A green approach to fabricating nacre-inspired nanocoating for super-efficiently fire-safe polymers via one-step self-assembly
Graphical abstract
Introduction
Fire hazard of combustible polymers remains one of the biggest threats for human life. Seventy-nine people were burned dead in London Grenfell Tower fire last year, which has drawn more and more attentions on the fire-safe polymers [1,2]. At present, to improve the flame retardancy of polymers, the commonly used method is incorporating flame retardants into the polymer matrices [3]. However, this method has some intractable disadvantages: (1) halogen and phosphorus-containing flame retardants are most commonly used because of their high efficiency, but they incur a lot of environmental problems [[4], [5], [6], [7]]; (2) the existing flame retardants do not have the universal applicability, and each flame retardant can only be applied to a small number of polymers [8]; (3) the efficiency and compatibility of flame retardants are not satisfactory, hence the mechanical properties and processability of the polymer matrices are sacrificed to a great degree due to the heavy addition [9]. Therefore, a green, high efficient and universal applicable strategy to improving the fire-safety of polymers is urgently needed [10].
Instead of incorporating flame retardants inside the polymer matrices by blending, fire-retardant coating shows some unique advantages because it concentrates the flame retardancy on the matrices surface (where the flame occurs), thus perfectly preserves the inherent properties of the polymers [11]. Intumescent coatings were intensively studied and displayed good flame retardancy on many substrates, such as cotton fabric [12], epoxy [13], polycarbonate [14], and so on. However, the phosphorous-containing additives were always used as the main components of intumescent coatings, which increased the risk of exposing to environment and caused air and water pollution [7,[15], [16], [17]].
In recent years, some researches reveal that the distinct brick-and-mortar structure of nacre exhibits exceptional hardness, toughness, thermostability and barrier effect, which endows nacre with outstanding mechanical properties and flame retardancy [[18], [19], [20]]. Accordingly, fabricating nacre-like brick-and-mortar structural nanocoating on substrates surface is considered to be a promising strategy to flame-retard polymers [21]. A number of nacre-mimetic organic-inorganic hybrid materials have been fabricated through layer-by-layer self-assembly [22], filtration [23], electrophoretic deposition [24] or freeze casting [25]. However, most of their fabricated processes were complicated, time-consuming and energy-intensive, which severely restricted their practical applications. Recently, Sun and co-workers [26] fabricated nacre-mimetic nanocoatings based on MMT and polyvinyl alcohol through one-step co-assembly method, which provided a convenient method to fabricate fire-safe coating.
With people's growing concern on environmental issue, some biopolymers such as chitosan [27], cellulose [28] and lignin [29], are receiving increasing attention and have been applied in many fire-safe systems due to their non-toxicity and sustainability. Among them, chitosan is considered to be one of the most desirable substitutes for traditional flame retardant because it contains plenty of hydroxyl groups and amino groups, possessing excellent charring property [30,31]. Chitosan is the deacetylation product of chitin, which is the second most abundant biopolymer and widely presents in the shells of marine crustaceans and insect cuticles [32]. The food industry around the world produces 6–8 million tons of marine crustacean shells each year, and most of them are disposed as garbage, buried into soil or poured into sea, which cause environment pollution [33]. Considering the great demand for highly efficient environment-friendly flame retardants, fabricating a green fire-safe coating with high efficiency and universal applicability based on chitosan is of great economic benefits and environmental value. However, to the best of our knowledge, the flame retardant efficiency of chitosan mainly depended on its cooperation with phosphorus-containing compound [30,31,34]. Inspired by nacre, we proposed that fabricating brick-and-mortar fire-safe coating might be a promising way to take the advantages of chitosan: (1) chitosan can play a role to bond the inorganic nanosheets together easily by forming hydrogen bonding, due to its large amount of ether bonds, hydroxyl groups and amino groups [35]. Meanwhile, the active groups also make it easy to build covalent bonding between chitosan and inorganic nanosheets. These can ensure the stability, tightness and continuity the microstructure of the nanocoating [24]. (2) Combing the excellent charring ability of chitosan and outstanding barrier property of inorganic nanosheets can endow the nanocoating with high efficient flame retardancy.
Herein, in order to develop a high efficient, environmental-friendly and universal fire-safe strategy for combustible polymers, we fabricated a green nacre-inspired nanocoating based on carboxymethyl chitosan (CCS) and montmorillonite (MMT) via one-step self-assembly. The scanning electron microscope (SEM) displayed that the as-prepared coating possessed well-arranged nacre-like microstructure. Importantly, the nacre-inspired coating endowed polyester film, cotton fabric and polyurethane foam with excellent flame retardancy by dip-coating or spray-coating. The fabrication method is convenient, low cost, environmental sustainable, and feasible for large-scale preparation, thus it is a new strategy to fabricating super-efficient and universal applicable nanocoating for fire-safe polymers.
Section snippets
Materials
Montmorillonite (MMT) was provided by Zhejiang Fenghong New Material Co., Ltd., China. Carboxymethyl chitosan (CCS, viscosity: 10 mPa·s, 83.4% carbxylation degree) was purchased from Zhejiang Aoxing Biotechnology Co., Ltd., China. [3-(2,3-Epoxypropoxy)propyl]trimethoxysilane (epoxy silane, ES) and ethanol were purchased from Nanjing Chemical Reagent Co., Ltd., China. All the reagents were used as received without further purification.
Three kinds of polymer products were applied as the coating
Microstructure of MMT/CCS nanocoatings
Fig. 1 shows the photos and cross section SEM images of nacre, PET-MMT/CCS-50/d6 and PET-MMT/CCS-50/s. As shown in Fig. 1b1 and c1, the coated PET films not only exhibited high transparency (also as shown in Fig. S3), but also exhibited distinct iridescence like nacre when they were viewed from the side. The iridescence of the coated PET films even was observed under optical microscope (insets in Fig. 1b1 and c1). The iridescence of nacre comes from the Bragg diffraction of its well-arranged
Conclusion
A green nacre-inspired nanocoating based on carboxymethyl chitosan (CCS) and montmorillonite (MMT) was successfully fabricated via one-step self-assembly. MMT nanosheets showed a good arrangement in the nanocoatings, forming nacre-like hierarchical microstructure and exhibiting high transparency and specific nacre-like iridescence. More importantly, the nanocoatings endowed polyester film, cotton fabric and polyurethane foam with super-efficient flame retardancy. The film, fabric and foam all
Acknowledgements
We greatly acknowledge the National Natural Science Foundation of China (51773064 and 51403067), the Natural Science Foundation of Guangdong Province (2017A030313303), the Pearl River S&T Nova Program of Guangzhou (201710010062), and the Fundamental Research Funds for the Central Universities (2018MS04) for their financial supports.
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