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Polymer Degradation and Stability 94 (2009) 996–1000

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Polymer Degradation and Stability

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

The anti-dripping intumescent flame retardant finishing for nylon-6,6 fabric

Lingyao Li a, Guohua Chen b, Wei Liu a, Jianfu Li c, Sheng Zhang a,d,*

a Beijing Key Laboratory on Preparation and Processing of Novel Polymeric Materials, Department of Polymer Engineering, Beijing University of Chemical Technology,Beijing 100029, Chinab College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing 100029, Chinac China National Chemical Corporation, Haidian District, Beijing 100080, Chinad State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e i n f o

Article history:Received 15 December 2008Received in revised form19 February 2009Accepted 25 February 2009Available online 6 March 2009

Keywords:Anti-drippingIntumescent flame retardantNylon-6,6 fabricChar formation

* Corresponding author. College of Materials andBeijing University of Chemical Technology, Beijinþ86 (10)64436820.

E-mail address: sheng1999@yahoo.com (S. Zhang)

0141-3910/$ – see front matter � 2009 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2009.02.009

a b s t r a c t

Nylon-6,6 fabric has been widely used in military and civilian area for many years. However, the meltingdrip problem has not been effectively solved despite the efforts made in the last two decades. Anintumescent flame retardant system, containing ammonium polyphosphate, melamine and pentaery-thritol, has been proved to be effective on preventing melting drip during burning of nylon-6,6 fabric inthis study. The LOI and the vertical flammability test indicate that this IFR (intumescent flame retardant)system could improve the flame retardancy and impart dripping resistance to nylon-6,6 fabric. Thermalbehaviour of nylon-6,6 fabric treated with IFR system was investigated by thermogravimetric (TG) anddifferential scanning calorimetric(DSC) experiments. The results indicate that char residue of treatedsamples are above 13%, and the highest value could reach up to 24% at 750 �C which is much higher thanthat of the untreated fabric. SEM graphs of residue of treated and untreated nylon-6,6 fabric show thatIFR could promote formation of residual char which impart anti-dripping property to nylon-6,6 fabric.The tensile property test shows that tensile strength of treated fabric decreased.

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1. Introduction

Flame retardant finishing of nylon fabric has been paid more andmore attention in recent years because of its excellent end-useproperties in both military and civilian areas. One difficulty in flameretardant finishing of nylon is the low reactivity of nylon whichusually results water solubility, another one is the formation ofmelting drip during burning which usually leads to spread offire [1].

A number of articles on flame retardant finishing of nylon fabricsbased on various formulations have been published. Halogenatedflame retardants, such as brominated flame retardants, thiourea–formaldehyde resin and cyclic phosphoryl chloride derivatives,have been used on nylon fabrics due to their effective flameretardancy [2–6]. A lot of research about modification of nylon withformaldehyde which could impart possible reactivity to nylon havebeen published elsewhere [7,8]. However, halogen and formalde-hyde flame retardant system has been undergoing more and morepressure because of the increasing concern about the possible

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hazard caused to the environment. Therefore, phosphorus-basedand intumescent flame retardant systems have been attracted moreand more attention for flame retardant treatment of nylon textiles[9–11]. However, none of those technologies has achieved anysubstantial commercial success.

In this study, anti-dripping effect of intumescent flame retar-dant (IFR) system which includes ammonium polyphosphate,melamine and pentaerythritol on nylon fabrics has been investi-gated. Flame retardant performance and washing durability oftreated samples were evaluated by LOI and the vertical flamma-bility tests. Thermogravimetric (TG) and differential scanningcalorimetric(DSC) experiments were carried out to study the charformation and thermal property. The structure of residual char wasobserved by scanning electron microscopy.

2. Experimental

2.1. Materials

Nylon fabric: 100% nylon-6,6 woven fabric with the density of165 g/m2 was supplied by Hai rong Technical Textiles Co. Ltd.,Cheng Du, China.

Ammonium polyphosphate (APP): (1) APPI, with the polymer-ization degree of 10–20, was supplied by TongLi Agent, Cheng Du,

Table 1The LOI (%) values of the nylon-6,6 fabric treated with different formulae.

Samples APPI:APPII:melamine:pentaerythritol mass ratio

Add-on(%w/w)

LOI (%)

1 3:0:0:0 30.2 23.52 3:0:1:0 38.0 27.53 3:0:0:1 34.9 25.34 3:0:1:1 40.6 27.95 0:3:0:0 2.5 21.76 0:3:1:0 3.8 22.17 0:3:0:1 3.4 21.18 0:3:1:1 4.6 22.9

Table 2The vertical flammability test result of samples treated with APPI.

Samples After-flame time(s)

Afterglow time(s)

Char length(cm)

Dripping

1 26.3 0 27 None2 5.6 0 4.3 None3 8.0 0 4.5 None4 5.3 0 4.3 NoneUntreated sample 11.6 0 7.8 Dripping

L. Li et al. / Polymer Degradation and Stability 94 (2009) 996–1000 997

China. (2) APPII, with the polymerization degree of more than 20,was supplied by LeTai Chem, Beijing, China.

Melamine and pentaerythritol were commercial productssupplied by FuChen Chem, Tian Jin, China. All chemicals areanalytical grade.

2.2. Flame retardant treatment

The fabric was first immersed in a finishing solution, thenpassed through a padder with two dips and two nips, and finallydried at 135 �C for 4 min. All concentrations in this study werebased on weight of bath solution (w/w, %). The wet pick-up ofnylon-6,6 fabric was about 70%. The pressure between the two rollsis 0.1 Mpa.

2.3. Evaluation of flame retardant performance

The limiting oxygen index (LOI) is measured according to GB/T2403-1993 by using JF-3 LOI instrument, Jiangning, China. Thevertical flammability is measured according to GB/T 5455-1997 byusing CZF-3 instrument, Jiangning, China.

2.4. Thermal analysis

Thermogravimetric (TG) and differential scanning calorimetric(DSC) experiments were carried out by using HENVEN HCT-1 TG-DSC analyzer. Sample weight is in the range of 2–3 mg. All samplesfor TG and DSC were measured from 25 �C to 800 �C at the heatingrate of 10 �C/min in static air atmosphere.

2.5. Scanning electron microscopy (SEM)

SEM analysis was carried out on HITACHI S4700 instrument inorder to investigate the surface of residual char of treated samplesand the melting drip of untreated sample. Treated and untreatedfabric samples were burnt in air and their residual char sampleswere used for SEM tests.

2.6. Measurement of tensile property

Samples with the size of 25� 4 cm were subjected to a test oftensile property by using GM T4104 electronic tensioner made inShengzheng, China according to FZ 65001-1995. The drawing speedis 10 cm/min and the clamping distance is 20 cm.

3. Results and discussion

3.1. Flame retardant performance

The nylon-6,6 fabric was treated with two IFR systems con-taining APPI and APPII respectively. The concentration of APPI was90% due to its good solubility in water, while the concentration ofAPPII was 30% because of its limited solubility. The LOI valuesof treated samples are presented in Table 1. LOI values of samplestreated with APPI are significantly higher than that of samplestreated with APPII. The highest LOI value occurs in the formulacontaining APPI, melamine and pentaerythritol (with massproportion of 3:1:1). It is the result of synergic effect of APP as acidsource, melamine as gas source and pentaerythritol as char sourcein IFR system. No melting drip has been observed during burningprocess of all treated samples. Flame spread on the top of thetreated sample in the initial stage of burning, a char zone wasformed afterward on the sample surface which finally preventedthe flame from spreading.

The same phenomenon of anti-dripping has also been observedin vertical flammability test (see Table 2). Compared to untreatedsample, samples treated with two components and three compo-nents have shorter after-flame time, shorter char length and nodripping. The melting drip from untreated sample could burn thepledget and it might cause another fire in fact. Treated nylon-6,6fabric solved this problem due to its dripping resistance. However,the sample treated with only one component APP had longer after-flame time and char length than untreated one. It is suggested thatthis is because of the ‘‘scaffolding effect’’ when nylon blends withchar-promoting substances [12]: the nylon fabric burns stronglybecause the molten nylon wicks onto the char of the othercomponent. When IFR is insufficient, the fabric will exhibit theburning phenomenon described above. If IFR is sufficient, the fabriccould show not only anti-dripping but also flame retardantproperty.

The conventional halogen and formaldehyde system used inpolyamide fabric flame retardant finishing, such as tetrabromoph-thalate and thiourea-formaldehyde systems, usually have high LOIvalues of over 30. However, these two systems cannot pass thevertical flammability test due to the melting drip problem.Although the highest LOI of this intumescent system is only 27.9, itis high enough to meet the industrial needs in most cases;furthermore, the excellent performance in vertical flammabilitybecause of anti-dripping property gives this system a distinguishedadvantage compared to the conventional methods.

3.2. Thermal analysis

TG, DTG and DSC were used to investigate the thermal proper-ties of nylon-6,6 fabric treated with IFR system. The TG, DTG andDSC curves of the treated samples are presented in Figs. 1–3,respectively.

The untreated nylon-6,6 fabric starts to lose weight at 350 �C(Fig. 1). DTG curve shows that there is a peak at 410 �C which meansthe rate of weight lost reaches its maximum at this point (Fig. 2). Inthe same temperature range DSC curve appears as an exothermalpeak (Fig. 3). The untreated nylon-6,6 fabric lost 98% of its originalweight at 750 �C(Fig. 1).

For treated nylon-6,6 fabric samples, the onset degradationtemperature moved forward. Sample 4 treated with APP, melamine

Fig. 1. TG curves of the untreated nylon-6,6 fabric and that treated with IFR containingAPPI, melamine and pentaerythritol.

Fig. 3. DSC curves of the untreated nylon-6,6 fabric and that treated with IFR con-taining APPI, melamine and pentaerythritol.

L. Li et al. / Polymer Degradation and Stability 94 (2009) 996–1000998

and pentaerythritol started to lose weight at 120 �C and othersamples at 140 �C (see Fig. 1). This phenomenon can be explainedby evaporation of water. Because APPI, melamine and pentaery-thritol in this system are hygroscopic. The first step weight loss ofSample 4 with 40.6% of add-on is more obvious than that of othersamples, and it is suggested that this is mainly due to its moremoisture absorption. The second step weight loss appeared around270 �C for all samples’ TG curves. This may correspond with thedecomposition of APPI which starts at 250 �C and the initialdecomposition of melamine and pentaerythritol which are botharound 300 �C. Three components of this system start to decom-pose and react to form char layer which could prevent theinflammation during this step. Sample 2 lost 76% of its originalweight with 24% residual solid at 750 �C and other samples’residual solid were all above 13%.

The DTG curve shows that treated samples had lowertemperature of maximal rate of weight loss than that ofuntreated sample. The sample 4’s rate of weight loss reached itsmaximum at 310 �C which was lowest in those samples. The

Fig. 2. DTG curves of the untreated nylon-6,6 fabric and that treated with IFR con-taining APPI, melamine and pentaerythritol.

DTG curve also indicates that treated samples’ maximal rate ofweight loss is smaller than that of untreated one. The sampletreated with APP and pentaerythritol had the smallest value ofmaximal weight loss rate in all samples (see Fig. 2). Treatedsamples show not only smaller exothermal areas but also earlierexothermal peaks than untreated sample in the DSC curve (seeFig. 3). All the data presented above indicate that the presence ofIFR could lower the decomposition temperature and enhancethe formation of char after burning of nylon-6,6 fabric. Consid-ering the fact that there is no dripping observed when treatedsamples burns in LOI and vertical tests, it is suggested that thegood formation of char could be the main attribution for thisphenomenon.

3.3. Microstructures of the char residue

SEM photographs in Fig. 4 show the microstructures of theresidue of treated and untreated nylon-6,6 fabric. The surface ofuntreated sample is relatively flat and smooth compared with othergraphs of treated samples. This may suggest that the components ofdrip are mainly melted nylon-6,6 and a little partially decomposednylon-6,6. However, compared with untreated sample, graph (B)–(E) show the different residue microstructures. Graph (B) showsthat residual char of sample treated with APP has micro-convexitieson its surface, which might because of the APP promotion for theformation of char. Residual char of sample treated with APP andmelamine in graph (C) has frothy structure and the reason ismelamine as gas source decomposes and releases gas which finallyformed a frothy structure. Graph (D) shows that residual char ofsample treated with APP and pentaerythritol is continuous andthick due to char-formation effect of pentaerythritol as char source.In graph (E), residual char of sample treated with APP, melamineand pentaerythritol is frothy and swollen, which is a typical intu-mescent char structure.

When nylon-6,6 fabric is burning, the char could form a barrierbetween flame and the fabric, which prevents nylon-6,6 fromcontacting the fire and inhibiting the release of fuel gas.

The residual char observed in SEM graphs can explain the anti-dripping phenomenon of treated nylon-6,6 fabric. Intumescentchar layer could support melted nylon-6,6 so as to prevent meltedcomponent from dripping during combustion, and this has beenproved in vertical flammability test.

Fig. 4. SEM photographs of residue of treated and untreated nylon-6,6 fabric. (A) untreated; (B) treated with APP; (C) treated with APP and melamine; (D) treated with APP andpentaerythritol; (E) treated with APP, melamine and pentaerythritol.

L. Li et al. / Polymer Degradation and Stability 94 (2009) 996–1000 999

3.4. Tensile strength of treated nylon-6,6 fabric

Tensile properties of treated and untreated fabric were testedand the data are presented in Table 3. The tensile strength ofuntreated sample is 190.1 Mpa and elongation at break is 26%. Thetensile strength of treated samples is lower than that of untreatedsample, while the sample containing APP has the lowest value of149.5 Mpa (see Tables 1 and 3). It is suggested that two proceduresof flame retardant finishing of nylon-6,6 fabric led to the decreasingof the nylon-6,6 fabric’s tensile property. When the fabric was

Table 3Tensile properties of treated and untreated nylon-6,6 samples.

Formulae Tensile strength (MPa) Elongation at break (%)

1 149.5 28.32 168.4 29.23 171.4 27.64 167.8 30.2Untreated 190.1 26.0Treated without flame retardants 167.0 28.9

dipped in flame retardant solution and passed through the twin-roll, some fibers of the fabric might be broken. Then the fabric wasdried at 135 �C for 4 min and the high temperature in this step couldcause damage to the fibers. The tensile strength of the sampletreated without flame retardants decreases, which further demon-strates the fibers of fabric can be broken during the process. Inaddition, the weight loss below 150 �C according to the TG curves oftreated samples may reduce the tensile property at this temperaturerange. All the samples except the sample containing APP retained89% of tensile strength of the original sample. Although the tensilestrength decreased, the elongation at break increased.

3.5. Flame retardant mechanism of the intumescent system

In this intumescent flame retardant system APP is the main acidsource, melamine is the gas source and pentaerythritol is the charsource. Three components could decompose and react to formintumescent char layer at the temperature range of 250–350 �C. It issuggested the flame retardant system effectively prevent theinflammation through the following processes:

L. Li et al. / Polymer Degradation and Stability 94 (2009) 996–10001000

i. APP starts to decompose at 250 �C and then releases poly-metaphosphoric acid.

ii. The polymetaphosphate acid esterifies the pentaerythritol.iii. Dehydration of polyhydric alcohol phosphate ester occurs

and it forms the char finally. Meanwhile, the water vaporproduced by esterification and the nonflammable gasproduced by melamine make the char layer intumescent andfrothy [11].

iv. The decomposition and reaction of flame retardants absorbheat significantly, at the same time the intumescent charlayer can prevent the flame from propagating.

4. Conclusions

The intumescent flame retardant system containing APP,melamine and pentaerythritol improved flame retardancy andreduced dripping tendency of nylon-6,6 fabric. The highest LOIcould reach up to 27.9. Thermal analysis showed that treated fabriccould form more residual char and release less heat than untreatedfabric. SEM photographs indicated treated fabric could form intu-mescent residual char. It is concluded that this intumescent flameretardant system could improve the charformation which is the keyfactor in dripping resistance. Further investigation is undergoing toimprove the tensile strength and washing durability in our labo-ratory. This flame retardant system can be applied to produce somecommercial products which washing durability is not highlyrequired, such as carpet and curtain.

Acknowledgement

The authors are grateful to National Natural Science Foundationof China for financial support and to Mr. Xinjun Zhu for his kindhelp during this investigation.

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