1. Introduction
Polymer materials are widely used in all walks of life nowadays owing to their excellent comprehensive properties, mainly composed of compounds formed by carbon and hydrogen. Therefore, they are prone to fire when exposed to a heat source continuously. Fire hazards caused by combustible polymers have become a serious threat to human life and health and may cause economic losses.[1] To improve the flame retardancy of polymer, several commonly used flame retardants are added to the polymer matrix, such as halogenated flame retardants,[2] and phosphorus-based flame retardants,[3] inorganic flame retardants.[4] Although effective, these methods have some obvious shortcomings or deficiencies. Polymers containing halogenated flame retardants will release heavy smoke and corrosive gases (like HBr) during the combustion process, which will bring secondary disasters to people’s life and pollute the environment.[5, 6] The use of halogen-free flame retardants can reduce the amount of smoke to a certain extent, but its applicability is not as good as that of halogen-based flame retardants, which will increase the processing difficulty.[7]In addition, although inorganic flame retardants such as aluminum hydroxide have the effects of smoke suppression and low toxicity, their flame retardant efficiency is low, and more additives are needed to achieve the desired requirement of flame retardant performance.[8] Consequently, a safe, effective, and environmentally friendly strategy for the preparation of polymers with good flame retardant properties is essential and has great potential for application.
Instead of conventional flame retardants, polymer nanocomposites can also be prepared to enhance flame retardancy. Commonly used are layered silicate,[9] graphene and its derivatives,[10] boron nitride[11] and so on. The flame retardant mechanism of nanocomposites is the barrier effect formed by nanomaterials, which not only blocks the diffusion of combustible small molecules produced by the thermal degradation of internal polymer chains to the combustion interface, but also delays the transfer of external oxygen to the combustion interior.[12]Unfortunately, small amounts of nanomaterials seem unlikely to form promising barrier effects.[12] Therefore, an appropriate amount of nanomaterials must be added to acquire a sufficient barrier effect. However, as the content of nanomaterials increases, the interface between organic and inorganic is un-neglected, and the fillers tend to aggregate and reduce uniformity. Consequently, the flame retardant performance cannot be improved effectively.
One such complex is found in natural materials, nacre, mainly consisting of extremely thin organic matter separated by orderly lamellar calcium carbonate.[13] Its hierarchical structure and excellent mechanical properties have attracted extensive attention.[14, 15] At present, a variety of preparation methods have been developed, hoping to obtain composite materials with excellent properties by artificially simulating the nacre structure, such as gravity-induced deposition,[16]layer-by-layer deposition,[17] freeze casting,[18] electrophoretic deposition,[19] etc. Typically, hydrophilic or water-soluble polymers, such as polyvinyl alcohol (PVA),[20] polyacrylic acid (PAA),[21] polyethyleneimine (PEI),[22] cellulose,[23]etc., are used to prepare nacre-like composites, to increase the interfacial compatibility with nano inorganic materials. Apart from the mechanical properties, several other useful functions have been developed, such as a separator for battery,[24]thermally conductive film,[25]sensor,[26] energy storage,[27] and so on. Based on the defect-dominated diffusion model first proposed by Prins and Hermans,[28] the addition of inorganic flake materials to polymers can effectively prolong the gas diffusion path in the material, thereby significantly improving its barrier properties. Isolate external oxygen to prevent it from entering into the material and avoiding the escape of internal pyrolytic gas can effectively improve the flame retardancy of the material, which shows that bioinspired nacre-like composite will have excellent flame retardant performance.[29] The raw materials that artificial bioinspired nacre-like composite is prone to swelling or even dissolution in water. So, when this kind of material is applied to the exterior wall, it is quite easy to be damaged in rainwater, lose its fire effect, and cause economic losses, limiting its scope of application.
Herein, to solve the above problems, we used polyimide (PI) as polymer matrix and montmorillonite (MMT) as lamellar inorganic nanomaterials and prepared water-insoluble bioinspired nacre-like composites with good fire retardancy. PI is a kind of polymer containing imide rings in the main chain[30] which has fabulous comprehensive properties, including high chemical stability, thermal stability, and high mechanical properties.[31, 32] Particularly, the water insoluble PI can be transformed from the water-soluble precursor poly (amic acid) salt (PAAS) by thermal imidization. Using this transformation, it is possible to make nacre-like composites that are not insoluble or swellable by water. By selecting the PI monomer containing polar groups, PAAS can form a strong interaction with the abundant hydroxyl groups on the MMT surface, elegantly resolving the interfacial problem between them, resulting in homogeneous composites. The MMT sheets attached to the PAAS will gradually sink and accumulate in an orderly manner due to the influence of gravity. Finally, by increasing the temperature to volatilize the solvents and complete thermal imidization, while preserving the ordered structure, the bioinspired polyimide composite was realized. Benefiting from the well-ordered layered structure, the composite shows excellent gas barrier properties, followed by the improvement of its flame retardancy, which is reflected in a significant increase in the limiting oxygen index (LOI). Besides, the composite framework is retained after combustion, which plays a role in delaying the spread of the fire. Surprisingly, even if the PAAS is used as a polymer matrix, composite coating with MMT’s still has good flame retardant properties. When PAAS is coated on PU foam, it gives flammable PU foam self-extinguishing performance, and the flame retardant and smoke suppression performance are significantly improved. This is a meaningful discovery. Through structural design, PAAS can be quantitatively imidized at 150oC.[33] When PAAS/MMT is coated on some 150 oC resistant foam, it can be converted into PI/MMT coating after quantitative imidization to obtain higher flame retardancy. Moreover, even PAAS/MMT coating can effectively improve the flame retardancy. To sum up, this work will further broaden the application of bioinspired nacre-like materials in the field of fire protection.