Dynamically bonded cellulose nanocrystals hydrogels: Structure, rheology and fire prevention performance

Flame retardants are chemical additives incorporated into materials to reduce their flammability and slow down the spread of fire. Conventional flame retardants such as, halogenated compounds and organophosphates have been utilized for a long period. While effective in enhancing fire safety, some of these traditional materials are proven to pose a risk to the health and environment. Sustainable alternatives are much needed to provide effective fire protection while minimizing adverse impacts. Bio-based materials, like starch-based compounds or cellulose derivatives, hold significant potential as sustainable options due to their biodegradability and lack of toxicity. Essentially, cellulose nanocrystals (CNCs) stand out for their superior strength, modulus, surface area, and liquid crystalline characteristics compared to bulk cellulose, and most importantly, their modifiability enables them to be utilized in innovative applications in material science. Hydrogels have a significant promise to be used as a green flame-retardant coating, considering their ability to absorb and retain water within their intricate structure, which is a characteristic that plays a pivotal role in reducing the flammability of materials. Dynamically bonded hydrogels are characterized by reversible physical interactions and offer key advantages. Their self-healing ability, shear-thinning behavior for easy processing, and responsiveness to external stimuli make them ideal for various applications. The versatility in design, reduced stiffness in the relaxed state, and biocompatibility highlight their potential for innovative and tailored solutions in diverse fields. Boron-based compounds favor intumescent systems, notably, they synergize effectively with cellulosic substances and improve their flame-retardant characteristics by promoting charring. In this study, we aimed to create a dynamic hydrogel network between CNCs by crosslinking with borax. Utilizing borax as a crosslinker yielded dual effects: it facilitated the dynamic network formation through boron-ester bonds and enhanced flame-retardant properties when combined with CNCs. First, we elucidated the gelation behavior of CNC-borax hydrogel network by varying the concentrations of CNC and borax separately. Therefore, the concentration dependency of morphological and viscoelastic properties was unraveled for both borax and CNC variables. We found that, both the CNC and borax play a pivotal role on improving the stiffness of the gels in different principles. Borax presence is essential to trigger CNC gelation in low concentrations by facilitating the boron-ester bond formation, which attributes dynamic properties to the network with remarkable self-healing properties. Higher borax concentrations in the gel were characterized by the presence of undissolved borax crystals in polarized optical microscopy images. Therefore, the composition of these gels involved both the borate ions originated from dissolved borax, and the undissolved borax crystals in different sizes. On the other hand, CNCs played an important role in improving the stiffness of the network owing to their ability to form self-supported networks through intermolecular interactions and H-bonding. The ease of applicability of gels were assessed by spreading on pine wood surface. The gels with higher borax content provided better control and distribution, while weaker gels tend to drip. These findings provided us valuable insight into the development of effective, readily applicable, and sustainable hydrogel materials. Secondly, we assessed the performance of flame-retardant coatings on the flammability of pine wood substrates. Similar to the previous study, the contribution of both components to the flame-retardant properties were also evaluated by varying their concentrations. Fire test results revealed that, CNC-Borax hydrogel coating significantly improved the flammability of the wood substrate. Specifically, hydrogels containing both undissolved and dissolved borax parts exhibited remarkable fire prevention performance. This was due to the harmonious work of boric acid and borax; the smoldering and glowing behavior was controlled by boric acid, while the prevention of flame was ensured by borax. In combination, they promoted the glassy and robust char layer formation with high thermal stability and shielded the flammable wood substrate. As a result, hydrogel coating notably delayed the ignition time while accelerating the flameout time. Limiting oxygen index (LOI) and fire performance index (FPI) of the hydrogel coated woods were drastically increased. Examining the sustained effectiveness of the gels post-application is another vital aspect to consider in flame-retardancy applications. Therefore, a second fire case was simulated by reigniting the samples to evaluate the effectiveness of the remaining film residue from the first application of the gels. The extended fire protection effect demonstrated its efficacy in preventing damage to the integrity of the substances. The CNC-Borax hydrogels have demonstrated considerable promise as effective solutions for flame retardancy applications. Their effectiveness in these scenarios opens new avenues for the development of environmentally conscious flame retardants, particularly relevant for combatting forest fires or wildfire situations. Given the prevalence of wildfires in extremely hot and dry climates, where cellulosic materials like wood and grass serve as combustible fuel, ensuring the efficiency of CNC-Borax gels after the drying process becomes crucial. This prolonged effectiveness makes them specifically well-suited to use in challenging situations, contributing to enhanced fire protection and safety measures.