Is flame retardant equal to non-flammable? What is the concept of flame retardant?
Many people who do not understand flame retardant will have this concept: flame retardant is to make the material non-combustible. In fact, this is not the case. Flame retardant is the process of preventing or inhibiting the speed of physical or chemical changes in the burning process of polymer materials.
Plastic flame retardant is divided into grades, and the plastic flame retardant grade is gradually increased from HB, V-2, V-1 to V-0, and the higher flame retardant grades than V-0 grade are 5V, 5VB, 5VA .
Isn't the higher the flame retardant grade, the better? Generally speaking, people will think so, and hope to achieve the highest flame retardant level. But the actual situation is not what you want. First of all, the existing flame retardant test methods have limitations, and the conclusions obtained are also relative. In addition, under some application conditions, plastics with low flame retardant grades are better
For example, in some electrical products, the material is required to have resistance to arc ignition. In this case, if a V-2 grade plastic is used, it will be better than a V-0 grade plastic.
This is because the V-2 grade plastic will not form electricity and become coked under the action of electricity, thereby reducing the possibility of fire, but the V-0 grade plastic has the possibility of fire. Therefore, the actual application needs comprehensive consideration.
Flame retardant mechanism of polymer materials
The combustion of polymers is a very intense and complex thermal oxidation reaction, which is characterized by the emission of dense smoke or blazing flames.
The general process of combustion is that under the constant heating of an external heat source, the polymer first undergoes a free radical chain degradation reaction with oxygen in the air to produce volatile combustibles, which will ignite and burn when it reaches a certain concentration and temperature. A portion of the heat released supplies the degrading polymer, which further aggravates its degradation and produces more flammable gases. The flame will quickly spread and cause a fire in a short time.
The flame retardant exerts its flame retardant effect through several mechanisms, such as endothermic effect, covering effect, chain reaction inhibition, suffocation effect of non-combustible gas, etc. Most flame retardants achieve the purpose of flame retardant through a combination of mechanisms.
1. Heat absorption
The heat released during the heating process heats up the raw materials and quickly reaches the ignition point. The principle of endothermic and flame retardant is that the material needs to absorb a lot of heat during the combustion process, which can reduce the heating process of the material, so that the gasified combustible molecules are cracked It is difficult for the free radicals to reach the ignition point, thus inhibiting the combustion to a certain extent.
For example, the flame retardant mechanism of Al (OH) 3 flame retardant is to increase the heat capacity of the polymer to absorb more heat before reaching the thermal decomposition temperature, thereby improving its flame retardant performance. This type of flame retardant fully exerts its characteristics of absorbing a large amount of heat when combined with water vapor, and improves its own flame retardant ability.
The high temperature generated when the material burns causes the flame retardant to form a glass-like or stable foam covering layer, which isolates oxygen. This not only insulates the heat and oxygen, but also prevents the combustible gas from escaping outside, thus achieving the purpose of flame retardancy. Organic phosphazene flame retardants crosslink or form a carbonized layer when heated. This can prevent the polymer from further pyrolysis, on the other hand, it can prevent the internal thermal decomposition products from entering the gas phase to participate in the combustion.
3. Inhibition of chain reaction
According to the chain reaction theory of combustion, it is free radicals that are needed to maintain combustion. The flame retardant can act on the gas-phase combustion zone to capture the free radicals in the combustion reaction, thereby preventing the flame from spreading, reducing the flame density in the combustion zone, and ultimately reducing the combustion reaction rate until it terminates.
Such as: halogen-containing flame retardant, its evaporation temperature and polymer decomposition temperature are the same or similar, when the polymer is thermally decomposed, the flame retardant also volatilizes at the same time. At this time, the halogen-containing flame retardant and the thermal decomposition products are in the gas-phase combustion zone at the same time, and the halogen can capture the free radicals in the combustion reaction, thereby preventing the propagation of the flame, reducing the flame density in the combustion zone, and finally reducing the combustion reaction rate until the termination .
4. Suffocation of non-combustible gas
The flame retardant decomposes non-combustible gas when heated, and dilutes the concentration of combustible gas decomposed by combustibles below the lower combustion limit. At the same time, it also has a dilution effect on the oxygen concentration in the combustion zone, preventing the continuation of combustion and achieving the effect of flame retardancy.
Flame retardant classification
According to the relationship between the flame retardant and the flame-retardant material, flame retardants can be divided into two categories: additive and reactive. The former is only physically dispersed in the substrate, and is mostly used in thermoplastic polymers. The latter is mainly used as a monomer or an auxiliary reagent to participate in the synthesis reaction of the polymer, and finally becomes a structural unit of the polymer, which is mostly used in thermosetting polymers.
According to the classification of flame retardant elements, commonly used flame retardants are divided into halogen, phosphorus, nitrogen and halogen-phosphorus, phosphorus-nitrogen, antimony, magnesium-aluminum, boron, silicon, molybdenum, etc. . There is also a class of intumescent flame retardants, most of which are composites of phosphorus-nitrogen compounds; and a nano-inorganic substance, mainly layered silicate; the latter can be flame retardant with a series of high polymers Polymer / inorganic nanocomposites.
How to choose flame retardant
There are many types of flame retardants, and their flame retardant effects are also different. Different polymer materials have different flame retardants, but the basic requirements are the same.
1. High flame retardant efficiency and high efficiency / price ratio;
2. The ecological and environmental protection requirements are strict, so it should have the following ecological and environmental characteristics:
(1) Harmless to people, animals and plants;
(2) Difficult to migrate;
(3) The amount of toxic and corrosive gases (including smoke and dust) released during thermal cracking or combustion should be small;
(4) Easy to recycle, no deterioration or minor deterioration of the performance of mechanical recycling products;
(5) Compatible with the environment.
3. Good compatibility with the flame-retardant substrate;
4. Good thermal stability, the decomposition temperature is preferably between 250 ℃ ~ 400 ℃;
5. Little influence on the processing performance and physical mechanical properties of the flame-retardant substrate;
6. Has a certain light stability;
7. The source of raw materials is sufficient, and the price can be accepted by downstream users.