Evaporation-Induced Flame Retardant Coating: A One-Step Polyelectrolyte Complex for Durable Fire Protection of Cotton

Cotton is an essential material woven into the fabric of everyday life, prized for its softness, breathability, and comfort. Its natural origins and widespread use in textiles, upholstery, and industrial applications make it a staple in homes and industries worldwide. However, this ubiquitous material has a critical drawback—it is highly flammable. With a limiting oxygen index of approximately 18%, cotton ignites easily, burns rapidly, and leaves little residue after combustion. This fundamental weakness poses a significant safety hazard, increasing the risk of fire-related property damage and personal injuries.  For decades, scientists and engineers have sought ways to reduce the flammability of cotton. Historically, flame retardancy was achieved by incorporating halogen-based chemicals that interfered with combustion by releasing radicals to suppress flame propagation. While effective, these compounds raised serious concerns due to their environmental persistence and toxicity, leading to their gradual phase-out in many regions. The shift away from halogenated flame retardants has driven research toward alternative, safer, and more sustainable solutions. Among the most promising approaches are halogen-free flame retardant chemistries based on phosphorus, nitrogen, silicon, and boron. These elements are often integrated into intumescent systems that form protective char barriers when exposed to heat, preventing the spread of fire. One such approach involves using polyelectrolytes, which have been extensively explored for their ability to create flame-retardant coatings. Polyethylenimine, a nitrogen-rich polymer, releases nonflammable gases such as ammonia and nitrogen upon heating, helping to suppress flames. Meanwhile, sodium hexametaphosphate functions as an acid source, breaking down into phosphoric acid during decomposition, which catalyzes char formation and enhances flame resistance. When combined with a carbon-based substrate like cotton, these materials can create an effective intumescent barrier. While traditional layer-by-layer (LbL) assembly methods for depositing polyelectrolytes have shown promise, they present scalability challenges. The process requires multiple alternating deposition steps, making it time-consuming and less practical for large-scale applications. A more efficient approach is needed to achieve the same protective effect in fewer steps. This challenge motivated the researchers, led by Professor Jaime Grunlan and his team, to explore an evaporation-induced polyelectrolyte complexation strategy. By using ammonia as a volatile base, the team was able to induce complexation on the cotton surface in a single step, eliminating the need for time-intensive buffer treatments. This novel method streamlines the flame retardant coating process, offering a more scalable and cost-effective solution. New research paper published in ACS Applied Polymer Materials and conducted by  Maya Montemayor, Danixa Rodriguez-Melendez, Edward Chang, Dallin Smith, Natalie Vest, Alexandra Moran, Bethany Palen, and led by Professor Jaime Grunlan developed innovative polyelectrolyte complex composed of PEI and PSP, applied in a one-step process. Their study demonstrates that cotton treated with this coating exhibits a remarkable 93% reduction in total heat release while maintaining flame-retardant behavior even after multiple water rinses. This breakthrough represents a significant step forward in developing durable, eco-friendly, and easily deployable fire-resistant textiles, paving the way for safer everyday materials.

The researchers tested the effectiveness of their newly developed polyelectrolyte complex coating for flame-retardant cotton. Their first step was to prepare the cotton fabric by immersing it in a solution containing polyethylenimine (PEI) and sodium hexametaphosphate (PSP). Unlike traditional multi-step methods, this process was designed to work in a single step by using ammonia as a volatile base, which evaporates and lowers the pH, triggering the formation of the complex. The researchers explored different PEI-to-PSP molar ratios—1:1, 1:2, and 2:1—carefully observing how each formulation interacted with the cotton fibers. Once coated, the fabrics were dried at 70°C overnight, ensuring a uniform and stable application. The resulting textiles gained significant weight, with the 1:2 ratio demonstrating the most effective coverage. To evaluate how well the coating adhered to the cotton and whether it could withstand washing, the team subjected the treated fabric to multiple rinse cycles in deionized water. After five rinses, the 1:2 and 1:1 coatings retained around 7% weight gain, indicating strong adhesion, while the 2:1 coating nearly disappeared. This revealed an important finding—the higher ratio of PSP played a crucial role in forming a more resilient, ionically cross-linked network, ensuring durability even after repeated exposure to water. The scanning electron microscopy (SEM) images of the fabric confirmed that the coating was deposited conformally, with minimal aggregation, reinforcing the idea that the complexation process was successful. The true test of the coating’s performance came when the research team conducted flame tests on the treated cotton. They exposed samples to a standardized 12-second vertical flame test, carefully monitoring the afterburn time and residue left behind. Untreated cotton burned completely, turning to ash within seconds. However, all coated samples exhibited a self-extinguishing effect, with the flame dying out almost immediately upon removal of the ignition source. Particularly impressive was the 1:2 PEI-PSP sample, which left behind 97% of its original mass after burning, compared to the mere 8% residue of untreated cotton. Even after multiple rinses, this sample retained its flame-retardant properties, proving the robustness of the coating. To understand why the coating performed so well, the team analyzed its thermal degradation using thermogravimetric analysis and differential scanning calorimetry. The data revealed that the coated cotton decomposed at lower temperatures than untreated fabric, suggesting that the coating acted as a catalyst for early char formation. This process effectively slowed down the heat transfer to the fabric, reducing its ability to sustain combustion. Microscale combustion calorimetry further validated these results, showing a dramatic 93% reduction in total heat release for the 1:2 coated sample compared to untreated cotton. Even after multiple washes, the coated samples maintained an 89% reduction in heat release, confirming their long-term effectiveness.

One of the most compelling observations came from the visual evidence of the fabric’s reaction to fire. High-magnification SEM images of the charred samples revealed the formation of microscopic bubbles within the protective char layer. These bubbles, a hallmark of intumescent behavior, indicated that the coating expanded and reinforced itself during combustion, providing a physical barrier that insulated the cotton fibers. The authors noted that this expansion was crucial for the fabric’s ability to resist flames, further supporting the effectiveness of the polyelectrolyte complex system. To compare their innovative approach with traditional methods, the researchers tested a buffer-cured polyelectrolyte complex system, in which the coating was applied and then cured using a citric acid buffer. While this method also imparted flame retardancy, it resulted in weaker adhesion and significantly less char residue after burning. This highlighted the superiority of the ammonia-induced evaporation method, which not only simplified the coating process but also enhanced the fabric’s durability and fire resistance.

In conclusion, Professor Jaime Grunlan and colleagues devveloped safer, more efficient, and environmentally friendly flame-retardant treatments for cotton. By demonstrating a scalable, one-step deposition method for polyelectrolyte complex coatings, the research eliminates the time-consuming and multi-step processes that have historically been a barrier to widespread adoption of flame-retardant textiles. The ability to impart self-extinguishing behavior to cotton in a single treatment without relying on halogen-based chemicals is an important advancement, as it directly addresses concerns over toxicity and environmental sustainability. Unlike conventional approaches, which often require multiple coating cycles or harsh chemical treatments, this method simplifies the process while maintaining strong flame resistance. One of the most significant implications of this work is its potential impact on textile manufacturing. Cotton is used in clothing, furniture, and industrial applications, where fire hazards remain a persistent concern. With this new coating approach, manufacturers could integrate flame-retardant protection into their fabrics without introducing complex chemical processing steps or significantly altering the properties of the material. Since the coated fabric retains its flame-resistant behavior even after multiple washes, it paves the way for long-lasting safety in everyday applications, reducing both the risk of fire-related accidents and the need for frequent reapplications of fireproofing agents. Beyond its immediate applications in textiles, this study also introduces a broader concept for material science and engineering. The evaporation-induced polyelectrolyte complexation technique could be adapted for use in other flammable materials, such as wood, paper, and synthetic fabrics. This flexibility opens doors for future research into protective coatings that extend beyond textiles, potentially benefiting industries such as construction, aerospace, and automotive manufacturing, where fire-resistant materials are critical for safety.

Furthermore, the durability of the coating suggests long-term cost savings for consumers and businesses alike. Many fireproofing treatments degrade over time, requiring frequent replacement or reapplication. The rinse-resistant nature of this coating addresses that limitation, making it a viable candidate for large-scale production without significantly increasing costs. This is particularly relevant for institutional settings such as hospitals, hotels, and transportation sectors, where fire safety regulations mandate the use of flame-resistant materials. At a fundamental level, the study also enhances the understanding of polyelectrolyte complexation mechanisms, particularly in the context of flame retardancy. The observed formation of an intumescent char layer, coupled with the catalytic role of phosphorus and nitrogen in the protective coating, provides valuable insights into how chemical interactions can be harnessed to enhance material resilience. This knowledge could drive the development of next-generation coatings with even greater efficiency and broader applicability.