preparation of flame retardant polyamide 6 composite with melamine cyanurate nanoparticles in situ...

Polymer Degradation and Stability 91 (2006) 2632e2643www.elsevier.com/locate/polydegstab

Preparation of flame retardant polyamide 6 composite with melaminecyanurate nanoparticles in situ formed in extrusion process

Yinghong Chen*, Qi Wang, Wei Yan, Hongmei Tang

The State Key Laboratory of Polymer Materials Engineering, Polymer Research Institute of Sichuan University, 24, Southern Section 1,Yihuan Road, Chengdu 610065, Sichuan, PR China

Received 7 March 2006; received in revised form 29 April 2006; accepted 8 May 2006

Available online 12 June 2006

Abstract

This paper is focused on in situ preparation of melamine cyanurate (MCA) nanoparticles from reaction of melamine (MEL) and cyanuric acid(CA) and their flame retardant polyamide 6 (PA6) composite in the extrusion process through a novel reactive processing method. Fourier trans-form infrared (FT-IR), X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and scanning elec-tron microscopy (SEM) were utilized to characterize the in situ formed MCA nanoparticles and their blends with PA6. Introduction ofpentaerythritol (LTP) and water-bound plasticizer dioctyl phthalate (DPT) into the extrusion reaction system greatly inhibits the evaporationof water required for melamine and cyanuric acid reaction at high temperature (higher than 180 �C), laying a foundation for successful insitu preparation of MCA through reactive processing. XRD and FT-IR measurements indicate that under the effect of pentaerythritol, dioctylphthalate and water, melamine really reacts with cyanuric acid to in situ form MCA in extrusion process. The reaction degree is close to100%. A very important finding through SEM is that the in situ formed MCA particles, which were found to have aspect ratio of about 7.5,radial size in the range of 70e300 nm (mostly 70e90 nm) and crystallite size of less than 22 nm, are uniformly dispersed in the matrix PA6at nanoscale. The in situ formed MCA nanoparticles greatly improve the flame retardancy and the mechanical properties of flame-retardedPA6 materials, and the introduced plasticizer dioctyl phthalate also ameliorates the related impact property. The obtained flame-retarded PA6materials have good comprehensive performance with flame retardancy UL-94 V-0 rating at 1.6 and 3.2 mm thickness, tensile strength48.0 MPa, elongation at break 106.3% and Izod notched impact strength 8.92 kJ/m2. Compared with flame-retarded PA6 material with insitu formed MCA, the one prepared through conventional blending of PA6 with commercial MCA product has improved tensile strength butdeteriorated impact strength and flame retardancy.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Polyamide 6; Flame retardant; Melamine cyanurate; Nanoparticles; In situ formation

1. Introduction

Polyamide 6 (PA6) is one of the important engineeringplastics. Due to its good mechanical property, attrition resis-tance, oil resistance and ordinary organic solvent resistance,PA6 has been used widely [1,2]. However, PA6 with low lim-iting oxygen index (LOI) (23 or so) easily burns. In addition,the large quantity of heat, the high burning speed and the large

* 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.2006.05.002

amount of heavy smoke and melt drips occurring during thecombustion of PA6 lead to very easy spread of the flame. Asa result, how to improve the flame retardancy of PA6 becomesan important research topic [3e14].

Currently, several types of flame retardant (FR) additivesare used to impart flame retarding properties to PA6. Theseflame retardants can be divided into halogen FR, inorganicFR, phosphorous FR, nitrogen FR and nitrogenephosphorous(NeP) FR [1,5e7,9e16]. The halogen system is famous forits high efficiency, small additive dosage, reasonable costand small negative effect on the performance of materials.However, in recent years the application of halogen FRs is

mailto:[email protected]

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

2633Y. Chen et al. / Polymer Degradation and Stability 91 (2006) 2632e2643

being greatly restricted due to the possible dioxin and the largeamount of smoke and corrosive gases produced by pyrolysis orcombustion of the corresponding FR materials. In Europe, theapplications of some certain halogen FRs will even be pro-hibited to use in the near future. Halogen free system is beingbehalf of the developing trend of the flame retardants. Devel-opment of highly efficient halogen free flame retardants forPA6 has been the hot topic in flame retarding field. As far ashalogen free FRs are concerned, inorganic FRs and phospho-rous FRs are both confronted with many problems, e.g. theformer has disadvantages of high loading level, low flame re-tarding efficiency and heavy damage to the mechanical prop-erties of materials, and the latter has drawbacks of heavy color,toxicity and high exudation. By comparison, the nitrogenflame retardant, especially melamine cyanurate, has been suc-cessfully applied in flame retarding PA6 due to its many ben-efits such as non-toxicity, low loading level, goodcompatibility with PA6, etc. [1,6,7,12e14,17].

Melamine cyanurate (MCA), as an environmentallyfriendly flame retardant, was developed in early 1980s in Ja-pan, and is attracting more and more attention [6,7,12,17e23]. The traditional strategy of synthesizing MCA includesthe molecular complex reaction of melamine and cyanuricacid in a dispersing medium, i.e. hot water using alkali saltor alkali hydroxide as catalyst [17,24,25]. This method facesmany problems such as low production efficiency, high energyconsumption, too much waste water and off color of obtainedproduct. In addition, the flame retardancy of the final productcan be greatly impaired due to the introduction of the alkali-based catalyst, which cannot be easily removed from the sys-tem. As a result, the obtained original reaction product has tobe purified, leading to an increased cost and complicated pro-cess. In order to improve the traditional route of synthesizingMCA, a novel preparation method of MCA, i.e. molecularcomplexing modification, was established by Qi Wang’s group[26,27]. This technology not only greatly simplified the pro-duction process of MCA, but also improved the comprehen-sive performance of the flame-retarded material.

In this paper, based on the previous work [28,29], a technicalstrategy, i.e. reactive processing method, was proposed to syn-thesize melamine cyanurate in the presence of water in the ex-trusion process using melamine and cyanuric acid as rawmaterials. In this preparation process, in situ formation ofMCA and the preparation of its blend with PA6 were integratedinto one step to finish. So, the preparation of flame retardantPA6 composite can be greatly simplified without the first prep-aration of the MCA flame retardant. US patent 4,321,189 dis-closed a process for preparation of the improved flameretarding polyamide molding resins directly from melamine,cyanuric acid and polyamide and obtained melamine cyanuratewith crystallite size of less than 250 A [30]. Also, US patent5,037,869 disclosed a preparation process for flame retardantpolyamide molding material containing melamine cyanurateas a flame retardant which are prepared from melamine and cy-anuric acid in the presence of water, glycol and phthalate ester,a process similar to reactive processing [31]. However, in bothpatents, the structure and the morphology of the prepared flame

retardant polyamide composite are not clear. In this paper, mel-amine cyanurate with a certain aspect ratio and nano-radial par-ticle size was successfully obtained by the introduction ofa new type of modifier different from above mentioned patentsinto the reaction system and controlling the processing condi-tions. The structure and the property of the flame retardantPA6 composite were fully characterized and determined. In ad-dition, we also made a comparison of structure and perfor-mance of FR PA6 composite between the in situ preparedMCA and the commercial MCA product Melapur� MC50 ofCiba Specialty Chemicals.

2. Experimental

2.1. Materials

The following materials were used as received: melamine(MEL, chemically pure, supplied by Chengdu Kelong Chem-ical Plant, China), cyanuric acid (CA, industrially pure(�98.5%), supplied by Sichuan Chuanhua Stock Co., Ltd.),flame retardant modifier pentaerythritol (LTP, chemicallypure, supplied by Chengdu Kelong Chemical Plant, China), di-octyl phthalate (DPT, analytically pure, supplied by ChengduKelong Chemical Plant, China), melamine cyanurate (Mela-pur� MC50, with particle size D99%� 50 mm, supplied byCiba Specialty Chemicals Inc.), polyamide 6 (PA6, withrelative viscosity of 3.2 in 98% H2SO4 solvent, as granulateproduct supplied by Baling Petrochemical Co., China).

2.2. Flame-retarded samples’ preparation

For in situ preparation method, firstly, dioctyl phthalate(DPT), water and dried PA6 with various proportions weremanually well-mixed in a closed vessel, followed by 2 h ofstanding time. Then, calculated amounts of melamine(MEL), cyanuric acid (CA) and pentaerythritol (LTP) wereadded to above mentioned PA6 mixture and the obtained mix-ture was blended using high-speed mixer for another 10 min.For conventional preparation of FR PA6 material with Mela-pur� MC50, MC50 and dried PA6 with two proportionswere also mixed using high-speed mixer for 10 min. The finalwell-mixed ingredients of both methods were melt-blendedin a twin-screw extruder (f: 30 mm, L/D: 32, model: SLJ-30, Longchang Chemical Engineering Equipment Factory,China) at 235 �C. The residence time of the materials in ex-truder was determined as 2e5 min according to the set rota-tion rate. The extrudate was cut into pellets, dried and theninjection molded at 250 �C into various specimens for flam-mability and mechanical property tests. The compositionsand related numbers of the prepared FR composites are sum-marized in Table 1.

2.3. Determination of evaporation of water in hydratedPA6 system

The dried PA6, dioctyl phthalate (DPT), pentaerythritol(LTP) and water with various proportions shown in Table 2

2634 Y. Chen et al. / Polymer Degradation and Stability 91 (2006) 2632e2643

Table 1

The compositions of the flame-retarded PA6 materialsa

Samples In situ preparation method Conventional method

PA6 (parts)b H2O (parts)b LTP (parts)b DPT (parts)b MEL (parts)b CA (parts)b PA6 (wt%) MC50 (wt%)

PA6 100 0.0 0.0 0.0 0.0 0.0 e e

H0L0D0PA6 91.0 0.0 0.0 0.0 4.5 4.5 e e

HL0D0PA6 91.0 5.0 0.0 0.0 4.5 4.5 e eHL0DPA6 89.5 5.0 0.0 1.5 4.5 4.5 e e

H1LDPA6 89.5 1.0 1.8 1.5 3.6 3.6 e e

H2LDPA6 89.5 2.0 1.8 1.5 3.6 3.6 e e

H4LDPA6 89.5 4.0 1.8 1.5 3.6 3.6 e eH7LDPA6 89.5 7.0 1.8 1.5 3.6 3.6 e e

CB1PA6 e e e e e e 92.8 7.2

CB2PA6 e e e e e e 91.0 9.0

a For in situ preparation method, the total amount of all components of each sample excluding water, which will be removed in the subsequent process, is 100

parts by weight. The actual water content of formulation HL0D0PA6, HL0DPA6, H1LDPA6, H2LDPA6, H4LDPA6 and H7LDPA6 is 4.8, 4.8, 1.0, 2.0, 3.8,

6.5 wt%, respectively.b By weight.

were manually mixed well in a closed vessel, followed by 24 hof standing time. The obtained mixture was used to investigatethe effect of DPT and LTP on the evaporation of water in thepresence of PA6 in an electrical blast oven. The water loss ofhydrated PA6 system was recorded using electronic balance.

2.4. Characterization

The X-ray diffraction (XRD) patterns were obtained witha Philips company powder X’Pert X-ray diffractometer equip-ped with Cu Ka generator (l¼ 1.54178 A) at a scanning rateof 0.02 � per second in the 2q range of 10 �e50 �. The Fouriertransform infrared (FT-IR) spectra of samples were obtainedusing a Nicolet 20SXB FT-IR spectrometer. The differentialscanning calorimetry (DSC) measurements were conductedon a NETZSCH DSC 204 instrument thermal analyzer witha heating rate of 10 �C/min ranging from 25 to 350 �C (alsowith an identical cooling rate in the same temperature rangein the subsequent cooling process) and a dynamic nitrogenflow of 50 ml/min. The thermogravimetric analysis (TGA)curves were recorded on a General V 4.1c Dupont TA2100 in-strument thermal analyzer with a heating rate of 10 �C/min inthe range of 25e600 �C and a dynamic nitrogen flow of100 ml/min. The fractured surface of injection molded speci-men coated with a conductive gold layer was observed undera JEOL JSM-5900LV scanning electron microscope (SEM).

The vertical burning test was conducted on a CZF-3 hori-zontal and vertical burning tester, on sheets127� 12.7� 3.2 mm and 127� 12.7� 1.6 mm according to

Table 2

The compositions of the hydrated systems used in the evaporation experiment

Samples Components

PA6 (wt%) H2O (wt%) LTP (wt%) DPT (wt%)

EE01 95.5 4.5 0.0 0.0

EE02 93.5 4.5 2.0 0.0

EE03 93.5 4.5 0.0 2.0

EE04 91.5 4.5 2.0 2.0

the America National UL-94 test ASTM D3801. Tensile testswere carried out on an Instron universal testing machine 4302at room temperature and at a crosshead speed of 50 mm min�1

according to ASTM D638. Izod notched impact strength testswere conducted using an XJ40A impact tester according toASTM D256 standard.

3. Results and discussion

As far as the preparation of melamine cyanurate (MCA)from melamine (MEL) and cyanuric acid (CA) at moderateconditions is concerned, water is an absolutely necessary sub-stance for the reaction. The existence of water contributes tosalting reaction of MEL with CA and sufficient mixing andcontact of MEL with CA to a considerable degree, so doesthe reaction of MEL and CA conducted in extruder. We calledthe process involved above in extruder as reactive processing.As we know, the boiling point of water is 100 �C, the reactiveprocessing temperature must be at least more than the meltingpoint of PA6 (>220 �C), a temperature far higher than theboiling point of water. Consequently, how to prevent the evap-oration of water and how to prolong the residence time of wa-ter at high temperature become most important for the reactionof MEL and CA in the presence of water in extruder. In orderto solve above problems, dioctyl phthalate (DPT) and flameretardant modifier pentaerythritol (LTP) were introduced intothe reactive processing system to decrease the water loss athigh temperature, where, DPT was used as a water-bound plas-ticizer. The effect of DPT and LTP on the evaporation of waterin the hydrated PA6 system was investigated.

3.1. Evaporation behavior of water in the hydrated PA6system

Fig. 1 shows the effect of mixture of DPT with LTP and itsindividuals on the evaporation of water in the hydrated PA6system at different temperatures. For the hydrated PA6 systemwithout DPT and LTP, the weight loss of water increases rap-idly with increasing treating temperature, especially higher


than 150 �C, indicating that it is very difficult for water to bekept in the hydrated PA6 system if no measures are taken. Onthe contrary, the weight loss of water is greatly decreased uponaddition of 2.0 wt% LTP or 2.0 wt% DPT or their mixture tothe system. And with increasing treating temperature, the wa-ter loss also increases very slowly. Above experimental resultsshow that LTP, DPT and their mixture can all restrain the evap-oration of water. The possible reason for this can be explainedas: both LTP and DPT contain functional groups, which havethe capability of forming hydrogen bonding. For the former, itis eOH group, and for the latter, it is CeO and C]O group.Consequently, on one hand, water evaporation can be pre-vented by forming hydrogen bonding of LTP or DPT with wa-ter, especially the free water in the hydrated PA6 system; onthe other hand, the possibly enlarged confined space betweenPA6 molecular chains by the formation of hydrogen bondingbetween LTP or DPT molecules with larger size and abovemacromolecular chains contributes to accommodation ofmore water molecules, thus greatly restraining the water evap-oration. The suppression effect of LTP seems to be strongerthan that of DPT. However, the combination of LTP withDPT does not show a synergistic suppression effect on theevaporation of water, but presents a tendency to be closer tothe evaporation behavior of the hydrated system with onlyDPT. In the present study, the aim of the addition of LTPand DPT can be summarized as: (1) to effectively preventthe evaporation of water at high temperature with the promo-tion of the reaction of MEL with CA; (2) to take advantage ofthe possible synergistic effect in flame retarding PA6 betweenLTP and the formed MCA; (3) to improve the mechanicalproperty such as impact strength using the plasticizing prop-erty of DPT.

3.2. In situ formation of MCA through reactiveprocessing

Melamine cyanurate (MCA) is formed through the reactionof MEL and CA in the presence of water at a certain

Fig. 1. Effect of LTP, DPT and their mixture on the evaporation of water in

various hydrated systems with 10 min thermal treatment in air.

temperature. Obviously, increase of the temperature will con-tribute to the conduction of MELþCA reaction, which can beeasily satisfied by the processing conditions of extruder. Inview of the structure, on one hand, there is a condition of for-mation of hydrogen bonding between eNH2 group of MELmolecule and eOH group of CA molecule. So, MEL andCA molecules can form complex compound [20e23]. Onthe other hand, the conjugative effect of the big p electroncloud of benzenoid ring in molecules of MEL and CA endowsthe functional groups connected with these rings with propertyof positive or negative charge. We know, eNH2 group isa kind of electron repulsive one, but pC¼O group is a kindof electron attractive one. As a result, the MEL and CA mol-ecules with both groups mentioned above, respectively, can at-tract each other and form melamine cyanurate (MCA) witha very stable structure at a certain temperature condition [17].

In the present study, mixture of PA6, water, MEL, CA, LTPand DPT was reactively extruded in a twin-screw extruder. Theformed MCA was characterized using XRD and FT-IR. Fig. 2shows XRD result of the prepared FR PA6 composite contain-ing in situ formed MCA. For the purpose of comparison, therelated XRD curves of CA, mixture of MEL with CA andpure PA6 were included in Fig. 2. Comparing the XRD dataof CA (Fig. 2b), mixture of MEL with CA (Fig. 2a) and FRPA6 composite (Fig. 2c), it can be seen that in the FR PA6composite the diffraction peaks almost completely disappear(2q¼ 13.29 �, 17.88 �, 19.95 �, 22.22 �, 26.38 � 28.97 � and29.92 �) reflecting the characteristics of the mixture of MELand CA, but some new peaks appear especially at 2q of12.09 �, 20.57 �, 21.71 �, 23.44 �, 28.34 � and 37.49 � (the corre-sponding d value is 7.31, 4.32, 4.09, 3.79, 3.15 and 2.40, re-spectively), of which position agrees very well withliterature values of the main characteristic diffraction maxi-mums of MCA (2q¼ 10.91 �, 11.94 �, 20.44 �, 22.14 �,24.09 �, 28.12 �, 37.27 � and d¼ 8.10, 7.40, 4.34, 4.01, 3.69,3.17 and 2.41, respectively) [32]. Above characterizations

Fig. 2. XRD patterns of the mixture of CA with MEL (a), CA (b), FR PA6

composite containing in situ formed MCA through reactive processing (c)

and pure PA6 (d).


prove that the reaction product in situ formed in FR PA6 com-posite is MCA, and the reaction degree becomes close to100%. Also, partial broad diffraction peaks of FR PA6 com-posite in the range of 2q values 19e23 � (Fig. 2c) are causedby the a-type crystal of PA6 (Fig. 2d, two peaks at 19.8 �

and 23.7 �).Fig. 3 includes the FT-IR spectra of MEL (a), CA (b), the

mixture of MEL with CA (c), pure PA6 (d), FR PA6 composite(e) and MCA synthesized via traditional method (f). In Fig. 3a,the double peaks at 3468 and 3418 cm�1 are ascribed to thesymmetrical and asymmetrical stretching vibrations of eNH2 group in MEL molecule, respectively. The absorptionsat 1653e1437 cm�1 are caused by vibration of the frameworkof MEL molecules. For the spectrum of CA (Fig. 3b), there aresome absorptions of enantiotropic isomers between alcoholand ketone existing in the molecules of CA, e.g. the absorp-tions at 3403 cm�1 and at 1720e1760 cm�1 can be attributedto the vibrations of pNeH group and pC]O group in the ke-tone isomer of CA, respectively. However, the absorptions at3027 cm�1 and at 1467e1397 cm�1 are caused by the vibra-tions of eOH group and the framework of triazine ring inthe alcohol isomer of CA, respectively. The spectrum of themixture of MEL with CA (Fig. 7c) just rises from the simplesuperposition of the FT-IR spectrum of MEL and CA. Somesignificant changes of some bands in the range of 1800e1100 cm�1 can be found on comparing FT-IR spectrum ofthe reaction product (Fig. 7e) with that of reactants MELand CA, e.g. relative to MEL and CA, the peak intensity of re-action product set at 1736, 1640, 1542 and 1202 cm�1 is en-hanced, but the one set at 1452 and 1377 cm�1 is weakened.Also, it can be noted that there are some big peak shifts occur-ring between the spectra of reactants and the reaction product.These shifts include change from CA 1723 cm�1 to product

1736 cm�1, from MEL 1660 cm�1 to product 1640 cm�1,from CA 1467 cm�1 to product 1452 cm�1, etc. All thechanges mentioned above indicate that MEL really reactswith CA through strong interaction (hydrogen bonding or elec-trical charge attraction) in the presence of water in the extru-sion process and the reaction degree is very high. The reactionproduct was confirmed as MCA by comparing its absorptionswith the characteristic absorptions of the traditionally synthe-sized MCA (Fig. 3f) at 1781, 1739, 1665, 1536, 1448 and1203 cm�1. However, there are still some differences betweenthe spectra of the in situ formed MCA in FR PA6 and that ofpure MCA (Fig. 3f) especially at 1780, 1738, 1263 and1201 cm�1, indicating the possible strong interaction, e.g. hy-drogen bonding between the in situ formed MCA and matrixPA6, which needs further confirmation by solid state NMRspectra. The absorptions at 3300, 3072, 2937 and 2865 cm�1

occurring in the spectra of FR PA6 material are contributedby the matrix PA6 according to the spectra of pure PA6(Fig. 3d). The occurrence of 1640e1542 cm�1 matrix PA6 ab-sorptions are also superposed with that of the formed MCA inFR PA6 material.

3.3. Structure characterization of PA6/MCA flameretardant composite

3.3.1. FT-IR analysisThe absorptions at 3300e2800 cm�1 and 1640e

1542 cm�1, which are the characteristic bands of pure PA6,can all be found in the FT-IR spectra of H0L0D0PA6(Fig. 4a), HL0D0PA6 (Fig. 4b) and H4LDPA6 (Fig. 4c). ForH0L0D0PA6 and HL0D0PA6, their spectra are similar: bothhave the characteristic absorptions of the simple mixture ofMEL and CA at 3468e3400, 1653e1397 and 1088e522 cm�1

Fig. 3. FT-IR spectra of MEL (a), CA (b), the mixture of MEL with CA (c), pure PA6 (d), FR PA6 composite with in situ formed MCA (e) and MCA synthesized

via traditional method (f).


Fig. 4. FT-IR spectra of FR PA6 composites numbered with H0L0D0PA6 (a), HL0D0PA6 (b) and H4LDPA6 (c).

and also have the very weak absorptions of MCA at 1640e1204 cm�1, indicating the occurrence of the reaction betweenreactants MEL and CA only in a very small amount in bothsystems. Obviously, for H0L0D0PA6, due to the absence ofwater in the system, only a very small amount of reactantscan react under the effect of high processing temperature(240 �C). However, for HL0D0PA6, despite the presence ofwater in the system, the reaction of most reactants MEL andCA cannot be similarly ensured to occur due to the lack ofsomething that can prevent the violent evaporation of waterat high temperature. This is because the water in the systemhas almost completely evaporated before the reaction ofMEL with CA at extrusion processing temperature(T> 230 �C). It is very important to add something that canrestrain the violent evaporation of water at high temperaturein the extrusion system for the MEL and CA reaction to besmoothly conducted. According to previous discussion, addi-tion of modifier LTP and water-bound plasticizer DPT to thehydrated PA6 system can prevent the evaporation of water athigh temperature to a considerable degree. As a result, for

H4LDPA6 system, the full reaction of MEL and CA can be en-sured by the effective delay of water evaporation, as proved bythe very strong absorptions of MCA appearing in its FT-IRspectra (Fig. 4c).

Fig. 5 shows the FT-IR spectra of FR PA6 material preparedfrom reaction extrusion system containing H2O, LTP, and DPTwith different water content. It can be seen that according tothe change of absorption strength of in situ formed MCA at1780 and 1732 cm�1, proper increase of water content in ex-trusion reaction system contributes to the formation ofMCA. This is consistent with the effect of water content in re-action system on water evaporation. Our investigation showsthat proper increase of water content in the hydrated PA6 sys-tem can also help to slow down the water evaporation at hightemperature.

3.3.2. TG analysisFig. 6 shows the thermal weight loss (TG) and differential

weight loss (DTG) of pure PA6 and the prepared FR PA6 ma-terial (H4LDPA6). PA6 starts to decompose at 400 �C with

Fig. 5. FT-IR spectra of FR PA6 composites with 2.0 wt% H2O (a) and 3.8 wt% H2O (b), in the extrusion reaction system containing H2O, LTP and DPT.


only one weight loss region and the decomposition ends atabout 500 �C. Compared to pure PA6, H4LDPA6 has twoweight loss regions, i.e. 316e340 and 390e500 �C. The deg-radations occurring in these two regions can be reasonablyspeculated as following: first, the weight loss in the range of316e340 �C can be possibly attributed to the following fac-tors: (1) the escape of small molecules of LTP and DPT intro-duced into the extrusion reaction system, (2) thedecomposition of MCA itself, (3) the degradation of matrixPA6 and (4) the reaction of MCA (or its degradation products)with the degradation products of PA6 [7]. Second, the weightloss ranging from 390 to 500 �C is mainly caused by the deg-radation of formed MCA, the degradation of PA6 polymerchain into low molecular weight substances and the reactionof MCA (or its degradation products) with caprolactam (orits oligomers) produced from the degradation of PA6. WhenFR PA6 composite suffers from high temperature, the gener-ated PA6 degradation products in the form of monomer capro-lactam or its oligomers with lower molecular weight can flowto drip due to its low viscosity to remove a great deal of heatreleased from combustion of FR PA6 material, protecting thebase material from continual burning. On the other hand, someinert gases generated from the degradation of MCA at hightemperature can dilute the oxygen in air and hence playa role of flame retardation. DTG can reflect the decompositionrate of a chemical substance. The DTG curves in Fig. 6 showthat the decomposition rate of FR PA6 composite is lower thanthat of pure PA6, indicating that the in situ formed MCA slowsdown the high decomposition rate of PA6 at high temperature,i.e. decreases the release rate of the combustible gases pro-duced by the decomposition of PA6 and also contributes tothe protection of matrix.

3.3.3. DSC analysisThe crystallization behavior of FR PA6 composite was also

investigated using DSC technique. Fig. 7 shows the heatingcurves of pure PA6 (a), H0L0D0PA6 (b), HL0D0PA6 (c)and H4LDPA6 (d) and Fig. 8 just shows their cooling curves.For convenience, the melting and crystallizing parameters are

Fig. 6. TGeDTG curves of pure PA6 and FR PA6 composite (H4LDPA6).

included in Table 3. From Fig. 7, it can be seen that for purePA6 (Fig. 7a), a weak shoulder endothermic peak just appearsat a lower temperature (about 210 �C) near the main meltingpeak. The shoulder peak corresponds to the melting heat ab-sorption of g type crystal of pure PA6 [33] (as confirmed bythe weak peak in the XRD pattern of pure PA6 at2q¼ 21.3 � in Fig. 2), while the main peak corresponds tothe one of a-type crystal of pure PA6. When the mixture ofMEL and CA is blended with pure PA6, the obtained FR

Fig. 7. DSC heating curves of pure PA6 (a), and FR PA6 composites numbered

with H0L0D0PA6 (b), HL0D0PA (c) and H4LDPA6 (d).

Fig. 8. DSC cooling curves of pure PA6 (a), and FR PA6 composites numbered

with H0L0D0PA6 (b), HL0D0PA6 (c) and H4LDPA6 (d).


PA6 material (H0L0D0PA6) has an enhanced shoulder endo-thermic peak at the lower temperature (Fig. 7b), indicatingmore g type crystals of PA6 formed in the system. The mixtureof MEL with CA in PA6 has a promoting effect on the forma-tion of g type crystal. However, for the hydratedPA6þMELþCA (HL0D0PA6) and the hydratedPA6þMELþCAþ LTPþDPT (H4LDPA6) systems, theshoulder endothermic peak in their melting region disappears,indicating that the in situ formed MCA in extrusion reactionsystem has an inhibitive effect on the formation of g type crys-tal of PA6. When the content of the formed MCA in FR PA6material exceeds a certain degree, the g type crystal of PA6 isno longer formed. From another side, this indicates that rela-tive to anhydrous PA6þMELþ CA system (H0L0D0PA6),the hydrated system (HL0D0PA6) has a slightly more amountof MCA formed. Consequently, the change in peak shape ofDSC heating curves of above systems reflects the compositionchange in the corresponding FR composite to a certain degree.

Samples a (PA6), b (H0L0D0PA6), c (HL0D0PA6) andd (H4LDPA6) were first melted and then cooled to room tem-perature at a rate of 10 �C min�1. The obtained DSC curvesare shown in Fig. 8. Combined with Table 3, relative to purePA6, the onset crystallization temperature ðTc0

Þ, crystallizationtemperature (Tc) and onset melting temperature ðTm0

Þ of sam-ples b, c and d are all increased to a greater degree, indicatingthat the mixture of MEL and CA with a very small amount offormed MCA in sample b, the mixture of MEL and CA witha small amount of formed MCA in sample c and the formedMCA with almost 100% conversion in sample d all play a het-erogeneous nucleation role in the crystallization process ofPA6. From samples b to d, the MCA content increases andthe corresponding crystallization temperature also increasesfrom 188 to 193 �C, indicating that the reaction productMCA has a more remarkable heterogeneous nucleation effecton the crystallization of PA6 than the reactant mixture of MELwith CA. In the mean time, it is also noted that with increas-ingly formed MCA content (samples bed), the crystallizationdegree decreases from 32.7 to 25.8%, while the crystallizationbandwidth increases from 9 to 15 �C. It is known that reactionproduct MCA possesses a stronger interaction or compatibilitywith PA6 polymer chain than the reactants MEL and CA. So,with increasing MCA content in samples, the movement ofPA6 polymer chain segments is restrained more greatly,

Table 3

DSC analytical results of FR PA6 composites

Samples Tc0

( �C)

Tc

( �C)

DTc

( �C)

DHc

(J/g)

Tm0

( �C)

Tm

( �C)

DTm

( �C)

DHm

(J/g)

Xc

(%)

PA6 control 176 182 11 62.1 205 221 21 51.0 26.8

H0L0D0PA6 183 188 9 69.2 205 218 17 62.2 32.7

HL0D0PA6 182 189 12 63.4 208 220 18 56.8 29.9

H4LDPA6 185 193 15 54.5 209 220 16 49.1 25.8

Note: Tc0, Tc, DTc, DHc, Tm0

, Tm, DTm, DHm and Xc are onset crystallization

temperature, temperature of crystallization peak, temperature width of crystal-

lization band, enthalpy of crystallization, onset melting temperature, tempera-

ture of melting peak, temperature width of melting band, enthalpy of melting

and crystallinity, respectively. DHf(PA6)¼ 190 J/g.

holding back the formation of crystal. As a result, the incom-plete degree of PA6 crystal increases and the deficiency aug-ments too, leading to a decreasing crystallization degree anda broadening crystallization bandwidth.

3.3.4. XRD analysisFig. 9 shows the XRD patterns of H4LDPA6 (a),

HL0D0PA6 (b) and H0L0D0PA6 (c). At 2q of 26.2 �, 28.8 �

and 29.8 � (the corresponding d value is 3.38, 3.08 and 2.98)appear the characteristic diffraction peaks of the mixture ofMEL and CA in both samples b and c, indicating that thereare still a great deal of un-reacted mixtures of MEL and CAcontained in both FR PA6 materials prepared from anhydrousMELþ CA system and hydrated MELþ CA system, respec-tively. In curve a, there are strong featured diffraction peaksof MCA and almost no featured ones of the mixture of MELwith CA appearing, indicating that for sample a, the reactionof MEL with CA becomes almost complete. Above analyticalresults are in agreement with the FT-IR results.

Fig. 10 shows the effect of water content in the extrusionreaction mixture on the XRD result of the corresponding FRPA6 material. With increasing water content in the reactionmixture, the characteristic diffraction peak intensity of theformed reaction product MCA at 2q of 28.34 � first increasesand then decreases. The peak value of the intensity appearsat 3.8 wt% water content. In addition, with increase of the wa-ter content from 1.0 to 6.5 wt%, the characteristic diffractionpeak intensity of mixture of reactants MEL and CA at 2q of29.92 � first decreases (even close to zero) (3.8 wt% water con-tent) and then augments (6.5 wt% water content). Above ex-perimental results indicate that proper increase of the watercontent in the reaction system contributes to the formationof MCA in FR PA6 material. However, excessive water con-tent can be detrimental to the formation of MCA instead.The reason deserves further investigation.

Based on the XRD data (Fig. 10), the crystallite size (L) ofthe formed MCA in FR PA6 material can also be calculatedusing Scherrer’s equation, which has the following formula

Fig. 9. XRD patterns of FR PA6 composites numbered with H4LDPA6 (a),

HL0D0PA6 (b) and H0L0D0PA6 (c).


L¼ kl

b cos q

wherein, L¼ average crystallite dimension of MCA measuredalong the direction perpendicular to the lattice planes withd spacing 3.17 A, l¼wavelength of the employed X-ray(A), b¼ half width of the diffraction peak corresponding tothe lattice planes with d spacing 3.17 A, q¼ diffraction angleof the lattice planes with d spacing 3.17 A and k¼ Scherrer’sconstant 0.9 [34].

The calculated results are shown in Table 4. As can be seen,the formed MCA particles in the prepared FR PA6 compositesat various water content all have a substantially small averagecrystallite size of less than 22 nm and hence an expectedly ex-cellent dispersion in matrix PA6, which can be further provedby the following morphological analysis. It can also be seenthat the crystallite size of MCA formed in FR PA6 compositeprepared at 3.8 wt% water content is slightly bigger than thatof MCA formed in FR PA6 composite prepared at other watercontent. This is possible because this water content (3.8 wt%)is advantageous to the growth of MCA crystal except for con-version of reactants to MCA at the selected processing condi-tions stated previously.

3.3.5. Morphology analysisFig. 11 shows the morphology of the fractured surface of

H4LDPA6 (sample a, Fig. 11a), HL0DPA6 (sample b,

Fig. 10. XRD patterns of FR PA6 composites with 1.0 wt% H2O (a), 2.0 wt%

H2O (b), 3.8 wt% H2O (c) and 6.5 wt% H2O (d), in the extrusion reaction sys-

tem containing H2O, LTP and DPT.

Table 4

The crystallite size (L) of MCA in FR PA6 composite in situ formed in extru-

sion process

Samples H1LDPA6a H2LDPA6b H4LDPA6c H7LDPA6d

L (nm) 17.5 19.2 21.6 19.1

a Prepared at 1.0 wt% H2O.b Prepared at 2.0 wt% H2O.c Prepared at 3.8 wt% H2O.d Prepared at 6.5 wt% H2O.

Fig. 11b), H0L0D0PA6 (sample c, Fig. 11c) and HL0D0PA6(sample d, Fig. 11d). It is very surprising that for sample a,the reaction of MEL and CA in extrusion process generatesMCA particles with a certain aspect ratio (Fig. 11a). The as-pect ratio was estimated as high as 7.5. The radial size ofthe in situ formed MCA particles was found in the range of70e300 nm, mostly in the range of 70e90 nm and is 3e4times larger than their crystallite size (Table 4). Hence, theformed MCA particle can be regarded as the congeries ofsome crystallites. Above results have not been reported sofar. In addition, the formed MCA particles are dispersed uni-formly in PA6 matrix and have a good compatibility withPA6. The formation of MCA nanoparticles with a certain as-pect ratio can be explained as that the in situ formed MCAcrystallites can orientationally align along the direction ofthe very strong shear force of twin-crew extruder. It can bepredicted that the correspondingly prepared FR PA6 material(H4LDPA6) has good mechanical properties and flame retard-ancy. The addition of LTP and DPT to the extrusion reactionsystem plays an important role in decreasing the domainsize of the in situ formed MCA particles and in improvingtheir dispersing morphology in matrix PA6, which was con-firmed in Fig. 11b. Compared to the reaction system involvedin Fig. 11a (H4LDPA6), the one involved in Fig. 11b(HL0DPA6) only lacks LTP. However, the domain size ofthe latter obviously increases to 2e4 mm. Its flame retardantdispersion is not uniform and the compatibility with matrixresin is also poor. In the FR PA6 materials involved inFig. 11c and d, the FR particle size is even bigger and theFR dispersion is more uneven. As a result, the domain sizeof FR PA6 material can be controlled by the introduction ofFR modifier LTP and water-bound plasticizer DPT and changeof their content.

It is well known that Melapur� MC series are famous mela-mine cyanurate (MCA) flame retardants developed by CibaSpecialty Chemicals Inc. in recent years and are widely usedin unfilled and mineral filled polyamide 6 (or 66), thermoplasticpolyurethanes and epoxies. It is interesting to make a compari-son between Melapur� MC product and our in situ formedMCA based on the structure and performance of their flame-re-tarded PA6 materials. Here, we used Melapur� MC50 for inves-tigation. The fractured surface morphology of PA6/MC50 FRcomposite was obtained by SEM observation as displayed inFig. 12. The morphology of FR PA6 material at both concentra-tions (7.2 and 9.0 wt.%) is similar: compared with the in situformed MCA (Fig. 11a), the particle size of MC50 in PA6 ma-trix is much larger and the particle size distribution is wider(ranging from 0.4 to 1.0 mm), and the FR dispersion is not uni-form. In addition, the clear interfacial lines between MCA par-ticles and PA6 matrix show the poor compatibility in the system.However, it is interestingly found that a part of MCA particleshave a certain aspect ratio, and relative to the original particlesize of MC50 (D99%� 50 mm), the particle size of MC50 hasbeen decreased too much after melt blending with PA6, indicat-ing that MC50 particles can be further broken down and dis-torted under the shear force of screw during processing. Therelated performance was given by the following part.


Fig. 11. SEM images of FR PA6 composites numbered with H4LDPA6 (a), H0L0D0PA6 (b), HL0DPA6 (c) and HL0D0PA6 (d).

3.4. Performance of PA6/MCA flame retardantcomposite

Table 5 compares the flame retarding properties of FRPA6 material prepared from mixture of MEL, CA and PA6(H0L0D0PA6), mixture of water, MEL, CA and PA6(HL0D0PA6), mixture of water, LTP, DPT, MEL, CA andPA6 (H4LDPA6), mixture of 7.2 wt% MC50 with PA6(CB1PA6) and mixture of 9.0 wt% MC50 with PA6(CB2PA6). Obviously, the FR PA6 composite with in situformed MCA nanoparticles can achieve V-0 flame retardinglevel at both 1.6 and 3.2 mm thickness in the UL-94 test,and the dripping particles did not ignite the cotton fibers.

But, H0L0D0PA6 and HL0D0PA6 can only achieve V-2 ratingand the former cannot even be classified at 1.6 mm thicknessin UL-94 test because there are only a very small amount ofMCA particles formed in PA6 matrix. Above experimental re-sults reveal that the in situ formed nano-MCA particles playa very important role in improving the flame retardancy ofPA6 material. In comparison with sample containing in situformed nano-MCA (H4LDPA6), the FR PA6 materials atboth 7.2 wt% (CB1PA6) and 9.0 wt% (CB2PA6) MC50 load-ing have poorer flame retardancy (only V-2 for the former andV-0(t) for the latter at 1.6 mm thickness), due to their poorerdispersion of MCA particles. The mechanical performanceof FR PA6 material with nano-MCA particles formed

Fig. 12. SEM images of FR PA6 composite with 7.2 wt% (a) and 9.0 wt% (b) MCA (Melampur� MC50) produced from Ciba Specialty Chemicals.


Table 5

The flame retardancy of FR PA6 materials prepared at different conditions

FR PA6 material H0L0D0PA6 HL0D0PA6 H4LDPA6 CB1PA6 CB2PA6

UL-94 at 1.6 mm

thickness

tf a (s) e 104.7 7.8 90.1 34.1

Rating NC V-2 V-0 V-2 V-0(t)b

The amount of specimens

of no ignition of cotton

0/5 0/5 5/5 0/5 1/5

UL-94 at 3.2 mm

thickness

tf a (s) 119.4 95.0 0 256.9 49.0

Rating V-2 V-2 V-0 NC V-2

The amount of specimens

of no ignition of cotton

0/5 0/5 5/5 0/5 0/5

a Means the total duration (five specimen) of flaming combustion.b V-0(t) means that the tested sample only meets the requirements of combustion duration for UL-94 V-0 level.

(H4LDPA6) is shown in Table 6. For the convenience of com-parison, the mechanical properties of pure PA6, H0L0D0PA6,HL0D0PA6, CB1PA6 and CB2PA6 are also included in Table6. Compared with pure PA6, the elongation and notched im-pact strength of H4LDPA6 are improved greatly and reach106.3% and 8.92 kJ/m2, respectively. The tensile strength ofH4LDPA6 does not decrease much (only from control 51.2to 48.0 MPa). The increase in impact strength and the decreasein tensile strength can be explained by the plasticizing role ofwater-bound plasticizer DPT added into the extrusion reactionsystem. For H0L0D0PA6 and HL0D0PA6, except for the ten-sile strength, both their elongation and notched impactstrength decrease too much relative to that of pure PA6. Thisindicates that the in situ formed nano-MCA particles also con-tribute to the improvement of the mechanical properties of FRPA6 materials.

Similarly, compared with pure PA6 and also withH4LDPA6, PA6/MC50 FR composites present an increasedtensile strength (53.7 MPa at 9.0 wt% FR loading) and a de-creased impact strength (6.59 kJ/m2 at 9.0 wt% FR loading),which can be ascribed to the reinforcing effect of the filledMC50 particles with a certain aspect ratio and to the poorcompatibility of MC50 with PA6 matrix, respectively. If sur-face modification is done on MC50 particles, the flame retard-ancy and the mechanical performance, especially the impactstrength of the FR PA6 material can be improved.

Reactive processing method is an attractive novel technol-ogy to prepare FR PA6 composite with good comprehensiveperformance.

Table 6

The mechanical properties of FR PA6 materials prepared at different

conditions

FR PA6

material

Tensile

strength

(MPa)

Young’s

modulus

(MPa)

Elongation

at break

(%)

Izod notched

impact strength

(kJ/m2)

PA6 control 51.2 2167 169.4 7.58

H0L0D0PA6 49.2 2028 22.0 6.13

HL0D0PA6 51.7 2938 28.6 6.01

H4LDPA6 48.0 2235 106.3 8.92

CB1PA6 55.7 2182 33.2 6.85

CB2PA6 53.7 2310 35.2 6.59

4. Conclusions

Twin-screw extruder was demonstrated to be a good reactorto conduct the reaction of melamine (MEL) and cyanuric acid(CA) in the presence of water in the process of blending withmolten PA6 in this study. In order to retain the necessary waterrequired for the reaction of MEL with CA at high temperature,the flame retardant modifier LTP and water-bound plasticizerDPT were introduced into the extrusion reaction system, whichsuccessfully prevents the evaporation of water at high temper-ature to a considerable degree. X-ray diffraction (XRD) andFourier transfer infrared (FT-IR) analyses showed that duringreactive extrusion process MEL and CA really react to insitu form melamine cyanurate (MCA) with a reaction degreeclose to 100% conversion, proving the feasible strategy of insitu preparation of MCA and the corresponding flame retar-dant PA6 composite in extruder. In the extrusion reaction sys-tem, the presence of only water cannot ensure the formation ofMCA. Only when there are proper amounts of LTP, DPT andwater all existing in the reaction system, the reaction of MELwith CA can really occur to form MCA with an almost fullconversion. Proper increase of water content contributes tothe formation of MCA. Both the mixtures of MEL with CAand their reaction product MCA have a heterogeneous nucle-ation effect on PA6, but the influence of the latter is more re-markable. The mixture of MEL with CA can promote theformation of g crystal of PA6. The reaction product MCA,however, prevents the formation of g crystal. It was foundthat in the FR PA6 material prepared through reactive process-ing, the in situ formed MCA particles have a certain aspect ra-tio as high as 7.5, radial size in the range of 80e300 nm(mostly 70e90 nm) and crystallite size of less than 22 nm,and disperse uniformly in the matrix PA6. The in situ formedMCA nanoparticles contribute to the improvement of both theflame retardancy and the mechanical properties of FR PA6 ma-terials, while the introduction of DPT contributes to improve-ment of the impact performance of FR PA6 materials. Theobtained FR PA6 materials possess good comprehensive per-formance with flame retardancy UL-94 V-0 rating at 1.6 and3.2 mm thickness, tensile strength 48.0 MPa, elongation atbreak 106.3% and Izod notched impact strength 8.92 kJ/m2

and have a broader application prospect. The comparison of


performance between in situ formed MCA and Melapur�

MC50 shows that the FR PA6 material with the latter has in-creased tensile strength and decreased impact strength andflame retardancy. Reactive processing is surely a simple, effi-cient, clean, and environmentally friendly technique deservingfurther development.

Acknowledgements

This work is supported by National Natural Science Foun-dation of China (20404009), National Basic Research Programof China (2003CB615600) and Youth Science Foundation ofSichuan University.

References

[1] Levchik SV, Weil ED. Polymer International 2000;49:1033e73.

[2] Luo Y, Xiao QS. China Plastics 1998;12(5):9.

[3] Jang BN, Wilkie CA. Polymer 2005;46(10):3264e74.

[4] Lei S, Yuan H, Lin ZH, Xuan SY, Wang SF, Chen ZY, et al. Polymer

Degradation and Stability 2004;86(3):535e40.

[5] Lewin M, Brozek J, Martens MM. Polymers for Advanced Technologies

2002;13(10e12):1091e102.

[6] Casu A, Camino G, De Giorgi M, Flath D, Morone V, Zenoni R. Polymer

Degradation and Stability 1997;58:297e302.

[7] Gijsman P, Steenbakkers R, Furst C, Kersjes J. Polymer Degradation and

Stability 2002;78:219e24.

[8] Schartel B, Potschke P, Knoll U, Abdel-Goad M. European Polymer

Journal 2005;41:1061e70.

[9] Levchik SV, Costa L, Camino G. Polymer Degradation and Stability

1992;36(3):229e37.

[10] Balabanovich AI, Levchik GF, Levchik SV, Schnabel W. Fire and Mate-

rials 2001;25(5):179e84.

[11] Levchik SV, Levchik GF, Camino G, Costa L, Lesnikovich AI. Fire and

Materials 1996;20(4):183e90.

[12] Levchik SV, Balabanovich AI, Levchik GF, Costa L. Fire and Materials

1997;21(2):75e83.

[13] Ou YX, Chen Y, Wang XM, editors. Flame-retarded polymeric materials.

1st ed. Beijing: National Defense Industrial Press; 2000.

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

Chemical Industrial Press; 2002.

[15] Hornsby PR, Wang J, Rothon R, Jackson G, Wilkinson G, Cossick K.

Polymer Degradation and Stability 1996;51(3):235e49.

[16] Jahromi S, Gabrielse W, Braam A. Polymer 2003;44(1):25e37.

[17] Yang SL, Xiao JJ, Li YT, Shi WJ, Song PL. Journal of the Hebei Acad-

emy of Sciences 2000;17(4):219e23.

[18] Miyamoto T, Sato I, Ando Y. IEEE Transactions on Magnetics

1987;23(5):2386e8.

[19] Tsuya Y, Watanabe M, Hirae T, Sato M, Kawakita A. ASLE Transactions

1981;24(1):49e60.

[20] Bielejewska AG, Marjo CE, Prins LJ, Timmerman P, De Jong F,

Reinhoudt DN. Journal of the American Chemical Society

2001;123(31):7518e33.

[21] Kimizuka N, Kawasaki T, Hirata K, Kunitake T. Journal of the American

Chemical Society 1998;120(17):4094e104.

[22] Arduini M, Crego-Calama M, Timmerman P, Reinhoudt DN. Journal of

Organic Chemistry 2003;68(3):1097e106.

[23] Seto CT, Whitesides GM. Journal of the American Chemical Society

1990;112(17):6409.

[24] Ohmura Y, Murakami Y, Hidaka R. US Patent 4,298,518; 1981.

[25] Yanagimoto A, Kumazawa S, Takakuwa Y. US Patent 4,180,496; 1979.

[26] Liu Y, Wang Q. Polymer Materials Science and Engineering (China)

2006;22(2):169e72.

[27] Liu Y, Wang Q, Hu FY. Polymer Materials Science and Engineering

(China) 2004;20(3):220e3.

[28] Chen YH, Liu Y, Wang Q, Yin H, Aelmans N, Kierkels R. Polymer Deg-

radation and Stability 2003;81:215e24.

[29] Wang Q, Chen YH, Liu Y, Yin H, Aelmans N, Kierkels R. Polymer In-

ternational 2004;53:439e48.

[30] Ohshita H, Tsutsumi T. US Patent 4,321,189; 1982.

[31] Sprenkle Jr., William E. US Patent 5,037,869; 1991.

[32] Siegel A. Cyanamide Company, Stamford, CI, USA, private

communication.

[33] Du QG, Wang RH, Chen WJ. Polymer Materials Science and Engineer-

ing (China) 1991;3:28e35.

[34] Li Q, Zhao ZD, Ou YC, Qi ZN, Wang FS. Acta Polymerica Sinica

1997;2:188e93.

preparation of flame retardant polyamide 6 composite with melamine cyanurate nanoparticles in situ...

Documents