PP/PS/PE/PVC Flame Retardant Technology And Application Tips

PP/PS/PE/PVC Flame Retardant Technology and Application Tips

General-purpose plastics have an extremely wide range of applications, covering key materials such as polypropylene (PP), polyethylene (PE), polystyrene (PS), and polyvinyl chloride (PVC), which play an important role in a wide range of industries, including packaging, automotive industry, daily necessities, electronic and electrical components, piping systems, and wires and cables.

Given the importance of flame retardant safety in all types of environments, polymer materials are extremely flammable at high temperatures, accompanied by the release of large amounts of toxic gases, which not only poses a serious threat to the natural environment, but also greatly jeopardizes people's lives, health and quality of life. Therefore, solving the flame retardant safety problem of polymer materials has become a key issue to be tackled urgently.

As a professional in the field of materials science, exploring the mystery of flame retardant mechanism and committing to the research, development and application of new flame retardant materials are not only important for the promotion of social scientific and technological progress, but also necessary for the protection of the country's economic security and the improvement of people's quality of life.

General plastics combustion characteristics and identification:

Polypropylene (PP) materials, whether formed through the polymerization of propylene monomer alone to form homopolymer PP, or propylene and a small amount of ethylene copolymerization to generate copolymer PP, show a highly regular structure and significant crystalline properties. With a melting point close to 167°C and a low density, PP is one of the lightest general-purpose plastics, and is widely used in the manufacture of appliance housings, automotive interior and exterior decorative parts, and electrical and electronic components because of its excellent surface rigidity and flexural fatigue resistance.

Regarding the combustion characteristics of PP, it is a highly flammable material with a low oxygen index of about 17%, which means that it does not require a high concentration of oxygen for combustion. During combustion, PP releases a large amount of heat energy, the flame spreads rapidly, and the CH-component in its chemical structure is not easily converted to charcoal, resulting in the material melting and dripping during combustion. The flame has a unique appearance, with a yellow color at the upper end and a blue color at the lower end, and produces almost no black smoke, accompanied by a faint odor of burning petroleum. It is particularly noteworthy that PP continues to burn even after it is removed from the ignition source, eventually leaving a black, gelatinous residue.

Polyethylene (PE) is a typical crystalline polymer formed by the polymerization of ethylene monomers, with differences in crystallinity resulting in a wide range of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and linear low-density polyethylene (LLDPE) containing small amounts of alpha olefins, etc. PE is well known for its excellent low-temperature resistance, and is stable even at extreme low temperatures below -70°C, while showing good resistance. PE is known for its excellent low-temperature resistance, even in extreme low temperatures below -70°C, and also exhibits good chemical stability and electrical insulation properties, as well as excellent processing performance. Therefore, PE is widely used in many fields such as plastic packaging bags, agricultural mulch, hollow blow molding and injection molding products.

In terms of combustion characteristics, PE is also highly flammable, with an oxygen index of around 17%. At high temperatures, PE cracks and burns extremely quickly. Similar to polypropylene, PE's CH-structure makes it difficult to form a carbon layer, causing the material to melt and drip during combustion. The flame has a distinctive shape, yellow at the top and turning blue at the bottom, and produces little black smoke and a characteristic paraffin burning odor. It is noteworthy that PE continues to burn even after it is removed from the ignition source, eventually leaving a black burning residue.

Polystyrene (PS) is a composite material made from the copolymerization of butadiene and styrene, which internally exhibits a two-phase coexisting system, with the polybutadiene content typically ranging from 5% to 15%. PS offers cost advantages over ABS materials, while exhibiting high strength, excellent rigidity and good dimensional stability. These properties have led to the use of PS in a wide range of applications in consumer packaging materials as well as injection molded parts such as housings for household appliances.

In terms of combustion characteristics, polystyrene has a high calorific value and the combustion process is very violent. Once close to the ignition source, PS will shrink rapidly and it is difficult to form a charcoal layer due to its chemical structure characteristics. During combustion, the surface of PS will soften and produce bubbles, and the flame presents a bright orange color, accompanied by thick black smoke and charcoal ash flying, showing a large smoke density. In addition, a special odor of styrene monomer is emitted during combustion. Even when removed from the ignition source, PS continues to burn and eventually leaves a black burning residue.

Polyvinyl chloride (PVC) is a synthetic polymer material carefully constructed from vinyl chloride monomer by a free radical polymerization process. The molecular chain consists of a continuous series of vinyl chloride units, each of which is unique and contains a carbon atom as the core, supplemented by two hydrogen atoms and a chlorine atom as the flanks.

This unique chemical structure gives PVC a series of compelling physical and chemical properties that make it shine in a variety of fields. not only does PVC exhibit excellent mechanical strength and chemical stability, able to resist a variety of chemical substances, but also naturally has a certain degree of flame retardancy, pure PVC oxygen index of up to 45%, showing a good tendency to self-extinguishing. However, it is worth noting that in the process, in order to improve its performance, often add a large number of plasticizers, this practice to a certain extent at the expense of its original flame retardant advantages, making the flammability increased.

It is based on these advantages, PVC in the construction of building materials, packaging materials, wire and cable insulation, as well as artificial leather manufacturing and other industries occupy a pivotal position, becoming one of the indispensable materials.

When it comes to the combustion characteristics of PVC, its self-extinguishing and charcoal-forming capabilities make the combustion process both challenging and unique. When burning, PVC will gradually soften, the flame shows a unique color change, the upper end of the yellow and the lower end of the green, accompanied by the generation of black smoke. In addition, irritating hydrogen chloride gas is released during combustion, but once removed from the fire, PVC tends to self-extinguish, leaving a black combustion residue.

Application Tips for General Purpose Plastic Flame Retardants:

Halogen flame retardants, despite the challenges of environmental concerns and higher smoke density, are still at the forefront of the global flame retardant market by virtue of their highly efficient flame retardant properties, rich variety options and wide applicability. Brominated flame retardants, in particular, are irreplaceable in terms of their high efficiency and importance as a leader in the halogen series.

Typical representatives of brominated flame retardants include decabromodiphenyl ether, decabromodiphenylethane, tetrabromobisphenol A, bromotriazines, brominated epoxy resins, and brominated styrene, etc., which play a crucial role in flame retardant technology.

And out of the growing importance of environmental protection, non-halogenated flame retardants are gradually becoming the industry's new favorite. They include inorganic flame retardants such as magnesium hydroxide and aluminum hydroxide, phosphorus-based flame retardants (in the form of red phosphorus, aluminum diethyl hypophosphite, inorganic hypophosphite, and phosphate esters), phosphorus-nitrogen synergistic flame retardants (e.g., ammonium polyphosphate, melamine polyphosphate) and nitrogen flame retardants (e.g., melamine and its cyanuronate, etc.).

Through the flexible application of these single or compound formulations of flame retardants, plastic materials can realize diverse flame retardant levels and performance requirements to meet the safety standards of different application scenarios.

• Polypropylene flame retardant applications

1. Flame retardant PP - UL standard V2 grade, the program selection is as follows:

Octabromo ether (also known as octabromosulfide) with the system, the introduction of antimony trioxide as a synergistic flame retardant, the total addition of the two ratios controlled between 6% and 8%, can effectively realize the self-extinguishing function of the material away from the fire, although in the process of combustion there will be a melting drip phenomenon, but the mechanical properties of the material is still maintained at a relatively stable level.

In the phosphorus-nitrogen-bromine compounding system, for homopolymerized PP, the amount of flame retardant added is set between 1% and 2%, while for copolymerized PP, the amount is increased to 4% to 6%. This system also realizes self-extinguishing of the material from the fire, but it is necessary to pay attention to the dripping phenomenon accompanied by combustion and the ignition phenomenon of skimmed cotton. Nevertheless, the mechanical properties of the material under this system are almost the same as those of pure PP without flame retardants, and good physical properties are maintained.

YINSU has specially developed two flame retardants for PPV2 grade, white masterbatch for PPV2-6 and black masterbatch flame retardant PPV2-8H for PP recycled materials. These two flame retardants can reach V2 level with low additive amount of 3%-10%.

2. Flame retardant PP - UL standard V0 grade, the program options are as follows:

The bromine-antimony synergistic system (DBDPE+Sb2O3) is capable of conferring UL-94 V0 level flame retardancy to the material at a total addition of about 25%. However, this high percentage of addition not only pushes up the cost of the material, but also inevitably affects its mechanical properties. To alleviate this problem, it is often necessary to introduce compatibilizers and tougheners to optimize the physical properties of the material, or to find a cost-effective balance by adding fillers such as talc.

For IFR halogen-free systems, i.e., phosphorus-nitrogen intumescent flame retardant systems, PPAP-15, additions in the range of 25% to 30% are also able to achieve UL 94 V0 flame retardancy. However, similarly, high additive levels have a significant impact on the mechanical properties of the material. Therefore, tougheners and other auxiliary additives are also needed to enhance the overall performance of the material to meet the actual application requirements.

On the other hand, although the inorganic magnesium hydroxide (MDH) system, as a traditional halogen-free flame retardant method, can be added in large quantities (at least 50-60%) to significantly enhance the flame retardancy and oxygen index of PP, such high additions will undoubtedly seriously impair the mechanical properties of the material. In order to mitigate this side effect, the strategy that can be adopted is to use it in combination with other flame retardants to keep the mechanical properties of the material within an acceptable range by reducing the amount of inorganic flame retardants.

• Polyethylene Flame Retardant Applications

Polyethylene flame retardant solution selection:

Red Phosphorus (RP) Flame Retardant Solution: In the flame retardant treatment of PE materials, the red phosphorus system is recognized as one of the most efficient flame retardants. However, due to safety considerations, the actual application is mostly in the form of modified and coated red phosphorus masterbatch, whose additive amount is about 15%, which can make the material reach the V0 flame retardant grade under the thickness of 1.6mm in UL94 standard.

YINSU Flame Retardant Company, has developed various red phosphorus flame retardants for PE, including FRP-950X, microencapsuled red phosphorus flame retardant, addition at 3%~5% for UL94-V0. PEG-14, specifically for PE pipe.

Antimony bromide (DBDPE/Sb2O3) synergistic flame-retardant system: the system through about 25% of the total amount of additives, can also make the material to meet the UL-94 standard under the thickness of 1.6mm V0 flame retardant requirements. In order to further optimize costs, mineral fillers can be added in appropriate amounts. In addition, through the addition of toughening agent, can effectively reduce the negative impact strength of the material, to ensure that the material in the flame retardant at the same time to maintain good physical strength.

IFR halogen-free flame retardant system: For PE system, it should be noted that the use of APP-containing flame retardant formulations should be avoided, so as not to adversely affect the flame retardant properties. On the contrary, the use of phosphorus and nitrogen compounding flame retardant, in the total addition of 25~26%, can achieve UL94 standard under the 1.6mm thickness of the V0 flame retardant grade. It is worth noting that this system is usually not recommended to add mineral filler, so as not to significantly affect the flame retardant effect.

Inorganic Magnesium Hydroxide (MDH) and Aluminum Hydroxide (ATH) Flame Retardant System: These two inorganic flame retardants in a large number of use (more than 60%), can significantly improve the oxygen index of the material to more than 30, and give it a low smoke density characteristics, suitable for low-smoke halogen-free flame retardant material needs. In order to further enhance the flame retardant effect, it can also be considered with red phosphorus (RP) or IFR system for compounding.

• Polystyrene Flame Retardant Applications

Flame retardant program selection:

Bromine-antimony system: Generally, the ratio of bromine and antimony is 3:1. There are more bromine flame retardants suitable for polystyrene, each with different advantages and disadvantages, and they can generally meet the requirements of flame retardancy, so it is necessary to select the flame retardant system according to the characteristics of the products.

IFR/expandable graphite system: The layer structure of expandable graphite can form a special type of intercalation compound. Some research shows that expandable graphite and phosphorus and nitrogen flame retardants can achieve better flame retardant effect when used in combination.

In YINSU company, the IFR flame retardants PPAP-15, and expandable graphite EG, are with long term development and innovations. Both items can achieve high performance.

IFR+PPO system: IFR and polyphenylene ether are chosen as the composite flame retardant system for compounding flame retardant PS, which can effectively improve the flame retardant performance of PS. PPO has very good charcoal formation performance and has good synergistic flame retardant effect with IFR. However, due to the poor UV resistance of PPO, the mobility is relatively low, making the application of products have limited occasions.

Inorganic magnesium hydroxide flame retardant system: by adding a large amount of magnesium hydroxide inorganic flame retardant, can achieve flame retardant effect, can also be compounded with red phosphorus flame retardant, to get higher flame retardant materials. However, due to the addition of large amounts of magnesium hydroxide, the toughness of the material has an impact on the need for toughening and compatibility modification, in order to obtain the desired mechanical properties.

l Application of Polyvinyl Chloride Flame Retardants

Flame retardant selection program:

Metal oxides are used as synergistic flame retardant enhancers for PVC. Given that PVC materials are rich in chlorine, their flame retardancy can be significantly enhanced by adding CPE (chlorinated polyethylene) and specific metal oxides to the PVC matrix in appropriate amounts. Of the metal oxides, zinc stannate shows the best performance in terms of its ability to improve the oxygen index of the material, followed closely by Sb2O3 (antimony trioxide), ammonium octamolybdate, and zinc borate, each of which shows a different gradient in flame retardancy.

The antimony trioxide replacement in YINSU Company, T series, can replace antimony completely. Not only lower the cost, but also keep the original performance.

Inorganic aluminum hydroxide and magnesium hydroxide systems:

Magnesium hydroxide and aluminum hydroxide, as inorganic flame retardant additives, not only effectively reduce the amount of smoke released from PVC materials during combustion, but also significantly enhance their flame retardant properties and allow for a reduction in the amount of other flame retardants used. These inorganic mineral flame retardants have a profound effect on the physical and mechanical strength, flame retardant level, and smoke reduction properties of rigid PVC.

The experimental results show that hard gypsum powder, as a filler reinforcing agent, significantly improves the oxygen index of PVC materials compared to conventional heavy calcium carbonate, while exhibiting better environmental characteristics. Further, when hard gypsum powder is synergized with magnesium hydroxide and metal oxide flame retardant synergists, flame retardant materials with higher oxygen index and more environmental friendly properties can be prepared.

In order to further optimize the flame retardant properties of PVC, it is an effective strategy to partially replace the combustible plasticizers with the flame retardant plasticizers TCPP or tetrabromophthalic anhydride ester (B45-Z), which is particularly effective due to its high bromine content, although it will correspondingly increase the hardness and density of the material and bring a certain cost burden. In contrast, TCPP, although lower in cost, is slightly less effective in flame retardancy. Therefore, when selecting alternative plasticizers, the relationship between cost and performance needs to be balanced according to specific needs.

Conclusion

General-purpose plastics, due to their wide application coverage, especially occupying an important position in many safety-critical areas, the optimization of their flame retardant properties has become a key issue to be solved. In this process, exploring and applying the most suitable flame retardant technology strategy is crucial to ensure the safety of materials in a wide range of applications.

For example, YINSU Flame Retardant may offer a series of flame retardant products. Their PP flame retardants such as PPV2-8H and PPAP-15, along with red phosphorus flame retardants like FRP-950X and PEG-14 for PE, as well as the antimony replacement T series, contribute effectively to enhancing the flame retardant properties of corresponding materials, providing reliable options to meet different application requirements and ensure safety in various industries.