Plastic products are easy and inexpensive to mold and process and have been widely used in home appliances, electronic products, and automotive parts. The market demand for plastic products is expanding by modifying plastic materials such as dyeing, toughening, glass fiber or carbon fiber reinforcement, etc. However, plastic is a flammable material. Different end-use products introduced different regulations for safety reasons, such as UL94 or LOI (Limiting oxygen index) test.
Some basic rules can determine the flame retardancy of plastic materials:
- Plastics containing halogen atoms (e.g., fluorine, chlorine, bromine) are not flammable. Thus, the higher the content, the better the flame retardancy. Besides, polymers containing sulfur (S) or nitrogen (N) atoms also have good flame retardant properties after oxidation. For example, PTFE, the king of plastics, is made from Teflon (which has four fluorines), so it is not flammable; PVC is also made from vinyl chloride and has a chlorine structure, so it is inherently flame retardant as well.
- If the chemical structure contains more benzene rings, the thermal cracking rate will be slower in the combustion process. The amount of carbon and hydrogen released after cracking will be less, and the amount of oxygen bonding will also be less, so it will have a better flame retardant effect.
- The proportion of inorganic filler in the plastic composite material also affects the flammability. Both fiberglass and inorganic powders are non-combustible materials. The more additives added to the plastic compound, the less plastic material there is, which means that the proportion of combustible material in the combustion process is also lower, thus improving the flame retardant effect.
In our daily life, each of the common plastics we use, each of them has a different degree of flammability, from flammable to non-flammable, they are PE<PP<PS<PET<PVC. PVC contains many chlorine atoms in its chemical structure, so the pure product is very non-flammable. In practice, however, PVC requires the addition of plasticizers to facilitate processing, which reduces its flammability. However, PVC’s flame retardancy is still sufficient for interior applications, such as curtains, window frames, flooring, wire covers, etc. As for the flame retardancy of general engineering plastics, POM<ABS<PBT<Nylon6<PC<PPO. Because of its molecular structure, POM is composed of carbon, hydrogen, oxygen and no benzene ring structure, so it is the most flammable.
However, not many plastic materials are inherently flame retardant, so adding flame retardants to get different flame retardant grades of modified plastic materials is still the direction that material researchers are actively developing.
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