preparation, properties and characterizations of halogen-free nitrogen–phosphorous flame-retarded...

Polymer Degradation and Stability 91 (2006) 2003e2013www.elsevier.com/locate/polydegstab

Preparation, properties and characterizations of halogen-freenitrogenephosphorous flame-retarded glass fiber reinforced

polyamide 6 composite

Yinghong Chen*, Qi Wang

The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, 24,

Southern Section 1, Yihuan Road, Chengdu, Sichuan 610065, PR China

Received 13 December 2005; received in revised form 8 February 2006; accepted 14 February 2006

Available online 21 April 2006

Abstract

Halogen free nitrogenephosphorous flame retardants (PMOP) were prepared through reaction of melamine and polyphosphoric acid in thepresence of flame retardant modifier CM with silicotungistic acid as a catalyst in aqueous solution. FT-IR, XRD, DSC and TGA techniques wereused to characterize the reaction product PMOP. The obtained flame retardants were then used to prepare flame retardant (FR) polyamide 6(PA6) composite reinforced with glass fiber (GF) and the factors affecting the flame retardancy of the material were also investigated. TheFR GF reinforced PA6 composite and the obtained charred layers were analyzed by utilizing TGA, SEM, FT-IR and XRD. The propertiesof the charred layer were connected with the flame retardancy of the corresponding material to reveal the flame retarding mechanism of FRGF reinforced PA6 composite. The experimental results show that PMOP flame retardant consists of melamine polyphosphate, melamine phos-phate and possible melamine pyrophosphate. The presence of CM was found to improve the flame retardancy of FR GF reinforced PA6 com-posite. It was experimentally found that PMOP flame retardant, which is comparatively stable in the range of processing temperatures of PA6, isparticularly suitable for flame retarding PA6 reinforced with GF. With increasing the flame retardant content, the flame retardancy of the FRreinforced material is not improved so obviously. However, the increase in the GF content greatly improves the flame retardancy of the com-posite, because GF greatly increases the char yield of material, decreases the maximal thermal decomposition rate, promotes the formation ofcharred layer with (PNO)x structure and greatly increases the strength of the charred layer. The prepared FR GF reinforced PA6 composites havegood comprehensive properties with flame retardancy 1.6 mm UL 94 V-0 level, tensile strength 76.8 MPa, Young’s modulus 11.7 GPa, Izodnotched impact strength 4.5 kJ/m2, flexural strength 98.0 MPa and flexural modulus 7.2 GPa, showing a better application prospect.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Flame retardant; Polyamide 6; Glass fiber; Melamine polyphosphate; Reinforcement; Intumescent

1. Introduction

Polyamide materials, as engineering thermoplastics, areplaying a more and more important role in modern industrialapplications. As well known, engineering thermoplasticspure polyamides (especially PA6 and PA66) are endowedwith relatively high tensile strength, high ductility, good chem-ical resistance, good abrasion, low friction coefficient, good

* Corresponding author. Tel.: þ86 28 85405133; fax: þ86 28 85402465.

E-mail address: [email protected] (Y. Chen).

0141-3910/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.polymdegradstab.2006.02.006

electrically insulating property and easy processing properties.However, polyamides also have some disadvantages such ashigh moisture absorptivity, poor dimensional stability, lowheat distortion temperature, poor low-temperature impactstrength and easy flammability [1,2]. So, reinforcing modifica-tion needs to be conducted on the pure polyamide materials.Most industrial areas require the reinforced polyamide com-posites. Generally, the used reinforcing fillers involve glass fi-bers [3,4], wollastonite [5,6], magnesium salt (M-HOS)whisker [7], carbon fiber [8,9], PET fiber [10], carbon nano-tubes [11,12] and Kevlar fiber [13]. Industrially, glass fiberis the filler which is mostly utilized in reinforcing polyamide

mailto:[email protected]

http://www.elsevier.com/locate/polydegstab

2004 Y. Chen, Q. Wang / Polymer Degradation and Stability 91 (2006) 2003e2013

materials (PA6 and PA66) due to its low cost, good availabilityand simple filling process. After the addition of glass fiber toPA6 or PA66, the dimensional stability, heat distortion temper-ature, impact resistance, resistance to chemical solvent, agingresistance and moisture absorption resistance of the corre-sponding composites all are improved markedly. In recentyears, the application volume of glass fiber reinforced PA6and PA66 presents a fast increasing trend. The typical applica-tions of glass fiber reinforced PA6 and PA66, which takes upan important station in the entire engineering plastics, canbe largely found in the car, mechanical instruments, electricindustries, national defense industries, aviation industries, etc.

PA6 and PA66 are flammable materials. The glass fiberreinforced PA6 or PA66 composites are even more combusti-ble than the pure polyamide material due to the ‘‘candlewickeffect’’ of glass fiber, which greatly limits their applicationsin electric industries, including electrical connectors, switchcomponents, wire ties, electrical housings and so on [1,14].So, how to enhance their flame retardancy becomes a big chal-lenge. Many attempts have been made to impart fire retardingproperty to glass fiber reinforced PA6 and PA66 materials andobtained a great success [1, 15e21].

Halogen-containing flame retardant additives are oftenindustrially used in decreasing the combustibility of glass fiberreinforced PA6 and PA66 and prove to be very effective [15e17]. These additives include chlorinated compounds suchas hexachlorocyclopentadiene [22], Dechlorane Plus [15],bis(hexachlorocyclopentadieno)-cyclooctane [23], etc. andbrominated compounds such as brominated epoxy [19,24],brominated poly(phenylene oxide) [16,19], brominated poly-styrene [25], decabromodiphenylether [17], etc. But haloge-nated flame retardants encounter many problems, e.g. somehalogenated additives can catalyze fuel-forming reactionwhich is detrimental to the fire retardancy. To obtain satisfac-tory flame retardancy, a relatively high amount of additives isrequired together with metal oxide synergists, resulting in de-creased mechanical properties and increased specific gravityof the obtained composite. In addition, the halogenated FRsalso cause reduction in tracking index [1], cause corrosion tothe processing equipment and generate corrosive and toxiccombustion products (especially the ‘‘dioxin’’).

As a result, halogen free retardants are the preferred addi-tives used in glass fiber reinforced PA6 and PA66. These halo-gen free retardants are divided into inorganic products[1,19,21,26,27], phosphorus-based products [1,18,28e31],nitrogen-based products [1,19,32] and phosphorusenitrogen-based products [1,20,26,33]. The investigation of inorganicadditives is concentrated on aluminum hydroxide (Al(OH)3)[26], magnesium hydroxide (Mg(OH)2) [21,26,27] and zinc bo-rate [19,26]. In order to achieve a relatively high flame retardinglevel, big inorganic FR loading (generally 50e60 wt%) and sur-face modification are needed, leading to inferior mechanicalproperties and complicated preparation process. Among phos-phorous additives, such as red phosphorus [18,28], phosphineoxide [29], phosphonate [30] and phosphonitride [31], etc.,which can effectively flame retard reinforced PA6 and PA66,only red phosphorus can be actually applied on a large scale

[1]; however, its red color and generation of highly toxic hydro-lysis product phosphine in use are also problems. Melamine andits derivatives such as melamine cyanurate, melamine sulfate,etc. are often used as nitrogen-containing additives in glass-reinforced PA6 and PA66 [1,19,32]. Sometimes, melamineand its derivatives need combination with other compounds toexert their optimum efficiency. The main advantages of mela-mine-based systems are that they have chemical structuresvery similar to that of polyamides and then possess good com-patibility with matrix polyamides. However, the main disadvan-tage of melamine-based additives such as melamine cyanurateis their increase in melt flow and promotion of dripping of poly-amides during combustion and hence relative ineffectiveness inglass fiber reinforced composites, particularly leading to asevere ‘‘candle effect’’ of glass fiber. Nitrogenephosphorous(NeP) systems seem to be very useful in flame retarding glassfiber reinforced PA66 [1,20,26,33]. These additives includemelamine phosphate [33], melamine polyphosphate [20], am-monium polyphosphate [26]. Due to their halogen free, rela-tively high efficiency and no release of toxic and corrosivegases during combustion, they are representative of the develop-ing trend of flame retardants applied in glass-reinforced poly-amides. There are many studies on flame retardant PA66 withNeP systems but only few reports on PA6 [1]. Wang et al.[34] investigated the flame retarding efficiency of melaminepolyphosphate in PA66 and PA6. They found that the fire retard-ing efficiency of melamine polyphosphate in glass-reinforcedPA66 is obviously better than that in glass-reinforced PA6 andthe flame retardancy of the latter was not improved at UL 94test.

In this study, halogen free melamine-based intumescentflame retardants were prepared to apply in glass fiber filledPA6 and a satisfactory flame retarding efficiency was obtained.The structure, mechanical and flame retarding performance,and flame retarding mechanism of the prepared flame retardedglass-reinforced PA6 composite were discussed. The influenceof the prepared flame retardant and glass fiber contents on theflame retardant properties (LOI and UL 94 flame retardancylevel) and the mechanical performance of the reinforced PA6composite were also investigated.

2. Experimental

2.1. Materials

The following materials were used as received: polyamide6 (PA6, with relative viscosity of 3.2 in 98% H2SO4 solvent, asgranulate product supplied by Baling Petrochemical Co.,China), melamine (MEL, chemically pure, supplied byChengdu Kelong Chemical Plant, China), polyphosphoricacid (PHPO, chemically pure, supplied by Chengdu KelongChemical Plant, China), glass fiber (GF, 4.5 mm in length,supplied by Tongxiang Zhenshi stock incorporation limited,China), silicotungistic acid (STA, reactant degree, suppliedby Hunan Xiangzhong Fine Chemical Plant, China), flameretardant modifier, i.e. caprolactam (CM, chemically pure,supplied by Shenyang Xinxi Reagent Plant).

2005Y. Chen, Q. Wang / Polymer Degradation and Stability 91 (2006) 2003e2013

2.2. Nitrogenephosphorous flame retardant (FR)preparation

A certain amount of polyphosphoric acid (PHPO) witha molecular structure H6P4O13 aqueous solution was droppedinto the heat melamine aqueous solution containing silicotun-gistic acid (STA) and caprolactam (CM) with a continual mix-ing, followed by a continued reaction in water bath for a giventime at a certain temperature after completion of addition ofPHPO solution. After completion of the reaction, the reactionmixture was cooled down to room temperature, filtrated andthen dried. The obtained products were finally pulverized toless than 100 mm for the subsequent use. We called the reac-tion product as PMOP.

2.3. Flame retarded glass fiber reinforced PA6 samplespreparation

The obtained NeP flame retardant PMOP, glass fiber, pro-cessing aids and dried pelletized PA6 were firstly well mixedand then blended in a twin-screw extruder (v: 30 mm, L/D:32, model: SLJ-30, Longchang Chemical Engineering Equip-ment Factory, China) in the range of 230e245 �C. The extru-date was quenched in a water bath, cut into pellets and thendried in a vacuum oven at 100 �C for 8 h. The obtained pelletswere then injection molded at 250 �C into various specimensfor flammability and mechanical performance test.

2.4. Characterization

The Fourier transform infrared (FT-IR) spectra of sampleswere obtained using a Nicolet 20SXB FTIR spectrometer.The thermogravimetric analysis (TGA) curves were recordedon a General V 4.1c Dupont TA2100 instrument thermalanalyzer with a heating rate of 10 �C/min in the range of25e600 �C and a dynamic nitrogen flow of 100 ml/min. Thedifferential scanning calorimetry (DSC) measurements wereconducted on a NETZSCH DSC 204 instrument thermal ana-lyzer with a heating rate of 10 �C/min ranging from 25 to450 �C and a dynamic nitrogen flow of 50 ml/min. TheX-ray diffraction (XRD) patterns using Cu Ka radiation(l¼ 1.542 A) were performed with a powder X’Pert X-raydiffractometer made by Philips Company in The Netherlandsat the scanning rate of 0.02 � per second in the 2q range of5e50 �. The fractured surface of injection molded specimenand the surface of residual charred layer were observed bya JEOL JSM-5900LV scanning electro microscope (SEM).The residual charred layer was from the burned specimen inthe UL 94 test. The samples to be observed were coatedwith a conductive gold layer in advance.

The vertical burning test was conducted on a CZF-3 hori-zontal and vertical burning tester, on sheets 127�12.7� 3.2 mm and 127� 12.7� 1.6 mm according to theAmerica National UL 94 test ASTM D3801. The LOI valuewas obtained using an ATLAS limiting oxygen index instru-ment on sheets 120� 6.5� 3.2 mm according to the standardoxygen index test ASTM D2863-70. Tensile tests were carried

out on an Instron universal testing machine 4302 at room tem-perature and a crosshead speed of 50 mm min�1 according toASTM D638. Izod notched impact strength tests wereconducted using an XJ40A impact tester according to ASTMD256 standard. Flexural measurements were done also on Ins-tron universal testing machine 4302 at room temperatureaccording to ASTM D790.

3. Results and discussion

3.1. Preparation and characterization of PMOP flameretardants

The halogen free PMOP flame retardant was prepared bythe reaction of melamine with polyphosphoric acid in the pres-ence of flame retardant modifier CM in aqueous solution for3 h using silicotungistic acid as catalyst.

3.1.1. FT-IR analysisThe FT-IR spectrum of the obtained product is shown in

Fig. 1. The appearance of absorptions of the characteristicbands [35,36] of melamine polyphosphate indicates that mel-amine polyphosphate is contained within the synthesized NePflame retardant. These bands include 3136e2828 cm�1 (thevibration absorption of NH3

þ), 2696 and 970 cm�1 (the vibra-tion absorptions of OeH with hydrogen bonding and PeO inPeOeH group, respectively), 1384 cm�1 (the stretchingvibration of CeN group of the melamine salt structure),1178 and 1284 cm�1 (the vibration of P]O group) and970e870 cm�1 (the vibration absorption of PeOeP group).From Fig. 1, it can be also seen that the characteristic absorptionsof melamine phosphate at 3393, 3309, 1558 and 1333 cm�1

appear, indicating the presence of melamine phosphate inthe reaction products. Above analysis shows that the preparedPMOP flame retardant is composed of melamine polyphos-phate, melamine phosphate and possible melamine pyrophos-phate. Because the reaction of melamine with polyphosphoricacid is conducted in the aqueous solution, the reactant poly-phosphoric acid will suffer hydrolysis to a certain degree.The partly hydrolyzed product orthophosphoric acid can reactwith melamine to form melamine phosphate. We cannotexclude the generation of dimer or trimer of phosphoric acidduring hydrolysis of polyphosphoric acid. The formation ofmelamine pyrophosphate and melamine triphosphate is thenpossible. Consequently, the reaction of melamine and poly-phosphoric acid in aqueous solution is very complicated.Jahromi et al.’s studies show that a composition of melaminepolyphosphate, melamine pyrophosphate and melamine phos-phate can be quantitatively analyzed by means of solid state31P NMR characterization [37]. So, the real composition ofPMOP can be determined by solid state 31P NMR analysis,which needs further characterization.

3.1.2. XRD measurementsX-ray diffraction analysis was used to investigate the crystal-

line structure of PMOP. The result is shown in Fig. 2. As can beseen in Fig. 2, the diffraction peaks are pungent and narrow,


Fig. 1. FT-IR spectrum of synthesized nitrogenephosphorous flame retardant (PMOP).

indicating that PMOP is highly crystalline material. Some dif-fraction maximums appearing at 2q values of 14.36, 16.36,18.32, 22.24, 26.15, 27.01, 28.36, 29.01 and 31.17 � (the corre-sponding d value: 6.164, 5.414, 4.838, 3.994, 3.404, 3.298,3.145, 3.075, and 2.867) agree well with most peaks of mela-mine polyphosphate reported by the literature [38] at 2q values.This indicates that the prepared PMOP flame retardant is mainlycomposed of melamine polyphosphate. But in Fig. 2 there arealso some diffraction peaks appearing at 2q values of 8.75,12.24, 17.38, 19.01, 20.93, 24.29, 24.54, 27.01 and 29.01 �,which can be ascribed to melamine phosphate based on thecomparison of experimental value with the literature values[39], indicating that melamine phosphate is also contained inthe synthesized products. Above analytical results are in agree-ment with those of FT-IR characterization.

3.1.3. Thermal studiesIn order to understand the thermal behavior of PMOP flame

retardant during heating process, differential scanning

Fig. 2. XRD pattern of the synthesized PMOP flame retardant.

calorimetry (DSC) measurement and thermogravimetric anal-ysis (TGA) were carried out. The DSC heating curves ofPMOP flame retardants are shown in Fig. 3, where cure a,b and c correspond to pure melamine phosphate (MP), synthe-sized FR with CM (PMOP) and synthesized FR without CM,respectively. The used CM was expected to improve the flameretardancy of glass fiber filled PA6 material (this will be dis-cussed in the following study). One can see that MP has endo-thermic peaks occurring at 282, 322 and 379 �C, which can beexplained by the lack of water with different properties at dif-ferent temperatures for MP. Comparing the DSC result ofPMOP (Fig. 3b) with that of MP (Fig. 3a), it can be seenthat both have three endothermic peaks, in which the positionsof the first two endothermic peaks of both cases are individu-ally very close, but the peak strength of the former is obvi-ously weaker than that of the latter and the thirdendothermic peak of the former has shifted to 405 �C relativeto the latter (379 �C). Above changes indicate that on one side,

Fig. 3. DSC curves of pure melamine phosphate (MP) (a), synthesized FR with

CM (b) and synthesized FR without CM (c).


the prepared PMOP flame retardant contains only a smallamount of melamine phosphate and on the other side, the shiftand the enhanced strength of the third peak are contributed bythe presence of a large amount of melamine polyphosphateproducts in the PMOP flame retardants. In addition, the endo-thermic peaks of FR without CM at 270 and 329 �C (Fig. 3c)are remarkably weakened in comparison with those of FR withCM (PMOP) at the similar temperature positions, revealingthat addition of flame retardant modifier CM promotes the for-mation of melamine phosphate to a certain degree.

The TGA data are shown in Fig. 4. It can be seen that theprepared PMOP flame retardants are comparatively stable inthe range of processing temperatures (220e260 �C) of PA6and hence suitable for application in flame retarding of PA6.The decomposition of PMOP can be divided into three temper-ature regions, i.e. 260e290, 290e370 and 370e500 �C.According to the investigation results of Costa and Camino[40], the weight loss in the range of 260e290 �C can beascribed to the dehydration reaction of the formed melaminephosphate in system into melamine pyrophosphate. Theweight loss in the range of 290e370 �C is well then contrib-uted by the dehydration of the formed melamine pyrophos-phate. The dehydration product is melamine polyphosphate.However, above 370 �C the weight loss of system is mainlycaused by the decomposition of melamine polyphosphateinto melamine and melam ultraphosphate with simultaneousrelease of incombustible gases such as vapour and ammonia.With increasing treating temperature, above mentioned melamultraphosphate undergoes self-condensation to form highmolecular weight substance with phosphatedultraphosphateemelamine type structure, while melamine undergoes furtherdecomposition into incombustible gases. The final 32.2% res-idue at 550 �C can be obtained.

3.2. The performance of the flame retardant GFreinforced PA6 composite

PMOP flame retardants, glass fiber and PA6 were firstlyblended to prepare flame retardant reinforced PA6 composites.

Fig. 4. TG curve of the synthesized PMOP flame retardant.

Then, these materials were evaluated by mechanical propertiestest (tensile, impact and flexural properties) and combustiontest (LOI and UL 94).

3.2.1. Mechanical performanceThe influence of flame retardant content on the mechanical

properties of FR reinforced PA6 materials at 25 wt% GF load-ing is shown in Table 1. As can be seen, with increase in FRcontent, the mechanical properties of the composites presenta decreasing trend except for the Young’s modulus and flexuralmodulus. The FR content has an important influence onthe mechanical properties. With increasing FR loading from15 to 30 wt%, the tensile strength, the flexural strength andthe notched impact strength decreases from 86.8 MPato 68 MPa, 110.2 MPa to 92.0 MPa and 5.04 kJ/m2

to 4.24 kJ/m2, respectively, and only the Young’s modulusand flexural modulus increase. Compared with PA6, for FR re-inforced PA6 composite both the tensile strength and flexuralstrength have improved to a different extent, but the impactstrength obviously decreases. Increase in FR content greatlyimpairs the mechanical performance of FR reinforced PA6material. This can be explained by the poor compatibility ofFRs with matrix PA6 resulting from the relatively great polar-ity of FRs. Surface modification of FRs is required to improvethe mechanical properties of flame retarded materials. So,increase in FR content can improve the flame retardancy butis at the expense of mechanical properties.

The influence of GF content on the mechanical propertiesof reinforced PA6 materials at 25 wt% FR loading (Table 2)was also investigated. The mechanical properties of FR PA6composite without glass fiber are very poor and decreasemuch (especially for the impact strength) relatively to thoseof pure PA6 due to the poor compatibility with matrix. Addi-tion of glass fiber greatly enhances the mechanical propertiesof FR reinforced composite, e.g. with increasing GF contentfrom 0 to 35 wt%, tensile strength, Young’s modulus, flexuralstrength, flexural modulus and Izod notched impact strength ofthe material are increased from 26.6 MPa, 3.8 GPa, 57.0 MPa,2.5 GPa and 1.87 kJ/m2 to 76.8 MPa, 11.7 GPa, 98.0 MPa,7.2 GPa and 4.49 kJ/m2, respectively. The mechanical proper-ties of the reinforced material at 35 wt% GF loading are evenmuch higher than those of pure PA6, indicating that the pre-pared FR reinforced material still has its potential applicationprospect.

Table 1

Effect of FR loading on the mechanical properties of FR GF reinforced PA6

composite at 25 wt% GF constant content

FR

loading

(wt%)

Tensile

strength

(MPa)

Young’s

modulus

(GPa)

Elongation

at break

(%)

Flexural

strength

(MPa)

Flexural

modulus

(Gpa)

Izod notched

impact strength

(kJ/m2)

15 86.6 4.9 2.1 110.2 5.0 5.04

20 80.0 7.3 1.9 109.2 5.3 4.82

30 68.1 8.0 1.2 92.0 6.4 4.24

PA6 control 56.5 2.8 153.2 61.38 1.5 8.00


3.2.2. Flame retardant performanceAs mentioned previously, the introduction of CM was

expected to enhance the flame retardancy of material. Table 3shows the effect of CM on the flame retardancy of the rein-forced material. When the CM content is 3.0 wt%, the LOIvalue of sample can reach 30.5, one unit higher than that with-out CM. In the vertical burning test, the sample with 3.0 wt%CM content achieves UL 94 V-0 level at 3.2 mm thickness; thesample without CM, however, can partly reach UL 94 V-0 rat-ing at 3.2 mm thickness. Above experimental fact indicatesthat CM was proved to be able to enhance the flame retardancyof PA6/GF/FR composite. DSC analysis reveals that introduc-tion of CM can promote the formation of melamine phosphate,which is disadvantageous for the improvement of the flameretardancy of material due to its greater polarity and lowerthermal stability, but Table 3 shows a contrary result. This ispossibly because introduction of CM prevents the violentdecomposition of PA6 in the FR reinforced composite to a cer-tain extent and improves the thermal stability of material,which needs further investigation.

The influence of PMOP flame retardant content on theflame retardancy of the reinforced PA6 material at 25 wt%GF loading is shown in Table 4. With increase in FR contentfrom 15 to 30 wt%, the LOI value of material is enhancedfrom 27.5 to 30 by 2.5 units; meanwhile, the correspondingUL 94 rating is improved from failure to part V-0 at 3.2 mmthickness and from failure to part V-2 at 1.6 mm thickness,and the extent of increase is not so much as the LOI valuedoes. Above results indicate that even if the content ofPMOP flame retardant increases to 30 wt%, the obtained FRGF reinforced PA6 composite cannot still achieve UL 94V-0 level at both 3.2 mm and 1.6 mm thicknesses, showingthat increase in FR content is not the effective approach toenhance the flame retardancy of the material. This is possibly

Table 2

Effect of GF loading on the mechanical properties of FR GF reinforced PA6

composite at 25 wt% FR constant content

GF

loading

(wt%)

Tensile

strength

(Mpa)

Young’s

modulus

(GPa)

Elongation

at

break (%)

Flexural

strength

(MPa)

Flexural

modulus

(Gpa)

Izod notched

impact strength

(kJ/m2)

0 26.6 3.8 0.93 57.0 2.5 1.87

20 67.1 6.6 1.5 87.2 5.7 4.23

30 74.2 8.9 1.0 96.3 6.6 4.35

35 76.8 11.7 1.2 98.0 7.2 4.49

PA6 control 56.5 2.8 153.2 61.38 1.5 8.00

Table 3

The effect of flame retardant modifier CM on the flame retardancy of FR GF

reinforced PA6 composite at 25 wt% GF and 25 wt% FR loading

CM content (wt%) 0.0 3.0

LOI 29.5 30.5

UL94 at 3.2 mm thickness tf (s)a e 28.5

Rating Part V-0 V-0

UL94 at 1.6 mm thickness tf (s)a e e

Rating Failure Failure

a The total duration (five specimen) of flaming combustion.

because the interested FR reinforced material at a low GFloading cannot form charred layer with high quality even usinga high FR loading. It was experimentally found that the FRreinforced PA6 materials with a lower GF loading or evenwithout GF at 25 wt% FR loading acutely burn, also accompa-nying with severe flamed dripping. As a result, the quality ofcharred layer formed from FR PA6 filled with only PMOP isvery poor; however, addition of GF can improve the qualityof charred layer, which will be discussed in the following part.

The influence of GF content on the flame retardancy of thereinforced PA6 materials was also investigated. The result islisted in Table 5. The LOI value of FR reinforced compositeat 25 wt% FR loading increases with increase in the GF con-tent, i.e. with increasing GF content from 0 to 35 wt%, the LOIvalue of FR reinforced PA6 material is augmented from 21.5to 32, about 12 units higher; simultaneously, the UL 94 levelis also improved from failure to V-0 at 3.2 mm and especially1.6 mm. As can be seen, the addition of glass fiber to the FRPA6 system can effectively enhance the flame retardancy ofFR reinforced PA6 material. Particularly, when the GF loadingis increased to 30e35 wt% in FR system, the UL 94 test levelof FR reinforced material has been able to reach V-0 rating at1.6 mm thickness. This is contrary to the flame retardantbehavior of other nitrogen-based additives such as melaminecyanurate (MCA) in the presence of GF, e.g. when a nitro-gen-containing flame retardant of MCA is applied in the GFreinforced PA6 with FR loading constantly kept, increase inGF content can result in the severe decrease in flame retard-ancy due to the increased flamed dripping caused by the GF‘‘candlewick effect’’. Consequently, PMOP and MCA havedifferent flame retardant mechanisms in flame retarding GFreinforced PA6. As we know, when nitrogen-based MCAflame retardant is used to flame retard pure PA6, its flameretardant mechanism acts through the following process, i.e.during combustion, PA6 is catalyzed by MCA to degrade

Table 4

The effect of FR content on the flame retardancy of FR GF reinforced PA6

composite at 25 wt% GF loading

FR content (wt%) 15 20 25 30

LOI 27.5 28 29.5 30

UL94 at 3.2 mm thickness tf (s)a e e e e

Rating Failure Part V-0 Part V-0 Part V-0

UL94 at 1.6 mm thickness tf (s)a e e e e

Rating Failure Failure Failure Part V-2


Table 5

The effect of GF content on the flame retardancy of FR GF reinforced PA6

composite at 25 wt% FR loading

GF content (wt%) 0 20 25 30 35

LOI 21.5 30 29.5 31 32

UL94 at 3.2 mm tf (s)a e 88.0 e 12.2 8.3

Rating Failure V-2 Part V-0 V-0 V-0

UL94 at 1.6 mm tf (s)a e e e 26.7 19.6

Rating Failure Failure Failure V-0 V-0



into low molecular weight substance, promoting the melt flowand dripping of PA6 to lower the temperature of material in theburning region; simultaneously, the endothermic decomposi-tion of MCA at high temperature can also decrease surfacetemperature of the burning material and the incombustible de-composition products additionally dilute the combustiblegases generated from PA6 and oxygen in burning region,finally leading to the self-extinguishment of PA6. But, forPA6/MCA/GF system, during combustion, PA6 decomposesto low molecular weight substances with very low viscosityunder the catalytic effect of MCA and would preferentiallyadsorb on the surface of GF. The wrapped GF hence becomesa ‘‘candlewick’’, leading to a very combustible material. How-ever, for PA6/PMOP/GF system, the case is completely differ-ent. PMOP flame retardant belongs to NeP intumescent flameretardant and is easily charred. Upon burning GF in FR rein-forced material can connect the instantaneously formed charparticles together to generate a solid, uniform and compactcharred layer. It is this charred layer that plays a role of heatinsulation, oxygen insulation and prevention of the degrada-tion products of matrix polymer into the burning region toeffectively improve the flame retardancy of material. The de-tailed characterization of the charred layer will be involvedin the following part.

3.3. Characterization of FR reinforced PA6 composite

3.3.1. TG studiesFig. 5 shows the TG curves of pure PA6 (a), FR PA6 with-

out GF (b), FR reinforced PA6 with 20 wt% GF (c) and FRreinforced PA6 with 30 wt% GF (d). Pure PA6 starts to decom-pose at 400 �C and subsequently experiences a violent degra-dation till 520 �C almost without any residue left. Thepresence of NeP flame retardants decreases the thermal stabil-ity of all FR materials, which start to decompose at 300 �C,showing that the flame retardants begin to function, i.e. dehy-drate to form char. The thermal decomposition behaviors of

Fig. 5. TG curves of pure PA6 (a), flame retarded PA6 without GF (b), with

20 wt% GF (c) and with 30 wt% GF (d), where the used FR loading is 25 wt%.

three FR materials (curves b, c and d) before 370 �C are close,indicating that the decomposition of material is mainly con-tributed by the PMOP flame retardant and PA6 in FR materialsbefore 370 �C. After that, with increasingtemperature, PA6/FR (curve b) material continues to degrade and obtains15.6 wt% residues. In addition, the thermal stability of FRPA6 without GF is always much lower than that of purePA6 before 470 �C, indicating that PMOP flame retardantspromote the decomposition of PA6 to a considerable degree.Unlike MCA, PMOP intumescent flame retardant has32.2 wt% residues at 550 �C (the former almost completelydecomposes at the same temperature), making PMOP FRs tohave a capability of forming charred layer with GF. For thereinforced FR materials with 20 wt% and 30 wt% GF, theGF starts to participate in char formation at 370 �C, and after450 �C the formation of charred layer tends to be stable withresidual char yield of 41.3 wt% and 43.6 wt%, respectively.Thus it can be seen that the introduction of GF greatly in-creases the residual char yield of FR material and plays themost important role in improving the quality of the formedcharred layer, leading to good flame retardancy.

The DTG curves of pure PA6 (a), FR PA6 without GF (b),FR reinforced PA6 with 20 wt% GF (c) and FR reinforced PA6with 30 wt% GF (d) (Fig. 6) show that the addition of flameretardants remarkably decreases the temperature at the maxi-mal thermal decomposition rate of FR material (from466.3 �C for pure PA6 to 370.1 �C for FR material), andmore importantly, decreases the maximal thermal decomposi-tion rate to a considerable degree. With increasing GF contentof the FR reinforced material from 0 to 30 wt%, the maximalthermal decomposition rate of the corresponding materialsdecreases according to the following order, i.e. pure PA6(21.8% min�1)< FR material with 0 wt% GF and 25 wt%FRs (17.2% min�1)< FR material with 20 wt% GF and25 wt% FRs (11.7% min�1)< FR material with 30 wt% GFand 25 wt% FRs (9.8% min�1). Combined with the previousflame retarding properties’ studies, it can be seen that theflame retardancy of material seems to be closely concernedwith the maximal decomposition rate of matrix resin but lessrelative to the thermal stability of FR material, e.g. purePA6 has the best thermal stability (do not decompose till400 �C), but contrarily has the poorest flame retardancy. Onthe contrary, the FR PA6 material with 30 wt% GF contentstarts to degrade at about 340 �C but achieves 1.6 mm and3.2 mm UL94 V-0 flame retarding level instead (Table 5). Inaddition, the order of flame retardancy of all FR samplescan agree well with the order of their maximal thermal decom-position rate, although their temperature (reflection of thermalstability) at the maximal thermal decomposition rate is veryclose (Table 5; Fig. 6). The possible reason is that the violentdecomposition of PA6 makes a great amount of combustiblegases being generated in a very short time and can easilypierce through the formed charred layers to diffuse to the burn-ing region, accelerating the burning of material. However, onthe other side, the presence of flame retardants even togetherwith increasing amount of glass fibers obviously decreasesthe release rate of combustible gases due to formation of


many charred layers with improved strength and the amount ofcombustible gases generated in unit time hence remarkablydecreases too. A notably decreased amount of combustiblegases, of course, do not easily break through the formedcharred layer, contributing to the improvement of flame retard-ancy of material. Consequently, the maximum decompositionrate of a material is also an important factor influencing theflame retardancy.

3.3.2. SEM observationsFig. 7 shows the SEM photograph of the fractured surface

of FR PA6 without GF. It can be seen that the fractured surfaceis even and smooth and only a very small amount of FRs isunevenly dispersed on it. This is possibly because the dis-persed flame retardant particles fall off upon cryogenic frac-ture. Above observational results show that the PMOP flameretardants have poor compatibility with matrix and poor dis-persion, interpreting the poor mechanical properties and flameretardancy of FR material with only PMOP flame retardant toa certain degree.

Fig. 8 shows the SEM photographs of the fractured surfaceof FR reinforced PA6 material with 20 wt% (a) and 35 wt%GF loading (b). One can see that the dispersion of GFs inmatrix is irregular and their orientation is inconsistent, whichis possibly concerned with the surface treatment and feedingmode of GF. The mechanical properties of material are un-doubtedly affected by such a dispersion of GF to a certaindegree. From Fig. 8, many holes formed by evulsion of GFare also found to be left in the fractured surface of FR rein-forced material upon cryogenic fracture, indicating the com-paratively poor adhesion of GF with matrix resin. Aboveresults can possibly explain why the mechanical propertiesof FR reinforced PA6 material cannot still reach the expectedvalue according to the study in the previous part. In addition,the presence of GF seems to improve the dispersion of FRscompared with that of unreinforced PA6 material. If some sur-face modifications are conducted on GF and flame retardants,

Fig. 6. DTG curves of pure PA6 (a), flame retarded PA6 without GF (b), with

20 wt% GF (c) and with 30 wt% GF (d), where the used FR loading is 25 wt%.

the comprehensive performance of the prepared FR reinforcedmaterial can be expected to be further improved.

3.4. Characterization of the residual charred layer

TG analytical result shows that PMOP flame retardant hasresidual char yield of 33.0 wt% and belongs to the typicalNeP intumescent flame retardant, which mainly functions inthe condensed phase through the formed multi-cellular charredlayer. Characterization of the residual charred layer of FR PA6and FR reinforced PA6 with PMOP was used to reveal theflame retarding mechanism of PA6/GF/FR composite duringcombustion process.

3.4.1. FT-IR analysisThe FT-IR spectra of the residual charred layers of FR PA6

materials without GF, with 20 wt% and 35 wt% GF are shownin Fig. 9. There are some typical featured bands of the charredlayer (with (PNO)x structure) of FR material containing NePIFRs appearing, e.g. 3424 cm�1 (the vibration absorption of eNHe group in pyrrole-based compounds), 2927e2856 cm�1

(eCH2e aliphatic group), 1632 cm�1 (NeNO2 group),1207 cm�1 (the stretching vibration absorption of PeOeCstructure in PeC complex compound [41]), 1088 cm�1 (thesymmetrical vibration absorption of PeO group in PeOeCstructure [41]) and 995 cm�1 (the vibration absorption ofP(]O)eOH group). With increasing the GF loading, thestrength of the characteristic absorptions of several groupsmentioned above also augments, indicating that increase inthe content of GF in FR system contributes to the formationof charred layer with (PNO)x structure, i.e. favors the stabilityof the charred layer. The flame retardancy can be consequentlyimproved.

3.4.2. SEM analysisThe morphologies of the residual char of PA6/FR (without

GF) and PA6/FR/GF (with 35 wt% GF) were observed bySEM instrument, showing in Fig. 10a and b, respectively.

Fig. 7. SEM photograph of flame retarded PA6 composite without GF at

25 wt% FR loading.


Fig. 8. SEM photographs of flame retarded PA6 composite reinforced with 20 wt% GF (a) and 35 wt% GF (b) at 25 wt% FR loading.

The morphology of the charred layer of FR PA6 material with-out GF (Fig. 10a) shows that the obtained charred layer hasdiscontinued and accidented agglomerations and even hascracks. In addition, the surface of the charred layer is notsmooth and its thickness is also not uniform. As a result, thequality of the total charred layer of FR PA6 material withoutGF is poor, making the transfer of heat and the penetration

of oxygen and combustible gases cannot be effectively pre-vented. At this rate, the combustible gases generated fromthe decomposition of matrix resin during combustion easilybreak through the charred layer to diffuse into the burningregion, leading to a poor flame retardancy of material. Itwas experimentally found that the specimen burns violentlywith simultaneous generation of a small amount of chars

Fig. 9. FT-IR spectra of the residual char of FR GF reinforced PA6 composite without GF (a), with 20 wt% GF (b) and with 35 wt% GF (c) at 25 wt% FR loading.


Fig. 10. SEM photographs of the residual charred layer of the flame retarded PA6 composite without GF (a) and with 35 wt% GF (b).

and flamed dripping, which is responsible for the poor charredlayer formed. On the contrary, the relatively smooth, uniform,continued, compact and thick charred layers with a greatamount of formed char and almost no holes can be found inthe samples of FR reinforced polyamide material with35 wt% GF (Fig. 10b), resulting in good quality of charredlayers. These intumescent charred layers, which have effectsof heat insulation and oxygen insulation, can prevent the deg-radation products of matrix polymer to enter the burningregion and correspondingly can hold back the external oxygento penetrate into the degradation zone, leading to the excellentflame retardancy of the FR reinforced PA6 specimens. How-ever, more importantly, in Fig. 10b it was found that manyglass fibers, which function like the reinforcing steel bars inconcrete, regularly distribute in the charred layers and firmlycombine with the char particles, greatly improving thestrength; hence the quality of the charred layer. Consequently,glass fibers play an extremely important role in the improve-ment of the flame retardancy of materials. The glass fibershave a more important role in reinforcement of the charredlayer except for their promotion of charred layer formation(shown by FT-IR analysis). This is the main reason why theflame retardancy of PA6/GF/FR composite improves withincrease in the GF content.

3.4.3. XRD analysisX-ray diffraction was also used to analyze the obtained re-

sidual charred layers of FR reinforced PA6 materials aftercombustion. The characterization results are shown inFig. 11, where a and b are representative of FR sample with20 wt% and 35 wt% GF, respectively. It can be seen that thecharred layers of both samples have the similar features with-out obvious X-ray diffraction maximums in XRD patterns andhave the structural characteristics of the normal amorphouscarbon, showing that the residual char of FR reinforced PA6material is a type of amorphous substance. This is becauseFR GF reinforced PA6 materials containing PMOP flameretardants have the thermal decomposition reaction of charformation during combustion and form organic substanceswith complicated structure. There are no diffraction peaks of

glass fibers left in the charred layer occurring in the XRD pat-terns, indicating that the glass fibers may be highly dispersedin the charred layers.

4. Conclusions

(1) FT-IR, XRD, DSC and TG studies indicate that the reac-tion products (PMOP) of melamine with polyphosphoricacid in the presence of CM using silicotungistic acid ascatalyst in aqueous solution are composed of melaminepolyphosphate, melamine phosphate and possible mela-mine pyrophosphate. Melamine polyphosphate is themain reaction product and melamine phosphate, however,is only the small part of reaction product. The addition ofCM promotes the formation of melamine phosphate toa certain degree. The obtained PMOP flame retardantsstart to decompose at 260 �C and are comparatively stablein the range of processing temperatures of PA6, indicatingtheir suitable applications in flame retarding of PA6.

(2) Introduction of flame retardant modifier CM into the flameretardant system can improve the flame retardancy of FR

Fig. 11. XRD patterns of the residual char of the reinforced flame retarded PA6

composite with 20 wt% (a) and 35 wt% (b) GF.


reinforced PA6 material. Increase in PMOP flame retar-dant content is not the effective way of enhancing theflame retardancy of the reinforced material, but increasein glass fiber content can greatly improve the flame retard-ancy of the reinforced material.

(3) Due to the great polarity of PMOP flame retardant, the me-chanical properties of FR PA6 material without GF is verypoor. For the FR reinforced PA6 composite, increase in FRcontent leads to poorer mechanical properties of material,but increase in GF content obviously improves themechanical properties of material, although the improve-ment is not so much as expected.

(4) Addition of flame retardants decreases the initial decompo-sition temperature of PA6, the temperature at the maximumthermal decomposition rate and the maximum thermaldecomposition rate of material. In the FR GF reinforcedPA6 composite, the presence of glass fibers furtherdecreases the maximum thermal decomposition rate,greatly increases the residual char yield, promotes the for-mation of charred layers with (PNO)x structure and greatlyenhances the strength of charred layers. All above factorsplay an important role in greatly improving the flame retard-ancy of material. Glass fibers constituting the charred layersfunction like the reinforcing steel bars in the concrete.

(5) The residual charred layers formed from the FR GF rein-forced PA6 material are amorphous substances. The glassfibers are highly dispersed in the residual charred layers.

(6) The obtained FR GF reinforced PA6 materials have goodcomprehensive performance. When FR loading and GFloading are 25 wt% and 35 wt%, respectively, the preparedFR reinforced materials can have flame retardancy 1.6 mmUL 94 V-0 level, tensile strength 76.8 MPa, Young’s mod-ulus 11.7 GPa, Izod notched impact strength 4.5 kJ/m2,flexural strength 98.0 MPa and flexural modulus 7.2 GPa.If surface treatment is conducted on flame retardants andglass fibers, the mechanical properties of materials areexpected to be further improved.

Acknowledgement

This work is supported by 863 Program of China(2002AA333070) and National Natural Science Foundationof China (20404009).

References

[1] Sergei VL, Edward DW. Polymer International 2000;49:1033e73.

[2] Song GJ, Yin LL, Li BY. Plastics (China) 2004;33(6):66e70.

[3] Tjong SC, Xu SA, Mai YW. Materials Science and Engineering A (Struc-

tural Materials: Properties, Microstructure and Processing) 2003;347(1e

2):338e45.

[4] Cho JW, Paul DR. Journal of Applied Polymer Science 2001;80(3):

484e97.

[5] Wang Q, Liu CS, Chen YH. Plastics, Rubber and Composites

2001;30(8):363e9.

[6] Unal H, Mimaroglu A, Alkan M. Polymer International 2004;53(1):

56e60.

[7] Tan FY, Yang XY, Jia YQ, Ye HM. Plastics Assistants (China)

2004;3:27e9.

[8] Lei CH. Journal of Advanced Materials 2004;36(3):64e7.

[9] Mohd Ishak ZA, Berry JP. Journal of Applied Polymer Science

1994;51(13):2145e55.

[10] Fakirov S, Evstatiev M, Schultz JM. Polymer 1993;34(22):4669e79.

[11] Liu TX, Phang IY, Shen L, Chow SY, Zhang WD. Macromolecules

2004;37(19):7214e22.

[12] Zhang WD, Shen L, Phang IY, Liu TX. Macromolecules

2004;37(2):256e9.

[13] Yu Z, Brisson J, Ait-Kadi A. Polymer Composites 1994;15(1):64e73.

[14] Zong RW, HU Y, Wang SF. Fire Safety Science (China) 2003;12(1):

46e50.

[15] Ilardo CS, Duffy JJ. US Patent 4,504,611.

[16] Williams IG. US Patent 4,696,966.

[17] Van Wabeeke L, De Schryver D. US Patent 5,674,972.

[18] Bonin Y, LeBlanc J. US Patent 4,985,485.


[20] Kasowski RV, Martens MM. PCT Patent application WO 98/45,364.

[21] Schmid E, Luedi D. US Patent 4,963,610.

[22] Theysohn R, Wurmb R, Leutner B, Schlipmer HU. US Patent

4,076,682.

[23] Pagilagan RU. US Patent 4,360,616.

[24] Tjahjadi M. EP Patent application 0,855,421 A1.

[25] Nakahashi J, Shigetomi T, Kai S. US Patent 4,788,244.

[26] Ou YX, editor. Applied flame-retarding technology. 1st ed. Beijing:

Chemical Industrial Press; 2002.

[27] EI Sayed A, Ostlinning E, Idel KJ. US Patent 5,378,750.

[28] Joua WS, Chenb KN, Chaoc DY, Lind CY, Yehd JT. Polymer Degrada-

tion and Stability 2001;74:239e45.

[29] Braksmayer DP, Hussain SN. US Patent 4,301,057.

[30] Hulskotte R. PCT Patent application WO 99/02,606.

[31] Gareiss B, Schneider HM, Weber M. German Patent DE 19,615,230eA1.


[33] Martens MM, Kasowski RV. US Patent 5,618,865.

[34] Wang JR, Liu ZG, Ou YX, Li J. Transactions of Beijing Institute of Tech-

nology 2004;24(10):920e3.

[35] Zhang ZJ, Mei XJ, Fen LR, Qiu FL. Chinese Journal of Applied Chem-

istry 2003;20(11):1035e8.

[36] Keyhani M, Krishnan V. Thermal response of a decomposing polymer-

heat transfer in porous media. New York: Wiley-Interscience; 1993.

p. 35.

[37] Jahromi S, Gabrielse W, Braam A. Polymer 2003;44:25e37.

[38] Sheridan RC. Inorganic Syntheses 1982:157.

[39] Frazier A, Waerstad K, Kim Y. J Chem Eng Data 1988;33:518.

[40] Costa L, Camino G, Luda di Cortemiglia MP. Fire and polymers. In: ACS

Symposium Series, 425. Washington, DC: American Chemical Society;

1990. p. 211e238.

[41] Xie RC, Qu BJ. Polymer Degradation and Stability 2001;71(3):395e402.

preparation, properties and characterizations of halogen-free nitrogen–phosphorous flame-retarded...

Documents