UL 94 is an internationally recognized standard for the flammability of plastics used in equipment and appliances, used to determine their ability to self extinguish, drip, and spread flames. It is a mandatory reference for electronic, new energy, and home appliance casings, with a total of four categories of testing and grading. The most commonly used in daily life are HB, V0/V1/V2, 5VA/5VB UL. Benchmarking international flame retardant standards: IEC 60695-11, national standard GB/T 2408
UL 94 flame retardant combustion test
V-level vertical combustion test
一、Basic universal samples&pretreatment requirements
1. Sample size (universal V/HB)
125mm x 13mm x actual thickness of the product (conventional test thickness: 0.8/1.6/3.2mm), with 5 samples per group, free of bubbles, delamination, and scratch defects.
2. Pre treatment (necessary, moisture affects flame retardant results)
- Standard Group A (Normal): Placed at constant temperature and humidity of 23 ℃ and 50% RH for 48 hours
- Aging Group B (commonly used for certification): 168h in a 70 ℃ oven (7 days), cooled in a dryer for 4h before testing
3. Standard burner
50W Bunsei lamp, blue flame height 20mm, flame temperature 950 ± 50 ℃; 5V test for 500W high-power flame (125mm)
二、Four mainstream testing methods+grading judgment
（一）HB horizontal combustion (minimum flame retardant level)
Sample 45 ° tilted clamping, single flame application for 30 seconds, measure combustion velocity between 25mm~100mm markers
- judgment criteria
- Thickness ≥ 3mm: Burning rate ≤ 40 mm/min
- Thickness<3mm: Burning rate ≤ 75 mm/min
- If the combustion extinguishes on its own before the 100mm mark, directly use HB
- Applicable: Ordinary shell, non electrified bracket, components without mandatory fire protection requirements
（二）V-0/V-1/V-2 50W vertical combustion (most commonly used in the electronics industry)
The sample is suspended vertically, with the lower end 300mm away from the degreased cotton. Two flames are applied for 10 seconds each, and the record is:
T ₁=residual flame time after the first transfer of fire; T ₂=residual flame after the second shift of fire; T ∝=second afterglow (flameless redness); Observe whether the dripping ignites the cotton
| Judgment indicators |
V-0（Highest universal level） |
V-1 |
V-2 |
| Single single residual flame t ₁/t ₂ |
≤10s |
≤30s |
≤30s |
| 5 total residual flames (total t ₁+t ₂) |
≤50s |
≤250s |
≤250s |
| Single secondary residual flame+afterglow (t ₂+t ∝) |
≤30s |
≤60s |
≤60s |
| Dripping with fire ignites the cotton below |
not allowed |
not allowed |
Allow |
| The flame burns to the top fixture |
Prohibited |
Prohibited |
Prohibited |
- V0：Fastest self extinguishing, no droplet ignition, standard for power supply/PCB/energy storage housing
- V1：Slow down the self extinguishing process, and there will also be no ignition droplets
- V2：Allow burning droplets to ignite cotton, low-cost ordinary plastic
（三）5VA / 5VB 500W High power combustion (highest flame retardant rating)
High power flame, 5 times, 5 seconds each time, with an interval of 5 seconds, harsh high-temperature conditions (distribution cabinet, high-power energy storage, high-voltage enclosure)
- 5VA（UL94 highest level）
- Burn for ≤ 30s after each shift of fire
- The sample is not allowed to burn through or have no ignition droplets
- 5VB
- Burn for ≤ 60s after each shift of fire
- Allow burning through holes without igniting droplets
（四）Special material subdivision testing
- VTM-0/1/2: Ultra thin flexible film (PCB film, insulation film), with the same grading logic as V-grade
- HF-1/HF-2/HBF: special flame retardant grade for foamed foam and sound absorbent cotton
三、Complete V-level testing operation process
- Sample constant temperature and humidity pretreatment for 48 hours
- Vertically clamp the sample and lay dry degreased cotton on the bottom 300mm
- Calibrate the 20mm blue flame, aim it at the center of the sample bottom and ignite for 10 seconds, then move the flame away
- Record t ₁, wait for it to completely turn off, then ignite again for 10 seconds
- Record t ₂ and afterglow t ∝, observe whether the dripping ignites the cotton
- Only when all 5 samples meet the standard can they be considered as passed; If a single piece is not qualified, a group of 5 pieces can be retested
四、Industry selection reference
- Consumer electronics adapters, chargers PCB：V0 1.6mm
- Energy storage cabinet, battery casing, high-voltage components: priority 5VA, minimum V0
- Ordinary plastic shell and decorative parts: HB/V2
- High power distribution cabinet, industrial controller: 5VA
五、Common causes of test failure
-
- The sample was not fully dried, resulting in reduced moisture and flame retardant effect
- Insufficient material thickness may result in a decrease in the flame retardant rating of thin materials (3.2mm V0, 1.0mm may only pass V2)
- Insufficient addition and uneven dispersion of flame retardants
- Molding defects (bubbles, delamination) accelerate combustion
- Molten droplets ignite cotton with fire, directly downgrading from V0/V1 to V2
Foshan Zhiwei Adhesive Products Co., Ltd. has over 20 years of professional matching solution experience in the adhesive and sealing industry. We welcome inquiries from major manufacturers. Contact email: yang@fszw-tapes.com
