flame retardant polyamide 6/nanoclay/intumescent nanocomposite fibers through electrospinning

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published online 16 January 2014Textile Research JournalHao Wu, Mourad Krifa and Joseph H Koo

Flame retardant polyamide 6/nanoclay/intumescent nanocomposite fibers through electrospinning

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Original article

Flame retardant polyamide 6/nanoclay/intumescent nanocomposite fibersthrough electrospinning

Hao Wu1, Mourad Krifa1 and Joseph H Koo2

Abstract

Flame retardant polyamide 6 (nylon 6) nanocomposite nanofibers containing montmorillonite clay (MMT) platelets and

intumescent non-halogenated flame retardant (FR) additives were processed by electrospinning. Different methods of

mixing nano fillers before electrospinning were explored and compared. It was found that high loadings of nanoclay

particles affected the electrospinnability of the nanocomposite material. Good dispersion and exfoliation of nanoclay

platelets within individual nanofibers was verified by transmission electron microscopy. The degradation temperature of

nanocomposite samples was lower than pristine nylon 6 samples. However the degradation of all nanocomposite

formulations was overall slower. Moreover, the difference in residual char weight after decomposition was significant.

Microscale combustion calorimeter results show that FR particles played a major role in reducing flammability of the

material in both solution- and melt-compounded samples, while MMT nanoclay was effective in improving char residue

and in reducing flammability in high-shear melt premixed samples.

Keywords

Nanocomposite fibers, nanofibers, nanoparticles, nanoclay, polymer, thermal and flammability properties, polyamide

Military, firefighting personnel, as well as civilians needeffective protection from fire. Fire protection is one ofthe major functions of protective clothing. Currently, anumber of high performance flame retardant fibers, suchas Nomex� and PBI,1–3 are widely used in low-volumeapplications such as firefighter and racer suits due totheir excellent fire resistance. These high performancefibers are cost-prohibitive and cannot be used in high-volume applications such as mass-market consumerproducts or military uniforms. Extensive studies haveshown the potential of producing low-cost flame retard-ant materials using nanocomposite systems based oncommonly used polymers, such as polypropylene orpolyamide 6 (PA6).4–9 A widely reported approach isto incorporate carbon nanotubes (CNTs) as additivesin the polymer matrix to enhance thermal stability andprevent melt dripping.4,5,7,10 Another category of nano-particles used to achieve thermal stability in polymers ismontmorillonite nanoclay (MMT).7,8,11–13 MMT con-sists of superposed platelets which, when completelyexfoliated, exhibit a thickness of approximately 1 nm

and lateral dimensions ranging from 100 to 200 nm.14

Thus, MMT platelets have large specific surface areaproviding substantial interfacial surface in a polymermatrix.15

Ever since the Toyota group successfully made clay/nylon 6 molecular composites by in-situ polymeriza-tion,16,17 polymer–clay nanocomposites have been thesubject of extensive studies for their improved mechan-ical, thermal, and biodegradable properties.18,19 One ofthe most promising applications of nanocomposites is

1School of Human Ecology – Textiles and Apparel, The University of

Texas at Austin, USA2Department of Mechanical Engineering, The University of Texas at

Austin. USA

Corresponding author:

Mourad Krifa, School of Human Ecology – Textiles and Apparel, The

University of Texas at Austin, 200 W 24th St, Stop A2700, Austin, TX

78712-1247, USA.

Email: [email protected]

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in the area of flame retardant (FR) materials, such ascoatings for electronics and electrical equipment, andprotective textiles.20 However, it was shown that add-ition of nano fillers alone (CNTs or MMT) often fallsshort of meeting minimum requirements of fire-safetyregulations and standards, such as UL-94, although itimproves thermal stability and reduces heat releaserate.12,21 On the other hand, combinations of conven-tional flame retardants such as intumescent additives inconjunction with nano-fillers have been shown toreduce total heat release in addition to peak heat releaserate, which results in the required performance accord-ing to UL-94 standards.7,8,22–25 Synergistic effects ofclay and intumescent FR additives help enhance FRperformance. With the addition of intumescent sys-tems, the self-sustained combustion of the polymerwill be interrupted at an early stage. Upon exposureto the flame, the intumescent and nanoclay particlesincorporated within the polymer matrix are trans-formed into a protective barrier which swells andforms a stable char at the surface of the materialacting as a thermal shield.7,8,11–13

Previous research has yielded improved bulk mater-ial properties in PA6 through the infusion of MMTnanoclay and intumescent particles via co-rotatingtwin-screw extrusion.7,8,26 Specifically, TGA results ofinjection-molded PA6 samples infused with MMTnanoclay and intumescent additives (FR) retainedbetween 14% and 20% more mass than neat PA6 atan exposure temperature of 500�C.8 This means theinfusion of additives into the nylon retards the polymerdegradation that feeds flame growth through pyrolysis.Moreover, UL 94 tests demonstrated the absence ofdrip and the generation of a protective char shield onthe specimen’s surface when exposed to flame. Afterexposure to the burner, all neat PA6 specimensburned for more than 10 s after the first flame applica-tion. On the other hand, the MMT- and FR-infusedPA6 nanocomposite formulations had no combustionafter the first flame application. Additionally, the poly-mer nanocomposite samples did not reignite after self-extinguishing, which is another indicator of improvedthermal properties and reduced flammability. The poly-mer nanocomposite samples passed the UL 94V-0rating.8 Based on these results, non-drip FR PA6 con-forming to the highest UL 94 rating was achievable ininjection-molded bulk materials.

The objective of the current study was to optimizethe nanocomposite formulations and transfer the prop-erties observed on the bulk material to a porous ultra-thin fibrous form obtained through electrospinning.Electrospinning allows the formation of thin andmicroporous nonwoven mats made of sub-micronfibers. This is valuable in applications such as multi-functional laminate systems or in conformal coating

of fabric surfaces.9 Electrospinning requires the dissol-ution of the compounded nanocomposite samples. It isimportant to determine whether the performanceobserved in bulk samples obtained using twin-screwmelt processing would be preserved in the nano- orsub-micron fibers after dissolution of the material forelectrospinning. Another important question we willattempt to answer is whether solution mixing couldsubstitute for high-shear melt compounding, given thefact that the ultimate electrospinning fluid is in solutionform. Therefore, two methods of compounding weretested to prepare the electrospinnable fluids for ourexperiments: (a) high-shear melt compounding, e.g.twin-screw extrusion, followed by dissolution of thecompound material; and (b) direct solution mixing ofthe polymer and additives in the solvent used forelectrospinning.

Materials and methods

Materials

Low/medium viscosity nylon 6 (Aegis� H8202NLB) wasprovided by Honeywell Co. It has a melting point of220�C and specific gravity of 1.13.27 Formic acid(>88%) was purchased from Sigma-Aldrich ChemicalCo. The nano-filler used in this study is montmorillonitenanoclay Cloisite� 30B from Southern Clay ProductsInc. Cloisite 30B is a natural MMTmodified with a qua-ternary ammonium salt.28 It is an additive used toimprove physical and thermal properties of plastics orrubbers, including heat deflection temperature (HDT),coefficient of linear thermal expansion (CLTE), and bar-rier capability.28 Flame retardant additives Exolit�

OP1312 were provided by Clariant Ltd. Exolit OP1312is a non-halogenatedFRadditive based on organic phos-phinates. The FR mechanism is through intumescencewhere thermoplastic polymers, such as nylon 6 withOP1312 will foam on exposure to flame and form astable char at the surface acting as a barrier. The protect-ive layer provides a heat insulation effect, reduces oxygenaccess, and prevents dripping of molten polymer.29 Toobtain fine FRparticles, OP1312were wetmilled in etha-nol by NETZSCH Premier Technologies, LLC beforeuse. Nylon 6, nanoclay, and FR particles were all driedat 80�C for 24 hours before processing.

Processing

Two methods of mixing before preparation of electro-spinning solutions were used in the study. The firstmethod we used was high-shear compounding topremix the polymer with nanoclay and FR additivesbefore dissolving the system in formic acid to preparethe electrospinning solution. High-shear mixing has

2 Textile Research Journal 0(00)




been proven to be an effective method for polymernanocomposite blending. Using this method, a fine dis-persion of both MMT and FR particles can beachieved. High shear premixing was performed by amicrocompounder manufactured by DSM Xplorewith 5ml capacity. Approximately 3 g of polymer mix-ture were fed into DSM microcompounder each time.The rotation speed was 100 rpm; compounding tem-perature and duration were 240�C and 10min. Thispremixing process mimics the compounding approachused to obtain the bulk injection-molded samplesdescribed in previous research.7,8,26

The second method we used was solution blendingto directly mix the polymer and additives in formicacid, the electrospinning solvent, with no prior meltcompounding. Solutions containing different loadingsof particles were thus blended using a magnetic stirrerfor at least 12 hours. Properties of these samples interms of morphological, thermal, and FR characteris-tics were compared.

Based on previous experiments,8,11–13 twin-screwextruded samples containing 20wt % FR and 5wt %MMT rendered desirable FR properties on bulk mater-ial. This composition constituted the starting point forthe selection of the range of formulations we tested andwas replicated as formulation #6 (Table 1). However, itwas found that the prepared electrospinning fluids withsuch high loading of fillers had poor electrospinnability.Electrospinnability is defined as the ability for a fluid tobe electrospun into a uniform fiber.30 In this case, poorelectrospinnability manifested through an unstablefluid jet and through the observation of splashing andaccumulated polymer deposit at the tip of the needle. Inorder to improve electrospinnability, formulations withlower filler concentrations were attempted using thetwo different mixing methods discussed above. All sam-ples processed in this study are shown in Table 1.

Electrospinning

Electrospinning solutions from all formulations shownin Table 1 were prepared by dissolving 25wt % of solidcompound with formic acid and gently stirring with amagnetic stirrer at room temperature. Pham et al. stipu-late that in order for a polymer solution to be electro-spinnable,31 there is a minimum concentrationthreshold to allow some chain overlap or entanglement.On the other hand, higher concentrations can result inexcessive viscosity and poor flow of the electrospinningjet, particularly when nano-fillers are added to thesystem. Therefore, two additional levels of concentra-tion were tested: neat nylon was spun at 30wt % informic acid; and formulation #6 which has the highestfiller loading (see Table 1) was spun at a 20wt % con-centration to allow stable spinnability.

Both neat PA6 and nanocomposite fibers were pro-duced using the same electrospinning parameters (volt-age, distance, feeding rate, etc.). A lab-builtelectrospinning apparatus was used to spin the fibers.The solutions were fed at a rate of 4� 1 ml/min using asyringe pump (Harvard Apparatus, 11 Plus) connectedto an 18 Gauge needle (Harvard Apparatus); the col-lecting distance was 8� 1 cm. The needle tip was con-nected to a high voltage power supply (Gamma HighVoltage Research, ES 100 P-10W/DAM). Preliminaryspinning tests with neat PA6 were conducted using dif-ferent voltage levels (ranging from about 20 to 35 kV)in order to observe the impact on spinnability. After theinitial tests, the voltage was set to 25� 2 kV for all sam-ples. A piece of grounded aluminum foil (15 cm by15 cm) was used to collect the fiber mats. The electro-spun fibers were carefully cut and removed from thesurface by peeling.

Characterization

The morphology of the electrospun fibers was observedby scanning electron microscopy (SEM, Hitachi S-5500and Quanta 650). The samples were attached to theSEM sample holder using double-sided carbon tapeand were sputter coated with gold. To investigate thedispersion of nanoclay platelets, a FEI Tecnai transmis-sion electron microscope (TEM) was used. Electrospunfibers were directly deposited on 400 mesh copper gridsfor TEM imaging. Thermal properties were character-ized by thermogravimetric analysis (TGA-50,Shimadzu Corporation) in nitrogen at a flow rate of20 cm3/min and a heating rate of 10�C/min. In additionto the electrospun fiber samples, TGA analyses werealso conducted on compression-molded films obtainedfrom the neat nylon control and from formulation #6,in order to confirm the effectiveness of compoundingand benchmark the results with reference to those pre-viously obtained on bulk materials.7,8,12,13,26 All sam-ples (both electrospun fibers and compression-moldedfilms) were heated up to 80�C and held for 10min toremove moisture, and then returned to room

Table 1. PA6–MMT–FR formulations compounded for

electrospinning

Formulation

#

PA6

(%)

FR

(%)

MMT

clay (%)

Control 1 100 – –

Solution blending 2 80 15 5

3 80 20 –

High-shear premixing 4 80 15 5

5 80 20 –

6 75 20 5

Wu et al. 3




temperature for a full scan. Combustion properties ofall samples were measured using a microscale combus-tion calorimeter (MCC2, Govmark, Inc.) according toASTM D7309-2007.32 First developed by the USFederal Aviation Administration,33,34 Microscale com-bustion calorimetry (MCC) is a technique that quan-titatively evaluates flammability by directly measuringheat release parameters of small amounts of materialsin the size of milligrams. This technique perfectly fitsthe needs to test the flammability of ultra-thin fiberswhere it would be very costly and time-consuming toproduce large samples. The samples were heated in theapparatus at a heating rate of 1�C/s to 700�C. Eachsample was tested in three repetitions to account forvariability and ensure accuracy. The MCC measure-ment provides combustion characteristics that includespecific Heat Release Rate (HRR) as a function oftemperature, Peak Heat Release Rate (PHRR), andHeat Release Capacity (�c). HRR is defined inASTM D7309 as ‘‘the rate at which combustion heatis released per unit initial mass of specimen duringcontrolled thermal decomposition’’.32 �c is defines as‘‘the maximum specific heat release rate during a con-trolled thermal decomposition divided by the heatingrate in the test’’. Further definitions and a detaileddescription of the method can be foundelsewhere.32,33,35–39

Results and discussion

Fiber morphology

As described in the methods section, we used transmis-sion electron microscopy (TEM) to verify the disper-sion and exfoliation of nanoclay platelets in the

electrospun fibers. TEM is an effective method forexamining the state of nano-filler dispersion withinsmall sections of the matrix. Figure 1 shows TEMmicrographs in increasing magnification showing anelectrospun PA6 fiber with embedded nanoclay plate-lets. An overall homogeneous dispersion of nanoclayplatelets in exfoliated state was achieved, which con-firms results obtained previously on MMT nanocom-posites compounded using the same approaches.13,40,41

In all TEM observations, the nanoclay platelets wereprimarily oriented along the fiber axis.

SEM images of samples electrospun from the neatnylon 6 solution at 27 kV (Figure 2(a)) show fibers witha smooth surface, and fiber diameter of about 200 nm.On the other hand, flat-sheet or ribbon-shaped fiberswith diameters as large as 1 mm were observed with thehigher voltage (Figure 2(b)). Ribbon-shaped electro-spun fibers have been reported for a number of differentpolymers and solvents.42,43 In particular, Fong et al.reported ribbon-shaped fibers of electrospun nylon-6/hexafluoroisopropanol (HFIP).43 The authorsexplained the ribbon structure by the collapse ofhollow tubes formed by the rapid solvent evaporationfrom the surface of the fluid jet.

All solution-blended formulations (see Table 1) weresuccessfully electrospun at 25� 2 kV. Figure 3 showsSEM images with different magnifications of fibers elec-trospun from formulation #2, which contains 15wt %of FR and 5wt % MMT. The overall fiber diameterappears uniform at roughly 100 nm with some ultra-finefibrous protrusions forming a web-like structure. It isalso noticed that there are some particles embeddedwithin the fiber mats, which indicates that the FR par-ticles are not small enough to be incorporated inside thefibers, but are trapped in between fiber layers.

Figure 1. Transmission electron micrographs of electrospun PA6 fibers containing nanoclay platelets in progressive magnification

with scale bars of 200 nm (a), 100 nm (b), and 50 nm (c).





Fibers electrospun from formulations #3 exhibitedsimilar structures as formulation #2 above. Overall, itwas observed that the fibers have a diameter in the100 nm range or below for all solution-blended formu-lations, which indicates that the addition of 5wt %MMT particles or the higher FR particle loading didnot negatively affect spinnability and did not limit theability to spin fibers in the nanometer scale (�100 nm).For all formulations, FR particles were found trappedin the nanofiber webs. In particular, Figure 4 shows alarger FR particle with about 1 micron in diameter,embedded in the fiber web. Close examination of suchparticles revealed agglomerate structures which sug-gested that FR additives may have re-agglomerated toform larger-sized particles in the current processingconditions.

The samples premixed using high-shear blendingwere successfully electrospun with no observableinstability during the entire process, with the exceptionof formulation #6 (the highest loading of FR andMMT particles), which as previously mentioned, exhib-ited some instability during the electrospinning process.

Fibers containing 15wt % FR and 5wt % MMT(formulation #4) are shown in Figure 5. It can beseen that FR particles are entrapped in the fiber mat(Figure 5, left micrograph). This is similar to what wasobserved in with solution-compounded samples. Thefiber diameter is in the range of 100–200 nm. Somefine texture or protrusions along the fiber axis can beobserved, which could correspond to exfoliated clayplatelets. For sample #5, which contains 20wt % FR(Figure 6), the fiber structure shows no significant dif-ference from sample #4 except that the surface wasfound texture-free and more fibrous nano-webs wereobserved. Similar nano-webs or nano-nets wereobserved for all FR-PA6 formulations. The formationof nano-nets has been reported in the literature in elec-trospun PA6 and polyurethane nanocompositefibers.44–46 These nano-nets were explained by thepotential occurrence of secondary electric fields pro-moted by the presence of the nano fillers.44–46

As mentioned above, the formulation with the high-est FR and MMT loading (#6) presented some difficul-ties and instability during electrospinning. The spinning

Figure 2. SEM images of fibers electrospun from neat PA6 (formulation #1) at 27 kV (a), and 35 kV (b).

Figure 3. SEM images of fibers electrospun from nanocomposite formulation #2 (PA6 + 15 wt % FR + 5 wt % MMT) at two

magnifications.

Wu et al. 5




process was not continuous; solution at the needlepoint splashed from time to time when voltage wasapplied. This may be due to the addition of clay andintumescent FR that increased the Rayleigh instability

of the solution and reduced its electrospinnability.47

Despite these difficulties, the spinning process yieldedenough fibers to observe the morphology (although notenough to test thermal properties). The fibers thus elec-trospun from formulation #6 had relatively rough sur-face structures (Figure 7). The protrusions distributedalong the fiber axis are likely to be exfoliated clay plate-lets. As with previous samples, it was also noticed thatsome fine fibrous web structures with diameters as smallas a few nanometers were formed between larger fibers(Figure 7).

Thermogravimetric properties of electrospun fibers

As mentioned in the methods section, TGA was firstperformed on compression-molded films from bulksamples #1 (neat PA6 control) and #6 (20wt % FRand 5wt % MMT) to confirm previous results. Themass loss behavior of the two samples is shown inFigure 8. The observed behavior is in agreement withprevious studies.8,48 The nanocomposite formulation#6 showed lower decomposition temperatures than

Figure 4. SEM image of a large FR agglomerate particle trapped

in the nanofiber web.

Figure 5. SEM images of fibers electrospun from formulation #4 (PA6 + 15 wt % FR + 5 wt % MMT, high-shear melt compounding) at

two magnifications.

Figure 6. SEM images of fibers electrospun from formulation #5 (PA6 + 20 wt % FR high-shear melt compounding) at two

magnifications.





neat PA6. The temperatures at 50% mass loss are 458–473�C for neat PA6 and 434–453�C for the nanocom-posite sample. Single-stage decomposition wasobserved for both samples (Figure 8(b)). The sampleinfused with FR and MMT particles had a lowerpeak mass loss rate. These results indicate thatalthough the nanocomposite sample degradationstarts earlier (at a lower temperature), the mass loss isslower than that observed for the neat nylon sample(lower mass loss rate). Moreover, the difference in resi-dual char weight after decomposition is significant.While the char residue was close to zero in the case ofthe control formulation, it increased to nearly 17% in

the case of the nanocomposite formulation. TGA of theadditives alone in the same conditions resulted in a40% char residue, which, given the proportions of theadditives in the nanocomposite formulation wouldexplain less than a 10% residue. It appears thereforethat the 17% char residue observed in the case of thePA6–MMT–FR formulation does correspond to achange in the overall behavior of the nanocompositesystem.

The TGA curves of the electrospun fiber samples areshown in Figures 9 and 10 for both solution-mixed andmelt-compounded formulations, respectively. TGAcurves for the electrospun fiber-form neat nylon wereincluded in all plots for comparison purposes. Asobserved with the bulk samples, with the addition ofFR and MMT particles, the samples tend to startdegrading at lower temperatures compared to neatPA6. In addition, the overall mass loss rates were effect-ively decreased in all samples infused with FR andMMTparticles. However, the thermographs of the electrospunnanocomposite samples in Figures 9 and 10 are ratherdifferent from those of the bulk sample (Figure 8).Unlike the bulk sample, the patterns observed on theelectrospun fibers suggest a three-stage decompositionprocess. Mass loss rate curves (Figures 9(b) and 10(b))reveal that both solution-blended and melt-com-pounded electrospun nanocomposite samples havethree peaks, whereas neat PA6 samples only have onemajor peak. Based on these observations, it appears thatthe addition of FR particles resulted in higher mass lossrate peaks at low temperatures (at approx. 340�C) cor-responding to the degradation of FR additives, and

Figure 8. Thermographs (a) and mass loss rate curves (b) for bulk samples from formulations #1 (neat PA6) and #6 (PA6 + 20 wt %

FR + 5 wt % MMT – high-shear melt compounding).

Figure 7. SEM image of fibers electrospun from formulation #6

(PA6 + 20 wt % FR + 5 wt % MMT, high-shear melt compounding).

Wu et al. 7




lower peaks between 400�C and 500�C where nylon 6starts degrading.

A summary of the 10% and 50% mass loss tempera-tures (T10% and T50%) for all formulations can befound in Table 2. Decomposition temperaturesdecrease with the addition of FR particle.Furthermore, higher MMT clay loading resulted inhigher char residue, as was expected. For the electro-spun fibers, the method used to premix or dispersethe additives (solution versus high-shear melt mixing)does not appear to impact the decompositiontemperatures.

Figure 10. Thermographs (a) and mass loss rate curves (b) for electrospun fiber samples from formulations #1 (neat PA6), #4

(PA6 + 15 wt % FR + 5 wt % MMT), and #5 (PA6 + 20 wt % FR) – high-shear melt compounding.

Table 2. Thermal properties of neat PA6–FR–MMT

formulations

Sample #

Decomposition temperature (�C)Residue at

700�C (%)T10%�C T50%

�C

1 406 457 6.7

2 338 444 12.4

3 322 434 6.5

4 321 426 9.2

5 333 443 9.0

6 397 434 16.2

Figure 9. Thermographs (a) and mass loss rate curves (b) for electrospun fiber samples from formulations #1 (neat PA6), #2

(PA6 + 15 wt % FR + 5 wt % MMT), and #3 (PA6 + 20 wt % FR) – solution compounding.





Microscale combustion calorimeter (MCC) results

As previously mentioned the sample with the highestadditive loading (formulation #6) was not readily elec-trospinnable and was only tested on MCC in bulk-form. Figure 11 depicts MCC curves for bulk-formneat PA6 and melt-compounded nanocomposite for-mulation #6. The peak heat release rate of the PA6–20wt % FR–5wt % MMT nanocomposite (formula-tion #6) is 328.5W/g, which represents more than a50% decrease from the 685W/g observed on the con-trol PA6 sample. This result, in conjunction with theobserved reduction in heat release capacity (discussedsubsequently) confirmed the effective enhancement inflame retardant effect that was found using UL 94test.8 In addition to the lower heat release parameter,Figure 11 shows a lower ignition temperature of thePA6–20wt % FR–5wt % MMT nanocomposite(approximately 350�C). This temperature correspondsto the degradation of the intumescent FR particleswhich ignites first then forms a protective foam thatisolates the fuel source from heat and oxygen, thusslowing the combustion. The observation of a lowerignition temperature coupled with lower heat releaseparameters is consistent with the mechanism of intu-mescent flame retardants, and is typical in theliterature.49,50

Figure 12 depicts the MCC curves for all electrospunfiber samples, prepared by both solution blending (leftpanel) and high-shear melt mixing (right panel).

The patterns observed for the electrospun nanocompo-site samples appear slightly different from thoseobserved in the injection-molded bulk sample. Indeed,in the case of the electrospun fibers, there is a distinctsmall heat release local peak that appears at about350�C, i.e. at the temperature corresponding to the deg-radation of the intumescent FR particle. This change inbehavior in comparison to the bulk samples is likelydue to the fact that dissolving the nanocompositematerial and processing it through electrospinningresulted in a separation of the FR particles whichwere entrapped between the fibers (see SEM images)and not embedded inside the material mass as is thecase in the bulk samples. Despite this slightly differentbehavior, the presence of the intumescent FR particlesin the fiber mat did result in a similar reduction inPHRR (compare formulations #3 and #5 in Figure 12to formulation #6 in Figure 11). In addition, it isobserved that the weight percentage of FR additivesaffects the peak heat release rate in those samples. Inthe solution blending case, formulations #2 (15% FR)exhibited a peak HRR of around 450W/g, while for-mulations #3 (20% FR) had a peak HRR close to350W/g. This suggests that the substitution of FRwith MMT particles did not result in the same reduc-tion of peak HRR in the solution-blended formulations(Figure 12(a)). On the other hand, when observing thehigh-shear mixed formulations (Figure 12(b)), theaddition of MMT particles appears to have a moresizeable effect considering that the difference between

Figure 11. Comparison of MCC curves of neat PA6 (#1) and formulation #6 (PA6 + 20 wt % FR + 5 wt % MMT) in bulk forms.

Wu et al. 9




the 20wt % FR loading (formulation #5) which exhi-bits a peak HRR of 363.5W/g, and the 15wt % FR–5wt % MMT (formulation #4) with a 385.3W/g peakHRR, is not as important as the difference observedbetween the MCC curves of the corresponding solu-tion-compounded formulations. In addition, the for-mulation with 20wt % FR and 5wt % MMT had an

even lower peak HRR (328W/g), as mentioned above.Overall, the PHRR reduction observed here appearsanalogous to that observed in the literature usingcone calorimetry.20,40

Among the parameters measured by MCC, heatrelease capacity (HRC or �c) is a true material propertywhich only depends on thermodynamic state values and

Figure 12. Comparison of MCC curves of neat PA6 (#1) and nanocomposite formulations #2–5 in electrospun fiber form – solution

blending (a); high-shear melt mixing (b).

Nanocomposite Formulations200

250

300

350

400

450

500

550

600

650

700

750

800

Hea

t rel

ease

cap

acity

ηc

(J/g

-K)

Compression-molded bulk samples Melt premixed elctrospun fibers Solution mixed elctrospun fibers

PA6 control

PA6+5wt% MMT+20wt%FR



PA6+20wt%FR PA6+20wt%FR

Figure 13. Heat release capacity, �c (J/g K) for PA6–FR–MMT formulations with three processing sequences: compression molded

bulk samples (circle point-markers), melt premixed electrospun fibers (square markers), and solution mixed electrospun fibers

(rhombus markers).





is independent of sample size or heating rate.39 Heatrelease capacity (�c) results for the samples tested bythe MCC are depicted in Figure 13. It shows meanplots with 95% confidence intervals. The results areshown in separate plots with differing point-markersfor the three processing sequences: circle point-markersfor the compression molded bulk samples, square mar-kers for the melt premixed electrospun fibers, andrhombus markers for the solution mixed electrospunfibers.

Again, the compression-molded film and electrospunfibers show similar results for the neat PA6. The resultsof the FR–MMT–PA6 formulations appear to bedependent on the weight percentage of the additives.A compression-molded film of formulation #6 contain-ing 5wt % MMT and 20wt % FR resulted in a 50%reduction of �c. Figure 13 also shows that even with lowamounts of FR additives, the electrospun formulationswith MMT particles achieved a significant decrease in�c. Again, the MMT clay particles appear to have amore sizeable effect when the formulations are com-pounded using high-shear melt mixing prior to dissol-ution in formic acid for electrospinning.

Overall, the results above suggest that for the MMTparticles to be effective in reducing peak heat releaseand heat release capacity, high-shear melt compound-ing may be required to ensure exfoliation and disper-sion of the clay platelets inside the nanofiber structures.In other words, the fact that electrospinning requiresdissolving the compounded formulations in formicacid, does not appear to preclude the need for high-shear premixing of the nanocomposite. When exfoli-ation and good dispersion are achieved (e.g., throughhigh-shear mixing), it appears feasible to reduce theamount of FR additives and substitute it with MMTparticles while maintaining flame retardant perform-ance. On the other hand, when exfoliation and gooddispersion are uncertain (solution blending in thiscase), the flame retardant effect appears determined pri-marily by the wt % of FR additives and adding MMTnanoclay has little effect. The effect attributable to theFR particles, on the other hand, does not appear to beaffected by the compounding method (high-shear meltblending vs. solution blending). This observation maybe due to the overall larger particle size of the FR addi-tives and to the fact that those particles are not dis-persed inside the nanofiber core, but are rathertrapped in the fiber web and between fiber layers.

Conclusions

Electrospun flame retardant nanocomposite fibers werespun and characterized. For most samples, the fibershad diameters in the 100 nm range. For some samplesexhibiting instability during spinning (high nano-filler

loading), the fiber diameters generally ranged between100 nm and 200 nm. Two methods of mixing FR andMMT additives, namely solution blending and high-shear premixing were explored. Although TGA analysisindicates a lower starting temperature of decompositionfor nanocomposite samples, the mass loss rate wasoverall decreased for all nanocomposite samples. Inaddition, the residual char mass at 700�C increased sig-nificantly with the incorporation of the additives in thepolymer. MCC results on both bulk and fiber materialsproved that the FR and MMT helped suppress peakheat release rate during combustion. Moreover, FRparticles played a major role in reducing flammabilityof the material in both solution and melt compoundedsample, while MMT was effective in improving charresidue and in reducing flammability in high-shearmelt premixed samples. This result underscores the crit-ical importance of exfoliation and good dispersion toensure effective enhancement of the material withMMT nanoclays.

For future work, other aspects of the nanocompositefibers will be characterized, i.e. more analysis will focuson the dispersion of these nano fillers mainly usingTEM, as well as on the potential to control the particlesize distribution of the FR additive. In addition, futureplans include testing the mechanical properties of thefibrous structure and the incorporation of the mem-branes onto or within fabric systems.

Funding

This work was supported, in part, by KAI, LLC.

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