Flame Retardant Electrospun Fibers

Electrospun mesh coating on substrate as fire retardant.

In fire protection, one method is to have a thin fire resistant coating that reduces the underlying material's flammability without the need to incorporate flame retardant additives. This preserves the protected material's composition and properties since there is no additive introduced. There are two basic ways which a material may function as a flame retardant. First is to prevent ignition by increasing the heat capacity of the material. However, when that fails, the next step is to prevent spreading of the fire. Some ways of preventing fire spreading are,

1. Char formation which prevent fuel release and function as a thermal insulation layer for underlying substrate
2. Additives that scavenge for free radicals during burning and disrupts the combustion process.
3. Fillers that releases water at high temperature
4. Fillers that are non-combustible that takes up volume and act as thermal sink.

Given that electrospinning is able to produce a thin nanofibrous film out of a wide variety of materials and composites, electrospun nanofibrous membrane or coating has been tested for its capability as flame retardant. Most attempts to improve the thermal stability and flame resistance of electrospun nanofibers are by mixing additives and filler to the polymer.

Prevention of ignition by increasing the heat capacity of electrospun material and having non-combustible fillers are commonly used approaches. Fillers such as MgO [Dhineshbabu et al 2014], TiO2, melamine, ammonium phosphate and pentaerythritol [Lee et al 2008] have been added to electrospun nanofibers to increase its flame resistance. Heat release capacity of less than 300 J/g-K for pure polymers is expected to be self-extinguishing [Walters et al 2001]. Moon et al (2013) used carbon nanotubes and ochre as fillers to electrospun polyacrylonitrile nanofibers. With electrospun polyacrylonitrile nanofibers, the heat release capacity was found to be 253 J/g-K. Addition of carbon nanotubes and/or ochre reduces the heat release capacity as these fillers are non-combustible. Polyacrylonitrile nanofibers with 0.1% single wall carbon nanotubes and 1% ochre stabilized at 240 °C reduces the heat release capacity to 24 J/g-K. Thermal stabilization was also found to increase char formation which reduces fire spreading. Boric acid, which is a common flame retardant additive, has been added to electrospun polyamide fibers as nanocomposite. The electrospun polyamide/boric acid nanocomposites fibers coated on cotton fabric showed increased time to ignition and char formation [Selvakumar et al 2012]. Combination of additives such as intumescent non-halogenated flame retardant (FR) and montmorillonite clay (MMT) platelets has also been to reduce flammability of polyamide 6 with the later additive found to improve char residue [Wu et al 2014].

Having additives that scavenged free radicals during burning is known to disrupt combustion process. Cai et al (2011) used this method to improve flame resistance of electrospun polyacrylonitrile by adding FeCl3. Investigation into the heat release rate (HRR) of the composite showed a higher value during initial combustion but the peak HRR is lower than pure polyacrylonitrile when the FeCl3 induces macroradicals recombination and intermolecular crosslinking as the burning continues.

Thermal insulation due to the highly porous electrospun nanofibers mesh may delay ignition of the underlying material. In a separate application, polyethylene oxide membrane was found to offer better thermal insulation than cast film of the same thickness and material [Jiang et al 2012]. Gallo et al (2011) compared the flame retardancy of poly(amic acid) (PAA) nanofibrous mats and polyimide (PI) nanofibrous mats with PAA film and PI film as coating on polyamide-66. Time to ignition was longer for the nanofibrous mats compared to film thus demonstrating better flame retardancy. However, there is no clear advantage of nanofibrous mat over film in terms of peak heat release rate, fire growth rate (FIGRA), maximum average rate of heat emission (MAHRE). More studies need to confirm whether structural advantage is limited to delay in ignition and not in fire spreading.

The location of the fire resistant additives on the fiber will also have a significant impact on the fibers' fire resistant capability. Zheng et al (2014) tested the fire resistance of electrospun cellulose fibers with magnesium hydroxide nanoparticles either blended within the fibers or coated on the outside. Pure cellulose fibers combust in air at a temperature of 239°C while cellulose fibers with magnesium hydroxide nanoparticles blended within its matrix combust at 267°C. Cellulose fibers with magnesium hydroxide nanoparticles coated on its surface combust at a temperature of 276°C. Surface coating of the cellulose fibers were prepared by collecting the electrospun fibers in an aqueous suspension of magnesium hydroxide nanoparticles.

One potential application of electrospun fibers with fire retardant property is in rechargeable batteries. Some cases of lithium ion battery catching fire is due to failure in the membrane separating combustible electrolytes in the battery which sets off an exothermic reaction when they come into contact with each other. Liu et al (2017) constructed a core-shell electrospun fibrous membrane with a fire retardant, triphenyl phosphate (TPP), a popular organophosphorus-based flame retardant, in its core. This thin membrane can be used as the separation layer in the lithium battery. The shell was made of poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) due to its chemical inertness to the battery electrolytes and relatively low melting point of 160°C. Their tests showed that the electrospun membrane was able to effectively extinguishes fire. Liang et al (2019) showed that core-shell electrospun fibers with flame retardant triphenyl phosphate (TPP) at its core and poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) as the shell (TPP@PVDF-HFP) was able to extinguish the fire in an ignition test. This composite electrospun fibers may also be coated on commercial battery membrane (TPP@PVDF-HFP@commercial) separator to give it flame retardant property as shown by the same ignition experiment. Both TPP@PVDF-HFP and TPP@PVDF-HFP@commercial separator were tested as membrane separator in Li-batteries and it was found that both the cells showed good electrochemical cycling stability during the repeating charge and discharge although TPP@PVDF-HFP separator demonstrated higher specific capacity than that of TPP@PVDF-HFP@commercial separator. Both cells were also able to deliver a stable capacity of more than 120 mAh/g on average for over 100 cycles. C-rate test showed that both cells exhibited good rate capabilities. At higher discharge rate, the cell Coloumbic efficiency of each cycle gradually declined when using TPP@PVDF-HFP as separator.

In thermal management of chips, it is useful for the material to exhibit flame retardant property. Phase change materials (PCM) have high potential use for thermal management but most polymeric PCMs are flammable. Lin et al (2023) constructed a phase change nanocomposite (PCN) film with flame retardant property using electrospinning, electrospraying and mold-pressing. Boron nitride nanosheets (BNNS) were loaded into the phase change materials (PCM) for thermal management applications. Using coaxial electrospinning, the PCM material, polyethylene glycol (PEG) forms the core and the sheath consists of boron nitride and thermoplastic polyurethane (TPU). The composite fibers were aligned on a rotating drum during electrospinning and an additional layer of boron nitride was electrosprayed on the nanofiber layer. A final step of mold-pressing was used to bind the electrosprayed boron nitride nanosheet (BNNS-es) to the nanofiber layer. Without boron nitride, TPU and PEG@TUP films ignited immediately by an alcohol lamp. With PEG@TPU/BNNS, the film can still be ignited but the combustion stops after burning droplets fall off. The final assembly with PEG@TPU/BNNS fibers and BNNS-es compressed on the surface, the composite film failed to ignite by the flame within 6s and self-extinguished upon flame removal. The flame retardancy of this film has been attributed to the presence of BNNS which shielded the interior of the PCNS from the heat.

Scheme of the preparation process of the PEG@TPU/BNNS-es nanocomposite films [Lin et al 2023]