Hydrogels Protect Critical Infrastructure During Wildfires

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Oct 24 2024

A recent article published in Advanced Materials introduced an innovative biomimetic hydrogel using sustainable cellulosic polymers crosslinked with colloidal silica particles. These hydrogels possess ideal viscoelastic properties, and when exposed to heat, they transform into highly porous, thermally insulative silica aerogel coatings. This transformation creates a strong barrier that protects surfaces from ignition.

Background

Catastrophic wildfires are increasingly common in the United States, Europe, and Australia due to climate change, historical fire suppression policies, and limited vegetation management. This has created an urgent need for new classes of environmentally friendly fire retardants to effectively combat wildfires.

Wildland fire chemicals are categorized as long-term retardants, foam suppressants, and water-enhancing gels. Among these, water-enhancing gels offer a greener approach, with the ability to adhere well to surfaces and retain water. The cross-linked polymer structure of hydrogels is ideal for fire suppression, as it allows them to absorb and hold significant water. However, these gels lose effectiveness when dried by high heat or wind, limiting their practical use.

This study introduces a new water-enhancing hydrogel that transforms into a highly insulating silica aerogel when exposed to heat, improving its performance during wildfires.

Methods

Hydroxyethyl cellulose (HEC) and methylcellulose (MC) were selected to create stable polymer-particle (PP) hydrogels, mixed with colloidal silica particles (CSPs) for their rapid self-healing, shear-thinning, and long-lasting properties.

Two PP hydrogel formulations, HEC+MC/CSP and methyl 2-hydroxyethyl cellulose (MHEC/CSP), were tested as water-enhancing gels. The proportion of polymer (X) and CSP (Y) was varied in formulations labeled as X-Y.

Rheological properties were measured using a stress-controlled rheometer for flow sweeps, large amplitude oscillatory shear (LAOS), and stress relaxation tests. These hydrogels were compared to AquaGel-K, a commercial water-enhancing gel.

To test performance, the hydrogel was applied to a plywood substrate and exposed to flames reaching 2054 °C (3730 °F). The resultant aerogel foam was examined with electron microscopy to observe foam structure and film thickness. The fire retardancy of AquaGel-K, HEC+MC/CSP 1-5, and MHEC/CSP 1-5 was analyzed using thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FTIR).

Results and Discussion

Replacing HEC+MC polymers with MHEC in the PP hydrogel reduced aging effects that typically stiffen hydrogels over time. Rheology tests showed that MHEC/CSP 1-5 experienced minimal aging after 455 days, while HEC+MC/CSP 1-5 became harder to spread, breaking into chunks.

Both MHEC/CSP 1-5 and HEC+MC/CSP 1-5 formulations showed changes in mechanical properties due to aging, but these were minor in MHEC/CSP 1-5, whose yield stress remained stable over time.

In burn tests, the HEC+MC/CSP formulations effectively prevented charring, protecting the substrate for over seven minutes of direct flame exposure, while MHEC/CSP 1-5 protected it for over five minutes. Both formulations formed an insulating crust upon exposure to flames, turning into a silica aerogel that shielded the substrate. AquaGel-K gels, in contrast, only protected the substrate for under 90 seconds.

Conclusion

This study successfully demonstrated a novel hydrogel design with heat-activatable properties, using dynamic polymer-particle interactions between cellulosic biopolymers and colloidal silica particles. These hydrogels, scalable and easy to fabricate, transform into thermally insulative aerogels upon heating, offering enhanced protection against ignition even when dry.

By tuning polymer-particle interactions, these materials overcome the aging limitations of some existing hydrogels, making them suitable for practical applications. This bioinspired, eco-friendly material has strong potential to protect lives, property, and the environment from severe wildfires.

Journal Reference

Dong, C. et al. (2024). Water‐Enhancing Gels Exhibiting Heat‐Activated Formation of Silica Aerogels for Protection of Critical Infrastructure During Catastrophic Wildfire. Advanced Materials. DOI: 10.1002/adma.202407375, https://onlinelibrary.wiley.com/doi/10.1002/adma.202407375

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