By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Oct 24 2024A recent article published in Advanced Materials introduced unique biomimetic hydrogels fabricated from sustainable cellulosic polymers crosslinked by colloidal silica particles. These hydrogels ideal viscoelastic properties transitioned into highly porous and thermally insulative silica aerogel coatings under heat activation, robustly protecting substrates against ignition.
Study: Water‐Enhancing Gels Exhibiting Heat‐Activated Formation of Silica Aerogels for Protection of Critical Infrastructure During Catastrophic Wildfire. Image Credit: RODKARV/Shutterstock.com
Background
Catastrophic wildfires are becoming more frequent and severe in the United States, Europe, and Australia due to climate change, historical fire suppression policies, and inadequate vegetation management. Thus, new classes of environmentally friendly wildland fire retardants are required to address and combat wildfires effectively.
Wildland fire chemical systems are classified as long-term retardants, foam suppressants, and water-enhancing gels. Water-enhancing gels offer a more environmentally friendly wildland fire suppression strategy as their characteristics can be tuned to obtain excellent substrate adherence and water retention.
The cross-linked polymer structure of hydrogels is specifically relevant for fire suppression efforts as it allows them to absorb and retain substantial amounts of water. However, they become ineffective after drying under high heat and wind conditions, limiting their practical applications.
Water-enhancing gel technologies, therefore, require critical advancements to retain their effectiveness during wildfires. Thus, this study introduced a water-enhancing hydrogel transformable into a silica aerogel with exceptional insulating properties during heat activation.
Methods
Based on rapid self-healing, shear thinning, and long-term stability, hydroxyethyl cellulose (HEC) and methylcellulose (MC) were selected to form robust polymer-particle (PP) hydrogels when mixed with colloidal silica particles (CSPs).
Two PP hydrogel formulations comprising CSPs and a mixture of HEC and MC (HEC+MC/CSP) or methyl 2-hydroxyethyl cellulose (MHEC/CSP) were evaluated for water-enhancing gel application. The total weight percent loading of polymer (X) and the weight percent loading of CSPs (Y) were varied in these formulations as X-Y.
The rheological properties of the hydrogels were measured using a stress-controlled rheometer. Different rheometer configurations were employed for flow sweeps, large amplitude oscillatory shear (LAOS), and stress relaxation tests. The rheological properties of various PP hydrogel formulations were compared to those of a commercial water-enhancing gel, AquaGel-K.
The hydrogel was applied onto a plywood plate, and a fuel based on propylene and propane, which could burn up to 2054 °C (3730 °F), was used for burn tests. The aerogel foam was collected post-burn for electron microscopy analysis to image the foam structure and measure the film thickness.
The morphology and thickness of the burnt aerogel films were characterized via scanning electron microscopy (SEM). The fire retardancy mechanisms and combustion of AquaGel-K, HEC+MC/CSP 1-5, and MHEC/CSP 1-5 were compared through thermogravimetric analysis (TGA and Fourier transform infrared spectroscopy (FTIR), respectively.
Results and Discussion
Replacing HEC+MC polymers with MHEC in the PP hydrogel formulation made the MHEC/CSP system less susceptible to irreversible chemical aging, which stiffens the hydrogels over time. The hydrogel stiffened with age and exhibited higher storage and loss moduli, as demonstrated by rheology experiments.
MHEC/CSP 1-5 aged negligibly compared to HEC+MC/CSP 1-5 over 455 days. This observation was validated by the easy spread-ability of aged MHEC/CSP 1-5. Alternatively, aged HEC+MC/CSP 1-5 did not spread and broke into small solid chunks upon spreading.
In the steady-state flow sweep tests, both MHEC/CSP 1-5 and HEC+MC/CSP 1-5 formulations exhibited mechanical property changes due to aging. However, the extent of these changes was negligible for MHEC/CSP 1-5 (Day 455) compared to HEC+MC/CSP 1-5. Additionally, the yield stress for HEC+MC/CSP 1-5 hydrogels more than doubled with aging from the initial value, whereas that for MHEC/CSP 1-5 hydrogels remained the same.
During burn tests, HEC+MC/CSP 1–5 and HEC+MC/CSP/SDS 1-5-0.1 were most effective at preventing charring. They protected the substrate for more than seven minutes of continuous flame exposure. Alternatively, MHEC/CSP 1-5 PP formulations could protect the substrates for over five minutes.
The PP hydrogels formed a solid opaque crust almost immediately upon direct flame impingement, scattering the heat from the flame. This crust turned into a foam, desiccated, and formed a silica aerogel in situ upon continued flame exposure, insulating the coated substrates from the flame. Among all, the HEC+MC/CSP 1-5 formulation exhibited the highest foaming index. However, the AquaGel-K gels could protect the substrates from charring for less than 90 seconds.
Conclusion
Overall, the researchers successfully demonstrated a novel heat-activatable hydrogel design leveraging dynamic, multivalent polymer-particle interactions between cellulosic biopolymers and colloidal silica particles, which could be fabricated in a facile and scalable manner.
When exposed to heat, the prepared hydrogels transformed into a thermally insulative aerogel in situ. This innovative feature magnified the protective capabilities of these materials as coatings on substrates, ensuring continued efficacy against ignition even when desiccated.
Thus, the polymer-particle interactions can be tuned to overcome challenges with irreversible aging observed for some existing hydrogel systems, enabling the formulation of materials with enhanced properties for practical applications. The proposed innovative, bioinspired, and environmentally friendly materials have the potential to safeguard lives, property, and the environment from extreme wildfire.
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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