Thermoplastics are materials that do not start to burn in direct contact with the flame. While most plastics continue to burn after the flame is removed, self-extinguishing plastics have the further capability to stop self-combustion after a certain period of time. Regardless of abundant standards that classify the flammability of polymers, UL94 is the internationally accepted flammability standard. This standard encompasses all areas of application, but especially electrical engineering.

The flammability class of any plastic material is tested vertically and horizontally using real flame. The evaluation classes for increasing flame resistance are HB (horizontal burning), V1, V2, V0 vertical ratings.

• UL 94- HB (Horizontal Burning): The sample is fixed in a horizontal position and exposed to direct flame at one end for 30 seconds. Then the flame is removed. The rate of burn in the material is followed because the material is rated HB if it burns slower than the defined burn rate.

• UL94 V2 Plastics (Vertical Burning):The sample is fixed in a horizontal position and exposed to direct flame at one end for 30 seconds. Then the flame is removed. If the total extinguishing time is less than 250 seconds and if flame dripping takes place, the material is rated UL94 V2.

• UL94-V1 Plastics (Vertical Burning): The sample is fixed in a horizontal position and exposed to direct flame at one end for 30 seconds. Then the flame is removed. If the total extinguishing time is less than 250 seconds and if no flame dripping takes place, the material is rated UL94 V1.

• UL94-V0 Plastics (Vertical Burning): The sample is fixed in a vertical position and exposed to direct flame at the lower end for 10 seconds. After repeated 10 seconds of burning, the time required for the burning to stop after the removal of the flame is summed up. If the total extinguishing time is less than 50 seconds and if no flame dripping takes place, the material can be rated UL94 V0.

• Non-flammability additives are usually used to enhance the non-flammability degree of polymers. Polymers are hydrocarbons composed of carbon and hydrogen. In the presence of flame, they decompose into carbon dioxide and water. Flame resistance is boosted with the addition of additives. Additives should be used to meet the required extent of flammability property in accordance with international standards. The type of additive varies depending on the polymer variety to be used. Halogen-based additives are usually used. However, the use of halogen has been restricted in recent years since halogen atoms are found to be effective during fire and detrimental to human health. For this reason, additives with inorganic content are also used to provide flame retardant properties. Zinc Borate is a good example of an inorganic flame retardant.