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Polymer Degradation and Stability 170 (2019) 108998

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Flame retardant polyester fabric from nitrogen-rich low molecularweight additives within intumescent nanocoating

Igor Jordanov a, **, Eva Magovac b, Abbas Fahami c, Simone Lazar d, Thomas Kolibaba d,Ryan J. Smith d, Sandra Bischof b, Jaime C. Grunlan c, d, e, *

a Department of Textile Engineering, Faculty of Technology and Metallurgy, Ss. Cyril and Methodius University, Ruger Boskovic 16, 1000, Skopje, Macedoniab Department of Textile Chemistry and Ecology, Faculty of Textile Technology, University of Zagreb, Prilaza baruna Filipovica 28a, Zagreb, Croatiac Department of Mechanical Engineering, Texas A&M University, 3123 TAMU, College Station, TX, 77843, United Statesd Department of Chemistry, Texas A&M University, 3123 TAMU, College Station, TX, 77843, United Statese Department of Materials Science and Engineering, Texas A&M University, 3123 TAMU, College Station, TX, 77843, United States

a r t i c l e i n f o

Article history:Received 16 July 2019Received in revised form2 October 2019Accepted 12 October 2019Available online 14 October 2019

Keywords:Flame retardancyIgnitionChitosanAmmonium polyphosphateLayer-by-layerAssembly

* Corresponding author. Department of Material ScA&M University, 3123 TAMU, College Station, TX, 778** Corresponding author. Department of Textile Engogy and Metallurgy, Ss. Cyril and Methodius UniversSkopje, Macedonia.

E-mail addresses: jordanov@tmf.ukim.edu.mk (I. Jo(J.C. Grunlan).

https://doi.org/10.1016/j.polymdegradstab.2019.108990141-3910/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

An intumescent nanocoating composed of chitosan (CH) and ammonium polyphosphate (APP), with andwithout low-molecular weight nitrogen and nitrogen/sulphur based derivatives, was deposited onpolyester fabric using the layer-by-layer technique. Guanidine sulfamate (GSM), urea and thiourea wereadded to the aqueous chitosan deposition solution in an effort to improve flame retardancy. Exceptionalself-extinguishing behavior is observed with a 10 bilayer (BL) CH:GSM/APP coated fabric. This water-based coating added 19.6wt% to the polyester and reduced peak heat release rate by 61.7%, relative touncoated fabric. Furthermore, the polyester is not fully ignitable because of earlier decomposition of GSMthat helps to generate a thicker sponge-like char that acts as a physical barrier. This unique addition oflow-molecular weight molecules in traditional intumescent polyelectrolyte pairs is a new opportunityfor imparting flame retardancy to highly flammable synthetic fibers used in clothing and homefurnishing.

© 2019 Elsevier Ltd. All rights reserved.

1. Introduction

Polyester is a widely used synthetic textile fiber that iscompatible with cotton in wearable fabric blends due to its highmechanical stability, durability and chemical, abrasion, shrinkageand wrinkle resistance [1]. Despite these numerous advantages,polyester is highly flammable and can spread fire through moltendrops produced during burning [2]. Flame retardant (FR) polyesterfibers can be obtained by incorporating co-monomers in the pro-cess of chain polymerization [3], mechanical incorporation of FRadditives into the filament during extrusion [4], and through sur-face modification by coating [5]. Halogens and halogen-based de-rivatives are the most efficient flame retardants [6], but theirtoxicity has caused them to be banned or restricted in many places.

ience and Engineering, Texas43, United States.ineering, Faculty of Technol-ity, Ruger Boskovic 16, 1000,

rdanov), jgrunlan@tamu.edu

8

As a result, significant development of FR using phosphorous, ni-trogen, silica, sulphur and their mixtures has occurred [7e12].

Flame retardant coatings can be applied to fibers by traditionalfinishing processes, resulting in a thick coating [13], or by nano-coating deposition. Layer-by-layer (LbL) assembly has emerged asan effective FR nanocoating technique in recent years [9,14]. This isa simple, highly tailorable, water-based technique, performed byalternate exposure of a substrate to oppositely-charged poly-electrolyte solutions or suspensions, giving rise to a multilayer thinfilm. The variety of different ingredients that can be used for thistechnique (polyelectrolytes, inorganic nanoparticles, organicnanomaterials, and renewable macromolecules) makes it possibleto design an unlimited combination of assemblies [15,16]. LbL-coated foam [17e19], cotton [20,21], polyester [22,23], polyamide[24,25] and fiber blends [26,27] have been shown to be flameretardant. Several studies using LbL nanocoatings to improve theflame retardancy of polyester fabric have been conducted. Oppo-sitely charged silica particles were applied to PET as a first attemptto improve FR. This coating eliminated melt dripping and reducedtime to ignition and peak heat release rate (pkHRR) [28]. Zirconiumphosphate was paired with polydiallyldimethylammonium chlo-ride (PDDA)-functionalized polyhedral oligomeric silsesquioxane

I. Jordanov et al. / Polymer Degradation and Stability 170 (2019) 1089982

(POSS), and alumina-coated silica [23]. Zirconium phosphate pairedwith PDDA increased the time to ignition, while when paired withPOSS it reduced pkHRR andwhen pairedwith alumina-coated silicananoparticles it reduced smoke.

Intumescent chemistries have been used to impart flameretardancy to textiles for many years. Early studies mixed ammo-nium polyphosphate, pentaerythritol and urea into bulk poly-propylene [29e31]. This system creates an inorganic acid uponheating that creates a char when heated, which is aided by thepresence of the blowing agent [12,29e31]. Intumescent LbL nano-coating systems have also been studied on cotton, polyester andpolyamide [20,25,26,32e34]. In the present study, LbL deposition ofa chitosan/ammonium polyphosphate intumescent system, incombination with low-molecular weight nitrogen and nitrogen/sulphur based derivatives (guanidine sulfamate, urea and thiourea)was evaluated. Low molecular weight ingredients were solubilizedin the chitosan solution to impart better flame retardancy to thepolyester. CH:GSM/APP achieves exceptional flame retardancy afterdeposition of 10 bilayers (BL). Without these low-molecular weightmolecules, 30 BL are needed to achieve the same performance.

2. Materials and methods

2.1. Substrate and chemicals

Polyester 720H knitted fabric, with a weight of 206 g/m2, wasused as the substrate (Testfabrics, Inc., Pittston, PA). Chitosan (CH)was purchased from G.T.C. Bio Corporation (Qingdao, China), withMw~ 60,000 g/mol. Exolit AP422 ammonium polyphosphate (APP)was supplied by Clariant Corp. (Charlotte, NC). Hydrochloric acid37% (HCl), sodium hydroxide-pellets (NaOH), urea (U) and thiourea(THU) were purchased from Sigma-Aldrich (Milwaukee, WI), whileguanidine sulfamate (GSM) was purchased from Tokyo ChemicalIndustry Co., Ltd. (Tokyo, Japan). All chemicals were used asreceived. All aqueous solutions were prepared with 18.2MUdeionized (DI) water. 1 wt% CH was prepared in a pH 1.5 solution(using HCl) and then adjusted to pH 3 using 0.1M NaOH. 1wt% APPsolution was prepared according to a previously reported proced-ure and used for coating immediately upon preparation [35]. 1 wt%APP, 11.1wt% 1M NaOH, 11.1 wt% 1M HCl, and 76.8wt% DI waterwere prepared by first mixing APP in water until a homogenoussuspension was obtained. NaOH was then added and the solutionwas mixed until completely dissolved (a noticeable increase inviscosity occurs). Finally, HCl was added to reduce viscosity. Thecontrol flame retardant recipe consists of a positively chargedaqueous solution of 1wt% CH at pH 3 and a negatively chargedsolution of 1wt% APP at pH 4.2. From this recipe, three combinationsolutions were prepared by dissolving 13wt% each of GSM, U andTHU in the chitosan solution [(1% CH - 13% GSM)3/1% APP4.2], [(1%CH - 13% U)3/1% APP4.2] and [(1% CH - 13% THU)3/1% APP4.2]. 13wt%was chosen because the maximum solubility of THU at roomtemperature is 142 g/L. pH is indicated by the subscript on a givensolution.

2.2. Layer-by-layer (LbL) deposition

LbL deposition was carried out by hand dipping, as shownschematically in Fig. 1. The polyester fabric was first immersed intothe CH solution for 5min, followed by rinsing in DI water for 1min.The same dipping and rinsing procedure was carried out with theAPP solution. This cycle creates one bilayer. The solution dips werereduced to 1min for additional layers and the cycle was repeateduntil the desired number of bilayers was deposited. Each solutionand the rinse water were renewed after every five bilayers. Oncethe desired numbers of BL were deposited, the fabric was rinsed

with DI water and dried for 2 h at 70 �C.

2.3. Characterization

Vertical flame testing (VFT) was performed on three76� 300mm samples, according to ASTM D 6413, by applying apropane flame for 12 s at the bottom of the fabric. The combustionbehavior was tested with micro cone calorimeter (MCC) accordingto ASTM D 7309, using MCC-2 instrument (Govmark, Farmingdale,NY). A 5mg sample (three samples per each recipe) was evaluatedfrom 25 to 650 �C at a heating rate of 1 �C/s in a nitrogen gas streamof 80ml/min. Limiting oxygen index (LOI) was measured in aDynisco LOI chamber (Heilbronn, Germany), according to ISO4589:1996, using fabric strips (14 cm� 5.2 cm). The presented re-sults are average values of three measurements. Thermal stabilityof control and coated polyester (z15mg) were also evaluated bythermogravimetric analysis (TGA) using a Q-50 thermogravimetricanalyzer (TA Instruments; New Castle, DE) under a controlledheating ramp of 10 �C min�1, from ambient temperature to anisothermal hold at 100 �C for 30min and then ramping up to 850 �C.A sample purge flow of 60mLs�1 air and a balance purge flow of40mLs�1 nitrogen were used for testing in an air atmosphere. Asample purge flow of 40mLs�1 nitrogen and a balance purge flow of60mLs�1 nitrogen were used for testing in a nitrogen atmosphere.Scanning electron microscope (SEM) images were collected using aLYRA3 TESCAN scanning electron microscope (Brno, Czech Repub-lic) at a beam voltage of 5 kV. Prior to imaging, a 5 nmPt/Pd coatingwas applied to the surface of the fabric to minimize charging.

3. Results and discussion

Polyester fabric coatedwith 10, 25 and 30 BL of CH/APP is shownin Fig. 2, following vertical flame testing. By increasing the numberof bilayers from 10 to 25, weight gain nearly doubles (from 16.1 to31.8%), and burning rate decreases (from 7.8 to 5.9mm/s), but thesamples are not self-extinguishing. Samples with 10 and 25 BLcreate significant char after VFT, indicating the intumescent char-acter of the CH/APP pair. With 30 BL of CH/APP, the polyester has38.8%weight gain and exhibits self-extinguishing behavior. Fig. 2(c)shows this fabric after vertical flame testing. It has residual weightof 98.8% and LOI of 26%, as summarized in Table 1. Limiting oxygenindex is a widely used parameter for measuring the flammability oftextiles. Generally, fabrics with a LOI greater than 25 vol% are said tobe flame retardant [36,37]. Without the multilayer coating, the LOIof the polyester fabric is 20e21% [36,38].

In an effort to achieve better flame retardant behavior withfewer bilayers deposited, low molecular weight nitrogen and ni-trogen/sulphur based derivatives were added to the CH/APP intu-mescent system (solubilized in the CH solution). Guanidinesulfamate, urea and thiourea were each added as 13wt% into thechitosan solution for deposition. Fabric coated with 10BL ofCH:GSM/APP has a weight gain of 19.6%, as shown in Table 1. TheVFT results summarized in Table 1 show that only the CH:GSM/APPsystem exhibits good flame retardancy, achieving self-extinguishing behavior with only 10 BL and nearly half theweight gain required for CH/APP to achieve the same behavior.Polyester coated with 10 BL CH:GSM/APP has a LOI of 26%, it is notignitable and does not burn under exposure to flame in a verticalconfiguration. Although the other two systems, containing urea andthiourea, did not achieve this behavior, they have lower burningrate and higher LOI than polyester fabric coated with 10 BL CH/APP.

Weight percent and derivative weight loss (measured in air) as afunction of temperature are shown in Fig. 3 and the data aresummarized in Table 2. The uncoated fabric exhibits two degra-dation steps: the first starts around 430 �C and the second starts

Fig. 1. Schematic of LbL assembly of [(1% CH-13% X)3/1% APP4.2] nanocoatings.

Fig. 2. Images of LbL coated polyester fabric after vertical flame testing: (a) 10 BL 1% CH3/1% APP4.2, (b) 25 BL 1% CH3/1% APP4.2, (c) 30 BL 1% CH3/1% APP4.2, (d) 10 BL (1% CH e 13%GSM)3/1% APP4.2, (e) 10 BL (1% CH e 13% U)3/1% APP4.2, (f) 10 BL (1% CH e 13% THU)3/1% APP4.2.

Table 1Results of VFT of uncoated and LbL-coated polyester fabric.

Coating BL Weight gain (%) Burning rate (mm/s) Self-extinguishing Residual weight after VFT (%) LOI (%)

Uncoated Polyester e No e N/A1% CH3/1% APP4.2 10 16.1 7.8 No 38.7 23

25 31.8 5.9 No 42.5 2530 38.8 e Yesa 98.8 26

(1% CH - 13% GSM)3/1% APP4.2 10 19.6 e Yesa 95.4 26(1% CH - 13% U)3/1% APP4.2 10 17.8 6.9 No 32.7 24(1% CH - 13% THU)3/1% APP4.2 10 17.4 6.7 No 34.9 24

a The samples did not truly ignite during the 12 s exposure to flame, but rather produced char during flame exposure.

I. Jordanov et al. / Polymer Degradation and Stability 170 (2019) 108998 3

around 560 �C. The first step is due to degradation of PETchains intosmaller fragments by an initial scission of the chain end. The secondweight loss is related to thermo-oxidative degradation of the smallfragments into volatile products, leading to a final residue of 0.6%[23,39]. The presence of the nanocoating reduces the decomposi-tion temperature of polyester. The Tonset10% decreases as the numberof deposited CH/APP bilayers increase. Fabric with 10 BL is 8% andwith 30 BL it is almost 30% lower than the Tonset10% of uncoatedpolyester. The initial temperature of decomposition (Tmax1) isaround 220e230 �C, which corresponds to the starting temperatureof decomposition of APP [40]. The CH/APP pair represents a tradi-tional intumescent-like system. Chitosan is a polysaccharide thatacts as the carbon source, while its amino groups serve as a blowingagent. APP is an acid source that generates in situ phosphoric acid athigh temperature that dehydrates chitosan, generating water vaporand promoting char formation. The residue at Tmax1 decreases,while the residue at Tmax4 and at 600 �C increases as number ofdeposited bilayers increase. The greater weight of APP with addedbilayers degrades more at Tmax1, so less residue is left on the poly-ester fiber surface. Additionally, this greater amount of APP and CHproduce more residue at Tmax4 and at 600 �C because more acid is

produced that acts on the CH by forming more char at elevatedtemperatures, as shown in Table 2.

The thermo-oxidative behavior of CH/APP enriched with low-molecular weight additives are also summarized in Table 2. Poly-ester coated with 10 CH:GSM/APP BL shows an additional Tmax2 at385 �C (Fig. 3(b)), which likely corresponds to the decomposition ofGSM. Similar data were obtained for polyamide filament extrudedwith 5% GSM [41]. Polyester fabric is self-extinguishing whencoatedwith 30 BL CH/APP and 10 BL CH:GSM/APP, but the latter has34% lower residue at Tmax4 and about 40% lower residue at 600 �C.The extinguishing behavior of 10 BL CH:GSM/APP coated fabric isbelieved to be the result of decomposition of GSM that acts as ablowing agent to create gases that will create thicker char underexposure to flame.

Thermal degradation of the samples under pyrolysis conditionsin a nitrogen atmosphere are shown in Fig. 4 and the data aresummarized in Table 3. These results are similar to TGA in an airatmosphere (Table 2). The uncoated fabric tested in nitrogen ex-hibits two degradation steps, CH/APP samples exhibit three, andCH:GSM/APP exhibits four, degradation steps. The shoulder at385 �C of the CH:GSM/APP derivative weight loss curve measured

Fig. 3. Weight loss (a) and derivative weight loss (b) as a function of temperature foruncoated and LbL-coated polyester obtained in an air atmosphere. Fig. 4. Weight loss (a) and derivative weight loss (b) as a function of temperature for

uncoated and LbL-coated polyester obtained in a nitrogen atmosphere.

I. Jordanov et al. / Polymer Degradation and Stability 170 (2019) 1089984

in nitrogen (Fig. 4(b)) corresponds to the temperature of GSMdecomposition in air (Fig. 3(b)) and confirms decomposition ofGSM. Final residual weight measured in nitrogen is more intensethan that measured in air. Uncoated polyester has 17.4% residualweight and it rises as a number of deposited CH/APP bilayers in-creases (Table 3). CH:GSM/APP has the lowest final residual weightcompared with 10 and 30 BL CH/APP on polyester.

The thermal behavior of coated and uncoated polyester wasfurther probed with micro-cone calorimetry, with the resultssummarized in Fig. 5 and Table 4. Uncoated polyester fabric de-grades at 450 �C and exhibits a peak heat release rate of 482W/g,

Table 2Thermogravimetric data of uncoated and LbL-coated polyester obtained in an air atmosp

Recipe BL Tonset10% [oC] Tmax1 [oC] Tmax2 [oC] Tma

Uncoated Polyester 396 e e 4291% CH3/1% APP4.2 10 363 225 e 422

25 330 227 e 42630 278 232 e 427

(1% CH-13% GSM)3/1% APP4.2 10 345 225 385 421(1% CH-13% U)3/1% APP4.2 10 355 230 e 420(1% CH-13% THU)3/1% APP4.2 10 360 232 e 420

while CH/APP coated fabric degrades earlier and produces less heat.Fabric coated with 10, 25 and 30 CH/APP BL yields pkHRR of 240,206 and 187W/g, respectively. TpkHRR of these samples decreasewith increasing bilayers and are similar to the Tmax3 (Table 2) whenweight loss is greater than 50%. Polyester coated with 10 CH:GSM/APP BL has the lowest TpkHRR and produces the lowest pkHRR of184W/g. This fabric has 4% lower TpkHRR and about 62% lowerpkHRR than uncoated fabric. This CH:GSM/APP-coated fabric hassimilar TpkHRR and pkHRR to 30 CH/APP BL coated fabric, which hasmuch higher weight gain (38.8%) and triple the bilayers. Fig. 5compares MCC data for 30 BL CH/APP and 10 BL CH:GSM/APP

here.

x3 [oC] Tmax4 [oC] Residue at

Tmax1 [%] Tmax2 [%] Tmax3 [%] Tmax4 [%] 600 [%]

560 e e 53.6 5.4 0.6533 96.1 e 48.3 16.5 10.9535 94.7 e 47.2 20.9 15.9535 92.7 e 47.1 22.2 17.3532 97.1 67.8 37.2 14.6 10.4529 95.0 e 44.9 15.4 9.4527 96.0 e 45.0 16.2 9.4

Table 3Thermogravimetric data of uncoated and LbL-coated polyester obtained in a nitrogen.

Recipe BL Tonset10% [oC] Tmax1 [oC] Tmax2 [oC] Tmax3 [oC] Tmax4 [oC] Residue at

Tmax1 [%] Tmax2 [%] Tmax3 [%] Tmax4 [%] 600 [%]

Uncoated Polyester 409 e e 435 595 e e 53.2 17.6 17.41% CH3/1% APP4.2 10 395 230 e 438 599 97.6 e 52.8 24.8 24.6

30 364 232 e 436 585 95.8 e 52.6 29.5 28.5(1% CH-13% GSM)3/1% APP4.2 10 354 232 390 430 584 96.9 73.6 45.5 23.9 22.9

Fig. 5. Heat release rate as a function of temperature (measured by MCC) for coatedand uncoated polyester.

I. Jordanov et al. / Polymer Degradation and Stability 170 (2019) 108998 5

deposited on the polyester fabric. These systems exhibit similarpkHRR, but the GSM-containing coating has an additional shoulderaround 385 �C that correspond to the Tmax2 from the TGA data,shown in Tables 2 and 3. These data suggest that degradation of theGSM at a lower temperature creates gas that is responsible forcreating a thicker sponge-like char that improves flame retardancyof the CH:GSM/APP coated polyester. The heat release rate datafrom MCC (Fig. 5) and TGA in nitrogen (Fig. 4(b)) match wellbecause micro-cone testing is based on a controlled pyrolysis in aninert nitrogen atmosphere. The gases produced are mixed withexcess oxygen and combusted. The heat of the combustion ismeasured as heat release rate by oxygen consumption calorimetry.

Fig. 6 shows SEM images of uncoated and coated polyesterfabric. 10 and 30 BL CH/APP-coated samples are shown in Fig. 6 (b)and (c), respectively. More coating is clearly evident with morebilayers, with 10 BL havingmore coating into the yarn (between thefibers) and between the loop structure of the knitted fabric andnegligible amount between the stitch columns. The 30 BL coatingcovers the whole knitted structure, making a continuous film on

Table 4MCC data for uncoated and LbL-coated polyester fabrics.

Recipe Number of bilayers TpkHRR (C

Uncoated Polyester 450.11% CH3/1% APP4.2 10 440.2

25 435.430 434.5

(1% CH - 13% GSM)3/1% APP4.2 10 432.1(1% CH - 13% U)3/1% APP4.2 10 439.8(1% CH - 13% THU)3/1% APP4.2 10 439.3

the fabric surface. The fabric coated with 10 BL of CH:GSM/APP(Fig. 6(d)), shows conformal coverage into the yarns and into theloop structure, without coating between the knit columns.

Fig. 7 shows SEM images of the fabric samples following verticalflame testing. Polyester coated with 10 and 30 BL CH/APP have thesame chemical composition and therefore their char looks similar,although greater char is generated with more coating. With morebilayers deposited a more continuous char is formed with smallerholes. Comparing 10 BL CH/APP (Fig. 7(b)) with 10 BL CH:GSM/APP(Fig. 7(c)) shows a contrast in char structure. The GSM-containingsample appears to have denser char than 10 BL CH/APP. Smallholes in the GSM-containing char are likely due to gas formationand release during burning. This char has a sponge-like structure(Fig. 7(d)).

Asmentioned above, the intumescence process is a combinationof charring and foaming at the surface of the substrate duringexposure to flame. The formed char layer acts as a physical barrierto slow heat and mass transfer between the gas and condensedphases. The formation of an effective char occurs with gas forma-tion and expansion at the surface. Adding low-molecular nitrogenand nitrogen/sulphur based derivatives into the CH layer providesblowing agents that further expand the char structure on polyester.The sponge-like structure of CH:GSM/APP char illustrates this effect(Fig. 7(c) and (d)). The GSM shows better performance than U andTHU. It decomposes earlier (Figs. 3(b) and Fig. 4(b)), creating moregas that serves as a blowing agent, making thicker char with asponge-like structure that acts as a physical barrier that slows heatand mass transfer between the gas and condensed phases.

4. Conclusions

Layer-by-layer deposition of the traditional intumescent CH/APPsystem, with and without low-molecular weight nitrogen and ni-trogen/sulphur based derivatives is shown to render polyesterfabric self-extinguishing. A 30 BL CH/APP coating adds 38.8wt% tothe polyester fabric, resulting in a 61.1% reduction in pkHRR (and itis not ignitable in vertical flame testing). With only 10 BL ofCH:GSM/APP the weight of polyester fabric increases 19.6% andpkHRR is reduced 61.7% relative to uncoated polyester. This 10 BLcoating is also non-ignitable in VFT, with only one third of thecoated layers and half the weight gain of CH/APP. Improved flame

�) △TpkHRR (%) pkHRR (W/g) △pkHRR (%)

481.8�2.2 239.7 �50.2�3.3 205.6 �57.3�3.5 187.3 �61.1�4.0 184.2 �61.7�2.3 258.9 �46.3�2.4 245.4 �49.1

Fig. 6. SEM images of (a) uncoated polyester fabric and fabric coated with (b) 10 BL CH/APP, (c) 30 BL CH/APP and (d) 10 BL CH:GSM/APP.

Fig. 7. SEM images of polyester fabric coated with (a) 10 BL CH/APP, (b) 30 BL CH/APP, (c) 10 BL CH:GSM/APP after vertical flame testing. (d) is (c) with higher magnification.

I. Jordanov et al. / Polymer Degradation and Stability 170 (2019) 1089986

retardancy with GSM addition is believed to be due to its earlierdecomposition at a lower temperature (around 385 �C), makingthicker char with a sponge-like structure that acts as a physicalbarrier that slows heat and mass transfer between the gas andcondensed phases. Adding low-molecular weight derivatives totraditional intumescent polyelectrolyte pairs is an effectiveapproach for imparting flame retardancy to synthetic textiles,

while also reducing weight gain and processing steps.

Declaration of competing interest

The authors declare that they have no known competingfinancial interests or personal relationships that could haveappeared to influence the work reported in this paper.

I. Jordanov et al. / Polymer Degradation and Stability 170 (2019) 108998 7

Acknowledgments

The authors acknowledge to the Fulbright Visiting Scholar Pro-gram at the US Department of State for supporting Professor IgorJordanov stay/research at Texas A&M University (Grant numberE0581973).

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