Background
Most polymer materials are flammable or combustible materials. When burning, they have the characteristics of fast burning speed, high heat generation, large smoke production and release of toxic gases. Statistics show that the main cause of casualties in fires is not the fire, but the smoke and toxic gases released during combustion, which seriously endanger the safety of people’s lives and property. Therefore, the research on smoke suppression and flame retardancy of polymer materials is equally important. How to improve the flame retardancy and suppress the generation of gunpowder of synthetic polymers and natural polymer materials has become an urgent problem to be solved, which has important social and economic significance.
Classification and characteristics of flame retardants
At present, there are many types of flame retardants. According to different classification standards, flame retardants can be divided into the following categories:
Classification criteria | Specific content | Features |
According to the flame retardant elements contained | Halogen series , phosphorus series, nitrogen series, sulfur series, phosphorus-halogen series, phosphorus-nitrogen series, | Halogen series has the largest output, small addition amount, and significant flame retardant effect . |
By flame retardant component | Inorganic salts: inorganic water, metal compounds, antimony series, silicon series | Inorganic flame retardants are halogen-free flame retardants that have the advantages of high safety, smoke suppression, non-toxicity, and low price. |
By usage method and polymer form | Two categories: additive and reactive | Additive type is mainly mechanical flame retardant, and online reactive type is mainly organic flame retardant |
Flame retardant mechanism
A combustion reaction generally requires three elements: combustible materials, oxygen and a certain temperature, and none of them can be missing. The mechanism of action of a flame retardant should be to inhibit the production of one or more elements when the material burns, so as to achieve the purpose of preventing or slowing down combustion.
The basic principle is to reduce the generation of combustible gases during thermal decomposition and hinder the basic reactions in the gas combustion process, absorb the heat in the combustion area, dilute and isolate the air, and also have a certain effect on preventing combustion.
Detailed explanation of flame retardant process
Process Classification | Specific details | Features |
Endothermic cooling |
| Physical Effects |
Gas phase dilution | During the combustion process of materials, a large amount of combustible gases , such as carbon monoxide, will be produced. The presence of flame retardants can produce a large amount of non-flammable gas, effectively diluting the flammable Gas or air, thereby achieving flame retardant effect on materials | Physical Effects |
Forming a thermal insulation layer | Some flame retardant materials (such as phosphoric acid and boric acid) melt when heated, forming A glass-like membrane that blocks the supply of oxygen and acts as a heat insulator and reduces The amount of combustible gas released, thus producing a flame retardant effect | Physical Effects |
Termination of free radical chain reactions | During the combustion of polymers, a large number of free radicals are generated to accelerate the gas phase combustion reaction. By capturing and eliminating these free radicals and reducing the free radical density, combustion can be controlled . Convert these high-energy free radicals into stable free radicals, inhibiting excessive To achieve flame retardant purpose | Chemical effect (carbon Chemical action, elimination Free radical action and Phosphate |
From the perspective of polymer combustion, the flame retardancy of materials is mainly achieved through three pathways:
First, it inhibits the free radicals that play a role in chain growth in the combustion reaction and isolates oxygen;
The second is to prevent the thermal decomposition of polymers and the release of combustible gases from polymers in the solid phase, such as grafting and cross-linking modification or catalytic carbonization;
The third is to slow down heat generation and heat transfer, such as cooling and flame retardancy.
Flame retardant properties of various flame retardants
Flame retardant properties of inorganic flame retardants
The main classifications, action mechanisms, advantages and disadvantages of inorganic flame retardants are shown in the following table. At present, the research on inorganic flame retardants is very hot. On the one hand, it is due to the demand for environmental protection, and on the other hand, the cost of this type of material itself is relatively low.
Inorganic classification | Specifically include | Mechanism of action | Advantage | Shortcoming | Remark |
Inorganic water alloy Generic compounds | Magnesium hydroxide Mg ( OH ) ₂ , aluminum hydroxide Al ( OH ) ₃ , hydrotalcite , Mg ₆ Al ₂ (OH)1 ₅ CO ₃ ·4 H₂O , calcium aluminate 3CaO ·Al ₂ Oz · 6H₂O | Reduce the concentration of combustible polymers; dehydration absorbs heat and inhibits the temperature rise of polymers; the water vapor generated by decomposition dilutes the concentration of combustible gases and oxygen, which can prevent combustion; the generation of oxides on the surface of combustibles can prevent combustion | Good thermal stability, non-toxic , non-volatile, no corrosive gas, small smoke, no secondary pollution | Large amount of addition High dehydration temperature | Al(OH)3 The earliest and most widely used |
Red Phosphorus |
Various red phosphorus particles and microgels bag | Make most polymers have good flame retardancy at relatively low concentrations Yes, free radicals can be generated in the gas phase Flame retardant, can form foam in condensed phase Flame retardant | The processing is stable. The physical properties of the matrix No effect on electrical performance | Easy to generate by heat Highly toxic phosphorus gas Hydrogen 2 PH ₃ , powder dust easy burst fry In polymer Not easy to disperse, etc. | Inorganic Widely used, research Most |
Boron compounds |
Mainly zinc borate products |
Heat absorption and dilution, as well as the formation Insulation layer | Excellent flame retardant, Smoke, arc extinguishing Performance, reduce product Release toxic and harmful |
Combined with halogen Combination of | The surface of the combustible material will be covered with Some non-volatile Zinc and Boron acid |
Antimony compounds |
Antimony trioxide, colloidal Antimony oxide and sodium antimony | Antimony oxide is covered in liquid or solid form On the surface of the burning object, isolate the air Gas; due to melting and volatilization absorption Heat reduces the combustion temperature; The decomposition products can dissipate the energy of the flame amount, changing the chemical process of combustion |
Applicable materials Many types | After becoming vapor of oxygen change antimony Formed in flames Antimony Object and element map By the base | For plastic Products, Textiles, Rubber, wood |
Flame retardant mechanism of organic flame retardants
The specific classification, mechanism of action, advantages and disadvantages of organic flame retardants are shown in the following table. Among them, halogen-based organic flame retardants are cheap and stable, but they are too toxic and cause serious damage to the human body, and have been gradually abandoned by various countries.
Organic classification | Specifically include | Mechanism of action | Advantage | Shortcoming | Remark |
Halogen | Nitrogen paraffin, CPE , etc. Decabromodiphenyl ether, Decabromodiphenyl ether Diphenylethane, tetrabromobis(2-bromobis(2-phenylethane) Phenol A, octabromoether, bromine Polystyrene, brominated Epoxy resin, etc. | Decomposition upon heating produces hydrogen halides. Through physical processes (including heat absorption and Temperature, gas phase dilution, and free radical mechanisms Flame retardant | Mature application, reasonable price Cheap, good stability, Small dosage, good compatibility |
| Production and use Maximum usage Currently already Listed as prohibited series |
Organophosphorus | APP and phosphates | Condensed phase flame retardant mechanism, through endothermic Cooling, gas phase dilution, thermal insulation Layer and terminate the white radical chain reaction diameter to achieve flame retardancy of the material. | Good flame retardant performance Has dual functions of flame retardant and plasticizer able | Sometimes factorization is used | Research prospects broad |
Nitrogen series | Dicyandiamide, diurea Guanidine compounds, trimer Cyanamide and its salts |
noodle | Halogen-free, low toxicity, low smoke Heat and UV stable Good flame retardant efficiency Cheap endothermic decomposition produces non- flammable gas body ( N H ₂ H₂ NCN , N₂ , NOx.CO.H₂O ) |
The gases produced are still toxic and polluting. Infectious
|
Research prospects broad
|
Intumescent flame retardant is a new type of composite flame retardant that has attracted wide attention at home and abroad in recent years. It is a composite flame retardant mainly composed of nitrogen and phosphorus. It has a flame retardant mechanism and unique characteristics of halogen-free, low smoke and low toxicity. It is in line with the research and development direction of flame retardants in the future and has become one of the most active flame retardant research fields at home and abroad.
Intumescent flame retardants have three basic elements, namely acid source, carbon source and gas source.
Basic Elements | Related explanation | Specific representatives | Remark |
Acid source | Dehydrating agent or carbonization accelerator, generally an inorganic acid or a compound that can generate acid in situ during combustion | Phosphoric acid, boric acid, sulfuric acid and phosphorus Acid esters, etc. | Main, largest proportion, Contains flame retardant elements |
Carbon source | Also called carbonizing agent, it is used to form the foam carbonization layer The basis is mainly some polyhydroxy Compound | Starch, sucrose, dextrin, Pentaerythritol, ethylene glycol, phenolic acid Resin, etc. | Synergist |
Gas source | Foaming source, which is a nitrogen-containing compound | Urea, melamine Amine, etc. | Synergist |
Its flame retardant mechanism is relatively complex. Endothermic cooling, dilution, formation of thermal insulation layer and termination of free radical chain reaction can all be reflected in the flame retardant process. Its specific manifestations are:
1. Forming a porous foam carbon insulation layer through expansion. At a relatively low temperature, the non-bicyclic phosphorus in the compound produces phosphoric acid that can be used as a dehydrating agent; the water vapor produced by the reaction and the non-flammable gas produced by the gas source cause the molten system to expand and foam. At the same time, the polyol is dehydrated and carbonized to form inorganic matter and carbon residues, and the system further expands and foams; when the reaction is nearly completed, the system gels and solidifies, and finally forms a porous foam carbon layer, which prevents heat transfer, reduces the amount of combustible gas released and isolates oxygen, so as to achieve the purpose of flame retardancy.
2. Intumescent flame retardants can release various non-flammable gases at the combustion temperature. Such as: CO2, NH3, N2, HX, these gas components dilute the concentration of combustible gas in the gas phase, reduce it to below the ignition limit, and play a gas phase flame retardant effect.
3 Phase change occurs at high temperature! Endothermic decomposition reactions such as dehydration or dehydrohalogenation reduce the temperature of the polymer substrate and the flame zone, slow down the speed of thermal cracking reaction, and then reduce the amount of flammable gas volatilization, and finally destroy the conditions for maintaining continuous combustion of the polymer to achieve the purpose of flame retardancy.
4 Polymers produce many high-energy free radicals during combustion. Intumescent flame retardants produce substances (PO•) that can capture free radicals during the decomposition process, thereby cutting off the free radical chain reaction to achieve the purpose of flame retardancy.
Flame retardant basic parameters
UL-94 vertical burning test
Under specified conditions, the standard sample is hung vertically, and the length or time of burning of the sample is measured after ignition to characterize the flame retardancy of the material. Including: UL94V0, UL94V1, UL94V2, UL94V5.
UL94V0 evaluation method: After the flame is removed after ignition, the sample can quickly self-extinguish and no burning melt drips within a certain time interval (that is, the burning melt drips onto a cotton pad one foot below the test sample and cannot ignite the cotton).
The UL94V1 evaluation method is similar to V0, except that it requires a longer self-extinguishing time. This test allows the melt to drip onto the cotton pad, but the cotton cannot be ignited.
UL94V2 is the same as V1, except that it allows burning molten droplets to ignite cotton one foot below.
UL94 V5 is a strict test method, which involves the actual life of plastic products in flames. The experiment requires a flame length of 5in, and the test sample is subjected to five combustions, during which no molten droplets are allowed, the test sample is not allowed to have obvious distortion, and no burnt holes are allowed.
Horizontal burning test
It is divided into flaming combustion and flameless combustion. Flaming combustion time: Under the specified sample conditions, after the ignition source is removed, the material flame continues to burn for a certain time. Flameless combustion time: Under the specified sample conditions, when the smoky combustion stops or the ignition source is removed, the material continues to burn without smoke.
Limiting oxygen index test
The limiting oxygen index refers to the volume fraction concentration of oxygen in a polymer in a mixture of oxygen and nitrogen that can just support its combustion. It is an index that characterizes the combustion behavior of a material. The limiting oxygen index can be determined by a candle test, where a polymer stick is burned downward under specific conditions. It is generally believed that when the limiting oxygen index is 22, the substance can burn in air.
Thermal performance analysis
With the help of thermogravimetric analyzers and cone calorimeters, the decomposition temperature of the material is mainly measured to determine the safe temperature for use and processing temperature.
Development direction of flame retardants
With the increasing application of polymer materials and the growing calls for environmental protection, higher and higher requirements are being placed on the flame retardant properties of materials. In order to better meet the flame retardant requirements of synthetic materials and improve the effect and scope of flame retardant use, the focus should be placed on the following aspects in the future:
1 For some traditional flame retardants, due to certain defects in their flame retardant properties, research on this type of flame retardants (defect repair) should be carried out, such as surface modification of inorganic flame retardants, nano-technology, etc.
2 Exploring the synergistic flame retardant mechanism of flame retardants% A single flame retardant may not have a good flame retardant effect, but if used in combination with other flame retardants, a more ideal effect may be achieved.
3. Research environmentally friendly flame retardants. With the continuous development of modern science and technology and the enhancement of environmental protection awareness, environmentally friendly flame retardants will surely receive more and more attention. This is the only way for future development.
4 Developing a new type of composite flame retardant that can flame retard materials from multiple aspects such as condensed phase, gas phase, and liquid phase is also an important direction for the future development of flame retardants.
