Analysis of Flame Retardant And Smoke Inhibition Mechanisms of Polymer Materials And Research Status of Smoke Suppressant

Analysis of flame retardant and smoke inhibition mechanism of polymer materials and research status of smoke inhibitors

I. Combustion process and mechanism of polymer materials

II. Flame retardant mechanism of polymer materials

III. Smoke generation process and mechanism of polymer materials

IV. Smoke suppression mechanism of polymer materials

V. Common Typical Smoke Suppressant Materials

VI. Conclusion

A variety of synthetic polymer materials represented by plastics, rubber, fibers have greatly improved people's lives, but their flammability has brought about increasingly serious fire hazards. At the same time, polymer combustion will produce a large number of toxic, corrosive chemicals containing smoke, and inhalation of toxic gases leading to unconsciousness and asphyxiation is the main cause of casualties in the fire. Therefore, the flame retardant and smoke suppression performance of polymer materials has become one of the key indicators for evaluating the performance of materials, which is extremely important and cannot be ignored.

At present, the flame retardant research has achieved fruitful results, the formation of halogenated, organic phosphorus (silicon, boron) and other types of organic flame retardants and inorganic phosphorus compounds, molybdenum compounds, metal hydroxides and layered silicates, and other major categories of inorganic flame retardant system. In recent years, with the increasing requirements of environmental protection, halogenated flame retardants have been gradually reduced or banned, which has facilitated the emergence and rapid growth of halogen-free environmentally friendly flame retardant materials.

Compared with the rapid development of flame retardants, the research and development of smoke suppressants at home and abroad started late and progressed slowly. Due to the wide variety of polymer materials, the differences in the structure and functional groups of different materials lead to different degradation and smoke generation mechanisms during combustion, and their corresponding combustion products and smoke generation are also different.

Combustion process and mechanism of polymer materials

The combustion process belongs to the intense thermal oxidation process. When the temperature of polymer material reaches the cracking temperature under the action of external heat source, thermal cracking reaction occurs, releasing low molecular weight gaseous organic combustibles, and gradually spreading to the surface of the material, and O₂ intense combustion reaction occurs, and at the same time, light and heat are released. Part of the heat generated by combustion is transferred to the polymer material in pyrolysis, which intensifies its pyrolysis process and makes the fire spread rapidly.

Due to the differences in the composition, structure and chemical bonding of the materials, the mechanism of thermal cracking and the resulting gas phase combustible gases and decomposition products are also different.

For example, polyvinyl chloride (PVC) in the C-Cl bond energy is the smallest, in the thermal degradation will be the first to break, the main chain is relatively stable. In polyolefins, the C-C bond is more likely to break during pyrolysis to form free radicals of smaller molecular weight. The free radicals generated during pyrolysis are highly reactive and can easily react with neighboring groups to induce further degradation and generate more reactive free radicals. Larger molecular weight radicals can be reorganized to produce cross-linked structures by migrating and colliding with each other. The combustible gas generated by thermal degradation can continue to decompose into active -H and -OH radicals, and the reaction between -OH radicals and CO is one of the main reactions of gas-phase combustion, which is very closely related to the combustion rate.

Therefore, combustion includes a number of complex physical and chemical processes such as solid-phase pyrolysis, gas-phase combustion, gas diffusion at the junction of gas and solid phases, and heat conduction in the solid phase.

Flame retardant mechanism of polymer materials

The flame retardant mechanism of polymer materials can be categorized into gas phase flame retardant and condensed phase flame retardant.

Gas-phase flame retardant mainly includes two mechanisms:

(1) Free radical scavenging mechanism. The flame retardant captures and removes the active radicals such as -H and -OH in the gas phase, or converts them into low combustion activity radicals, interrupts the growth of the combustion reaction chain, and reduces the decomposition of the polymer matrix triggered by the free radicals, thus inhibiting the combustion. Halogenated flame retardants belong to the typical gas-phase radical capture mechanism.

(2) Dilution effect. Flame retardants release H₂O, NH₃, CO₂, N₂, HX and other non-combustible gases during combustion. These gases enter the gas-phase combustion zone and dilute the concentration of O₂ and flammable gases produced by polymer decomposition, reducing them below the combustion threshold and achieving flame retardancy.

Coalescence phase flame retardance can be specifically summarized as:

(1) Slow down or stop the thermal decomposition of the polymer matrix in the condensed phase, and inhibit the generation of flammable gases and free radicals.

(2) The flame retardant uses its own thermal decomposition and the gasification process of the decomposition products to absorb a large amount of heat, reduce the internal or surface temperature of the material, and slow down or terminate the thermal decomposition of the material.

(3) Reduce the temperature of the material by means of heat storage or thermal conductivity, and play the role of flame retardant.

(4) Forming or promoting the formation of a dense protective charcoal layer during the combustion process. The coked carbon layer covers the surface of the polymer material, prevents the combustible gases generated by the matrix from entering the gas phase combustion zone, inhibits the transfer of external oxygen and heat to the interior, delays the further thermal decomposition of polymers, slows down the rate of combustion, and realizes flame retardancy.

At present, it is generally believed that the use of condensed phase reaction flame retardant and thus reduce the generation of combustible volatile gases, is more ideal flame retardant than gas phase flame retardant.

Smoke generation process and mechanism of polymer materials

Polymer material combustion degradation will produce a large amount of toxic smoke, at the same time, its processing process to add certain additives or plasticizers, will also contribute to the further increase in the amount of smoke.

The smoke released during combustion is an aerosol, including gaseous substances such as CO and CO₂, solid particles such as charcoal particles, and liquid droplets such as water. Typically, the oxidation of polymers competes with the char formation reaction, so that materials with a high oxygen content have a lower smoke emission when burned.

For example, aliphatic hydrocarbon polymers containing oxygen atoms in the main chain produce less smoke when burned or thermally degraded; polymers containing more double bonds and aromatic compounds with benzene rings on the side chain produce more smoke; halogen-containing polymers, especially PVC, produce particularly high amounts of smoke.In addition, the plasticizer added in the material can not only participate in the high temperature chemical reaction of the intermediate product, its own pyrolysis can also generate a large amount of smoke, which will make the material's smoke significantly higher.

PVC is widely used and combustion smoke, so more research on PVC smoke suppression.PVC belongs to self-extinguishing materials, not easy to burn in the air, the main thermal degradation reaction occurs at high temperatures, the combustion will produce a large amount of smoke and HCl and Cl₂ and other toxic and corrosive gases. Among them, conjugated polyene molecular cyclization of aromatic compounds obtained from easy to burn, in the gas phase can be further polymerization and the generation of thick ring aromatic compounds, which is an important reason for the production of smoke.

Under anaerobic conditions, polyvinylidene chloride, polyvinylidene fluoride and other materials when heated, can occur with the PVC system similar to the elimination reaction, respectively, to produce HCl, HF and other toxic and harmful gases. Polyurethane combustion also produces a large amount of smoke and toxic gases, the main components of which include CO, HCN, NH₃, NOx and so on.

Therefore, adding smoke suppressants to change or regulate the formation mechanism of volatile products of thermal decomposition of polymer systems, and thus change the proportion and composition of volatile products, is a key factor in reducing their smoke generation.

Smoke suppression mechanism of polymer materials

So that the intermediate products of polymer combustion molecules do not occur in each other's ring-forming, polymerization reaction, so as not to generate such as acetylene, benzene, polycyclic aromatic compounds and other intermediates with a high ratio of hydrocarbons, is a key factor in smoke suppression.

PVC, for example, the smoke is mainly generated by the thermal degradation process of the system decomposition of HCl and polycyclic aromatic compounds, so the formation of volatile pyrolysis products is the formation of the mechanism to determine the effectiveness of the smoke suppressant key. At present, the mechanism of PVC smoke suppressant is generally believed to contain Lewis acid mechanism, reduction coupling mechanism, as well as dehydration and adsorption.

(1) Lewis acid mechanism

It is generally believed that the Lewis acid site can promote the isomerization of cis and trans double bonds, because the trans configuration is more stable thermodynamically, so under the catalytic effect of the Lewis acid site, the formation of trans polyene products is prioritized after PVC dehydrogenation of HCl, avoiding the process of conjugated polyene cyclization to generate aromatic compounds, and accelerating intermediate product intermolecular cross-linking and charring, and at the same time, the Lewis acid site can adsorb a portion of HCl, reducing the escape of HCl. part of HCl and reduce its escape.

(2) Reduction coupling mechanism

The low-priced transition metal contained in the smoke suppressant can promote the reduction coupling reaction of allyl chloride, an intermediate product of PVC degradation, to reduce the thermal degradation of the polymer in the early stage, and allyl in the coupling of the polyene chain molecular fragments are relatively short, reducing the generation of benzene and other aromatic compounds, to achieve effective smoke suppression.

(3) Dehydration and adsorption

Some smoke suppressants with large surface area have good adsorption effects on carbon smoke and other harmful fumes generated during thermal decomposition, and can be converted into the corresponding compounds to inhibit the fuming of the material.

Common Typical Smoke Suppressant Materials

(1) Molybdenum-based smoke suppressants

Molybdenum compounds are currently known as one of the excellent performance of smoke suppressant materials, mainly including molybdenum trioxide and ammonium octamolybdate, etc., and its smoke suppressant mechanism is generally considered to be a Lewis acid mechanism.

The molybdenum element has empty 4d valence orbitals, which can accept the lone electron pair of the Cl atom on the PVC to form a strong coordination bond, so that the overall stability of the material is enhanced. In addition, the formed metal chloride MClx prevents intramolecular ring formation of polyolefin chains after release of HCl and promotes the formation of intermolecular cross-linking reaction products, which reduces the proportion of aromatic compounds in the pyrolysis products and improves the amount of residual carbon in the solidified phase.

In addition, MoO₃ or its derivatives can also catalyze the intermolecular Diels-Alder cyclization or Friedel-Crafts alkylation to destroy the olefinic precursors of benzene, thus promoting the cross-linking reaction during the thermal decomposition of PVC chain segments.

(2) Zinc-based smoke suppressants

Magnesium-zinc composite smoke suppressant is a white dispersible powder composed of MgO and ZnO, is mainly used as an additive for PVC materials, which can effectively inhibit the amount of smoke.

The mechanism of smoke suppression is believed to be magnesium and zinc complex and PVC cracking HCl reaction, the generation of solid-phase metal chlorides, inhibit Cl and polyene compounds for the next reaction, to prevent further cracking of the carbon chain, prompting the production of a dense carbon layer to reduce the volatile hydrocarbon compounds volatile amount of smoke suppression purposes.

(3) Copper-based smoke suppressants

The main mechanism of copper-based smoke suppressants is reduction coupling mechanism.

Research has proved that Cu (Ⅰ) and Cu (Ⅱ) compounds in PVC pyrolysis can significantly inhibit the generation of benzene, Cu (Ⅰ) of the compounds in the high temperature cracking of PVC disproportionation reaction occurs, a part of the generation of Cu (Ⅱ) compounds, but more inclined to generate a stable Cu (0) monomers.

This reaction will cause the PVC chain reduction coupling reaction, promote the polymer crosslinking each other, so that the generation of benzene and other aromatic compounds to reduce the amount of effective smoke suppression.

(4) Layered metal hydroxide

Metal hydroxides or layered composite metal hydroxides (LDHs) thermal decomposition will release a large number of water molecules and carbon dioxide, while the decomposition reaction absorbs heat, which can effectively reduce the rate of heating of the system and the thermal degradation of the material, and promote carbonization, significantly improve the thermal stability of the system, reduce the amount of smoke.

At the same time, LDHs laminates contain a large number of alkaline sites, which can adsorb acidic HCl gas, preventing the release of elemental Cl into the air and inhibiting the production of white smoke.

LDHs smoke suppression materials in PVC, EVA and polyurethane elastomers (PUE) and other materials also show excellent smoke suppression performance, with broad application prospects.

(5) Flame retardant smoke suppressants

Compared with flame retardants, there are relatively few studies on single-function smoke suppressants, and most of them are based on the flame retardant properties of flame retardants with the additional study of their smoke suppressant properties, forming “flame retardant smoke suppressant” dual-function materials.

Inorganic flame retardants usually have better smoke suppression properties, and therefore become the preferred choice for “flame retardant smoke suppression” dual-function materials. For example, molecular sieves and phosphate flame retardants have been reported to have excellent flame retardant and smoke suppressant effects.

By mixing or compounding multiple flame retardant and smoke suppressant components, synergistic effects between different flame retardant and smoke suppressant elements and structures can also be realized, effectively reducing the amount of flame retardant added to the polymer matrix, thus weakening the negative impact of a single flame retardant on other properties of the polymer substrate.

Conclusion

Flame retardant and smoke suppression of polymer materials are two complex processes with different characteristics. Due to the different mechanisms of the two processes, the chemical composition and microstructure requirements of flame retardant and smoke suppression materials are also different. It is difficult to maximize the advantages of the two functions by using “flame retardant and smoke suppressant” dual-function materials at the same time.

In addition, due to the differences in the composition and structure of different types of polymer materials, their combustion and smoke generation mechanisms are different, and the combustion characteristics and the amount of smoke generated are also quite different. The development needs of flame retardants and smoke suppressants for different kinds of polymers are also significantly different.

For example, for linear polymers such as polyolefins, due to the small amount of smoke, the main consideration should be flame retardant requirements, while for self-extinguishing materials such as PVC and other large amount of smoke, the main consideration should be the need for smoke suppression in combustion.

On the basis of the development of efficient smoke suppressants, the optimized flame retardant and smoke suppressant can be reasonably compounded to achieve the best performance of both, and at the same time to achieve efficient flame retardancy and smoke suppression.

While exploring the future development of flame retardants, YINSU Flame Retardant has also developed innovative products. In particular, we have launched T3, an antimony trioxide replacement agent, and FRP-950X, K100 for PE, which are known for their low additives and low smoke density, which effectively enhance the safety and environmental protection of materials. Both T3 and FRP-950X can be used with K100 flame retardant, which further reduces the heat release value and smoke generation of the material, providing a double guarantee for fire safety protection.