm |
| Suitable sample types | Self-supporting samples | Samples that shrink or distort in the presence of the flame |
Some considerations need to be made when understanding how to apply these testing conditions to your materials. UL94 V is a precursor to UL94 VTM. Meaning, that a sample would need to be tested using the UL94 V test procedure and fail, before it can move on to the UL94 VTM method. Another item worth mentioning is, what these tests can tell you about your material. These methods demonstrate the material’s ability to extinguish a flame and the behavior of a material under a controlled flame.
In Tables 2 and 3, the criteria for each of the tests are listed. At first glance, the tables appear to be the same. While the criteria are almost identical, there is one distinct difference. The distance the flame can travel in UL94 V is just before it reaches the clamp holding the sample. If the flame reaches the clamp, the sample fails to meet any UL94 V criteria. For UL94 VTM, the samples are marked with a line 125 mm from the bottom edge. If this marking is reached by the flame, the sample has failed to meet any of the UL94 VTM criteria.
TABLE 2:
| UL94 V | |||
|---|---|---|---|
| Rating | V-0 | V-1 | V-2 |
| Afterflame of each individual specimen | ≤ 10 sec | ≤ 30 sec | ≤ 30 sec |
| Total burning time | ≤ 50 sec | ≤ 250 sec | ≤ 250 sec |
| Afterflame + Afterglow (2nd application) | ≤ 30 sec | ≤ 60 sec | ≤ 60 sec |
| Flame up to clamp | No | No | No |
| Cotton ignited | No | No | Yes |
TABLE 3:
| UL94 VTM | |||
|---|---|---|---|
| Rating | V-0 | V-1 | V-2 |
| Afterflame of each individual specimen | ≤ 10 sec | ≤ 30 sec | ≤ 30 sec |
| Total burning time | ≤ 50 sec | ≤ 250 sec | ≤ 250 sec |
| Afterflame + Afterglow (2nd application) | ≤ 30 sec | ≤ 60 sec | ≤ 60 sec |
| Flame up to 125 mm marking | No | No | No |
| Cotton ignited | No | No | Yes |
Conclusion
Comparing the individual ratings, the difference from V-1 to V-2 or VTM-1 to VTM-2, is that the lower rating (“-2”) allows for drips to ignite the cotton, but the other criteria is the same (i.e burning times, travel distance of the flame). Ideally, having a V-0 or VTM-0 would be the best, since it does not have any type of dripping or flaming particles and extinguishes rather quickly after the flame is withdrawn.
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