Before conducting the flame retardant test, let’s first understand what V 0 is, what the acceptance criteria for V 0 are, and what constitutes a V 0 pass.
What is UL94 V 0? UL94 is the vertical burning flammability standard for plastics / polymeric materials. V 0 is the most stringent flame retardant rating within the V class. For the vertical burning test, the specimen shall be vertically secured on a fixture. It is not acceptable to hold the sample horizontally or test it at any arbitrary angle during ignition. Horizontal testing falls under the HB flammability rating.
What are the requirements to achieve V 0? A set consists of 5 specimens. Each specimen is subjected to two 10 second ignition applications. All criteria below must be satisfied for a pass:
1. After flame removal from any ignition, the afterflame time of a single specimen ≤ 10 seconds
2. For all 10 ignition applications on the 5 specimens, the total afterflame time ≤ 50 seconds
3. After the second ignition and flame removal, the sum of afterflame time plus afterglow time of the specimen ≤ 30 seconds
4. Melt drips shall not ignite the standard cotton placed 300 mm below the specimen
5. Flame or carbonization shall not propagate up to the top clamping fixture
The specimen shall be fixed vertically, exposed to standard flame twice, self-extinguish rapidly without sustained combustion. Molten drips must not ignite the cotton below, and flame shall not reach the top fixture. If the test is not performed in accordance with the standard, do you think the flame retardant result is reliable?
Even specimen thickness matters: too thin or too thick is invalid, and improper length is also unacceptable. Standard specimen conditioning is mandatory. Otherwise, no matter how good the material is, your test results will most likely differ from the report. Worse still, even with a well-formulated material, non-standard testing will lead to repeated failures.
Flame retardant testing hinges on many simple details! Next, let’s go over how to properly prepare before testing.
1. How to perform specimen pre-conditioning?
Standard conditioning: After cutting the specimens, place them in a constant temperature & humidity chamber at 23°C ±2°C, 50% ±5% RH for a minimum of 24 hours. This step balances moisture content and relieves molding internal stress.
If this step is skipped, moisture will reduce the melt strength of the material, accelerate burning speed and dripping, and cause continued burning after the flame is removed. The test data will be unstable — one set may pass while the next fails.
Special requirements (e.g. for wires and cables): UL94 and IEC 60695 certifications require double pre-treatment. First, condition the samples under the above conditions for at least 48 hours. Then place them in a forced-air circulating oven at 70°C ±1°C for 168 hours (7 days) of high-temperature aging. After taking them out, they still need to be conditioned for no less than 48 hours before testing.
For freshly produced TPU material that has not fully cooled after high-temperature processing, the crystalline regions within the molecules have not formed and remain mobile. Such samples tend to drip severely during combustion testing.
This 7-day aging step simulates long-term service conditions. It can expose hidden risks in advance, such as thermal migration of flame retardants, precipitation of low-molecular-weight substances and material embrittlement. Some materials pass the lab test at first but get rejected by customers one year later — because the initial test only captured a temporary qualified state.
2. What requirements apply to the size and shape of test specimens?
Specimens shall be molded from the same batch and cut uniformly, with clean cuts, free of burrs, scratches, oil contamination and deformation. Flame retardant tests are different from casual handling: grease and sweat on the surface will alter the combustion behaviour. Do not stretch or squeeze the material during cutting, otherwise additional internal stress will be introduced. This causes cracking and warping during burning, leading to unreliable test results.
Thickness tolerance must be strictly controlled. Standard specimen thicknesses for routine UL94 testing are 3.2 mm, 1.6 mm and 0.78 mm (1/8”, 1/16”, 1/32”). Even a minor deviation can change the rating. Thin and thick specimens produce different results — much like wood shavings ignite easily, while a solid log is hard to ignite.
3. Requirements for specimen storage environment
During pre-conditioning, place specimens vertically on racks with adequate spacing. Do not stack them or let them contact one another to ensure uniform ventilation. Uneven local moisture absorption will cause scattered test data.
Storage notes: Keep specimens away from solvents, oils, release agents and lubricants, which may cause surface contamination. Avoid direct sunlight, high-temperature storage and long-term exposure to high-humidity air.
A special reminder for material suppliers: All specimens in one test set must come from the same batch with identical formulation, molding machine and production process. Cutting dimensions and pre-conditioning duration shall be consistent. If you mix old and new materials or samples from different processes, the discrepancies you observe may not stem from flame retardancy issues.
4. Requirements for ambient conditions during testing
Routine tests shall be carried out at an ambient temperature of 15–30°C and relative humidity of 15%–75% RH. The test area shall have still air with no forced airflow. Wind will deflect the flame and alter the burning duration, dripping behaviour and afterglow time.
UL and IEC certifications impose stricter requirements: the test environment shall be maintained at 23°C ±2°C, 50% ±5% RH, with no significant fluctuations in temperature and humidity throughout the test. Once the specimens are taken out from the pre-conditioning environment, the test must be completed within 30 minutes. Otherwise, secondary moisture absorption will occur and compromise the accuracy of test results.
Site requirements: The test area shall be free of dust, oil fume and volatile gases, and kept away from open flames and high-temperature sources. The burner shall be calibrated periodically, and the purity of test gas shall be no less than 98%.
Real case sharing
1. A cable manufacturer cut extruded samples and conducted flame retardant tests immediately. The material had not fully cooled, and the hard segments were uncrystallized. The test failed.
2. An injection-molded part supplier had residual stress that was not fully relieved. The residual stress accelerated dripping during combustion, resulting in a test failure.
3. To achieve a glossy surface, a customer used an excessively high extrusion temperature. This caused decomposition and volatilization of flame retardants. Under high shear and high temperature, phase separation occurred between the flame retardant and the base material. The flame retardancy became inconsistent — effective in some areas and poor in others, leading to unstable performance and a failed test.
5. Pre-test self-checklist for flame retardant testing
✅ Samples molded from the same batch, with standard dimensions, smooth cuts, free of oil contamination and deformation. ✅ Sufficient pre-conditioning time: minimum 24 hours for routine sampling inspection; for certification tests: 48 hours conditioning + 4 hours drying and cooling. ✅ Specimens are vertically racked with proper spacing, not stacked or mixed, and free from surface contamination. ✅ Test area: still air, compliant temperature and humidity, stable oxygen level, and calibrated test equipment. ✅ Final visual check: no moisture-induced whitening, no blooming/precipitation, no aging or deformation.
Only carry out the test after all five items are satisfied.
Post time: Nov-02-2026