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.