Flame Retardant + Smoke Reduction, An Article for You To Understand The Classification And Selection of Polymer Flame Retardant Smoke Suppressants

Flame Retardant + Smoke Reduction, An Article for You To Understand The Classification And Selection of Polymer Flame Retardant Smoke Suppressants

In a fire, most people do not die because of the high temperature, but mostly due to the combustible material in the combustion process releases toxic, harmful gases, resulting in poisoning or insufficient oxygen intake and death. Therefore, the research and development of new non-toxic, harmless, combustion process does not produce or less toxic, harmful gases and soot, flame retardant materials with good flame retardant properties is the current flame retardant field focus on the direction of research.

At present, mainly through the addition of transition metal oxides, magnesium-zinc complexes, metal hydroxides, tin oxide, ferrocene, copper oxide and other smoke suppressants to solve the problem of excessive smoke in the fire. New smoke suppressants, combustion without toxic, harmful gases produced by the flame retardant is the future trend of flame retardant materials research and development.

General polymers in the combustion process will produce a lot of smoke, which part of the polymer combustion smoke is toxic. When the polymer is added to the flame retardant, especially halogenated flame retardants and antimony trioxide, combustion will produce more smoke and toxic gases.

Smoke Composition

Black smoke is the solid particles and agglomerates suspended in the products of combustion gases.

It is generally believed that there are three ways to reduce the concentration of black smoke: one is to draw on the coverage effect, transfer effect, inhibition of free radicals, accelerate the formation of carbon and other flame retardant principles to change the combustion pattern; the second is a large number of filling inorganic materials, by reducing the content of combustible materials to reduce the amount of smoke, but the dosage of too large will result in serious damage to the performance of the product; the third is to use synergistic effect, the composite of flame retardant.

White smoke is mainly due to water vapor generated when the material is burning, and water vapor condensate and other tiny particles suspended in the air. There are also some invisible parts as gases, such as HCl, CO₂, CO, HCN and methane.

Although water vapor is harmless to humans, it makes the light transmission rate decrease and the smoke density increase. Aluminum hydroxide, magnesium hydroxide due to the absorption of heat of combustion and play a flame retardant role, but the generation of water vapor is the main cause of white smoke. Therefore, how to reach a balance between flame retardant and smoke suppression is crucial.

Which materials tend to produce smoke

Measure the size of the material burning smoke generally use the maximum specific optical density (Dm), also known as the maximum smoke density. The greater the maximum specific optical density of polymers, the greater the smoke, the thicker the black smoke when burning, the greater the pollution of the environment. The maximum specific optical density of commonly used polymers is shown in the table below.

From the table can be seen:

1. polyolefin structure or polymers with benzene ring on the side chain, the amount of smoke is more, this is because the polyene carbon chain can be cyclized, polycondensation, the generation of graphitized carbon particles;

2. while the side chain has a benzene ring on the polymer (such as polystyrene) combustion is easy to generate conjugated double-bonded unsaturated hydrocarbons, and then the carbon chain cyclization polycondensation into charcoal, the amount of smoke is high.

3. Polyvinyl chloride dehydrogen chloride cyclization is also easy to form a large amount of combustion smoke polymer.

Because of the polymer combustion smoke-free standard for the maximum specific optical density Dm below 300, so PET, PC, PS, ABS and PVC and other large amount of smoke emitting resins, flame retardant at the same time also need to carry out smoke modification, of which PVC is particularly important.

Selection of flame retardant and smoke suppressant

In contemporary flame retardant technology, “flame retardant” and “smoke suppression” are comparable, for some polymers, “smoke suppression” is more important than “flame retardant”. For some polymers, “smoke suppression” is more important than “flame retardancy”, so it is very important to develop smoke-suppressing flame retardants. What kind of material has the conditions of smoke suppressant?

When a material burns, the flame is diffused, and the convection of the air brings the generated carbide into the air, which is the fundamental reason for the increase in the amount of smoke. If the generated carbide can be solidified on the surface of the burning material, rather than floating in the air, it will greatly reduce the smoke density of the material. The key to realizing this idea is to synthesize or seek a compound that can be melted between 700~1000 ℃, the performance of the temperature similar to the adhesive.

At the same time, in the design of smokeless flame retardant formulations, as far as possible to choose a low smoke flame retardant, the maximum specific optical density of various types of flame retardants is shown in the table below. Halogen/antimony flame retardant system and coated red phosphorus increase the amount of smoke and the diffusion of toxic gases, so in the use of bromine flame retardant system, you need to add smoke suppressant at the same time.

To summarize, smoke suppressant additives can be divided into two categories: inorganic and organic, and inorganic smoke suppressant additives are most commonly used.

(1) Inorganic smoke suppressant additives

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

a. Molybdenum is commonly used metal oxides smoke suppression aids, the main varieties of molybdenum trioxide, octamolybdate amine, calcium molybdate, calcium phosphomolybdate, zinc molybdate, etc., as well as molybdenum compounds and antimony, copper oxide, iron oxide, cadmium oxide, etc. and with the best varieties of smoke suppression effect. A large number of commercialized high-efficiency molybdenum compounds smoke suppressants are molybdenum trioxide and ammonium octamolybdate.

Molybdenum compounds for the principle of smoke suppression in the condensed phase through crosslinking to promote the formation of charcoal, to play the role of smoke suppression. For example, molybdenum compounds form residual charcoal with PVC and other resins during the combustion process, covering the surface of the polymer to achieve the effect of flame retardant and smoke suppression. Add molybdenum-based smoke suppressant general amount of 2% -3% between the amount of smoke can be reduced by 30% -80% of the amount of smoke.

b. Ferrochemical smoke suppression mechanism in the condensed phase through crosslinking to promote the formation of carbon, but also as an oxidation catalyst, the polymer will be converted into carbon monoxide and carbon dioxide carbon dioxide. The main varieties are ferrocene, ferrocene 1,1-dicarboxylic acid, ferric oxide, potassium iron oxalate, ferrous oxalate, etc., and halides and use.

Ferrocene is a cyclopentadienyl complex of ferrous iron. Ferrocene is orange-red color, can not be used with phosphorus-containing flame retardants. And because of the color, it is not easy to promote the application. Ferrocene is mainly sold as smoke suppressant for rigid PVC. Every 100 PVC with 0.5 copies, can make the hard PVC smoke reduction of 30% to 70%. Ferrocene is rapidly converted to α-Fe2O3 in the process of PVC de-HCl, which exists in the carbon layer. α-Fe2O3 can cause the carbonized layer to scorch and catalyze the oxidation of the carbon layer to become CO and CO2, which reduces the amount of carbon black formation.

FeCl2 and FeCl3 are the predecessors of α-Fe2O3 generation, they are also effective smoke eliminators, they change the cracking process of PVC for good, so that it is easy to generate light tar, thus reducing the generation of black smoke.

c. The main varieties of metal hydroxide are aluminum hydroxide and magnesium hydroxide. Specific smoke suppression principle is formed in the heating process of alumina and magnesium oxide has a large surface area, can adsorb soot; in the solid phase to promote the formation of charcoal; exothermic water into water vapor, can be diluted combustible gases, dilution of smoke; can be heat decomposition of halogen-containing compounds with the release of hydrogen halide reaction (capture of hydrogen halide), thereby reducing the amount of smoke in the toxic gases hydrogen halide. Single metal hydroxide smoke suppression effect has been very good, but the two direct compound use or with molybdenum compounds, metal oxides, metal composite use of better results.

d. The main varieties of metal salts are carbonates such as calcium carbonate, borates such as zinc borate, phosphates such as zinc phosphate, oxalates such as chromium and copper oxalate oxalate, etc., sulphates such as zinc sulfate, etc., stannates such as zinc stannate, aluminate such as zinc aluminate.

CaCO3 smoke suppression principle is that it can be and smoke in the hydrogen halide reaction (capture), so that the generation of stable CaCl2. Because of the reaction is solid - gas non-homogeneous reaction, can only be carried out on the surface of the solid particles, so the size of CaCO3 particles become a major factor in the effect of smoke suppression. Only tiny particles have a much larger specific surface. According to the above smoke suppression principle, all polymers that produce hydrogen halide when burning, such as vinyl chloride, chlorosulphonated polyethylene, neoprene rubber, etc., can use CaCO3 as a smoke suppressant.

e. Nano flame retardant smoke suppressant nano double hydroxyl composite metal oxides (LDH) is a class of composite metal oxides with a layered structure, add 3-5 parts of LDH in PVC can make the maximum smoke density of PVC combustion decreased by 30% -50%, and less impact on its mechanical properties. Filled with nanoscale CaCO3 method to carry out smoke suppression, the filling amount is only about 10%, it will produce the desired effect.

f. Reduction coupling smoke suppressant, can promote the coupling process of additives in the polymer cleavage can produce zero-valent metal compounds, including a series of transition metal carbonyl compounds, transition metal formate and oxalate, monovalent copper complexes; and monovalent copper complexes containing phosphite or other ligands, and so on, of which the copper compounds are one of the most effective additives of this kind.

Cu (II) compounds can greatly reduce the amount of benzene generated during cracking, and the degree of cross-linking of PVC is greatly increased when Cu2O is present at 200℃~300℃. Copper compounds cause cross-linking by reductive coupling, although copper salts do not readily catalyze isomerization (cis-trans isomerization) of polyenes, although they can also act as weak acid catalysts to promote Friedel-Crafts alkylation.

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

1.The electrochemical activity of the metal should be low, i.e., the metal ion should be reducible to a zero oxidation state;

2. In the metal oxide, the metal should be a lower oxidation state, or the metal complex should have an oxidizable ligand and be capable of generating a low- or zero-valent metal by thermal reductive elimination;

3. The metal ion should be above the polymer processing temperature in order to be reduced;

4. Must be inexpensive, as far as possible colorless, and no adverse effects on the polymer formulation.

(2) Organic smoke suppressant additives

a. Organic silicon system, is a new type of halogen-free flame retardant, but also a charcoal-type smoke suppressant, which gives polymers excellent flame retardant smoke suppressant at the same time, but also to improve the processing properties of the material and improve the mechanical strength of the material, in particular, low-temperature impact strength. Currently on the market silicone flame retardant SFR100 produced by General Electric, it is a transparent, viscous silicone polymer, with a variety of synergistic agents (stearate, amine polyphosphate and pentaerythritol mixtures, aluminum hydroxide, etc.) and has been used for flame-retardant polyolefins, low dosage to meet the general requirements of flame retardancy, high dosage can be endowed with the substrate with excellent flame retardant and smoke suppression.

b. Ferrocene system, the main varieties of ferrocene and some salts of organic acids. Commonly used is ferrocene and some organic iron compounds, they are most suitable for PVC smoke suppressant, adding amount of 1.5 parts.

(3) Synergistic flame retardant system

Synergistic flame retardant system refers to the flame retardant system composed of two (one of which is a flame retardant and the other is a synergistic agent) or more than two components, whose flame retardant effect is better than the sum of flame retardant effect of each component. The advantages and disadvantages of the synergistic system are often expressed as “synergistic efficiency” (SE), which is defined as the ratio of the flame retardant efficiency (EFF) of the synergistic system to that of a single flame retardant (without synergistic agent) in the system (with the same additive amount), and the EFF is defined as the increase in the oxygen index (LOI) of the flame retarded substrate per unit mass of the flame retardant element (within a certain range of additive amount). the amount added is within a certain range). In most cases, the SE value is based on the results obtained from the synergistic system with the best flame retardant efficiency.

For halogenated flame retardants in flame retardant polystyrene, ABS and some other plastics, antimony oxide - inorganic silicate complex can be used as a synergist. This synergist is inexpensive and has low color strength. In addition, antimony compounds / magnesium compounds - zinc compounds composed of the complex as some of the flame retardant system synergistic agent, both to improve the flame retardant effect, but also can play a smoke suppression effect.

Zinc stearate / talc / iron compounds composed of the complex is also a low-smoke synergistic agent, adding the appropriate amount of this synergistic agent, can make some halogen-containing flame retardant plastics, smoke density decreased, light transmittance increased.