Research on Flame Retardancy And Smoke Suppression Technology of Polyvinyl Chloride (PVC)

Research on Flame Retardancy and Smoke Suppression Technology of Polyvinyl Chloride (PVC)

Polyvinyl Chloride (PVC) resin possesses many excellent properties, such as good processability, excellent insulation, low cost, easy modification, wear and corrosion resistance, good comprehensive mechanical properties, and long service life. It is widely used in various fields including industry, agriculture, transportation, construction, communication, and healthcare. PVC resin itself is highly flame retardant and self-extinguishing. PVC products without plasticizers have good flame retardancy, with a chlorine content of 56% and a Limiting Oxygen Index (LOI) of up to 50, making it an excellent flame-retardant material. However, during processing and use, large amounts of plasticizers are often incorporated to improve the flexibility and durability of the products. Unfortunately, the incorporation of large quantities of plasticizers significantly reduces its flame retardancy, causing PVC to lose its self-extinguishing properties and become a combustible material. Furthermore, both rigid and flexible PVC generate large amounts of smoke during combustion. This dense smoke contains numerous toxic and harmful gases such as carbon monoxide, benzene compounds, hydrogen chloride, etc., causing suffocation and reduced comprehensive discernment, ultimately hindering the ability to identify and choose escape routes. This creates environmental pollution and threatens human life, property safety, forming a serious fire hazard. Statistics show that 82.5% of fire fatalities are caused by suffocation from toxic gases produced during combustion. Therefore, strengthening research on flame retardancy and smoke suppression of PVC holds significant practical importance.

I. Smoke Generation Mechanism of Polyvinyl Chloride
The main products generated after the pyrolysis of aromatic polymers include HCl, polyenes, benzene, aromatic compounds, low molecular weight olefins, and alkanes. These products further pyrolyze to produce large amounts of black smoke. During PVC combustion, HCl is eliminated, forming conjugated double-bond unsaturated hydrocarbons. Due to the cleavage of the carbon chain, low molecular weight products and free radicals are produced. Molecular rearrangement and cyclization form benzene nuclei, which further polymerize into resin with a condensed aromatic ring structure. Consequently, PVC produces substantial black smoke during combustion. Particularly when large amounts of plasticizers are incorporated into PVC, the plasticizer molecules participate in high-temperature chemical reactions of intermediate products, generating copious amounts of black smoke during combustion, significantly increasing the smoke yield of burning PVC.

II. Discussion on the Smoke Suppression Mechanism of PVC
The combustion process of PVC products can be divided into three stages: decomposition of the product upon heating to produce combustible gases, combustion of these gases in an oxygen-containing environment, and the heat generated sustaining the combustion of molten or solid material. Therefore, flame retardancy can be achieved by interrupting one or more of these stages through mechanisms such as condensed-phase flame retardancy, gas-phase flame retardancy, interruption of heat exchange, and intumescent flame retardancy. Condensed-phase flame retardancy involves retardants that delay or prevent thermal decomposition in the solid phase. Gas-phase flame retardancy involves retardants that delay or interrupt the chain combustion reaction in the gas phase. Interruption of heat exchange involves retardants that remove part of the heat generated during PVC combustion, reducing the heat absorbed by the material below its decomposition temperature, thereby reducing flammable gas generation and causing self-extinguishment. Intumescent flame retardancy combines both condensed-phase and gas-phase mechanisms, with reactions primarily occurring in the condensed phase but also involving the gas phase.

III. Methods for PVC Flame Retardancy and Smoke Suppression

1. Using Flame Retardant Plasticizers Instead of Conventional Ones

The addition of commonly used flammable plasticizers for PVC, such as dioctyl phthalate (DOP), decreases the oxygen index of PVC. Therefore, partially or fully replacing conventional flammable plasticizers with flame-retardant plasticizers commonly used for flexible PVC, such as triaryl phosphates and alkyl diaryl phosphates, is one method to improve the flame retardancy of plasticized PVC products. However, to achieve better flame retardant effects, phosphates are generally used in combination with other flame retardants, leveraging synergistic effects for enhanced performance.

2. Using Inorganic Additives

Inorganic additives are among the most researched and applied flame retardants and smoke suppressants in recent years. Those applied to PVC mainly include transition metals, their oxides, salts, etc., such as copper, molybdenum, iron, tin, zinc, and lead compounds.

Key systems include:

• Magnesium-Aluminum-Boron System: Al(OH)₃ and Mg(OH)₂ are the most widely used flame retardants globally.

• Antimony-Bismuth and Silicon-Tin Systems: Antimony-based flame retardants are traditional options. Silicon-tin compounds are often used as smoke suppressants for PVC in combination with other flame retardants.

• Copper-Zinc System: Extensive research shows copper compounds are highly effective smoke suppressants. Copper-zinc system compounds offer advantages like high flame retardant efficiency, low toxicity, and relatively low cost, making this one of the faster-developing flame retardant systems.

• Iron-Cobalt-Nickel System: Iron compounds mainly include Fe₂O₃, Fe₃O₄, Fe₂O₃·H₂O, FeSO₄, cobalt compounds include Co(acac)₂ (cobalt acetylacetonate), CoCO₃, nickel compounds include NiO, Ni(acac)₂, NiC₂O₄.

• Molybdenum-Tungsten-Manganese System: Molybdenum compounds are mainly its inorganic salts and oxides, with MoO₃ (molybdenum trioxide) being predominant in its hexavalent state. Manganese compounds include MnCO₃, Mn₂O₃, MnSO₄.

• Titanium-Zirconium-Vanadium System: TiO₂, Zr(acac)₄ (zirconium acetylacetonate), and VO(acac)₂ (vanadyl acetylacetonate) show good smoke suppression effects.

3. Using Crosslinking Technology

Crosslinking technology is a relatively new method applied to PVC flame retardancy in recent years. Methods such as radiation, adding metal oxides, or adding peroxides can crosslink polymers. The synergistic flame retardant effect of crosslinking on flexible PVC is particularly practically useful. Improving the flame retardant efficiency of inorganic systems through crosslinking technology is receiving increasing attention and represents a new, potentially high-growth area in flame retardant research.

In summary, PVC is prone to generating large amounts of black smoke and toxic gases during combustion, posing significant safety hazards. Employing methods such as flame-retardant plasticizers, introducing inorganic additives like copper and zinc, and crosslinking modification can effectively enhance its flame retardancy and smoke suppression properties. Combining multiple flame retardant mechanisms for synergistic effects is key to improving PVC safety, holding great importance for safeguarding human life and the environment.

In this field, Guangzhou Yinsu Flame Retardant are continuously dedicated to developing efficient and environmentally friendly flame retardant solutions. Their product includes the T-series, antimony composite and ATO replacement T30, which can replace 50%-60% . These innovations provide valuable options for enhancing the flame retardancy and smoke suppression of PVC materials while addressing cost and environmental considerations.