Analysis of PA6 Flame Retardant Mechanism And Common Flame Retardants

Analysis of PA6 Flame Retardant Mechanism And Common Flame Retardants

As the preparation technology becomes more and more mature, PA6 has become a popular polymer material in many fields such as electrical and electronic, automotive, and communication. Especially PA6 composites have more diverse structures and functional parts. When applied in these fields, PA6 composites often face extreme working conditions such as high temperature, flammability, leakage, short circuit, etc., in which flammability becomes one of the important indexes of whether PA6 composites can work safely and normally.

Unmodified PA6 is inherently flame retardant up to UL94 V-2 with an ultimate oxygen index between 20-22%. This means that in the case of contact with an open flame, PA6 will burn quickly, while there is a low drop that causes the open flame to spread. PA6 composites complicate this metric: some of the composite components help PA6 burn, such as common glass fibers that make the material burn faster due to the candlewick effect.

As we all know, the industry of automobiles, electrical appliances and other products on the use of materials will have very strict flame retardant requirements. Therefore, taking into account the good flame retardant and mechanical properties of PA6 is very research and commercial value, especially in the PA66 price remains high today, high flame retardant PA6 composites have great potential.

In this article, we will analyze the strategies to inhibit PA6 combustion from the principle, and the current application of common flame retardants.

1. Combustion mechanism of PA6

To extinguish the fire, the most critical thing is to know how the fire burns. Combustion is generally divided into evaporation combustion, thermal decomposition combustion and solid surface combustion in three forms, PA6 and most polymerized materials belong to the thermal decomposition combustion.

The main combustion processes are as follows:

First of all, the material is heated up by heat, when the overall temperature of the material rises to about 200℃, the material will have obvious melting and softening, and the polymer molecules on the surface of the material begin to undergo thermal oxidative decomposition;

At further elevated temperatures, the thermo-oxidative decomposition reaction is more adequate and generates a large number of free radicals, which bind to the methylene groups in the molecular structure of PA6, thus accelerating decomposition;

PA6 in a large number of polar bonds so that this material has a strong water absorption, high temperature action of the amide bond hydrolysis will also occur simultaneously, the end product of hydrolysis for the carbon-containing small molecules of combustibles, mainly has been lactam and cyclopentanone and so on;

These small combustible molecules mix well with oxygen by diffusion and convection at high temperatures and eventually ignite. The heat generated during this process is not only released to the outside world, but also acts on the PA6 itself, i.e. the combustion process continues even if the external heat source is removed.

This is how PA6, and the vast majority of polymers, burn. Understanding this process leads to a look at how PA6 should be designed in terms of flame retardancy.

2. Design of flame retardant PA6

As we all know, the essence of flame retardation is to stop or slow down the action of combustion factors through physical and chemical actions. In the case of PA6, it is the 4 major factors of heat source, air, combustibles and free radical reaction.

Without changing the PA6 matrix, the addition of flame retardants is an important method to eliminate the PA6 burning conditions. Different flame retardants play their flame retardant mode of action varies, according to the specific mode of action of flame retardants, flame retardants can be divided into condensed phase flame retardant mode, gas phase flame retardant mode and synergistic flame retardant mode.

Vapor phase flame retardant mode

It means to act as a flame retardant in the gas phase to inhibit or interrupt the combustion reaction of a flammable gas mixture.

Vapor phase flame retardation can be done in two specific ways:

One of them is the thermal decomposition of flame retardants to produce free radical trapping agents, thus interrupting the free radical reaction and thus inhibiting the combustion reaction;

The second is the thermal decomposition of the flame retardant releases inert gases, filled in the vicinity of the combustion center, which will be near the center of the combustion of oxygen and gas-phase combustible concentration of significant dilution, thereby inhibiting the formation of combustion conditions, and play a role in flame retardant.

Condensed phase flame retardant mode

Cohesive phase flame retardant means that the corresponding flame retardant mainly plays a flame retardant effect within the cohesive component, thus delaying or preventing the thermal decomposition process of the polymer, and then play a role in inhibiting the combustion of the polymer.

There are also two specific types of condensed phase flame retardation:

One of them is that the flame retardant is decomposed by heat during the combustion process, thus absorbing a large amount of heat generated in the combustion, so as to prevent the combustion from taking place;

The second is the chemical reaction of flame retardants at high temperatures, thereby generating solid metal oxides (such as aluminum oxide, boron oxide and magnesium oxide, etc.) or high-density vapors, the above products can be covered in the surface of the combustion material, blocking the polymer material and the outside world of the exchange of substances and energy, in order to inhibit the combustion process.

Synergistic flame retardant model

In addition, some flame retardants have both gas-phase and condensed-phase flame retardant mechanisms, and these flame retardants are considered to exhibit synergistic flame retardant mechanisms. As the flame retardant acts in both the gas phase and condensed phase, the combustion of the polymer is more strongly inhibited.

Therefore, from the point of view of specific effects, the flame retardant that exerts a synergistic flame retardant effect can provide a more efficient flame retardant effect, thereby reducing the amount of flame retardant in PA.

3. Application of different flame retardants

According to the combination of flame retardants and PA6 matrix, the flame retardants used in PA6 can be divided into two categories: reactive flame retardants and filler flame retardants.
Reactive flame retardants

Among them, reactive flame retardants are added during the polymerization and preparation of PA6 or processing and molding, and such flame retardants can be chemically grafted in the molecular chain of PA6 so as to introduce flame retardant elements or groups into PA6.

Reaction-acting flame retardants have good stability and less influence on the performance of PA6 itself, however, the process of reaction-acting flame retardants has complicated process conditions and high cost, so these flame retardants are not easy to be applied in large-scale industrial production of flame retardant PA6 composites.

Filler-acting flame retardants

Comparatively speaking, filling-type flame retardant is more economical, easy to use, and is the main flame retardant for the preparation of flame-retardant PA6 composites in the current industry, while in the filling-type flame retardant, according to the chemical structure of its effective components, it can be subdivided into halogenated, phosphorus, nitrogen, and inorganic flame retardant and other major categories.

Different classes of flame retardants have different flame retardant efficiencies, and at the same time, the structure of the flame retardant has a certain effect on the basic physical and mechanical properties of PA6.

Therefore, the key point of preparing high-performance flame retardant PA is to consider both flame retardant and mechanical factors, and reasonably select the type of flame retardant to be used.

Halogenated flame retardants

Halogenated flame retardants are widely used in PA6 due to their good compatibility with PA6 and high flame retardant efficiency.

At the same time, halogenated flame retardants can also be used together with metal oxide flame retardants, phosphorus-containing flame retardants, carbon forming agents, etc. to play a synergistic flame retardant role. At present, bis(hexachlorocyclopentadienylcyclooctene)decabromodiphenyl ether (DBDPO), 1,2-bis(pentabromophenyl)ethane (BPBPE), brominated polystyrene (BPS), pentabromodiphenyl ether (PBDO), poly(dibromostyrene) (PDBS), pentabromopentabromopentaacrylic acid (PPBBA), brominated epoxy resin (BER) are the commonly used flame retardant for flame-retardant PA6 materials.

On the basis of the above flame retardants, some domestic scholars have tried to develop decabromodiphenyl ethane to replace decabromodiphenyl ether, in order to solve the problem of dioxin produced by the flame retardant, but also decabromodiphenyl ethane and antimony trioxide and used to improve the flame retardancy of PA6, when the two use the ratio of 13:5, flame retardant modified PA6 flame retardant grade can be reached UL94 V-0 level, at the same time, the other properties of the material are with the At the same time, the other properties of the material are comparable to those of pure PA6.

Phosphorus flame retardants

Halogenated flame retardants in the use of the process there is a "secondary disaster" risk, and such flame retardants there is a very serious environmental pollution problems, non-halogenated flame retardant instead of halogenated flame retardant is the current trend of flame retardant development.

Among the non-halogen flame retardants, phosphorus flame retardants have the largest production volume and the widest application range. In terms of flame retardant mechanism, phosphorus flame retardants mainly play a cohesive phase flame retardant mechanism.

I. Red phosphorus

Red phosphorus is a typical inorganic flame retardant, because its composition contains only phosphorus, so in 7% of the addition amount, can significantly improve the flame retardancy of PA6, so that it reaches UL94 V-0 grade.

However, red phosphorus is chemically active and susceptible to oxidation when stored under conventional conditions, and at the same time, pure inorganic phosphorus is not compatible with organic PA matrix; to solve the above problems, red phosphorus is usually prepared as a microencapsulated flame retardant for use.

Studies have shown that adding 16% microencapsulated red phosphorus to 15% glass fiber reinforced PA6 can increase the ultimate oxygen index of the material to 28.5%, and the material's flame retardant rating can reach UL94 V-0.

II. Ammonium polyphosphate

Ammonium polyphosphate is another important inorganic phosphorus flame retardant, which is commonly used in PA6 materials, and studies have shown that when ammonium polyphosphate is used alone, its dosage is more than 30% before the flame retardant effect is sufficiently obvious.

Ammonium polyphosphate and other phosphorus flame retardants and can improve its flame retardant efficiency, the results show that in the amount of ammonium polyphosphate added to 25%, the peak rate of heat release of the material decreased by 44.3%, the total heat release decreased by 20.2%, the flame retardant properties of PA6 can be significantly improved.

However, the researchers also found that simply increasing the amount of ammonium polyphosphate is difficult to overcome the flaming drop phenomenon in PA6 combustion, so it is necessary to consider adding certain anti-drip additives to PA6 using ammonium polyphosphate as a flame retardant.

Nitrogen-based flame retardants

Nitrogen-based flame retardant is also a commonly used non-halogenated environmentally friendly flame retardant with the advantages of low toxicity, good thermal stability, low price, non-corrosive and so on.

Nitrogen compounds containing triazines in their molecular structure are a class of nitrogen flame retardants widely used in PA6 flame retardant modification, and melamine (MA) and its inorganic and organic acid salts are typical representatives of this class of compounds.

I. MA

Among them, the improvement effect of MA on PA6 flame retardancy is more obvious. In order to overcome the poor dispersion of MA in PA6 matrix, generally need to be compounded with other components for use.

BASF compounded MA with fluoride to prepare the KR4025 series of flame retardants, which are used in PA6 to give the material both high toughness and good flame retardancy.

II. MCA

MCA is essentially a large planar structural complex composed of MA and cyanuric acid under the action of hydrogen bonding, and in recent years, the use of MCA as a flame retardant modification of PA6 has been a hot topic.

Melamine polyphosphates are capable of acting as flame retardants alone or in combination with inorganic oxides. It has been found that a nitrogen-phosphorus-containing synergistic flame retardant was produced from melamine and polyphosphates, and when the flame retardant was used in 25% of glass fiber reinforced PA6, the flame retardancy of the corresponding glass fiber reinforced PA6 could reach UL94 V-0, while the tensile strength, tensile modulus of elasticity, notched impact strength, bending strength and bending modulus of elasticity of the material could reach 76.8 MPa, 11.7 GPa, 4.5 kJ/m2, 98 MPa and 7.2 GPa, respectively. 11.7GPa, 4.5kJ/m2, 98MPa and 7.2GPa respectively.

Inorganic flame retardants

Inorganic flame retardants utilize the characteristics of inorganic materials that are difficult to burn, and have the advantages of low generation of harmful soot, good thermal stability, and no susceptibility to degenerative failure. Currently, metal hydroxides and inorganic nanofillers are the two main types of inorganic flame retardants used in flame retardant PA6.

Magnesium hydroxide and other flame retardant components and can also play a good synergistic flame retardant effect. Domestic scholars will be magnesium hydroxide and aluminum hydroxide in the ratio of 3:1 compound as a flame retardant, glass fiber reinforced PA6 in the use of tensile strength of the material can be maintained at more than 100MPa, bending strength of more than 150MPa, the limit of the oxygen index reaches 31.7%.

In addition to improving the flame retardancy of PA6, inorganic nano-fillers can also improve the abrasion resistance of the material, improve the electrical and thermal conductivity of the material, and improve the coloring effect of PA6. In addition, inorganic nano-fillers are inexpensive, and filling in PA6 has a significant effect on reducing the overall cost of the material.

Currently, commonly used inorganic nano-fillers include limestone, montmorillonite, talc, silica, silicone, wollastonite, and calcium sulfate. These inorganic fillers themselves are incombustible, and at the same time can play a role in accelerating the charring of PA6 combustion, reducing the PA6 melt droplets, and blocking the transfer of heat and small molecules. Inorganic nano-fillers and other types of flame retardants used in the flame retardant PA6, can achieve the desired flame retardant effect, there are a lot of research results in this regard.

4.The development trend of flame retardant PA6

At present, researchers tend to use physical compounding of flame retardants, chemical combination of flame retardants, and modified flame retardants to solve the above problems, and the related research has made some progress.

Through the design of in-situ reaction, the effective flame retardant components accounted for a higher proportion, contains a variety of effective flame retardant structure, flame retardant process does not produce toxic and harmful substances, and better compatibility with the amide structure of the flame retardant filled with PA6 is one of the trends of the future development of flame retardant PA6 materials.

In addition, the development of customized flame retardant solutions for reinforced PA6 and functional PA6 materials is also a direction for the development of flame retardant PA6 composites.

With the continuous progress of science and technology and the rapid development of material science, we have higher and higher requirements for the safety and functionality of materials. Especially in the wide application of plastic products, flame retardant performance has become one of the important indexes to measure the safety of materials.

Relying on its profound technical accumulation and innovation ability in the field of flame retardants, YINSU Flame Retardant Company has launched a series of PA6 flame retardant products, which not only include traditional red phosphorus flame retardants, but also cover bromine antimony flame retardants and more environmentally friendly bromine antimony replacements. These diversified flame retardant solutions provide customized solutions for different industries and application scenarios, ensuring that the materials meet the flame retardant requirements while being environmentally friendly and cost effective. This series of products from YINSU Flame Retardant Company undoubtedly makes an important contribution to the safety enhancement and sustainable development of plastic products.