Why do polyvinylchlorides need flame retardants?
Polyvinylchloride, commonly known as PVC, is a versatile thermoplastic polymer used in a wide range of products and industries such as construction, automotive, electrical, and healthcare. PVC products can be divided into rigid PVC and soft PVC. Generally, those with a plasticizer content of less than 10% are called hard PVC, and those with a plasticizer content of more than 30% are called soft PVC. Rigid PVC itself has excellent flame retardant properties, LOI>45%, no special requirements do not need to add flame retardants. Due to the presence of plasticizers in soft PVC, flame retardants need to be added to meet flame retardant requirements.
In addition, PVC products can release toxic gases and particles when burned. Flame retardants act as a barrier to prevent the release of harmful chemicals, such as dioxins, furans, and heavy metals, which can pose serious health risks when inhaled or ingested.
Therefore, adding flame retardants to PVC formulations can prevent PVC materials from catching fire and reduce the possibility of fire accidents. Second, flame retardants can delay the spread of fire, giving more time for evacuation and firefighting efforts. Moreover, by adding flame retardants, the generation of these harmful substances can be minimized, thereby improving the safety of personnel in the event of a fire.
Flame Retardants for PVC
There are several flame retardants available for polyvinyl chloride PVC. Some commonly used flame retardants for PVC include:
These are chemicals that contain chlorine and are effective flame retardants for PVC. They act by releasing chlorine gas when exposed to heat, which can suppress the combustion process.
These chemicals contain bromine and are used in various forms, such as polybrominated diphenyl ethers (PBDEs) or tetrabromobisphenol A (TBBPA). Brominated flame retardants are effective in reducing the flammability of PVC.
- Phosphorus-based Flame Retardants
Organic phosphates and phosphorus esters are often used as flame retardants for PVC, such as tricresyl phosphate (TCP), tris(2,3-dichloropropyl) phosphate, tris(2,3-dibromopropyl) phosphate, etc. They work by releasing phosphoric acid when exposed to fire, forming a protective char layer on the material's surface.
Antimony oxide (antimony trioxide) has almost no flame retardant activity when used alone, but it has a synergistic effect when used together with halogens. Polyvinyl chloride is a halogen-containing resin, so antimony oxide can be used alone to obtain flame retardancy. When antimony oxide and chlorinated paraffin are used together, the flame retardant effect will be better.
Zinc borate (ZB) is a cheap flame retardant, and its flame retardant effect is not as good as antimony oxide. In PVC formulations, it is generally used together with antimony oxide to reduce the amount of antimony oxide, and has good smoke suppression and flame retardant effects. The mechanism of accelerated dehydrochlorination and crosslinking of ZnCl2 formed by the interaction of ZB with HCl is shown as bellow. [1]
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Reference
- Levchik S V, et al. Polymers for Advanced Technologies, 2005, 16(10): 707-716.
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