Classification And Selection of Polymer Flame Retardant And Smoke Inhibitors

Classification And Selection of Polymer Flame Retardant And Smoke Inhibitors

In many fires, a large number of people do not die from high temperatures. Instead, most of them perish due to poisoning or suffocation caused by oxygen deprivation, as a result of the release of toxic and harmful gases from combustible materials during the combustion process.

Therefore, the development of new types of non-toxic, harmless, and flame-retardant materials with good flame-retardant properties, which do not produce or produce fewer toxic and harmful smoke during combustion, has become a key research direction in the current field of flame retardancy.

At present, the main approach to addressing the issue of excessive smoke and dust in fires is by adding smoke suppressants such as transition metal oxides, magnesium-zinc compounds, metal hydroxides, tin oxide, ferrocene, and copper oxide.

The development of new smoke suppressants and flame-retardant materials that do not produce toxic and harmful gases during combustion is the trend for future flame-retardant material research.

I. Composition of Smoke

Black smoke refers to the suspended solid particles and aggregates in the combustion gas products.

It is generally believed that there are three ways to reduce the concentration of black smoke:

First, by drawing on flame-retardant principles such as the covering effect, transfer effect, inhibition of free radicals, and acceleration of carbonization to change its combustion mode.

Second, by filling in large amounts of inorganic materials to reduce the amount of combustibles and thus decrease the smoke emission. However, excessive use can cause serious damage to the properties of the products.

Third, by utilizing synergistic effects to achieve composite flame retardancy.

White smoke is mainly caused by the water vapor generated during the combustion of materials, as well as the tiny particles of water vapor condensate suspended in the air. There are also some invisible parts that are gases, such as HCl, CO₂, CO, HCN, and methane, etc.

Water vapor, although harmless to the human body, reduces the transmittance and increases the smoke density. Aluminum hydroxide and magnesium hydroxide play a flame-retardant role by absorbing combustion heat, but the water vapor they produce is the main cause of white smoke. Therefore, achieving a balance between flame retardancy and smoke suppression is crucial.

II. Which Materials Are Prone to Producing Smoke?

The amount of smoke emitted by materials during combustion is generally measured by the maximum specific optical density (Dm), also known as the maximum smoke density. The larger the maximum specific optical density of a polymer, the greater its smoke emission tendency, and the thicker the black smoke it produces during combustion, resulting in more severe environmental pollution. The maximum specific optical densities of common polymers are shown in the table below.

From the table, it can be observed that:

(1) Polymers with polyene structures or benzene rings in the side chains tend to produce more smoke. This is because polyene carbon chains can undergo cyclization, condensation, and the formation of graphitic carbon particles.

(2) Polymers with benzene rings in the side chains (such as polystyrene) easily form conjugated double bonds during combustion, which then cyclize and condense into carbon, resulting in high smoke production.

(3) Polyvinyl chloride (PVC), after releasing hydrogen chloride, can also cyclize and form polymers that produce a large amount of smoke during combustion.

Since the standard for smoke-free combustion of polymers is a maximum specific optical density (Dm) of less than 300, resins with high smoke production such as PET, PC, PS, ABS, and PVC need to be modified for both flame retardancy and smoke suppression, with PVC being particularly important.

III. Selection of Flame Retardants and Smoke Suppressants

In contemporary flame retardant technology, "flame retardancy" and "smoke suppression" are considered together. For some polymers, "smoke suppression" is more important than "flame retardancy," making the development of smoke-suppressing flame retardants very important.

What qualities should a material have to be considered a smoke suppressant?

During combustion, the flame is of a diffusive type, and air convection carries the already formed char into the air, which is the fundamental reason for the increase in smoke production.

If it were possible to anchor the already formed char to the surface of the burning material instead of allowing it to float in the air, it would significantly reduce the smoke density of the material.

The key to realizing this concept is to synthesize or find a compound that can melt between 700~1000℃ and act like an adhesive at that temperature.

At the same time, when designing smoke-free flame retardant formulations, it is essential to choose flame retardants with low smoke production whenever possible. The maximum specific optical densities of various flame retardants are shown in the table below.

Both halogen/antimony flame retardant systems and coated red phosphorus increase the amount of smoke and the diffusion of toxic gases, so when using bromine flame retardant systems, it is necessary to add a smoke suppressant at the same time.

IV. Classification of Flame Retardants and Smoke Suppressants

In summary, smoke suppressants can be divided into two major categories: inorganic and organic, with inorganic smoke suppressants being the most commonly used.

• Inorganic Smoke Suppressants

Inorganic smoke suppressants are primarily metal oxides, hydroxides, and metal salts.

1. Molybdenum compounds are commonly used metal oxide smoke suppressants. The main types include molybdenum trioxide, ammonium octamolybdate, calcium molybdate, calcium phosphomolybdate, zinc molybdate, and the combination of molybdenum compounds with antimony trioxide, copper oxide, iron oxide, and cadmium oxide. These are among the most effective for smoke suppression. Many commercialized and highly efficient molybdenum compound smoke suppressants are molybdenum trioxide and ammonium octamolybdate.

The smoke suppression mechanism of molybdenum compounds involves promoting carbonization through cross-linking in the condensed phase, thereby exerting a smoke suppression effect. For example, molybdenum compounds form char with PVC and other resins during combustion, covering the polymer surface to achieve flame retardancy and smoke suppression.

The general addition level of molybdenum-based smoke suppressants is between 2% and 3%, which can reduce smoke production by 30% to 80%.

2. The smoke suppression mechanism of iron compounds involves promoting carbonization through cross-linking in the condensed phase and also serving as oxidation catalysts, converting carbon in the polymer into carbon monoxide and carbon dioxide. The main types include ferrocene, ferrocene-1,1'-dicarboxylic acid, iron(III) oxide, potassium ferrate, and ferrous oxalate, used in combination with halides.

Ferrocene is a cyclopentadienyl complex of iron(II). Ferrocene is orange-red in color and cannot be used in conjunction with phosphorus-based flame retardants. Also, due to its color, it is not easily promoted for widespread application.

Ferrocene is mainly sold as a smoke suppressant for rigid PVC. Using 0.5 parts per 100 parts of PVC can reduce the smoke production of rigid PVC by 30% to 70%.

During the HCl release process in PVC, ferrocene quickly transforms into α-Fe2O3, which is present in the carbon layer. α-Fe2O3 can cause the carbonized layer to burn and catalyze the oxidation of the carbon layer into CO and CO2, thereby reducing the amount of carbon black formed.

FeCl2 and FeCl3, which are precursors to α-Fe2O3, are also effective smoke suppressants. They improve the pyrolysis process of PVC, making it easier to produce light tar, thus reducing the generation of black smoke.

3. The main types of metal hydroxides are aluminum hydroxide and magnesium hydroxide.

The specific smoke suppression mechanism involves the formation of aluminum oxide and magnesium oxide with a large surface area during heating, which can adsorb smoke and dust. it promotes the formation of char in the solid phase. the exothermic reaction of water turning into steam can dilute flammable gases and smoke. it can react with hydrogen halides released from the thermal decomposition of halogen-containing compounds (capturing hydrogen halides), thereby reducing the amount of toxic gas hydrogen halides in the smoke.

The smoke suppression effect of a single metal hydroxide is already good, but the direct combination of the two or their use in conjunction with molybdenum compounds, metal oxides, and metal complexes can achieve even better results.

4. The main types of metal salts include carbonates such as calcium carbonate, borates such as zinc borate, phosphates such as zinc phosphate, oxalates such as chromium oxalate and copper oxalate, sulfates such as zinc sulfate, stannates such as zinc stannate, and aluminates such as zinc aluminate.

The smoke suppression mechanism of CaCO3 involves its reaction with hydrogen halides in the smoke (capturing) to form stable CaCl2. Since the reaction is a solid-gas heterogeneous reaction, it can only occur on the surface of solid particles, making the particle size of CaCO3 a significant factor in smoke suppression. Only fine particles have a much larger specific surface area.

According to the above smoke suppression principles, any polymer that produces hydrogen halides during combustion, such as vinyl chloride, chlorosulfonated polyethylene, chloroprene rubber, etc., can use CaCO3 as a smoke suppressant.

5. Among nano flame retardant and smoke suppressants, nano double hydroxide composite metal oxides (LDH) are a class of composite metal oxides with a layered structure. Adding 3-5 parts of LDH to PVC can reduce the maximum smoke density during PVC combustion by 30%-50%, with minimal impact on its mechanical properties. Using the method of filling with nano-scale CaCO3 for smoke suppression, a filling amount of only about 10% can produce an ideal effect.

6. Reductive coupling smoke suppressants are additives that promote coupling reactions and can produce zero-valent metals when polymers pyrolyze. These include a range of transition metal carbonyls, formate and oxalate salts of transition metals, complexes of monovalent copper. as well as complexes of monovalent copper with phosphites or other ligands, etc. Among these, copper compounds are one of the most effective types of additives.

Copper (II) compounds can significantly reduce the amount of benzene produced during pyrolysis, and in the presence of Cu2O, the degree of cross-linking of PVC is greatly increased at temperatures between 200℃ and 300℃.

Copper compounds cause cross-linking through reductive coupling. Although copper salts are not prone to catalyzing the isomerization of polyenes (cis-trans isomerization), they can also act as weak acid catalysts to promote Friedel-Crafts alkylation.

As a suitable reductive coupling agent, it should generally have the following properties:

- The electrochemical activity of the metal should be relatively low, meaning that the metal ion should be capable of being reduced to a zero oxidation state.

- In metal oxides, the metal should be in a lower oxidation state, or the metal complex should have an oxidizable ligand that can be removed through thermal reduction to form a lower-valent or zero-valent metal.

- The metal ion should only be reducible at temperatures higher than the processing temperature of the polymer.

- It must be inexpensive, as colorless as possible, and have no adverse effect on the polymer formulation.

• Organic Smoke Suppression Auxiliaries

1. Organosiloxane Series, a new type of halogen-free flame retardant, is also a carbon-forming smoke suppressant. It not only provides polymers with excellent flame retardancy and smoke suppression but also improves the processing performance and mechanical strength of the materials, especially the low-temperature impact strength.

Currently available on the market, the organosiloxane series of flame retardants include SFR100 produced by General Electric Company of the United States. It is a transparent, viscous silicone polymer that can be used in conjunction with various synergists (stearate salts, mixtures of polyphosphoric amines and pentaerythritol, aluminum hydroxide, etc.). It has been used for flame retarding polyolefins, meeting general flame retardancy requirements with low dosages, and providing excellent flame retardancy and smoke suppression with higher dosages.

2. Ferrocene Series, the main types include ferrocene and some salts of organic acids.

The commonly used compounds are ferrocene and some organic iron compounds, which are most suitable as smoke suppressants for PVC. The amount added is about 1.5 parts.

• Synergistic Flame Retardant Systems

Synergistic flame retardant systems refer to flame retardant systems composed of two (one being a flame retardant and the other a synergist) or more components, whose flame retardant effect is superior to the sum of the individual components' effects.

The quality of a synergistic system is often represented by the "Synergistic Efficiency" (SE). SE is defined as the ratio of the flame retardant efficiency (EFF) of the synergistic system to the flame retardant efficiency of a single flame retardant (without a synergist) at the same addition level, and EFF is defined as the increase in the oxygen index (LOI) value of the flame-retarded matrix per unit mass of flame retardant element (within a certain range of addition levels). In most cases, the SE value is calculated based on the results obtained from the synergistic system with the best flame retardant efficiency.

Halogenated flame retardants in flame-retarded polystyrene, ABS, and other plastics can use antimony oxide-inorganic silicate complexes as synergists. This synergist is inexpensive and has low color intensity.

Additionally, compounds composed of antimony compounds/magnesium compounds-zinc compounds, when used as synergists in certain flame retardant systems, can enhance flame retardancy and also provide smoke suppression.

Antimony trioxide and silicon dioxide complexes can be used in polyolefins, ABS, PVC, synthetic rubber, and coatings. Mixtures of antimony oxide and fluoroborates can be used in various thermoplastic resins and engineering plastics.

The complex composed of zinc stearate/talc/iron compounds is also a low-smoke synergist. The appropriate addition of this synergist can reduce the smoke density of some halogen-containing flame retardant plastics and increase the transmittance.

Antimony trioxide also has a synergistic effect on MCA flame retardant PA. MCA can significantly improve the flame retardant performance of PA12 and increase the oxygen index.

The development of flame retardant materials has been a key focus area in the field of fire safety. Traditional flame retardant methods usually use substances that, while effective in slowing the spread of fire, produce large amounts of smoke and toxic gases during the combustion process. Recognizing this, the flame retardant industry has begun to turn to the production of new, non-toxic, environmentally friendly flame retardant materials that minimize the production of smoke and the release of harmful substances.

YINSU Flame Retardants is dedicated to the research and development of halogen-free, low smoke flame retardant solutions. Our commitment to innovation has led to the development of a range of flame retardant products that not only comply with stringent safety standards, but also improve processability and mechanical strength, making them a perfect match for the material. In short, YINSU Flame Retardants is committed to providing the market with advanced flame retardant solutions that prioritize safety, environmental protection and material performance. Through continuous R&D efforts, we are committed to setting new standards in the industry and ensuring that our products meet the evolving needs of our customers and the global community.

In the realm of flame retardancy, the development of materials that mitigate smoke emission and avoid the use of halogens is of paramount importance. This is due to the fact that many fire-related fatalities are not caused by heat but by the toxic and harmful gases released during combustion. The classification and selection of flame retardants and smoke suppressants are crucial, with a focus on inorganic and organic compounds that can effectively reduce smoke production without compromising the material's properties.

YINSU Flame Retardant Company is at the forefront of this industry, dedicated to the research and development of low-smoke, halogen-free flame retardants. Our products, such as the Microencapsulated red phosphorus flame retardants FRP-950X and FRP-8050, are designed to be highly efficient in flame retardancy while producing smoke density. Additionally, our composite T series offers an environmentally friendly, halogen-free alternative to antimony trioxide, allowing for equivalent substitution and maintaining the same level of flame retardancy. YINSU's commitment to innovation in flame retardant technology not only addresses the immediate safety concerns but also contributes to the long-term environmental health by providing sustainable solutions for various applications in the polymer industry. With a focus on excellence and a dedication to environmental responsibility, YINSU is setting new standards in the field of flame retardancy.