Layered Double Hydroxide Nanosheets Incorporated Hierarchical Hydrogen Bonding Polymer Networks for Transparent and Fire-Proof Ceramizable Coatings

Abstract

In recent decades, annual urban fire incidents, including those involving ancient wooden buildings burned, transportation, and solar panels, have increased, leading to significant loss of human life and property. Addressing this issue without altering the surface morphology or interfering with optical behavior of flammable materials poses a substantial challenge. Herein, we present a transparent, low thickness, ceramifiable nanosystem coating composed of a highly adhesive base (poly(SSS1-co-HEMA1)), nanoscale layered double hydroxide sheets as ceramic precursors, and supramolecular melamine di-borate as an accelerator. We demonstrate that this hybrid coating can transform into a porous, fire-resistant protective layer with a highly thermostable vitreous phase upon exposure to flame/heat source. A nanosystem coating of just ~100 μm thickness can significantly increase the limiting oxygen index of wood (Pine) to 37.3%, dramatically reduce total heat release by 78.6%, and maintain low smoke toxicity (CITG = 0.016). Detailed molecular force analysis, combined with a comprehensive examination of the underlying flame-retardant mechanisms, underscores the effectiveness of this coating. This work offers a strategy for creating efficient, environmentally friendly coatings with fire safety applications across various industries.

1. Introduction

Fire, a cornerstone of civilization, has dramatically shaped the trajectory of human development, facilitating the transition of our ancestors from primitive to advanced societies [1]. Nevertheless, flames can quickly lead to devastating fire disasters, causing irretrievable losses of property and life once they are out of bounds [2]. Indeed, both human society and the natural world experience numerous fire incidents each year, with solid wood and various polymer products often serving as fuel sources [3, 4]. For example, the Notre-Dame de Paris fire in 2019 highlighted the vulnerability of wooden structures to fire, resulting in severe damages to this historic architecture [5]. On the other hand, the buildings incorporating photovoltaic are receiving increased attention due to the prevalence of green lifestyle [6]. However, these systems pose potential fire risks due to electrical arcing [7], which can release toxic chemicals and explosive gases [8]. As a result of such events, it is imperative to develop reliable and effective fire-retardant coating for flammable wooden products and other materials to ensure safety and protect lives.

In general, the design of fire-proof materials follows two main strategies: mechanical mixing with fire retardants and surface fire-retardant coating techniques. While the incorporation of traditional fire-retardant fillers (e.g., layered double hydroxide (LDHs) [9], and ammonium polyphosphate (APP) [10]) into a matrix through physical mixing at a high content (~4 wt%, even >20 wt% [11]) is widely used, it often leads to increased system viscosity. This rise in viscosity can restrict subsequent manufacturing processes, such as the foaming of foam materials and the blow molding of films. Additionally, the introduction of bulk flame retardants can degrade tensile strength and elongation, making it challenging to achieve ideal low-density foams or highly flexible films [12]. Moreover, natural materials like wood products struggle to achieve effective flame retardance through above method. In contrast, surface fire-retardant coatings offer a promising means of providing robust fire protection for various substrates, including wood products, fabric, steel, polymer foams, and more [13–20]. Since these coatings localized on the surface, they have minimal impact on the mechanical or processing performance of the substrate. Typically, they form a thermostable fire-protective layer that acts as a physical barrier against mass and heat transfer when exposed to flame resource [21–23], thus preventing damage to the underlying materials. For this reason, some impressive efforts have been made to achieve a fire safety, especially combining nano-/microscale filler (e.g., nanocellulose, MXene, LDHs, graphene oxide (GO)) with adhesive macromolecule to create a composite coating [16, 22, 24–26].