Types of Flame Retardants in PVC Plastics, Flame Retardancy Rating Standards, Mechanisms of Action, and Applications

I. Flame Retardant Properties and Challenges of PVC

PVC (polyvinyl chloride) contains up to 56% chlorine by weight. When burned, it releases hydrogen chloride (HCl) gas and exhibits some self-extinguishing properties (limit oxygen index, LOI ≈ 45%). However, the following challenges exist in practical applications:

1. Flexible PVC: The addition of plasticizers (such as DOP and DINP) significantly reduces the LOI (to below 20%), requiring the addition of flame retardants;

2. Smoke and Toxicity: Combustion releases HCl, dioxins, and dense smoke;

3. Thermal Stability: It is prone to decomposition during high-temperature processing, requiring the synergistic use of thermal stabilizers and flame retardants.

II. Types of Flame Retardants in PVC Plastics

1. Halogen-based Flame Retardants

• Chlorinated flame retardants: Such as chlorinated paraffin; those with 50% chlorine content are primarily used as auxiliary plasticizers for PVC plastics, while those with 70% chlorine content are primarily used as flame retardants.

2. Phosphorus-based Flame Retardants

• Inorganic phosphorus-based flame retardants: such as red phosphorus and ammonium polyphosphate (APP). Red phosphorus is highly effective as a flame retardant but has poor compatibility with resins and requires microencapsulation.

• Organic phosphorus-based flame retardants: such as phosphate esters and phosphaphenanthrene. Phosphate esters serve both as flame retardants and plasticizers and are commonly used in flexible PVC.

3. Nitrogen-based flame retardants: such as melamine and melamine cyanurate (MCA), which are often used in combination with phosphorus-based flame retardants.

4. Inorganic flame retardants: such as aluminum sulfate trihydrate and nano-silica.

III. Commonly Used Synergistic Flame Retardants

1. Antimony trioxide flame retardant: Typically used in conjunction with halogenated flame retardants. During the initial stage of combustion, antimony trioxide melts first, forming a protective film on the surface of the PVC material that isolates it from the air; it lowers the combustion temperature through an endothermic reaction within the material; at high temperatures, antimony trioxide vaporizes, diluting the oxygen concentration in the air and thereby providing a flame-retardant effect; when used in conjunction with chlorides generated by combustion in PVC, the resulting antimony chloride captures free radicals in its gaseous state, further aiding in flame retardancy. For example, adding antimony trioxide to PVC cable compounds increases the oxygen index, enhancing their flame-retardant properties. Antimony trioxide that has undergone special processing not only provides flame retardancy but also reduces smoke emission, making it particularly suitable for use in PVC cable compounds.

2. Aluminum hydroxide: It reduces the smoke emission of PVC and improves its flame-retardant properties. When decomposed by heat, it absorbs a large amount of heat, and the resulting aluminum oxide forms a protective film on the material’s surface, isolating it from oxygen and heat. At the same time, the water vapor produced during decomposition dilutes the concentrations of flammable gases and oxygen.

3. Magnesium hydroxide: It decomposes to produce magnesium oxide and water, providing good smoke suppression. As particle size decreases, the oxygen index of PVC increases, while the maximum smoke release rate and smoke density rating decrease. Furthermore, it has minimal impact on the mechanical properties of PVC and can meet certain application requirements.

4. Red phosphorus: Microencapsulated red phosphorus is typically used, offering advantages such as low addition levels, high flame-retardant efficiency, low smoke emission, and low toxicity. When burned, it forms metaphosphic acid, which then polymerizes into a stable polymeric phase, creating a protective layer on the plastic that isolates it from oxygen. At the same time, red phosphorus can also be blended with other inorganic flame retardants to reduce the amount of flame retardant required and improve the processing and physical-mechanical properties of PVC.

5. Silicone-based flame retardants: These use organosilicon and organoboron as carriers and chelate various functional groups. When used in combination with various flame-retardant systems, they form small-molecule gases that isolate oxygen during combustion, dilute flammable gases, and absorb heat to reduce temperature. This promotes the carbonization and film formation of the polymer matrix after combustion, enhancing flame-retardant effectiveness and suppressing melt dripping. Additionally, they improve the phase compatibility between the various components and the polymer matrix in composite materials, thereby enhancing the material’s mechanical properties.

6. Nitrogen-based flame retardants: Examples include triazines and their derivatives, as well as melamine. While their effectiveness is limited when used alone, they produce a synergistic effect when combined with phosphorus-based flame retardants. Composite flame retardants, such as those composed of melamine and polyphosphates, can improve the flame retardancy of PVC.

IV. Flame Retardancy Rating Standards for PVC Plastics

Common standards include the UL 94 standard, which is divided into V-0, V-1, V-2, and HB ratings. The V-0 rating has the highest requirements: the test specimen must self-extinguish within 10 seconds without any dripping material igniting lint or similar materials; the HB rating has the lowest requirements and is based on a horizontal burning test, with the rating determined by the burning rate.

V. Mechanism of Action of Flame Retardants

1. Heat Absorption: Flame retardants undergo an endothermic reaction at high temperatures, lowering the surface temperature of the combustible material and reducing the generation of flammable gases, thereby suppressing combustion.

2. Coating Effect: During combustion, flame retardants form a solid or liquid coating that isolates the material from oxygen and prevents combustion.

3. Inhibition of Chain Reactions: Halide ions generated by the decomposition of halogen compounds react with free radicals during combustion, reducing the concentration of free radicals and thereby slowing the combustion rate.

4. Suffocation by Non-flammable Gases: For example, nitrogen-based flame retardants decompose to produce non-flammable gases, which dilute flammable gases and reduce the combustion rate.

5. Charring Effect: Phosphorus-based flame retardants produce phosphoric acid, metaphosphoric acid, and other compounds during combustion, promoting charring on the surface of plastics and forming a char layer that insulates against heat and blocks oxygen.

VI. Applications of Flame Retardants

1. PVC-U Profiles: In PVC-U profiles, the addition of chloramine phosphate esters significantly improves flame retardancy, while inorganic flame retardants such as nano-silica and aluminum sulfate trihydrate can also be used to enhance thermal insulation and thermal stability.

2. PVC Cable Compounds: Flame retardants such as chlorinated paraffins and phosphate esters are commonly used to improve the flame retardancy and heat resistance of cables.

3. PVC Pipes: In PVC pipes used in construction, inorganic flame retardants such as nano-silica can effectively suppress flame spread during combustion.

VII. Analysis of Classic Case Studies

1. Flame-retardant modification of PVC cable compounds: By adding an appropriate amount of chlorinated paraffin and Yinsu Flame Retardant Company’s red phosphorus masterbatch FRP-950X—at a concentration of 2%—to PVC cable compounds, the flame-retardant performance of the cables is significantly improved, enabling them to meet the UL 94 V0 rating and fully replace antimony trioxide. Through this modification, the heat generated and smoke density produced during cable combustion are reduced, while maintaining good mechanical properties.

2. Flame Retardant Applications for PVC-U Profiles: In PVC-U profiles used in construction, chloramine phosphate and nano-silica are added. During combustion, chloramine phosphate releases nitrogen and chlorine gases, forming a char layer that prevents the spread of fire; nano-silica, with its fine particle characteristics, inhibits flame spread and heat conduction. This composite flame-retardant system significantly enhances the flame-retardant performance of PVC-U profiles, making them safer for use in construction applications.

As shown by the above analysis, flame retardancy in PVC must take into account its inherent characteristics (high chlorine content, processing sensitivity) and achieve high efficiency, low toxicity, and low cost through the synergistic use of flame retardant blends (such as phosphorus-metal hydroxide-smoke suppressants). The application of flame retardants in PVC plastics is of great significance, as it not only enhances the material’s fire safety but also preserves its excellent physical and mechanical properties.