The flame retardancy of a product has a direct impact on the safety and reliability of electronic and electrical equipment. UL94 V-0 is an internationally recognized standard for high flame retardancy and is widely used. It has how to delineate the standard, what specific performance? This article will be introduced specifically for your selection of materials for detailed reference.
What Is UL94v-0?
UL94v-0 is a standard developed by Underwriters Laboratories and is one of the highest flame-retardant ratings in the UL 94 flammability test standard. It is used to test and evaluate the burning behavior of polymer materials such as plastics and silicone when exposed to open flame. By measuring the flammability of these materials, it evaluates their resistance, ignition, a nd self-extinguishing ability when exposed to flame.
If this standard is reached, it means that the flame retardancy of this polymer material will be excellent. It will extinguish itself within 10 seconds after leaving the flame, and the total burning time of the two tests cannot exceed 50 seconds. The residue that falls from its burning is not flammable and cannot ignite the cotton. And the fire is small and will not burn quickly.
Key Properties of UL94V-0 Silicone Rubber
Flammable Retardance
The most important feature of silicone rubber that meets the UL94V-0 standard is its high flame retardancy. Silicone rubber can extinguish itself after the flame is ignited, usually in 3-8 seconds, which is much faster than the time set by the standard. In this burning process, the flame does not have a very strong fire, but a slow rise, and does not produce a continuous flame; the flame control is good.
The combustion is not fueled by drips like organic plastics. When the flame stops, the surface of the silicone rubber will have a grayish-white or light brown carbonized layer, which protects the internal structure from further damage and burning.
Low Smoke and Toxicity
The main chain of silicone rubber is a silicon-oxygen bond, not the typical carbon chain structure. When burned, it produces mainly silica, water vapor, and a very small amount of simple decomposition products. It does not produce large amounts of black smoke and toxic gases like carbon-based polymers. Even with the addition of flame retardants, you can rest assured that the combustion products are far safer than organic materials.
| Property | Silicone Rubber | Organic Flame-Retardant Plastics |
| Main Combustion Products | Silicon dioxide (SiO₂), water vapor | CO, CO₂, soot, VOCs |
| Smoke Density | Very low | High |
| Toxicity | Low | High (depends on composition) |
| Flaming Drips | None | Possible |
| Tendency to Sustain Burning | Low | Higher than silicone rubber |
Thermal Resistance
Silicone materials can maintain good properties at temperatures ranging from -60°C to 250°C or higher. Even at high temperatures, they retain their shape and mechanical properties without melting or severe deformation. A protective layer of silica forms on the surface at high temperatures, insulating it from oxygen and reflecting heat, thus preventing the spread of flame. This property protects electronic equipment or electronic components in case of overheating.
Electrical Insulation
UL94V-0 tested silicone rubber remains electrically insulating after being burned. A short flame causes a protective film of silicon dioxide to form on the surface of the silicone rubber. Silicon dioxide itself is a high-resistance material. It prevents the passage of electric current. The remaining unburned parts and the internal structure remain unchanged, so the insulating properties are hardly affected.
If there is a long time of combustion or high temperature decomposition, the surface of the silicone rubber will be completely charred. At this time, the internal structure will be destroyed, the thickness of the material will become thin, or cracks will form, and the insulation performance will decline. But still not easy to conduct electricity. Even if the silicone rubber is in a humid and wet environment, it can also maintain low insulation properties.
Mechanical Properties
You may think that by adding some flame retardant, the mechanical properties of the material will be reduced. Some silicone may be a little bit short of passing the UL94V-0 test standard, and then they will choose to add some flame retardant. Even with the flame retardant, it is still an elastomer and can maintain about the same flexibility and structural strength as the original. It also has stable tensile and tear strength. It can withstand many deformations unchanged. For example, it does not crack when bending and is resistant to vibration.
Environmental Resistances
UL94 V-0 silicone rubber not only has good flame-retardant ability, but also has a lot of resistance to harsh factors in different environments. It has good heat aging resistance, and can maintain elasticity and mechanical strength even under high temperature environment for a long time. It can also resist the damage of ultraviolet rays, ozone, and air pollutants.
Silicone rubber is very hydrophobic and hardly absorbs water. So it is also not prone to insulation failure and material expansion. It is also more resistant to some chemicals, such as hydrochloric acid, alkalis, salt spray, and industrial oils. The weather resistance of silicone can maintain stable performance for a long time and does not require frequent maintenance.
How UL94 V-0 Testing Is Conducted for Silicone Rubber
Specimen Preparation
Material Selection
The sample selected should be the final formulation of the silicone rubber material. This includes whether or not flame retardants have been added, the percentage of fillers, the vulcanization system, and the hardness level. Because the results of the test are only valid for fixing that formulation, it is important to have all the information.
Sample Conditioning
Place the selected samples in a specific temperature and humidity environment. Because silicone rubber is a polymer material that absorbs a small amount of moisture in the air, the rate of thermal decomposition of silicone rubber materials in contact with the flame will be different at different temperatures, affecting the afterburning time and flame spreading behavior of the silicone rubber material.
Therefore, it is important to control the variables in a relatively fixed environment so that samples of the same formulation can be accurately evaluated during testing. 23°C ±2°C and 50% ±5% relative humidity is the international standard recommended “neutral room conditions”; this condition environment can avoid the material dry and wet difference caused by the error, but also can make the combustion performance in the actual application scenarios comparable.
Dimension and Thickness
Sample dimensions are specified to be 125 ±5 mm in length, 13 ±0.5 mm in width, and 0.8-3.2 mm in thickness, and it is not appropriate to select samples that are either too thick or too thin, as too thick a sample will tend to form a protective layer of silica, which can be easily extinguished. Thin samples may not achieve V-0.
Edge Quality
The edges of the sample should be idiotically smooth and free of burrs and cracks. This ensures that the flame will burn evenly on contact.
Number of Specimens
With at least 5 samples, the test result sample group performance prevails over a single sample. This is because the test performance of multiple samples reduces the effect of chance error.
Mounting Method
The sample is mounted vertically to hold the sample in place, with the value of the cotton wool underneath the sample to detect drips. The sample is placed naturally straight and not bent.
Testing Apparatus
Burner Flame
Using a Bunsen burner with a flame height of about 20mm, the fuel is usually methane or natural gas
Specimen Holder / Clamp
Fix the strip vertically to keep the sample straight and stable, leaving space at the bottom for the cotton.
Cotton Indicator
Placed underneath the sample, it is used to test for the ignitability of combustion droplets.UL94 V-0 requires that the droplets not ignite the cotton, which is a key indicator of a passing rating.
Measurement and Timing Tools
Record afterflame time and afterglow time with a stopwatch or chronograph.
Safety and Environmental Equipment
Prepare fire shelters or combustion boxes to avoid uncontrolled fires affecting the surrounding environment. Smoke extraction or ventilation devices should also be prepared to ensure the safety of the laboratory. A constant temperature and humidity chamber is required during the sample pre-treatment stage.
Test Procedure for UL94 V-0 Silicone Rubber
The vertical combustion method is used to evaluate the combustion performance of silicone under open flame conditions, mainly to determine whether the material can extinguish itself quickly after losing the external source of ignition and whether any combustible drips appear. Why the vertical combustion method? Can’t it be a horizontal or any angle combustion method? You may have this question.
In a real fire environment, many electronic devices, power assemblies, industrial components, etc., are overwhelmingly installed in a vertical or near-vertical condition. If the material is ignited, the flame will spread upwards. At the same time, the high temperature may also trigger thermal decomposition, accelerating the burning speed. Therefore, the experiment selects the vertical angle burning direction, which will have a more severe combustion path than in reality, and if the laboratory can pass the test, it will be safer in the real one.
Vertical Burning Test
In order to make the same material unaffected by the environment under multiple repetitions of the experiment, it is also necessary to exclude the interference of the airflow with the flame. Otherwise, the same formulation of material may give completely different results in different experiments. The silica gel was fixed vertically in a combustion chamber without interference from air currents. Then a Bunsen burner was used to produce a 20 mm high blue flame, and the bottom of the flame was controlled to be 10 mm away from the lower end of the sample.
Flame Removal & Observation
The first contact of the flame with the bottom of the sample lasted 10 seconds. Then, immediately after removing the flame, the time for which the material continues to burn is recorded. This indicator is the residual flame time. After the sample has stopped burning, a second burn is performed on the same sample, again using the same method and time as the first burn, with the flame removed after 10 seconds. Then remove the flame, this second time not only to record the material afterglow time, but also to observe whether the sample afterglow phenomenon. Whether to produce combustible droplets.
This is for a complete set of experiments, and more sets of experiments, at least five, are needed to minimize error.
Success Determinants of a UL94v-0 Test on Silicone Rubber
Excellent Thermal Stability
The main chain of silica gel is the Si-O bond, which has a high bond energy, higher than the C-C bond of most organic polymers. This structure makes silica gel less likely to break quickly at high temperatures and less likely to release large amounts of flammable gases, which don’t help the flame sustain enough fuel. This makes silica gel not so easy to burn up and extinguish quickly.
Protective Layer of Silicon Dioxide
Thermal decomposition occurs when silicone is burned at high temperatures, and some of the silicon-oxygen bonds in the main chain break, and then some of the silicon elements oxidize to produce a residual layer of silica that adheres to the surface of the material. It is like a wall that prevents the outside from entering and the inside from going out. This protective layer isolates the material from oxygen, reducing the amount of oxygen needed for combustion reactions.
It also reduces the rate of thermal decomposition by preventing the transfer of heat to the interior. When the surface is covered, the thermal decomposition of the material inside is slowed, and the release of flammable gases is limited so that the flame cannot be sustained for long. This protective layer on the surface makes the material less likely to melt and flow, and less likely to drip.
Rational Flame Retardant Formulation Design
Although the molecular chain of the silica gel already provides a certain degree of flame retardancy, it needs to be further optimized through scientific formulation in order to pass the UL94-0 test stably. The main purpose of this formulation is to maintain the original material properties, but also to improve the self-extinguishing ability, shorten the afterburning time, and reduce the combustion droplets.
Silicone commonly used flame-retardant fillers are aluminum hydroxide, magnesium hydroxide, and expanded graphite. Then add some auxiliary agent optimization, such as an antioxidant, a carbonation promoter, and a dispersant.
| Component Category | Typical Ingredients | Function / Mechanism | Effect on UL94 V-0 Test |
| Flame-retardant fillers | Aluminum Hydroxide (ATH) | Decomposes at high temperature to absorb heat and release water; promotes char layer formation | Slows flame spread, reduces burn time; lowers afterflame and total burn time |
| Flame-retardant fillers | Magnesium Hydroxide (MH) | Decomposes at high temperature, absorbs heat, forms magnesium oxide residue; creates thermal barrier | Enhances self-extinguishing ability, reduces dripping, and shortens afterflame |
| Flame-retardant fillers | Expandable Graphite | Expands upon heating to form a thermal insulation barrier | Prevents flame penetration; helps form a continuous protective layer |
| Flame-retardant fillers | Silicates (e.g., Talc, Montmorillonite) | Provide physical insulation; reinforce char structure | Reduces combustible gas release rate; helps form a dense protective layer |
| Crosslinking agents | Silane Crosslinkers (e.g., Vinyl-functional silanes) | Forms a 3D network, improves high-temperature stability | Reduces material melt flow and dripping; protects the integrity of the silica layer |
| Char-promoting additives | Phosphate salts | Decomposes at high temperature to produce phosphoric acid species, promoting surface char | Accelerates protective layer formation; enhances flame retardancy, shortens afterflame |
| Additives | Antioxidants, Thermal stabilizers | Prevent premature degradation during processing or combustion | Maintains thermal stability, reduces combustible gas release, and improves V-0 pass rate |
| Plasticizers / Dispersants | Polyethers, Silicone oils | Improve processability and filler dispersion | Optimizes filler distribution, prevents local overheating and dripping |
| Intrinsic feature | Si–O backbone | High bond energy, resists chain scission, slow combustible gas release | Slow thermal decomposition; insufficient fuel for sustained flame → favors self-extinguishing |
| Post-combustion protective layer | Silica (SiO₂) | Forms a dense layer at high temperature; provides oxygen and a heat barrier | Blocks oxygen, insulates heat, reduces dripping → key factor for V-0 success |
Why Does the UL94V-0 Rating Matter for Silicone Rubber?
UL94 V-0 rating is one of the highest flame retardant ratings, and is a very important reference standard for measuring the flame retardancy of silicone. The electrical and electronic equipment industries have very strict requirements for product safety. Silicone with UL94 V-0 rating is an important material to protect circuits from short-circuits or overheating that can cause fires, which can keep the products safe for use.
It meets the requirements of international standards, such as UL certification, IEC standards, and home appliance safety standards, to improve product compliance and competitiveness in the global market.
Application of UL94v-0 Certified Silicone Rubber Products
Consumer Electronics
UL94 V-0 flame-retardant silicone can be used to make sealing and insulating parts for cell phones and laptops. There are also wearable device seals and charging port protection parts. These products are compact, with dense circuitry, and are prone to short circuits that can cause fires.UL94 V-0 flame-retardant silicone can be quickly extinguished after the source of ignition is removed, preventing the flame from spreading and improving the safety of the equipment.
Electric Vehicles and Battery Systems
In new energy vehicles and battery systems, UL94 V-0 silicone is commonly used for battery pack seals, battery module seals, and high voltage connector protection parts. These devices operate in high-voltage and high-temperature environments for a long period of time, and are also prone to gathering heat and posing a fire risk. UL94 V-0 silicone can greatly reduce the fire hazard.
Power Supply and Energy Storage
In power supply and energy storage equipment, the power supply will continue to generate heat during operation, the sealing material needs to be a flame-retardant material. UL94 V-0 silicone can reduce the possibility of sustained combustion of the material, to improve the safety of the power supply system in the presence of abnormal conditions. That’s why it’s used in power supply seals, charger seals, and energy storage battery sealing components.
Industrial and Electrical Equipment
UL94 V-0 silicone rubber can be used as industrial connector seals, electrical insulation gaskets, and control system protection parts. Industrial and electrical equipment operate at high currents and high temperatures, and UL94 V-0 silicone products can improve sealing and insulation.
FAQ
What Applications Are Suitable For Silicone Rubber That Meets UL94 V-0 Standards?
UL94 V-0 silicone is suitable for use in products that may come into contact with sources of ignition, where there is a risk of electrical failure, or in applications that require flame-retardant safety protection. Examples include electrical connections, batteries, new energy vehicle systems, charging equipment, LEDs, and industrial and control equipment.
What Are The Advantages Of UL94 V-0 Silicone Rubber Compared To Other Flame Retardant Grades Like V-1 Or V-2?
UL94 V-0 silicone offers a higher level of fire safety than V-1 or V-2 rated silicone. The main difference is in the length of time it takes for each to self-extinguish after the flame is removed. V-0 silicone extinguishes in less than 10 seconds and does not produce drips that can ignite. V-1 or V-2 silicone takes longer to self-extinguish, and V-2 silicone allows for flame droplets. In comparison, UL94 V-0 silicone has the highest fire safety rating and is better suited for electrical, battery, and high-risk applications to prevent the spread of fire.
Do Silicone Rubbers of Different Thicknesses All Pass UL94 V-0 Testing?
Of course not, silicone generally needs to be a certain thickness to pass the V-0 test. Thinner silicone heats up faster. It is also easier to keep burning and cannot meet the requirement of self-extinguishing within 10 seconds after removing the ignition source. Therefore, the thickness of the silicone has a great impact on the flame-retardant properties. However, it is not the thickness of silica gel does not affect flame retardant performance is the same. And the specific material formulation of silicone also has a relationship.
| Material | Minimum Certified Thickness | UL94 Flame Retardant RatingUL94 |
| Standard Silicone Rubber | ≥1.5 mm | V-0 |
| High-Performance Silicone Rubber | ≥2.0 mm | V-0 |
| Specially Modified Silicone Rubber | ≥0.75 mm | V-0 |
Does UL94 V-0 Silicone Rubber Completely Prevent Fire, Or Is It Only Flame Retardant?
UL94 V-0 silicone does not completely stop fires. Its greatest effect is that it burns slowly when an ignition source is present and can self-extinguish within 10 seconds after the source is removed, and it does not produce combustible drippings, which reduces the risk of flame propagation. Its main value is to limit the extent and speed of the spread of fire, but it does not prevent combustion from being triggered by an external source of ignition or a high-energy incident.
Final Thought
UL94 V-0 listed silicone offers not only excellent flame retardancy, but also low smoke, low toxicity, high temperature resistance, electrical insulation, and environmental resistance for electrical equipment, battery systems, industrial control, and high-risk applications. You need to choose a silicone that meets UL94 V-0 standards to reduce the risk of fire, improve product safety and compliance, and meet the mechanical properties of materials for complex designs.
Legensilicone‘s UL94 V-0 certified silicone products are formulated with stable, controlled thicknesses that can be customized for different applications. They also maintain excellent mechanical elasticity and weatherability. If you are looking for good quality UL94 V-0 certified silicone products, you can come to us; we can produce UL94 V-0 certified silicone products to meet the needs of various industries.
