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In a paper published in the journal Polymers, researchers reviewed the combination of synergistic phosphorus-based flame retardants (P-FRs) and expandable graphite (EG). They discussed the flammability and smoke toxicity of polyurethane foams (PUFs) when burnt, the types of PUFs, the effective approach for flame-retardancy, and the efficiency of EG and P-FRs combinations.

Study: It Takes Two to Tango: Synergistic Expandable Graphite–Phosphorus Flame Retardant Combinations in Polyurethane Foams. Image Credit: prapann/Shutterstock.com

PUFs are synthetic polymers, rich in hydrocarbons and thus highly inflammable. One of the main methods used to improve fire protection involves the addition of flame retardants to polymers. Flame retardancy is defined by the flame retardant’s reaction mechanism with polymers as well as other additives, synergists, and even other flame retardants. The ignition scenario is also one of the factors to be considered.

Fire protection can be approached in view of rising fires or completely developed fires. The same polymer can favor different flame retardants depending on whether the application is as a composite, in bulk, or fiber or foam form. Thus, various combinations of flame retardants protect consumer goods containing PUFs.

One of the highly sought-after flame retardants applied in present-day goods is a synergistic combination of EG with P-FR. Their specific interactions provide effective flame retardancy through charring and maintaining a thermally insulated residue morphology.

Since flame retardants can deter the thermal decomposition of PUFs, their recyclability becomes a more apparent issue than pure PUFs. PUFs on ignition release poisonous gases such as nitrogen oxides, carbon monoxide, and hydrogen cyanide, posing a threat to human life. As a result of current trends as well as future environmental regulations, further breakthroughs are warranted regarding the manufacturing of PUFs and their flame retardants.

Depending on the chemical characteristics of polyols and isocyanates, PUFs are divided into rigid polyurethane foams (RPUFs) and flexible polyurethane foams (FPUFs). RPUFs are commonly used in transportation, construction, and refrigeration due to the greater crosslink density. FPUFs are flexible, offer various degrees of cushioning, and are often used in mattresses, furnishings, and packaging.

Regardless, both forms of PUFs are highly flammable. FPUFs show higher flammability than RPUFs due to the open-cell structure and greater surface-to-mass ratio. FPUFs produce pool fires due to a lesser tendency to char.

Flame retardants are selected based on processing, structure-property relationship, polymer matrix compatibility, costs, and PUF applications. Both additive and reactive flame retardants are introduced into the foam matrix to suspend ignition and the release of heat, thus retarding flame spread. Multiple studies have reported that a mixture of two or more flame retardants in one compound can elevate flame-retardant efficiency, a phenomenon termed synergism.

Additive flame retardants depreciate the morphology and mechanical properties of PUFs, while reactive flame retardants do not damage the structure and improve the flame retardancy of PUFs. Aromatic polyols also enhance flame retardancy by promoting char yield while burning.

Flame retardants act in a condensed phase or the gas phase. Those working in the condensed phase increase carbonaceous char while reducing the release of flammable volatiles, while those working in the gas phase release non-flammable gases during decomposition, thus reducing the effective combustion heat. 

When the protective layer and flame inhibition are combined, EG and P-FRs complement each other and a synergy between the two occurs. Upon ignition, the top layer of EG expands, and glassy polyphosphate is formed from the decomposition of the phosphorus compound. This polyphosphate glues the char together and strengthens it. Thus, a layer is formed against the external heat flux protecting the underlying material.

The expanded graphite particles are linked to the carbonaceous char by the phosphorus residue. This adhesion also prevents crack formation while burning, subsequently protecting the underlying materials. As a result of incomplete burning, the total heat release decreases significantly.

Overall, the review demonstrated that the synergistic effects between EG and P-FRs provide improved flame retardancy while balancing the mechanical properties of the PUFs, considering the fire safety regulations. Higher flame retardancy can be achieved for PUFs with the appropriate types and amounts of EG and P-FRs. This can be further enhanced by including other flame-retardant elements with adjustments to the PUF chemistry in line with the combinations.

Although effective, phosphorus-based flame retardants pose noteworthy hazards to human life, affecting reproduction as well as the occurrence of allergies and asthma. The review also discussed some natural materials that hold the potential to act as flame retardants due to their chemical structure or flame-retardant properties. Notable examples include deoxyribonucleic acid (DNA), phytic acid, chitosan, and lignin among others.

Chan, Y.Y.; Schartel, B. It Takes Two to Tango: Synergistic Expandable Graphite–Phosphorus Flame Retardant Combinations in Polyurethane Foams. Polymers 2022, 14, 2562. https://doi.org/10.3390/polym14132562

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Chinmay Saraf is a science writer based in Indore, India. His academic background is in mechanical engineering, and he has extensive experience in fused deposition-based additive manufacturing. His research focuses on post-processing methods for fused deposition modeling to improve mechanical and electrical properties of 3D printed parts. He has also worked on composite 3D printing, bioprinting, and food printing technologies. Chinmay holds an M.Tech. in computer-aided design and computer-aided manufacturing and is passionate about 3D printing, new product development, material science, and sustainability. He also has a keen interest in "Frugal Designs" to improve the existing engineering systems.  

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