Analysis Of PP Flame Retardant System And Its Application Prospect

Analysis Of PP Flame Retardant System And Its Application Prospect

Polypropylene (PP), as one of the five general-purpose plastics, has a wide range of applications in all walks of life, however, the flammable characteristics of PP also limits its application space, hindering the further development of PP materials, so the flame retardant modification of PP has been the focus of attention.

Next we will take a look at the combustion process and flame retardant mechanism of polymer materials represented by PP, the inventory of flame retardant PP, the outlook for the application of flame retardant PP in the field of packaging and the current problems.

I. Combustion Process And Mechanism Of Polymer Materials

1.Combustion Process

Polymer materials are polymer compounds containing carbon, hydrogen, oxygen and other elements in the molecular chain, and most of the polymers are combustible.

The combustion of polymer materials is the synthesis of a series of physical changes and chemical reactions, so in the combustion process of polymer materials will show special phenomena such as melting and softening, volume changes and so on.

The combustion process of polymer materials is shown in Figure 1, which can basically be divided into the following three steps:

(1) With the gradual increase in temperature, the weaker bonds in the molecular chain will be broken, and the material begins to undergo thermal decomposition. As the thermal decomposition of polymer materials continue to carry out and intensify, the surface of the material gradually produce small molecules of gas, most of these gases are flammable, these small molecules of flammable gases mixed with oxygen in the air, thus forming a flammable gas mixture.

(2) With the decomposition reaction, the combustible concentration of the gas mixture on the surface of the polymer material gradually increases, and when the concentration of the combustible gas mixture and the external ambient temperature reaches the critical conditions required for combustion, a violent chemical reaction occurs, and the surface of the material is rapidly ignited.

(3) The rapid combustion of the combustible gas mixture releases a large amount of heat, which not only spreads to the bottom of the material, but also further raises the temperature of the surrounding environment of the material, thus accelerating the decomposition of the material, which produces more combustible gases, and ultimately makes the combustion reaction continue. Therefore, the combustion of polymer materials can be viewed as a process of gradual promotion and cyclic reaction.

As a hydrocarbon, PP's oxygen index is only 17.4, easy to burn, poor flame retardancy, and greater heat when burning, accompanied by dripping easily caused by fire, posing a threat to life and property.

In the field of electronic and electrical appliances, this flammable property of PP limits its wider application, so it is necessary to carry out research and development of flame retardant PP materials.

2.Flame Retardant Mechanism

The flame retardant mechanism can be roughly divided into two categories: chain reaction termination mechanism, surface isolation mechanism and interruption of heat exchange mechanism.

(1) Chain reaction termination mechanism: When PP burns, it first decomposes into hydrocarbon, and then further thermal oxidative cleavage into free HO- at high temperature, the chain reaction of HO- is the reason why the combustion can be sustained, and the termination of the chain reaction is to consume the HO- produced in the process of combustion.

(2) surface isolation mechanism, PP in the combustion, flame retardant not only absorbs heat, but also in the PP surface to generate solid compounds, the compounds play a role in blocking the matrix and air contact, thus preventing combustion.

(3) Interruption of heat exchange mechanism. This mechanism refers to the flame retardant in the combustion process can absorb a large amount of heat of combustion, so that the combustion reaction lacks sufficient heat, and then self-extinguishing phenomenon, to achieve flame retardant effect.

II. PP flame Retardant Modification

1. Metal Hydroxide Flame Retardant

The activated carbon in the metal hydroxide flame retardant has a large specific surface area and is rich in functional groups, which can be well combined with the hydroxyl group on the particles of sodium magnesium hydroxide, effectively weakening the surface polarity of the magnesium hydroxide, thus reducing the possibility of agglomeration, improving the compatibility of sodium magnesium hydroxide and PP matrix, and enhancing the flame retardant properties of the material.

In addition by testing the change of oil absorption value, the optimal ratio and the optimal activation degree of the flame retardant can be further adjusted, and finally it was found that the limiting oxygen index reached the maximum value of 28.9% when 25 wt% activated charcoal-modified magnesium hydroxide flame retardant was added to the PP matrix.

Metal hydroxide flame retardant, which was used to improve the flame retardancy of PP materials, was added with polyolefin elastomer (POE) and nano-calcium carbonate CaCO3 to improve the mechanical strength of the materials. The results show that the modified PP composites can have both strong flame retardant properties and high mechanical strength.

2. Boron Flame Retardant

In PP/BN@MGO composites, due to the encapsulated structure and alkylation modification of BN@MGO flame retardant, its alkyl linkage branching efficiency is high, and carbon can be enriched on the surface of the filler, which significantly enhances the affinity between the BN@MGO flame retardant and the PP body, so that it can be uniformly distributed in the PP matrix.

Meanwhile, the modified treatment BN@MGO has zigzag path effect and high thermal stability, which makes the material have lower thermal expansion coefficient and higher flame retardant performance, and these characteristics can make the composite material of PP/BN@MGO have a wide application space in the field of high-efficiency heat dissipation electronic devices, household appliances and thermal management.

With 25 wt% APP/MCA-K-ZB filler (APP/MCA-K-ZB mass ratio of 3/1), the PP composites can achieve V-0 rating in UL-94 test, while the ultimate oxygen index is as high as 32.7%.

Meanwhile, thermogravimetric analysis (TGA) and scanning electron microscope (SEM) tests show that the addition of APP/MCA-K-ZB can form a dense graphite carbon layer, and this dense carbon layer can effectively protect the PP matrix underneath from further combustion, thus improving the thermal stability and carbon formation ability of PP composites.

3. Phosphorus Flame Retardant

Phosphorus flame retardant in sorbitol with a large number of hydroxyl groups, easy to form a charcoal layer in the combustion, and ammonium polyphosphate decomposition of the moment of heat to produce phosphate compounds, further enhance the charcoal effect of sorbitol, the production of charcoal layer slows down the propagation of heat and isolate the oxygen, improve the flame retardant properties of the material.

In addition, sorbitol as a shell can play a good dispersion role, hindering the aggregation of MCAPP particles, and better distribution can simultaneously improve the flame retardant properties and mechanical strength of the material.

SPDEB is compounded with ammonium polyphosphate as a flame retardant to improve the flame retardancy of PP materials. In the system, SPDEB will decompose amino radicals and alkyl radicals when subjected to heat, and both of them can capture the highly active radicals generated by the thermal decomposition of polymers, blocking the decomposition of PP chains and reducing the production of combustible substances, thus playing the role of delaying and terminating combustion.

When SPDEB is used together with ammonium polyphosphate, the ammonium polyphosphate can promote the dehydration of SPEDB into charcoal and solidify its charcoal layer, which can reduce the leakage of charcoal particles during combustion and reduce the emission of combustible gases.

4. Nitrogen-based Flame Retardant

MPP will produce non-combustible gases (including NH3, NO and H2O) and some phosphorus-containing substances during combustion, while AP can release aluminum phosphate Al2(HPO4)3 and phosphine (PH3) gases at high temperatures, which not only dilute the combustible gases in the air, but also act as gas shielding on the surface of the material, thus reducing combustion.

In addition, this MPP can volatilize phosphorus-oxygen reactive radicals into the gas, trapping highly reactive radicals and thus terminating the main chain breaks of PP.

Supramolecular self-assembly is a method to synthesize compounds with specific functions and well-defined structures using non-covalent bonds such as hydrogen bonds and ionic interactions. In the APP@MEL-TA system, MEL-TA interconnects with ammonium polyphosphate through electrostatic interactions to cover the surface of ammonium polyphosphate, which improves the dispersion of ammonium polyphosphate in PP materials.

At the same time, MEL-TA has high nitrogen content and decomposes by heat to release a large amount of non-combustible gases, which cover the surface of the material to reduce the concentration of oxygen on the surface of the material and further improve the flame retardant ability.

5. Intumescent Flame Retardant

NiCO₂O₄ has the advantages of controllable morphology, large specific surface area, multiple active sites, and easy and diverse preparation methods, which, as a nickel-based compound, possesses excellent carbon-catalyzing ability, both in terms of reducing combustion products and improving flame retardancy.

This is mainly attributed to the presence of Ni+, which accelerates the thermal decomposition of PER and enhances the charring of ammonium polyphosphate, contributing to the formation of an expanded carbon layer in the PP/IFR system. Meanwhile, the bimetallic oxides, with their high stability and strong catalytic ability at high temperatures, are able to promote the formation of a dense and homogeneous carbon layer in the PP/IFR material, and to improve the thermal stability of both the layer and the carbon residue.

In addition, the flower-like NiCO₂O₄ has a large number of folds, and the contact surface with the polymer is large and rough, which enhances the bonding force, and the flower-like structure has strong stability, which can avoid it being damaged during processing and maintain the structural integrity.

At the same time, the charcoal-forming substances in the combustion process can be fixed between the flower-like structure to improve the stability of the charcoal layer, thus effectively playing the role of a barrier to realize the flame retardant and protection of the substrate.

OS-MCAPP is a kind of APP treated with SiO₂ gel, which, while acting as a gas and acid source, also helps PP to form a protective char layer and protects the PP substrate from further decomposition.PEIC, as an excellent source of char, its presence plays a great role in the formation of high-quality expanded char and promotes the obtaining of excellent flame-retardant composites.

PPA-C reacts with PER during combustion to form P-O-C bonds and P-C bonds, which contribute to the formation of virtually defect-free char layers. In addition, PPA-C can cause PP to thermally decompose earlier and form more char residues at higher temperatures.

Meanwhile, PPA-C and PER have good synergistic effect, and the flame retardancy of PPA-C/PER is superior to the traditional APP/PER system. When the content of PPA-C/PER (3:1) reaches 18wt%, PP/IFR composites reach V-0 rating in UL-94 test, and the ultimate oxygen index can reach 28.8%.

III. Application of Flame Retardant PP in Packaging Field

PP plastic has low density, good transparency, non-toxic and tasteless, easy processing and molding, low price and other characteristics, so that it has a huge application value in the field of packaging, but the defects of PP plastic such as flammability, poor high temperature resistance and other defects limit its development in the field of packaging. Therefore, in recent years, many scholars are committed to researching PP packaging materials with high flame retardant properties.

1.Automobile Battery Shell

Battery is one of the most important parts of new energy vehicles, and the battery shell responsible for protecting the safety of the battery is also particularly important, requiring the battery packaging with insulation, impact resistance, corrosion resistance, good flame retardant properties.

Traditional battery packaging is mainly made of metal materials and sheet molding compound (SMC). However, some of these two materials have complex molding process and high density, which affects the lightweight of new energy vehicles, so low-density, impact-resistant PP materials have received attention.

Using PP resin as the matrix, ammonium polyphosphate/triazine complex system as the flame retardant, ethylene-octene copolymer, propylene-based elastomers and EPDM as the toughening agent, a PP material with flame retardant properties is prepared by melt blending method, which is used as the battery shell of new energy vehicles.

This PP material has good flame retardant properties and impact strength while maintaining low density, and has good sealing and waterproof performance, which is now put into production in batch.

2.Parts Packaging

PP/MHSH/Al2O3/N-P composites were prepared by melt blending method by modifying magnesium sulfate whisker (MHSH) and alumina (Al₂O₃), introducing cross-linking agent KH-550 to the surfaces of both of them, and adding nitrogen-phosphorus complex flame retardant and PP matrix, and were further processed into films.

The nitrogen-phosphorus flame retardant, in addition to promoting the formation of an expanded carbon layer in the PP matrix at high temperature, reacts with MHSH to form a magnesium phosphate salt with good thermal stability, and the presence of the magnesium phosphate salt enhances the strength of the expanded carbon layer and plays the role of skeleton support.

The addition of Al₂O₃ can improve the thermal conductivity of the material, so that the internal heat of the material is quickly transferred to the surface, which plays a role in heat dissipation, thus improving the heat resistance of the material. Meanwhile, MHSH and Al₂O₃ are rigid fillers with good mechanical strength, which can improve the mechanical properties of PP/MHSH/Al₂O₃/N-P composite film.

Therefore, PP/MHSH/Al₂O₃/N-P composite film has both excellent flame retardant properties and high mechanical strength, expanding the scope of application of PP composites.

3.Food Containers

Using ammonium polyphosphate as the gas and acid source, triazine carbonic agent as the carbon source, and then with the synergistic agent, together constitute IFR, and then with the clean treatment of recycled polypropylene lunch boxes through the melt blending method to prepare PP composite materials with high flame retardant properties, proving that the PP lunch boxes have a huge potential in recycling.

IV. Current Issues in PP Flame Retardant Research

In recent years, more and more people have begun to study the flame retardant PP composites, but the flame retardant research of PP is mainly characterized by the following problems:

(1) Flame retardant additive is large, poor compatibility with the matrix, causing too much damage to the mechanical properties of the material, affecting the use of PP composites.

(2) Most of the current flame retardants with high flame retardant efficiency contain halogens, which do not meet the requirements of green environmental protection.

(3) Flame retardants are more expensive, increasing the production cost of flame retardant PP materials.

Conclusion

YINSU Flame Retardant specializes in providing comprehensive flame retardant solutions for PP materials, and its product line covers a wide range of highly effective flame retardants to meet the needs of different application scenarios. For applications seeking the highest flame retardancy standards, the company's PP V0 grade flame retardants, such as PPV0-P-32M, which utilizes red phosphorus flame retardant technology, can ensure that the material achieves a UL-94 V0 rating under rigorous testing, and is suitable for use on PP new, reclaimed, and homopolymerized and copolymerized materials. Meanwhile, for cost-sensitive applications, PP V2 grade flame retardant PPV2-8H provides a cost-effective and efficient flame retardant option for PP recycled materials, which not only reduces material costs but also maintains good flame retardant properties.

In addition, YINSU Flame Retardant's T3 product, which is an ideal partner for bromine flame retardants, can synergize with them to significantly enhance the flame retardant effect of PP materials, easily reaching the V0 grade standard. The company has also launched a non-halogenated flame retardant, piperazine pyrophosphate PPAP-15, a product that is not only environmentally friendly but also has a high flame retardant efficiency, providing the market with a green flame retardant option.

YINSU Flame Retardant has always been committed to providing efficient and cost-effective flame retardant solutions for mainstream PP materials, both new and recycled. Through continuous technological innovation and product optimization, YINSU Flame Retardant helps customers to ensure product quality and safety while achieving cost reductions and promoting the sustainable development of the industry.